Jason Tye, Prof Craig Underwood Surrey Space Centre, University of Surrey, UK

1. Spaceborne Detections of Reflected SBAS Satellite Signals 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

2. Contents • Background • Coverage Simulations • The Software Receiver • Results of SBAS search • Discussion of Direct Signal Crossover 2 SBAS Reflectometry, SPACE REFLECTO 2013

3. 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 Image courtesy of SSTL 3 SBAS Reflectometry, SPACE REFLECTO 2013

4. 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 4 SBAS Reflectometry, SPACE REFLECTO 2013

5. Coverage Simulation • UK-DMC orbit and antenna pattern over 1 day • Maximum 4 SPs • SBAS (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 5 SBAS Reflectometry, SPACE REFLECTO 2013

6. Coverage Simulation • Clear advantage seen for GPS+SBAS especially focusing around the 4 specular point threshold 6 SBAS Reflectometry, SPACE REFLECTO 2013

7. 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 7 SBAS Reflectometry, SPACE REFLECTO 2013

8. 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 8 SBAS Reflectometry, SPACE REFLECTO 2013

9. Results • First identified SBAS reflection • Two MSAS satellites in one collection • Coincident with successful targeting of a GIOVE-A reflection (Jales) T NMEA 50 TxSBAS T NMEA 42 SPSBAS Papua New Guinea SP NMEA 42 SP NMEA 50 Rx Map Images: Google Earth 9 SBAS Reflectometry, SPACE REFLECTO 2013

10. Results Delay (Chips) (B) Doppler (Hz) (A) Doppler (Hz) • (A) MT-SAT 2 (MSAS) with NMEA ID 50 • (B) MT-SAT 1R (MSAS) with NMEA ID 42 10 SBAS Reflectometry, SPACE REFLECTO 2013

11. Results Delay (Chips) Doppler (Hz) TxSBAS • ESA Artemis (EGNOS) on 18/02/2009 SPSBAS 11 SBAS Reflectometry, SPACE REFLECTO 2013

12. Results Delay (Chips) (A) Doppler (Hz) (B) Doppler (Hz) (C) Doppler (Hz) • 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 12 SBAS Reflectometry, SPACE REFLECTO 2013

13. 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 13 SBAS Reflectometry, SPACE REFLECTO 2013

14. Direct Signal Overlay • GPS Code repeats every 1 ms => 298 km wrap-around Direct Signal Overlay is geometric effect based on the difference between reflected and direct paths as a multiple of code length • MOD[|TxS|+|SRx|-|TxRx|, 298(km)] Tx Rx S 14 CYGNSS TIM

15. Direct Signal Overlay • 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 15 CYGNSS TIM

16. Direct Signal Overlay • 30 chip wide window of bad data during ambiguity unwinding • Simple analysis using spherical Earth approximation 16 SBAS Reflectometry, SPACE REFLECTO 2013

17. Direct Signal Overlay GPS PRN 15 from R102, 1s SBAS PRN 134 from R7, 200ms • Direct signal travels through the DDM over the collection period due to vRx • Direct signal more prominent over short incoherent integration periods • Effect was noted in GPS-R DDMs from UK-DMC also 17 CYGNSS TIM

18. Direct Signal Overlay Delay (Chips) • 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) 18 SBAS Reflectometry, SPACE REFLECTO 2013

19. 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 19 SBAS Reflectometry, SPACE REFLECTO 2013

20. Thank You for Listening! 20 SBAS Reflectometry, SPACE REFLECTO 2013