CHAMP Long-Term Analysis of the Power and Thermal Subsystems

Abstract

CHAMP is a German geo-scientific satellite, operated by the German Space Operations Centre (DLR/GSOC). The satellite was launched on 15 July 2000 by a Russian Cosmos launcher and successfully delivered into a polar orbit of 450 km altitude. The main scientific goal of the mission is to obtain data for improved models of the Earth’s gravitational, magnetic and electric fields. The large amount of housekeeping data collected since the beginning of the mission allows some initial trend analyses of the on-board subsystem behaviour to be performed. It is important to distinguish between near-real-time and long-term analysis: • Near-real-time analysis: based on individual data dumps, is normally performed immediately after a ground contact. This is necessary to observe and evaluate the recent behaviour of the on-board subsystems and to detect failures or anomalies. • Long term analysis: This is necessary to detect and to predict degradation or ageing effects, to observe seasonal variations, drifts and other tendencies. It is a true offline task and does not aim to deal with immediate effects or anomalies. It allows a broader understanding of the inorbit behaviour to be gained, by observing long time spans. This kind of investigation can also be carried out in collaboration with the spacecraft manufacturers, who can benefit from the results in order to improve their models (e.g. thermal) and their future designs. This paper focuses on some trend analyses performed on CHAMP, in particular with respect to the power and thermal subsystems. The main aspects under study were: • Solar generator and battery performance at different sun-to-orbit angles. • Solar generator and battery efficiency degradation. • Temperature behaviour under various conditions. • Comparison of predicted vs. in-orbit behaviour. The experience gained from these analyses is also relevant for the GRACE mission (launch March 2002), which is also operated by GSOC. GRACE is CHAMP’s follow-on mission. The main scientific goal is to collect data for static and timevarying gravitational field models of the Earth of unprecedented accuracy. It consists of two twin satellites flying approximately 220 km apart on a low-Earth polar orbit similar to CHAMP’s. The spacecraft’s bus architecture is derived from CHAMP’s, and the power subsystems are almost identical. The paper also illustrates GSOC’s general concept of processing and archiving the housekeeping data for scientific satellites. Additional topics discussed include our experience in the generation of data products and the extraction of particular data points necessary for the long-term analyses and investigations carried out to date.