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Precision astrometry is an integral component of successful pulsar timingcampaigns. Astrometric parameters are commonly derived by fitting them asparameters of a timing model to a series of pulse times of arrival (TOAs). TOAsmeasured to microsecond precision over several-year spans can yield positionmeasurements with sub-milliarcsecond precision. However, timing-basedastrometry can become biased if a pulsar displays any red spin noise, which canbe compared to the red noise signal produced by the stochastic gravitationalwave background. We investigate how noise of different spectral types isabsorbed by timing models, leading to significant estimation errors in theastrometric parameters. We find that commonly used techniques for fittingtiming models in the presence of red noise (Cholesky whitening) prevent theabsorption of noise into the timing model remarkably well if the time baselineof observations exceeds several years, but are inadequate for dealing withshorter pulsar data sets. Independent of timing, pulsar-optimized very longbaseline interferometry (VLBI) is capable of providing position estimatesprecise to the sub-milliarcsecond levels needed for high-precision timing. Wecompute a necessary transformation between the International CelestialReference Frame and pulsar timing frames and quantitatively discuss how thetransformation will improve in coming years. We find that incorporating VLBIastrometry into the timing models of pulsars for which only a couple of yearsof timing data exist will lead to more realistic assessments of red spin noiseand could enhance the amplitude of gravitational wave signatures in post-fittiming residuals by factors of 20 or more.

The Benefits of VLBI Astrometry to Pulsar Timing Array Searches for Gravitational Radiation