Very-Long-Baseline Interferometry

Abstract

In 1967, a new technique of interferometry was developed in which the receiving elements were separated by such a large distance that it was expedient to operate them independently with no real-time communication link. This was accomplished by recording the data on magnetic tape for later cross-correlation at a central processing station. The technique was called very-long-baseline interferometry (VLBI), a term recalling the earlier long-baseline interferometers at Jodrell Bank Observatory, in which the elements were connected by microwave links that had reached 127 km in length. The principles involved in VLBI are fundamentally the same as those involved in interferometers with connected elements. The tape recorder and its successor, disk storage, can be considered as an IF delay line of limited capacity with an unusually long propagation time, weeks instead of microseconds. The use of tape and disk recording media is motivated entirely by economics and places substantial limitations on the system. Satellite links have been demonstrated (Yen et al. 1977), but their high cost discourages their use.Tape recorders have been entirely replaced by compact disks. Data can also be transmitted to correlation facilities via the Internet in quasi real time. However, latency and throughput are significant issues, and data buffering is usually required.

Radio Interferometric Observation of an Asteroid Occultation

“…The fringe visibilities were counter-rotated to the contribution of the group delay and the phase delay rate, and coherently averaged over frequency. For details on the fringe fitting procedure, we refer the reader to Petrov et al (2011) and Thompson et al (2017).…”

Reading M87's DNA: A Double Helix revealing a large scale Helical Magnetic Field

“…Currently, the highest angular resolutions which allow the study of the formation and collimation of AGN jets can only be reached by using the Very Long Baseline Interferometry (VLBI) technique (for historical and technical details, see Thompson et al 2017). Very recently, observations performed with the Event Horizon Telescope (EHT) obtained polarized data on event-horizon scales, mapping, for the first time, the magnetic field close the supermassive BH at the center of M87 and favouring launching of the jet from the ergosphere of the BH (Event Horizon Telescope Collaboration et al 2021a).…”