The U.S. Nuclear Regulatory Commission (NRC) and the National Geodetic Survey (NGS) of the National Oceanic and Atmospheric Administration (NOAA) have entered into an interagency agreement to determine strain in the crust of the eastern two thirds of the contiguous United States. A network of 44 Global Positioning System (GPS) stations is planned, and distance measurements are expected to be accurate within 1 part in 107.
The station spacing (see the distribution, Figure 1) should permit the establishment of relative movements between major tectonic units in the central and eastern United States. GPS measurements use radio frequency signals emitted by several satellites and precise timing to establish the position of a GPS receiving instrument on the ground. Various measurement procedures are possible, but for the strain network the number of cycles from a starting signal and the phase of the last cycle received will be measured for high accuracy. For even greater accuracy, clock effects will be eliminated by differencing phase measurements between satellites and between stations in an interferometric mode.