The JPL Near-IR Laser Hyrometer is a new instrument for in-situ measurements of atmospheric water vapor from aircraft platforms such as the ER-2, the DC-8, and the WB57F aircraft. It is based upon near-IR tunable diode laser sources operating near 1.37 m m. These spectrometers feature open-path absorption cells [double-pass, or multipass (Herriott)] for true in situ monitoring of H2O concentrations with precision levels exceeding those of Lyman-a and frost-point hygrometers. External sampling outside the aircraft boundary layer minimizes ambiguities. Measurement precision is ± 0.05 ppmv in the stratosphere for a 2 s measurement integration period. The flight-ready instrument weight is 18 lbs., and power consumption, exclusive of structural heaters, is 7.5 Watts. The picture below shows the ER-2 instrument as mounted on the right-hand super pod for the POLARIS mission. This same instrument was flown on the NASA WB57 as part of WAM (WB57 Aerosol Mission) during April, 1998, and was mounted on an underwing hatch on the right wing of the aircraft.
"Open-Path, Near-IR Tunable Diode Laser Spectrometer for Atmospheric Measurements of H2O", R.D. May, J. Geophys. Res., 103, 19,161-19,172 (1998).
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The DC-8 version of the laser hygrometer is shown here mounted on a window blank of the DC-8 at fuselage station 490 right. The optical path is 50 cm and is a "there and back" double pass arrangement. The upper blue housing holds the laser and detector, and the lower piece holds a single 0.5" diameter mirror for returning the beam to the detector. Both the laser and detector are mounted on a single Al disk that is stabilized in temperature using a thermoelectric cooler (which actually operates as a heater most of the time during flights).
This instrument is currently (Fall 1998) mounted on the DC-8 as part of the Convection and Moisture Experiment (CAMEX-3) where flights are underway from Patrick Air Force base near Cocoa Beach, Florida.