The evolution of the frequency standards on the GPS program started with the Block I concept validation program, beginning with a proposal program in 1973 and followed by the (GPS 1 through 8) prototype space vehicle contract in 1974. The full-scale developmental (GPS 9 through 11) models contracted In 1978 provided both navigation and nuclear detection capability. The production qual vehicle (GPS 12) was contracted in 1980, and the production vehicles (GPS 13 through 40) were contracted in 1982. During the proposal phase of this program, the on-board frequency Standards were considered the most critical item within the GPS navigational system for achieving user position accuracy. Therefore, a considerable amount of effort was devoted to the frequency standards. The initial GPS vehicles (GPS 1 through 3) incorporated three rubidium frequency standards, each with a backup mode, to minimize the risk to the GPS program on this critical item. This was achieved by operating a high performance VCXO without its rubidium reference. This design concept resulted in the redundancy potential of six frequency standards per vehicle. Later space vehicles, starting with GPS 4, included an additional cesium frequency standard, also with a backup VCXO mode. This extended redundancy was deemed necessary in lieu of the more conventional dual redundancy because if the rubidium loop failed, then the VCXO could be utilized as a backup device that would maintain frequency stability for a specific period of time, as required, to maintain navigational accuracy over the test area. The actual on-orbit GPS frequency standard operating history shown in Figure 1 illustrates the results of this hardware implementation.