Investigating the Variability of Earth Gravity Field’s J2 Spherical Harmonic Coefficient Using Satellite Laser Ranging Data

Spatial-temporal variations in the Earth’s gravity field (expressed as a set of spherical or ellipsoidal harmonic coefficients: the geo-potential model) are caused by mass redistribution within the Earth system. Accurate determination of the Earth’s gravity field is essential for a variety of geophysical applications such as oceanography, hydrology, geodesy, solid Earth science as well as being essential for precise orbit determination. As a result, improved knowledge of the variations of the Earth’s gravity field could yield improved understanding of the Earth’s system dynamics and associated long term climate change. The main objective of the current analysis is to investigate the variability of coefficients computed using a new Satellite Laser Ranging (SLR) program being developed at Hartebeesthoek Radio Astronomy Observatory (HartRAO). In the analysis program, Earth tide, ocean tide, pole tide and atmospheric loading models are included in SLR station position adjustment in order to improve calculated range and therefore minimise the observed minus computed (O-C) residuals. The time series for lower degree and order harmonics has been recovered utilising a priori J2 coefficients based on the GRACE gravity model GGM03C. We present results of J2 coefficients computed from about 20 SLR stations tracking LAGEOS-1 and LAGEOS-2. In addition we estimate a known secular decrease in J2. The ability to estimate gravity coefficients will enable further investigations into seasonal annual variations and other longer term inter-decadal and decadal variations which can be linked to global mass redistributions.