Spacecraft Charging at High Altitudes - The Scatha Satellite Program

Satellites at synchronous altitude exhibit unexplained behavior in the operation of electronic circuits and in the performance of thermal controls. A possible explanation for this behavior is the fact that satellites can be charged to large negative voltages by energetic electrons in the space environment. A space measurements program entitled SCATHA has been formulated to determine the characteristics of the charging process, to measure the response of the satellite when charging occurs and to evaluate the utility of various corrective techniques which can minimize differential charging on the satellite. The instrumentation will measure charging levels and rates of twenty samples of satellite materials, some of which will be modified to prevent buildup of electrostatic charge. The electromagnetic interference background on the satellite will be measured for comparison with MIL STD 461, Electromagnetic Interference Characteristics Requirements for Equipment. Simultaneous measurements of electron and ion properties will be performed over an energy range of 1 to 10
⁷
electron volts to correlate with the electrical charging data. The purpose for these measurements is to specify design criteria and test procedures which can be used to assure that satellites will not be adversely affected when exposed to electrical charging by the space environment. The model by which satellites can be electrically charged will be described and the SCATHA instrumentation, which has been formulated on the basis of the charging model, will be discussed.