Orbit Determination System for Low Earth Orbit Satellites
- Paper number
IAC-07-C1.5.02
- Author
Dr. Haim Shyldkrot, Israel Aerospace Industries LTD, Israel
- Year
2007
- Abstract
The IAI/MBT Orbit determination system for Low Earth Orbit satellites is introduced. The concept includes both on-board and ground station systems. The on-board system is based on ROKAR GPS receiver, whereas, the ground segment orbit determination is mostly based on GPS samples, but there is a backup mode, which uses antenna measurements in case GPS signals are not available. The real-time navigation position and velocity accuracy provided by the ROKAR GPS receiver is about 5-10 meters and 2-3 cm/sec respectively. Since this velocity may be inadequate for geolocation implementations and for on-board orbit prediction, a real time orbital filter is implemented in the receiver. The orbital filter implements an Extended Kalman Filter (EKF) algorithm, which generates the refined estimates on the basis of the 3D fixed PVT solution supplied by the GPS receiver and the orbit dynamic equations. The orbital filter reduces the velocity error to 1 cm/sec and in the absence of sufficient visibility conditions (e.g. when the antenna is obscured) the GPS receiver uses the orbital filter to generate the extrapolation estimate of the orbit for aided navigation (i.e. faster reacquisition). The ROKAR GPS receiver provides L1 C/A code and carrier tracking on 12 channels with accuracies of 0.8 meters and 1 mm respectively. These measurements are downlinked and processed offline by Least Squares Fit (LSF) for precise orbit determination. By using the arithmetic mean of code and carrier measurements, the ionospheric path delays can be fully eliminated. This measurement called GRAPHIC(Group And Phase Ionospheric Calibration) [1], exhibits a noise level of about half the C/A code noise. However, an unknown bias has to be estimated as part of the orbit determination process. This bias is different for each observed GPS Satellite but constant between epochs during uninterrupted carrier-phase tracking. The additional use of orbit knowledge from the equations of motion may substantially improve the orbit determination accuracy. The dynamic model includes the EGM96S of degree and order 70 as the Earth gravity model, CIRA72 as the atmospheric drag density model, and takes into consideration the Moon and Sun gravity. To compensate for any deficiencies in the employed dynamical model, the reduced dynamic method with piecewise constant empirical accelerations in the radial,tangential, and normal directions is used [2]. The amplitude of these acceleration components is estimated as part of the orbit determination process. The combination of GRAPHIC measurement with the dynamic model has been shown to improve the precision of the position and velocity knowledge up to a 3D accuracy of 0.2 meters and 0.1 mm/sec respectively, which are appropriate for most Satellite applications. [1] T.P. Yunck, Orbit determination, in: B.W. Parkinson, J.J. Spilker (Eds.), Global Positioning System: Theory and Applications, AIAA Publications, Washington, DC, 1996. [2] Montenbruck O, van Helleputte T, Kroes R, Gill E (2005) Reduced dynamic orbit determination using GPS code and carrier measurements. Aerospace Sci. Technol. 9(3): 261-271
- Abstract document