GPS ACCURACY AND ERRORS: NON-DIFFERENTIALHORIZONTAL SINGLE POINT POSITIONING USING A HIGH GRADE DUAL FREQUENCY RECEIVER

Since the US Department of Defense switched off "Selective Availability" the accuracy of the non-differential single point positioning (standalone) technique of collecting GPS measurements has improved significantly. The accuracy is amplified with the combination of dual frequency receivers to create almost ionosphere-free observables. Differential methods were used to institute control co-ordinates to establish the errors and accuracies inherent in the University of Plymouth's GPS basestation. hi observing raw non-differential data, a minimal period of 20 hours is sufficient in obtaining averaged co-ordinates with a known approximate accuracy of two metres. This is approximate at best because conditions vary in the long term. Position accuracy is a function, not only of the GPS receiver and antenna, but also a fiinction of the geometry and status of the satellites, the surroundings of the antenna and ionospheric conditions. The dilution of Precision and number of satellites is found to be predictable, based on the cyclic behaviour in regard to each satellite and the earth's orbits. Results indicate that error conforms to a rayleigh distribution where 40% of values fall between one and two metres with, 83.56% less than three metres and 63.39% less than two metres from the differentially computed co-ordinates. In general, the maximum error falls with an increase in the number of satellites with eight or more satellites producing an error of less than four metres. The best resuhs for positioning for any type of GPS measurements assuming similar ionospheric conditions are found to be within an 11-hour window that varies with sidereal time.Faculty of Scienc