Observational and Analytical Capability at SGF Herstmonceux Graham
Observational and Analytical Capability at SGF Herstmonceux Graham Appleby, Rob Sherwood, Jose Rodriguez Space Geodesy Facility, Herstmonceux © NERC All rights reserved
The Space Geodesy Facility, Herstmonceux ACI Workshop, EOARD, 1/23/2022 © NERC All rights reserved Ruislip, UK 2
Outline • The Herstmonceux site • Weather, laser ranging tracking: • Quantity, quality • Description of SGF satellite tracking capabilities • Laser ranging; • optical (astrometry) • Data analysis • Precise orbit determination • Application to astrometric data ACI Workshop, EOARD, 1/23/2022 © NERC All rights reserved Ruislip, UK 3
Satellite Tracking • • Satellite laser ranging transmit/receive system: • 10 cm refracting telescope transmits laser pulses at 1 k pps or 10 pps • 50 cm Cassegrain reflector receives few-photon return from retroreflector-carrying satellites Co-mounted on high-rate alt/az system • • Rated up to 20°s-1 Tracking day and night at ranges of 400 – 42, 000 km at mm precision and accuracy* • Geodetic spheres, active Earth Observation and GNSS satellites ACI Workshop, EOARD, 1/23/2022 © NERC All rights reserved Ruislip, UK 4
Effectiveness of site as an optical observatory • • • Weather- and, to some extent, funding-driven laser and astrometry observations on a seven-day basis • Laser ranging 24 hours/day Cloud-cover data from MODIS gives somewhat pessimistic picture, but: Global Int Laser Ranging Service (ILRS) shows high data yield from Herstmonceux from perspective of full network: © NERC All rights reserved
© NERC All rights reserved
© NERC All rights reserved
Optical astrometry in LEO/MEO • Use of SLR telescope in astrometry mode; • • ideal for LEO/MEO objects ( up to 20° s-1 track rates) Additional wide field (2°) intensified camera for acquisition Narrow field (1 mr) on 50 cm telescope for data taking Predictions derived from TLEs using in-house code ACI Workshop, EOARD, 1/23/2022 © NERC All rights reserved Ruislip, UK 8
Optical astrometry in LEO/MEO • • • Image of object manually centred in camera fov during tracking Encoder readings taken Telescope point-model removed • Intrinsic max accuracy ~3 arc-sec via star-pointing model Post-processing via orbit fit to retrieve Alt/Az or RA/Dec observations and data precision Conversion to TDM format as part of process ACI Workshop, EOARD, 1/23/2022 © NERC All rights reserved Ruislip, UK 9
LEO/MEO astrometric observations - pre-processing scheme • • Solve for along-track and across-track corrections to the predicted ephemeris Reject outliers at ± 3σ Resulting post-fit residual RMS is about 10 arc-sec This value depends upon quality of predictions • determines ease with which observer can centre the image • Note that this is observation precision: • More on accuracy later ACI Workshop, EOARD, 1/23/2022 © NERC All rights reserved Ruislip, UK 10
Sample post-fit residual plot ACI Workshop, EOARD, 1/23/2022 © NERC All rights reserved Ruislip, UK 11
Observations of MEO and GEO objects • • • The automated GEOF system CCD images of ~10 s exposure Tracks target satellite • Min brightness observable mv ~15 or 16 ACI Workshop, EOARD, 1/23/2022 © NERC All rights reserved Ruislip, UK 12
Astrometry of satellite images from GEOF • • System operational: At intervals during satellite tracking, GEOF autocommanded to track at sidereal rate for 20 s: Delivers sharp star images and satellite trail Use of open-access astrometry s/w to search for star field and extract positions: ACI Workshop, EOARD, 1/23/2022 © NERC All rights reserved Ruislip, UK 13
Consecutive frames from GEOF ACI Workshop, EOARD, 1/23/2022 © NERC All rights reserved Ruislip, UK 14
Example of astrometric solution ACI Workshop, EOARD, 1/23/2022 © NERC All rights reserved Ruislip, UK 15
Astrometry • • Practical problems: poor predictions and small FOV Observers’ time limited by other tasks Newly developed real-time monitoring and TCP/IP interface with legacy system for remote control Automatic corrections (& search) in the future? Remote control of GEOF system enabling observers to quickly apply corrections © NERC All rights reserved
Astrometry • • • © NERC All rights reserved Astrometry solver: Astrometry. net • FOSS package • Well tested • Easy integration in our processing chain Processing of solved frames • Currently done by hand (literally) • Way forward is clear: automate everything Some issues to tackle (e. g. signal to noise) 17
