Ames Airborne Tracking Sunphotometer, AATS-14

The 14-channel NASA Ames Airborne Tracking Sunphotometer (AATS-14, Fig. 1) measures the transmission of the solar beam in 14 spectral channels. Azimuth and elevation motors controlled by differential sun sensors rotate a tracking head, locking onto the solar beam and keeping detectors normal to it. The AATS-14 tracking head mounts outside the aircraft skin, to minimize blockage by aircraft structures and avoid data contamination by aircraftwindow effects. Each channel consists of a baffled entrance path, interference filter, photodiode detector, and preamplifier. Filters are at wavelengths 354 2139 nm (Fig. 2), chosen to allow separation of aerosol, water vapor, and ozone transmission. The filter/detector sets are temperature-controlled to avoid thermally-induced calibration changes. Detectors in the two longestwavelength channels incorporate thermoelectric coolers. The other 12 channels are maintained at an elevated temperature by foil heaters.