Fringe Detection Methods for Very Long Baseline Arrays

In VLBI observations, atmospheric turbulence and local oscillator phase noise often limit the coherence time to a few seconds. When such coherence losses severely limit the integration time, detection thresholds rise and weak sources cannot be detected. We show here that under these conditions, the detection methods and measurements of visibility amplitude and phase can be reformulated in terms of incoherently averaged quantities. The theory is presented by examining the properties of averaged amplitudes and triple products. This paper reviews current detection methods, discusses new techniques, presents applications to recent 3 mm VLBI experiments, and shows that signal processing can significantly improve the probability of signal detection.