5.3. Very Long Baseline Interferometer Systems

Abstract

This chapter provides an overview of very long baseline interferometer (VLBI) systems. In VLBI, the undetected signals received at remote, unconnected radio telescopes are stored in such a way that they can be recovered at a later time and processed in a fashion similar to conventional interferometry. Several essential components are required to make two remote telescopes into an interferometer which include: a stable local-oscillator system for signal frequency conversion; a video converter for conversion of the intermediate frequency (i.f.) signal to a video band; a means of synchronizing the clocks at the stations; a device for storing the received signal and precise timing information; and a playback system. The antennas and front-end amplifiers used in other applications of radio astronomy are generally adequate for VLBI measurements. There are no special tracking requirements for the antennas. Local oscillators in single-telescope continuum studies can be free running and, in spectral-line studies, need only be frequency locked, those in VLBI applications must be phase locked to high-performance frequency standards. A general block diagram of a VLBI system is provided in the chapter along with an overview of single-sideband video converter. Details of IBM-compatible, digital-recording system are also presented.