Knowledge of the Line-of-Sight (LoS) is a key parameter in retrieving correct and reliable geolocation information. Both the ASM and ESM scanner contribute to the accuracy of LoS pointing. Although the design of the ASM and ESM permitted not to consider both mechanisms as life limited in the nominal mission lifetime, the mission extension until the end of 2013 requires to monitor both scanners regularly.
Scanner monitoring has been implemented using the scanner currents in the HK telemetry when executing particular states. For each specified state the maximum clockwise current (ASM: I0116 / ESM: I0126), the maximum counterclockwise current (ASM: I0117 / ESM: I0127) and the mean current (ASM: I0118 / ESM: I0128) are analyzed. Since nominal HK telemetry is only recorded every 16 sec, the execution of a single state generates too few readings. However retrieving regularly all states of a certain type provides data which form a regular pattern. This pattern is characteristic of the particular state. As long as the pattern does not change, scanner degradation is considered not to be relevant.
We selected 4 states for the scanner current monitoring:
The table below provides the mean current for these states since 2002. Individual years are distinguished via the colour coding.
|Scanner Mean Motor Current|
|ASM||state 65||state 02||state 33||state 49|
|ESM||state 65||state 02||state 33||state 49|