Traditionally DGPS was only received by a mobile GPS receiver through some form of radio link. The nature of this link varies according to the DGPS service being used and can include terrestrial radio beacons or communications satellites. The corrections themselves are usually transmitted in a format known as the RTCM SC-104 protocol. This is an internationally agreed standard that enables any mobile GPS receiver equipped with the appropriate box to receive, understand and apply the corrections.
The problems associated with terrestrial radio beacons are more pronounced due to the properties of the MF frequency. This is more susceptible to electromagnetic noise and weather conditions such as thunderstorms, atmospheric distortion, multipath, masking and the receiver simply being beyond the range of the radio transmissions. Because of the curvature of the Earth, these may typically only be received up to 200 km away.
Communication satellites are now providing a solution to this problem. Instead of the correction messages being sent from a beacon at ground level, they are uplinked to a satellite in geostationary orbit above the earth.
The satellite then re-transmits the corrections, which may be received by users anywhere within a vast area of the Earth's surface. Together, they cover the Earth’s entire surface with the exception of the north and south poles, which are masked by the curvature of the planet. These signals are relatively low powered and require a big antenna comparable to a satellite TV receiver.
This is not a problem for large, relatively slow moving users such as ships or offshore oil drilling rigs. They are, however, unsuitable to be used by GPS receivers in the agriculture, mapping, or survey/construction industries.
Since 1995, a new type of communications satellite has become available that transmits telephone, television and other services via powerful spot beams. These are focused on specific geographical areas and are now being used by services such as LandSTAR-DGPS and SkyFix to transmit differential corrections. Because of their power, the signals can be received by modern antennas.