A Frequency-Agile System for VLBI Bandwidth Synthesis.

Mathematics – Probability

Scientific paper

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Scientific paper

The bandwidth synthesis technique has been successfully used in Very Long Baseline Interferometry (VLBI) for over two and a half decades for the precise determination of interferometer group delay measurements, with formal measurement precisions currently at the level of tens of picoseconds or centimeter to sub-centimeter delay accuracies over intercontinental distances. This thesis investigates a VLBI system for bandwidth synthesis based on a frequency-agile local oscillator. Although 'frequency-switched' VLBI has been successfully demonstrated in the late 1960's (28), the system suffered primarily from instrumental difficulties, and since that time the frequency-switched system type has been generally superseded with what has become the de-facto standard in current VLBI bandwidth synthesis systems: the 'simultaneous-acquisition' system type utilizing multiple, parallel, fixed-frequency channels across the spanned bandwidth. In this thesis we re-examine the 'frequency-switched' bandwidth synthesis technique, investigating effects created by the non-simultaneity in data acquisition across the spanned bandwidth. We examine issues related to: the effects of 'correlated phase noise,' noise which is common to all acquisition channels and which is intrinsically minimized in traditional simultaneous-acquisition bandwidth synthesis systems; two-dimensional acquisition sequences, their criteria and opitmization; the implications of two-dimensional delay functions or 'processing beams'; an extension of the classical probability of error calculation, motivated by the desire to estimate the effect of two-dimensional signal sidelobe structure in the processing beam and the possible degradation in VLBI detection threshold; and some basic issues related to the post-correlation 'fringe' processing of frequency -switched datasets. Results from this thesis will be applied directly to the design and implementation of the S2 VLBI frequency-switched bandwidth synthesis system for geodesy/astrometry under development in Canada at the time of writing (mid 1995).

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