  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 station
employs a commercial low noise mixing block coupled to a 0.7 m off-axis
satellite dish which receives a broadband, white noise-like signal from a
geostationary satellite. The signals are processed by an analog correlator to
produce the phase delays between all pairs of stations with projected baselines
ranging from 33 to 261 m. Each baseline's amplitude and phase is measured
continuously at a rate of 8 kHz, processed, averaged and output at 10 Hz.
Further signal processing and data reduction is accomplished with a Linux
computer, including the removal of the diurnal motion of the target satellite.
The placement of the stations below ground level with an environmental shield
combined with the use of low temperature coefficient, buried fiber optic cables
provides excellent system stability. The sensitivity in terms of rms path
length is 1.3 microns which corresponds to phase deviations of about 1 degree
of phase at the highest operating frequency of the SMA. The two primary data
products are: (1) standard deviations of observed phase over various time
scales, and (2) phase structure functions. These real-time statistical data
measured by the API in the direction of the satellite provide an estimate of
the phase front distortion experienced by the concurrent SMA astronomical
observations. The API data also play an important role, along with the local
opacity measurements and weather predictions, in helping to plan the scheduling
of science observations on the telescope.

Instrumentation and Methods for Astrophysics;Atmospheric and Oceanic Physics;Instrumentation and Detectors