Assessing Reduced-Dynamic Parametrizations for GRAIL Orbit Determination and the Recovery of Independent Lunar Gravity Field Solutions

Abstract

Orbit determination of probes visiting Solar System bodies is currently the main source of our knowledge about their internal structure, inferred from the estimate of their gravity field and rotational state. Nongravitational forces acting on the spacecraft need to be accurately included in the dynamical modeling (either explicitly or in the form of empirical parameters) not to degrade the solution and its geophysical interpretation. In this study, we present our recovery of NASA GRAIL orbits and our lunar gravity field solutions up to degree and order 350. We propose a systematic approach to select an optimal parametrization with empirical accelerations and pseudo-stochastic pulses, by checking their impact against orbit overlaps or, in the case of GRAIL, the very precise inter-satellite link. We discuss how parametrization choices may differ depending on whether the goal is limited to orbit reconstruction or if it also includes the solution of gravity field coefficients. We validate our setup for planetary geodesy by iterating extended lunar gravity field solutions from pre-GRAIL gravity fields, and we discuss the impact of empirical parametrization on the interpretation of gravity solutions and of their error bars.