AVIGATION AND NTEGRITY UTONOMOUS ATELLITE AVIGATION...

S T A T E M E N T

O F W O R K

document title/ titre du document

AVIGATION AND NTEGRITY UTONOMOUS ATELLITE AVIGATION YSTEM

prepared by/ reference/réference issue/édition 3 revision/révision 6 date of issue/date d’édition 23-01-2006 status/état Issue Document type/type de document Statement of Work Distribution/distribution

préparé par

a

ESTEC Keplerlaan 1 - 2201 AZ Noordwijk - The Netherlands Tel. (31) 71 5656565 - Fax (31) 71 5656040

GSP_SoW 3.6.doc

Navigation and Integrity Autonomous Satellite Navigation System issue 3 revision 6 -

page ii of iv

A P P R O V A L

Title Navigation and Integrity Autonomous Satellite Navigation System titre

issue issue

3 revision revision

6

author auteur

date date

23-01-2006

approved by Activity TEB approuvé by

date date

23-01-2006

C H A N G E L O G

reason for change /raison du changement issue/issue revision/revision date/date

C H A N G E R E C O R D

Issue: 3 Revision: 6

reason for change/raison du changement page(s)/page(s) paragraph(s)/paragraph(s)


page iii of iv

T A B L E O F C O N T E N T S

1 INTRODUCTION ......................................................................................................................1 1.1 Scope..................................................................................................................................................1 1.2 Applicable Documents .......................................................................................................................2 1.3 Reference Documents ........................................................................................................................2 1.4 Acronyms ...........................................................................................................................................3

2 BACKGROUND AND OBJECTIVES .......................................................................................5 2.1 Background ........................................................................................................................................5 2.2 Objectives...........................................................................................................................................5

3 TASKS AND DEVELOPMENT LOGIC.....................................................................................8 3.1 Scope of activities ..............................................................................................................................8 3.2 Definition Phase (General Tasks) ......................................................................................................9 3.3 Deliverables of the Definition Phase .................................................................................................9 3.4 Detailed Tasks of the Definition Phase............................................................................................10 3.5 SW Tool Implementation, Test Definition and Execution Phase, (General Tasks) ........................15 3.6 Deliverables of the SW Tool Implementation Phase .......................................................................16 3.7 Detailed Tasks of the SW Tool Implementation Phase ...................................................................17 3.8 Intermediate Experimentation Phase (General Tasks).....................................................................18 3.9 Deliverables of the Intermediate Experimentation Phase ................................................................18 3.10 Detailed Tasks of the Intermediate Experimentation Phase ............................................................19 3.11 Final Experimentation Phase (General Tasks).................................................................................21 3.12 Deliverables of the Final Experimentation Phase ............................................................................21 3.13 Detailed Tasks during the Final Experimentation Phase .................................................................22

4 MANAGEMENT......................................................................................................................23

TECHNICAL APPENDIX A (TECHNICAL REQUIREMENTS) ...................................................25

TECHNICAL APPENDIX B (ECSS-E-40 TAILORING) ..............................................................28

TECHNICAL APPENDIX C (TECHNICAL FEATURES AUTONOMY-RELATED EXISTING IN THE GPS SYSTEM AND FORESEEN FOR THE GALILEO SYSTEM)........................................34

TECHNICAL APPENDIX D (OBSERVABLES FOR THE ON-BOARD OD&TS PROCESS) ....41

TECHNICAL APPENDIX E (ON BOARD ORBIT DETERMINATION PROCESS) .....................67


page iv of iv

TECHNICAL APPENDIX F ON BOARD CLOCK DETERMINATION PROCESS......................76

TECHNICAL APPENDIX G (ISL LINKS PHYSICAL DEFINITION)............................................85

TECHNICAL APPENDIX H (GROUP DELAY CALIBRATION)..................................................91


page 1 of 101

1 INTRODUCTION

1.1 Scope The proposed study will assess the feasibility, preliminary system definition and performance for a Global Satellite Navigation System, targeting very stringent accuracy requirements as well as a significant level of satellite autonomy, beyond the specifications of the Galileo System, currently under development by the European Space Agency.

This document describes:

The activity development logic, in terms of activity phases, with their respective tasks and deliverables. Four different project phases are defined and described within the document, namely the:

Definition Phase. SW Tool Implementation, Test Definition and Execution Phase. Intermediate Experimentation Phase. Final Experimentation Phase.

The project specific management requirements, including schedule and reporting aspects

Finally the document contains:

One applicable appendix on technical requirements (appendix A)

One applicable appendix on general SW requirements (ECSS-E40 tailored) (appendix C)

One reference appendix describing the technical autonomy-related features existing in the GPS System and those foreseen for the Galileo System (appendix D)

Four reference appendixes (appendixes E, F, G and H) which should be considered by the

Bidder as concrete technical guidelines describing a certain orientation of the design. These appendixes aim merely to define a starting point for the development, and to prove its feasibility. The critical assessment and consolidation of these inputs during the activity development, could lead to deviations from the original guidelines, either enhancing system performances or reducing system complexity.


page 2 of 101

1.2 Applicable Documents

1. ECSS-E-40 Part1B " Space engineering - software" and Part2B " DRDs" as tailored in Appendix C

1.3 Reference Documents

1- Robert Wolf: Satellite Navigation, GPS, GLONASS, Galileo, Orbit Determination, Orbit Estimation, Orbit Simulation, Inter-satellite Links, Onboard Ephemeris Determination, Integrity Monitoring. (http://137.193.200.177/ediss/wolf-robert/inhalt.pdf)

2- “J. Hammesfahr, A. Hornbostel, J. Hahn, H. L. Trautenberg, B. Eissfeller, R. Wolf, H.

Malthan, P. Souty, P.Tavella, W.Shafer: Inter-satellite Ranging and Autonomous Ephemeris Determination for Future Navigation Systems.

3- María Dolores Laínez Samper: Galileo Test Bed V1 Orbit Determination and Time

Synchronization Computation Performance Assessment - Detailed Processing Model Volume 1: Pre-Processing and Validation. GT-DS-GMV-TC-0123

4- María Dolores Laínez Samper: Galileo Test Bed V1 Orbit Determination and Time

Synchronization Computation Performance Assessment - Detailed Processing Model Volume 2: OD&TS. GT-DS-GMV-TC-0123

5- Kenneth R. Brown: The Theory of the GPS Composite Clock, proc. ION-GPS, 1991, pages

221-241

6- D.R. Cox, H.D.Miller, The theory of stochastic processes, Science Paperbacks Chapman and Hall, London 1965, Cap. 2,5

7- J. Hahn, S. Bedrich: Common view, Clock synchronization of remote atomic clocks using

GPS and PRARE onboard ERS2, proc 10th European Frequency and Time Forum, Brighton UK, 1996, pp. 393-398

8- Michael P. Scardera: The NAVSTAR GPS Master Control Station’s KAlman Filter

experience, Flight Mechanics / Estimation Theory Symposium 1990, NASA conference proceedings, CP 3102, 1991


page 3 of 101

1.4 Acronyms EDDN External Data Dissemination Network ERP Earth Rotation Parameters ESA European Space Agency G/S Ground Segment GACF Ground Assets Control Facility GCS Ground Control Segment GLONASS Global Navigation Satellite System (Russian) GMS Ground Mission Segment GNSS Global Navigation Satellite System (generic) GPS Global Positioning System GSP General Studies Programme GSS Galileo Sensor Station GST Galileo System Time ISL Inter Satellite Links ITRF International Terrestrial Reference Frame LEO Low Earth Orbit MCF Mission Control Facility MDDN Mission Data Dissemination Network MEO Medium Earth Orbit MGF Message Generation Facility MKMF Mission Key Management Facility MNE Monitoring Network Equipment MSF Mission Support Facility MUCF Mission Uplink Control Facility NASA National Aeronautics and Space Administration ODTS Orbit Determination and Time Synchronization OSPF Orbit and Synchronization Processing Facility PRSKMF Public Regulated Service Key Management Facility PTF Precise Timing Facility S/C Spacecraft S/S Space Segment SKMF Satellite Key Management Facility SPF Satellite Processing Facility SPR Software Problem Report SRP Solar Radiation Pressure SVN Space Vehicle Number SW Software TWTT Two Way Time Transfer


page 4 of 101

ULS Uplink Station UTC Universal Time Coordinated VSAT Very Small Aperture Terminal WAN Wide Area Network


page 5 of 101

2 BACKGROUND AND OBJECTIVES

2.1 Background This study is intended a valuable input for improving present Navigation Systems and future evolutions of GNSS Systems (e.g. Galileo II) The Galileo Satellite Navigation System currently under development by European Space Agency, is intended to provide a significantly improved performance level, in terms of accuracy, integrity and continuity when compared with existing Systems. Nevertheless current performance objectives are on one hand not sufficient for a certain number of highly demanding applications, e.g. absolute surveying (not differential technology based); and on the other hand have yielded to a considerable level of complexity and dependability on the deployment of Ground Segment infrastructure, mostly in terms of tracking network (Galileo Sensor Stations,(GSS)), mission uplink network (Uplink Stations (ULS)) and Mission Data Dissemination Network. Despite these current difficulties, performance targets for future Global Navigation Satellite System have to be more ambitious, while increasing or maintaining System availability, but reducing dramatically the dependability on Ground Segment assets. All these targeted improvements, are very challenging, for the existing and mature technology, as applied within the development of the Galileo Satellite Navigation System. Alternative and innovative technologies and methods, in the navigation area, could be the key to fulfil these more demanding requirements.

2.2 Objectives The objectives of this activity are to investigate alternative, innovative technologies and methods in the navigation area, with the view of bringing, as a minimum, the following benefits on current Systems:

Enhanced satellite autonomy. Reduced G/S infrastructure, in terms of tracking network and uplink stations Satisfy severe security constrains, in terms of geographical location of the G/S facilities. Enhanced accuracy for the orbit and clock navigation data Higher refresh rate of navigation data Reduced operational cost


page 6 of 101

In view of the above objectives, the Contractor shall investigate, as a minimum, the following technologies and methods:

Inter-satellite links ranging signals Inter-satellite ranging measurements processed in the Ground Segment (G/S) On board orbit and clock determination Inter-satellite communication links, to exchange between spacecrafts navigation, telemetry,

telecommand and ranging data In the above defined high level frame this Study targets two different scenarios, namely:

The first scenario, named as “Galileo-like constellation” analyzes the benefits of ISL, when navigation processing is either entirely or partially located on board each spacecraft, in a Galileo-like constellation.

The second scenario, named as “Galileo-like + LEO constellation” analyzes the benefits of

ISL, when navigation processing are either entirely or partially located on board each spacecraft, in a Galileo-like scenario in which a few LEO satellites are incorporated for calibration and monitoring purpose. In this Study the Galileo-like constellation is named as Principal Constellation, while the LEO satellites are named as Auxiliary satellites.

The Study shall explore and clarify the advantages and drawbacks of different techniques for the on-board processing considering ISL input observables, generating the satellite orbits and clocks. The Study shall analyze the technical feasibility of enhancing satellite accuracy and autonomy in the provision of navigation data, by means of on-board processing considering ISL input observables; with or without information exchanges between processors in different satellites. It shall clarify the:

Strategy for on-board satellite navigation data (orbit and clock) determination and prediction, including on-board satellite orbit and clock determination and prediction algorithms

The level of independence from ground based assets, in determining an adequate clock reference strategy (e.g. on-board clock ensemble).

Approach for the navigation data exchange between satellites of the Space Segment. Approach for the navigation data exchange between Space Segment and Ground Segment

(e.g. for linking the navigation data to UTC and ITRF).


page 7 of 101

Performance assessment in relevant scenarios, covering both nominal and degraded

constellation scenarios For this purpose the activities that are necessary have been identified and have been structured in four high level sets, which are listed hereafter:

Tasks regarding the on board orbit determination process

Tasks regarding the on board clock determination process

Tasks regarding ISL physical characterization

Tasks regarding group delay real time calibration


page 8 of 101

3 TASKS AND DEVELOPMENT LOGIC

3.1 Scope of activities [3.1.1] The contractor shall provide a justified design (architecture and technologies) for an

navigation system implementing ISL, that provides navigation observables orbit and clock prediction. A design and architectural definition is already provided with the ITT (please see appendixes), however the tenderer may challenge these suggestions and is invited to propose alternative solutions or modifications with adequate justification.

[3.1.2] Within the architectural design activities, the following activities shall be performed by

the contractor:

Selection of the technical solution for the ISL-based architecture, including a detailed definition of its components, the technologies and design drivers.

Confirmation and justification of the feasibility of the recommended solution. Identification of SW tool to be employed in the activity, and justification for the

definitive choice taken. Development of the SW tools to perform the activity (as far as possible by tailoring of

existing SW tools) Definition of the experimentation to be performed. Analysis of the results and conclusions.

[3.1.3] These activities shall be backed up by the assessment of techniques, technologies and

means to implement the ISL-based navigation system. [3.1.4] The selection, justification and confirmation of a solution for an ISL-based navigation

system, shall be supported by quantitative analysis [3.1.5] The activities shall be carried out in 4 sequential phases, identified as follows:

Definition Phase. SW Tool Implementation, Test Definition and Execution Phase. Intermediate Experimentation Phase. Final Experimentation Phase.

[3.1.6] Each one of these phases shall be subject of a review by the Agency who will declare the

objectives defined for this phase as achieved or failed.


page 9 of 101

3.2 Definition Phase (General Tasks) [3.2.1] The contractor shall analyze the reference material provided by the Agency (Appendixes

E, F, G, H and I). [3.2.2] The Definition Phase shall include at least the following activities:

Analyze the technical specification and applicable documents. Perform the functional decomposition and the architectural design of the system

including design trade offs and selection of the baseline concept. Quantitative engineering analyses to justify the architecture and design. Quantitative engineering analyses comparing different architectural solutions Identification of elements which would be new design. Assessment of feasibility for new design elements Identification of critical technologies and quantification of key parameters, .for new

design elements Preliminary definition of experimentation test cases and test scenarios. Selection and justification of ODTS SW tool to be used for the activity (e.g. Bernese). Definition of the modifications required on the SW tool in order to perform the foreseen

experimentation Autonomy capability versus G/S complexity G/S simplification in the enhanced System

[3.2.3] The Definition Phase shall end after successful completion of the Definition Review

(DR). Successful completion of this review will lead to authorization by the Agency to start the activities corresponding to the next phase.

