Measuring the Deflection of the Vertical via Local Reference Point Surveying and Pointing Calibration of a VLBI Telescope: A Case Study at the Urumqi Station

Abstract

The deflection of the vertical (DOV), the angle at a given point on the Earth between the vertical and the direction of the normal to the reference ellipsoid through that point (Herrmann and Bucksch, 2014), is critical to different directions (Barzaghi et al., 2016) in geodesy, for example, the transformation between astronomical and geodetic results including coordinates and azimuth angles, the transformation between different height systems, the reduction of horizontal and vertical angles to an ellipsoid surface, geodetic net calculations and geoid detection. Some critical applications of the DOV and its variation, that is, plumb line variations (Li and Li, 2009;Li et al., 2001;Tanaka et al., 2001), probably are national defense, aerospace applications, and geophysics research. In particular, geophysics can reveal the migration of materials underground, which has a significant impact on embodied earthquake signal analysis and could contribute to earthquake monitoring. Both the International Astronomical Union and the International Association of Geodesy pay considerable attention to studying the DOV.The methods for determining the DOV include astro-and geodetic observations. For example, the observation equipment has been updated from traditional astronomical theodolite or zenith tubes to digital zenith telescopes in recent years (which have an absolute axis pointing error of 1  E 2ʺ and 0.2ʺ observing internal coincidence, respectively); other methods include early global navigation satellite system (GNSS) and leveling measurements (e.g., 0.7ʺ calculation accuracy could be obtained using the geoid in an accuracy level of several centimeters), which are available for small and linear terrain features (Ceylan, 2009), and gravitational methods (e.g., the mean accuracy of the DOV in the 2000 China gravitational field and quasi-geoid system ([CGGM] could reach up to 1.5ʺ [Sun et al., 2005]).

The European GeoMetre project: developing enhanced large-scale dimensional metrology for geodesy

“…The reference point is derived by modelling trajectories of mounted markers at the rotatable part of the telescope. Beside the reference point, additional important parameters are estimated, for instance, orientation parameters that are used to reduce the pointing error of telescopes (Zhang et al 2021), or the axis-offset which, if unconsidered, biases the global position (Combrinck and Merry 1997).…”

Operator-software impact in local tie networks

“…( 25) does not require predefined telescope orientations, and is suitable for an automated reference point determination during the regular station process (Ning et al 2015;Lösler et al 2016). Moreover, the model is not restricted to a specific telescope type and contains important additional parameters like the axis-offset e AO , which, if unconsidered, biases the global position (Combrinck and Merry 1997), or the zero-pointing direction κ 0 , which is used to reduce the pointing error of the telescope (Zhang et al 2021).…”

Sphero-Conical Modeling for the Estimation of Very Long Baseline Interferometry Invariant Point

“…Therefore, the local tie vector is represented with respect to the GNSS pillar (SEJN). The orientation of the local reference frame will be defined using the local geodetic coordinate system of the CORS (SEJN); thus, there might be an offset called the “deflection of the vertical” between the geodetic vertical direction and the local gravity vector [ 25 ]. However, the magnitude is very small at the project site (approximately 6 arcseconds in the absolute sense), and it should be absorbed into the Helmert transformation parameters while transforming the local tie vector into the global reference frame.…”