Transit 5B5 was a US Navy navigation satellite launched by a Thor Able Star rocket. It carried a nuclear power source. Transit, one of the first operational satellite systems, was also known as the Navy Navigation Satellite (NNS). The Transit spacecraft were developed for updating the inertial navigation systems on board US Navy Polaris submarines, and later for civilian use. Transit receivers used the known characteristics of the satellite's orbit, measured the Doppler shift of the satellite's radio signal, and thereby calculated the receivers position on the earth. As a single spacecraft travelled overhead, the user measured the Doppler shift over a 15 minute period by receiving timing marks and satellite orbital information on two separate frequencies, 149.99 and 399.97 MHz. These signals were corrected for ionospheric refraction and the information was then fed into the users navigation system. Individual Transit satellites operated for over 10 years. Technical break- throughs during the program included gravity gradient stabilization, the use of radio-isotope thermoelectic generators (RTG), and navigation satellite technologies later used in the GPS system. Transit was superseded by the Navstar global positioning system. The use of the satellites for navigation was discontinued at the end of 1996 but the satellites continued transmitting and became the Navy Ionospheric Monitoring System (NIMS).