  Very long baseline interferometry (VLBI) provides the resolution needed to
make precision measurements of a pulsar's parallax and proper motion. In making
these measurements, the astronomer is faced with difficult calibration problems
and a paucity of strong point-like calibrators that ultimately limit the
accuracy of parallax measurements of even the brightest pulsars. A new
technique to calibrate away the effects of the ionosphere, the dominant source
of phase error at frequencies below 5 GHz, has led to the new measurements of
nine pulsar parallaxes to 100 microarcsecond or better precision.
