A broadband VLBI system using transportable stations for geodesy and metrology: an alternative approach to the VGOS concept

Mamoru Sekido,Hidekazu Hachisu,Gérard Petit,Nils Nemitz,Hideki Ujihara,Tetsuro Kondo,J Leute ,Masahiro Tsutsumi ,Federico Perini,G Maccaferri ,Monia Negusini,Cecilia Clivati,Tomonari Suzuyama,Kunitaka Namba,Kazuhiro Takefuji,Eiji Kawai,C Bortolotti ,J Komuro ,Ryuichi Ichikawa,Marco Pizzocaro,Mauro Roma,R Ricci ,K Watabe ,Davide Calonico,T Ido

doi:10.1007/s00190-021-01479-8

Abstract

We have developed a broadband VLBI (very long baseline interferometry) system inspired by the concept of the VLBI Global Observing System (VGOS). The new broadband VLBI system was implemented in the Kashima 34 m antenna and in two transportable stations utilizing 2.4 m diameter antennas. The transportable stations have been developed as a tool for intercontinental frequency comparison but are equally useful for geodesy. To enable practical use of such small VLBI stations in intercontinental VLBI, we have developed the procedure of node-hub style VLBI: In joint observation with a large, high sensitivity ‘hub’ antenna, the closure delay relation provides a virtual delay observable between ‘node’ stations. This overcomes the limited sensitivity of the small diameter node antennas, while error sources associated with large diameter antennas, such as gravitational deformation and delay changes in necessarily long signal cables, are eliminated. We show that this scheme does not result in an increased sensitivity to radio source structure if one side of the baseline triangle is kept short. We have performed VLBI experiments utilizing this approach over both short range and intercontinental distance. This article describes the system components, signal processing procedure, experiment, and results in terms of baseline repeatability. Our measurements reveal signatures of structure effects in the correlation amplitude of several of the observed radio sources. We present a model of the frequency-dependent source size for 1928+738 derived from correlation amplitude data observed in four frequency bands.

Highlights

Very long baseline interferometry (VLBI) has utilized the S-band (2.3 GHz) and X-band (8.4 GHz), which have been assigned for deep space communication and for geodetic very long baseline interferometry (VLBI) observations
Part of the discrepancy between the zenith delay estimated by GPS and VLBI represents the anisotropic atmosphere: The azimuthal directions for MARBLE1M and MARBLE2 stations were in the range of − 40◦ to + 50◦, and − 50◦ to + 130◦, respectively
A broadband VLBI system has been developed inspired by the VLBI Global Observing System (VGOS) concept

Summary

Introduction

Very long baseline interferometry (VLBI) has utilized the S-band (2.3 GHz) and X-band (8.4 GHz), which have been assigned for deep space communication and for geodetic VLBI observations. The International VLBI Service for Geodesy and Astrometry (IVS) is promoting the VLBI Global Observing System (VGOS) (Petrachenko et al 2012; Niell et al 2018) as the generation geodetic VLBI system, which is targeting 1 mm geodetic precision by observing four 1 GHz bands spread over the 2–14 GHz frequency range. The concept of VGOS was developed to improve the estimation of the atmospheric delay contribu-. To enable measurement of the delay observable with sufficient accuracy, broadband observation, high rate data acquisition, and dual polarization observation were included in the specification. Since the precision of group delay measurements is inversely proportional to the effective bandwidth (Rogers 1970) [see later Eq (6)], a ten times wider frequency range enables a one order of magnitude improvement in delay precision with the same signal-to-noise ratio (SNR). The SNR itself improves with bandwidth B as SNR ∝ D1

Results

Discussion

Conclusion