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The system alternates between three viewing modes directed by
a rotating plane mirror, with an uncoated gold surface, which is set
at a 45
angle to the source. Observation modes include an
ambient blackbody calibration source (dwell time average of 1.5 minutes), hot
reference blackbody calibration source (1.5
minutes) maintained at 334 K, and zenith atmosphere view (3 minutes).
This combines to yield a data sample rate of approximately 7 minutes
after adding data transfer time. The scan time for each mode was
reduced by one-third, relative to standard operation in the field, to
obtain a greater temporal resolution for the instrument. This
facilitates comparison to HSRL data. The blackbody calibrations are
required to interpolate the atmospheric observation from a pair of
spectrally characterized targets.
The blackbody references are high emissivity cavities developed at the UW, consisting of copper with a diffuse black overcoat. They are thermally monitored with high precision thermistors and controlled with a feedback servo (Revercomb et al., 1993). A third blackbody source, maintained at 77 K, is used to periodically monitor the stability of the hot and ambient calibration, and to adjust for nonlinear effects.
A data system of two networked IBM PC computers handle data acquisition, system control, and data analysis. The first computer, located with the interferometer system, controls timing of the viewing modes and storage of the raw interferometric data. This raw data is transferred, following each view mode, to a second computer; where the data is processed, stored, and displayed.