James M. Johnson

Improved Mapping of Tropospheric Delays

Abstract The authors compare several methods to map the a priori tropospheric delay of global positioning system (GPS) signals from the zenith direction to lower elevations. This is commonly achieved with so-called mapping functions. Dry mapping functions are applied to the hydrostatic delay; wet mapping functions are used to map the zenith wet delay to lower elevation angles. The authors compared the following mapping techniques against raytraced delays computed for radiosonde profiles under the assumption of spherical symmetry: (a) the Niell mapping function; (b) mapping through the COSPAR International Reference Atmosphere with added water vapor climatology; (c) the same as b with added use of surface meteorological temperature, pressure, and humidity; and (d) use of the numerical reanalysis model of the National Centers for Environmental Prediction–National Center for Atmospheric Research. Based on comparisons with all available global radiosondes (∼1000 per day), for every fifth day of 1997 (73 days)...

The measurement of atmospheric water vapor: radiometer comparison and spatial variations

Two water vapor radiometer (WVR) experiments were conducted to evaluate whether such instruments are both suitable and necessary to correct for propagation effects that are induced by precipitable water vapor (PWV) on signals from the Global Positioning System (GPS) and Very Long Baseline Interferometry (VLBI). WVRs are suitable for these corrections if they provide wet path delays to better than 0.5 cm. They are needed if spatial variations of PWV result in complicated, direction-dependent propagation effects that are too complex to be parameterized in the GPS or VLBI geodetic solution. In the first experiment, the suitability of radiometers were addressed by comparing six WVRs at Stapleton International Airport in Denver, Colorado, for two weeks. The second experiment addressed the question whether radiometers are needed for the detection of inhomogeneities in the wet delay. Three JPL D-series radiometers were operated at three sites in Colorado approximately 50 km apart. The WVRs simultaneously sampled PWV at different azimuths and elevations in search of spatial variations of PWV. >

Calibration of antenna-radome and monument-multipath effect of GEONET—Part 2: Evaluation of the phase map by GEONET data

Phase maps for GSI/GEONET (Geographical Survey Institute/GPS Earth Observation NETwork) monuments and antennas obtained in Hatanaka et al. (2001) are evaluated by applying them to the analysis of GEONET. We used the same strategy as the GEONET routine analysis except for the antenna phase model. The coordinate solutions change by more than 10 cm in height when we apply the new phase maps. A scale change of up to 20 ppb is also observed for one of the sub-networks. The height bias is not constant but changing daily and seasonably, which implies that seasonal variations in the GPS solution are related to mis-modeling of the phase characteristics and that other environmental or geometric factors are coupled to the phase mis-modeling. Two more checks are done by comparing tropospheric delay estimates, and by conducting elevation angle cut off tests. Both tests show dramatic improvement when the new phase maps are applied, as compared to applying the standard antenna phase maps. It is concluded from this experiment with almost 1000 GEONET sites, that monument/antenna specific phase characteristic calibrations are essential for any application of GPS to achieve the highest accuracy for Earth science applications.

Pointed water vapor radiometer corrections for accurate global positioning system surveying

Delay of the Global Positioning System (GPS) signal due to atmospheric water vapor is a major source of error in GPS surveying. Improved vertical accuracy is impor- tant for sea level and polar isostasy measurements, geodesy, normal fault motion, subsidence, earthquake studies, air and ground-based gravimetry, ice dynamics, and volcanology. We conducted a GPS survey using water vapor radiometers (WVRs) pointed toward GPS satellites to correct for azi- muthal variations in water vapor. We report 2.6 mm vertical precision on a 50-km baseline for 19 solution days. Kalman filter or least-square corrections to the same data do not account for azimuthal distribution of water vapor and are degraded by 70%.