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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
Advanced Radio Technologies, Sep 8 - 10, 1999
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
Advanced Radio Technologies, Sep 8 - 10, 1999
Occultation GeometryOccultation Geometry
• Carrier phase (Doppler)measurement
• Atmosphere delay
• Bending angle
• Refractivity profile
• Temperature profile
Advanced Radio Technologies, Sep 8 - 10, 1999
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
Advanced Radio Technologies, Sep 8 - 10, 1999
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
Advanced Radio Technologies, Sep 8 - 10, 1999
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
Advanced Radio Technologies, Sep 8 - 10, 1999
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
Advanced Radio Technologies, Sep 8 - 10, 1999
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
Advanced Radio Technologies, Sep 8 - 10, 1999
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
Advanced Radio Technologies, Sep 8 - 10, 1999
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
Advanced Radio Technologies, Sep 8 - 10, 1999
Occultation AntennaOccultation Antenna
Advanced Radio Technologies, Sep 8 - 10, 1999
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
Advanced Radio Technologies, Sep 8 - 10, 1999
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
Advanced Radio Technologies, Sep 8 - 10, 1999
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
Advanced Radio Technologies, Sep 8 - 10, 1999
RFCU L1 RFCU L1 PreselectionPreselection Filter Filter
8-pole coaxial cavity filter
S&R transmitter suppressed 53 dB
Advanced Radio Technologies, Sep 8 - 10, 1999
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)
Advanced Radio Technologies, Sep 8 - 10, 1999
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
Advanced Radio Technologies, Sep 8 - 10, 1999
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
Advanced Radio Technologies, Sep 8 - 10, 1999
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