Enhancing Geo-location Precision for POES Imagery Calibration

1. 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

2. 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

3. 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

4. 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!

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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)

11. 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

12. Operational Implementation(continued)

13. User Impact Direct Data users can now use POES imagery directly, without any need for post-ingest navigation to correct for timing errors!

14. Backup Slides

15. Navigation Histogram

16. Legacy Architecture

17. New Architecture

18. PFS Blocking Cycle

19. Legacy Clock Delta Equation

20. New Clock Delta Equation

21. 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

22. 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