Atmospheric Propagation Delay Research Articles

GPS site testing at chajnantor in Chile

We present first results on the site coordinates and the atmospheric propagation delay parameters from the permanent Global Positioning System site established at Chajnantor, in the Chilean Andes, at 5000 m altitude. The site is designed to ensure a long-term monument stability. The primary reason behind the site establishment is the future construction of the Atacama Large Millimeter Array (ALMA), a radio astronomy facility, at the same location. There is an interest to observe the site coordinate stability as well as the total radio propagation path delay variations which is possible employing the GPS technique. The fact that the site is known to have mainly a dry delay component makes it also a unique site for GPS accuracy studies. We present first results on the estimated site coordinates and the zenith total and wet atmospheric delays. Structure function results of the delays are included as well as the impact of using different elevation cut off angles in the data processing. We also discuss the performance of the mapping functions used for the estimation of the zenith delay terms.

External atmospheric corrections in geodetic very-long-baseline interferometry

Three methods to correct for the atmospheric propagation delay in very-long-baseline interferometry (VLBI) measurements were investigated. In the analysis, the NASA RD (2) inferring the wet delay from water vapour radiometer (WVR) data, and (3) using independent estimates based on data from the global positioning system (GPS). Optimum elevation cutoff angles were 22∘ and 26∘ when using WVR and GPS data, respectively. The results were found to be similar in terms of reproducibility of the estimated baseline lengths. The shortest baselines tend to benefit from external measurements, whereas the lack of improvement in the longer baselines may be partly due to the large amount of data thrown away when removing observations at low elevation angles. Over a 2 week period of intensive measurements, the two methods using external data showed an overall improvement, for all baseline lengths, compared to the first method. This indicates that there are long-term systematic errors in the wet delay data estimated using WVR and GPS data.

Sensing atmospheric structure using small‐scale space geodetic networks

We describe two ways in which horizontal atmospheric structure affects GPS observations. For a single site, such structure results in azimuthal variations in the atmospheric propagation delay. For a network of sites, the structure will induce an intersite variability in the atmospheric propagation delays. The former effect is more sensitive to high‐ than low‐altitude atmospheric gradients above the site, whereas the latter is insensitive to the altitude of the gradient above the network. This difference in sensitivity can be utilized to probe the local vertical structure of the atmosphere. We demonstrate this technique using GPS data from a small network on the Swedish west coast. We infer for one observing session the presence of a strong horizontal gradient which varies with height.

Usage of Geosat Altimeter Data for Offshore Oil Exploration off Bombay, India

Geosat altimeter Exact Repeat Mission (ERM) data over a part of western offshore, India have been used to determine the prospecting geoid and the free‐air gravity anomaly. The sea surface height measured by satellite altimeter, when corrected for atmospheric propagation delays and other dynamic (time‐varying) oceanic variabilities, conforms to an equipotential surface known as the geoid. Geoidal anomalies contain information regarding mass distribution in the entire earth, including those due to bathymetric variations. The geoidal contribution due to the lithospheric zone (called prospecting geoid as it is obtained after removing the contributions for the deeper earth and the bathymetric variation) has been interpreted in terms of major geological structures significant to occurrences of petroleum. The prospecting geoidal anomalies can be used to compute the free‐air gravity anomaly which is useful for locating offshore hydrocarbon-bearing structures. Spectral analysis of the prospecting geoid and gravity anomaly field provide short, medium and long wavelength components which bring out density variations in the sedimentary layer of the earth's crust. One year of Geosat altimeter data have been processed over western offshore, India and a number of geological features e.g. Bombay high, Saurastra Platform, Ratnagiri basin etc. could be demarcated.

GPS FOR HEIGHT DETERMINATION

The Global Positioning System (GPS) has considerable potential for providing high accuracy height measurement over considerable distances. At long range it has higher precision than conventional levelling and, at all but the shortest ranges, it is much more efficient. However, GPS levelling has its problems, regarding for example geoid determination, atmospheric propagation delay, and ocean loading effects. This paper reviews the state of GPS levelling with specific reference to both engineering and large scale environmental applications.

