Measures of Accuracy in the Navstar/GPS: 2drms Vs. CEP

Rudolph M. Kalafus, Gerald Y. Chin

Abstract:	The Federal Radionavigation Plan, among other documents, uses the accuracy measure known as "2drms" to indicate the accuracy of the various navigation systems.There are other measures in use as well, such as "Circular Error Probable" (CEP) and "Spherical Error Probable" (SEP}, which are preferred by the Department of Defense. There are situations in which each measure can overstate or understate the accuracy of a navigation system. The 2drms measure is simple to compute from dilution-of-precision parameters but is not associated with a fixed probability level. A useful measure is described which is similar to the CEP, but which is set at the 95 per cent probability level. This paper is intended to clarify the _circumstances under which the different measures are most appropriate. In addition, the accuracy achieved by GPS over the CONUS by each measure is presented. It is shown that the average ellipticity ratio is about 0.7 for the GPS over the US. It is also shown that the east-west accuracy of GPS is better than the north-south accuracy.
Published in:	Proceedings of the 1986 National Technical Meeting of The Institute of Navigation January 21 - 23, 1986 Long Beach, CA
Pages:	49 - 54
Cite this article:	Kalafus, Rudolph M., Chin, Gerald Y., "Measures of Accuracy in the Navstar/GPS: 2drms Vs. CEP," Proceedings of the 1986 National Technical Meeting of The Institute of Navigation, Long Beach, CA, January 1986, pp. 49-54.
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