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A High-frequency Geodetic VLBI Experiment for Optical Clock Comparison

Monia Negusini, Myoung-Sun Heo, Cecilia Clivati, Shuangjing Xu, Roberto Ricci, Taehyun Jung, Buseung Cho, Matteo Stagni, Claudio Bortolotti, Giuseppe Maccaferri, Federico Perini, Mauro Roma, Do-Young Byun, Do-Heung Je, Marco Pizzocaro, Davide Calonico, Elena Cantoni, Giancarlo Cerretto, Stefano Condio, Giovanni A. Costanzo, Simone Donadello, Irene Goti, Michele Gozzelino, Alberto Mura, Filippo Levi, Matias Risaro, Huidong Kim, Won-Kyu Lee, Chang Yong Park, Dai-Hyuk Yu, Young Kyu Lee, Joon Hyo Rhee, Chanjin Park, Minseong Lee, Hyo Ryoung Kim, Sung-Moon Yoo, Jungho Cho, Jongsoo Kim, Sang-Oh Yi, Ha Su Yoon, Pablo de Vicente, Javier González, Cristina García Miró

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. 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 (FPT) 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.

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 s/s. 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 (FPT) 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.
Paper Structure (20 sections, 1 equation, 3 figures)

This paper contains 20 sections, 1 equation, 3 figures.

Table of Contents

  1. Introduction
  2. VLBI-clock comparison experiment
  3. VLBI basics
  4. Experiment setup
  5. VLBI setup
  6. Optical clocks and fiber distribution
  7. VLBI observation and data analysis
  8. Observation overview
  9. VLBI data correlation and fringe fitting
  10. Geodetic data analysis
  11. Results
  12. Comparison via VLBI
  13. Comparison by optical clocks
  14. Comparison via GNSS
  15. Discussion
  16. ...and 5 more sections

Figures (3)

  • Figure 1: Schematic view of the full experiment setup. a) At INRIM, in Italy, radiation from an infrared laser is referenced to IT-Yb1 clock via an optical comb and sent to Medicina with a phase-stabilised fiber. IT-Yb1 was also compared to KRISS-Yb1 via a GPS link, using INRIM H-maser as flywheel oscillator. b) In Medicina, the incoming radiation is coherently converted to a radiofrequency and used to calibrate the local H-maser, which is used for the VLBI observation. c) In Korea, a 10 MHz signal referenced to a H-maser is sent to Sejong VLBI antenna with a phase-stabilised fiber; here, after demodulation, it seeds the antenna synthesis chain and is used for VLBI observation. KRISS H-maser is also calibrated by KRISS-Yb1 and compared to INRIM via GPS. d) and e) Map of Europe and Korea with involved telescopes and Metrological Institutions.
  • Figure 2: Group delay residuals as a function of observing time during the Dec 2021 geodetic session obtained by $\nu$Solve (top) and by VieVS (bottom). Different colors represent the results of distinct baselines.
  • Figure 3: Closure delays during the experiment. Blue and orange dots represent closure delays for Korean baselines (KVN and Sejong) and Korean-European baselines, respectively.