Introduction to VLBI

Abstract

The principles of Very Long Baseline Interferometry are essentially the same as those in connected-element interferometry. Both systems can be used to make images of extended radio sources through the application of various calibration and image restoration techniques or to measure geometric quantities such as the positions of point sources or baseline vectors. The distinguishing characteristic of VLBI, as currently practiced, is that the interferometer elements, or stations, operate without any communication in real time. At each station, the signals are converted to a baseband of frequencies and are recorded on magnetic tape along with timing information for later correlation. The frequency standard at each station controls the phase of the local oscillator signal, which is necessary to convert the received signal to a baseband, and the station clock (see Fig. 1). The frequencies of the frequency standards are not precisely determined, and the clocks cannot be set exactly. Hence, there is a frequency offset and epoch error associated with the data recorded at each station. Thus, the processing of the data from a VLBI experiment must begin with a two-dimensional search for fringes, which involves the alignment of the data streams in time and frequency. The recording of the received signals on tape limits the bandwidth and presents some interesting problems concerning how to represent the signal.