accurate positioning by low frequency (DGPS) (ALF)

To increase the usable accuracy of the GPS system, there are several systems that calculate correction signals and then transmit them terrestrially or via satellites.

All systems work with one or more reference stations whose coordinates are known exactly and calculate a correction signal by means of Differential Global Positioning System( DGPS), which is transmitted to the mobile GPS receivers.

The Accurate Positioning by Low Frequency (ALF) service, developed by the Federal Agency for Cartography and Geodesy in Frankfurt, Germany, is a wide- area service that transmits DGPS correction signals in the RTCM standard, Radio Technical Commission for Maritime Services (RTCM), at regular intervals from a fixed transmitter. The ALF service is transmitted in longwave and therefore has a transmission range of 600 km; position accuracy is one meter and below.