Our goal is to assess the potential of differential-VLBI (D-VLBI, known as phase referencing in the astronomical community) for the establishment of frame ties for geodesy and astrometry.

Distant celestial sources provide the most stable reference frame known.  Standard geodetic VLBI observations tie the telescope positions in the kinematic International Terrestrial Reference Frame (ITRF) to the sources in the International Celestial Reference Frame (ICRF).  Positions of spacecraft and planetary system bodies (planets, moons, asteroids) are typically realized in various dynamic reference frames that rest upon dynamical theories.  Tying these dynamical frames to the celestial frame is essential in order to provide the most correct and reliable long-term results for applications including monitoring global change and climate variation and spacecraft navigation. D-VLBI, a technique that provides accurate, relative astrometric measurements by nearly canceling effects introduced by the instruments, troposphere, ionosphere, and delay models, has the capability to provide the most accurate positions of spacecraft and planetary bodies with respect to the ICRF, and thereby enables these dynamical frames to be tied to the ICRF. Dynamic reference frames are typically realized specifically for each satellite or satellite constellation; when it comes to the combination of various missions or follow-on satellites, precise ties between the various frames are essential for correct and long-term reliable results.  D-VLBI provides the measurements that can tie all of these different frames to a single, highly stable frame, the ICRF.