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We have constructed a five station 12 GHz atmospheric phase interferometer(API) for the Submillimeter Array (SMA) located near the summit of Mauna Kea,Hawaii. Operating at the base of unoccupied SMA antenna pads, each stationemploys a commercial low noise mixing block coupled to a 0.7 m off-axissatellite dish which receives a broadband, white noise-like signal from ageostationary satellite. The signals are processed by an analog correlator toproduce the phase delays between all pairs of stations with projected baselinesranging from 33 to 261 m. Each baseline's amplitude and phase is measuredcontinuously at a rate of 8 kHz, processed, averaged and output at 10 Hz.Further signal processing and data reduction is accomplished with a Linuxcomputer, including the removal of the diurnal motion of the target satellite.The placement of the stations below ground level with an environmental shieldcombined with the use of low temperature coefficient, buried fiber optic cablesprovides excellent system stability. The sensitivity in terms of rms pathlength is 1.3 microns which corresponds to phase deviations of about 1 degreeof phase at the highest operating frequency of the SMA. The two primary dataproducts are: (1) standard deviations of observed phase over various timescales, and (2) phase structure functions. These real-time statistical datameasured by the API in the direction of the satellite provide an estimate ofthe phase front distortion experienced by the concurrent SMA astronomicalobservations. The API data also play an important role, along with the localopacity measurements and weather predictions, in helping to plan the schedulingof science observations on the telescope.

A Multi-Baseline 12 GHz Atmospheric Phase Interferometer with One Micron Path Length Sensitivity