A high-resolution spherical harmonic degree 1500 lunar gravity field from the GRAIL mission

Abstract:

The highest resolution lunar gravity field to date has been generated by analyzing Gravity Recovery And Interior Laboratory (GRAIL) data from the Primary and Extended Missions. The Extended Mission Ka-band inter-spacecraft range-rate data have a precision near 0.05 micron/second with spacecraft altitudes as low as a few kilometers above the lunar surface. This new spherical harmonic degree 1500 field involves solving for nearly 2.3 million parameters in a least-square estimation procedure with 5 million observations. This results in an upper triangular 20 TB covariance matrix, computed using the NASA Pleiades Supercomputer. The first figure compares RMS unconstrained gravity field coefficients with uncertainties. The constrained global gravity spectrum (magenta) is determined to about n=900, whereas the Bouguer spectrum is accurate to about n=600. The correlation with gravity derived from constant density topography in the second figure shows that the high-order coefficients (n>700) are improved significantly over the previous degree 1200 field. Moreover, the Ka-band residual RMS is significantly improved for the low-altitude orbit solutions of the last month of the extended mission. The maximum local resolution of this new gravity field corresponds to a surface resolution of 3.6 km.