3.3 Deliverables of the Definition Phase [3.3.1] The contractor shall deliver at the Definition Review a data package, with content

accordingly to the table below:

REVIEW

DELIVERABLE (BY DEFAULT DOCUMENT TITLE)

ISSUE

System Architecture Definition File (SADF) Issue 1.0

System Architecture Justification File (SAJF) (Including a detailed description of the traded off technologies)

Issue 1.0

DR

Experimentation Platform Definition File (EPDF)

Issue 1


page 10 of 101

Experimentation Platform Acceptance Test Procedures (EPAP)

Issue 1 (Final)

Test Case Experimentation Plan (TCEP) (Describing the Test Cases and Scenarios)

Issue 1.0

On-board Orbit Determination Detailed Processing Model

Issue 1.0

On-board Clock Determination Detailed Processing Model

Issue 1.0

Experimentation Platform User Manual Draft

[3.3.2] The contractor shall update the data package for Agency approval as a result of the

review discussion and agreements.

3.4 Detailed Tasks of the Definition Phase The Definition Phase shall cover, as a minimum, the following tasks by means of analysis: Regarding the constellation definition

[3.4.1] The Bidder shall optimize the auxiliary constellation definition. The constellation

parameters, described in the Appendix A (Technical Requirements), shall be optimized by the Contractor, including the number of auxiliary satellites, which are merely in charge of performing some system internal functions

[3.4.2] The contractor shall select an existing ODTS tool (e.g. Bernese) allowing the required

adaptations for the activity Regarding the orbit determination process shall be understood as both orbit restitution and orbit

prediction throughout the entire document.

The Study shall explore and clarify the advantages and drawbacks of the different alternatives for the on-board orbit determination and prediction process, analyzing in detail at least the following related areas:

[3.4.3] The sub-set of S/S to S/S observables to be processed on board each spacecraft for orbit

determination


page 11 of 101

[3.4.4] The sub-set of S/S to G/S observables to be processed on board each spacecraft for orbit

determination [3.4.5] The type of orbit estimation process (e.g. Batch, Kalman filter, etc) [3.4.6] The estimated parameters. The analysis shall include an assessment on the benefit of

estimating: Solar Radiation Pressure coefficients Earth Rotation Parameters delta coefficients

[3.4.7] The on-board CPU time consumption, indicating the minimum time span between

consecutive refreshments of the broadcast orbit [3.4.8] The power consumption of the on-board Orbit Determination board [3.4.9] The achievable performance in terms of broadcast orbit error in nominal scenarios (e.g.

availability of the full S/S and G/S) [3.4.10] The performance degradation in terms of broadcast orbit error, in nominal scenarios (e.g.

availability of the full S/S and G/S), when contact with the G/S is interrupted Regarding the clock determination process shall be understood as both clock restitution and

clock prediction throughout the entire document

The Study shall explore and clarify the advantages and drawbacks of different alternatives for the on-board clock determination and prediction process, analyzing in detail at least the following related areas:

[3.4.11] The sub-set of S/S to S/S observables to be processed on board each spacecraft for clock

determination [3.4.12] The definition of the navigation system time reference, independently from G/S clocks [3.4.13] The relativistic effects on the satellites on board clocks, when observed from either other

spacecrafts or from ground stations [3.4.14] The type of clock estimation process (e.g. Batch, Kalman filter, etc) [3.4.15] The estimated clock parameters [3.4.16] The on-board CPU time consumption, indicating the minimum time span between

consecutive refreshments of the broadcast clock [3.4.17] The power consumption of the on-board Clock Determination board


page 12 of 101

[3.4.18] The achievable performance in terms of clock error in nominal scenarios (e.g. availability

of the full S/S) [3.4.19] The performance degradation in terms of broadcast clock error, in nominal scenarios (e.g.

availability of the full S/S), when contact with the G/S is interrupted [3.4.20] The sub-set of S/S to G/S observables to be processed on board each spacecraft for the

estimation of the steering parameters necessary to refer the navigation system time to G/S based clocks or time external references

[3.4.21] The achievable performances in terms of steering parameters error in nominal scenarios

(e.g. availability of the full S/S) [3.4.22] The performance degradation in terms of steering parameters error, in nominal scenarios

(e.g. availability of the full S/S), when contact with the G/S is interrupted Regarding the ISL link physical definition the Study shall explore and clarify the advantages

and drawbacks of different alternatives. The Study shall clarify the: [3.4.23] Optimization of the ISL connectivity scheme between spacecrafts, or in other words with

what satellites is a given satellite supposed to establish a link [3.4.24] Optimization of the connectivity scheme between spacecrafts and ground [3.4.25] Number of “system internal” frequencies for ranging between principal satellites (see

Appendixes) [3.4.26] Frequency band for the “system internal” frequencies, as a compromise between key

parameters such as the antenna size or the propagation losses, and ITU allocated bands for ISLs. Selected band shall be compatible with an accurate control of the antenna group delay.

[3.4.27] Number of “system external” frequencies carrying ranging signals to the users (see

Appendixes) [3.4.28] Frequency sub-band for the “system external” frequencies. The frequency band for the

“system external” frequencies is the L band, including by default the Galileo frequencies Regarding the ranging signals they shall consist on pseudo-random sequences which are

generated in a synchronized way by both transmitter and receiver; and transmitted modulated on a radio-frequency carrier. The design is oriented to cross-link observables. The Study shall clarify the:


page 13 of 101

[3.4.29] Characteristics of the ranging signal, in terms of power density function [3.4.30] Characteristics of the ranging signal generation unit [3.4.31] Link budget [3.4.32] Characteristics of the ranging measurements quality [3.4.33] The ranging signals, leading to a ranging accuracy compatible with the Study’s objectives

in terms of orbit and clock determination accuracy. [3.4.34] The selection of the ranging signals shall target fast re-acquisition Regarding the cross-links antenna design the study shall analyze the:

[3.4.35] Feasibility and convenience of a phase array based antenna with its radiating elements

placed symmetrically on a spherical surface [3.4.36] Feasibility and convenience of alternative designs proposed by the Contractor, such as a

passive pattern [3.4.37] Group delay introduced by the antenna subsystem in the ranging signals, and its potential

dependency versus beam orientation Regarding the communication links.

[3.4.38] The Study shall analyze the possibility of re-using the ranging frequencies for

communication purposes either while ranging is being performed or during a different time slot exclusively dedicated for this purpose; or whether it is necessary to establish a different set of “system internal” frequencies for communication purposes.

The Study shall clarify the: [3.4.39] Exchanged information, including navigation, ranging data, telemetry and telecommand [3.4.40] Necessary bandwidth [3.4.41] Communication signal characteristics (including multiple access schemes) [3.4.42] Technology in terms of on-board transmitting and receiving chains [3.4.43] Performance assessment in relevant scenarios, in terms of BER and communication

delays


page 14 of 101

[3.4.44] Transmitting and receiving on board antennas [3.4.45] Compatibility with other RF links [3.4.46] Link budget Regarding the group delay calibration

[3.4.47] The Study shall explore and clarify the advantages and drawbacks of different methods

for the “system external” ranging signals group delay calibration, either based on the auxiliary satellites observations, or on satellite local means or on any other alternative method.

[3.4.48] The description of the on-board equipment required for performing this calibration Regarding the operational concept

[3.4.49] The Study shall identify the different operations modes, initialisation, switch between

ground-supported and autonomous modes [3.4.50] The Study shall propose a preliminary high level system operational concept [3.4.51] The Study shall identify the necessary adaptations to the navigation algorithms in order to

support the proposed mode transitions. All tasks shall consider the design guidelines described in the Appendixes E, F, G and H. These appendixes aim, either to define the starting point for the development, or merely to prove its feasibility. The study shall analyze the benefits from deviating from these guidelines if as result either performance improves or system complexity diminishes.


page 15 of 101

3.5 SW Tool Implementation, Test Definition and Execution Phase,

(General Tasks) [3.5.1] The contractor shall select an existing ODTS tool (e.g. Bernese) allowing the required

adaptations for the activity. [3.5.2] The SW tool shall be adapted, as necessary, in order to make possible the processing of

ISL measurements, in addition to the conventional space to ground measurements. [3.5.3] The contractor shall develop the auxiliary tools necessary to generate the input

measurements to the SW Tool, more precisely synthetic ISL measurements, and space to ground measurements..

[3.5.4] The SW tool user interface shall allow a flexible definition of the ISLs in terms of

number and in terms of type amongst the following:

Both S/Cs are part of the navigation constellation, One S/Cs is part of the navigation constellation and the other S/C is an auxiliary

LEO Both S/Cs are part of the auxiliary constellation,

Allowing simulations not only on the baseline architecture resulting from the Definition Phase but also with other combinations of ISLs.

[3.5.5] The SW tool output shall provide the accuracy of the orbit and clock determination and

prediction for each S/C. [3.5.6] The contractor shall perform a validation of the tool according to the needs of the study [3.5.7] The contractor shall define the test cases and scenarios for experimentation, covering

both the baseline architecture and other possible architectures derived from different combinations of ISLs and ground to space observables. The experimentation shall include, at least the following three scenarios:

ISLs between MEO navigation satellites ISLs between MEO navigation satellites, and between MEO and LEO

satellites, ISLs between MEO navigation satellites, between MEO and LEO satellites,

and between LEO satellites [3.5.8] The contractor shall update during this phase the documentation produced in previous

phases in order to input the modifications that arise as the study evolves.


page 16 of 101

[3.5.9] The SW Tool Implementation, Test Definition and Execution Phase shall end after

successful completion of the SW Implementation and Test Review/Experimentation Test Review (ERR).

3.6 Deliverables of the SW Tool Implementation Phase [3.6.1] The contractor shall deliver at the SW Implementation and Test Review/Experimentation

Test Review a data package, with content accordingly to the table below:

REVIEW


ISSUE

Experimentation Platform Acceptance Test Results (EPAR) (Including all implemented adaptations)

Issue 1 (Final)


System Architecture Justification File (SAJF) Issue 2.0

Test Case Experimentation Plan (TCEP)

Issue 2.0


Issue 2.0


Issue 2.0

ERR

Experimentation Platform User Manual Issue 1.0


page 17 of 101

[3.6.2] The contractor shall update during this phase the documentation produced in previous

phases in order to input the modifications that arise as the study evolves. [3.6.3] The contractor shall update the data package for Agency approval as a result of the


3.7 Detailed Tasks of the SW Tool Implementation Phase The SW Tool Implementation, Test Definition and Execution Phase shall cover, as a minimum, the following tasks, by means of analysis: [3.7.1] The review and refinement of the outputs of Tasks [3.4.9] up to [3.4.34] [3.7.2] The review and refinement of the outputs of Tasks [3.4.47] up to [3.4.48]


page 18 of 101

3.8 Intermediate Experimentation Phase (General Tasks) [3.8.1] This phase shall provide the first experimentation of the ISL based architecture for

navigation systems. The contractor shall perform during this phase the experimentation defined in previous phases.

[3.8.2] The experimentation plan shall be modified and adapted as required during the activity

according to the needs and evolution of the study. [3.8.3] The contractor shall perform simulations for different scenarios in order to assess the

performance achieved trade off with the complexity of the proposed architecture. [3.8.4] Software Problem Report (SPR) in the SW tool, cannot result in a reduction of the

experimentation scope. [3.8.5] The contractor shall update during this phase the documentation produced in previous

phases in order to input the modifications that arise as the study evolves. [3.8.6] The Intermediate Experimentation Phase shall end after successful completion of the

Intermediate Experimentation Phase Review (IER)

3.9 Deliverables of the Intermediate Experimentation Phase [3.9.1] The contractor shall deliver at the Intermediate Experimentation Phase Review a data

package, with content accordingly to the table below:

REVIEW


ISSUE

Test Case Experimentation Results (TCER) Issue 1



IER

Test Case Experimentation Plan (TCEP) Issue 2.1


page 19 of 101

On-board Orbit Determination Detailed Processing Model Issue 2.1

On-board Clock Determination Detailed Processing Model Issue 2.1





3.10 Detailed Tasks of the Intermediate Experimentation Phase The Intermediate Experimentation Phase shall cover, as a minimum, the following tasks mostly by means of experimentation: [3.10.1] The review and refinement of the outputs of Tasks [3.7.1] and [3.7.2] [3.10.2] The detailed and exhaustive experimentation of the design elaborated in the Definition

Phase and consolidated during the SW Tool Implementation Phase. For this purpose all analysis from [3.4.9] up to [3.4.22] and from [3.4.47] up ro [3.4.48] shall be replaced by experimentation results obtained with the help of the SW tool.

[3.10.3] The achievable performance in terms of broadcast orbit error in degraded scenarios (e.g.

availability of merely a reduced S/S or/and a reduced G/S) [3.10.4] The performance degradation in terms of broadcast orbit error in degraded scenarios (e.g.

availability of merely a reduced S/S or/and a reduced G/S), when contact with the G/S is interrupted

[3.10.5] The detail processing model of the orbit estimation algorithms at equation level [3.10.6] Robustness of the preferred alternative to external perturbations (e.g. contaminated

observables) [3.10.7] The achievable performance in terms of broadcast clock error in degraded scenarios (e.g.

availability of merely a reduced S/S)


page 20 of 101

[3.10.8] The performance degradation in terms of broadcast clock error in degraded scenarios

(e.g. availability of merely a reduced S/S), when contact with the G/S is interrupted [3.10.9] The achievable performances in terms of steering parameters error in degraded scenarios

(e.g. availability of merely a reduced S/S) [3.10.10] The performance degradation in terms of steering parameters error, in degraded scenarios

(e.g. availability of merely a reduced S/S), when contact with the G/S is interrupted [3.10.11] The detail processing model of the clock estimation algorithms, at equation level. Note

that clock estimates are relative to the Navigation System Reference Time [3.10.12] The detail processing model of the Navigation System Reference Time steering

algorithms, at equation level [3.10.13] Robustness of the preferred alternative to external perturbations e.g. contaminated

observables


page 21 of 101

3.11 Final Experimentation Phase (General Tasks) [3.11.1] During this phase the contractor shall carry out the activities as described in the previous

phase, endorsing and implementing the inputs provided by the Agency in the Intermediate Experimentation Results Analysis Phase.

