The Impact of Current and Future

Global Navigation Satellite Systems

on Precise Carrier Phase Positioning

R.Murat Demirer

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Potential Applications of the

Global Positioning Systems

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GPS

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Functions & Products of

the GPS Segments

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The GPS Satellite Constellation

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The GNSS (Global Navigation Satellite

System)

From ancient times people's effort for positioning and navigation has never been stopped

Currently there are two GNSS systems in operation: the United Currently there are two GNSS systems in operation: the United States' Global Positioning System (GPS) and the Russian's Global Orbiting Navigation Satellite System (GLONASS)

Europe's Gallieo system, another promising GNSS, will reach its full operational capacity in 2010

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Principles of RTK Satellite Navigation

• Navigation -is a process of determination “Where I am now?”-> current position and velocitynow?”-> current position and velocity

• Radionavigation-user (passive) receives signals transmitted by satellites (GPS, GLONASS, Galileo)

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Main Characteristics of GPS, GLONASS

& Galileo

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The main idea of Navigation: triangulation

Different signal structure, different carrier frequency, different satellite orbit, they share the similar idea to

locate the user: triangulation.

The theory of triangulation is simple: as long as the receiver can measure the radial distance between itself and several radio transmitter, and the position of all the

transmitters, the user's location can be determined by the intersection of all the spheres centered the transmitters.

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How GPS works.

• The idea of the GPS system is that the user is

located at the intersection of the four spheres

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G e o m e t r i c P o s i t i o n

f r o m R a n g e O b s e r v a t i o n s A b s o l u t e P o s i t i o n i n g

Main features

�Each satellite broadcasts signals on two carriers:

the major carrier L1 with frequency f1 = 1575.42 MHz, and the

secondary carrier L2 with frequency f2 = 1227.60 MHz.

�Two pseudorandom noise (PRN) codes are modulated on the

two base carriers.

�The first code is the coarse acquisition code (C/A code) which is

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�The first code is the coarse acquisition code (C/A code) which is

available for civilian use with the Standard Positioning Service

(SPS).

� The second code is the precision code (P code or Y code) for

military use and designated as the Precision Positioning Service

(PPS).

�The P-code is modulated on

both carriers L1 and L2, whereas the C/A code is modulated

upon only L1.

The three types of measurements can be

obtained from GPS receivers based on

the GPS signals• Pseudo range Measurements: These are derived from

the PRN codes

• Carrier Phase Measurements: These are obtained by measuring the phase of the incoming carrier (L1 and/or L2), and the range to a satellite can be computed by L2), and the range to a satellite can be computed by measuring an ambiguous number of cycles

• Doppler Measurements: The derivative of the carrier phase measurement is the Doppler shift due to the relative motion between the receiver and the GPSsatellite

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GNSS Observations

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Distance Measurement Using GNSS

• 1. Satellite positions are known

• 2. Each satellite transmits a different code

• 3. Time measurements rely on an accurate time reference (clock) accurate time reference (clock)

• 4. Satellite and user clocks should be synchronized

• 5. The differences between generated and received codes corresponds to the time difference

• 6. Several error sources affect the measurement accuracy

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Schematic Presentation of GNSS

Codes & Carrier

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The signals has three parts:

Carrier wave with fL1 and fL2

Navigation Data (50 bps)

Spreading Sequence

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The GPS signal transmitted from the

satellite can be modeled

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There are three component: C/A code on L1, P(Y) code on L1 and P(Y) code on L2

Comparison of the Present GPS Signals

and the Post-Modernization GPS

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Signal transmitted from satellite k

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Generation of GPS Signals at the

satellites

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C/A Code Generator

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Carrier Phase and Pseudo-range

Measurement

Pseudo-range measurement is noisy but unambiguous.

The carrier phase measurement is the difference between the phases The carrier phase measurement is the difference between the phases of the local generated carrier signal and the phase of the carrier from

the input signal of each satellite.

Carrier phase measurement is cleaner but ambiguous because it only contains the information of the change of the carrier cycles between

the time that phase lock was achieved until the present time.

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Sources of Range Measurement

Errors

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Measurement Simulation Diagram

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Simulation Approach

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The basic observation equations for GPS satellite

positioning at pseudo range and carrier phase

measurements from a receiver A to a GPS satellite i or k

The geometrical range between satellite

and receiver is computed as

Model equations

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GPS Carrier Phase measurements (Lachapelle 2003)

The carrier phase based double

difference observable in meters

(Lachapelle 2003)

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Time scale representation of GNSSS31

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Linearization of observation Equation 32

Nonlinear term

Linearization starts by finding an initial position for the receiver

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Earth(0,0,0)

The partial derivatives in Taylor expansion

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Linearized observation equation

where

Using Least Squares Method 34

Observations:

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m≥4 unique solution

Kalman Filter Equations35

(from Brown et al. 1992)

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A Kalman filter estimation algorithm has been used to estimate the unknown statesin a sequential fashion

Like standard least squares estimation, the Kalman filter is based on anoptimality criterion of minimizing the squared estimation residuals.

Kalman Filter Approach 36

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Kalman Gain

Keplerian Orbit Elements 37

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GM=3.986005.1014

M3/s2

Orbit Estimation Strategy38

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Ephemeris Parameters39

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Necessary variables for determining geometric coordinates of satellite k at tj

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Tropospheric models

• According to Parkinson et al. (1995), simple troposphere models remove about 90% of the troposphere error on an undifferencedmeasurement.

