The successful launch of NTS-2 (Navigation Technology Satellite) marked the beginning of a new era in navigation and timekeeping history. NTS-2 was launched on 23 June 1977 as the first NAVSTAR GPS Phase I satellite.

NTS-2 technological features included cesium frequency standards, a nickel-hydrogen battery, three-axis gravity-gradient stabilization with momentum-wheel unloading, control of the spacecraft orbit, laser retroreflectors, solar-cell experiments, radiation dosimeters, and a worldwide network for data acquisition. The satellite experiment has verified Einstein's relativistic clock shift.