Initial position is provided in cellular networks by using a database of cell IDs linked to positions. Each cell tower has a cell ID, which is used by the cellular network to facilitate handoffs as the mobile phone moves from one tower to the next. An A-GPS system makes use of a database that links cell ID to the location of the cell tower. While every cell tower will broadcast a cell ID, it is not an inherent property of cellular networks that the location of the towers is available. For an A-GPS system to be implemented across a network, a database of locations has to be created and maintained. Sometimes this is done by the network operator, but it is also done by third parties. An A-GPS reference station comprises at least a GPS receiver that receives and decodes the broadcast satellite navigation data ( Rear View Mirror Camera ) and a computer that converts the data into industry-standard formats.
Commercial A-GNSS reference stations existed that comprised combined GPS+GLONASS+SBAS receivers, the operational GNSS systems available at that time. In the future, we expect to see A-GNSS reference stations that have one or several receivers to collect the data from all operational GNSS satellites, including Galileo, IRNSS, Compass, and QZSS. Commercial A-GNSS reference stations are usually situated in cities where there is good communication infrastructure to the networks that they serve. The reference stations are usually deployed so that their antennas have a clear view of the sky. Before May 2000, the GPS signals had deliberate errors on them. This was known as selective availability (SA), and led to position errors of up to 100m. At this stage, it was standard industry practice that any A-GPS reference station would also serve as a differential GPS (GPS Rear View Mirror ) reference station and thus would be located in the same region as the A-GPS receivers that it supported.
However, on May 1, 2000, SA was stopped by a presidential order from President Bill Clinton. This may be the single most significant moment in the history of GPS, since the launch of the first satellite in 1978. Without SA, the broadcast ephemeris provides the satellite orbit and clock data to accuracies of the order of 1m. (Remember, when we say clock accuracy to 1m, we mean the accuracy of the clock in units of time multiplied by the speed of light, so 1m of clock accuracy is approximately 3 ns.) One result of the removal of SA is that an entire industry of portable car-navigation systems has arisen. But the significance for A-GPS is that an A-GPS reference station does not also have to serve as a DGPS reference station. The idea of regional A-GPS reference stations went away and was replaced by the concept of worldwide reference networks. In May 2000, a global network of GPS reference stations was coordinated by the International GNSS Service, formerly the International GPS Service (GPS Phone Tracker ), and still exists today. This network comprises GPS and GPS+GLONASS reference stations for Earth-science research. For example, one of the participants in IGS is the Scripps Orbit and Permanent Array Center, with the role of supporting high-precision GPS measurements, particularly for the study of earthquake hazards and tectonic plate motion.