Abstract
The receiving properties of radio telescopes used in geodetic and astrometric very long baseline interferometry (VLBI) depend on the surface quality and stability of the main reflector. Deformations of the main reflector as well as changes in the sub-reflector position affect the geometrical ray path length significantly. The deformation pattern and its impact on the VLBI results of conventional radio telescopes have been studied by several research groups using holography, laser tracker, close-range photogrammetry and laser scanner methods. Signal path variations (SPV) of up to 1 cm were reported, which cause, when unaccounted for, systematic biases of the estimated vertical positions of the radio telescopes in the geodetic VLBI analysis and potentially even affect the estimated scale of derived global geodetic reference frames. As a result of the realization of the VLBI 2010 agenda, the geodetic VLBI network is currently extended by several new radio telescopes, which are of a more compact and stiffer design and are able to move faster than conventional radio telescopes. These new telescopes will form the backbone of the next generation geodetic VLBI system, often referred to as VGOS (VLBI Global Observing System). In this investigation, for the first time the deformation pattern of this new generation of radio telescopes for VGOS is studied. ONSA13NE, one of the Onsala twin telescopes at the Onsala Space Observatory, was observed in several elevation angles using close-range photogrammetry. In general, these methods require a crane for preparing the reflector as well as for the data collection. To reduce the observation time and the technical effort during the measurement process, an unmanned aircraft system (UAS) was used for the first time. Using this system, the measurement campaign per elevation angle took less than 30 min. The collected data were used to model the geometrical ray path and its variations. Depending on the distance from the optical axis, the ray path length varies in a range of about pm ,1,hbox {mm}. To combine the ray path variations, an illumination function was introduced as weighting function. The resulting total SPV is about - ,0.5 mm. A simple elevation-dependent SPV model is presented that can easily be used and implemented in VLBI data analysis software packages to correct for gravitational deformation in VGOS radio telescopes. The uncertainty is almost 200,upmu hbox {m} (2sigma ) and is derived by Monte Carlo simulations applied to the entire analysis process.
Highlights
Radio telescopes are large technical facilities, that are, among other possible applications such as radio astronomy and astrophysics, deep space tracking or astrometry, used as space geodetic instruments for geodetic very long baseline interferometry (VLBI) observations
Photogrammetric measurement campaigns were carried out two times from elevation 0◦ up to 90◦ using a step-size of 10◦, as well as once at 34◦
The Signal path variations (SPV) approach depends on several input quantities, i.e., the derived point set of the bundle adjustment, the campaign temperature, and the elevation angle of the telescope
Summary
Radio telescopes are large technical facilities, that are, among other possible applications such as radio astronomy and astrophysics, deep space tracking or astrometry, used as space geodetic instruments for geodetic very long baseline interferometry (VLBI) observations. Deviations in the signal path affect the accuracy of VLBI and result in systematic biases of the estimated vertical positions of the radio telescopes and, potentially even affect the estimated scale of derived global geodetic reference frames such as the International Terrestrial Reference Frame (ITRF) (cf Sarti et al 2011). The International VLBI Service for Geodesy and Astrometry as well as the IERS Working Group on Site Survey and Co-location focus on measuring and modeling gravitational deformations for as many VLBI radio telescopes as possible (cf Bergstrand 2018; Gross and Herring 2018). The OTT are equipped with ring-focus paraboloids as the main reflectors and rotational-symmetric Gregorian sub-reflectors Often this kind of telescope type is referred to as ringfocus telescope.
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