VLBI Techniques in Pulsar Astronomy
  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. 
Walter Brisken (National Radio Astronomy Observatory)