VARIATIONS IN THE TELEMETRY OF AN OFFEQ SATELLITE'S SUN SENSORS

摘要

One of the Israeli Offeq satellites has two fine solar aspect sensors which are analog devices based on a silicon quadrant detector. They exhibited variations in the telemetry since the beginning of mission that were similar for the four quadrants of the same detector. The telemetry signals of sun sensor no.1 declined ～14% and the signals of sun sensor no.2 declined ～10% during the first day of mission. The signals of sun sensor no.1 gradually stabilized at ～60% variation (～40% of the expected signals) while those of sun sensor no.2 stabilized at ～40% variation (～60% of the expected signals). Such values were sufficient for the normal continuous operation of the sensors. The angular difference of the sun vectors, calculated from the data of the two sensors, was small but a slight increase was observed from 0.2°-0.3° during the first days of mission to 0.4°-0.6° later. The goal of our investigation was to find out the possible causes for this behavior and suggest how to prevent them in future satellites. It included preliminary screening of all the possible causes, experimental and theoretical work done in order to support or exclude the remaining causes, conclusions and recommendations. The preliminary screening identified the most probable reasons for the telemetry variations as either the direct interaction of the optical window of the sun sensor with the space environment or its contamination. Our extensive computations and experiments indicated the importance of the contamination mechanisms but excluded direct interaction with the space environment as a major cause for the behavior. The most probable contamination sources were either outgassing from internal sources of the satellite and/or outgassing from one of the solar panels before their deployment. Partial fixation of the contaminants could occur by the combined effects of the space environment, especially by atomic oxygen and UV radiation. The conclusions led to recommendations for future missions.