Phasor Measurement Units (PMUs) calculate voltage and current phasors based on digital sampling of alternating current (AC) waveforms and a precise time signal provided by a GPS clock. A PMU provides output data in a standard protocol at rates of 30 or more samples/second, communicating to remote locations. The time-stamped measurements allow collected synchrophasor data to be accurately compared to time synchronized conditions at distant locations. Devices with PMU capability are also considered PMUs.