DGPS MAX

1.1.2 GPS ServicesThe positioning accuracy offered by GPS varies depending upon thetype of service and equipment available. For security reasons, twoGPS services exist: the Standard Positioning Service (SPS) and thePrecise Positioning Service (PPS). The US Department of Defense(DoD) reserves the PPS for use by its personnel and authorizedpartners. The DoD provides the SPS free of charge, worldwide, to allcivilian users.In order to maintain a strategic advantage, the US DoD used toartificially degrade the performance of the SPS so that the positioningaccuracy was limited to 100 meters 95% of the time. This intentionaldegradation is called Selective Availability (SA). The effect of SA hasbeen turned to zero since mid-2000, however, it has not been officially‘turned off’.Currently, autonomous GPS is able to provide accuracy on the orderof 10 meters, depending on the sophistication of the GPS engine. Formany positioning and navigation applications, this level of accuracy isnot sufficient, and differential techniques must be employed.1.2 Differential GPSThe purpose of differential GPS (DGPS) is to remove the effects ofSA, atmospheric errors, timing errors, and satellite orbit errors, whileenhancing system integrity.1.2.1 How it WorksDGPS involves setting up a reference GPS receiver at a point ofknown coordinates. This receiver makes distance measurements, inreal-time, to each of the GPS satellites. The measured ranges includethe errors present in the system. The base station receiver calculateswhat the true range, without errors, knowing its coordinates and thoseof each satellite. The difference between the known and measuredrange for each satellite is the range error. This error is the amount thatDGPS MAX Reference Manual 2