Many features of the Very Long Baseline Array (VLBA) contrive to make it the best telescope for pulsar astrometry. The measured proper motions and parallaxes allow distances and transverse velocities to be determined. These in turn provide clues to questions spanning nuclear astrophysics on scales of 10−23 m to the distribution of gas in the Galaxy on scales of 10 m. Three pulsars are discussed...

Attaining the limit of sub-microarcsecond optical resolution will completely revolutionize fundamental astrometry by merging it with relativistic gravitational physics. Beyond the sub-microarcsecond threshold, one will meet in the sky a new population of physical phenomena caused by primordial gravitational waves from the early universe and/or di erent localized astronomical sources, space-time...

The SIM Lite Astrometric Observatory is a mission concept for a space-borne instrument to perform micro-arcsecond narrow-angle astrometry to search 60 to 100 nearby stars for Earth-like planets, and to perform global astrometry for a broad astrophysics program. The main enabling technology development for the mission was completed during phases A & B. While the project is waiting for the result...

The Space Interferometry Mission (SIM), with its launch date planned for 2005, has as its goal astrometry with ∼ 1 μ arcsecond accuracy for stars as faint as 20th mag. If the SIM lives to expectations it can be used to measure astrometric displacements in the light centroid caused by the gravitational microlensing in the events detected photometrically from the ground. The effect is typically ∼...