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This paper summarizes a continuing effort to develop, design, construct, and evaluate the performance of an airborne autonomous wavemeter for laser tuning. The wavemeter supports tunable solid-state lasers that are used for atmospheric remote sensing. This sensing approach is the Differential Absorption Lidar (DIAL) technique. One atmospheric species, water vapor is measured by tuning one laser to line center of a water vapor line and by tuning another laser off the line. The two sets of received backscattered radiation are ratioed and corrected to determine the vertical profiles of water vapor. On a spacecraft platform, an advanced system could monitor the global water vapor profiles. This would provide a technology improvement for meteorological forecasting. One experiment to make water vapor DIAL measurements from a down-looking airborne platform is the NASA-LASE (Lidar Atmos pheric Sensing Experiment) Project instrument1 to be flown on a NASA ER-2 aircraft. Breadboard results have been obtained that predict that a wavemeter can be built to survive the harsh air-borne environment of low temperatures and low pressures that exist in high altitude flight. Based on the breadboard results, the airborne wavemeter consist of three stages of Fabry-Perot interferometers. Stages #1 and #2 provide the necessary information to provide laser wavelength centroid measurements to the required accuracy of less than 0-.25 picometers. This provides the real-time information necessary for tuning the two tunable alexandrite lasers for the water vapor measurement. A high resolution stage #3 interferometer provides the laser spectral profile measurement with an instrumental profile less than 0.5 picometers full-width at half-maximum amplitude for the post-flight science data reduction. To maintain interferometric stability, the first two stages are thermally controlled and contained in a vacuum chamber. A two level control approach is used to stabilize the interferometers above the highest ambient temperature. Techniques have been developed to evaluate the accuracy of the wavelength centroid algorithms. By using a single wavelength stabilized He-Ne laser as a perfect source, the end-to-end random errors (short term stability) are determined in a period too short for any thermal/mechanical response. The end--to-end sys
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