[G03-3] Time-varying gravity field methods and solutions

[G03-3-03] On computation of along-track potential and Line-of-Sight (LOS) acceleration difference using GRACE inter-satellite ranging data for time-variable gravity analysis

^○Khosro Ghobadi Far¹, Shin-Chan Han¹, Bryant Loomis², Scott Luthcke² (1.School of Engineering, University of Newcastle, Callaghan, New South Wales, Australia, 2.Geodesy and Geophysics Laboratory, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA)

In situ along-track potential difference and LOS acceleration difference can be computed from GRACE orbit and inter-satellite range-rate and range-acceleration data. We study the systematic errors in deriving along-track potential difference from inter-satellite range-rate and LOS acceleration difference from inter-satellite range-acceleration. Since GRACE can be considered as a long single-axis gradiometer, approximating gradiometric observable (i.e. LOS acceleration difference divided by inter-satellite range) by along-track gravitational gradient is also discussed. In all three cases (potential, gravity and gravity gradient), we formulate the errors explicitly in terms of cross-track and radial components of the inter-satellite velocity vectors. Based on a simulation study, we found that, except for low frequency part of the spectrum (0-1.85 mHz, or equivalently, spherical harmonic degrees <10), for which the radial component of inter-satellite velocity vector is the main source of the errors, the approximation error of range-rate and range-acceleration remains <8% of the signal. Considering the KBR ranging noise, it is feasible to accurately compute the along-track potential and LOS acceleration difference directly from band-pass filtered range-rate and range-acceleration, respectively, and employ them for analyses concerning the local time-variable gravity fields of the Earth. We present the results from real GRACE KBR data analysis and applications to local refinement of the GRACE monthly gravity solutions. We also discuss the increased sensitivity to gravity signals by virtue of altitudes lowered down to 350 km in 2016 from the nominal altitude of 500 km.