Spacecraft Performance

5. Spacecraft Performance

Spacecraft performance continues to be excellent. All Galileo subsystems continue to perform in an exemplary manner. During the past year, two significant thermal-related changes have been made to the electrical heater configuration. A despun electronics structural mounted heater was powered on continuously to protect sensitive despun electronics from experiencing a large temperature excursion and remain within acceptable limits should adjacent electronics become unpowered. The additional heater power raises the thermal environment into the louver range where power dissipation-temperature sensitivity is less. Also, the flash heater on the SSI radiator plate was turned on to maintain the SSI detector temperature limits rather than have the SSI instrument main power on.

Galileo has superbly performed all TCMs, attitude maintenance maneuvers, propulsion maintenance flushing pulses, calibrations, telecommunications tests, and other activities in support of the Ida encounter and data return. In July, the spacecraft flawlessly executed the SL-9 impact observation stored sequence and began early return of some comet fragment impact data; more impact data will be returned through the balance of this year and in January '95. In addition, computer memory checkout tests were successfully performed in August and several special telecommunication characterization tests are scheduled for later this year.

Though spacecraft performance has been excellent, some unpredicted events have occurred. Noteworthy events were the occurrence of a Command and Data Subsystem (CDS) transient bus reset in the all-spin mode, an Attitude and Articulation Control Subsystem (AACS) autonomous inertial-to-cruise mode change, and a significant change on the Direct Current (DC) bus imbalance measurement. None of these unexpected events poses a threat to the spacecraft health. The following paragraphs briefly summarize each of these three notable events in more detail.

5.1 All-Spin Bus Reset

A CDS spurious transient bus reset occurred on September 24, 1993 in the all-spin mode. This was a major surprise and immediately prompted intensive sophisticated computer analyses efforts to understand the dynamics at the slip ring-brush interface as a function of spacecraft spin mode (all-spin, dual-spin, and quasi all-spin). It had been thought that the absence of significant relative motion between the spacecraft spun and despun sections precluded these bus resets in all-spin mode. The computer modeling revealed that mechanical "dither" motion between the spun and despun sections as small as 30 to 40 micro-radians is enough to cause momentary (10-20 microsec) simultaneous brush "lifting" or brush "heel-toe rocking". This condition in conjunction with existing brush-debris-formed spurious electrical paths in the Spin Bearing Assembly (SBA) can cause bus resets. Furthermore, the analysis indicated that all-spin seems to be the most likely (from a dynamics view) mode for enabling these bus resets. Because transient bus resets are thought to be caused by electrical debris paths in the SBA coupled with momentary, drastically lower brush-ring electrical conductivity (due to rocking or lifting), the spacecraft is now being operated primarily in the quasi-all-spin mode (relative motion between sections of 0.2 deg/sec). This operating mode minimizes the generation of additional brush debris, ensures adequate bearing lubrication, helps preclude possible slip-ring contamination and is mechanically less sensitive to "rocking or lifting." The all-spin and dual-spin operating modes will be used only when required for mission/spacecraft activities (e.g., remote sensing) until orbital operations. Dual-spin will be primary in orbital operations where the new flight software will automatically recover from resets.

5.2 Gyro Fault Protection Trip

On August 28, 1993, about 5 hours before Ida closest approach, the spacecraft gyros were autonomously powered off via fault protection in response to detection of high rates (outside of preset limits) sensed by both gyros. As a consequence, the Ida encounter was performed in cruise mode rather than the inertial mode (gyros on) which provides better scan platform pointing (see Ref. 1). Despite the slightly degraded scan platform pointing, the encounter was a spectacular success. Because of the low (10 bps) real-time telemetry rates available during the encounter, it was not immediately possible to determine why the gyros were turned off. An extensive test and analysis effort, including testbed simulators and a thorough review of the flight software code and timing, provided no clue to the anomaly. Subsequent tape recorder playback of the Ida data provided some additional information for the anomaly investigation but still no clue. The Ida playback data, however, did provide some important information revealing that unexpected scan platform (where gyros are mounted) motions occurred several times in both control axes as evidenced from higher than expected power consumption and other telemetry data.

Because the anomaly could not be re-created with the test simulators, explained via software analyses, or analyzed via diagnostic flight data; a special flight test was performed in May 1994 using identical portions of the original Ida sequence where the initial anomaly and other slew-related anomalies occurred. The anomaly did not recur during the flight test. It is noted that ever since the Ida anomaly, the AACS gyros, electronics, and scan platform have operated flawlessly with no hint of a problem. At this writing, it has just been determined that a transient error in the cone encoder data word can result in all the anomaly symptoms in a remarkable pathology. The source of such an error is being sought.

5.3 DC Bus Imbalance Change

During the past year the AC/DC power bus imbalances continued to fluctuate. The Alternating Current (AC) imbalance measurement exhibited only minor changes remaining fairly stable near its March 1992 level of 4.5 volts. The DC bus imbalance measurement, after about 7 months of near-stable operation, exhibited significant changes over a three day period in mid-May 1994, shortly after transition to quasi all-spin. During this period, several other engineering measurements also changed. Changes were observed on the AC bus current, DC bus current, shunt current, CDS +10 volt power supply current, SBA temperature, and the Ultra-Stable Oscillator (USO) oven current. Analyses showed that all the changes can be explained by the clearing of spurious slip-ring brush debris paths in the SBA. Previous ground tests demonstrated that debris paths are cleared with low current levels (50 to 100 mA). The flight observed current, power, and temperature changes are internally consistent and consistent with ground test data.

5.4 Galileo-Mars Observer Comparison Study

As a consequence of the permanent loss of signal from the Mars Observer (MO) spacecraft, an intensive effort was undertaken by a multi-discipline team to verify that the Galileo spacecraft is not susceptible to the MO type failure modes identified by the failure review process. Based on the NASA, JPL, and Martin Marietta failure reports and JPL audit, Project Galileo performed a comparison study covering all the identified MO failure modes, including propulsion, telecommunications, power, etc. The comparison study results and conclusions were independently reviewed by a special board (including several members from the MO failure analysis team) in May 1994. The study concluded that Galileo is not susceptible to MO type failures, there is no reason to change planned operational use, and there are no risk areas that had not been previously accounted for. The Review Board unanimously endorsed the study findings.

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