VLBI requires large directional antennas (see Figure 4.2) that are able to move rapidly to obtain the required geometrical distribution of observations. This requirement makes VLBI instruments technologically complex and costly, and it is difficult to determine the location of the antenna’s reference point at the desired one-millimeter-level for such large antennas (see Chapter 5).
The requirements outlined in the Global Geodetic Observing System project of the International Association of Geodesy, combined with the science goals specified in the NASA Solid Earth Science Working Group Report, establish three main criteria for the next generation of geodetic VLBI systems (VLBI2010) (Niell et al., 2006). These include one millimeter measurement accuracy, continuous measurements for station positions and Earth orientation, and a turnaround time of less than 24 hours.
Recommendation: To pursue these system enhancements, the United States should invest in the following future developments to make VLBI more effective and less expensive:
Radio telescope apertures should be reduced (to 10-12 meters) by increasing the recorded signal bandwidth. The benefits of smaller telescope apertures include: lower manufacturing and maintenance cost; higher attainable slew rates; lower instrument distortions associated with temperature changes and gravitational and wind loading; increased ease of locating the effective reference point; and reduced cost of piers and domes at observing stations. The optimal radio telescope parameters for geodetic applications are different than those for astronomical applications. The geodetic infrastructure must, therefore, include a dedicated network of geodetic VLBI observatories to obtain continuous measurements. In addition, using multiple VLBI antennas at some locations may yield improvements in accuracy and lead to better separation of atmospheric delay and clock parameter estimates. Although there is a need for a VLBI network dedicated to geodetic applications, correlator centers can efficiently process both geodetic and astronomic VLBI