The Mars Global Surveyor (MGS) spacecraft reached a low-altitude near-circular orbit on February 4, 1999, after the termination of the second phase of aerobraking[1,2]. The spacecraft carries the Mars Orbiter Laser Altimeter (MOLA) whose primary goal is to derive a global, geodetically referenced 0.2 x 0.2 topographic grid of Mars with a vertical accuracy of better than 30 meters[3]. During the interim science orbits in the Hiatus mission phase (October to November 1997) and the Science Phasing Orbits (March - April, and June - July 1998), 208 passes of altimeter data were collected by the MOLA instrument. Starting on March 1, 1999, MOLA obtained altimeter measurements from the low-altitude mapping orbit. Altimeter data will be collected throughout the mapping phase of the MGS mission, or for at least one Mars year (687 days). Whereas the interim orbits of Hiatus and SPO were highly eccentric, and altimeter data were returned only when the spacecraft was below an altitude of 785 km, the MGS mapping orbit is near circular with altitudes ranging from 370 to 430 km. In the mapping orbit, altimeter data are returned continuously at a rate of 10 Hz.
Since the MGS orbit serves as a reference from which the topography of the planet is deduced, the proper analysis of the altimeter data requires that the orbits be known as accurately as possible. In this paper, we will discuss the force modelling required to achieve the best possible orbits during the mapping phase of the MGS mission.