• Some examples of effective troposphere models include the Saastamoinen model (Saastamoinen, 1972), the UNB3 model (Collins et al. 1996) and the Modified Hopfield Model (Goad et al. 1974), while mapping function can be found in Neill (1996), Lanyi (1984), and Ifadis (2000)

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Hopfield Model (total tropospheric

error D at zenith)

N…Refractivity

P…pressure

T….temperature

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2-5 meters error at zenith

25 meters at low elevation

Tropospheric Delay versus Humidity for Satellites with

5 and 80 Degree

Elevation Angles

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Ionosphere delay

• the ionosphere delay is treated as a state to

be estimated.

• Since the ionosphere delay is highly correlated

with the initial carrier phase ambiguity, a with the initial carrier phase ambiguity, a

pseudo-observation is used to enable the

ionosphere delay state to converge

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Ionospheric delay

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GNSS User Range Error Budget

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Real Accuracy of Position

Determination

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GNSS Positioning Accuracy

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GN S S E r r o r S o u r c e s

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GPS Modifications (GPS III)

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Why do We Need the

Augmentations?

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Differential Navigation

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Comparison of Normal and

Differential GNSS

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Potential Applications

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Galileo

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Galileo Signal Structure and Services

( Hein et al. 2002)

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Positioning Scenarios

Galileo will also transmit

freely available satellite

navigation signals on three

frequencies and is scheduled

to be fully operational as

early as 2008

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early as 2008

(Wibberley, 2004)

the impact that the

new signals will have on

precise kinematic

positioning

Galileo Services

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Potential Services of Galileo System

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GPS and Galileo Together Enhanced geometry of a combined GPS/Galileo system greatly improve the ability to estimate ionosphere delays quickly and precisely

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GPS and Galileo systems in an

integrated manner

• The L1/E1 (1575.42 MHz) and L5/E5a (1176.45 MHz) frequency bands will be shared.

• This will allow the systems to be used together because receiver manufacturers will be able to use the same receiver front-ends for multiple use the same receiver front-ends for multiple signals.

• This will keep the cost of future dual-system receivers economically viable. However, the systems will remain autonomous by keeping the GPS and Galileo control segments completely separate

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Homogenous Double Differences

Heterogeneous Double Differences

Homogenous Double Differences

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Pseudorange Measurement Covariance Matrix for

GPS/Galileo Triple-

Frequency Data

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Developing Near Real-Time Kinetic Algorithms for estimating accurate

position using GPS carrier phase observations

To develop real-time kinetic algorithms defined on a dynamical model (H) for increasing accuracy of coordinates of moving vehicle.

To identify the relationship between problems sourced by orbit, ionosphore,

To introduce a new RTK mathematical model

To identify the relationship between problems sourced by orbit, ionosphore, troposphere as well as multipath (fading) and antenna phase center variations (z) on a

dynamical model (RK) by competing with other estimators

To develop new techniques that can be applied to a wide range of problems like Autonomous Helicopter Landing, missile navigation in the RK model.

To evaluate performance of accuracy of centimeters of some duration of seconds

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Diagram of Finite-Difference Nonlinear

FilteringThe Ito equation characterizes how target states and their probability densities evolve in time due to deterministic and random target motion effects.

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What will be the goals with GPS and

Galileo systems together in future?

1. To describe and demonstrate effective processing techniques for GNSS data from

multiple systems and on multiple frequencies;

2. To show the impact that current and future GNSS signals will have on the ability

to estimate ionosphere delays and GPS signal fading effects

3. To provide a realistic and quantitative analysis of the reliability of ambiguity

resolution with current and future GNSS signals;

4. To elucidate the benefits of using linear combinations of GNSS data and to test

various optimally chosen combinations for better accuracy for example dual-system

scenario using triple-frequency GPS and triple-frequency Galileo measurements

together;

5. To develop a realistic mathematical model for simulation of future GNSS

measurements

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GPS Array Monitors Deformation of

the Earths Crust Daniel Abrams

Institution Name: University of Illinois at Urbana-Champaign

• The earths crust bends or deforms before it breaks, just as a stick bends before it breaks across your

knee. When the earths crust breaks, it is called an earthquake.

• Deformation of the earth between earthquakes invisible to scientists

• Relative movements of less than 3mm between two points separated by hundreds of kilometers can now

be measured by GPS. be measured by GPS.

• Networks of GPS stations currently monitor the relatively rapid movement of the earths plates and the

slower deformations due to the interaction of two plates at their boundaries.

• The ability of GPS to measure both small and slow deformations naturally leads to its use in studying the

problems of earthquakes and attendant seismic hazard.

• Preliminary results suggest that deformations may be occurring inside the seismic zone, and these

deformations are consistent with the types expected based on the earthquake activity, models for stress

• Improvements in the measurement of this deformation over the next few years will help in developing

better explanations for the causes of New Madrid earthquakes and will contribute to improvements in

seismic hazard estimation.

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Seismic instrumentation network

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Defense Application

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A Block 50 F-16D of the Eglin based 46th Test Wing carrying a pair

of EDGE test weapons during captive carry trials. One bomb is

measuring its position using standard GPS, the other is using

differential GPS updates datalinked to the F-16 from a ground

station, using an encrypted channel. The bomb using DGPS

achieved accuracies as high as 3.5 ft in horizontal position (USAF).