We present the concept, some of the approaches used, and the capabilities of the technique referred to as GPS tomography. It is used for retrieval of the 3-dimensional distribution of the refractivity due to atmospheric water vapor. We discuss the presently used methods for retrieval of the primary observable in the GPS tomography, the slant path delay, as well as their shortcomings. Comparisons of GPS slant delays to independent data from a microwave radiometer are included. From a tomographic point of view we concentrate on the capabilities to retrieve the vertical structure of the wet refractivity. For this purpose we present and apply two methods for tomographic inversion. Both are based on the Kalman filtering technique, where the expected statistical behavior of the refractivity is utilized. The difference between the two is in the way the covariance matrix of the Kalman filter is constructed. We base our study on simulated and real data from the ground network of 8 GPS receivers operating in Göteborg, Sweden. The results demonstrate that at present the limitations of the GPS tomographic technique are errors in the retrieved wet slant delays and their poor geometric distribution.
Abstract. 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.
An analysis of microwave measurements of the complex dielectric constant of different mixtures of pharmaceutical materials using an open-ended coaxial probe is presented. Using the probe in combination with a network analyser, measurements in the frequency range of 1–19 GHz were conducted. Calibration measurements on conditioned samples were first acquired in a controlled laboratory environment, and then in situ measurements, taken in a small-scale high-shear mixer, were also obtained. The dominating material in the investigated mixtures was microcrystalline cellulose. By using the suggested microwave method, a novel possibility for in situ measurements of the initial moisture content of the powder mixture before and at the beginning of the water addition stage is demonstrated. In situ density-independent estimation of the moisture content having a relative error of below 10% for the moisture interval of 2–14% is demonstrated. The possibility of performing an adaptive control of the evolution of the mixing process by utilizing the microwave sensor information is also presented.
Very Long Baseline Interferometry (VLBI) is collocated with a permanent Global Positioning System (GPS) receiver and a Water Vapor Radiometer (WVR) at the Onsala Space Observatory in Sweden. Both space geodetic techniques are affected by the propagation delay of radio waves in the atmosphere, while the remote sensing technique is sensitive to the atmospheric emission close to the center of the 22 GHz water vapor emission line. We present a comparison of estimated equivalent zenith wet delay and linear horizontal delay gradients from an independent analysis of simultaneous VLBI, GPS, and WVR observations. Using different constraints for the variability of the delay and the horizontal gradient in the analysis of the VLBI and the GPS data did not have a large influence on the agreement with the WVR estimates. We found that the weighted rms differences between wet delay estimates from the geodetic techniques and the WVR estimates generally increased for an increased variability in the atmosphere.
We present an analysis of the correlation between the atmospheric slant wet delays in different directions using data from a microwave radiometer. The correlations between wet delays observed in different directions using different temporal constraints are compared to a model derived from theories of turbulence. The agreement between the model and the radiometer data was good, and the average squared difference between zenith mapped slant wet delays could be predicted with an accuracy of 0.01-0.04 cm 2. Our analysis shows a large short-term variability which variance has a seasonal dependence of about 26%, largely depending on the refractivity structure constant. We also demonstrate how the model can be used to characterize the stability of a microwave radiometer.
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