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.