GPS

GPS is the United States Global Positioning System, a constellation of about 31 medium-Earth-orbit satellites broadcasting timing signals a receiver uses to trilaterate position. Civilian standard positioning gives horizontal accuracy of roughly 3 to 5 m, tightened to sub-meter with SBAS or DGPS corrections. Marine receivers output position in the WGS84 datum, the basis of ECDIS and electronic charts. GPS is one of several GNSS alongside GLONASS, Galileo, and BeiDou; multi-constellation receivers combine them for redundancy and faster fixes.