Almost overnight GPS has revolutionized the way civil engineers do their surveying and setting out. ICE joint engineering survey board member M. Oliver explains the principal techniques involved. GPS—the Global Positioning System—operates by simultaneously measuring distances from a receiver (Fig. 1) to at least four US Government satellites. Using one receiver gives a positional accuracy of 2-300 m depending on the duration of site occupation. Since this is quite inadequate for civil engineering surveying purposes it is necessary to use two or more receivers at the same time in order to determine the vector between them. This is called differential GPS or DGPS. The two types of measuring method are classed as code measurement and phase comparison. This article focuses on DGPS using phase comparison receivers.
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