ICESat Reference Orbit Ground Tracks

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To acquire the necessary 91-day orbit phasing, the NASA ICESat orbit transitioned to the 8-day repeat orbit from 05 August (02:00 UTC) to early September 2004.

The ICESat spacecraft attitude changed by 180 degrees on 30 June 2005. On 08 July 2005, the orbit transitioned to the 8-day repeat cycle to facilitate transition into the desired subcycle of the 91-day orbit in preparation for the Laser 3D fall campaign.

The first several months of ICESat operations were in an orbit that repeated ground tracks every eight days for calibration and validation purposes. During the subsequent mission phase, ICESat has been in an orbit that repeats ground tracks every 91 days. The geodetic latitude and longitude location of the reference orbit ground tracks for the 8-day and 91-day orbits are defined in available binary files (t8p.reforb and t91p.reforb). Reference ground track locations are reported at 1 sec intervals, corresponding to an along-track spacing of approximately 7 km. A Fortran program (read_orbit.f90) and an IDL program are also available to ingest these data files.

The actual ICESat orbit is maintained so that the nadir ground track is within ±1 km of the reference orbit track. Beginning with Laser 2 operations on 25 September 2003 at latitudes north and south of 59 degrees, precise spacecraft attitude control was used to point the laser beam within ±150 m of the reference orbit ground tracks. Precision spacecraft pointing control is not normally used in mid-latitudes between 59 degrees, so the data tracks at these latitudes typically remain within ±1 km of the reference orbit ground tracks; however, spacecraft pointing control is used several times a day to target specific off-nadir locations or conduct attitude calibration maneuvers. During those times when pointing at specific targets, the ground tracks depart from the reference orbit tracks by as much as 60 km. Pointing to the reference track north and south of 59 degrees continued through the Laser 3B operations period. Starting with the Laser 3C period, the northern boundary for pointing to the reference track will be moved south to approximately 47° N.

The reference orbit ground track files are provided as direct-access binary files in the format defined in Table 1. Each record consists of data for one track, which represents one orbit revolution starting and ending at the ascending node equator crossing. The initial record in each file corresponds to track 1, and the 8-day and 91-day repeat orbits are composed of 119 and 1,354 orbit tracks, respectively.

Table 1. Reference Orbit Ground Track File Structure
Record # Bytes Variable Type Description
1 - 23204 R*4 array (5801) Latitude for each second along track 1
-999.0 fill extra values at end of array
23205 - 46408 R*4 array (5801) Longitude for each second along track 1
-999.0 fill extra values at end of array
46409 - 46412 R*4 Time increment between array values
nominally 1 second
2 through n
Same as above for tracks 2 through n
n = 119 for 8-day orbit
n = 1354 for 91-day orbit

Orbit tracks acquired during the ICESat operation periods are identified in Table 2. During the Laser 1 operation period, the reference tracks were observed on four complete repeat cycles of the 8-day tracks and portions of two repeat cycles. During the Laser 2A operation period, slightly more than one complete cycle of the 8-day orbit was repeated, followed by 45 days in the 91-day repeat orbit. During the Laser 2B operation period and subsequent periods, the reference tracks observed during the last 33 days of Laser 2A operations are being repeated. Acquisition of altimeter channel surface returns along these tracks depends on cloud cover conditions at the time of data collection. The information in Table 2 and the fact that there are approximately 14.8 orbits per day helps determine the day specific tracks were acquired.

Table 2. Reference Orbit Tracks Acquired During ICESat Observation Periods
Operation Period Start Date End Date Starting Track Ending Track
Laser 1A 2003-02-20 2003-03-21 8-day #72 8-day #24
Laser 1B 2003-03-20 2003-03-29 8-day #25 8-day #23
Laser 2A 2003-09-25 2003-10-04 8-day #88 8-day #100
Laser 2A 2003-10-04 2003-11-19 91-day #1098 91-day #421
Laser 2B 2004-02-17 2004-03-21 91-day #1284 91-day #421
Laser 2C 2004-05-18 2004-06-21 91-day #1283 91-day #434
Laser 3A 2004-10-03 2004-11-08 91-day #1273 91-day #452
Laser 3B 2005-02-17 2005-03-24 91-day #1258 91-day #426
Laser 3C 2005-05-20 2005-06-23 91-day #1275 91-day #421
Laser 3D 2005-10-21 2005-11-24 91-day #1282 91-day #421
Laser 3E 2006-02-22 2006-03-27 91-day #1283 91-day #424
Laser 3F 2006-05-24 2006-06-26 91-day #1283 91-day #420
Laser 3G 2006-10-25 2006-11-27 91-day #1283 91-day #423
Laser 3H 2007-03-12 2007-04-14 91-day #1279 91-day #426
Laser 3I 2007-10-02 2007-11-05 91-day #1280 91-day #421
Laser 3J 2008-02-17 2008-03-21 91-day #1282 91-day #422
Laser 3K 2008-10-04 2008-10-19 91-day #1283 91-day #145
Laser 2D 2008-11-25 2008-12-17 91-day #96 91-day #423
Laser 2E 2009-03-09 2009-04-11 91-day #1286 91-day #424
Laser 2F 2009-09-30 2009-10-11 91-day #1280 91-day #84