This week, flight team members concentrated their efforts on determining what event caused the Mars Global Surveyor spacecraft to enter safe mode early in the morning on May 8th. Since then, Surveyor has been operating in a configuration that ensures that the spacecraft has adequate power, thermal, and communications margins. Flight software on the spacecraft automatically commands entry into this safe mode if it detects an unexpected event in one or more of Surveyor's subsystems.
One of the major diagnostic activities involved commanding the spacecraft to transmit portions of its computer memory back to Earth for analysis. An examination of a region of memory called the Audit Queue revealed that entry into safe mode occurred when a flight software task timed out and failed to report back to Surveyor's central processor.
Each software task executed by Surveyor's computer is allocated a certain amount of time to complete. Timeouts occur when a task fails to complete in the allocated time. Members of the flight team at the Lockheed Martin facility in Denver traced the source of this timeout to an infinite loop that occurred in flight software. A timeout resulted because infinite loops are impossible to complete.
The infinite loop resulted from the corruption of an area of computer memory called the Active Script Table. This table contains a list of programs executed by Surveyor's central processor, and corresponding links to the locations in computer memory where those programs are stored. A software task that was executing prior to safe- mode entry caused the infinite loop when it incorrectly updated one of the entries in the table by linking that entry back to itself.
Over the last few days, engineers on the flight team reproduced the safe-mode entry conditions in the spacecraft simulator. Subsequent analysis indicates that the action that created the infinite loop is uncommon, but predictable. Consequently, the Flight Operations Manager has decided to allow the flight team to begin procedures that will return the spacecraft back to its normal operating state. Commands to perform this recovery will be sent starting in the afternoon on Monday, May 19th. Once recovery is complete, the flight team will transmit modifications to Surveyor's flight software that will prevent this infinite loop condition from occurring again. Normal operations should be restored by mid-week.
In other news not related to safe-mode operations, the flight computer powered down gyroscope #2 on Tuesday, May 13th. This power down occurred automatically when the electrical current used by the gyroscope exceeded a preset limit. Gyro #2's functions were automatically assumed by Gyro #1 and #3, the transition was smooth, and there is no performance degradation with respect to Surveyor's ability to point at targets in space. The powered-down gyroscope will be reactivated after normal operations recommence.
After a mission elapsed time of 190 days from launch, Surveyor is 110.33 million kilometers from the Earth, 31.07 million kilometers from Mars, and is moving in an orbit around the Sun with a velocity of 23.29 kilometers per second. This orbit will intercept Mars 118 days from now, slightly after 6:00 p.m. PDT on September 11th (01:00 UTC, September 12th). Although the spacecraft is currently operating in safe mode, all systems are functioning properly, and no spacecraft hardware problems exist that pose a threat to the mission.
Status report prepared by:
Office of the Flight Operations Manager
Mars Surveyor Operations Project
NASA Jet Propulsion Laboratory
California Institute of Technology
Pasadena, CA 91109