|
The primary objective of the TOPEX/POSIEDON satellite is to monitor the world's oceans for scientific study of ocean circulation leading to weather and climate prediction, coastal storm warning and maritime safety. TOPEX/POSIEDON was launched on August 10, 1992 from the Kourou Space Center in French Guyana on a nominal circular orbit with an altitude of 1336 Km with a 66 degree inclination. Selection of this orbit imposed challenging requirements on the on- board electronics. At this altitude, South Atlantic Anomaly (SAA) covers a large area. During many orbits the satellite may spend up to 40 minutes in the SAA region, all the time being bombarded by heavy protons and other charged particles. The on-board electronics were required to endure an estimated total dose radiation of 70 KRAD, with an RDM of 2, over the prime mission life of three years. In addition to the total dose radiation requirement, the spacecraft is required to perform within specifications in-spite of the heavy protons present at this orbit. TOPEX/POSIEDON is the first NASA satellite to carry two CCD based star trackers on a long duration mission. TOPEX/POSIEDON star trackers, known as advanced star tracker (ASTRA), were designed and built by Hughes Danbury Optical Systems (HDOS). These trackers have experienced single-event upsets and possible radiation induced radiation changes in their characteristics. One of the trackers has been in an in-operable state since being hit by a suspected SEU in November 1992. The second tracker has also had anomalies indicative of an SEU, but has managed to recover and is performing within the TOPEX specifications. JPL has been monitoring and evaluating the performance of these star trackers, during the last forty months. The data show change in certain tracker characteristics such as the magnitude of the detected stars, the background counts and the hot pixel data. This paper addresses the CCD tracker performance and the change in their characteristics in the high radiation environment.
|
