James F. Zumberge

Precise point positioning for the efficient and robust analysis of GPS data from large networks

Networks of dozens to hundreds of permanently operating precision Global Positioning System (GPS) receivers are emerging at spatial scales that range from 100 to 103 km. To keep the computational burden associated with the analysis of such data economically feasible, one approach is to first determine precise GPS satellite positions and clock corrections from a globally distributed network of GPS receivers. Then, data from the local network are analyzed by estimating receiver-specific parameters with receiver-specific data; satellite parameters are held fixed at their values determined in the global solution. This “precise point positioning” allows analysis of data from hundreds to thousands of sites every day with 40-Mflop computers, with results comparable in quality to the simultaneous analysis of all data. The reference frames for the global and network solutions can be free of distortion imposed by erroneous fiducial constraints on any sites.

Shortening and thickening of metropolitan Los Angeles measured and inferred by using geodesy

Geodetic observations using the Global Positioning System (GPS) and other techniques record a high rate of north-south shortening in an east-southeast–trending, 5–40-km-wide belt in northern metropolitan Los Angeles, California. Downtown Los Angeles is observed to be converging upon the southern San Gabriel Mountains at 6 mm/yr. Aside from the elastic strain that will be released during earthquakes rupturing the San Andreas and San Jacinto faults, east-west lengthening across northern metropolitan Los Angeles is minor, <2.5 mm/yr. Therefore north-south shortening is accommodated mainly by vertical crustal thickening.

Probing Europa's Hidden Ocean From Tidal Effects on Orbital Dynamics

Recent observations of Europa suggest that the Jovian satellite may have a liquid ocean underneath its icy surface. Geophysical models indicate that the tidal Love number k2 has a strong dependence on the presence or absence of an ocean. The k2 dependence on the ice shell thickness is also significant. Measurements of the static and tidal gravity fields through their dynamic effects on the trajectory of a low Europan orbiter can be essential in the detection of an ocean and inference of other internal structures. Covariance analyses have been carried out to assess accuracies using simulated Doppler tracking data. With 15 days of tracking from 2 Earth stations, the uncertainties for k2, mantle libration amplitude and the epoch radial position of the spacecraft are expected to be 0.0004, 2.8 arcsec and 5.7 m, respectively. These tight constraints will strongly contribute to ocean detection and ice thickness determination when combined with altimeter measurements.

The coseismic geodetic signature of the 1999 Hector Mine earthquake

The M = 7.1 Hector Mine earthquake ruptured the Lavic Lake fault near Twentynine Palms, CA at 09:46 UTC October 16, 1999. Because it occurred near the eastern edge of the Southern California Integrated GPS Network (SCIGN), a network of permanent, continuously recording GPS receivers for measuring the crustal deformation field around Los Angeles, CA, it was possible to determine the deformation associated with the earthquake with unprecedented speed and reliability. Thirty-four stations recorded displacements over the 3-sigma level. The displacements measured with GPS can be modeled by a fault 46.2±2.6 km long, 8.2±1.0 km wide, striking 330°, dipping 84° east, with 301±36 cm right lateral strike-slip, and 145±36 cm of east-up dip-slip, yielding a potency of 1.3 km³ and geodetic moment of 3.8 × 1026 dyne-cm. The trace and dip of the model fault is consistent with the observed ground rupture and seismic focal mechanisms.

Few millimeter precision for baselines in the California permanent GPS Geodetic Array

A microcomputer-based, closed-loop electronic temperature control system and method for an electrically heated cold weather garment having a plurality of independently heated zones. In one preferred embodiment the system comprises a potentiometer for selecting the desired temperature at which a plurality of areas of the garment are to be maintained, a thermistor for determining a reference temperature of the interior areas of the garment, and a plurality of conductive heating wires operable to heat each area of the garment independently. The system further includes a plurality of solid-state semiconductor power devices for controllably allowing current flow through each heating wire and, a plurality of current sensing circuits for sensing the current flow through each heating wire. The microcomputer receives information from the potentiometer, the thermistor and the current sensing circuits, and determines therefrom the level of heating required for each zone. The microcomputer then generates and applies a plurality of pulse width modulated output signals to the solid-state switches to thereby control the amount of heat produced by each heating wire, thus controlling the temperature of each zone.

Rate change observed at JPLM after the Northridge Earthquake

Geodetic time series determined with the Global Positioning System indicate that the geodetic rate of a permanent site in Pasadena, California (JPLM) changed significantly after the 17 January 1994 Northridge California earthquake. Subtracting the pre-quake rate and co-seismic offset leaves 30±4 mm of integrated eastward excess motion observed in the three years following the earthquake. North and vertical components show excess motion of −11 plusmn;3 mm and 25±11 mm respectively. Local surveys to three additional points near JPLM changed by no more than 6 mm E, 3 mm N, and 15 mm V during the two years after the earthquake, ruling out the possibility of a local effect at the JPLM monument. The direction and size of the post-seismic displacements at JPLM are not consistent with additional slip on the fault which ruptured. The most rapid accumulation of excess motion occurs immediately after the earthquake, suggesting a relationship between the two events.