[3.11.2] See requirement [4.4.2] [3.11.3] See requirement [4.4.3] [3.11.4] See requirement [4.4.4] [3.11.5] See requirement [4.4.5] [3.11.6] See requirement [4.4.6] [3.11.7] The Final Experimentation Phase shall end after successful completion of the Final

Experimentation Phase Review (FER). Successful completion of this review will lead to the completion and close out of the study.

3.12 Deliverables of the Final Experimentation Phase [3.12.1] The contractor shall deliver at the Intermediate Experimentation Phase Review a data

package, which content accordingly to the table below:

REVIEW


ISSUE

ODTS SW procured during activity, including all necessary licenses and libraries.

N/A

Experimentation Platform (HW) Final version

SW developed during activity, including all necessary licenses and libraries.

Final version

FER

Test data produced in the simulations and used for the analysis.

Final version


page 22 of 101

Final report (including a synthesis of the results and the conclusions)

Issue 1.0

System Architecture Definition File (SADF)

Issue 2.2



Issue 2.2


Issue 2.2





3.13 Detailed Tasks during the Final Experimentation Phase The Final Experimentation Phase shall cover, as a minimum, the following tasks, by means of experimentation: [3.13.1] The review and refinement of the outputs of Tasks [3.10.1] up to [3.10.13] by means of

additional experimentation


page 23 of 101

4 MANAGEMENT The standard requirements for management, reporting and meetings, which are described as an appendix to the contract, sections 1, 2 and 3 respectively, shall apply to this activity. Sections 4 and 5 of the mentioned appendix, on deliverables and commercial evaluation respectively, are not applicable to this study. The following additional requirements are incorporated to the mentioned appendix, section 1, on management: The contractor shall exercise an effective and transparent management of the work, providing

ESA at any time with all information necessary to undertake corrective measures if needed. The contractor shall organise the activity, propose a Work Breakdown Structure (WBS) and

provide an adequate allocation of tasks. The overall activity shall be completed in 12 months with the following intermediate milestone

dates:

Milestones Date (months)

Kick –off (KO) KO Definition Phase Review (DR) KO + 04.0 m SW Tool Implementation, Test Definition and Execution Phase Review (ERR) KO + 09.0 m

Intermediate Experimentation Phase Review (IER) KO + 10.5 mFinal Experimentation Phase Review (FER) KO + 12.0 m

After successful completion of each of the intermediate reviews (Definition Phase Review, SW

Tool Implementation, Test Definition and Execution Phase Review, and Intermediate Experimentation Results Analysis Phase Review) the Agency will authorise the continuation of the activity and the initiation of the following phase.

The following additional requirements are incorporated to the mentioned appendix, section 2, on reporting: Minutes shall be distributed in Word or/and PDF format. Any presentations done at the

meeting shall be attached to the minutes in PPT format.


page 24 of 101

The following requirements replace entirely the mentioned appendix section 4, on deliverables: All items procured or developed under the contract shall be the property of the Agency at the

closure of the contract The Contractor shall submit to the Agency for information or approval (as applicable) all

technical notes, specifications, test and demonstration plans and other documents as they become available during the execution of the contract, at the latest by the agreed date of delivery

Any technical documentation to be discussed at a meeting with the Agency shall be submitted

at least 5 working days weeks prior to such a meeting The Contractor shall submit technical documents from subcontractors to the Agency normally

only after review and acceptance The Contractor shall give to the Agency prior notice without delay of any meetings with third

parties to be held in connection with the contract. The Agency reserves the right of participation in such meetings.

With due notice to the Contractor and with the Contractor's agreement, the Agency reserves the

right to invite third parties to meetings to facilitate information exchange. For all meetings the Contractor shall ensure that proper notice is given at least 2 weeks in

advance. The Contractor shall be responsible for ensuring the participation of his and of the sub-contractor(s)’ personnel as needed

For each meeting the Contractor shall provide an agenda and handouts of his presentation (if

any) If deemed necessary, the Agency or the Contractor may request ad hoc meetings.

Nominally, these documents and reports are to be provided in electronic form, in PDF

(including signatures) and in MS-WORD The list of contract output and deliverables given in previous chapter is neither exclusive nor

exhaustive and needs to be amended by the contractor as required.


page 25 of 101

TECHNICAL APPENDIX A (TECHNICAL REQUIREMENTS)

REQ-010: The Study shall be performed for two different constellations, associated to the Scenario I and Scenario II, which are described hereafter.

REQ-020: The Scenarios are defined in terms of Principal and Auxiliary satellites, being the Principal Satellites those which broadcast navigation signals to the GNSS user, and being the Auxiliary Satellites those merely in charge of some internal system functionalities, transparent to the GNSS user.

REQ-030: SCENARIO I

PRINCIPAL SATELLITES

Walker definition ppp fpt // 27/3/1 (GALILEO)

Number of satellites pt 27

Number of planes pp 3

Number of satellites per plane p

pp p

ts = 9

Pattern unit pp tu /360 o= 13.3°

Slot spacing

pp up * 40°

Node spacing

pp us * 120°

Satellite phasing

pp uf * 13.3°

Inclination 56° (GALILEO)

Orbit radius 29600 km (GALILEO)


page 26 of 101

Right ascension for the first plane node pα

AUXILIARY SATELLITES

Not applicable

The Scenario “I” considers exclusively Principal Satellites, and correspond exactly to the Galileo constellation. Tables above summarize all constellation parameters.

REQ-040: SCENARIO II

PRINCIPAL SATELLITES

As in Scenario I (GALILEO)

AUXILIARY SATELLITES

Walker definition aaa fpt // TBD (6/3/1)

Number of satellites at 6

Number of planes ap 3

Number of satellites per plane a

aa p

ts = 2

Pattern unit aa tu /360 o= 60°

Slot spacing

(spacing between spacecrafts in the same plane) aa up * 180°

Node spacing

(spacing between planes on the equator) aa us * 120°

Satellite phasing

(spacing between spacecrafts on consecutive planes) aa uf * 60°


page 27 of 101

Inclination TBD

Orbit radius TBD (LEO)

Right ascension for the first plane node 2* pp

p

us+α o60+pα

The Scenario “II” considers the same Principal Satellites as the Scenario I, this is the Galileo MEO constellation, plus a reduced LEO constellation of Auxiliary Satellites. Table above summarizes all auxiliary constellation parameters.

REQ-050: The number of auxiliary satellites shall be minimize as far as possible

REQ-060: The study shall target orbit prediction accuracies below 1 centimetre level

REQ-070: The study shall target clock prediction accuracies below 1 centimetre level

REQ-080: The group delay real time calibration shall target sub-centimetre level accuracy


page 28 of 101

TECHNICAL APPENDIX B (ECSS-E-40 TAILORING) Software process mapping to work packages

The software development processes introduced in ECSS-E40 part 1B are mapped on the work

packages and activities or tasks of the Statement of Work in the following way:

ECSS-E40 part 1B processes & activities Reference in the Statement of Work

System Engineering Processes related to software:

- System requirements,

- System architecture,

- System design & Hardware/Software partitioning,

Definition phase (part of*)

*: Experiment Platform Definition File , On Board Orbit Determination Detailed Processing Model ,

On Board Clock Determination Detailed Processing Model

SRR Merged with PDR

Software Requirements & Architecture Engineering Process:

- Software requirements specification,

- Interface Control Document

- Software architecture,

Definition phase (part of*)

*: On Board Orbit Determination Detailed Processing Model , On Board Clock Determination

Detailed Processing Model

PDR DR

Software Design & Implementation Engineering Process:

- Detailed design,

- Code,

- Unit tests,

- Integration tests,

SW tool implementation, Test definition and Execution Phase (part of)


page 29 of 101

DDR N/A

Software Validation Process:

- Software validation wrt TS

N/A

CDR N/A

Software Validation Process:

-Software Validation wrt RB

SW tool implementation, Test definition and Execution Phase (part of)

QR ERR

Software Delivery & Acceptance Implicitly included in the Final Experiment Phase

AR FER

Software Operation N/A

Software maintenance During the whole life cycle for evolutive maintenance if ODTS software is a COTS to be modified

During the 6 months warranty period for corrective maintenance of the new developed items (e.g. utilities)

Software management process Not formally required since management factors are not the keys drivers ( COST is Firm Fixed Price , DELAY is also fixed) of this project but a SDP ( that can be part of a global system development plan) is asked in the proposal

Software verification process - Experiment Platform RB/TS – SVTS/SATS traceabilty

- Timing & Sizing Budget Report

- Numerical Accuracy Report


page 30 of 101

List of ECSS-E40 applicable requirements

ECSS-E-40 part 1B from 28 November 2003 5.2 System Engineering Processes related to software 5.2.2.1 System requirements specification 5.2.3.1a System design 5.2.5.2 Control and data interfaces for system level integration 5.2.7.1 Software maintenance requirements 5.3 Software Management Process (only at proposal) 5.3.2.1 Definition of software life cycle 5.3.2.2 Software life cycle identification 5.3.2.3 Identification of inputs and outputs associated to each phases 5.3.2.4 Identification of documentation relevant to each 5.3.2.5 Identification of interface between the development and the maintenance processes 5.3.2.6 Requirements baseline at the SRR (at PDR in that case ) 5.3.2.7 Software technical specification phase 5.3.2.8 PDR (Preliminary Design Review) 5.3.2.11 software verification and validation process 5.3.2.12 QR (Qualification Review) 5.3.2.13 AR (Acceptance Review) 5.3.3.2 Support to software reviews 5.3.3.3 Technical reviews 5.3.4.1 Interface definition 5.3.5.1 Technical budget and margin philosophy 5.3.5.2 Technical budget and margin status at each milestone 5.4.2.1 Establishment and documentation of software requirements / software requirements specification: 5.4.2.1-a software requirements – functional and performance 5.4.2.1-e software requirements – data definition and DataBase requirements 5.4.2.1-f software requirements – Interfaces external to the software item 5.4.2.3 Identification of requirements unique identifier 5.4.3.1 Transformation of software requirements into a software architecture 5.4.3.2 Software design description 5.4.3.3 Software design documentation 5.4.3.4 Software architectural design contents 5.4.3.11 Evaluation of reuse of predeveloped software 5.4.3.12 Analysis of potential reusability 5.4.3.14 Conducting a preliminary Design Review (PDR) 5.5.2.1 Detailed design of each software components 5.5.2.2 Development and documentation of the software interface detailed design


page 31 of 101

ECSS-E-40 part 1B from 28 November 2003 5.5.2.8 Development and documentation of the software user manual 5.5.3.1 Development and documentation of the software units, test procedures and test data 5.6 Software validation process 5.6.4.1 Development and documentation of a software validation testing specification (SVTS) wrt RB 5.6.4.2 Conducting the validation wrt RB 5.6.4.3 Updating the software user manual 5.6.4.5 Conducting a Qualification Review (QR) 5.7 Software delivery and acceptance 5.7.2 Software delivery and installation 5.7.2.1 Preparation of the software product 5.7.3 Software acceptance 5.7.3.1 Acceptance test planning 5.7.3.2 Acceptance test execution 5.7.3.3 Executable code generation and installation 5.7.3.4a Supplier’s support to customer’s acceptance 5.7.3.4c Acceptance testing documentation 5.7.3.5 Evaluation of acceptance testing 5.7.3.6 Conducting an Acceptance Review (AR) 5.8 Software verification process 5.8.3 Verification activities 5.8.3.7 Verification of test specifications 5.8.3.12a / as support for verification of software requirements & architectural design / sizing (memory) and timing (CPU load) estimation 5.8.3.12c/ as support for verification of software coding and testing / sizing (memory) and timing (CPU utilization in WCET) calculation 5.8.3.13 Behaviour modelling verification 5.10.2.3 problem reporting and handling 5.10.2.4 Implementation of configuration management process 5.10.3.1 Problem analysis 5.10.3.2 Problem verification 5.10.3.3 Development of options for modifications 5.10.3.4 Documentation of problem, analysis and implementation 5.10.3.5 Customer approval of selected modifications options 5.10.4.1 Analysis and documentation of product modification 5.10.4.2 Documentation of software product changes 5.10.4.3 Invoking of software engineering process for modification implementation


page 32 of 101

Document Requirement List [as specified in ECSS-E-40 part 2B (15 November 2004)]

The ECSS software standards are completed with some DRDs, describing the most important software documents. The DRD list is a subset of the exhaustive list of documents to be produced in order to cover all the work output required by the ECSS standards.

The expected output of the requirements resulting of this tailoring shall be placed in the documents identified in the right column of the table below “GSP Study deliverable”.

It is highlighted that the left column of the table below, “ECSS Documentation” does not refer to the actual deliverables of the GSP activity, but should be understood as a list of outputs, which should appear within the real activity deliverables (found in the right column).

Note that most of the requirements outputs should imply merely a brief section within one of the identified GSP deliverables, result of a limited effort, which should not jeopardize the experimentation activities which are the core and objective of the GSP activity.

Note as well that this section refers exclusively to any new SW developed within the activity, and necessary for the experimentation.

ECSS Document ECSS Acronym in DRD

GSP Study Deliverable

SW System Specification SSS SW Interface Requirements Document

-

System partition with definition of items

System Configuration Item List

Software Requirements Specification

SRS

Software Interface Control Document

-

Software Design Document: Software static and/or dynamic architecture

SDD

System Architecture Definition File Experimentation Platform Definition File On-Board Orbit Determination Detailed Processing Model On board Clock Determination Detailed Processing Model

Software Release Document SRD ODTS & utilities software release notes

Software Delivery - ODTS software executable, licenses, libraries, documentation , input and output data

Software User Manual SUM Experimentation Platform User Manual

Software Validation Testing SVTS Experimentation Platform


page 33 of 101

Specification wrt RB Acceptance Test Plan - Software Validation Test Report wrt RB

-

Acceptance Testing Documentation

-

Acceptance Test Report -

Acceptance Test Procedures Experimentation Platform Acceptances Test Report

(Analyses, Inspection & RoD) verification report wrt RB

-

Software Traceability Matrices -

Experimentation Platform Acceptance Test Procedures

Software Reuse File (if any) SRF ODTS Software Reuse

File Software Budget Report - Numerical Accuracy Analysis Report

-

Validation Evaluation Report wrt RB

-

Experimentation Platform Acceptance Test Procedures (special test) Experimentation Platform Acceptances Test Report

Software Acceptance Data Package - SW delivery notice Procured Software Component Lists

- System Architecture Definition File Experimentation Platform Definition File

PR & NCR - Modification analysis report -Problem analysis report

- SPR and CR

Software Development Plan (at proposal only)

SDP As part of the Proposal


page 34 of 101

TECHNICAL APPENDIX C (TECHNICAL FEATURES AUTONOMY-RELATED EXISTING IN THE GPS SYSTEM AND FORESEEN FOR THE GALILEO SYSTEM) GALILEO Galileo will be an independent, global European-controlled satellite-based navigation system. It will have a constellation of satellites monitored and controlled by a Ground Control Segment.

