A 5-year campaign is in progress in central and southern California to monitor tectonic motions using observations of the Global Positioning System (GPS) satellites. Evidence from geological and space-geodetic data indicates that significant horizontal deformations of order 10 mm/yr are occurring west of the San Andreas fault along the California margin. We demonstrate, using data from initial epoch measurements in January 1987, that the goal of measuring horizontal deformations with an accuracy of 5 mm/yr over the duration of the campaign should be readily achievable. The GPS network was designed to span a wide range of baseline lengths within California and across North America to aid in phase ambiguity resolution, an important factor in achieving highest-accuracy baseline determinations. We present an efficient algorithm for multisession network analysis of GPS data with simultaneous orbit determination and ambiguity resolution. Using this algorithm with the California data, we demonstrate the improvement obtained in station position and satellite orbit estimation when the phase ambiguities are resolved. We demonstrate several millimeters +1 part in 108 short-term repeatability over five single-session (daily) analyses for horizontal baseline components and length and several tens of millimeters for the vertical. Comparison with the independently determined very long baseline interferometry baseline between Mojave and Owens Valley suggests that the accuracy is at the same level as the repeatability. We show that simultaneous multisession analyses with ambiguity resolution improve the baseline and satellite orbit estimates compared to the single-session analyses. With the single-session analyses we were able to resolve all phase ambiguities on baselines within California. With the multisession analyses we were able to resolve the phase ambiguities for most of the network, over regional and continental scales. ¿ American Geophysical Union 1989