Irregularities in the Earth’s rotation speed are quantified by the difference between Universal Time (UT1) and Coordinated Universal Time (UTC). This important observable is expressed as UT1−UTC and only estimated through Very Long Baseline Interferometry (VLBI). Typically, 1 h sessions, known as Intensives, are conducted daily between two to three stations, with the primary goal of determining UT1−UTC with a short latency. With the proposed plan of India having its own VLBI Global Observing System (VGOS) telescope, it is necessary to identify the optimal location for it and the ways it can help the International VLBI Service for Geodesy and Astrometry (IVS) to improve the precision of UT1−UTC estimates. This study investigates the change in the precision of different existing baseline solutions when a third station from India is added either in tagalong or in regular mode. Additionally, it identifies a new two-station intensive baseline including one Indian station, which could be part of future intensive sessions. Extensive simulations were conducted using VieSched++ software, varying the VGOS telescope’s location in India on a regular 5 <InlineEquation ID="IEq1"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="40623_2025_2214_Article_IEq1.gif" Format="GIF" Height="13" Rendition="HTML" Resolution="72" Type="Linedraw" Width="19" /> </InlineMediaObject> <EquationSource Format="TEX">$\times$</EquationSource> <EquationSource Format="MATHML"><math> <mo>×</mo> </math></EquationSource> </InlineEquation> 5 degree grid to test potential locations. The study reveals that adding an Indian telescope as a regular third station only improves the precision of UT1−UTC in certain cases. In particular, when the initial baselines already have a good geometry compared to the new baselines with the Indian station, no improvement can be expected. In contrast, using tagalong mode is always beneficial due to the increased number of observations. For example, adding an Indian station in regular mode to the KOKEE12M-WETTZ13S and European-ISHIOKA baselines does not improve the UT1−UTC precision. On the other hand, an Indian station added in tagalong mode with the MACGO12M-WETTZ13S baseline improves the UT1−UTC precision by a factor of 0.5. The study further highlights that UT1−UTC precision depends on baseline length, baseline geometry, and common sky coverage. Moreover, it is shown that an Indian station paired with U.S. stations, except KOKEE12M, results in a high precision of UT1−UTC, comparable with current VGOS intensive sessions. Overall, in both strategies, the study concludes that the optimal location for a VGOS telescope in India for UT1−UTC estimation is in the northeastern part of the country. Graphical Abstract

Evaluating the potential of Indian stations in augmenting the UT1−UTC precision of VGOS Intensive sessions

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2025年6月13日

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Arnab Laha,
Matthias Schartner,
Benedikt Soja,
Sigrid Böhm,
Hana Krásná,
Nagarajan Balasubramanian,
Onkar Dikshit

摘要

Irregularities in the Earth’s rotation speed are quantified by the difference between Universal Time (UT1) and Coordinated Universal Time (UTC). This important observable is expressed as UT1−UTC and only estimated through Very Long Baseline Interferometry (VLBI). Typically, 1 h sessions, known as Intensives, are conducted daily between two to three stations, with the primary goal of determining UT1−UTC with a short latency. With the proposed plan of India having its own VLBI Global Observing System (VGOS) telescope, it is necessary to identify the optimal location for it and the ways it can help the International VLBI Service for Geodesy and Astrometry (IVS) to improve the precision of UT1−UTC estimates. This study investigates the change in the precision of different existing baseline solutions when a third station from India is added either in tagalong or in regular mode. Additionally, it identifies a new two-station intensive baseline including one Indian station, which could be part of future intensive sessions. Extensive simulations were conducted using VieSched++ software, varying the VGOS telescope’s location in India on a regular 5 $\times$ $\times$ 5 degree grid to test potential locations. The study reveals that adding an Indian telescope as a regular third station only improves the precision of UT1−UTC in certain cases. In particular, when the initial baselines already have a good geometry compared to the new baselines with the Indian station, no improvement can be expected. In contrast, using tagalong mode is always beneficial due to the increased number of observations. For example, adding an Indian station in regular mode to the KOKEE12M-WETTZ13S and European-ISHIOKA baselines does not improve the UT1−UTC precision. On the other hand, an Indian station added in tagalong mode with the MACGO12M-WETTZ13S baseline improves the UT1−UTC precision by a factor of 0.5. The study further highlights that UT1−UTC precision depends on baseline length, baseline geometry, and common sky coverage. Moreover, it is shown that an Indian station paired with U.S. stations, except KOKEE12M, results in a high precision of UT1−UTC, comparable with current VGOS intensive sessions. Overall, in both strategies, the study concludes that the optimal location for a VGOS telescope in India for UT1−UTC estimation is in the northeastern part of the country.

Graphical Abstract