Geodetic baseline measurements using the Global Positioning System (GPS) were acquired in November 1985 across the Gulf of California between Loreto (Bahia Concepcion) and Cabo San Lucas in Baja California and Mazatlan on the mainland of Mexico. This experiment had two main objectives. The first was initiation of a research effort, hopefully spanning a decade or more, aimed at determining spreading rates in the Gulf and, with the eventual expansion of the network to northern Baja California, understanding the complex pattern of strain distribution along this part of the Pacific-North American plate boundary. The second objective, the focus of this paper, was assessment of deployment strategies, analytical techniques, and the inherent precision of GPS-based geodetic measur ments in the region. We review several approaches for calibration of path delays due to variable tropospheric water vapor content, and investigate the effect of varying the number and geometry of fixed GPS tracking sites (the fiducial network).

Day-to-day repeatability using optimum tropospheric calibration methods and four fiducial stations in the U.S. is better than 2.0 cm in horizontal baseline components for baselines up to 650 km in length spanning the Gulf of California. In the absence of any slowly varying systematic error sources, this performance should permit detection of spreading across the southern Gulf of California in less than 5 years, and estimation of spreading rates to better than 0.3 cm/yr in less than 10 years, assuming short-term (geodetic) and long-term (geologic) rates are comparable. Similar precision should be achievable throughout the region as the network is expanded.

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