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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