Lubomir Gradinarsky

Climate monitoring using GPS

Abstract We present results on long-term trends of integrated precipitable water vapor (IPWV) over the Scandinavian region based on data from the Swedish permanent Global Positioning System (GPS) network, obtained during the period August 1993 to the end of 2000. We assess the magnitude of the effects on the estimated IPWV caused by antenna radome changes by comparisons with other independent techniques, such as microwave radiometry and radiosondes. The agreement between the techniques is at 1 mm level for the IPWV content and at 0.1 mm/yr for the estimated linear trend. Using the IPWV differences between the techniques, we assess the effects of radome changes to be in the interval 0–1.8 mm depending on the type of radome used. The estimated trends of IPWV over Scandinavia show a general increase of 0.1–0.2 mm/yr, and are more pronounced in the south–west region. We also estimate trends based on summer and winter periods. We find them to be larger for the winter periods compared to the summer in the southern parts and the opposite in the northern regions of Scandinavia.

Water vapor tomography using GPS phase observations: simulation results

Global positioning system (GPS) tropospheric tomography normally requires that the slant wet delays between the GPS satellites and the ground receivers are estimated with high accuracy, which may be difficult given the presence of a number of error sources. This paper presents an alternative approach, namely to estimate the three-dimensional structure of the atmospheric water vapor directly from raw GPS phase observations. The method is tested in a number of simulations, where the impact of network size, the possible horizontal and vertical resolutions, the observation noise, and the inclusion of additional global navigation satellite systems were studied. The simulation results indicate that the refractivity field can be obtained with an accuracy of ~20% or better up to around 4 km with a height resolution of 1 km provided that a sufficient number of receivers and satellites is available

In situ monitoring and control of moisture content in pharmaceutical powder processes using an open-ended coaxial probe

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.

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.

Wet path delay and delay gradients inferred from microwave radiometer, GPS and VLBI observations

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.

The atmospheric influence on the results from the Swedish GPS network

Abstract The SWEPOS network currently consists of 21 continuously operating GPS stations. The GPS receiver at the Onsala site is collocated with a microwave radiometer measuring the integrated amount of water vapour along the line of sight. We have studied the correlation between GPS estimates of position and variations in the atmosphere using data from the autumns of 1995 and 1996, and found only small correlations between the estimated positions and the atmospheric quantities studied. We are, however, aware of other systematic errors, e.g. caused by radomes above the GPS antennas which may presently dominate the error budget

MM5 derived ZWDs compared to observational results from VLBI, GPS and WVR

Abstract Modelled values of zenith wet delay (ZWD) from the non-hydrostatic numerical weather prediction (NWP) model MM5 are compared to estimated values retrieved from observations by geodetic very long baseline interferometry (VLBI), global positioning system (GPS) receivers, and water vapour radiometers (WVRs). In addition, sparse radiosonde (RS) data are used to augment the available data sets. The comparison is done for three stations of the European geodetic VLBI network for six observing sessions during the year 1999. The stations (Madrid, Onsala, and Wettzell) were primarily chosen to have the maximum number of collocated measuring techniques. In general, the time series for the different techniques show a good agreement. The correlation values between the techniques amount to 75–95%. The RMS differences of MM5 with respect to the other techniques obtain values of ±1.3–1.6 cm. The bias between MM5 and VLBI lies at about 1.0 cm, the bias between MM5 and GPS varies in the range of 0.0–0.6 cm and appears to be station dependent.

Sensing atmospheric structure: Tropospheric tomographic results of the small-scale GPS campaign at the Onsala Space Observatory

Tropospheric tomography using data from local networks of Global Positioning System (GPS) receivers is producing encouraging spatio-temporal representations of the wet refractivity field. In this work we present the results from a small-scale geodetic experiment that we carried out at the Onsala Space Observatory. Seven GPS receivers distributed within a radius of 3 km from the center, were deployed during 21 days in the summer 1998. The limited number of sites and their spatial configuration present a challenge for tropospheric tomography. Using novel GPS techniques to determine the vertical structure of the atmosphere, we observed, for one session, a strong horizontal water-vapor gradient with a leading edge at higher altitude than the trailing edge, entering from the north. The vertical structure obtained independently using tomographic techniques matched such situation. These results suggest tomography is a promising technique for the determination of the spatio-temporal structure of the atmosphere. We will present preliminary results of the tropospheric tomography, using simulations and experimental data, together with some comparisons with radiosonde data.

Correlations between slant wet delays measured by microwave radiometry

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/sup 2/. Our analysis shows a large short-term variability which variance has a seasonal dependence of about /spl plusmn/26%, largely depending on the refractivity structure constant C/sub n/. We also demonstrate how the model can be used to characterize the stability of a microwave radiometer.

Horizontal gradients in the wet path delay derived from four years of microwave radiometer data

We have analyzed four years of inferred wet path delay data from a microwave radiometer operating at 21.0 and 31.4 GHz. We have applied a four parameter gradient model to the wet delays, using different lengths of the time series for the gradient estimation. The mean gradient, averaged over 15 to 1440 minutes, varies between 0.9 and 0.4 mm and has a preferred direction towards the north-east. Increasing the averaging time causes the estimated gradient to decrease. The 15 minutes mean gradient is 1.3 mm for the summer months and 0.7 mm for the winter months. Structure function results are also presented.