DGPS MAX

1.4 Factors Affecting Positioning AccuracyMany factors affect the positioning accuracy that a user may expectfrom a DGPS system. The most significant of these influencesinclude:! Proximity of the remote user to the reference station! Age of the received differential corrections! Atmospheric conditions at the beacon and remote user locations! Satellite geometry, often expressed as a Dilution of Precision (DOP)! Magnitude of multipath present at the remote station! Quality of the GPS receiver being used at both the reference andremote stations.The distance between a remote user and the reference station is oftenconsiderable when using 300 kHz DGPS radiobeacons. Broadcastranges may be as great as 450 km (280 miles) or more, dependingprimarily upon transmission power and surface conductivity.Consequently, some of the errors associated with GPS at the basestation differ somewhat from those at the remote user’s location. Thisspatial decorrelation of errors can result in a relative position offsetfrom the absolute coordinates of the remote receiver. This offset maybe as much as one meter for every 100 km (62 miles) between thebase station and remote receiver. The WAAS and OmniSTARservices are not as susceptible to this error, as it they use a wide-areacorrection format. However, when operating away from the referencestations in this network, accuracy may degrade, as correction datamay not model errors in your geographic area as thoroughly.The latency of differential corrections to a lesser extent affects theachievable positioning accuracy at the remote receiver since themagnitude of SA was turned to zero in year 2000. Latency is afunction of the following:! The time it takes the base station to calculate corrections! The data rate of the radio link! The time it takes the signal to reach the user! The time required for the remote differential receiver to demodulatethe signal and communicate it to the GPS receiver.DGPS MAX Reference Manual 4