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intuition than on a quantitative discussion of the various options, a few of which are listed above, that must be worked out soon to specify the design.

III.  Site Selection

The two principal sites under study are Mauna Kea in Hawaii and Mt. Graham in Arizona.  Major considerations for the site are sub-millimeter transparency of the atmosphere (a function of the water vapor above the site and the atmospheric pressure) phase fluctuations in the transmission of signals through the atmosphere, air mass for observation of the inner galactic plane and timely availability of the site.

The water vapor data available to the SAO is presently more complete for Mauna Kea.  It is based on measurements from the NRAO 225 GHz radiometer, correlations with long term radiosonde data, and the earlier Westphal survey.  The Mt. Graham water vapor data is based on University of Arizona radiometry measurements and radiosonde data.  It appears from a comparison of these data that the time available when the water vapor is less than 1.5mm, as needed for the shortest wavelength observing, is 1.5 times greater on Mauna Kea than on Mt. Graham.  The ratio mainly is due to the fact that for a few months in the summer season Mt. Graham is not usable for sub-millimeter observations.

The most interesting new data regarding the effectiveness of high altitude sites for sub-millimeter interferometry come from the observations at Mauna Kea of atmospheric chase fluctuations of a satellite signal.  Differential phase fluctuations due to the atmosphere were measured by the SAO group on a 100m baseline with an 11 GHz satellite as the source.  The experiment was well planned and executed. Although only the first few months data are available, it is already clear that the atmospheric phase fluctuations are substantial.  The somewhat surprising result is that the median phase fluctuations are roughly the same as they are at other lower altitude sites.  There is also a strong diurnal effect, with the daytime fluctuations much larger. There are two further facts which emerge from all the data.  The RMS differential phase fluctuations are generally lower on the driest days but otherwise do not correlate well with the average column of water vapor, and the phase RMS increases with approximately the 0.6 power of the antenna separation.  The latter indicates that better resolution will be obtained at longer wavelengths.  All of these facts underscore the premium placed on the nights that have the lowest water vapor.  They argue that the array must be able to obtain complete observations in just one night or less.  Phase fluctuation measurements have not been carried out on Mt. Graham.