A High-frequency Geodetic VLBI Experiment for Optical Clock Comparison

Abstract An intercontinental metrological clock comparison between Italy and the Republic of Korea was performed by means of geodetic K -band VLBI observations. The comparison involved the hydrogen masers (H-masers) used at Medicina and Sejong radio telescopes. The same clocks were simultaneously compared by a satellite link and by high-precision optical clocks maintained at the National Metrology Institutes, KRISS in Korea and INRIM in Italy, and delivered to VLBI antennas via optical fiber. The H-masers frequency difference was estimated by extrapolating the clock rate from VLBI data using two geodetic VLBI software. This was subsequently compared with clock differences derived by satellite link and by local optical clocks. Results obtained with different approaches were in agreement at the level of 10 −15 s s −1 . This pilot study demonstrates that standard high-frequency ( K -band) geodetic VLBI campaigns could be a viable approach to conduct intercontinental clock comparisons, now only possible via satellite links. This uncertainty can be reduced thanks to the planned installation of new-generation, broadband, high-frequency receivers on the involved telescopes. K / Q / W -band geodetic observations will allow an improvement of the accuracy of the resulting group delays through broad bandwidth synthesis from 20 to 100 GHz. Furthermore, the Frequency Phase Transfer method will also be explored together with the use of PCAL systems installed at the radio telescopes to improve phase stability and thus allow a better estimation of the station clock parameters.

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Publication Details

Journal
Publications of the Astronomical Society of the Pacific
Published
2026-02-01
DOI
https://doi.org/10.1088/1538-3873/ae36d1
Primary Topic
Advanced Frequency and Time Standards
Type
article
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Publications of the Astronomical Society of the Pacific
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article en

Abstract

Abstract An intercontinental metrological clock comparison between Italy and the Republic of Korea was performed by means of geodetic K -band VLBI observations. The comparison involved the hydrogen masers (H-masers) used at Medicina and Sejong radio telescopes. The same clocks were simultaneously compared by a satellite link and by high-precision optical clocks maintained at the National Metrology Institutes, KRISS in Korea and INRIM in Italy, and delivered to VLBI antennas via optical fiber. The H-masers frequency difference was estimated by extrapolating the clock rate from VLBI data using two geodetic VLBI software. This was subsequently compared with clock differences derived by satellite link and by local optical clocks. Results obtained with different approaches were in agreement at the level of 10 −15 s s −1 . This pilot study demonstrates that standard high-frequency ( K -band) geodetic VLBI campaigns could be a viable approach to conduct intercontinental clock comparisons, now only possible via satellite links. This uncertainty can be reduced thanks to the planned installation of new-generation, broadband, high-frequency receivers on the involved telescopes. K / Q / W -band geodetic observations will allow an improvement of the accuracy of the resulting group delays through broad bandwidth synthesis from 20 to 100 GHz. Furthermore, the Frequency Phase Transfer method will also be explored together with the use of PCAL systems installed at the radio telescopes to improve phase stability and thus allow a better estimation of the station clock parameters.

Publications of the Astronomical Society of the PacificVol. 138(2)
University of Rome Tor Vergata (IT), Politecnico di Torino (IT), Korea Research Institute of Standards and Science (KR), Korea Astronomy and Space Science Institute (KR), National Institute of Ecology (KR), Istituto di Radioastronomia di Bologna (IT), Yebes Observatory (ES), Istituto Nazionale di Ricerca Metrologica (IT), National Institute for Astrophysics (IT), Korea Institute of Science & Technology Information (KR), Korea University of Science and Technology (KR)
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Advanced Frequency and Time Standards
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