Metrological provision of GNSS-leveling on working standards

2015;
: pp. 29-40
https://doi.org/10.23939/istcgcap2015.02.029
Received: November 05, 2015
1
Lviv polytechnic National University
2
Lviv Polytechnic National University
3
Lviv Polytechnic National University

Widespread use of GPS / GNSS technology sets the task of determining the normal (orthometric) heights of points. We know that it is necessary to determine the initial level surface – the surface of the geoid / quasigeoid. Determination of the quasigeoid surface can be performed by: 1) a gravimetric readout in the relevant region and building the regional geoid / quasigeoid; 2) a model of potential of gravitational field of the Earth; 3) an interpolation of quasigeoid’s heights on a surface given by leveling reference points, on which GNSS-leveling is carried. Most often the quasigeoid surface іs determined with a global gravity a model, for example EGM08, the standard deviation of this quasigeoid from the adopted system of heights, depending on the region, is from 3.5 to 25 cm. In the previous studies authors of the article show that the determination of the relative heights from processing of GNSS-observations is performed with the mean square error of about 5 mm at a distance of 3 km. The goal of the article is the calculation of the possible accuracy of determination of heights of the basic geodetic network points of the reference polygon both from geometric leveling with the Program of II class and from GNSS-definitions. The task of the practical metrology is in providing the compliance of the units of measurement of the device to the reference ones and in using the methods or methodologies of measuring performance that retain this unit of measurement. In case of using indirect measurements, which are satellite observations, it is necessary to provide the preservation of the standard unit of measurements during the measurements themselves, while processing the array of observations and during obtaining the resulting value. Then you can compare the distance or elevations between points, measured by ground-based methods and by processing satellite observations. It also necessary to consider that the terrestrial methods and satellite technologies of measurements are not equally influenced by the Earth’s gravity and the atmosphere. The accuracy of determination of points heights with the use of GNSS technology is influenced by several key factors, but the size of the errors mostly depends on the duration of observations. Therefore we will explore the change of the error of height determination caused by the change of the observation session duration. The accuracy of GNSS-leveling attestation as a method is influenced by such errors: definition of geodetic point’s height with processing of GNSS-observations; determination of  the point’s height by geometric leveling; restoration of the reference surface of the sea level to determine the quasigeoid heights. To analyze the value of the error of point’s height determination using GNSS technology, daily observations of three GNSS-vectors, whose length were respectively – 10, 14 and 20 km, were performed. Geodetic coordinates of points were determined by sessions of GNSS-observations, the duration of which was increased gradually by 1 hour. For accurate values of points heights have been taken those, which were determined by the daily sessions of observations. The errors in determining the heights of points during a day describe sinusoids, moreover, with up to 6 hours of the session of observations the oscillation amplitude can reach 30 mm, for sessions lasting up to 16 hours – 5–10 mm and for sessions lasting more than 19 hours – about 1 mm. Therefore, to recieve accurate definitions of geodetic points’ heights and, accordingly, heights of quasigeoid using GNSS-leveling, the observation session should last from 19 to 24 hours. Between the daily sessions of GNSS-observation, the error the height determination is about 5 mm and it depends not only on the distance between points. The heights of quasigeoid, as the heights of points from GNSS-leveling, are always defined as absolute, that is, from the accepted level surface. The position of the level surface (Baltic system of heights 1977) is determined with bigger error, than the geodetic heights H from the surface of the ellipsoid (WGS-84), what can be concluded from the research. This way, it is proposed to determine the relative heights or elevations over the reference point. The study of definition accuracy of  points’ heights from processing GNSS-observations prove that if daily sessions of observation are performed, the error of determination of geodetic point’s  height can reach up to 5 mm. Thus, the accuracy of GNSS-leveling, that is, of determination of the geoid’s height, can increase the error by 5 mm at a distance of 10 km.

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