improved geo-location accuracy for poes imagery 11 december 2002 noaa/nesdis office of satellite...
TRANSCRIPT
Improved Geo-location Accuracy For POES Imagery
11 December 2002
NOAA/NESDIS
Office of Satellite OperationsOffice 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)
NOAA-16 Median Clock Deltasfrom GDP HRPT Timing System
-600
-525
-450
-375
-300
-225
-150
-75
0
75
150
225
300
375
450
525
600
114 121 128 135 142 149 156 163 170 177 184 191 198 205 212 219 226 233 240 247 255
Julian Day
Clo
ck
Del
ta,
Mil
lise
cond
s
Start of Daily DriftCorrection
13
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
15
Navigation Histogram
40 Day Histogram, 2001NOAA-16 Clock Error from PIDES Nav Correction
With Constant PACS Clock Delta of +600 milliseconds
0
1
2
3
4
5
6
7
8
Clock Error, Seconds
16
Legacy Architecture
RF EQUIP.
BIT SYNCH
POLAR FRAME SYNCH
COMM CONTROLLER DEC
HUB
PACS TCS
GPS TIME
TIP
BEACON TIP
ON-BOARD TIMING EQUIP.
(Latch GRT) (Calculate Delta)
17
New Architecture
RF EQUIP.
BIT SYNCH
225WA FRAME SYNCH
COMM CONTROLLER
DEC HUB
PACS TCS
GPS TIME
TIP
CROSS-STRAP UNIT
ON-BOARD TIMING EQUIP.
HRPT
MIRP
18
PFS Blocking Cycle
0
0
0
1
1
1
2
2 3
0 1 2 3 4
0 1 2 3 4
0
TIP Frame (n=0) Received and Held in PFS
TIP Frame (n=1) Received and Held in PFS
TIP Frame (n=2) Received and Held in PFS
TIP Frame (n=3) Received and Held in PFS
TIP Frame (n=4) Received in PFS
Block Transferred to CC
0
TIP Frame (n=0) Received and Held in PFS
Cycle Repeats as Shown Above
19
Legacy Clock Delta Equation
DELTABEACON (SCTIMETIP n *100 OFFSETTIP ) GRTCC
where
SCTIMETIP Spacecraft Time Code from TIP (milliseconds)
n Position of TIP Time Code Frame in Block (n 0,1,2,3, 4)
OFFSETTIP Constant to Account for Transfer Delays (milliseconds)
GRTCC Receipt Time of Block in Communications Controller (milliseconds)
20
New Clock Delta Equation
DELTAHRPT SCTIMEMIRP (GRTPFS OFFSETGRT )
where
SCTIMEMIRP Spacecraft Time Code from MIRP Frame Header (milliseconds)
GRTPFS Receipt Time of MIRP Frame (milliseconds)
OFFSETGRT Constant to Account for Transfer Delays (milliseconds)
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