Assessing tropospheric turbulence impact on VGOS telescope placement in the Indian subcontinent for the estimation of earth orientation parameters

Abstract

The complete set of five Earth Orientation Parameters (EOP) can only be estimated accurately using geodetic Very Long Baseline Interferometry (VLBI). Their precision and accuracy depends on network geometry and station-dependent properties. Atmospheric turbulence poses one of the largest error sources for geodetic VLBI, impacting the precision of EOP. Thus, it becomes imperative to consider this factor while choosing the optimal locations for geodetic VLBI. The magnitude of tropospheric turbulence is approximated through the refractive index structure constant, \(C_\textrm{n}^\textrm{2}\). In this study, we simulate the optimal locations for geodetic VLBI in India, considering individual tropospheric turbulence parameters per telescope location. The study identifies 14 potential VLBI stations, co-located with GPS stations and homogeneously distributed all over India, and computes the \(C_\textrm{n}\) values from zenith wet delay variances over 24 h obtained from GPS data. These locations are simulated in addition to three different reference networks, which show the current and future VLBI Global Observing System (VGOS) networks. Multiple schedules have been generated and simulated for each configuration using VieSched++, and the precision of EOP is compared when constant and station-specific tropospheric turbulence parameters are used. The study shows that, for the investigated networks, southern stations are optimal for polar motion and celestial pole offsets estimation, whereas an eastern station is optimal for UT1−UTC estimation. Furthermore, the study highlights that for reference networks with fewer stations, utilizing station-specific \(C_\textrm{n}\) values significantly influences the determination of optimal locations. It further demonstrates how station-specific \(C_\textrm{n}\) values impact the positioning of VGOS telescopes in each network for each EOP differently. The findings show that higher \(C_\textrm{n}\) values generally lead to a degradation in EOP precision. Geometrically, a station might be at a good location, but if the \(C_\textrm{n}\) value is too high, that location is not favorable.