TRANSIT RTG The TRANSIT RTG was developed specifically as the primary power for the TRIAD navigational satellite, with auxiliary power to be provided by four solar-cell panels and one 6-Ah NiCd battery. The 13.6-kg TRANSIT RTG, shown in Fig. 14, was a modular RTG with a 12-sided converter surrounding the radioisotope heat source. The low hot side temperature (673 K) allowed operation of the PbTe thermoelectric elements in a vacuum [18]. To accomplish its mission of improving the accuracy of orbital determinations the TRIAD spacecraft was designed with three (‘triad') main units as shown in Fig. 15. These units are the power unit, the disturbance compensation system (DISCOS), and the main electronics unit. The TRANSIT RTG was the primary power source in the power unit. DISCOS, which was located at the satellite's center of mass, was designed to minimize the effects of aerodynamic drag forces and solar radiation pressure experienced in lower altitude orbits. DISCOS performed very successfully leading to the provision of excellent navigational capabilities to a wider variety of users. In addition, TRANSIT TRIAD provided very important measurements of the Earth's magnetic field. TRANSIT TRIAD operated for over 13 years—well beyond the design requirement of 5 years. Multihundred Watt (MHW) RTG The designs of the Lincoln Experimental Satellites 8 and 9 (LES 8/9) and NASA's
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