An empirically derived lunar gravity field

Abstract

A new method has been devised to determine spherical harmonic coefficients of the lunar gravity field. This method uses a two-step data reduction and estimation process. The first step applies a weighted least-squares empirical orbit determination process to Doppler tracking data to estimate long-period Kepler elements and rates. In the second step, lunar gravity coefficients are determined using another weighted least-squares processor which fits the long-period Lagrange perturbation equations to the estimated Keplerian rates. This method has been applied to tracking data from the Lunar Orbiter missions. A gravity potential of degree and order four is presented and error sources discussed. Plots of lunar equipotential surfaces are shown. Gravity field results are applied to various physical properties of the Moon such as moments and products of inertia. This gravity field has been investigated using data from several Apollo missions. Solutions from these data, in all cases except that of Apollo 15, result in improved orbit predictions as compared to those using other fields. All solutions indicate that the field models are still imcomplete.