The impact of the Atmospheric Parameters at Electrooptical Distance Measurements

Abstract

The electro-optical distance measurement (EDM) instruments are widely used not only in geodesy but also in almost all engineering disciplines and related projects. High accuracy length measurements are required especially for different engineering works in urban areas. The paper will present the sources of errors which originate from the atmospheric conditions present in the process of measuring lengths with electro-optical light wave EDM. First, the working principle based on phase difference measurements and the basic components of EDM are described. Atmospheric influence parameters, temperature, atmospheric pressure and air humidity, their impact on the EDM measurements and the formulae recommended by IAG Resolution 3 adopted at the XXIIth General Assembly for the calculating the atmospheric influences are presented. The atmospheric influence parameters were treated as well-understood effect from a metrological point of view in according with the GUM - Part 6: Developing and using measurement models. Sensors with an appropriate resolution for measure temperature, atmospheric pressure and air humidity are used in this research. The first velocity correction estimate is based in part on the a priori known wavelength of the oscillator of the electro-optical EDM instrument and in part on the measured atmospheric parameters. Depending on the frequency bandwidth, as well as, the values of the measured lengths, the group refractive index of air and the first velocity correction is calculated for two EDM instruments Wild DI 20 and Leica TS30. The values of errors in distance measurements both for the shorter distances up to 500 m and longer distances up to 2 km in the case that temperature, pressure and air humidity influence are not accurately represented in real conditions are given and discussed. It is shown that first velocity corrections increase with temperature for all measured distances for both types of EDM instruments. Based on the comparison of correction errors for Wild DI 20 and Leica TS 30 instruments, it was concluded that the correction errors in both cases of measurement at lower and higher temperatures than nominal are less for the Wild DI 20 instrument.