Continuous GPS measurements at Long Valley caldera have been made since early 1993 (for more information go to <http://quake.wr.usgs.gov/research/deformation/twocolor/lv_continuous_gps.html>) . Seventeen GPS receivers are now operating within and near the Long Valley. The data are processed using the GIPSY software with point positioning. Prior to plotting, "bad" measurements of position, those that deviate significantly relative to position measurements made within a 3 month window, are identified and removed. Measurement precisions (1 St Dev) for "absolute" position coordinates, i.e. relative to a global reference frame, are 3-4 mm (north), 5-6 mm (east), and 10-12 mm (vertical) using 24 hour solutions. Corresponding velocities uncertainties for a 12 months period are about 2 mm/yr in the horizontal components and 3-4 mm/yr in the vertical component.
References M. Battaglia, P. Segall, J. Murray, P. Cervelli, J. Langbein, 2002. The mechanics of unrest at Long Valley caldera, California: 1. Modeling the geometry of the source using GPS, leveling and 2-color EDM data. J Volcanol. Geotherm. Res. (in press).
Dixon, T.H., Mao, A., Bursik, M.I., Heflin, M.B., Langbein, J., Stein, R.S., and Webb, F.H., 1997, Continuous monitoring of surface deformation at Long Valley Caldera, California, with GPS: Journal of Geophysical Research, v. 102, no. B6, p. 12,017-12,034.
Marshall, G.A., Langbein, J., Stein, R.S., Lisowski, M., and Svarc, J., 1997, Inflation of Long Valley caldera, California, Basin and Range strain, and possible Mono Craters' dike opening from 1990 to 1994 GPS surveys: Geophysical Research Letters, v. 24, no. 9, p. 1003-1006.