Simulations of lunar gravity field determination for Lunar Observer

The current plan for the Lunar Observer (LO) mission is to launch in the late 1990s and insert LO into a 100 km polar circular mapping orbit. However, prior to the mapping orbit, LO will be placed in a gravity calibration orbit (GCO) at a higher altitude to determine the gravity field of the moon. This paper examines the abilities of two GCO orbits (at 200 and 500 km altitudes) to recover a high degree and order gravity truth model that includes spherical harmonics and mascons by estimating different degree and order gravity fields with spherical harmonics only. This is achieved by comparing radial accelerations from the true and estimated models at the mapping altitude and by comparing trajectory propagations. For the gravity fields estimated (up to 30th degree and order), the 500 km GCO was just as successful as the 200 km GCO in determining the gravity field.