The VLBA is a series of ten 25 m diameter radio antennas located at sites across the continental United States, and on Mauna Kea, Hawaii, and St. Croix in the U.S. Virgin Islands. Work on its construction began in 1985 and the last VLBA station on Mauna Kea, Hawaii, was completed in 1993. Because of its extremely long baseline and large collecting area, the VLBA has a maximum angular resolution of less than one-thousandth of an arcsecond (at A = 7 mm), and covers a wavelength range of 3 mm to 90 cm in ten bands. Clearly, to achieve its high resolution the data from the antennas must be combined interferometrically, but since the sites are so far apart, this is not easy to do in real time. Instead, each site records its data onto magnetic tape and the data "time-tagged" with a reference signal generated by a hydrogen maser at each site. The tapes are then sent to the VLBA station in Socorro, New Mexico, where the tapes from each station are read and combined with the appropriate time delays consistent with the station's different positions across the globe to form a single interferometric device.
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