SUMMARY

We have improved orbit modelling for Mars Global Surveyor in the low-altitude and near-circular mapping orbit by developing a detailed nonconservative force model for the spacecraft and by developing improved models of the Mars geopotential. A ten plate macromodel was developed for MGS. Telemetered spacecraft and solar array quaternions were used define the spacecraft attitude and solar array orientation in the macromodel. Two months of tracking data in February 1999 and March 1999 were processed using this macromodel. These data formed the basis for the order of magnitude improvement in the modelling of the Mars geopotential. The improved gravity model, MGM0964C20, a model complete to 70x70 in spherical harmonics results in arc to arc overlap consistency of a few meters in total position, and under a meter in the radial direction. One of the best gravity models available prior to entry in the mapping orbit, MGM0890, which included the MGS data from Hiatus and SPO, had arc to arc overlap consistencies of 38 to 40 meters in the radial direction and 220 to 260 meters in total position. Orbits were calculated used the improved nonconservative and geopotential modelling, and these orbits were distributed to the MOLA science team.

The MOLA altimeter data were used validate the improvement in the Mars geopotential modelling. We have also demonstrated that the altimeter crossovers can be used in the near-circular mapping orbit to improve the orbit determination of MGS.

The single outstanding issue in orbit modelling for MGS remains properly correcting the Doppler data after deployment of the High Gain Antenna for the motion of the antenna over each orbit. Only a mean offset has been used so far in the data analysis for the MGS orbits following HGA deployment.

As stated in the MGS mission plan, tracking will consist of one 34-meter ground station pass per day, and one 70-meter ground station pass every third day[33]. Gaps of up to sixteen hours per day in the tracking will be possible, especially as the Deep Space Network faces increasing demands for its time from the Mars 1998 Orbiter and Lander, as well as other commitments. The incorporation of altimeter crossovers into the orbit determination for MGS will prove especially valuable, as the tracking becomes less dense. A second order benefit from the incorporation of crossovers, is that they will allow the adjustment of additional empirical parameters that will help to further remove residual errors in the orbit modelling.