From Reference Frames to Relativistic Experiments: Absolute and Relative   Radio Astrometry
  Reference systems and frames are crucial for high precision absolute astrometric work, and their foundations must be well-defined. The current frame, the International Celestial Reference Frame, will be discussed: its history, the use of the group delay as the measured quantity, the positional accuracy of 0.3 milliarcsec, and possible future improvements. On the other hand, for the determination of the motion of celestial objects, accuracies approaching 0.01 milliarcsec can be obtained by measuring the differential position between the target object and nearby stationary sources. This astrometric technique uses phase referencing, and the current techniques and limitations are discussed, using the results from four experiments. Brief comments are included on the interpretation of the Jupiter gravity deflection experiment of September 2002. 
E. B. Fomalont