Abstract
The primary goal of the geodetic Very Long Baseline Interferometry (VLBI) technique is to provide highly accurate terrestrial and celestial reference frames as well as Earth orientation parameters. In compliance with the concept of VLBI, additional parameters reflecting relative offsets and variations of the atomic clocks of the radio telescopes have to be estimated. In addition, reality shows that in many cases significant offsets appear in the observed group delays for individual baselines which have to be compensated for by estimating so-called baseline-dependent clock offsets (BCOs). For the first time, we systematically investigate the impact of BCOs to stress their importance for all kinds of VLBI data analyses. For our investigations, we concentrate on analyzing data from both legacy networks of the CONT17 campaign. Various aspects of BCOs including their impact on the estimates of geodetically important parameters, such as station coordinates and Earth orientation parameters, are investigated. In addition, some of the theory behind the BCO determination, e.g., the impact of changing the reference clock in the observing network on the BCO estimate is introduced together with the relationship between BCOs and triangle delay closures. In conclusion, missing channels, and here in particular at S band, affecting the ionospheric delay calibration, are identified to be the dominant cause for the occurrence of significant BCOs in VLBI data analysis.
Highlights
Very Long Baseline Interferometry (VLBI) is a space geodetic technique which can be used to estimate the terrestrial and celestial reference frames together with all five Earth orientation parameters (EOP) (Schuh and Böhm 2012)
The reason for this publication is that the omission of baseline-dependent clock offsets (BCOs) in the functional VLBI model leads to significant deterioration of the geodetic VLBI results
We describe the procedure for accounting of the BCOs in the analysis of VLBI data, and we show the impact of unmodelled BCOs on the geodetic parameters, such as station coordinates and Earth orientation parameters
Summary
Very Long Baseline Interferometry (VLBI) is a space geodetic technique which can be used to estimate the terrestrial and celestial reference frames together with all five Earth orientation parameters (EOP) (Schuh and Böhm 2012). The reason for this publication is that the omission of BCOs in the functional VLBI model leads to significant deterioration of the geodetic VLBI results For this reason, we describe the procedure for accounting of the BCOs in the analysis of VLBI data, and we show the impact of unmodelled BCOs on the geodetic parameters, such as station coordinates and Earth orientation parameters. The article is structured on the basis that we accept the need for estimating BCOs without going into detail of possible causes since careful investigations of the reasons for BCOs will be subject of our future work These forthcoming studies will be very useful in terms of feedback for the fringe fitting and pre-processing procedures of VLBI.
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