The site test interferometers directly measure the tropospheric phase stability. They observe unmodulated 11.5 GHz beacons broadcast from geostationary satellites and measure the phase difference between the signals received by two 1.8 m diameter antennas 300 m apart. Because the atmosphere is non-dispersive away from line centers, the results can be scaled to millimeter and submillimeter wavelengths.
Three instruments have been constructed by NRAO's Tucson office. The first was operated near the VLBA antenna (3720 m) on Mauna Kea, Hawaii, from 1994 September to 1996 June and at the VLA since 1997 May. The second has been operating on Chajnantor (5000 m) near San Pedro de Atacama, Chile, since 1995 May. A third was built for the LSA project and installed at Pajonales in 1997 April.
The design and operation of these instruments are described in Site Test Interferometer (Radford, Reiland, & Shillue 1996, PASP 108, 441). Contact Simon Radford for more information.
From the phase time series, we obtain the r. m. s. path fluctuations on a 300 m baseline, the power law exponent of the phase structure function, and the velocity at which the turbulent water vapor moves over the array. MMA Memo 129 describes the site test interferometer data reduction in detail, and MMA Memo 130 illustrates the agreement between two different methods of deriving the mean velocity of the turbulent water vapor flow in the atmosphere.
Contact Mark Holdaway for more information about analysis of data from the site test interferometers.