Phase compensation experiments with the paired antennas method: 2. Millimeter-wave fringe correction using centimeter-wave reference

An experiment for compensating interferometer phase fluctuations due to the turbulent troposphere was conducted with the paired antennas method (PAM) using different sky frequencies for a target and reference source. A celestial source 3C279 was observed at 146.81 GHz with the Nobeyama millimeter array (NMA), while a geostationary satellite was observed as a reference source at 19.45 GHz with commercially available antennas. Each of the antennas was installed near one of the NMA antennas, thus giving us 10 pairs of nearly parallel baselines up to 316 m. Large fluctuations in the interferometer phase of 3C279 were mostly eliminated on almost all baselines by subtracting the reference phase multiplied by a ratio of the observing frequencies. The standard deviations of the compensated interferometer phase were under the level of 20° where the angular separation between the two sources was within 2°, while those of the original phase were typically at the level of 40°–50°. The phase compensation was much improved to the level of the differential excess path length of 0.06-mm rms (11° at 146.81 GHz) in inserting a time lag proportional to the separation angle to the reference phase time series. These results have practical significance for the millimeter- or submillimeterwave interferometry because there is rarely a suitable reference source in the vicinity of scientifically interesting sources at such high frequencies. The present experiment shows that it is quite effective in the PAM to use the reference phase at centimeter-wave for compensating the millimeter-wave phase for future large millimeter-wave arrays.