Control and Power Supply Problems of Instrumented Satellites

Abstract

Schemes for an attitude control system and for electric power supplies for an unmanned satellite of a few hundred pounds total weight are presented. The attitude of the satellite with respect to the earth's center is controlled within ±10 degrees by utilizing the shadowing effect of the earth on the isotropic cosmic radiation. A number of Geiger counters are arranged so that they sense the location of the shadow cone of the earth. Signals resulting from the counting rates of the counters control fly wheels that cause the satellite to rotate around its center of gravity. Three sources for electric power are described, each of which delivers an average of about 100 watts. The first converts the sun's radiating energy with a silicon junction photoelectric generator. A sun-seeking device keeps the generator oriented toward the sun during daytime. In the second system, the sun's radiation is directed toward a pile of thermocouples made of ZnSb and constantan. Thermocouples have been built of these materials which convert solar energy into electric energy with an efficiency of 5.6%. The third method uses a radioactive, isotope, strontium 90, and its daughter product, yttrium 90, as heating element for a pile of thermocouples. The half life of strontium 90 is 20 years. Each of these three sources has a specific power production of the order of 0.4 to 0.7 watts per pound of weight. The attitude control system and the methods of power supply described are applicable also to larger satellites.