# Presentation: 030316

## Improved Geo-location Accuracy  For POES Imagery11 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**

## Operational Implementation(continued) 

## User Impact

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

## Backup Slides

## Navigation Histogram  

## Legacy Architecture  

## New Architecture

## PFS Blocking Cycle 

## Legacy Clock Delta Equation 

## New Clock Delta Equation 

## 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**