A High Performance GPS Radio Occultation … High Performance GPS Radio Occultation Instrument...

Advanced Radio Technologies, Sep 8 - 10, 1999

A High PerformanceA High PerformanceGPS Radio Occultation InstrumentGPS Radio Occultation Instrument

Magnus BonnedalJacob Christensen

Saab Ericsson Space

GothenburgSweden


Radio OccultationRadio Occultation

• Probing the atmosphere andionosphere by measuring the L1and L2 signals from raising andsetting GPS S/C

• Determining the S/C positionwith high accuracy to extract theatmosphere/ionosphere impact

• Method developed by JPL andStanford, for sounding ofplanetary atmospheres (Mariner,Pioneer and Voyager)

Local Horizontal Plane

Tropospheric/StratosphericOccultations

IonosphericOccultations

Overhead Measurements-POD

-Navigation-Double Differencing

NPOESS OrbitRm=7198 km

GNSS OrbitRg=25520 km, 26560 km

Troposphere/stratosphereIonosphere


Occultation GeometryOccultation Geometry

• Carrier phase (Doppler)measurement

• Atmosphere delay

• Bending angle

• Refractivity profile

• Temperature profile


PROGRAMMESPROGRAMMES

EUROPE US

Customer EUMETSAT IPO (NOAA, NASA, DOD)

Platform METOP NPOESS

1:st Launch 2003 2008

No of sat 3 5

Instrument GRAS GPSOS

Status BB-Phase - 98 Risk Reduction Phase - 98

C/D Phase running EM Phase running

Service Experiment Operational


Occultation Instrument ObjectivesOccultation Instrument Objectives

• Deliver meteorological data to the scientific andmeteorological communities

• Data: temperature, pressure and electron contentvertical profiles of the atmosphere and ionosphere

• 500 - 1000 profiles per day and LEO S/C

• Measurements with very high accuracy and down to afew miles altitude

• Joint program between US and Europe


Instrument AccommodationInstrument Accommodation

• Two occultationantennas

• Zenith antenna fornavigation (PreciseOrbit Determination)

• RF units placedclose to antennas forbest NF

• Can accommodatelong IF cables toElectronics unit

Zenith antenna coverage

Forward looking occultationantenna coverage

Flight direction

Aft looking occultationantenna coverage


GPS SystemGPS System

Coarse AcquisitionC/A-code

PrecisionP-code

CDMApseudorandom

1.023 10.23 Msps

BW ±10.23 ±10.23 MHz

Frequencies L1=1575.42 L1=1575.42L2=1227.6

MHzMHz

Availability Public Encrypted(Y-code)

Measurement AtmosphereAtmosphereIonosphere

End product Bending angle Environmental DataRecords


Generic GPS Digital ReceiverGeneric GPS Digital Receiver

SIN COS

CarrierNCO

Integrate& dump

Integrate& dump

Integrate& dump

Integrate& dump

Integrate& dump

Integrate& dump

3-bit shift register

ReceiverDSP

Codegenerator

CodeNCO

Clock

ClockCarrier phase increment

Code phase increment

E P L

Q

I

QL

QP

QE

IL

IP

IE

DigitalIF

• Digital downconversion

• Correlationwith Early,Punctual andLate code

• Integration(despreading)

• Locked toNumericallyControlledOscillator


Instrument Functional Block DiagramInstrument Functional Block Diagram

PowerSupply

DISCAGGA

AGGA

DSP

SpacecraftInterface FMU A/B

AGGA

ReferenceOscillator

FrequencyGenerator

ANTENNAS RFCUs GEU

DBI

GVA

GZA

GAVA

DBU Relay OnDBU Relay Off

EQ-SOL

ICU Power A

ICU Power B

DBU Relay On

DBU Relay Off

DBU Power

DSPBUS

L0

L0

L0

IF

IF

IF

DC Supply and TM

DC Supply and TM

Thermistor

Thermistor

Thermistor

+5V TM

RFCU-Z

RFCU -V

LNA RF/IF

RFCU -AV

DC Supply and TM

RBI

ISAC

IF-FilterA/D

DISCIF-Filter

A/D

IF-FilterA/D

Test


Occultation AntennasOccultation Antennas• Dual frequency fixe beam

array with resonant ringelements

• Individual feed networks insuspended QFRP microstrip

• Antenna pattern shaped toearth rim

• Coverage ±50° in azimuth(captures most of the GPSsatellites)

• High antenna gain 11 dBi

• Suppressed interference andnoise

• Low ohmic loss, low mass


Occultation AntennaOccultation Antenna


Radio Frequency Conditioner UnitRadio Frequency Conditioner Unit

• Very highly selectiveRF and IF filters

• Single stage downconversion to IF

• Common LO frequency• Recombination of

L1/L2 at IF


RFCU Functional Block DiagramRFCU Functional Block Diagram

RF L1

RF L2

RFCU IF

RF LO

PowerRegulator

+5 V -5 V

Input Power

Thermistor

In Out

PreselectionFilters

LNA Board RFIF Board

LNA Board

Power Regulator Board

184.90 MHz

L2

Frequency[MHz]

162.92 MHz

L1


RFCU Key FeaturesRFCU Key Features

• Overall gain >85 dB• Accommodates both GPS and GLONASS• S&R transmitter located 20 MHz from L1 band edge

- suppressed >95 dB• LNA data:

– NF < 0.9 dB– RL > 20 dB– Gain > 38 dB


RFCU L1 RFCU L1 PreselectionPreselection Filter Filter

8-pole coaxial cavity filter

S&R transmitter suppressed 53 dB


GRAS Electronic UnitGRAS Electronic Unit

1 Power converter

2 S/C interface

3 IF, Sampler and channelprocessor

4 DSP

5 Spare

6 Frequency generator

not shown: USO (UltraStable Oscillator)


AGGAAGGA

P-code Unit 3

P-code Unit 2

P-code Unit 1

P-code Unit 0

Real-to-Complex

Converter

C/A -codeUnit

SignalLevel

Detector

TimeBase

Generator

AntennaSwitch

Controller

In Out

Micro-processorInterface

InterruptController

Time strobe Antenna switch

Code-rate

Gen.

CodeDelayLine

Int Epoch

CarrierGene-rator

Integrator

Seq Sel

SystemSupport

Functions

Ctrl

A

D

• Advanced GPS GLONASS ASIC• A custom designed ASIC for

space applications developedunder ESA contract

• Supports C/A, P and Y(semi)code tracking

• Twelve channels• Application:

– S/C control; Position, Velocity,Time and Attitude

– Support to Precise OrbitDetermination, POD

– Atmospheric sounding– Reference stations


Instrument SummaryInstrument Summary

High S/N, lowimplementation losses

⇒ Measurements at lowaltitudes

Wide antenna coveragemany channels

⇒ Covers a large no ofoccultation

Ultra stable oscillator ⇒ High measurement accuracy

RFC tolerant designDistributed front ends

⇒ Easy to accommodate

Modular design ⇒ Allows redundancy andexpansion

GLONASS capability ⇒ Prepared for GNSS-2


AcknowledgementsAcknowledgements

• IPO under which the GPSOS study has been conducted

• ESA under which the GRAS study has been conducted

• Reference:

– P Sinander, P Silvestrin, Development of an Advanced GPSGLONASS ASIC, ESTEC, ESA

A High Performance GPS Radio Occultation … High Performance GPS Radio Occultation Instrument...

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