The US Department of Defense (DoD) turned off the Selective Availability (SA) facility of the Global Positioning System (GPS) with effect from 1 May 2000. The GPS is now much more accurate, because the SA was one of its largest sources of error. This paper investigates the current accuracy of the GPS after the removal of the SA; it also considers whether the GPS is ready for Intelligent Transport System (ITS) applications, and differential GPS (dGPS) techniques still need to be used. Significant sources of error other than the SA are: (1) ionospheric and tropospheric effects; (2) satellite clock drift; (3) signal multipath; and (4) receiver noise. The paper briefly describes an experiment, with only static tests, to assess the accuracy of the GPS without the SA. This was found to be within about 10-12m without using the dGPS techniques, and within only a few metres with them. In particular, the accuracy can be increased by several meters if higher-end GPS receiver units are used, but their cost is high so that their usage is not usually justified in general ITS applications. They are normally used in surveying applications, where sub-metre accuracy is usually desirable. There were almost no accuracy improvements when low-end GPS units were used with augmented dGPS receivers.
Samenvatting