Sz. Rózsa

Towards a cm-geoid for Hungary: Recent efforts and results

Abstract Some steps were taken recently for Hungary aiming at the determination of geoid heights with a cm-accuracy. The present HGTUB98 gravimetric solution was based on terrestrial gravity data, height data and the EGM96 geopotential model, and was computed with the 1D Spherical FFT method. The gravity data were used in the area 45.5 ° ≤ϑ ≤ 49 °, 16 ° ≤ λ ≤ 23 °, the resolution of the grid was 30″ × 50″. The DTM used had a resolution of 1 km × 1 km. Our solution was evaluated using GPS/levelling data at 340 and 308 points respectively and at 138 vertical deflection points. We have compared our solution to the European EGG97 geoid solution, the gravimetric solution HGR97B developed by A. Kenyeres and the litospheric geoid solution by G. Papp. We have correlated our recent HGTUB98 solution to the Moho model of Central Europe. The comparison with GPS/levelling yielded respectively an accuracy of ±8.7 cm and ±4.4 cm (in terms of standard deviation) when a linear trend was removed. The comparison of the 1D planar FFT solution for the deflections of the vertical with 138 astrogeodetic deflections yielded an accuracy (in terms of standard deviation) of ±0.62″ and ±0.52″ for ξ and η, respectively.

Gravity field modelling by torsion balance data — a case study in Hungary

Two test areas with different characteristics of the terrain were selected in Hungary to model the gravity field. We have used point gravity gradients, their terrain effects and geopotential information to model geoid heights by numerical integration using kernel functions for specific gradient and curvature combinations which arise from the solution of the corresponding overdetermined geodetic boundary value problem. The truncation characteristics of these kernel functions were also taken into account. We have compared our results with the collocation solution as well

Uncertainty Considerations for the Comparison of Water Vapour Derived from Radiosondes and GNSS

The integrated water vapour (IWV) can be estimated from the tropospheric delays of GNSS signals. These estimations are usually validated by radiosonde observations. However, very limited information is available on the precision of the IWV determined by radiosondes. In this paper the methodology of the computation of IWV retrieved from radiosonde data is revised using the atmospheric profiles of pressure, relative humidity and temperature. The formulae to calculate the uncertainty of the estimated values are derived, where the correlation of the neighbouring atmospheric layers is also taken into account. The results show that the mean uncertainty of the IWV from radiosonde observations reaches the level of ±0.26 kg/m2 in case of the Vaisala RS-92 radiosondes in Central and Eastern Europe. However, it increases to ±0.7 to 0.8 kg/m2 in summertime.Since the zenith hydrostatic delay (ZHD) must be modeled accurately to estimate the IWV from GNSS observations, the Saastamoinen, the Hopfield and the Black tropospheric delay models have been validated with ZHD values computed from radiosonde observations in Central Europe. Moreover some local models have also been derived in order to minimize the bias in IWV caused by the existing tropospheric models. In order to take the effect of the masses above the topmost level of the radiosonde profile in consideration, the International Standard Atmosphere has been used. Since the radiosonde observations terminate at different altitudes and pressure levels, which certainly affect the accuracy of the computed ZHD values, the omission error has been modeled with a simple exponential function. The results showed that the best ZHD model fitted to the radiosonde observations with the bias and standard deviation of +0.8 and ±1.2 mm, respectively. This means that the GNSS derived IWV is biased by −0.1 kg/m2. This value is approximately 50 % lower than the bias caused by the Saastamoinen model.Finally, the calculation of the scale factor between the zenith wet delay (ZWD) and the IWV is studied. Various models exist to determine this scale factor. There are models that derive the scale factor as a direct function of the surface temperature, while other models use a linear regression model of the surface temperature to compute the mean temperature of water vapour in the troposphere and derive the scale parameter from physical equations. Radiosonde profiles were used to test the two approached in Central and Eastern Europe. The results showed that the prior model showed no bias, while the latter one showed a relative bias of approximately 0.3 %.

Near Real Time Estimation of Integrated Water Vapour from GNSS Observations in Hungary

Meteorological products derived from Global Navigation Satellite Systems (GNSS) observations have been routinely used for numerical weather prediction in several regions of the world. Hungary would like to join these activities exploiting meteorological usage of the dense GNSS CORS (Continuously Operating Reference Station) network operated by the Institute of Geodesy, Cartography and Remote Sensing for positioning applications.

Prediction of Vertical Gravity Gradients Using Gravity and Elevation Data

The vertical gravity gradients play an important role in the reduction of absolute gravity measurements and in the geoid determination, too. In order to enhance the precision of the gravity reductions and the geoid computations, the difference between the vertical gravity gradient of the real and the normal gravity fields should be taken into account.

Estimation of Integrated Water Vapour from GPS Observations Using Local Models in Hungary

This paper studies the estimation of integrated water vapour (IWV) from the zenith tropospheric delay (ZTD). In order to evaluate the technique, six mathematical models are compared using a stormy summer period and a calm and dry winter period. The mathematical models include locally derived models using more than 10,000 radiosonde observations. The GPS derived IWV values are compared to radiosonde observations and a linear regression prediction of IWV using surface observations, too. Moreover the computations were carried out during a heavy storm in the summer period, when the estimated IWV distribution is compared to radar observations, too.

The determination of the effect of topographic masses on the second derivatives of gravity potential using various methods

In the space gradiometry the determination of the effect of topographic masses is crucial for the validation and downward continuation of the gravitational signal. This paper focuses on the determination of the effect of topographic masses on the second derivatives of the potential.