Site Test Interferometer

Abstract

To evaluate possible sites for NRAO's proposed Millimeter Array, we constructed interferometers to directly measure the tropospheric phase stability. These instruments observe an unmodulated 11.5 GHz beacon broadcast from a geostationary satellite and measure the phase difference between the signals received by two antennas 300 m apart. Because the atmosphere is non-dispersive away from line centers, the results can be scaled to millimeter and submillimeter wavelengths. Novel design features of the instruments include a local oscillator phase locked to the received signal and digital correlation of the downconverted signals with a computer. Two instruments have been deployed, one at 3720 m on Mauno Kea, Hawaii, and the other at 5000 m near Cerro Chajnantor in northern Chile. With identical instruments operating simultaneously, we can directly compare the phase stability at the two sites.