P. B. Nagel

Precise orbit determination for GRACE

Abstract The twin, co-orbiting GRACE (Gravity Recovery and Climate Experiment) satellites were launched in March 2002. The primary objective of the GRACE mission is to determine the Earths gravity field and its temporal variations with unprecedented accuracy. To satisfy this objective as well as other applications (e.g. atmospheric profiling by radio occultation), accurate orbits for GRACE are required. This paper describes several results related to the use of the data collected by the GRACE GPS receiver, high precision accelerometer observations and precise attitude data from star trackers in the application of the GRACE Precise Orbit Determination (POD). The orbit accuracy is assessed using a number of tests, which include analysis of GPS tracking observation residuals, Satellite Laser Ranging (SLR) residuals, K-Band Ranging (KBR) residuals and external orbit comparisons. The results show that an accuracy of better than 5 cm in each direction for GRACE orbits can be obtained. The relative accuracy of the two GRACE satellites is about 1 cm in position and 10 micrometers per second in velocity.

Evaluation of GRACE and ICESat Mass Change Estimates Over Antarctica

The goal of this study is to examine some of the many corrections and processing strategies that can have a significant influence on the ice mass change estimates computed from GRACE and ICESat mission data. These two missions, when combined, have the potential to generate new insights into the mass balance and geophysical processes of regions such as Antarctica, where such quantities are currently not well understood. Key to this combination is the identification of the major sources of uncertainty in the data processing. For the ICESat data, this includes an analysis into the calculation of the campaign biases, assumptions regarding the firn density, and a comparison between height rates derived from crossover and repeat track analysis. For the GRACE data, the focus will be on the impact of various GIA models and other a priori input values (i.e., C 20, geocenter motion, etc.). Comparisons with the latest data releases for both missions will be presented for the 4 year period spanning from October 2003 to October 2007. Recommendations for future work will also be discussed.