CHANGING THE ECONOMICS OF SPACE Recent GNSS Reflectometry Results from the UK TDS-1 Satellite Martin Unwin ICGPSRO2018, Taipei, 18-20 April 2018 SSTL 2018 Ack: SSTL, NOC, ESA, Surrey, CEOI, UKSA, InnovateUK, CYGNSS
About Surrey Satellite Technology Ltd (SSTL) Spin-off company, 1985, from University of Surrey, UK Now owned by Airbus Pioneer of small satellites Early user of Spaceborne GPS on small satellite PoSAT-1 1993 Built GIOVE-A precursor to Galileo Launched 2005 Payload builders for Galileo navigation satellites Batch 1& 2: 22 satellites, Works Order 3: 8+ Formosat-7/Cosmic 2 satellite platform manufacture To be launched 2018 SSTL 2018 2
Spaceborne GNSS Reflectometry GNSS Reflectometry Detecting GPS / GNSS signals reflected off the Earth s surfaces Multipath signals should contain geophysical imprint Bistatic radar but no need for radar transmitter Using Earth-reflected GPS signals for ocean sensing first discussed in 1988 1993 ESA proposed reflectometry for ocean Altimetry PARIS US & European studies on Scatterometry in late 90s 00s First reflected signal detected 1998 (JPL using SIR-C data) First dedicated in-orbit experiment: SSTL s UK-DMC (2003) feasibility First on-board processing instrument: UK TDS-1 (2014) First GNSS-R Constellation: SSTL 2018 NASA CYGNSS (Dec 2016) 3
UK TDS-1 and SGR-ReSI SSTL TechDemoSat-1 Mission 160 kg UK Satellite Demonstration 8 different payloads from UK Includes SSTL s GNSS-R SGR-ReSI payload Launched July 2014 Operated by SSTL & Sat App Catapult SGR-ReSI COTS Based GNSS Receiver Co-processor for Reflectometry Zenith antenna: hemispherical dual patch Nadir antenna 13 dbi gain, LHCP 30 beamwidth flared spiral Also two single freq. zenith patch antennas 5-10 watts, 1.5 kg SGR-ReSI Unit Nadir Antenna 4 Zenith Antenna
On-board DDM collection On-board processing of data generates 4 x Delay Doppler Maps - Level 1A Shows spread of reflected GNSS signal, related to surface roughness Processed on ground into Level 1B Reformat, add meta-data, calibration information 1 Measurement per second per track Over ocean DDMs L1B processed into Level 2 Wind Speed and mean square slope Products of operational use Strong reflection => low winds 5
MERRByS Database 27 9 21 35 1 24 7 3 8 1 2 Over 120 users Over 140 users 5 www.merrbys.org website for dissemination of GNSS-R data Web site front-page, FTP server to access all data, User forum on Google Groups Data release v0.7 up to July 2017
TDS-1: Precursor to CYGNSS NASA CYGNSS mission 8 small satellites sensing hurricanes using GNSS Reflectometry SGR-ReSI payload DDMI Designed in SSTL UK Manufactured in SST-US Orbits: 35º incl. to target tropics Potential for follow-on polar constellation Launched 15th Dec 2016 Entered Operational Phase 23rd March 2017 Overflew hurricanes: Harvey, Irma, Jose, Maria Recent data release: Dec 2017 7
TDS-1 GNSS-R Measurements Example Video 8
Correction Parameters for Measurements Simplest inversion use of peak signal Stronger reflection means flat ocean, low winds Weaker reflection means rough ocean, high winds First correction receiver antenna pattern Adjust for antenna pattern Needs good knowledge of pointing / attitude of satellite TDS-1 has attitude accuracy limitations Next use absolute signal, not signal to noise TDS-1 discovered that noise varies across globe Disable Automatic Gain Mode (AGM) and used fixed gain Take reference noise measurements off black body load 9
Global Variation of Noise in GNSS Band Fixed gain mode allows assessment of noise over globe Example is Aug 2016 Noise after GNSS correlation Surprise hot spots visible over equatorial oceans Over 3 db variation Perhaps reflection of GNSS, SBAS and other signals? Also some other man-made interference visible E.g. The Caucasus & California Good reason to avoid using signal to noise ratio as measurement Noise varies too much Ascending passes Descending passes 10
