Mars Global Surveyor Resumes Aerobraking
23 Sep 1998
(Source: Jet Propulsion Laboratory)
Source: MSOP-MGS Aerobraking Status Report
Wednesday, September 23 (DOY 259/19:00:00 to DOY 266/19:00:00 UTC)
Last Orbit Covered by this Report = 573
Total Aerobraking orbits accomplished = 180
Total Science Phasing orbits accomplished = 290
Apoapsis altitude = 17866 km
Apoapsis altitude decrease since start of aerobraking = 36160 km
Periapsis altitude = 171.1 km
Current Orbit Period = 11:38:02
Orbit Period decrease since start of aerobraking = 33:21:31
The Mars Global Surveyor spacecraft is in excellent health following today's main engine maneuver that started Phase 2 aerobraking operations. The 4.8 second burn provided 11.62 m/s change in velocity and hit the target of 127 km periapsis altitude perfectly. The P573 drag sequence is on board and will be controlling all on-orbit S/C activities until this evening when the new P574 sequence takes over.
The burn maneuver was originally planned for last Thursday but was delayed due to an unexpected entry into contingency mode by the spacecraft. A sequence block error, that passed through STL testing undetected, placed the +Y solar array gimbal drive in the powered hold mode and left the panel pointed away from the Sun. The lack of power production caused energy to be drawn from the S/C batteries until fault monitoring on board reacted to their low state of charge. At 10:43 UTC (04:43 MDT) the S/C entered contingency mode, corrected the panel position and mode, pointed panels at the Sun and canceled the on-board sequence execution, including the already initiated main engine burn sequence.
The low power entry path into contingency mode called for swapping the power subsystem mode controller to the backup unit. This transition introduced a temporary power drop on the power bus causing the IMU DC power supply, PSE (Power Subsystem Electronics) and the I/O bus to swap to their respective backup units. After analysis, commands were generated and uplinked that restored the IMU, PSE and bus to their nominal configurations by Friday evening. On Saturday, the mode controller was swapped back to primary which, as expected, caused the swap of the PSE and bus but not the IMU.
The remainder of the recovery required a short mini-sequence to enable and disable the star processing algorithm to prevent Mars albedo interference with the CSA (Celestial Sensor Assembly). With this sequence (Z568) in place, inertial reference was re-established while the S/C was Sun pointed and the uplink and downlink was at low rate via the LGA. Because of the low uplink rate via the LGA path, a normal drag sequence would have required over 75 minutes of continuos uplink time. This was deemed unacceptable due to the low link margins available using the LGA uplink path. An intermediate sequence, P570 was built, tested and executed that transitioned the S/C from LGA / Sun pointing to HGA / Earth pointing and continued to manage the star processing as well as maneuver to warm the MOLA instrument. This provided a safe HGA uplink path from which the return to aerobraking could be accomplished.
The P573 sequence was loaded late Tuesday then took control early Wednesday morning and performed the drag pass activities while the S/C was still above the atmosphere as a rehearsal. This pass resulted in the +Y solar array drive electronics being swapped to side 2 due to fault protection. The fault was discovered to be caused by a gimbal drive acceleration parameter that caused the panel to slightly overshoot the commanded target before converging on the desired position. This overshoot did not present a problem during the SPO operations but in aerobraking, the panel is moving toward the hard stops where, from a low rate, the gimbal came to a halt while the drive control was still trying to converge on the target. Commands to reset the GDE to side 1 and to increase the persistence threshold for the fault response were developed, tested and uplinked. STL testing re-created the problem and confirmed the need to change the acceleration parameter back to the Phase 1 aerobraking value. That update will be radiated later tonight.
The main engine maneuver, A573, was loaded at the same time as the P573 sequence and executed the fully successful burn at 17:49 UTC (11:49 MDT). The first drag pass will occur tonight at 19:30 MDT.
All spacecraft subsystems report excellent health. The power subsystem reports strong performance, however the battery charge profile did not occur as expected following the rehearsal drag pass on orbit 573. Normally when the flight software detects full battery state of charge it will command the primary chargers to it's lowest setting of 0.85 amps. This command did not occur on the first orbit and analysis is under way to determine the cause. The sequence disconnects the primary chargers and attaches the trickle chargers 20 minutes following full battery state of charge, preventing any concern of battery overcharging.
Temperatures remain well within limitations with the minimum MOLA instrument value riding above 10?C. Telecommunications performance remains strong. The TES and MAG/ER were powered down by the contingency mode response and remain un-powered now. These instruments will be re-powered by the end of the week.
The 3rd aerobraking Phase 2 reset meeting was held this week to discuss period reduction goals, schedules and parameters for the first sequence and AB-1.
The STL performed various tests on the products to recover from the contingency mode entry and the repaired drag pass sequence.
Periapsis for Orbit 574 DOY267/01:09:54 UTC 1st Drag Pass
AB-2 Maneuver Apoapsis for Orbit 574 DOY267/06:58:40 UTC
AB-3 Maneuver Apoapsis for Orbit 576 DOY268/06:11:17 UTC
There were 57 command files radiated to the S/C during this period. The total files radiated since launch is now 2753. These commands were sent in support of the following activities:
Nominal star catalogs and ephemeris file updates.
Errant drag pass sequence (P560)
AB-1 maneuver sequence that was canceled by C'Mode (A560)
Recovery from C'Mode command (44 files)
AB-1 sequence (A573)
Nominal drag pass sequence (P573)
Swap the GDE back to side 1
Reset of battery charge counters
Command Loss timer resets
Memory readout of on-board computer memory