Global Navigation Satellite Systems or GNSS comprise all positioning systems using satellites including GPS, Glonass, Galileo and Beidou. These systems measure pseudo ranges between at least four satellites and the receiver on the ground. The distance measurement is essentially the same as for an EDM (speed times travel time) but with an additional uncertainty for the clock error at the receiver. However, where an EDM only measures a single distance, a GNSS measures a number at the same time (minimum of four, usually more than eight) and converts it to a 4D position. As such it is also a time measuring device.