Dec. 20, 1999
Turn-on of the MISR instrument and unlatching of the cover and calibration panels has been postponed until tomorrow morning due to an anomaly in a monitoring circuit for the spacecraft High Gain Antenna (HGA). (This has affected turn-on of all of the instruments, not just MISR.) The HGA antenna has a monitoring system which keeps track of slew rates, and during a slew activity associated with a TDRSS contact when the spacecraft was entering the South Atlantic Anomaly (a region of high radiation) a bit got flipped to a state that caused a shutdown of the HGA drive. The manufacturer, Spar Aerospace in Canada, has been provided the data and are analyzing the situation. Since the currents and other indicators of the HGA hardware performance appear normal, it looks like the problem is not with the HGA drive itself. It is not clear why the bit flipped, but given the location in the orbit a single event upset associated with the radiation environment is considered a distinct possibility. Reactivation of the HGA has not yet been attempted while the data are being analyzed, but could happen this evening or sometime tomorrow. If necessary, a number of workaround scenarios exist, including specialized procedures for handling of the monitoring circuit data. The HGA also has a redundant capability, though implementation of that option would require reconfiguration of the spacecraft flight software. There is also capability for direct downlink to ground stations (at reduced efficiency); however, we are quite a long way away from having to consider that scenario. I am told that this situation is not considered an emergency and it is apparently not unusual to encounter this type of occurrence during mission activations. In the meanwhile, downlink of spacecraft engineering data using the omnidirectional antenna through TDRSS has been working at better than expected performance. The guaranteed capability of the omni is a 1 kbps rate, however it has been working reliably at 16 kbps, the rate required for the telemetry associated with instrument activations. It is on this basis that MISR turn-on and unlatch is being planned for tomorrow morning, using the omni antenna for the data downlink. This puts off beginning of the outgas period to sometime on Wednesday. Terra is in a 695 km x 655 km orbit, and trim maneuvers will occur over the coming weeks to place us in the final orbit. At present, MISR survival heaters are continuing to keep the instrument at comfortable temperatures, at stable levels, and with significant margins. David Diner