  (Abridged) This thesis describes the development of DiFX, the first
general-purpose software correlator for radio interferometry, and its use with
the Australian Long Baseline Array (LBA) to complete the largest VLBI pulsar
astrometry program undertaken to date in the Southern Hemisphere. This two year
astrometry program has resulted in the measurement of seven new pulsar
parallaxes, more than trebling the number of measured VLBI pulsar parallaxes in
the Southern Hemisphere. The measurements included a determination of the
distance and transverse velocity of PSR J0437-4715 with better than 1%
accuracy, enabling improved tests of General Relativity, and the first
significant measurement of parallax for the famous double pulsar system PSR
J0737-3039A/B, which will allow tests of General Relativity in this system to
proceed to the 0.01% level. The DiFX software correlator developed to enable
this science has been extensively tested and is now an integral part of the
upgraded LBA Major National Research Facility; furthermore, it has been
selected to facilitate a substantial sensitivity upgrade for the US Very Long
Baseline Array.
