Miran Kuhar

GPS-derived Geoid using Artificial Neural Network and Least Squares Collocation

Abstract The geoidal undulations are needed for determining the orthometric heights from the Global Positioning System GPS-derived ellipsoidal heights. There are several methods for geoidal undulation determination. The paper presents a method employing the Artificial Neural Network (ANN) approximation together with the Least Squares Collocation (LSC). The surface obtained by the ANN approximation is used as a trend surface in the least squares collocation. In numerical examples four surfaces were compared: the global geopotential model (EGM96), the European gravimetric quasigeoid 1997 (EGG97), the surface approximated with minimum curvature splines in tension algorithm and the ANN surface approximation. The effectiveness of the ANN surface approximation depends on the number of control points. If the number of well-distributed control points is sufficiently large, the results are better than those obtained by the minimum curvature algorithm and comparable to those obtained by the EGG97 model.

The new fundamental gravimetric network of Slovenia

The paper presents all stages of the development and processing of the fundamental gravimetric network of Slovenia, which consists of a zero order network, which has six absolute gravity stations, and twenty nine first order gravimetric stations. Descriptions are given of the design of the network, the geological assessment of the gravimetric stations, the gravity survey of the first order network, and the post-processing and adjustment of the gravimetric observations, which was performed in two stages. First the observations in the zero order network were adjusted as a free network, and then a standard adjustment of the first order network was performed. Finally, the adjusted gravity values at the stations were analysed against the Potsdam system, which was the basis of all previous gravimetric calculations in Slovenia. In the analyses an equation for the transformation of gravity values between the Potsdam system and the IGSN71 system (International Gravity Standardization Network 1971) has been derived.

Modelling hysteresis effect in scintrex CG-3M gravity readings

Significant hysteresis effects after transport in the upright position are found during measurement for the purpose of calibration of Scintrex CG-3M gravimeters serial numbers 4372 and 4373. The effects amount to 0.60 and 0.15 μms−2 during the first 10 minutes, and an additional 0.35 and 0.10 μms−2 during the next 10 minutes after transport, for gravimeters 4372 and 4373 respectively. Such significant and not completely homogenous effects do not completely cancel out when forming gravity differences. An iterative MATLAB algorithm has been developed that models hysteresis effects using exponential functions that account for shifts in time and eliminates the hysteresis effects from the measurements. The method facilitates determination of an exponential function of hysteresis from multiple observation series with different unknown offsets and significant noise that prevents the application of conventional methods. Hysteresis elimination from measurements for the purpose of calibration resulted in (mostly significant) increases in the precision of adjusted calibration constants for both gravimeters. It is shown that in order to obtain reasonable results, modelling the hysteresis effects should be based on approximately 30 minutes of readings. Finally, one can conclude that hysteresis effects surely do build up during transport, in contrary to statements by Scintrex. This article is protected by copyright. All rights reserved