Astrometry • • Satellite traces identified with standard computer vision algorithms Challenge lies in the extraction of faint sources Issues: small FOV, low elevation, poor atmospheric conditions, intrinsically faint sources… Unsupervised extraction of positional data crucial for consistent error estimation © NERC All rights reserved Line identification with Radon transform preprocessing with different thresholding levels 18
Astrometry • Observing strategy for satellite clusters? © NERC All rights reserved
Orbit determination • • A laser range analysis capability was instigated at the start of the technological development of the SLR (1980 s) Several university groups collaborated with and benefited from the SGF work Continuous development of the code Now a competitive package at the core of the SGF ILRS Analysis Centre work • SATellite ANalysis package SATAN ACI Workshop, EOARD, 1/23/2022 © NERC All rights reserved Ruislip, UK 20
The ‘SATAN’ package • • • Numerical integration of satellite equations of motion (‘orbit’) • Gauss-Jackson 8 th order integrator Parameter estimation by weighted least-squares (‘rgodyn’) Associated modules for data pre-processing, EOP a-priori, planetary ephemerides, etc. Recent use in an investigation into the scale of the ITRF derived from global geodetic laser observations* Pressing requirement to determine orbits using directional (RA, Dec, Az, Alt) observations • Use of RADAR data would be straightforward • Some data from STFC currently being investigated * Appleby et al, Jo. G, 2016 ACI Workshop, EOARD, 1/23/2022 © NERC All rights reserved Ruislip, UK 21
QC of astrometry from SGF and Starbrook • • Towards being able fully to process astrometry data: SATAN has been modified to provide a QC tool for astrometric observations: • A short observation campaign on the Galileo satellites was conducted between Starbrook and SGF in Sept 2016 • A series of positions on several nights were taken: • Ra, Dec (J 2000) from Starbrook; Az, Alt (apparent) from SGF laser ranger in passive mode; • Both sets of observations delivered in TDM format • SATAN preprocessor modified to take the TDM files • Convert the SGF Az, Alt to RA, Dec J 2000 • Then form O-C RA, Dec using a Galileo precise orbit ACI Workshop, EOARD, 1/23/2022 © NERC All rights reserved Ruislip, UK 22
QC of astrometry from SGF and Starbrook • • Galileo precise orbit determined using SATAN • Global (ILRS) laser range observations used • Five-day orbital arcs computed and iteratively fitted to the range observations • Post-fit residual RMS ~20 cm • Simple model only for solar radiation forcing Resulting ephemeris (J 2000) used as test of the quality of the astrometry data Ephemeris interpolated to epochs of SGF and Starbrook data O-C (RA, Dec) formed using modified SATAN code © NERC All rights reserved
QC of Starbrook astrometry – Galileo 103 Mean -1. 7 arc-sec Sigma 2. 1 arc-sec Mean +3. 5 arc-sec Sigma 3. 7 arc-sec © NERC All rights reserved
QC of Herstmonceux astrometry – Galileo 103 Mean +6 arc-sec Sigma 12 arc-sec Mean -18 arc-sec Sigma 22 arc-sec © NERC All rights reserved
QC of Herstmonceux astrometry – Galileo 103 - one segment © NERC All rights reserved
QC of astrometry – conclusions, future • • • Results from other Galileo satellites very similar Starbrook data consistently at 2 or 3” level of accuracy (<300 m at 20, 000 km) • Possible small time-trend in RA O-C values? Clear systematic trends in O-C values from Herstmonceux • Related to drive control loop? Observational accuracy at 15” level Further tests possible using the GEOF sensor on Galileo and in GEO • Indian GEO GNSS constellation is laser tracked and one vehicle visible from Herstmonceux © NERC All rights reserved
Take home message • • • An effective geodetic observatory Positional data with SLR system ready Astrometry operational • Some aspects automated (data collection, batch plate solving, general IT house keeping) • Most suitable method for frame processing to be decided • QC work is giving information on quality of the astrometry data from the site • more to do to QC the GEO data © NERC All rights reserved 28
• • 1/23/2022 © NERC All rights reserved Thank You 29
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