I have developed expressions for calculating the ratios (mapping functions) of the “line of sight” hydrostatic and wet atmospheric path delays to their corresponding zenith delays at radio wavelengths for elevation angles down to 3°. The coefficients of the continued fraction representation of the hydrostatic mapping function depend on the latitude and height above sea level of the observing site and on the day of the year; the dependence of the wet mapping function is only on the site latitude. By comparing with mapping functions calculated from radiosonde profiles for sites at latitudes between 43°S and 75°N, the hydrostatic mapping function is seen to be more accurate than, and of comparable precision to, mapping functions currently in use, which are parameterized in terms of local surface meteorology. When the new mapping functions are used in the analysis of geodetic very long baseline interferometry (VLBI) data, the estimated lengths of baselines up to 10,400 km long change by less than 5 mm as the minimum elevation of included data is reduced from 12° to 3°. The independence of the new mapping functions from surface meteorology, while having comparable accuracy and precision to those that require such input, makes them particularly valuable for those situations where surface meteorology data are not available.

Troposphere mapping functions are used in the analyses of Global Positioning System and Very Long Baseline Interferometry observations to map a priori zenith hydrostatic and wet delays to any elevation angle. Most analysts use the Niell Mapping Function (NMF) whose coefficients are determined from site coordinates and the day of year. Here we present the Global Mapping Function (GMF), based on data from the global ECMWF numerical weather model. The coefficients of the GMF were obtained from an expansion of the Vienna Mapping Function (VMF1) parameters into spherical harmonics on a global grid. Similar to NMF, the values of the coefficients require only the station coordinates and the day of year as input parameters. Compared to the 6‐hourly values of the VMF1 a slight degradation in short‐term precision occurs using the empirical GMF. However, the regional height biases and annual errors of NMF are significantly reduced with GMF.

The accuracy of the Global Positioning System (GPS) as an instrument for measuring the integrated water vapor content of the atmosphere has been evaluated by comparison with concurrent observations made over a 14-day period by radiosonde, microwave water vapor radiometer (WVR), and Very Long Baseline Interferometry (VLBI). The Vaisala RS-80 A-HUMICAP radiosondes required a correction to the relative humidity readings (provided by Vaisala) to account for packaging contamination; the WVR data required a correction in order to be consistent with the wet refractivity formulation of the VLBI, GPS, and radiosondes. The best agreement of zenith wet delay (ZWD) among the collocated WVR, radiosondes, VLBI, and GPS was for minimum elevations of the GPS measurements below 10Њ. After corrections were applied to the WVR and radiosonde measurements, WVR, GPS, and VLBI (with 5Њ minimum elevation angle cutoff ) agreed within ϳ6 mm of ZWD [1 mm of precipitable water vapor (PWV)] when the differences were averaged, while the radiosondes averaged ϳ6 mm of ZWD lower than the WVR. After the removal of biases between the techniques, the VLBI and GPS scales differ by less than 3%, while the WVR scale was ϳ5% higher and the radiosonde scale was ϳ5% lower. Estimates of zenith wet delay by GPS receivers equipped with Dorne-Margolin choke ring antennas were found to have a strong dependence on the minimum elevation angle of the data. Elevation angle dependent phase errors for the GPS antenna/mount combination can produce ZWD errors of greater than 30 mm over a few hour interval for typical GPS satellite coverage. The VLBI measurements of ZWD are independent of minimum elevation angle and, based on known error sources, appear to be the most accurate of the four techniques.

Analysis of Global Positioning System (GPS) data from two sites separated by a horizontal distance of only ∼2.2 m yielded phase residuals exhibiting a systematic elevation angle dependence. One of the two GPS antennas was mounted on an ∼1‐m‐high concrete pillar, and the other was mounted on a standard wooden tripod. We performed elevation angle cutoff tests with these data and established that the estimate of the vertical coordinate of site position was sensitive to the minimum elevation angle (elevation cutoff) of the data analyzed. For example, the estimate of the vertical coordinate of site position changed by 9.7±0.8 mm when the minimum elevation angle was increased from 10° to 25°. We performed simulations based on a simple (ray tracing) multipath model with a single horizontal reflector which demonstrated that the results from the elevation angle cutoff tests and the pattern of the residuals versus elevation angle could be qualitatively reproduced if the reflector were located 0.1–0.2 m beneath the antenna phase center. We therefore hypothesized that the elevation‐angle‐dependent error was caused by scattering from the horizontal surface of the pillar, located a distance of ∼0.2 m beneath the antenna phase center. We tested this hypothesis by placing microwave absorbing material between the antenna and the pillar in a number of configurations and by analyzing the changes in apparent position of the antenna. The results indicate that (1) the horizontal surface of the pillar is indeed the main scatterer, (2) both the concrete and the metal plate embedded in the pillar are significant sources of scattering, and (3) the scattering can be reduced greatly by the use of microwave absorbing materials. These results have significant implications for the accuracy of global GPS geodetic tracking networks which use pillar‐antenna configurations identical or similar to the one used for this study at the Westford WFRD GPS site.

We have used a ground‐based microwave radiometer, known as a water vapor radiometer, to investigate the local spatial and temporal variation of the wet propagation delay for a site on the west coast of Sweden. The data were obtained from a wide range of azimuths and from elevation angles greater than 23.6° (air mass 2.5). Visual inspection of the data suggested a simple “cosine azimuth” variation, implying that a first‐order gradient model was required. This model was adequate for short time spans up to approximately 15 min, but significant temporal variations in the gradient suggested to us that we include gradient rate terms. The resulting six‐parameter model has proven adequate (rms delay residual ∼1 mm) for up to 30 min of data. Assuming a simple exponential profile for the wet refractivity gradient, the estimated gradient parameters imply average surface wet‐refractivity horizontal gradients of order of 0.1–1 N km−1. These gradients are larger, by 1–2 orders of magnitude, than gradients determined by others by averaging over long (∼100‐km) distances. This result implies that for applications that are sensitive to local gradients, such as wet propagation‐delay models for radio‐interferometric geodetic studies, the use of meteorological data from widely spread stations may be inadequate. The gradient model presented here is inadequate for times longer than about 30 min, even if no gradients are present, because of the complicated stochastic like temporal behavior of the wet atmosphere. When gradients are present, they can change magnitude by ∼50% over 10–15 min. Nevertheless, our ability to fit the radiometer data implies that on timescales <30 min and for elevation angles >23.6°, the local structure of the wet atmosphere can be described with a simple model. (The model is not limited to this range of elevation angles in principle.) The estimated gradient and gradient rate vectors have preferred directions, which indicates a prevailing structure in the three‐dimensional temperature and humidity fields, possibly related to systematic behavior in large‐scale weather systems and/or the local air‐land‐sea interaction at this site.