A. E. Niell

Global mapping functions for the atmosphere delay at radio wavelengths

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.

Geodesy using the Global Positioning System: The effects of signal scattering on estimates of site position

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.

Geodesy using the Swedish permanent GPS network: Effects of signal scattering on estimates of relative site positions

This paper presents results from a study of elevation-angle-dependent systematic effects on estimates of relative site positions within the Swedish permanent Global Positioning System (GPS) network. Two months of data from 16 sites have been analyzed with three different elevation cutoff angles, namely, 10°, 15°, and 20°. We present offsets between these solutions and demonstrate that estimates of the vertical component of several baselines strongly depend on the minimum elevation angle (elevation cutoff angle) of the data analyzed. Offsets of 22.3 ± 1.6 mm in the vertical component are evident when the elevation cutoff angle is changed from 10° to 20°. We investigate these offsets and conclude that a significant part is due to differential phase errors caused by scattering from structures associated with the mounting of the antenna to the pillar and with the pillar itself. The horizontal components of baseline are less affected. We found, however, that the offsets in the horizontal components increase with baseline length. For the longest baselines (∼1500 km) offsets of more than 5 mm are evident in the north component when the elevation cutoff angle is changed from 10° to 20°. These offsets are most likely due to differential phase errors caused by nonuniform antenna phase patterns ; an effect that presumably increases with baseline length and which also might increase because of scattering from the pillars and the antenna mounts. We identify the scattering structure and reduce associated errors in the vertical component of baseline to a significant degree on one of the sites by using microwave-absorbing material. The results presented are of importance for those analyzing data from existing networks and for those who intend to establish permanent GPS geodetic networks.

Development of an antenna and multipath calibration system for Global Positioning System sites

Received 3 November 2003; revised 21 May 2004; accepted 17 June 2004; published 29 September 2004. [1] Site-dependent errors such as antenna phase-center variations, multipath, and scattering can have a significant effect on high-precision applications of the Global Positioning System (GPS). Determination of these errors has proven to be elusive since no method has been developed to measure these effects accurately in situ. We have designed and constructed a prototype Antenna and Multipath Calibration System (AMCS) to obtain such in situ corrections. The primary components of the AMCS are a steerable parabolic antenna, two GPS receivers, and a computer for control and data-logging functions. We obtain phase corrections for site-dependent errors by forming the difference between the carrier-beat phases from the GPS antenna to be calibrated and from the AMCS antenna, which is relatively free of such errors. Preliminary ‘‘sky maps’’ of the antenna phase and multipath contributions show root-mean-square (RMS) phase variations that are a factor of 10 or more greater than the AMCS system noise, which is � 0.5 mm. To explore the source of this ‘‘noise,’’ we acquired observations over small (few degrees) patches of the sky. From the analysis of these experiments we concluded that the source of the phase variations was antenna and multipath errors that vary by � 5m m amplitude over small changes in satellite direction. Thus, for example, differences of 1� in elevation angle can result in several millimeter variations in phase. Similarly, small variations in azimuth angle can also result in significant phase variations. We have also observed day-to-day millimeter-level changes in the calibration. We hypothesize that these phase variations are due to changes in multipath caused by changes in the local electromagnetic environment associated with, e.g., weather. INDEX TERMS: 0609 Electromagnetics: Antennas; 1243 Geodesy and Gravity: Space geodetic surveys; 1247 Geodesy and Gravity: Terrestrial reference systems; 1294 Geodesy and Gravity: Instruments and techniques; 6994 Radio Science: Instruments and techniques; KEYWORDS: GPS multipath calibration, antennas, geodesy

IVS High Accurate Products for the Maintenance of the Global Reference Frames as Contribution to GGOS

VLBI provides highly accurate and unique products for the realization and maintenance of the celestial and terrestrial reference frames, ICRF and ITRF, as well as for the Earth Orientation Parameters. In 2001 the products were reviewed with respect to obtaining highest accuracy and to improving the observing sessions in order to make best use of the resources which are made available by the IVS member institutions. Since 2002 improved observing sessions were coordinated by the IVS aiming at fast turn-around products for EOP and better products for TRF and CRF. The number of observations increased by more than 30% from 2002 to 2004. New products, e. g. the troposphere zenith path delay, are now being generated. Concerns about the aging technology, which has been used for the past three decades, and about radio interference problems that decrease the number of useable observations, led to the establishment of IVS Working Group 3. The working group was asked to examine current and future requirements for geodetic VLBI, including all components from antennas to analysis, and to create recommendations for a new generation of VLBI systems. The results are summarized in a vision paper, VLB12010 (Niell et al., 2005), which is recommended for coordination of new developments in VLBI and for plans to invest in new components by member institutions. This paper reviews IVS activities of recent years and gives a perspective about new developments for meeting future requirements, which are set up by GGOS.