DGPS

DGPS, Differential GPS, raises GPS accuracy by broadcasting range corrections from a reference receiver at a precisely surveyed position. The reference station compares its known location with the GPS-derived range to each satellite and transmits the pseudo-range correction (PRC) and its rate of change. At sea the corrections come from IALA medium-frequency radiobeacons in the 283.5 to 325 kHz band, formatted to RTCM SC-104, received by a separate beacon receiver feeding the GPS set. Differential working removes the errors common to both stations, satellite clock, ephemeris, and most ionospheric and tropospheric delay, cutting horizontal error from 5 to 10 m down to roughly 1 to 3 m within a few hundred kilometers of the beacon.