Precise orbit determination for GRACE using accelerometer data
Introduction
Gravity recovery and climate experiment (GRACE) is a joint project between the National Aeronautics and Space Administration (NASA) and the Deutsches Zentrum für Luft- und Raumfahrt (DLR). The primary objective of the GRACE mission is to map, with unprecedented accuracy, the long- to medium-wavelength spherical harmonic coefficients of the Earth’s gravity field and to observe its temporal variations (Tapley and Reigber, 2002). To satisfy this objective as well as other applications (e.g., atmospheric profiling), accurate orbits for GRACE are required.
The twin GRACE satellites were launched on March 17, 2002 into near polar orbits with an initial altitude of about 500 km. For the Precise Orbit Determination (POD) and gravity field recovery, both GRACE satellites are equipped with the following key science instruments: a Black-Jack GPS onboard receiver, a SuperSTAR accelerometer, a star tracker, a K-Band Ranging (KBR) system and a laser retro reflector. The Black-Jack receiver is an advanced codeless, dual frequency flight GPS receiver developed by the Jet Propulsion Laboratory (JPL). The SuperSTAR (Super Space Tri-axis Accelerometer for Research missions) accelerometer, which is manufactured by ONERA, measures the non-gravitational accelerations due to surface forces acting on the spacecraft, such as atmospheric drag and solar radiation pressure. The star tracker measures the precise satellite attitude, which is needed to translate the accelerometer data from the instrument reference system to the inertial reference system. In addition, the data from the KBR system and laser retro reflector can be used for evaluation of GRACE POD results.
Since the launch of the GRACE satellites, data quality has been improved significantly. Particularly for the star camera and accelerometer data, the calibration parameters have been well determined and used in data processing. This paper describes the POD methodology for GRACE using the high accuracy GPS tracking and accelerometer data, along with the attitude data from the star trackers. The study was performed using the Center of Space Research (CSR) Multi-Satellite Orbit Determination Program MSODP, which is based on a dynamic orbit determination method utilizing the batch processing approach. The data used are GRACE level 1B products produced by the NASA JPL.
Because the GRACE accelerometer can measure non-gravitational accelerations very precisely, a new challenge has to be faced with using accelerometer data for POD when highly variable forces are acting on the satellites (such as during high solar activity). With this motivation, the precise orbit determination for GRACE using accelerometer data has been investigated. The orbit accuracy is evaluated by analyzing GPS tracking observation residuals, by confirmation of the orbit solution with independent SLR tracking, by computing KBR residuals and by external orbit comparison.
Section snippets
Dynamic orbit determination
For GRACE orbit determination, we have used a dynamic orbit determination method (Kang et al., 2002). Using this method, not only precise orbits can be determined, but also force models such as the Earth’s gravity field model can be determined and/or improved. However, the orbit accuracy depends on the quality of the force models used in the dynamic solution. Because the surface forces acting on the low-Earth satellites are currently not modeled precisely enough, the GRACE mission uses a
Accelerometer data processing
For the precise measurement of the non-gravitational accelerations, each GRACE satellite carries the SuperSTAR accelerometer. This three axis accelerometer is specified to measure the non-gravitational accelerations on GRACE satellites to an accuracy of approximately 1 × 10−10 m/s2 within the bandwidth of 2 × 10−4–0.1 Hz.
At CSR, the GRACE level 1B accelerometer data from JPL has been processed for POD. Because the accelerometer data have been preprocessed at JPL, the data need only be reformatted
POD test cases
First, GPS double-differenced (GPS DD) carrier phase measurements were formed using about 40 global IGS stations. Next, precise orbits for the GRACE satellites were determined using only GPS DD observations by fixing GPS satellite orbits to the IGS solution and solving for a set of drag coefficients (3-h sub-arc Cd) and one-cycle-per-revolution (1 cpr) transverse (T) and normal (N) empirical parameters to obtain the accelerometer initial values. The estimation of T and N empirical parameters is
Orbital fits and SLR residual
The orbital fits are evaluated by the difference between the observed and computed value of the GPS double-differenced carrier phase data. This difference, referred to as observation residual, allows evaluation of the quality of the force and observation models used in the orbit determination. The precision of the GPS DD RMS is about 1.0 cm. If the forces were modeled perfectly, the orbital fits would be at the level of the data precision.
The orbit fits are treated as internal tests and measure
Conclusion
The precise orbit determination for GRACE using accelerometer data has been successful. Because the GRACE satellites provide very high accuracy intersatellite range data, the orbit accuracy has been assessed in different frequency regimes using the KBR residuals. The main conclusions are:
- •
The advantage of using accelerometer data for GRACE POD is that the non-gravitational forces do not need to be modeled and more accurate orbits are obtained. The GRACE orbits generated using accelerometer data
Acknowledgments
The authors extend their appreciation to J.P.L. for providing their reduced dynamic GRACE orbits for comparison. This research was supported by NASA Contract NAS5-97213.
References (4)
- et al.
Atmospheric model based on satellite drag data
Annales de Geophysique
(1978) - et al.
Precise orbit determination for CHAMP using accelerometer data
Advances in the Astronautical Science
(2002)