152 OEM4 Family Installation and Operation User Manual Rev 12Appendix C GPS Overviewhairs, the centre of the shaded area is the "location" of the mean estimate, and the radius of the shadedarea is a measure of the uncertainty contained in the estimate.Figure 60: Accuracy versus Precision 1C.3.1 Single-Point vs. Relative PositioningIn single-point positioning, coordinates of a GPS receiver at an unknown location are sought withrespect to the earth's reference frame by using the known positions of GPS satellites being tracked.The position solution generated by the receiver is initially developed in earth-centered coordinateswhich can subsequently be converted to any other coordinate system. With as few as four GPSsatellites in view, the absolute position of the receiver in three-dimensional space can be determined.Only one receiver is needed.In relative positioning, also known as differential positioning, the coordinates of a GPS receiver at anunknown point (the “rover” station) are sought with respect to a GPS receiver at a known point (the“base” station). The concept is illustrated in Figure 61, Example of Differential Positioning onPage 153. The relative-position accuracy of two receivers locked on the same satellites and not farremoved from each other - up to tens of kilometers - is extremely high. The largest error contributorsin single-point positioning are those associated with atmospheric-induced effects. These errors,however, are highly correlated for adjacent receivers and hence cancel out in relative measurements.Since the position of the base station can be determined to a high degree of accuracy usingconventional surveying techniques, any differences between its known position and the positioncomputed using GPS techniques can be attributed to various components of error as well as thereceiver’s clock bias. Once the estimated clock bias is removed, the remaining error on eachpseudorange can be determined. The base station sends information about each satellite to the roverstation, which in turn can determine its position much more exactly than would be possible otherwise.The advantage of relative positioning is that much greater precision (presently as low as 2 mm,depending on the method and environment) can be achieved than by single-point positioning. In orderfor the observations of the base station to be integrated with those of the rover station, relativepositioning requires either a data link between the two stations (if the positioning is to be achieved in1.Environment Canada, 1993, Guideline for the Application of GPS Positioning, p. 22.© Minister of Supply and Services CanadaHigh accuracy,high precisionHigh accuracy,low precisionLow accuracy,low precisionLow accuracy,high precision
