Time-variable gravity signal in Greenland revealed by SWARM high-low Satellite-to-Satellite Tracking

Amid a termination of the Gravity Recovery and Climate Experiment ( GRACE) mission and before the launch of GRACE Follow-On (in 2017), There is an urgent need to have a new satellite for continuous monitoring the global time variable gravity field. SWARM mission, which has three satellites, will orbit Earth at an altitude of 300 similar to 500 km on near-polar and near-circular trajectories. This mission shares similarity to that of CHAllenging Minisatellite Payload (CHAMP) mission, and hence, capable of continuous monitoring of the global time variable gravity field. In this paper, we first analyze the error characteristics of the spherical harmonic coefficients up to degree 60 of the variable gravity field and investigate the effect of different Gaussian smoothing radii on the higher degree frequency error for SWARM simulation, CHAMP and GRACE data, followed by inversion of the global mass change from the time variable gravity field model of SWARM, CHAMP, and GRACE data. It shows that the error in the higher degree of SWARM is lower than that CHAMP with the inversion result better than that of CHAMP, while worse than that of GRACE; Second, we get the ice mass loss during January 2003 and December 2009 over the entire of Greenland from CHAMP is -50.2 +/- 2.0 Gt/a, whereas -41.2 +/- 1.6 Gt/a from GRACE. Their trends differ by 21.8% whereas the trend between SWARM simulation and 'True' model over the entire of Greenland differ by 19.2% only. Based on the above result, we conclude that SWARM hl-SST can be detected to the time-variable gravity signal in the 20% relative accuracy level and retrieve time-variable gravity information in the absence of the GRACE twin satellites, and before the launch of GRACE Follow-On.