Global positioning system measurements over a strain monitoring network in the eastern two-thirds of the United States

A 45-station geodetic network was established in 1987 using global positioning system (GPS) technology to provide a means of monitoring strain and deformation in the central and eastern United States. Reduction of the initial epoch data showed that accuracies of 1 to 3 cm can be achieved for horizontal position, provided sufficient observations are available and there are four or more fiducial stations whose positions are known a priori, for example from Very Long Baseline Interferometry measurements. Accuracies obtained provide the ability to determine strain at the 1:10{sup 7} to 1:10{sup 8} level. Vertical positions are less accurate because of problems in modeling refraction and are determined at the 5 to 7 cm level. It is planned to remeasure this network at regular intervals in the coming years to place bounds on the strain occurring in the central and eastern United States. This network is also expected to serve as a reference network for more detailed monitoring networks in areas of high risk such as the New Madrid area. Future measurements are expected to provide more accurate results because of increased numbers of GPS satellites available and improved computation software. The improved software will also allow future upgrading of the accuracy of the 1987 observations. 3 figs., 5 tabs