GPS FOR HEIGHT DETERMINATION

The Global Positioning System (GPS) has considerable potential for providing high accuracy height measurement over considerable distances. At long range it has higher precision than conventional levelling and, at all but the shortest ranges, it is much more efficient. However, GPS levelling has its problems, regarding for example geoid determination, atmospheric propagation delay, and ocean loading effects. This paper reviews the state of GPS levelling with specific reference to both engineering and large scale environmental applications.

Geodesy by radio interferometry: Water vapor radiometry for estimation of the wet delay

An important source of error in very‐long‐baseline interferometry (VLBI) estimates of baseline length is unmodeled variations of the refractivity of the neutral atmosphere along the propagation path of the radio signals. We present and discuss the method of using data from a water vapor radiometer (WVR) to correct for the propagation delay caused by atmospheric water vapor, the major cause of these variations. Data from different WVRs are compared with estimated propagation delays obtained by Kalman filtering of the VLBI data themselves. The consequences of using either WVR data or Kalman filtering to correct for atmospheric propagation delay at the Onsala VLBI site are investigated by studying the repeatability of estimated baseline lengths from Onsala to several other sites. The lengths of the baselines range from 919 to 7941 km. The repeatability obtained for baseline length estimates shows that the methods of water vapor radiometry and Kalman filtering offer comparable accuracies when applied to VLBI observations obtained in the climate of the Swedish west coast. For the most frequently measured baseline in this study, the use of WVR data yielded a 13% smaller weighted‐root‐mean‐square (WRMS) scatter of the baseline length estimates compared to the use of a Kalman filter. It is also clear that the “best” minimum elevation angle for VLBI observations depends on the accuracy of the determinations of the total propagation delay to be used, since the error in this delay increases with increasing air mass. For use of WVR data along with accurate determinations of total surface pressure, the best minimum is about 20°; for use of a model for the wet delay based on the humidity and temperature at the ground, the best minimum is about 35°.

Effects of atmospheric modeling errors on determinations of baseline vectors from very long baseline interferometry

We have performed a series of special very long baseline interferometry (VLBI) experiments that involve a significant fraction of group delay measurements made at very low elevation angles. Twenty‐two such low‐elevation experiments, each ∼26 hours in duration, were performed over a period of ∼2.5 years using VLBI antennas at the Goldstone complex in California and at Westford, Massachusetts. The purpose of these experiments was to expose and study errors in the formula for the elevation angle dependence of the atmospheric propagation delay, called the “mapping function.” Errors in this formula are a principal source of error in estimates of vertical coordinate of site position and make a significant contribution to the error in estimates of baseline length for transcontinental and longer baselines. For the low‐elevation experiments, estimates of baseline length had a root‐mean‐square (RMS) residual about the mean value of 7 mm, or about 1.7 parts per billion for this 3900‐km baseline. This scatter is about 20% greater than that calculated by propagating the random measurement noise through a Kaiman filter, which includes the noise contributions of the atmospheres and hydrogen maser “clocks” as well. We used specialized statistical tests to determine the contribution of mapping function errors to the estimates of vertical coordinate of site position and found this contribution to be less than 19 mm RMS. This value is roughly consistent with the observed larger‐than‐predicted scatter of the estimates of baseline length. We further found that for the 1–2 month spacing of our experiments, the mapping function errors do not exhibit a coherent annual signature but appear to be random in the long term.

Refraction and propagation delays in space geodesy

Abstract Many measurements to satellites, whether of direct scientific interest or merely for tracking purposes, require a knowledge of the atmospheric propagation delay which affects the observation, The delay has to be precisely determined for such purposes as geodetic or navigation positioning using the TRANSIT system or GPS (Global Positioning system) and gravity-field determination by precise orbit determination using satellite laser ranging. In particular, the high accuracy of satellite altimetry can be severely degraded by propagation delays not only in the altimeter measurement itself but also in the tracking-system observations. This paper describes the effects of the ionosphere and troposphere on both optical and microwave observations, and discusses both the instrumental and mathematic modelling approaches to the determination of the propagation delay, giving examples of the solution adopted for geodetic, surveying and navigation satellite observations.