GPS Monitoring Networks
The USGS occupied 31 stations in the Long Valley area
each summer from 1992 to 1994, and again in 1996, 1998 and 1999. The stations
were occupied for a period of 5 days during July or August. The USGS
recorded dual-frequency carrier-phase and C/A-code pseudorange data at
30s intervals for at least six hours at each site. Up to seven receivers
were deployed each day. At least one local station, usually CASA, was
observed every day to provide a tie between stations occupied on different
days. Relative station position were computed using the GIPSY software.
Integer-ambiguity, ionosphere-free, improved-orbit solution solution
were computed in the ITRF global reference frame by including simultaneous
data from several tracking stations within North America (Marshall
et al., 1997).
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 (see Dixon et al., 1997).
Rapid static GPS
In September 1997, the Cascade Volcano Observatory occupied
most of the existing geodetic benchmark exisisting in Long Valley caldera
with rapid static GPS.
GPS on leveling benchmarks
A crew of Stanford University surveyed 44 existing leveling
monuments in Long Valley caldera in July 1999, using dual frequency GPS
receivers. GPS and leveling were tied to a common reference frame
in the Long Valley area and the vertical deformation computed by differencing
GPS-based and leveled orthometric heights (Battaglia et al., 2003).
M. Battaglia, P. Segall, J. Murray, P. Cervelli, J.
Langbein, 2003. 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.