PSRPI: A large VLBA pulsar astrometry program

Obtaining pulsar parallaxes via relative astrometry (also known as differential astrometry) yields distances and transverse velocities that can be used to probe properties of the pulsar population and the interstellar medium. Large programs are essential to obtain the sample sizes necessary for these population studies, but they must be efficiently conducted to avoid requiring an infeasible amount of observing time. This paper describes the PSRPI astrometric program, including the use of new features in the DiFX software correlator to efficiently locate calibrator sources, selection and observing strategies for a sample of 60 pulsars, initial results, and likely science outcomes. Potential applications of high-precision relative astrometry to measure source structure evolution in defining sources of the International Celestial Referent Frame are also discussed.