The overall Galileo System is illustrated in the figure above. The Galileo Space Segment will comprise a constellation of thirty satellites in medium-Earth orbit (as defined in the table below). Each satellite will broadcast four ranging signals carrying clock synchronisation, ephemeris, integrity and other data, depending on the particular signal.


page 35 of 101

GALILEO SPACE SEGMENT

Walker definition ppp fpt // 27/3/1 (GALILEO)




pp p

ts = 9

Pattern unit pp tu /360 o= 13.3°

Slot spacing

(spacing between spacecrafts in the same plane) pp up * 40°

Node spacing

(spacing between planes on the equator) pp us * 120°

Satellite phasing

(spacing between spacecrafts on consecutive planes) pp uf * 13.3°

Inclination 56° (GALILEO)

Orbit radius 29600 km (GALILEO)

Right ascension for the first plane node ( )tGALα

The Galileo Ground Segment will control the whole Galileo constellation; monitor the satellite health and up-load data for subsequent broadcast to users. The key elements of this data such as clock synchronisation, ephemeris, will be calculated from measurements made by a network of approximately 40 Galileo receiving stations. The Ground Segment is split into: The Ground Control Segment (GCS) in charge of monitoring and & control of the Galileo

constellation, The Ground Mission Segment (GMS) in charge of the determination and dissemination of the

navigation and integrity data


page 36 of 101

The Ground Mission Segment (please see next figure) is composed of:

GSS: Galileo Sensor Stations: GSS are in charge of Galileo satellites navigation signals monitoring, associated arrival time measurements relative to their reference and results routing to the Ground Segment Processing Facilities. MDDN: Communications network

OSPF Orbitography and Synchronization Processing Facility in charge of Orbit determination

and time Synchronisation Processing (OD&TS) i.e. Galileo satellites ephemeris and clock correction parameters estimation and prediction as well as Signal In Space Accuracy (SISA) determination for Galileo satellites. MGF: Message Generation Facility is in charge of multiplexing and routing navigation/integrity

data to be sent for mission uplink. MUCF: Mission Uplink Control Facility in charge of determining the contact plan for each

satellite and for each up-link station antenna, etc. ULS: Up-link stations in charge of navigation, data transmission up to Galileo satellites

PTF: Precise Timing Facility in charge of Precision Time processing, including Two Way Time

Transfer (TWTT) with UTC (k), and Galileo System Time (GST) establishment and a number of additional facilities such as the MCF, MSF, SPF, GACF, MKMF, SKMF and PRSKMF, etc

The Galileo System does not consider ISL technology and has no demanding specification in terms of satellite autonomy in the Galileo System Requirements document. Concrete specifications can be found in the table below

GAL

SATELLITE

RELEVANT SPECIFICATIONS

Satellite Working Without Guarantee: The Galileo satellite shall automatically set the Satellite Navigation Service level status flag in each Navigation Data Message to “working without guarantee” if the satellite receives no valid up-link signals for a period which it shall be possible to pre-set to any value from 100 minutes to 10 orbits

.


page 37 of 101


page 38 of 101

GPS The NAVSTAR Global Positioning System is managed by the NAVSTAR GPS Joint Program Office at the Space and Missile Systems Centre, Los Angeles Air Force Base, California. The GPS space segment consists of into six orbital planes, requiring a minimum of four satellites in each, to operate. The Global Positioning Service Space Segment comprises a nominal constellation of twenty four satellites in medium-Earth orbit (as defined in the Table below), plus 6 in orbit spares. Each satellite broadcasts one open ranging signal carrying clock synchronisation, and ephemeris.

GPS SPACE SEGMENT




pp p

ts = 4

Pattern unit pp tu /360 o=

15°

Node spacing

(spacing between planes on the equator) pp us * 60°

Inclination 55°

Orbit radius 20200 km

Right ascension for the first plane node ( )tGPSα

The GPS constellation currently contains four different types of GPS satellites called Block II, Block IIA, Block IIR and Block IIRM (first satellite launched in September 26, 2005). The allocation of satellites to planes, labelled from A to F, and plane slots numbered from 1 up to 5, is detailed below:


page 39 of 101

CURRENT BLOCK II/IIA/IIR/IIR-M SATELLITES =========================================

LAUNCH LAUNCH FREQ US SPACE ORDER PRN SVN DATE STD PLANE COMMAND ** ----------------------------------------------------------------- *II-1 14 14 FEB 1989 19802 *II-2 13 10 JUN 1989 20061 *II-3 16 18 AUG 1989 20185 *II-4 19 21 OCT 1989 20302 *II-5 17 11 DEC 1989 20361 *II-6 18 24 JAN 1990 20452 *II-7 20 26 MAR 1990 20533 *II-8 21 02 AUG 1990 20724 II-9 15 15 01 OCT 1990 Cs D5 20830 *IIA-10 23 26 NOV 1990 20959 IIA-11 24 24 04 JUL 1991 Cs D6 21552 IIA-12 25 25 23 FEB 1992 Cs A2 21890 *IIA-13 28 10 APR 1992 21930 IIA-14 26 26 07 JUL 1992 Rb F2 22014 IIA-15 27 27 09 SEP 1992 Cs A4 22108 IIA-16 01 32 22 NOV 1992 Cs F6 22231 *IIA-17 29 29 18 DEC 1992 Rb F5 22275 *IIA-18 22 03 FEB 1993 22446 IIA-19 31 31 30 MAR 1993 Cs C5 22581 IIA-20 07 37 13 MAY 1993 Rb C4 22657 IIA-21 09 39 26 JUN 1993 Cs A1 22700 IIA-22 05 35 30 AUG 1993 Rb B4 22779 IIA-23 04 34 26 OCT 1993 Rb D4 22877 IIA-24 06 36 10 MAR 1994 Rb C1 23027 IIA-25 03 33 28 MAR 1996 Cs C2 23833 IIA-26 10 40 16 JUL 1996 Cs E3 23953 IIA-27 30 30 12 SEP 1996 Rb B2 24320 IIA-28 08 38 06 NOV 1997 Cs A3 25030 ***IIR-1 42 17 JAN 1997 IIR-2 13 43 23 JUL 1997 Rb F3 24876 IIR-3 11 46 07 OCT 1999 Rb D2 25933 IIR-4 20 51 11 MAY 2000 Rb E1 26360 IIR-5 28 44 16 JUL 2000 Rb B3 26407 IIR-6 14 41 10 NOV 2000 Rb F1 26605 IIR-7 18 54 30 JAN 2001 Rb E4 26690 IIR-8 16 56 29 JAN 2003 Rb B1 27663 IIR-9 21 45 31 MAR 2003 Rb D3 27704 IIR-10 22 47 21 DEC 2003 Rb E2 28129 IIR-11 19 59 20 MAR 2004 Rb C3 28190 IIR-12 23 60 23 JUN 2004 Rb F4 28361 IIR-13 02 61 06 NOV 2004 Rb D1 28474 IIR-M1 17 53 26 SEP 2005 C4 * Satellite is no longer in service. ** US SPACE COMMAND, previously known as the NORAD object number; also referred to as the NASA Catalog number. Assigned at successful launch. *** Unsuccessful launch.


page 40 of 101

The GPS Ground Segment consists of five monitoring stations (Hawaii, Kwajalein, Ascension Island, Diego Garcia, Colorado Springs), three ground antennas, (Ascension Island, Diego Garcia, Kwajalein), and a Master Control station located at Schriever AFB in Colorado. The GPS System considers ISL technology and has direct specifications in terms of satellite autonomy. Relevant information can be found in the table below:

GPS

SATELLITE

RELEVANT SPECIFICATIONS

Block II

Corresponds to the space vehicle numbers SVN 13 through 21. Block II satellites were designed to provide 14 days of operation without contact from the Ground Segment.

Note: The Block IIs were launched from February 1989 through October 1990.

Block IIA

Corresponds to the space vehicle numbers SVN 22 through 40. Block IIA satellites were designed to provide 180 days of operation without contact with the Ground Segment. During the 180 day autonomy, degraded accuracy will be evident in the navigation message

Note: The Block IIAs were launched from November 1990 through November 1997.

Block IIR

Corresponds to the space vehicle numbers SVN 41 through 62. Block IIR satellites are designed to provide at least 14 days of operation without contact from the CS and up to 180 days of operation when operating in the autonomous navigation (AUTONAV) mode. Full accuracy will be maintained using a technique of ranging and communication between the Block IIR satellites. The cross-link ranging will be used to estimate and update the parameters in the navigation message of each Block IIR satellite without contact from the Ground Segment Note: The Block IIRs launching started in January 1997

Block IIR-M

Corresponds to the space vehicle number SVN 53. Block IIR-M satellites incorporate respect Block IIR two new military signals and a second civil signal Note: The Block IIR-Ms launching started in September 2005


page 41 of 101

TECHNICAL APPENDIX D (OBSERVABLES FOR THE ON-BOARD OD&TS PROCESS) Principal satellite to principal satellite observable: Two types of observables are used for orbit determination, namely:

• Halved “Two way range cross-links” • Halved “Range rate cross-links”

which are obtained by linear combinations of other more elemental observables; concretely from conventional pseudorange and Doppler observables. The process yielding to the orbit determination input observables is explained hereafter, with the help of the figure below:

Where:

• iSV refers to the space vehicle “i” • z

wp refers to a one way range observable (pseudorange), being the transmitter “z” and the

receiver “w”


page 42 of 101

The study considers that the one-way observables are derived from the correlation between a PRN code generated by one spacecraft, in transmission mode, and an identical PRN code replicated by other spacecraft, in reception mode. Concretely the study might consider by default the following measurement scheme amongst Principal Satellite’s pairs (e.g. aSV and bSV ):

1. aSV starts the transmission of the PRN “A” code (first bit) towards bSV at time pt , modulated on system carrier 1sf

2. bSV starts the transmission of the PRN “A” code (first bit) towards aSV at time pt ,

modulated on system carrier 1sf

3. aSV ends the transmission of the PRN “A” code A (last bit) towards bSV at time maxtt p ∆+ , modulated on system carrier 1sf

4. bSV end the transmission of the PRN “A” code (last bit) towards aSV at time

maxtt p ∆+ , modulated on system carrier 1sf

5. aSV receives the transmission of the PRN “A” code (first bit) from bSV at time app tt ,∆+ , modulated on system carrier 1sf


page 43 of 101

6. bSV receives the transmission of the PRN “A” code (first bit) from aSV at time bpp tt ,∆+ , modulated on system carrier 1sf

7. aSV ends the reception of the PRN “A” code (last bit) from bSV at

time max, ttt app ∆+∆+ , modulated on system carrier 1sf

8. bSV ends the reception of the PRN code (last bit) from aSV at time max, ttt bpp ∆+∆+ , modulated on system carrier 1sf

In this scheme:

• PRN “A” is common to all satellites

• aptt ,max ∆<<∆ . For dimensioning purposes the shorter propagation time amongst satellites that is considered is 60 ms, while the maximum transmission duration has been limited to 30 ms.

• aSV and bSV are ether in transmitting or in receiving, but never doing both actions

simultaneously.

• The 0/ NC ratio of the received signal allows a very accurate tracking. This is ensured by:

o Narrow beam antennas, with small side lobes at both transmitter and receiver

ensuring:

High antenna gains Absence of interference Absence of multipath

o High transmission signal power

• aSV and bSV host real time information on:

o The position and velocity of each spacecraft o The clock offset and drift of each spacecraft

• aSV and bSV host an Atomic Frequency Standard


page 44 of 101

In the above described scheme the correlation of the PRN “A” code, received between

app tt ,∆+ and max, ttt app ∆+∆+ , by the aSV , with a replica of the PRN “A” code generated

internally in aSV is possible. The same statement can be done for the PRN “A” code, received between bpp tt ,∆+ and max, ttt bpp ∆+∆+ , by the bSV . The following two one way observables are obtained in the above scheme:

( )max, tttp appba ∆+∆+ and ( )max, tttp bpp

ab ∆+∆+ which are given by the following equations:

( ) ( ) ( ) ( )( ) ( ) ( )max,max,max

max,maxmax,max,

ttttttHttH

tttCttCtttdtttp

appbaapp

RXap

bTX

appapb

appbaapp

ba

∆+∆++∆+∆+−∆++

∆+∆+−∆++∆+∆+=∆+∆+

ε

( ) ( ) ( ) ( )( ) ( ) ( )max,max,max

max,maxmax,max,

ttttttHttH

tttCttCtttdtttp

bppabbpp

RXbp

aTX

bppbpa

bppabbpp

ab

∆+∆++∆+∆+−∆++

∆+∆+−∆++∆+∆+=∆+∆+

ε

Where

• ( )max, tttd appba ∆+∆+ refers to the distance traveled by the signal transmitted from

satellite bSV , at maxtt p ∆+ , till it reaches aSV , at max, ttt bpp ∆+∆+ .

• ( )max, tttd bpp

ab ∆+∆+ refers to the distance traveled by the signal transmitted from

satellite aSV , at maxtt p ∆+ , till it reaches bSV , at max, ttt app ∆+∆+ .

