Jason Tye, Prof Craig Underwood
Surrey Space Centre, University of Surrey, UK
Dr Martin Unwin, Dr Philip Jales
Surrey Satellite Technology LTD, UK
SPACE REFLECTO
Brest, France, 5th November 2013
Spaceborne Detections of Reflected SBAS Satellite Signals
Contents
Background Coverage Simulations The Software Receiver Results of SBAS search Discussion of Direct Signal Crossover
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Background
Spaceborne GNSS-R in its infancy SSTL’s influence:
UK-DMC, TDS-1, CYGNSS In orbit demonstration of GNSS-R
Remote sensing targets Number of signals of
opportunity growing Introduction to PhD
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Image courtesy of SSTL
About SBAS
Satellite Based Augmentation Service primarily for integrity of service data and corrections for aircraft using GPS services
Geostationary ~ 35,800km altitude Broadcast similar C/A PRN codes to
GPS Encoded NAV data at 500bps
• ~30 message types @ 1Hz
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Coverage Simulation
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UK-DMC orbit and antenna pattern over 1 dayMaximum 4 SPsSBAS (B), GPS (R)Appears to have a 'generous' bias with respect to antenna gain
– Should be consistent across both satellite systems
0.67dB average SP gain+24.5% Global cells covered
Coverage Simulation
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Clear advantage seen for GPS+SBAS especially focusing around the 4 specular point threshold
The Software Receiver
Adapted to process SBAS signals
– Changes to data structures and processing methods, particularly navigation
Re-processing of UK-DMC data with help of the WaveSentry catalogue
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Results
Multiple examples of SBAS reflections obtained from ‘challenging’ geometries and different constellations
DDMs are incoherently accumulated for 7s at 1ms coherent integration steps
Sacrifice surface resolution for correlation power for detection–No science is to be done immediately
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Results
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First identified SBAS reflection
Two MSAS satellites in one collection
Coincident with successful targeting of a GIOVE-A reflection (Jales)
Map Images: Google Earth
Rx
SPSBAS
TxSBAS
T NMEA 42T NMEA 42
T NMEA 50T NMEA 50
SP NMEA 50SP NMEA 42
Papua New Guinea
Results
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(A) MT-SAT 2 (MSAS) with NMEA ID 50(B) MT-SAT 1R (MSAS) with NMEA ID 42
Dela
y (
Chip
s)
(A) Doppler (Hz) (B) Doppler (Hz)
Results
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ESA Artemis (EGNOS) on 18/02/2009 SPSBAS
TxSBAS
Dela
y (
Chip
s)
Doppler (Hz)
Results
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Reflections of Inmarsat 3 f3 (WAAS) taken in 2004(A) and (B) share almost identical geometries taken on different days(C) is the most 'extreme' of reflections and shows a bias in Doppler typical of being on the antenna fringes
(A) Doppler (Hz) (B) Doppler (Hz) (C) Doppler (Hz)
Dela
y (
Chip
s)
Detection Conclusions
Developed and demonstrated SBAS processing capability of the software receiver
We have tracked and decoded navigation data from direct SBAS signals in the UK-DMC data and reflection DDMs have been plotted
Provided proof of concept for use of SBAS satellites as reflectometry sources
Work must be done to establish fully the data quality retrieved for science purposes in context of a link budget as typically SBAS signal strength is a few dB less than GPS at source
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Direct Signal Overlay
CYGNSS TIM14
GPS Code repeats every 1 ms => 298 km wrap-aroundDirect Signal Overlay is geometric effect based on the difference between reflected and direct paths as a multiple of code lengthMOD[|TxS|+|SRx|-|TxRx|, 298(km)]
S
Rx
Tx
Direct Signal Overlay
CYGNSS TIM15
Sensitivity to orbital height - UK-DMC Zenith reflection path difference: 680+680km = 4 fold ambiguity Ambiguities as multiples of code length “unwrap” from a zenith reflection to the Earth limb where the direct and reflected signal paths are equal
Earth limb
Direct Signal Overlay
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30 chip wide window of bad data during ambiguity unwinding
Simple analysis using spherical Earth approximation
Direct Signal Overlay
CYGNSS TIM17
Direct signal travels through the DDM over the collection period due to vRx
Direct signal more prominent over short incoherent integration periodsEffect was noted in GPS-R DDMs from UK-DMC also
SBAS PRN 134 from R7, 200msGPS PRN 15 from R102, 1s
Direct Signal Overlay
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Direct signal offset from predicted reflection at 139.86 - 141.19 µs – Verified-20 chip signal is a land reflection– Would expect negative delay as land is above Geoid
Doppler (Hz)
Dela
y (
Chip
s)
Direct Signal Overlay Conclusions
The direct signal is picked up in the UK-DMC nadir antenna Direct signal may regularly overlay reflected signals in DDM
Appearance of direct signal could potentially affect DDM inversion – depending on method
Direct and Reflected signals have different dynamics in DDM Direct signal attenuated if longer integration times used
An automated geometry check could flag potential risks to data quality owing to direct signal overlay
One could envisage that channel allocation might take this effect into account if quantified appropriately
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Thank You for Listening!
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