Relative and Differential GPS Data Transfer and Error Analysis

Richard Phillips

Abstract:	In this study the distinction is made between relative GPS (RGPS) and differential GPS (DGPS). Both of these terms imply a transfer of information from a referencereceiver to an active receiver. Expressions for the errors associated with three different data transfer “formats” are derived. A numerical evaluation of the errors associated with one of these formats is made. It is often stated that the DGPS reference receiver must be placed at a known location. It can be a major effort to accurately survey such a location. In the final analysis it is never perfectly known and location errors play a role in the resulting DGPS performance. This is in contrast to the RGPS strategy which admits that surveying errors exist but insures that they are correlated between reference receiver and target. With the RGPS concept, it is only necessary to know the target location relative to the reference receiver. This data can be obtained in a number of ways such as from existing maps which are accurate in a relative sense but whose relationship to a global coordinate system is poorly known. SAR target location also yields coordinates for points which are well known relative to one another, but whose absolute location may not be so well known. Such relative data might also come from instruments which measure the target position with respect to the reference receiver. Handheld ranging/direction-finding devices are an example of such instruments. In the first two examples the primary measurements are of two locations. In the latter example the primary measurements are of a single location and the vector from that location to another. Two major sources of error are the difference in GPS bias errors between reference receiver and active receiver and the difference in mapping error between reference receiver and target. Although the relative GPS information can be transferred by one of three methods, it will be shown that there are first order differences which depend on receiver separation distance. The proper transfer format in any situation is a function of this error as well as logistical considerations. It will be seen that it is possible to do relative GPS target location using pseudo-range bias transfer to a active receiverwhich can see only a ,s.&@ of the satellites in view at the reference receiver. This fact makes the logistics of relative targeting easier than if the target coordinates were to be computed and transmitted. Rapid location of a relocatable target provides a good example. An unattended referencestation/SAR transponder can be airdropped into place near the target area. It is immediately followed by a SAR reconnaisance aircraft which images and defines coordinates for both the reference station and targets. The reconnaisance aircraft in turn is closely followed by GPS guided weapons which receive target coordinates from the reconnaisance aircraft and pseudo-range bias corrections from the reference receiver. These bias corrections compensate for both pseudo-range errors and (absolute) SAR survey errors. It is not necessary to compute coordinates for each target based on the current reference receiver reading. Simply broadcasting the pseudo-range bias corrections suffices. Each weapon uses these bias corrections (or a subset of them) and the coordinates of its designated target determined once by the reconnaisance aircraft. The error on target incurred by using pseudo-range biases for relative GPS targeting was computed for several latitudes, two receiver separation distances and for both 4 and 8 satellite signals in lock at the active receiver. For all latitudes the horizontal error is about 0.5 m. at 500 km. separation and 1.1 m. at 1000 km separation.
Published in:	Proceedings of the 53rd Annual Meeting of The Institute of Navigation (1997) June 30 - 2, 1997 Albuquerque, NM
Pages:	735 - 741
Cite this article:	Phillips, Richard, "Relative and Differential GPS Data Transfer and Error Analysis," Proceedings of the 53rd Annual Meeting of The Institute of Navigation (1997), Albuquerque, NM, June 1997, pp. 735-741.
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