Simulation and graphical representation of the orbit and the imaging parameter of Earth observation satellites

Synthetic aperture radar (SAR) satellites are widely used for Earth observation applications as weather conditions and cloudiness do not affect them. However, in order to be usable, data provided by those satellites need to be processed. This processing requires determination of imaging parameters that are closely linked to the spacecraft position and velocity on its orbit. For example, the difference in velocity of the Earth's surface and the spacecraft produces Doppler shift in the received signal. This decay, called Doppler centroid, is one of the most important parameters required by spaceborne SAR signal processing. As an illustration, the Antarctic anomaly that occurred in ERS data processing induced severe azimuth ambiguities ("ghosting") in the whole image. Those parameters also vary in relation to the attitude angles of the platform. In order to estimate the Doppler centroid, determination of orbital parameters and their evolution has to be made. In this paper, the method to perform orbital osculating elements calculation and imaging parameters determination for SAR satellites, starting from state vectors or two-line elements files and attitude angles, will be introduced as well as the perturbation due to the Earth flatness, called the J2 perturbation. The near-circular orbit often used by Earth observation satellites makes the perturbation equations of the J2 parameter unstable, and the introduction of new orbital parameters is required in order to be able to determine the evolution of the orbital parameters. Simulations will be performed and the resulting osculating elements will be presented and explained for short-term and long-term variations for different kinds of orbits, such as GEO, SSO, LEO and Molnya-orbit. Full analysis of the imaging parameters will be made for the SAOCOM satellite with particular attention to the Doppler centroid phenomenon: its sensitivity with relation to the attitude angles and the way to cancel it by setting the yaw steering dynamically. (c) 2005 Elsevier Ltd. All rights reserved.