Improved Geo-location Accuracy For POES Imagery11 December 2002 NOAA/NESDIS Office of Satellite Operations Office of Systems Development Peter Phillips Cynthia Hampton James Valenti
Topics • Basics of POES Geo-location and Clock Management • The Problem • Legacy Architecture and Findings • Corrective Actions • New System Details • Calibration and Validation • Operational Implementation • User Impact
Geo-location and Clock Management • Timing information (Day of Year and Millisecond of Day) is embedded in POES imagery data • Imagery users geolocate pixels by projecting this time onto an ephemeris-based map of spacecraft location • POES spacecraft have no internal means to “know” what time it is--time is set by the ground, and an on-board crystal oscillator provides pulses to advance the clock • The on-board oscillator drifts relative to true time, requiring the NOAA/NESDIS Office of Satellite Operations (OSO) to measure the difference between the spacecraft time and a true timing reference--called a “clock delta” • OSO clock delta measurements are used in two ways: • Provided to users to correct timing information prior to geolocation • Used to correct the spacecraft clock to true periodically
The Problem OSO-measured NOAA-15 and -16 clock deltas did not match imagery: In example, OSO measured a clock delta of -400 ms, but imagery showed it was +1200 ms This caused a geolocation error of over 10 km!
Legacy Findings • Though synchronized to Global Positioning System (GPS) time, clock delta measurement system only processed spacecraft and ground reference times to nearest 100 milliseconds • Expected clock delta measurement error of ±100 milliseconds could not explain magnitude of problem • OSO and users had image navigation software which performed “best fit” of pixels using coastlines • For 40-day period in 2001 where OSO clock delta measurement for NOAA-16 remained constant at -600 milliseconds, average clock delta from navigation software was +440 milliseconds • Navigation results normally distributed, with standard deviation of 600 milliseconds • Results consistent between OSO and users
Legacy Findings(continued) • In 1999, NOAA-15 on-orbit tests to set spacecraft clocks resulted in time being 500 milliseconds off target value on 5 of 8 attempts • Clock delta measurement system extracted time code information from different spacecraft data stream than users • Used TIROS Information Processor (TIP) data--spacecraft time code only available once every 32 seconds • Users extract time code from High Resolution Picture Transmission (HRPT) frames, which is available 6 times per second
Corrective Actions • NASA engineers discovered “bug” in NOAA-15 and NOAA-16 on-board clock-setting software • Used 1 Hz instead of 2 Hz reference to determine hardware cycle • Corrected via flight software patch in 2000 • In mid-2001, NOAA/NESDIS Office of Systems Development (OSD) engineers discovered incorrect Polar Frame Synchronizer (PFS) TIP data blocking setting for KLM spacecraft • Caused clock delta error of -900 milliseconds • Reconciling brought clock deltas to within 1 of navigation results • OSD procured new clock delta measurement system in 2002 • Part of PFS upgrades to Wallops and Fairbanks Command and Data Acquisition Stations (CDAS) • Goal was to keep spacecraft clocks within ± 75 milliseconds of GPS reference--equal to dimension of 1 high-resolution pixel
New System Details • Uses HRPT data stream • PFS receives ground timing reference from GPS receiver and performs internal “Time Stamping” as follows: • Extracts spacecraft time from header of every third frame of HRPT • Latches GPS-based Ground Receipt Time (GRT) to end of frame • Passes spacecraft time/GRT data pair to main ground system computer • Since Time Stamping is internal to the PFS, blocking and transmission delays no longer impact clock deltas • Spacecraft time and GRT processed with 1-millisecond precision
New System Calibration • New system includes settable GRT offset • Accounts for delay from spacecraft time extraction to GRT latch • Must be correct for system to produce accurate clock deltas • Testing at WCDAS compared new to legacy system • Legacy system had proper PFS blocking factor for TIP data • New deltas highly consistent within and between contacts • New deltas differed from legacy by +1000 milliseconds in all cases • PFS vendor found 640 millisecond delay in GRT output • 360 milliseconds of difference remained between new and legacy • WCDAS tests in January 2002 of MIT/Lincoln Labs clock delta measurement system recorded similar differences with legacy • Inspection of legacy code showed incorrect block transfer delay term--value of -400 milliseconds, but should be -750 milliseconds
New System Validation • Timing system calibration validated by Aerospace Corporation and MIT/Lincoln Labs • Post-installation testing at WCDAS and FCDAS with corrected GRT offset showed clock deltas consistent with initial test results and identical between stations • Navigation of imagery with spacecraft time set to within ± 75 milliseconds of true validated by OSO HRPT ingest system • Navigation also validated in Local Area Coverage (LAC) data by Air Force Weather Agency (AFWA)
Operational Implementation • New system clock deltas first used operationally for NOAA-17 • Spacecraft time set to within 2 milliseconds of true on day following launch • Users report “excellent” geolocation of NOAA-17 imagery • Clock deltas form basis for daily clock corrections to compensate for on-board oscillator drift • Archived clock delta information used to determine rate, in milliseconds per day, of drift relative to GPS reference • Daily 24-hour clock decrement term in spacecraft stored command table modified to include drift rate correction • OSO maintains spacecraft clock deltas to ± 75 milliseconds • By August 2002, OSO using new system for clock management of NOAA-14, 15, 16, and 17 spacecraft
User Impact Direct Data users can now use POES imagery directly, without any need for post-ingest navigation to correct for timing errors!
Calibration Details • PFS software includes settable GRT offset • Accounts for delay from spacecraft time extraction to GRT latch • Must be correct for system to produce accurate clock deltas • Initial testing at WCDAS compared new to legacy system • Legacy system had proper PFS blocking factor for TIP data • GRT offset was -173 milliseconds--length of 1 HRPT frame plus average link transit time from spacecraft to ground • New deltas highly consistent within and between contacts • New deltas differed from legacy by +1000 milliseconds in all cases • PFS vendor notified of test results and reviewed design • Found delay in output of ground time code following receipt of IRIG-B clocking signal • Delay was 640 milliseconds, causing GRT to be less than expected at time of latch
Calibration Details(continued) • 360 milliseconds of difference remained between new and legacy • WCDAS tests in January 2002 of MIT/Lincoln Labs clock delta measurement system recorded similar differences with legacy • Inspection of legacy clock delta calculation code showed incorrect offset term to account for block transfer delay--value of -400 milliseconds, but should be -750 milliseconds • Findings explained differences, justified change of GDP 225WA GRT offset from -173 to +467 milliseconds • Post-installation testing at WCDAS and FCDAS with new GRT offset showed clock deltas consistent with initial test results and identical between stations • Calibration validated by Aerospace Corporation Geolocation Study and MIT/Lincoln Labs