Alignment Drift During Observations (Part 2)


Previous reports have noted a drift in fid light positions following a maneuver as the spacecraft settles into a new thermal equilibrium. This problem is becoming increasingly common as the spacecraft (and it's thermal insulation) age. In an effort to characterize this drift, we have analyzed the maneuvers leading in to observations with large (> 2 arc-seconds) drift. We find that the vast majority of these cases occur when the spacecraft performs a large maneuver (>60 arc-seconds) predominately along the pitch axis.


Using the drift values calculated for the above-mentioned report, all observations with a z-axis drift larger than 2 arc-seconds were collected. The maneuver angles and Sun (pitch) angles were collected for the preceding maneuvers from the MPS MMAN Processing Summary in the Mission Planning log. These data are presented in the table below.

ObsidLoadStart Sun Angle (degrees)End Sun Angle (degrees)Total Maneuver (degrees)SIM drift (arc-seconds)
3241DEC2502124.961.4 64.52.1
4880JUN2104139.187.6 62.32.0
4595MAR290473.2 153.486.73.2
4587MAR1904120.071.4 74.22.2
4189JAN200348.9 133.384.62.1
5044JUL190471.5 71.5 0.032.4
2505AUG060150.9 142.792.72.7
4842JUN1404150.856.6 99.62.0
3775MAR1703146.274.2 72.12.5
4500JUN070460.6 170.2109.72.6
2800DEC010152.1 157.4105.42.0
1012AUG1701140.050.0 92.12.0
6129SEP200459.1 179.3120.42.0
3776JUL140398.8 168.371.62.5
4585NOV2204132.757.5 83.42.1
3732OCT200355.6 143.390.13.5
3901FEB1003123.454.1 69.92.2
4848SEP0604129.862.1 77.62.7
2708OCT0502138.078.5 59.52.0
2555JUN170258.4 112.763.52.0
5316MAY0304156.286.6 70.92.4
5015MAR0104174.569.8 105.52.7
4559MAY310468.7 152.984.42.4
2755NOV1102115.949.8 66.92.7

The below image show the near linear relationship between the change in Sun angle and total maneuver angle for each of the observations.

The next image shows the beginning and final Sun angle for each of the 29 recorded observations. Note that all but two observations pass through an angle of 100 degrees.


The above data show that alignment drift during observations is most likely due to thermal settling. This confirms anecdotal evidence that large drifts are associated with large slews, but no operational changes are recommended at this time.

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Last modified:12/27/13