Zhigui Kang

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.

Determination of CHAMP Accelerometer Calibration Parameters

One of the key science instruments for gravity science aboard CHAMP is a three-axis STAR accelerometer, which measures the non-gravitational accelerations acting on the spacecraft. These measurements are used in precise orbit determination and for gravity field estimation. Usually, the accelerometer output is not an absolute value of non-gravitational accelerations and has to be calibrated by applying a bias and a scale factor for each axis. Since the bias and scale factor parameters cannot be obtained from ground tests, they have to be determined using the flight data. This paper describes different approaches for determination of the CHAMP accelerometer calibration parameters: comparison of observed accelerations with accelerations computed using models, and determination of the parameters in orbit determination and gravity field recovery schemes. The impact of these calibration parameters on the gravity field recovery are also discussed.