Sorry to back-track to a long past thread, but read this in an RYA book on navigation:
Accuracy and Selective Availability The inherent accuracy of Coarse Acquisition (CA) code GPS is about 20 metres ... But it is owned and operated by the American Department of Defence, who do not want potential enemies to derive the full benefit of a system that has cost them over five billion dollars. Getting that five billion dollars out of their taxpayers, however, involved making a commitment to Congress that the system would be available to civilians. This posed a dilemma that was solved by a policy known as Selective Availability (SA). SA has nothing to do with availability: it involves the introduction of deliberate errors into the time and position information carried by the CA signal, producing position errors of about 100 metres ... Differential GPS All errors can be allowed for as long as you know about them, and SA is no exception. The snag is that SA is apparently random and rapidly changing, so calibration (as for logs) or correction tables (as for compasses) do not work. The solution is to use fixed reference stations ashore to monitor the GPS signals and broadcast correction messages that can be received and automatically applied by suitably equipped GPS receivers. An advantage of this system is that it also measures and corrects several of the smaller, naturally occurring errors that affect GPS, giving accuracies in the order of 5-10 metres ... This supplementary system is called differential GPS (DGPS) and the reference stations "differential beacons". ... Most countried, including the USA, transmit DGPS as a public service, free of charge, to anyone with the necessary equipment. At present the UK is an exception, with DGPS run as a commercial venture which endeavours to make a profit by encrypting its transmissions and charging a licence fee for the software required to interpret them. I hope this doesn't duplicate too much of what was discussed before, but thought that it might explain why people's back yards tended to "wander" to some extent! All the best, David Higgon
