FIGURE 4.1 The VLBI technique uses multiple radio telescopes to measure natural radiometric noise from distant radio sources, such as quasars. The differences in the arrival time for the same radio noise at separate antennas are later correlated to determine the time delay between the two antennas to millimeter precision. By observing radio sources in multiple directions, the VLBI network can be used to determine Earths geometric shape and orientation in space. SOURCE: NASA, Goddard Space Flight Center.

FIGURE 4.1 The VLBI technique uses multiple radio telescopes to measure natural radiometric noise from distant radio sources, such as quasars. The differences in the arrival time for the same radio noise at separate antennas are later correlated to determine the time delay between the two antennas to millimeter precision. By observing radio sources in multiple directions, the VLBI network can be used to determine Earth’s geometric shape and orientation in space. SOURCE: NASA, Goddard Space Flight Center.

in place. The two Canadian VLBI stations at Yellowknife and Algonquin have been shut down. The U.S. VLBI site in Gilmore Creek, Alaska was closed by NASA, and the geodetic VLBI site at Green Bank, West Virginia (originally part of the U.S. Naval Observatory program for Earth orientation) closed in 2001. There are currently no prospects for restoring either of the closed U.S. sites. Furthermore, those U.S. sites that are operational are not all operating routinely. Except for the sites at Kokee Park, Hawaii and Westford, Massachusetts, most U.S. VLBI sites collected less than ten days of data during 2009. If four non-colinear U.S. sites were routinely operational for geodetic observations, this would be sufficient for USNO to determine Earth’s orientation using U.S. resources only. Currently, USNO has to rely on international partners for its Earth orientation determination.

Several factors make the VLBI technique challenging and costly. First, the requirement that data from VLBI observatories must be physically shipped to a special correlator center for processing can introduce delays in determining Earth-orientation parameters. The signals from quasars far from Earth are weak and it is necessary to process the data from VLBI observatories centrally in order to separate the signals from background noise. In addition, obtaining usable signal-to-noise levels using



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