Improved Geolocation Accuracy For POES Imagery 11 December 2002

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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
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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

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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

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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!
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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

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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

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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

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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

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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

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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)

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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

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Operational Implementation(continued)
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User Impact
Direct Data users can now use POES imagery
directly, without any need for post-ingest
navigation to correct for timing errors!
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Backup Slides
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Navigation Histogram
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Legacy Architecture
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New Architecture
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PFS Blocking Cycle
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Legacy Clock Delta Equation
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New Clock Delta Equation
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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

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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