• ( ) ≠∆+∆+ max, tttd app

ba ( )max, tttd bpp

ab ∆+∆+


page 45 of 101

• ( )maxttC p

b ∆+ refers to the on board clock of bSV at maxtt p ∆+

• ( )max, tttC bppb ∆+∆+ refers to the on board clock of bSV at max, ttt app ∆+∆+

• ( ) ≅∆+ maxttC p

b ( )max, tttC bppb ∆+∆+ for the maximum bpt ,∆ which for dimensioning

purposes can be set in 300 ms (equivalent to 90000 Km), assuming a conventional AFS on board bSV

• ( )maxttC p

a ∆+ refers to the on board clock of aSV at maxtt p ∆+

• ( )max, tttC appa ∆+∆+ refers to the on board clock of aSV at max, ttt bpp ∆+∆+

• ( ) ≅p

a tC ( )max, tttC appa ∆+∆+ for the maximum apt ,∆ which for dimensioning

purposes can be set in 300 ms (equivalent to 90000 Km), assuming a conventional AFS on board aSV

• ( )maxttH p

bTX ∆+ refers to the on board PRN “A” code signal group delay, within the

bSV payload, from its generation, coherent with the on board clock, till it reaches the transmitting antenna phase center for the system frequency 1sf at maxtt p ∆+

• ( )max, tttH app

RXa ∆+∆+ refers to the on board PRN “A” code signal group delay, within

the aSV payload, from its reception at the receiving antenna phase center for the system frequency 1sf , till it reaches the PRN “A” code correlator on board, at

max, ttt app ∆+∆+

• ( )maxttH p

aTX ∆+ refers to the on board PRN “A” code signal group delay, within the

aSV payload, from its generation, coherent with the on board clock, till it reaches the transmitting antenna phase center for the system frequency 1sf at maxtt p ∆+

• ( )max, tttH bpp

RXb ∆+∆+ refers to the on board PRN “A” code signal group delay, within

the bSV payload, from its reception at the receiving antenna phase center for the system frequency 1sf , till it reaches the PRN “A” code correlator on board, at


page 46 of 101

max, ttt bpp ∆+∆+

• ( ) ( )max, tttHtH app

bTXp

bTX ∆+∆+≅ assuming conventional group delay stabilities for the

on board bSV transmitting chain.

• ( ) ( )max, tttHtH appaTXp

aTX ∆+∆+≅ assuming conventional group delay stabilities for

the on board aSV reception chain.

• ( ) ( )max, tttHtH bppRXbp

RXb ∆+∆+≅ assuming conventional group delay stabilities for

the on board bSV transmitting chain.

• ( ) ( )max, tttHtH appRXap

RXa ∆+∆+≅ assuming conventional group delay stabilities for

the on board aSV transmitting chain.

• ( ) ≠pbTX tH ( )p

RXb tH as one the first is a group delay associated to the bSV transmitting

chain of the PRN “A” code, modulated on the system frequency 1sf , while the second

is a group delay associated to the bSV reception chain.

• ( ) =paTX tH ( )p

RXa tH as one the first is a group delay associated to the bSV transmitting

chain of the PRN “A” code, modulated on the system frequency 1sf , while the second

is a group delay associated to the bSV reception chain.

• ( )max, ttt appba ∆+∆+ε refers to the one way observable error at max, ttt app ∆+∆+ ,

which is labeled as ( )pba tε , as it does not introduce any confusion

• ( )max, ttt bpp

ab ∆+∆+ε refers to the one way observable error at max, ttt bpp ∆+∆+ , which

is labeled as ( )pab tε , as it does not introduce any confusion

Therefore the basic one way observables, under the above mentioned conditions can be expressed in the following simplified form:


page 47 of 101

( ) ( ) ( ) ( ) ( ) ( ) ( )p

bap

RXap

bTXpap

bapp

baapp

ba ttHtHtCtCtttdtttp ε+−+−+∆+∆+=∆+∆+ max,max,

( ) ( ) ( ) ( ) ( ) ( ) ( )p

abp

RXbp

aTXpbp

abpp

abbpp

ab ttHtHtCtCtttdtttp ε+−+−+∆+∆+=∆+∆+ max,max,

The distances ( )max, tttd app

ba ∆+∆+ and ( )max, tttd bpp

ab ∆+∆+ can be converted to the

( ) ( )pbap

ba tdtd = observable, which corresponds with the geometrical distance between aSV

and bSV at time pt . For this purpose, it can be used, the a priori knowledge of:

• ( )( )

dttttdd app

ba max, ∆+∆+

, which can observed from Doppler observables or computed

• The satellite positions and clocks at time pt Assuming all the above corrections are applied, and adapting the terminology for ( )tp a

b , so that it refers to the common transmission time pt , it finally results in:

( ) ( ) ( ) ( ) ( ) ( ) ( )pbap

RXap

bTXpap

bp

bap

ba ttHtHtCtCtdtp ε+−+−+=

( ) ( ) ( ) ( ) ( ) ( ) ( )p

abp

RXbp

aTXpbp

ap

abp

ab ttHtHtCtCtdtp ε+−+−+=


page 48 of 101

In terms of simultaneity between different observations:

• zwp and w

zp are simultaneous, ( )wz,∀

• zwp and '

'wzp are not simultaneous, unless

⎩⎨⎧

==

wwzz

''

Consistently, the one way observables are:

• ( ) ( )[ ]pabp

ba tptp ,

• ( ) ( )[ ]r

acr

ca tptp ,

Each pair ( ) ( )[ ]p

abp

ba tptp , can be transformed in a pair ( ) ( )[ ]p

abp

ba tCtd ˆ,ˆ in the following way:

( ) ( )[ ]( ) ( ) ( )( )

( ) ( ) ( )( )⎪⎪

⎩

⎪⎪

⎨

⎧

−=

+=

=

2ˆ

2ˆ

ˆ,ˆ

pabp

ba

pab

pabp

ba

pba

pabp

ba

tptptC

tptptd

tCtd

( ) ( ) ( ) ( )( ) ( ) ( )( ) ( )

( ) ( ) ( ) ( ) ( )( ) ( ) ( )( ) ( )⎪⎪

⎩

⎪⎪

⎨

⎧

+−−−

+−=

+−+−

+=

=

−

+

pbap

RXbp

aTXp

RXap

bTX

papbpba

pbap

RXbp

aTXp

RXap

bTX

pabpab

ttHtHtHtH

tCtCtC

ttHtHtHtH

tdtd

ε

ε

2ˆ

2ˆ

( ) ( ) ( ) ( )( ) ( ) ( )( ) ( )

( ) ( ) ( ) ( ) ( )( ) ( ) ( )( ) ( )⎪⎪

⎩

⎪⎪

⎨

⎧

++

++

−−=

+−

+−

+=

=

−

+

pbap

RXbp

bTXp

RXap

aTX

papbpba

pbap

RXbp

bTXp

RXap

aTX

pabpab

ttHtHtHtH

tCtCtC

ttHtHtHtH

tdtd

ε

ε

22ˆ

22ˆ


page 49 of 101

Where:

• ( ) =pab td ( ) ( )pabp

ba tdtd = is the distance between satellites ""a and ""b at pt

• ( )pba t+ε refers to average between ( )pa tε and ( )pb tε • ( )pba t−ε refers to the semi-difference between ( )pa tε and ( )pb tε

The observable ( )pab td is used for orbit determination, after a number of manipulations described afterwards. This observable has the following characteristics:

• It is a purely ionosphere-free observable. • It is a purely troposphere free observable • It is a purely on board clocks free observable • It is not affected by any ambiguity • It is biased by the amount (slowly varying along time)

( ) ( )( ) ( ) ( )( )2

pRXbp

aTXp

RXap

bTX tHtHtHtH −+−

• It is affected by the local multipath caused by the satellite structure nearby the antenna

receiving the one way signal • It is affected by the local on-board receiver noise

The observable ( )p

ba tC is an on board clock observable, which nevertheless is considered just an

auxiliary measurement for the clock estimation process. This observable has the following characteristics:

• It is a purely ionosphere-free observable. • It is a purely troposphere free observable • It is a purely orbit/geometry free observable • It is not affected by any ambiguity • It is biased by the amount (slowly varying along time)

( ) ( )( ) ( ) ( )( )22

pRXbp

bTXp

RXap

aTX tHtHtHtH +

++


receiving the one way signal


page 50 of 101

• It is affected by the local on-board receiver noise

The errors ( )pba t+ε and ( )pba t−ε affecting the observables ( )pab td and ( )p

ba tC are uncorrelated

as demonstrated below, at least as far as the errors ( )pba tε and ( )p

ab tε affecting the one way

observables ( )pba tp and ( )p

ab tp are:

• Uncorrelated • Identical from a statistical perspective

( ) ( )b

apba Nt σε ,0≈

( ) ( )a

bpab Nt σε ,0≈

( ) ( )p

abp

ba tt σσ =

This is demonstrated below:

( )

( )( )( )

( )( ) ⎟⎟

⎠

⎞

⎜⎜

⎝

⎛=

⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛

2

2

00

pab

pba

pba

pba

tt

t

tCov

σ

σ

ε

ε

( )

( )

( )

( )⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−=⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛

−

+

pba

pba

pba

pba

t

t

t

t

ε

ε

ε

ε

21

21

21

21

What yields to the following covariance matrix:

( )

( )( )( )

( )( ) ⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−=⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛

−

+

21

21

21

21

00

21

21

21

21

2

2

pab

pba

pba

pba

tt

t

tCov

σ

σ

ε

ε


page 51 of 101

( )( )

( )( ) ( )( ) ( )( ) ( )( )

( )( ) ( )( ) ( )( ) ( )( )⎟⎟⎟⎟⎟⎟

⎠

⎞

⎜⎜⎜⎜⎜⎜

⎝

⎛

+−

−+

=⎟⎟⎠

⎞⎜⎜⎝

⎛

−

+

44

44

2222

2222

pbap

bap

bap

ba

pbap

bap

bap

ba

pba

pba

tttt

tttt

tt

Cov

σσσσ

σσσσ

εε

Under the mentioned assumption that ( ) ( )p

abp

ba tt σσ = it follows:

( )( )

( )( )

( )( )⎟⎟⎟⎟⎟⎟

⎠

⎞

⎜⎜⎜⎜⎜⎜

⎝

⎛

=⎟⎟⎠

⎞⎜⎜⎝

⎛

−

+

20

02

2

2

p

p

pba

pba

t

t

tt

Covσ

σ

εε

The standard deviation of the errors ( )pba t+ε and ( )pba t−ε is smaller than that of the originals

errors ( )pba tε and ( )p

ab tε .


page 52 of 101

The observable ( )pab td requires, as mentioned before, some further manipulations before it can be entered in the orbit determination process, in order to remove the bias due to group delays, both in the satellites transmission and reception chains. Concretely the following differencing scheme is proposed: Given the following observables:

( ) ( ) ( ) ( )( ) ( ) ( )( ) ( )11111

11 22ˆ

pbap

RXbp

bTXp

RXap

aTX

pabpab ttHtHtHtH

tdtd ++−

+−

+= ε

( ) ( ) ( ) ( )( ) ( ) ( )( ) ( )22222

22 22ˆ

pcap

RXcp

cTXp

RXap

aTX

pacpac ttHtHtHtH

tdtd ++−

+−

+= ε

( ) ( ) ( ) ( )( ) ( ) ( )( ) ( )33333

33 22ˆ

pbdp

RXbp

bTXp

RXdp

dTX

pdbpdb ttHtHtHtH

tdtd ++−

+−

+= ε

( ) ( ) ( ) ( )( ) ( ) ( )( ) ( )44444

44 22ˆ

pcdp

RXcp

cTXp

RXdp

dTX

pdcpdc ttHtHtHtH

tdtd ++−

+−

+= ε

Where 1pt , 2pt , 3pt and 4pt are close enough to consider that the all the terms referring to biases stay constant. The following new observable can be derived:

( ) ( ) ( )( ) ( ) ( )( )( ) ( )( ) ( ) ( )( )4321

43214321 ,,,ˆ

pcdpbdpcapba

pdcpdbpacpabppppbcad

tttt

tdtdtdtdttttd

++++ −−−

−−−=∇∆

εεεε

This observable has the following characteristics:

• It is a purely ionosphere-free observable. • It is a purely troposphere free observable • It is a purely on board clocks free observable • It is not affected by any ambiguity • It is affected by the local multipath caused by the satellite structure nearby the antenna

receiving the one way signal


page 53 of 101


The above observable is very similar to a double difference, except in the fact that the four observations are not simultaneous except from the perspective of the terms it intends to cancel. The observable ( )p

bcad td∇∆ yields to the following observation equation:

( ) ( )

( ) ( ) ( )

( ) ( ) ( )

( ) ( ) ( )

( ) ( ) ( )d

ad

ppppbcada

dpppp

bcad

ca

cpppp

bcada

cpppp

bcad

ba

bpppp

bcada

bpppp

bcad

aa

apppp

bcada

apppp

bcad

ppppbcadpppp

bcad

apr

vttttd

rr

ttttd

apr

vttttd

rr

ttttd

apr

vttttd

rr

ttttd

apr

vttttd

rr

ttttd

ttttdttttd

00

0

43210

0

4321

00

0

43210

0

4321

00

0

43210

0

4321

00

0

43210

0

4321

43214321

...,,,,,,

...,,,,,,

...,,,,,,

...,,,,,,

,,,,,,ˆ

rr

rr

r

rr

rr

r

rr

rr

r

rr

rr

r

∂∂

+∆∂

∇∆∂+∆

∂

∇∆∂+

∂∂

+∆∂

∇∆∂+∆

∂

∇∆∂+

∂∂

+∆∂

∇∆∂+∆

∂

∇∆∂+

∂∂

+∆∂

∇∆∂+∆

∂

∇∆∂+

∇∆=∇∆

Where

• ir0r refers to the a priori initial position for iSV at time 0t

• iv0r refers to the a priori initial velocity for iSV at time 0t

• iap 0r refers to the a priori auxiliary parameter vector for iSV at time 0t

• ( )4321 ,,, ppppbcad ttttd∇∆ refers to the computed version of the observation, based on the a

priori knowledge of the satellite orbits and clocks. This equation is further detailed to better understand the difference respect other observation equations commonly used for orbit determination.


page 54 of 101

( ) ( )

( )( )

( ) ( )( )

( )

( )( )

( ) ( )( )

( )

( )

( )( )

( ) ( )( )

( )

( )( )

( ) ( )( )

( )

( )

( )( )

( ) ( )( )

( )

( )( )

( ) ( )( )

( )

( )

( )( )

( ) ( )( )

( )

( )( )

( ) ( )( )

( )

( )d

ddp

d

pd

ppppbcad

dp

d

pd

ppppbcad

ddp

d

pd

ppppbcad

dp

d

pd

ppppbcad

c

ccp

c

pc

ppppbcad

cp

c

pc

ppppbcad

ccp

c

pc

ppppbcad

cp

c

pc

ppppbcad

b

bbp

b

pb

ppppbcad

bp

b

pb

ppppbcad

bbp

b

pb

ppppbcad

bp

b

pb

ppppbcad

a

aap

a

pa

ppppbcad

ap

a

pa

ppppbcad

aap

a

pa

ppppbcad

ap

a

pa

ppppbcad

ppppbcadpppp

bcad

ap

vvtr

trttttd

vtr

trttttd

rrtr

trttttd

rtr

trttttd

ap

vvtr

trttttd

vtr

trttttd

rrtr

trttttd

rtr

trttttd

ap

vvtr

trttttd

vtr

trttttd

rrtr

trttttd

rtr

trttttd

ap

vvtr

trttttd

vtr

trttttd

rrtr

trttttd

rtr

trttttd

ttttdttttd

0

00

4

4

4321

0

3

3

4321

00

4

4

4321

0

3

3

4321

0

00

4

4

4321

0

2

2

4321

00

4

4

4321

0

2

2

4321

0

00

3

3

4321

0

1

1

4321

00

3

3

4321

0

1

1

4321

0

00

2

2

4321

0

1

1

4321

00

2

2

4321

0

1

1

4321

43214321

...

