Equipment for Differential GPS

Abstract

It could be claimed that in one sense all navigational systems are differential by nature. The navigator is interested in where he or she is relative to other points of interest (e.g. the destination) and not particularly where he or she is in some arbitrary pattern of grid lines produced by a set of radio transmitters. Such a pattern has to be related to a more universal system that can be used for position definition irrespective of whether that radio system is in use or not. These “universal” systems, such as latitude and longitude, have themselves to be defined, usually by having a specified origin and reference framework. Thus, the radio system acts as a “position transponder”, relaying this universal origin and reference via its own grid. Were the radio grid to be absolutely accurate and unchanging, it could become totally “transparent” to the user, who would obtain position in the universal reference without even realizing it had been “transponded” through another system. Unfortunately, this is not so, because the radio system, whatever it is, has its own inaccuracies and although they may be quite well-known and calibrated, they cannot be ignored. There is, for satellite-based systems, the additional problem that although they may nominally produce positions in the same universal grid as is used at the Earth’s surface, their definition of this system, as seen from space, is necessarily different from that seen from the Earth’s surface, and sometimes the errors this produces are not well-calibrated. One way around these problems is to calibrate them accurately by providing a monitor station whose position is known precisely in the universal grid, that measures the position indicated by the radio system and relates one to the other in such a way that a navigator using the radio system anywhere in an area fairly close to the monitor can apply a correction transmitted to him by the monitor.