From: Goddard Space Flight Center
Posted: Tuesday, August 6, 2002
Checkout of AQUA continued nominally until Monday (7/29). At 20:18:44 GMT (4:18:44 pm EDT) on Monday (DOY 210), spacecraft fault management flight software autonomously transitioned AQUA from SCIENCE state (Fine Point Mode) into SAFE state (Earth Point Mode). The transition into SAFE state proceeded nominally and all spacecraft and instruments attained their appropriate SAFE configuration.
The flight software triggered the transition to SAFE state in response to a GNC level 2 "loss of Nadir" fault. The limit for this trigger is set at 8 degrees, with a 10 second persistence. At the time of the fault, the spacecraft was almost directly over the South Pole and telemetry up to that point indicated that there was no abnormal attitude error. However, earth sensor telemetry indicated a Nadir angle of 12.6 degrees.
Further examination of the earth sensor data indicates that there is software discontinuity between the two algorithms utilized to convert the earth sensor "sensed" voltage to "Dip In" angle (the amount of Earth limb visible to the sensors). The discontinuity is suspected to be exacerbated by an environmental condition associated with the Poles. Landsat-7 is flying in the same orbit as Aqua with the same type of earth sensor. Trending of Landsat-7 data reveals essentially the same orbital variations in its earth sensor raw outputs. When the software switched from the "standard" algorithm to the "low Dip In" algorithm over the South Pole, the discontinuity caused a calculated error of 12.6 degrees, resulting in the fault. This has been confirmed by design verification simulation. A software patch to fix the error is currently being tested and is planned for uplink early next week, followed by a recovery back to SCIENCE state.
Activities unrelated to the transition to SAFE state continued during the week. On Wednesday (7/31), we uplinked two flight software patches to the Command and Telemetry Controller (CTC). The first adjusted the CTC calibration analog limits while the second provided the CTC with stored command sequences to load patches into the Star Trackers. On Thursday (8/1), the patch (to refine the trackers performance regarding background noise) was transferred from the CTC into the flight software within the Star Trackers. To date, the performance of the Star Trackers with the patch has been nominal.
In spite of being in SAFE state most of the week, there was also some progress in instrument activation with the start of the AIRS defrost cycle. In the first few months after launch, a build-up of a small amount of water ice on the AIRS optics and the cold heads is normal. The defrost cycle will warm up the optics and cold heads, causing the ice to sublime and the water vapor to escape into space. AIRS completed phase one and two of the cycle this week and plan to complete the remaining phases over the next week or two. The recovery from SAFE state is not expected to impact these activities.
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