Precision VLBI astrometry: instrumentation, algorithms and pulsar parallax determination

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 to complete the largest Very Long Baseline Interferometry (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, which has more than trebled the number of measured VLBI pulsar parallaxes in the Southern Hemisphere. These 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; 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 and also offers insights into its formation and high-energy emission; and a factor of four revision to the estimated distance of PSR J0630-2834, which had previous appeared to possess extremely unusual x-ray emission characteristics. Additionally, the ensemble of refined distance and transverse velocity estimates have enabled a widely applicable improvement in knowledge of pulsar luminosities in several wavebands and the Galactic electron distribution at southern latitudes. Finally, the DiFX software correlator developed to enable this science has been extensively tested and verified against three existing hardware correlators, and is now an integral part of the upgraded Long Baseline Array Major National Research Facility used by astronomers throughout Australia and the world; furthermore, it has been selected to facilitate a major upgrade of the world's only full-time VLBI instrument, the Very Long Baseline Array operated by the National Radio Astronomy Observatory in the US.