  Precision astrometry is an integral component of successful pulsar timing
campaigns. Astrometric parameters are commonly derived by fitting them as
parameters of a timing model to a series of pulse times of arrival (TOAs). TOAs
measured to microsecond precision over several-year spans can yield position
measurements with sub-milliarcsecond precision. However, timing-based
astrometry can become biased if a pulsar displays any red spin noise, which can
be compared to the red noise signal produced by the stochastic gravitational
wave background. We investigate how noise of different spectral types is
absorbed by timing models, leading to significant estimation errors in the
astrometric parameters. We find that commonly used techniques for fitting
timing models in the presence of red noise (Cholesky whitening) prevent the
absorption of noise into the timing model remarkably well if the time baseline
of observations exceeds several years, but are inadequate for dealing with
shorter pulsar data sets. Independent of timing, pulsar-optimized very long
baseline interferometry (VLBI) is capable of providing position estimates
precise to the sub-milliarcsecond levels needed for high-precision timing. We
compute a necessary transformation between the International Celestial
Reference Frame and pulsar timing frames and quantitatively discuss how the
transformation will improve in coming years. We find that incorporating VLBI
astrometry into the timing models of pulsars for which only a couple of years
of timing data exist will lead to more realistic assessments of red spin noise
and could enhance the amplitude of gravitational wave signatures in post-fit
timing residuals by factors of 20 or more.
