  Recent discoveries of dispersed, non-periodic impulsive radio signals with
single-dish radio telescopes have sparked significant interest in exploring the
relatively uncharted space of fast transient radio signals. Here we describe
V-FASTR, an experiment to perform a blind search for fast transient radio
signals using the Very Long Baseline Array (VLBA). The experiment runs entirely
in a commensal mode, alongside normal VLBA observations and operations. It is
made possible by the features and flexibility of the DiFX software correlator
that is used to process VLBA data. Using the VLBA for this type of experiment
offers significant advantages over single-dish experiments, including a larger
field of view, the ability to easily distinguish local radio-frequency
interference from real signals and the possibility to localize detected events
on the sky to milliarcsecond accuracy. We describe our software pipeline, which
accepts short integration (~ms) spectrometer data from each antenna in real
time during correlation and performs an incoherent dedispersion separately for
each antenna, over a range of trial dispersion measures. The dedispersed data
are processed by a sophisticated detector and candidate events are recorded. At
the end of the correlation, small snippets of the raw data at the time of the
events are stored for further analysis. We present the results of our event
detection pipeline from some test observations of the pulsars B0329+54 and
B0531+21 (the Crab pulsar).
