Abstract
Abstract The modern Very Long Baseline Interferometry (VLBI) relativistic delay model, as documented in the IERS Conventions, refers to the time epoch when the signal passes one of two stations of an interferometer baseline (selected arbitrarily from the pair of stations and called the ‘reference station’ or ‘station 1’). This model consists of the previous correlation procedure used before the year 2002. However, since 2002 a new correlation procedure that produces the VLBI group delays referring to the time epoch of signal passage at the geocenter has been used. A corresponding correction to the conventional VLBI model delay has to be introduced. However, this correction has not been thoroughly presented in peer reviewed journals, and different approaches are used at the correlators to calculate the final group delays officially published in the IVS database. This may cause an inconsistency up to 6 ps for ground-based VLBI experiments between the group delay obtained by the correlator and the geometrical model delay from the IERS Conventions used in data analysis software. Moreover, a miscalculation of the signal arrival moment to the ‘reference station’ could result in a larger modelling error (up to 50 ps). The paper presents the justification of the correction due to transition between two epochs elaborated from the Lorentz transformation and the approach to model the uncertainty of the calculation of the signal arrival moment. Both changes are particularly essential for upcoming broadband technology geodetic VLBI observations.
Highlights
The paper presents the justification of the correction due to transition between two epochs elaborated from the Lorentz transformation and the approach to model the uncertainty of the calculation of the signal arrival moment
The Very Long Baseline Interferometry (VLBI) technique measures the difference between the arrival times of a signal from a distant radio source at two radio telescopes (Schuh & Behrend 2012)
Due to separation of the radio telescopes by a few hundred or thousand kilometres, the plain wave front passes first telescope earlier the second one. This difference in the arrival time of the signal at both radio telescopes is known as time delay, and the frequency shift due to the relative motion of the telescopes around the geocentre is known as delay rate
Summary
The Very Long Baseline Interferometry (VLBI) technique measures the difference between the arrival times of a signal from a distant radio source at two radio telescopes (Schuh & Behrend 2012). Data analysis The second term in Equation (A15) (of the Appendix) is the diurnal variation of the Earth scale’s factor that replaces the diurnal aberration applied for the traditional astronomical observations This is the only term due to the Earth’s rotation implemented by the FX-correlator software developers (in accordance to Corey 2000). A first solution produces a standard set of parameters for estimating – (i) corrections to the positions of radio telescopes in the ITRF2014 frame (Altamimi et al 2016), (ii) Earth orientation parameters, (iii) wet troposphere delay and two gradients, (iv) three parameters to model the clock instability for each station except the reference (clock offset, clock rate and second derivative) and (v) corrections to the ICRF3 positions of several radio sources that expose a high level of astrometric instability in the past. This results in additional, hidden, source of systematic error for all other parameters
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