Sure-Path utilises a very accurate version of differential GPS technology known as Real Time Kinematic GPS (RTK GPS).
Like all forms of differential GPS the system consists of a GPS "Base" receiver and a GPS "Rover" receiver. The Base receiver is in a known fixed position and generates error signals which are transmitted to the Rover which then utilises those error signals to calculate its position RELATIVE to the Base more accurately than standard GPS. The Rover receiver is used to measure buoy positions and to dynamically track the position of the boat.
Standard differential GPS (DGPS) can improve an innate GPS accuracy of +/- 5 metres down to about +/- 2 metres. In addition to DGPS, RTK GPS measures variations in the phase of the GPS carrier waves from the satellites due to perturbations in the ionosphere and troposphere, and can achieve a typical accuracy of +/- 1 centimetre if the Rover is not too far from the Base (within 10km or 6miles).
It is important to understand that the Rover's positional accuracy is RELATIVE to the Base. Since the Base's absolute position is only accurate to standard GPS accuracy (+/- 5 metres), then so is the Rover's absolute position. Fortunately, for waterskiing, absolute position is not important, since all buoy/boat positions are required to be accurate relative to certain buoys (entrance/exit gates for slalom, "start time" and "mid time" buoys for jump). Just as with traditional homologation methods using a Total Station, buoy positions are measured accurately RELATIVE to the position of the Base and therefore their positions relative to each other can be said to be accurate.