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.
Figure 1 shows the scatter in Monte Carlo simulations (equivalent to the expected measurement uncertainty) of VLBI observations of the E-GRASP satellite using five different global station network configurations with increasing numbers of stations to determine the 7 standard Helmert frame transformation parameters (3 translational parameters, 3 rotational parameters, and a scale parameter). The simulations used continuous 14-day sessions, with VLBI stations observing the spacecraft when it was visible and could be tracked, and with standard quasar measurements filling the rest of the observing time.
The main result is that standard VLBI observations with a reasonable future network size can come close to, but do not quite meet, the GGOS accuracy goal of 1 mm for the frame accuracy.
Anderson, J.M., G. Beyerle, S. Glaser, L. Liu, B. Maennel, T. Nilsson, R. Heinkelmann, H. Schuh (2018): Simulations of VLBI observations of a geodetic satellite providing co-location in space. Journal of Geodesy, doi:10.1007/s00190-018-1115-5, published online
For more information on the D-VLBI project, including a list of publications, see http://ww2.erdrotation.de