On the consideration of frequency-dependent illumination functions in modelling signal path variations

Investigations on the deformation behaviour of the receiving unit of large radio telescopes used for Very Long Baseline Interferometry (VLBI) indicate several elevation-dependent deformation patterns. These include, for instance, elevation-dependent deformations of the main reflector dish, variations in the position of the sub-reflector or the main reflector, or tilts of the sub-reflector. The deformation of the receiving unit yields signal path variations and, if unconsidered, distorts the vertical station position and, hence, the scale of the obtained global geodetic reference frame. Signal path variations result from a weighted combination of the overlapping deformation patterns. The corresponding weights are obtained by the intrinsic illumination function of the radio telescope. The gain of the feed horn and, thus, the illumination of the aperture depends on the frequency. In contrast to legacy radio telescopes, which observe in X-band at about 8.4 GHz, the new generation of radio telescopes participating in the VLBI Global Observing System (VGOS) is designed for broadband reception between 2 GHz and 14 GHz having corresponding illumination functions. This contribution investigates the impact of frequency-dependent illumination functions on signal path variations for the first time. For that purpose, several data-sets obtained from different feed-horns are analysed, and the impact on the signal path variations is studied.