,,,,,,

,,,,,,

...

,,,,,,

,,,,,,

...

,,,,,,

,,,,,,

...

,,,,,,

,,,,,,

,,,,,,ˆ

r

rr

r

rr

r

r

rr

r

rr

r

r

r

rr

r

rr

r

r

rr

r

rr

r

r

r

rr

r

rr

r

r

rr

r

rr

r

r

r

rr

r

rr

r

r

rr

r

rr

r

r

∂∂

+

∆⎥⎥⎦

⎤

⎢⎢⎣

⎡

∂

∂

∂

∇∆∂+

∂

∂

∂

∇∆∂+

∆⎥⎥⎦

⎤

⎢⎢⎣

⎡

∂

∂

∂

∇∆∂+

∂

∂

∂

∇∆∂+

∂∂

+

∆⎥⎥⎦

⎤

⎢⎢⎣

⎡

∂

∂

∂

∇∆∂+

∂

∂

∂

∇∆∂+

∆⎥⎥⎦

⎤

⎢⎢⎣

⎡

∂

∂

∂

∇∆∂+

∂

∂

∂

∇∆∂+

∂∂

+

∆⎥⎥⎦

⎤

⎢⎢⎣

⎡

∂

∂

∂

∇∆∂+

∂

∂

∂

∇∆∂+

∆⎥⎥⎦

⎤

⎢⎢⎣

⎡

∂

∂

∂

∇∆∂+

∂

∂

∂

∇∆∂+

∂∂

+

∆⎥⎥⎦

⎤

⎢⎢⎣

⎡

∂

∂

∂

∇∆∂+

∂

∂

∂

∇∆∂+

∆⎥⎥⎦

⎤

⎢⎢⎣

⎡

∂

∂

∂

∇∆∂+

∂

∂

∂

∇∆∂+

∇∆=∇∆


page 55 of 101

The orbit determination might also process the Doppler observations. An improvement, in terms of accuracy, robustness and autonomy could result from the processing of this additional information.


page 56 of 101

Principal satellite to auxiliary satellite observables: Two types of observables are used for orbit determination, namely:

• Halved “One way minus one way range cross-links” • Halved “Range rate cross-links”

which are obtained by linear combinations of other more elemental observables; as described in section Technical Appendix D. The nomenclature followed for these observables is described hereafter, with the help of the figure below:

Where:

• iSV refers to the space vehicle “i”, which belongs to the principal constellation • iSV ˆ refers to the space vehicle “i”, which belongs to the auxiliary constellation • z

wp refers to a one way range observable, being the transmitter “z” and the receiver “w”

In the figure above:

• zwp and w

zp are simultaneous, ( )wz,∀


page 57 of 101

• zwp and '

'wzp are not simultaneous, unless

⎩⎨⎧

==

wwzz

''

The one way observables depicted in the previous figure are ( ) ( )[ ]q

aaq

aa tptp ˆˆ , .

Each pair ( ) ( )[ ]q

aaq

aa tptp ˆˆ , is transformed in a pair ( ) ( )[ ]q

aaq

aa tCtd ˆˆ ˆ,ˆ in an analogous way to the

transformation from pair ( ) ( )[ ]pabp

ba tptp , can be transformed in a pair ( ) ( )[ ]p

abp

ba tCtd ˆ,ˆ , described

in section Technical Appendix D The observable ( )qaa td ˆ

ˆ is used for orbit determination. This observable has the same

characteristics as the ( )pab td observable described in previous section. The observable ( )q

aa tC ˆˆ could be used for clock determination. This observable has the same

characteristics as the ( )pba tC observable described in previous section.

The errors ( )qaa tˆ+ε and ( )qaa tˆ−ε affecting the observables ( )qaa td ˆ

ˆ and ( )qaa tC ˆˆ are uncorrelated

assuming that the errors ( )qaa tˆε and ( )q

aa tˆε fulfil the same type of conditions that were identified

for ( )pba tε and ( )p

ab tε in the previous section.

As described in section Technical Appendix D, the Doppler observable could be considered as an additional orbit determination observable.


page 58 of 101

In addition to the above described observables, the auxiliary satellites perform additional one-way ranging observations based on the ranging signals at the different user frequencies. These additional observables are further described with the help of the figure below:

These additional observables have the same characteristics:

• Type: zwp , with nomenclature accordingly to that followed in section Technical

Appendix D, being wSV an auxiliary satellite. • Observations per user frequency:

o z

wp could be any of the following observables ( )1uzw fp , ( )2u

zw fp , ( )3u

zw fp , ...

being 1uf , 2uf , 3uf , … the different user frequencies, broadcast by the principal

satellites

• Simultaneity of satellite observations:

o ( )uxzw fp and ( )''

uxzw fp are simultaneous, ',' xz ∀∀

o ( )ux

zw fp and ( )'' ux

zw fp are simultaneous, ',' xw ∀∀


page 59 of 101

Principal satellite to ground station observables: Two types of observables are used for orbit determination, namely:

• Halved “Two way range cross-links” • Halved “Range rate cross-links”

which are obtained by linear combinations of other more elemental observables; as described in section Technical Appendix D. The nomenclature followed for these observables is described hereafter, with the help of the figure below:

These additional observables have the same characteristics:

• Type: zwp , with nomenclature accordingly to that followed in section Technical

Appendix D, being zSV a principal (not an auxiliary) satellite and yST a ground station.

• Observations per user frequency:

o z

wp could be any of the following observables ( )1szw fp , ( )2s

zw fp , ( )3s

zw fp , ...

being 1sf , 2sf , 3sf , … the different system frequencies, broadcast by the

principal satellites


page 60 of 101

• Simultaneity of satellite observations:

o ( )sx

zw fp and ( )''

' sxwz fp are simultaneous, ( ) ',',' xwwzz ∀==∀

o ( )ux

zw fp and ( )''

' uxzw fp are not simultaneous, ( )[ ] ',',' xTRUEwwzz ∀≠≠=∀

o ( )ux

zw fp and ( )''

' uxzw fp are not simultaneous, ( )[ ] ',',' xTRUEwwzz ∀≠=≠∀

Consistently the one way observables depicted in the previous figure are: • ( ) ( )[ ]sxrasxr

a ftpftp ,,, 11

Each pair of simultaneous one way observations responds to the following expression:

( ) ( )[ ]

( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( )

( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( )⎪⎩

⎪⎨

⎧

+−+++−+=

+−+++−+=

=

ra

sxraTXsxr

RXsxr

ar

arr

ar

asxr

a

rasxrRXasxrTXsxrarararrasxra

sxra

sxra

tftHftHftItTtCtCtdftp

tftHftHftItTtCtCtdftp

ftpftp

1111111

1111111

11

~,,,,

~,,,,

,,,

ε

ε

Where:

• ( ) ( )ra

ra tdtd 11 = refers to the distance between satellites ""a and ground station "1" at rt

• ( ) ( )rr tCtC 1

1 = refers to the ground station “1” clock ""b at rt • ( ) ( )rar

a tCtC = refers to the on board clock of the satellite ""a at rt

• ( ) ( ) ( )sxrasxr

asxra ftIftIftI ,,, 11

1 == is the propagation delay, due to the ionosphere, in the propagation of the ranging signals, at system frequency sxf

• ( ) ( ) ( )rara

ra tTtTtT 111 == is the propagation delay, due to the troposphere, in the

propagation of the ranging signals • ( )ra t1~ε refers to the one way ( )ra tp1 observable error at rt • ( )r

a t1~ε refers to the one way ( )r

a tp1 observable error at rt • ( )sxrTX ftH ,1 , ( )sxr

RXa ftH , , ( )sxr

RX ftH ,1 and ( )sxraTX ftH , follow the same nomenclature

used for ( )rbTX tH , ( )r

RXa tH , ( )r

RXb tH and ( )r

aTX tH , respectively, in section Technical


page 61 of 101

Appendix D; with the exception that in section Technical Appendix D the dependency of these terms with the carrier frequency (and in fact with the modulated ranging signal bandwidth) is not made explicit. Please note that in fact there is a further dependency with the modulated ranging signal bandwidth)

The set of observables:

( ) ( )[ ]1111 ,,, sr

asra ftpftp

( ) ( )[ ]212

1 ,,, sra

sra ftpftp

( ) ( )[ ]3131 ,,, sr

asra ftpftp

………………

are transformed by a linear combination geometry-preserved (Condition 1) one-way range, as described below:

{ }( ) ( )

{ }( ) ( )

∑

∑∑

∑∑

=

⎥⎥⎥⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢⎢⎢⎢

⎣

⎡

=⎟⎟⎠

⎞⎜⎜⎝

⎛=

=⎟⎟⎠

⎞⎜⎜⎝

⎛=

gg

gsgr

ag

gsggr

agr

a

gsgrag

gsggragra

k

ftpkfktpktp

ftpkfktpktp

1

,,,

,,,,

111

111

Which are detailed hereafter:


page 62 of 101

{ }( ) { }( )[ ]

( ) ( ) ( ) ( ) ( ) ( ) ( )[ ] ( )

( ) ( ) ( ) ( ) ( ) ( ) ( )[ ] ( )⎪⎪⎩

⎪⎪⎨

⎧

+−+++−+

+−+++−+

=

∑∑∑

∑∑∑

gsgr

ag

gsgr

aTXsgr

RXg

gsgr

agr

arr

ar

a

gsgrag

gsgr

RXasgrTXg

gsgragrararra

gra

gra

ftkftHftHkftIktTtCtCtd

ftkftHftHkftIktTtCtCtd

ktpktp

,~,,,

,~,,,

,,,

111111

111111

11

ε

ε

This observable is:

• An ionospheric-free observable (Condition 2) if the following additional condition is imposed:

( ) 0,1 =∑

gsgrag ftIk

• Optimum (from error contamination perspective) if the following additional conditions

are imposed

( ){ }

( )

sfrequenciesystemofnumberp

gisgragi

Rkgsgrag ftkEftkE

pgi

=

∈ ⎪⎭

⎪⎬⎫

⎪⎩

⎪⎨⎧

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛=

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛∑∑

2

1

2

1 ,~min,~ εε

where [ ]...E refers to the mathematical expectation

The gk optimum coefficients that fulfill Condition 1 and Condition 2 simultaneously can be derived by the Lagrange method for identifying the conditioned minimum; concretely by solving the follow set of equations:


page 63 of 101

{ }( ) ( )( )[ ] ( )

{ }( )

{ }( )

{ }( )⎪⎪⎪⎪

⎩

⎪⎪⎪⎪

⎨

⎧

=∂

Φ∂

=∂

Φ∂

∀=∂

Φ∂

++=Φ ∑∑∑

0,,

0,,

,0,,

,,~,,

2

21

1

21

21

121

21221

λλλ

λλλ

λλ

λλελλ

g

g

gg

g

gsgrag

gg

gsgragg

k

k

kk

k

ftIkkftEkk

Where • iλ is the Lagrange’s coefficient associated to Condition “i”

The proposed selection of gk coefficients yields to the following observables:

( ) ( )[ ]

( ) ( ) ( ) ( ) ( ) ( )[ ] ( )

( ) ( ) ( ) ( ) ( ) ( )[ ] ( )⎪⎪⎩

⎪⎪⎨

⎧

+−++−+

+−++−+

=

∑

∑

ra

gsgr

aTXsgr

RXgr

arr

ar

a

rag

sgrRXasgrTXgrararra

ra

ra

tftHftHktTtCtCtd

tftHftHktTtCtCtd

tptp

11111

11111

11

,,

,,

,

ε

ε

Where

( ) ( )

( ) ( )⎪⎪⎩

⎪⎪⎨

⎧

=

=

∑

∑

gsgr

agr

a

gsgragra

ftkt

ftkt

,~

,~

11

11

εε

εε


page 64 of 101

The observables ( ) ( )[ ]r

ara tptp 1

1 , can be finally transformed in ( ) ( )[ ]ra

ra tCtd 11 ˆ,ˆ as described

in section Technical Appendix D. The result is as follows:

( ) ( )

( ) ( )[ ] ( ) ( )[ ]( )

( ) ( ) ( )

( ) ( )[ ] ( ) ( )[ ]( )⎪

⎪⎪⎪⎪⎪⎪

⎩

⎪⎪⎪⎪⎪⎪⎪

⎨

⎧

+−−−

+

+−=

+−+−

+

+=

=

−

+

∑∑

∑∑

rbag

sgrRX

sgraTXg

gsgr

RXasgrTXg

rarbrba

rbag

sgrRX

sgraTXg

gsgr

RXasgrTXg

rabrab

tftHftHkftHftHk

tCtCtC

tftHftHkftHftHk

tdtd

ε

ε

2

,,,,

ˆ

2

,,,,

ˆ

11

11

( ) ( ) ( )

