Common-clock very long baseline interferometry using a coherent optical fiber link

Abstract

Among the most powerful techniques for the exploration of the Universe is very long baseline interferometry (VLBI), which is based on the simultaneous observation of radio sources in the sky with arrays of distant ground-based antennas. One of the effects currently limiting its ultimate sensitivity is the phase-instability of the reference clocks adopted at each antenna.This term can be made negligible delivering the same clock signal to multiple telescope sites using optical fibers. We realized such an infrastructure by disseminating a coherent optical frequency signal to two distant radio telescopes using a 1739-km-long fiber. We performed a 24 h geodetic VLBI campaign in which the same clock reference was used at both telescopes and analyzed it using standard VLBI procedures. The results were consistent with the expectations, confirming that the proposed approach is feasible and configures as a novel tool for studying the role of clocks, troposphere, and systematic effects in the ultimate VLBI resolution. ; Funding: Projects EMPIR-18SIB06-TIFOON and EMPIR-17IND14-WRITE, which have received funding from the EMPIR programme co-financed by the Participating States and from the European Union's Horizon 2020 research and innovation programme; Italian Space Agency (DTFMatera); Italian Ministry of Education, University and Research (MIUR) through the Progetti Premiali program (MetGeSp). V. Di Sarno thanks the grant support by Progetti di Ricerca di Interesse Nazionale (PRIN), Project No. 20152MRAKH of the Italian Ministry of University and Research.