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 in some arbitrary pattern of gird lines produced by a set of radio transmitters. Such a pattern has 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 define, 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 "transponder" 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 related 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. This method is, of course, of general application and not restricted to any one system, but in the case of GPS it has become known as Differential GPS (DGPS). GPS itself is so well-known now that it is not proposed to describe it here - there are many references available if a description is needed. Superficially it may seem an easy task to set up such a system - all that is needed is monitor receiver and a method of transmission but, in practice, as usual, it is not. Not only does the actual design and implementation of the monitor need care, but data transmission has to be done within some very stringent requirements. It is the purpose of this paper to outline some of there and illustrate them by reference to measurements on representative equipment. A representative DGPS system block diagram is given in Fig 1.

Fig 1 (available in full paper)

Correctable Errors

Table 1 shows the variety of sources that can cause GPS errors. They contribute in two way

  • they affect either the geometry of the system itself or

Table 1 SPS error budget with ephemeris degraded by accuracy demal (available in full paper)

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