The gain change commands at the time of launch were issued at the start time of a scene, which due to various error accumulations was placing the gain change 1 to 2 seconds into the start of the scene. The MOC was informed that the U.S. ground processing system could not handle this band change location. A scheduler modification was subsequently made to move the gain change commanding back by 4 seconds, so it would occur during the trailing end of the preceding scene. This was initially incorporated between July 14 to July 26, 1999. This change was made permanent starting on August 2, 1999.

The rational for setting the offset to -4 seconds was:

1. Payload command timing error: up to 1 second quantization error because the SCP executes commands on integer second boundaries.
   
   
2. Gain change command execution time: 0.1 second/band X 8 bands = 0.8 seconds
   
   
3. Orbital along-track position uncertainty: typically <1.0 km, equivalent to 0.2 seconds (except on days following delta-V orbit correction maneuver, in which the along track error could reach 2 seconds.)
   
   
4. Scene center-to-center time variance: typically 23.92 +/- 0.09 seconds due to orbital eccentricity.
   
   

The -4 second gain change offset exists for all scenes acquired after August 2, 1999 (1999/128:22:07 zulu). A list of scenes with leading edge gain changes (i.e. pre August 2, 1999) was compiled for the record.

Back to Chapter 6

Last Update: on July 2, 2002