In 1978, the US Department of Defense launched the first navigation satellite that could provide precision timekeeping through on-board atomic clocks and distance measurements, following up by eventually deploying a complete fleet of 24 satellites, which began full-scale operation in the 1990s.
1978 年美國國防部發射第一枚可以測時、測距的導航衛星，並且陸續建置完成 24 枚衛星，90 年代後開始運作。