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August 25, 1999

Mars Climate Orbiter (MCO)

Two separate in-flight checkout activities were conducted on Friday and Saturday (August 20 & 21), exercising both the primary encoder (encoder "A") on the solar array inner gimbal drive and the inner gimbal assembly itself. These tests indicated no anomalous behavior and confirmed that the inner gimbal and the "A" encoder are fully functional. Special high-rate telemetry data did indicate larger than expected structural oscillations of the solar panel-the attitude and articulation control group at Lockheed Martin Astronautics (LMA) in Denver is investigating via comparison of ground-based simulations of the test with flight data. Subsequently, evaluation of the temporary stoppage of the inner gimbal drive after Trajectory Correction Maneuver #3 (TCM-3) on July 25 is now focusing on the gimbal movements performed to move the solar array out of its passive restraint, shortly after the TCM-3 burn was completed.

On Monday (August 23) the Deep Space Network (DSN) initiated the demonstration phase of its Near-Simultaneous Tracking (NST) program, using the Mars Global Surveyor (MGS) spacecraft already orbiting Mars as a form of calibration target for navigation of the MCO spacecraft during its approach to Mars. In NST tracking passes a single ground station will first track one spacecraft for 2-4 hours, then switch over in 20 minutes to track a second spacecraft for a similar (2-4 hours) time period. Two-way Doppler and range data are recorded from the two spacecraft sequentially, then processed within the navigation software system to obtain estimates of the approaching spacecraft's flight path relative to the other spacecraft (and hence Mars itself) that are substantially more accurate than would otherwise be possible. Following a 30 day demonstration phase, the DSN will begin NST tracking of Mars Polar Lander and MGS, in preparation for the critical 45 day period leading to landing, in which the NST technique will be used operationally for precision guidance of the lander toward its target entry conditions.

Yesterday (Tuesday, August 24), the flight team conducted its second test and training exercise for MCO aerobraking, simulating the development and execution of sequences for three successive drag passes during the final "end-game" phase of aerobraking, including a simulated "jack" procedure to perform a maneuver rapidly following a yellow alarm limit violation on drag pass constraints. Several minor problems were identified for correction in the operation of the uplink system for flight. Otherwise, the team successfully met all key objectives for aerobraking test and training, and is now beginning preparations for the final Mars Orbit Insetion (MOI) operational readiness test, which will take place on September 2.


Mars Polar Lander (MPL)

In addition to the MCO training activites yesterday, NASA formally approved a target landing site for Mars Polar Lander, following a briefing given to senior management from the Office of Space Science (OSS). OSS accepted the recommendation made by the flight team of a primary target area centered at 76 deg S Latitude, 195 deg W Longitude, which is believed to lie well onto the layered terrain in the south polar region of Mars. This site was chosen after a detailed evaluation of images and laser altimetry obtained from the mission target sector by the Mars Global Surveyor (MGS) spacecraft. The Mars Orbiter Camera (MOC) onboard MGS will continue to image the target area at high resolution through August and September, given the opportunities provided by MGS orbit geometry.

Following NASA's approval of the target site, the flight team proceeded with final targeting computations for the upcoming site adjustment maneuver. Development of the command products for this maneuver have begun, in preparation for transmission to the spacecraft early next week. The site adjustment maneuver is scheduled to begin at 10:00 PDT on Wednesday, September 1; the maneuver will be 2.3 meters/second in magnitude.

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