Utilization of Mobile VLBI for Geodetic Measurements

Three mobile VLBI systems have been fabricated under the direction of the Jet Propulsion Laboratory of the California Institute of Technology for the NASA Crustal Dynamics Project. These include the 9-m-diameter MV-1 telescope, the 3.6-m MV-2 telescope, and the 5-m-diameter MV-3 telescope. Since 1980, mobile systems have been operated in conjunction with several fixed-base stations in the Western United States as part of a geodetic survey program determining relative motions and regional strain fields near the tectonic plate boundaries in California and Alaska. In this paper, we present a description of the three mobile systems and the environment in which they must function. The inherent accuracy of moble VLBI measurements is assessed, based on a considcration of major sources of error. Some recent results are presented. which serve to illustrate various aspects of the error model and are of geodetic interest as they span the broad region surrounding the surface trace of the San Andreas Fault. These results indicate that baseline measurements utilizing the current mobile VLBI systems have attained an accuracy of 2 cm or better in the horizontal plane. Since average geological rates of horizontal motion are of order 5 cm/year across the plate boundary regions being studied, it is likely that crustal motions, will be detected within the next few years, provided they are presently occurring at the geological rates.