( ) ( )[ ] ( ) ( )[ ] ( )

( ) ( ) ( ) ( )

( ) ( )[ ] ( ) ( )[ ] ( )⎪⎪⎪⎪⎪⎪⎪

⎩

⎪⎪⎪⎪⎪⎪⎪

⎨

⎧

++

−+

+

+−=

+−

+−

+

+=

=

−

+

∑∑

∑ ∑

rbag

sgrRXasgr

aTX

gg

sgrRX

sgrTXg

raspapbpba

rbag g

sgrRXasgr

aTX

gsgr

RXsgrTX

g

rarabrab

tftHftH

kftHftH

k

tTtCtCtC

tftHftH

kftHftH

k

tTtdtd

ε

ε

2,,

2,,

ˆ

2,,

2,,

ˆ

11

11

1

Where:

• ( ) =ra td 1 ( ) ( )ra

ra tdtd 11 = is the distance between satellites ""a and ""b at rt

• ( )ra tT 1 is the propagation delay, due to the troposphere, in the propagation of the

ranging signal • ( )ra t1+ε refers to average between ( )ra tε and ( )rt1ε • ( )ra t1−ε refers to the semi-difference between ( )ra tε and ( )rt1ε


page 65 of 101

The observable ( )ra td 1

ˆ is used for orbit determination. This observable has the following characteristics:

• It is an ionosphere-free observable. • It is a not an troposphere free observable • It is a purely on board clocks free observable • It is not affected by any ambiguity • It is affected by the local multipath caused by the satellite structure nearby the antenna

receiving the one way signal • It is biased by the amount (slowly varying along time)

( ) ( )[ ] ( ) ( )[ ]∑ ∑

−+

−

g g

sgrRXasgr

aTX

gsgr

RXsgrTX

g

ftHftHk

ftHftHk

2,,

2,, 1

1


The observable ( )ra tC1ˆ could be used for clock determination. This observable has the following characteristics:

• It is a purely ionosphere-free observable. • It is a purely troposphere free observable • It is a purely orbit/geometry free observable • It is not affected by any ambiguity • It is biased by the amount (slowly varying along time)

( ) ( )[ ] ( ) ( )[ ]∑∑

+−

+

g

sgrRXasgr

aTX

gg

sgrRX

sgrTXg

ftHftHk

ftHftHk

2,,

2,, 1

1


receiving the one way signal • It is affected by the local on-board receiver noise

The errors ( )ra t1+ε and ( )ra t1−ε affecting the observables ( )ra td 1

ˆ and ( )ra tC1ˆ are uncorrelated assuming that the errors ( )qa t1ε and ( )q

a t1ε fulfil the same type of conditions that were identified

for ( )pba tε and ( )p

ab tε in section Technical Appendix D.


page 66 of 101

The observable ( )pab td requires, as in section Technical Appendix D, some further manipulations before it can be entered in the orbit determination process, in order to remove the bias due to group delays, both in the satellites/ ground station transmission and reception chains. The final orbit determination observable is:

( ) ( ) ( )( ) ( ) ( )( )( ) ( )( ) ( ) ( )( )4321

43214321 ,,,ˆ

rcdrbdrcarba

rdcrdbracrabrrrrbcad

tttttdtdtdtdttttd

++++ −−−−−−=∇∆

εεεε

Which observation equation can be found in section Technical Appendix D.


page 67 of 101

TECHNICAL APPENDIX E (ON BOARD ORBIT DETERMINATION PROCESS)

The figure above describes a possible scheme for the orbit measurement process for the “One way range cross-links”, which, is described in section Technical Appendix D. It considers a sequential two steps approach, which are described below. The scheme feasibility can be understood under the tracking conditions described in section Technical Appendix D, which are repeated hereafter for commodity:

• The shorter propagation time (for dimensioning purposes) amongst satellites is 60 ms

• The maximum transmission duration has been limited to 30 ms.

• Spacecrafts are either in transmitting or receiving, but never doing both actions simultaneously.

• Excellent 0/ NC ratio of the received signal

• Spacecrafts host accurate real time information on the position and velocity of each

spacecraft, and on the clock offset and drift of each spacecraft

• Spacecrafts host an Atomic Frequency Standard


page 68 of 101

1. First Step: The spacecraft to spacecraft measurements are performed amongst all satellites, except potentially for those in which the propagation path could cross the ionosphere. It has been assumed that given the above described tracking conditions, an accurate range measurement is obtainable in a few milliseconds, e.g. 15 ms. This would yield, for the constellation defined in 0, to an overall observation period of:

( ) ( )p

apap

ktttt 1

21

maxτ⋅−+⋅+

Where:

• pt is the number of principal satellites, which is 27. • at is the number of auxiliary satellites, which is 6. • maxτ is the maximum time span to obtain an accurate one-way range

measurement, which is 30 ms.

• ( ) ( )

21−+⋅+ apap tttt

is the number of independent pair of satellites

• pk is a parallelism factor, which reflects the possibility to perform simultaneously spacecraft to spacecraft ranging measurements for different pairs of satellites, with no common satellite. This factor, and for an even number of

satellites can be as high as( )

2ap tt +

.

For the more demanding scenario (Scenario II) within these analyses, in terms of number of spacecrafts (please refer to 0), the overall measurement process would last 1t∆ :

mst 150050 max1 =<∆ τ

The First Step process is represented in the next figure, in which:

• pkN =1 • "","","","","","" fedcba are principal satellites


page 69 of 101

2. Second Step: The spacecraft to ground station measurements are performed amongst

principal satellites and ground stations. In this case the ionospheric effects on the ranging signals can be corrected by making use of the ranging signals transmitted simultaneously on different frequencies. It has been assumed, as in the case of the First Step that given the above described tracking conditions, an accurate range measurement is obtainable in a few milliseconds, e.g. 15 ms. This would yield, for the constellation defined in 0, to an overall observation period of:

maxmax τ⋅n

Where:

• { }sp ttn ,maxmax = • pt is the number of principal satellites, which is 27. • st is the number of ground stations, which for dimensioning purposes can be

considered 15<< • maxτ is the maximum time span to obtain an accurate one-way range

measurement, which is 30 ms.

For the more demanding scenario (Scenario II) within these analyses, in terms of number of spacecrafts (please refer to 0) and ground stations, the overall measurement process would last 2t∆ :

mst 90030 max2 =<∆ τ

The Second Step process is represented in the figure after to the next, in which:

• max2 nN = • "","","","","","" fedcba area sub-set of satellites amongst the overall set of

satellites, including both principal and auxiliary satellites.

• "4","3","2","1" are ground stations


page 70 of 101

This measurement process, Step 1 → Step 2, is repeated periodically every 3t∆ ms,

cept

ne

rom the ed by

and

atical

being 3t∆ given by the following condition:

( ) msttt 2500213 =∆+∆≥∆

Note that ( )213 ttt ∆+∆=∆ . The presented scheme intends merely to identify “a priori”potential overall conlimitations, but is not necessarily optimum or even adequate. The Study should analyze carefully the most convenient scheme for the measurement process yielding to the “Oway range cross-links” between either two satellites, or between one satellite and one ground station. Note that the measurement process has been kept, intentionally, independent fobservations exchange scheme, which a priori is considered to be performdedicated on-board communication equipment, transmitting on different frequenciesthrough different antennas. The convenience and even the need of such independence should be carefully analyzed within the Study.

The above described scheme guarantees ( ) ( )

spapap tt

tttt⋅+

−+⋅+

21

observables every

epoch, being an epoch any time interval with duration 3t∆ . Note that the above scheme implies a set of measurements which is:

• Refreshed at least once every 5 seconds 50003 <<∆t ms

• Clock free, by generating of “Halve two way range observables” abd .

• Transmitters and receivers group delay free, by applying the mathemoperator ∇∆ to adequate groups of “Halve two way range observables”


page 71 of 101


page 72 of 101


page 73 of 101

The on-board orbitography process is briefly described hereafter.

1. The observations type ( )pbcad td∇∆ are pre-processed. The processing includes blunder

detection and isolation. 2. The observations type ( )p

bcad td∇∆ are corrected from relativistic effects, which modify the

apparent frequency transmitted from each satellites as observed either by other satellites or by the ground stations.

3. The observation equation for each ( )p

bcad td∇∆ measurement is formed.

4. The “a priori” synthetic observation ( )p

bcad td∇∆ for each ( )p

bcad td∇∆ measurement is

formed, based on the “a priori” knowledge on the satellite orbits, as well as in a number of problem parameters.

5. The “a priori” synthetic observation (for each ( )p

bcad td∇∆ ) sensitivity to modifications on

the “a priori” knowledge on the satellite orbits (as well as in a number of problem parameters) is formed.

6. The observation residuals ( )p

bcad td∇∆ - ( )p

bcad td∇∆ are formed

7. Modifications on the “a priori” knowledge on the satellite orbits as well as in a number of

problem parameters, are estimated by minimizing a certain mathematical quadratic form of the observation residuals:

The following new elements respect current ground-based orbit determination processes, such as the one implemented for Galileo, point towards a significant improvement in terms of orbit accuracy:

1. The orbit determination observations are physical clock free 2. The orbit determination observations are physical group delay free

3. The orbit determination observations are propagation effects free. Therefore the observables are purely geometrical. Orbit determination can be separated from Clock determination


page 74 of 101

4. The orbit determination observables are highly accurate, with accuracy at sub-

centimeter level.

5. The orbit determination observables are not ambiguous

Therefore the observables are GPS carrier-phase-like in terms of accuracy, but not ambiguous. Note that the ambiguity resolution is frequently the most consuming process by far in the highly accurate orbit determination algorithms.

6. The relative geometry amongst principal satellites provides a significantly better

observability, on the along track and across track orbit error component. 7. The relative geometry between principal and auxiliary satellites provides a good

observability, on the radial error component (without the need of ground stations). The inclusion of the low orbit auxiliary satellites (together with the processing of long enough batches), much higher exposed to the irregularities of the force models, facilitates the linkage of the inertial positions to the ECEF system


page 75 of 101


page 76 of 101

TECHNICAL APPENDIX F ON BOARD CLOCK DETERMINATION PROCESS

The clock measurement process considers parallel feeding to the on-board timing algorithms, of the following observables: a) input principal spacecraft on-board-clock to auxiliary spacecrafts on–board-clock offsets, and b) input principal spacecraft on-board-clock to ground station clock offset. Two different and independent timing algorithms are considered:

• Generation of the Navigation System Reference Time algorithm, based on the

processing of the principal spacecraft on-board-clock to auxiliary spacecrafts on–board-lock offsets (and potentially drift) performed on each the user frequencies.

The observables principal spacecraft on-board-clock to principal spacecrafts on–board-lock offsets performed on each the system frequencies could complement the information given by the basic algorithm input observables

• Generation of the steering parameters from the Navigation System Reference

Time relative to a ground based time reference algorithm; based on the processing of the principal spacecraft on-board-clock to ground station clock offset (and potentially drift). The clock steering process is based on principal spacecraft to ground stations measurements.

The objective of the Generation of the Navigation System Reference Time Generation Algorithm is to estimate the satellite on-board clock relative to the Navigation System Reference Time which is defined implicitly within the process by means of auxiliary conditions. As result of this process the following information, expressed in the Navigation System Reference Time, is available for each satellite:

• The clock offset at a reference time 0t • The clock drift at a reference time 0t • The clock drift rate at a reference time 0t • The reference time 0t expressed in Navigation System Reference Time

The objective of the Steering of the Navigation System Reference Time Steering Algorithm is to estimate the Navigation System Reference Time steering parameters relative to UTC. As result of this process the following information, expressed in the Navigation System Reference Time, is available for each satellite:


page 77 of 101

• The System Time Reference offset relative to TAI at a reference time

0t • The System Time Reference drift relative to TAI at a reference time 0t • The reference time 0t expressed in Navigation System Reference Time

The following new elements with respect to current ground-based clock determination processes, such as the one implemented for Galileo, point towards a significant improvement in terms of clock accuracy:

1. The clock determination observations are physically geometry free

2. The clock determination observations are propagation effects free. Therefore clock determination can be conceptually speaking separated from the orbit determination. This however is not in contradiction with the fact that a minimum knowledge of the satellite orbits is necessary in order to properly pre-process the observables (e.g. potential relativistic corrections)

3. The clock determination observables are highly accurate, with accuracy

at sub-centimeter level.

4. The composite clock algorithms could provide a higher robustness, based on the reduced weight of any of the individual clock contributors, than a master clock algorithm (even including redundancy).