Allowance for GPS Transmit Powers and Patterns Variations in GPS transmit signal power can add errors to wind speed measurement Both absolute signal power, and antenna pattern View of GPS antenna pattern / power obtained from SGR-GEO experiment on GIOVE-A Visible differences in GPS transmit patterns, incl. azimuth (Paper by Verde, Unwin, et al, ESA GNC May 2017) Also Tx power may vary with time (BIIR-M 1.5 db, Feb 2017, GPSWorld Apr17) Block IIR(A) Block IIR-M 11
Finding Wind Speed from DDMs SSTL partnered with NOC National Oceanography Centre (NOC) NOC developed inversion algorithm using measurement of wind speed from ASCAT scatterometer Find best model fit, then apply to whole data Progress with inversion: Initially agreement of 3.8 m/s After corrections, refinements, & filtering, NOC is achieving < 2 m/s agreement Good enough performance for new global wind sensor, and still improving 12
Land Applications: North Africa Land Collections show geophysical patterns Water, soil moisture, soil roughness, salinity, biomass, Strong reflections Salt lakes in Tunisia and Algeria Some desert areas Weak reflections Vegetation Mountain ranges Other deserts Some temporal variations visible, but TDS data is sparse Many applications hydrology, flooding, climate, etc. 13
Reflections off ice very strong Can detect edges, resolution few kms Also potential for altimetry over ice E.g. Northwest Passage iced over in March 2016 Recent work by IEEC shows TDS-1 signal coherency < 5cm agreement with height of thin sea ice over Hudson Bay Ice Sensing 14
ESA-funded TGSCATT study (May 16-May 18): End-to-end scientific assessment of GNSS reflectometry scatterometric measurements from TDS-1 and data products Seeks to establish the physical relation between GNSS-R signals and ocean wind and roughness properties Objectives/tasks Revise and adapt simulation framework for TDS-1 (Wavpy) Define GNSS-R observables using simulation framework Develop/consolidate physical/empirical GMFs Consolidation of Level1 & Level2 products (MERRByS) Impact analysis on global NWP (O-B, preliminary OSEs & OSSEs) Workshop planned for 24 th May 2018, Southampton, UK https://conference.noc.ac.uk/gnss-r-workshop 15
TDS-1 Current Activities TDS-1 demonstrated tracking new GNSS signals in orbit: First position fixes from in-orbit GPS L2C signals, orbit recovery < 1 metre First in-orbit tracking of Galileo E1 signals, 4 signals tracked TDS-1 mission provisionally finished in July 2017 But life extension approved for 12+ months, de-orbit sail postponed New ESA-funded study for TDS-1 TDS-1 re-commenced operating GNSS Reflectometry February 2018 Careful data compression permits continuous operation 24/7 Plan for pilot demonstration of low latency wind speed products via MERRByS - Demonstration of Galileo E1 Reflectometry 13 16
SSTL GNSS Receiver Developments SGR-ReSI / SGR-Axio available for GPS L1 reflectometry Also doubles up as platform GNSS receiver Used on 2 orbiting satellites for positioning Miniature GNSS Receiver - SGR-Ligo Cubesat compatible, approx 0.5 W, also an integrated version Due to fly on Cubesat in mid-2018 Dual antenna could be applied to Reflectometry ESA ORORO Concept instrument combining Reflectometry with Radio Occultation Constellation of small satellites dedicated to weather Multi-frequency breadboard development supported by ESA 17
Future of GNSS-Reflectometry UK TechDemoSat-1 demonstrated, at low cost, practicality of GNSS Reflectometry Datasets enabled research across the world into many areas Ocean wind speed can be measured globally EO Applications for ocean, land and ice Following missions researching GNSS-R include CYGNSS, WNISat-1R, FS-7R, FSSCat, etc. New capabilities and techniques to be uncovered Then, challenge to move beyond research Sustainable service for operational weather measurement Need constellation to give coverage (12+ satellites) Need customers for service, may not be expensive TDS-1 pilot data service being rolled out Stepping stone for customer involvement Steering future spaceborne service mission 18
CHANGING THE ECONOMICS OF SPACE Thank You www.merrbys.co.uk Surrey Satellite Technology Ltd. SSTL 2018 Tycho House, 20 Stephenson Road, Surrey Research Park, Guildford, Surrey, GU27YE, United Kingdom Tel: +44(0)1483803803 Fax:+44(0)1483803804 Email: info@sstl.co.uk Web:www.sstl.co.uk