Mission Summary and Data Coverage

The MGS spacecraft was tracked by the antennae of the Deep Space Network (DSN) using two-way and three-way ramped Doppler, and two-way ramped range data[4,5]. In addition, as described by Lemoine et al.[6], one-way Doppler data were obtained from the Ultrastable Oscillator (USO) aboard MGS during SPO-1 (March - April 1998) and SPO-2 (June - July 1998). No one-way Doppler data were used outside of these time periods. The MGS mission phases and orbit characteristics are summarized in Table 1.

After the end of aerobraking on February 4, 1999, most of February 1999 was devoted solely to acquisition of tracking data to allow for improvement of the gravity models of Mars. During this mission phase (known as the Gravity Calibration Orbit), the spacecraft was maintained in a quiescent state to minimize disturbances in the tracking and obtain as much high quality tracking data as possible.

A contingency period of science data collection took place in March 1999, prior to the deployment of the High Gain Antenna (HGA) on March 29, 1999. In this mode, (designated Fixed High-Gain Antenna Mapping) the spacecraft pointed its instruments at Mars 18 to 20 hours per day, and then turned toward the Earth four to six hours per day to return the science data, and obtain tracking. The first MOLA data after HGA deployment were obtained on April 2, 1999. After HGA deployment, normal mapping operations involved pointing the instruments at nadir while the High Gain Antenna independently tracked the Earth. The High Gain Antenna is located at the end of a two meter boom and two axis gimbals maintain the antenna pointed at the Earth over each orbit.

Mapping operations were interrupted on April 16, 1999 when the azimuth gimbal of the HGA became stuck at 41.5. No MOLA altimeter data were collected from April 16 to April 28, 1999, while the causes of the anomaly were investigated. It was determined that the gimbal would not rotate further than 41.5, precluding simultaneous collection of science data, and pointing of the High Gain Antenna at the Earth during certain phases of the mission.

Science data collection resumed on April 28, 1999. From April 28 to May 5, 1999, the spacecraft pointed the instruments at nadir to collect science data 18 to 20 hours per day, and then pointed the HGA antenna at Earth for four to six hours per day to transmit these data to the Earth. By May 5, 1999 the Earth-Mars-MGS relative geometry had changed sufficiently to allow resumption of normal mapping operations at least through February 2000.

Following, HGA deployment, a degradation in the quality of the tracking data was observed[7], including an increase in the data noise, as well as a 19 second periodicity in the tracking data. Thus, none of the tracking data after HGA deployment were used to help refine the models of the Mars geopotential.

Table 1: MGS Orbit Characteristics during MOLA Data Acquisition
Mission	Dates	Orbital	Periapsis	Periapsis
Phase		Period (hr)	Ht. (km)	Latitude (deg)
Insertion	September 12-16, 1997	44.993	263	31 N
Hiatus	Oct. 13 - Nov. 7, 1997	35.43	170	35 N
SPO-1	March 27 - May 1, 1998	11.64	171	61 - 71 N
SPO-2	May 28 - July 30, 1998	11.64	171	82 - 86.3 N
				86.3 - 76 N
GCO &	February 4, 1999 -	1.97	377	90 S
Mapping