ANALYSIS OF THE EXPECTED LASER RANGING SYSTEM PERFORMANCE ON GRACE-C

Since its launch in 2002, the GRACE mission has provided time-variable gravity field solutions for nearly a decade. The monthly GRACE solutions clearly show the continental scale hydrological cycle, while the long-term time series reveal trends in deglaciation in Antarctica and Greenland, the post glacial rebound in Canada as well as the sea level rise. GRACE solutions are widely used in hydrology, oceanography, ice, atmosphere, solid earth and other related Earth science studies.

GRACE has already passed the designed life of 5 years and it may terminate due to technical reasons. Given the great importance and excellent achievements of GRACE, GRACE-C has been planned to continue the observation of the time-variable gravity signals. The GRACE-C mission will consist of 2 satellites in a pendulum constellation.The distance between the two satellites and its 1^st time derivative will be measured by Laser Ranging Instrument (LRI) system with a very high precision. Comparing to the K-band ranging system equipped on the GRACE mission, laser ranging system will allow the distance between satellites to be measured with about 100 times better accuracy. Besides the Laser Ranging System, GRACE-C will also benefit from the drag-compensation system. In comparison to the accelerometers implemented on the GRACE mission, such system may reduce proof mass acceleration noise since the noise associated with the acceleration measurement can be avoided. Additionally, the drag-free system enables the satellite to fly at a lower altitude, thus resulting in a better sensitivity to the mass transport and distribution in the Earth system as well as an improved spatial resolution.

Comparing to the current GRACE mission, the GRACE-C mission will greatly improve the quality of global gravity field recovery. On one hand, GRACE-C will benefit from a more isotropic measurement principle due to combination of the cross-track with the along-track information. On the other hand, advanced sensor techniques will lead to significant improvement in the measurement accuracy, taken the advancement of the geophysical modeling into consideration. In total an improvement of about two orders of magnitude can be expected, which will greatly benefit the research activities in Earth science related studies.