5. The definition of the System Time Reference through an ensemble of on-

board clocks provides a high level of autonomy.


page 78 of 101

A sequential two steps approach, coherent with the one described in Appendix A, has been considered within this document, (again merely for feasibility assessment). The mentioned approach is depicted in the next two figures, where:

• Basic observables which correspond to the additional observables described in 0, and are indicated in red colour. Their properties are repeated hereafter for easing the understanding:

o Type: z

wp , with nomenclature accordingly to that followed in section Technical Appendix D, being wSV an auxiliary satellite and zSV a principal satellite.

o Observations per user frequency: z

wp could be any of the following observables ( )1u

zw fp , ( )2u

zw fp , ( )3u

zw fp , ... being 1uf , 2uf , 3uf , …

the different user frequencies, broadcast by the principal satellites

o Simultaneity of satellite observations:

( )uxzw fp and ( )''

uxzw fp are simultaneous, ',' xz ∀∀

( )uxzw fp and ( )'' ux

zw fp are simultaneous, ',' xw ∀∀


page 79 of 101

• Complementary observables ( )rzw tC , which correspond to the “Halved

“One way minus one way range cross links” described in 0, and are indicated in blue colour. Their properties are repeated hereafter for easing the understanding:

o Type: z

wC , with nomenclature accordingly to that followed in section Technical Appendix D, being wSV an auxiliary satellite and zSV a principal satellite.

o Observations per system frequency: z

wp could be any of the following observables ( )1s

zw fp , ( )2s

zw fp , ( )3s

zw fp , ... being

1sf , 2sf , 3sf , … the different system frequencies, broadcast by the

principal satellites

o Simultaneity of satellite observations:

( )sxzw fp and ( )''

sxzw fp are simultaneous, ',' xzz ∀=

( )uxzw fp and ( )ux

zw fp ' are not simultaneous, ww ≠∀ '

In the next two Figures:

aSV , bSV , fSV are principal satellites cSV , dSV , eSV are principal satellites

The above basic observables require further manipulations before it can be entered in the clock generation process, in order to remove the bias due to auxiliary satellite receiver chain group delay. Concretely the following differencing scheme is proposed:

yw

zw

zyw ppp −=∆

Yielding to the following type of observation equation:

( ) ( ) ( ) ( )[ ] zywugr

yTXugr

zTXryrz

zyw ftHftHtCtCp ε+−+−=∆ ,,

Where:


page 80 of 101

• ( )rz tC refers to the principal satellite “z” clock at rt

• ( )ry tC refers to the principal satellite “y” clock at rt

• ( )ugrzTX ftH , refers to the group delay of the transmitting chain of principal

satellite “z” at rt , at the user frequency ugf

• ( )ugry

TX ftH , refers to the group delay of the transmitting chain of principal satellite “y” at rt , at the user frequency ugf

• ( )rzyw tε refers to the zy

wp∆ observable error at rt

Note that at a given point in time, e.g. rt , the minimum number of zy

wp∆ observables

derived from is given by the formulae:

( ) ap tt 1* −

Where • *

pt is the minimum number of principal satellites observable from an

auxiliary satellite • at is the number of auxiliary satellites

The above mentioned complementary observables do not require further manipulations before it can be entered in the clock generation process. They obey the following type of observation equation:

( ) ( ) ( ) ( )[ ] ( ) ( )[ ]yz

sgrRXysgr

yTXsgr

RXzsgr

zTX

ryrzzy

ftHftHftHftHtCtCC −+

+−

++−= ε

2,,

2,,ˆ

The analysis of the above observation equations implies:

• The set of complementary observations is biased respect to a purely onboard physical clock observable.

o The bias is the difference between the two selected satellites of the

accumulated group delay of the ranging signal through the satellite transmitting and receiving chains.

o The bias refers to the system frequency used for the derivation of the observable.


page 81 of 101

• The set of basic observations is biased respect to a purely onboard physical

clock observable.

o The bias is the difference between the two selected satellites of the group delay of the ranging signal through the satellite transmitting chains.

o The bias refers to the user frequency used for the derivation of the observable

• (Conclusion) Each set of complementary observations at the same frequency

is biased respect to the set of basic observables, at a given selected user frequency. The frequency dependability is not always visible in previous Figures.


page 82 of 101


page 83 of 101


page 84 of 101

The clock steering process is based on principal spacecraft to ground stations measurements. The objective of the Generation of the Navigation System Reference Time Generation Algorithm is to estimate the satellite on-board clock relative to the Navigation System Reference Time which is defined implicitly within the process by means of auxiliary conditions. As result of this process the following information, expressed in the Navigation System Reference Time, is available for each satellite:

• The clock offset at a reference time 0t • The clock drift at a reference time 0t • The clock drift rate at a reference time 0t • The reference time 0t expressed in Navigation System Reference Time

The objective of the Steering of the Navigation System Reference Time Steering Algorithm is to estimate the Navigation System Reference Time steering parameters relative to UTC. As result of this process the following information, expressed in the Navigation System Reference Time, is available for each satellite:

• The System Time Reference offset relative to TAI at a reference time 0t • The System Time Reference drift relative to TAI at a reference time 0t • The reference time 0t expressed in Navigation System Reference Time


page 85 of 101

TECHNICAL APPENDIX G (ISL LINKS PHYSICAL DEFINITION) Frequencies of the principal satellites to principal satellites links:

• Frequencies carrying a ranging signals

o These frequencies are labeled as saf , s

bf , scf , s

df ,…etc, where the subscript “s” means “system internal”.

o The number of “system internal” frequencies for ranging between principal

satellites is not limited a priori. Nevertheless it is assumed that one single frequency would be sufficient, in this case as no ionospheric correction is necessary.

o Potential frequency bands accordingly to ITU regulations.


page 86 of 101

o Ranging signals: The ranging signals consist on pseudo-random sequences, which

are generated in a synchronized way by both transmitter and receiver, and transmitted modulated on a radio-frequency carrier.

o The proposed scheme considers simultaneous cross-links between transmitter and

receiver.

• Frequencies for communication links

o These frequencies are labeled as caf , c

bf ,…etc, where the subscript “s” means “system internal”

o The number of “system internal” frequencies for communication between principal

satellites is not limited a priori. Nevertheless it is assumed that one single frequency could be sufficient.

Frequencies of the principal satellites to auxiliary satellites links:


o The frequencies labeled as saf , s

bf , scf , s

df ,…etc, are “system internal” frequencies.


page 87 of 101

o The number of “system internal” frequencies for ranging between principal and

auxiliary satellites is not limited a priori. Nevertheless it is assumed that these frequencies are common to those “system internal” frequencies.

o These frequencies labeled as u

af , ubf , u

cf , udf ,…etc, are “system external”

frequencies or simply “user” frequencies. The subscript “u” means “user”. They carry the ranging signals available to the user.

o The number of “system external” frequencies is not limited a priori. It is considered

that the minimum number would be 4.

o The frequency band for the “system external” frequencies is the L band, including as a minimum the Galileo frequencies.

• Ranging signals: The similarity to the Galileo signal is intended, nevertheless a certain

tailoring might be necessary.


o These frequencies are labeled as caf , c

bf ,…etc, where the subscript “s” means “system internal”.

o The number of “system internal” frequencies for communication between principal

satellites is not limited a priori.


page 88 of 101

Frequencies of the principal satellites to ground station links


It is assumed that “system internal” frequencies for ranging between the principal satellites and the ground stations are those “system internal” frequencies.

• Ranging signals:

o It is assumed that signals for ranging between the principal satellites and the ground stations are those specified for other links


o It is assumed that these frequencies are common to those “system internal” frequencies already specified

Satellite antennas:

A navigation antenna sub-system is proposed hereafter. The satellite hosts at least the following antennas:

o Antenna 1: Ultra narrow beam, controllable in azimuth and elevation, for

transmitting the ranging signals on the saf , s

bf , scf , s

df frequencies. As specified in Technical Appendix D, these frequencies have to be selected amongst those identified by the ITU regulations for this purpose.


page 89 of 101

This antenna is used both for the transmission of the ranging signals on the “system internal” frequencies to either other satellites (principal or auxiliary) or to ground stations, and for the reception of ranging signals on the “system internal” frequencies from either other satellites (principal or auxiliary) or from ground stations. The antenna design would be such that any potential group delay introduced by the antenna remains stable both versus time for a fixed beam orientation, and versus beam orientation for a fixed time. The antenna design would consist on a phase array based antenna with its radiating elements placed symmetrically on a spherical surface, as indicated in the Figure below.

o Antenna 2: Fixed pattern, for transmitting the ranging signals on the u

af , ubf , u

cf , u

df frequencies. As specified in Technical Appendix D these frequencies are within the L band. This pattern would be isoflux, in other words such that it basically illuminates uniformly, in terms of constant C/No ratio, the Earth Surface, while it minimizes, in an effective way, the power transmitted in any other spatial direction.

o Other Antennas: To be defined within the study. These antennas would serve the

follow communication links:

- Between principal spacecrafts and principal satellites - Between principal satellites and auxiliary satellites - Between principal satellites and ground stations - Between auxiliary satellites and ground stations


page 90 of 101

All the above elements are considered relevant, as the ISL technology for navigation has not been extensively explored in the European GNSS Systems. The proposed approach provides when compared with other alternatives the following advantages:

1- A reduced number of “system internal” frequencies 2- Homogeneity in the satellite definition, as all of them work with the same frequencies 3- The possibility of reducing receiver tracking error due to internal noise, down to

centimeter values 4- The possibility of reducing receiver tracking error due to local multipath, down to

centimeter values 5- The possibility of stabilizing the ranging signal group delay for each satellite


page 91 of 101

TECHNICAL APPENDIX H (GROUP DELAY CALIBRATION) A concrete real time calibration allocation scheme is proposed allowing the estimation of the differences amongst the observations performed with each of these ranging signals either those modulated in the “system external” navigation frequencies or those modulated in the “system internal” navigation frequencies. This calibration is intended at sub-centimeter level, and therefore allows considering a unique set of clock parameters per spacecraft, as well as an enhanced level of autonomy. Current global navigation systems either operational (as GPS) or under development (as Galileo) exhibit an apparent dependability of the on-board Atomic Reference Standard (AFS) offset relative to the Navigation System Reference Time, when observed through different broadcast navigation signals. Such variability is explained by both unavoidable differences in the signal generation, or by different effects on the different navigation signals when injected in the same microwave component. Current GPS signal shows inconsistencies in the range of a few nanoseconds, quite are considered quite stable along time. Such an inconsistency is also foreseen in the Galileo System, and has yielded to consider different clock parameters at navigation message level, one per Galileo service (Open Service (OS), Safety of Life Service(SoL), Public Regulated Service (PRS) and Commercial Service (CS)).

saf , s

bf , scf , s

df The effects which could be observable in global navigation system proposed are:

• Difficulties to define a single satellite on board clock, valid for the user ranging signals modulated in the “system external” navigation frequencies: u

af , ubf , … This is the case

even for half a meter level accuracy. • Difficulties to define a single satellite on board clock, valid for the system ranging signals

modulated in the “system internal” navigation frequencies: uaf , u

bf , … This is the case even for half a meter level accuracy.

• Difficulties to link the on board clock observations performed through navigation signals

modulated in the “system internal” navigation frequencies: uaf , u

bf , …with those performed through navigation signals modulated in the in the “system external” navigation frequencies: u

af , ubf , …

It is proposed a real time calibration scheme of the differences amongst the observations performed with each of these ranging signals either those modulated in the “system external” navigation frequencies or those modulated in the “system internal” navigation frequencies. This calibration is


page 92 of 101

intended aper spacecraft, as well as an The proposStudy, assumi The proposed approach is explained hereafter fointernal” navigation frequencies: For each navigation sigobservable is auxiliary sclock of

The clock estimsatemcoefficients of the unknowns in a The clockonly one ranging signal has been estimestimmGroup Delay for each of the process The estimcom

t sub-centimeter level, and therefore allows considering a unique set of clock paramenhanced level of autonomy.

ed approach, which has been carefully assessed and further developed withinme that the difference between clock parameters for each is stable over periods

nutes.

r the navigation signals modulated in the “system

nals the following observable, described in section 0, is formatellite clock and group delay free. It is a biased observation of the relative

fset between two principal satellites, and is frequency dependent:

( )

eters

this of a few

ed. This

a

the

n ck d

is

( ) ( ) ( )[ ] zywugr

yTXugr

zTXryrz

zyw ftHftHtCtCp ε+−+−=∆ ,,

ation process, is fed by all these observables for different pairs of principalllites and at different frequencies. It solve all the individual input clocks, by the imposition of

inimum variance condition which provides full rank to the first design matrix (matrix ofconventional least squares process).

estimates for a same satellite at different frequencies (for simplicity in the explanatioconsidered per frequency) are differenced respect the clo

ates for a given arbitrarily selected frequency. These observables are input to a seconation process, named as “Differential Group Delay Estimation Process” which

athematically speaking very similar to the “Clock Estimation Process”, and which provides theed frequencies.

ation process, biases slightly all the results in the same direction, what is however pletely transparent for positioning purposes.


page 93 of 101


page 94 of 101


page 95 of 101

The proposed approach provides the following advantages:

1- Definition of a single satellite on board clock, valid for all ranging signals modulated in either “system external” or “system internal” frequencies

2- Possibility to use at user level multiple frequencies combinations, mitigating more

efficiently the user tracking error due to local multipath and/or receiver noise The accuracy of existing navigation systems is driven by the accuracy of the ODTS processes used for the orbit and clock prediction the navigation satellites. The accuracy of the ODTS process depends on the fidelity of the ODTS algorithm models and, mostly, on the quality of the navigation signal observables. Navigation signal observables are typically collected on ground through a network of reference stations. The navigation signal observables always present a number of errors due to different contributions through the signal path and on its reception:

Unknown variation of the on-board group delay. Signal propagation through the ionosphere. Signal propagation through the troposphere. Multipath coming from the surrounding environment of the reference station. Interference generated by ground based transmitters.

Performing a measurement of the navigation signal in space (e.g. from a LEO satellite) will provide a measurement that is directly free of iono and tropo errors. Multipath and interference will also be mitigated by the LEO satellite RF environment and also by a proper selection of the antenna design and its location on the spacecraft. Finally the group delay of the navigation signal when traveling through the RF circuits of the navigation satellite can be easily calibrated once the other error contributors are mitigated. This is achieved then by having 2 navigation satellites in view of a LEO satellite. The minimum number of LEO satellite required for being able to perform group delay calibrations to any navigation satellite at any time is two. Additional LEO satellites would provide additional robustness. In order to perform these observations of the navigation signals from space the LEO satellites have to be equipped with a navigation receiver fitted with an atomic clock and an L-band antenna. In addition an ISL based on narrow beam steer able antenna between the LEO and the navigation satellites is needed to perform and provide the measurements to the navigation satellites for its processing.


page 96 of 101

Being the number of elements required for performing these measurements very limited the LEO spacecrafts required would be rather small. Alternatively these calibration equipments could also be part of the payload of other LEO satellites (e.g. GMES) instead of being on-board dedicated spacecrafts. In this case the navigation satellites will obtain a calibration measurement performed on the LEO satellites and allowing them to provide a highly accurate navigation service, while the LEO satellites could benefit from the navigation signals and receiver installed on-board for obtaining a highly accurate orbit and timing (additional navigation receivers would also allow the LEO satellites to perform highly accurate attitude determination based on the high accuracy achieved by the navigation signals through this process).


page 97 of 101

END OF DOCUMENT

AVIGATION AND NTEGRITY UTONOMOUS ATELLITE AVIGATION...

Documents

Transcript of AVIGATION AND NTEGRITY UTONOMOUS ATELLITE AVIGATION...