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NAVSTAR 46 (USA 145)

GPS Navigation Satellite (Block IIR) · US Space Force

Navstar 46 (USA 145), launched in 2000, is a Block IIR GPS satellite carrying rubidium atomic clocks accurate to within three nanoseconds — a precision that matters enormously, since GPS positioning works by timing how long a radio signal takes to travel from satellite to receiver at the speed of light. A clock error of just one microsecond translates into a position error of 300 meters on the ground. Block IIR satellites also introduced onboard cross-linking, letting satellites in the constellation communicate with each other directly to detect and flag their own clock or orbit errors, dramatically improving the system's autonomous reliability.