MODELING OF GPS SYSTEMATIC-ERRORS IN MONITORING AND CONTROL SURVEYS

The accuracy of Global Positioning System (GPS) relative positioning d epends on the geometric distribution of the observed satellites and on the quality of the observations. Different types of errors affect GPS relative positioning in different ways. Some of the errors may have s ystematic effects on the measured baselines, which may produce signifi cant scale errors and rotations. These effects, however, may change fr om one survey epoch to another due to a change in the error characteri stics and/or the sky distribution of the observed satellites. Therefor e, they cannot be completely canceled out in the computation of deform ations. In monitoring and engineering surveys of high precision, these effects must be modeled and eliminated. This paper discussed the effe cts and their modeling. As an example, a GPS ground-subsidence monitor ing network in Venezuelan oil fields was analyzed. Rotation of up to 6 ppm in a vertical plane was obtained between survey campaigns in 1990 and 1991. To significantly minimize these systematic effects, some re commendations are made for the design of monitoring schemes and contro l networks for large engineering projects.