For several years, tunable solid-state lasers have been used for atmospheric remote sensing. One approach is the Differential Absorption Lidar (DIAL) technique. With this technique, one atmospheric species, water vapor is measured by tuning one laser to the center of a water vapor line and by tuning another laser off of the line. The data are used by scientists to determine the vertical profiles of water vapor. One experiment, to make water vapor DIAL measurements from a down-looking airborne platform, is the NASA-LASE (Lidar Atmospheric Sensing Experiment) Project instrument to be flown on a NASA ER-2. The LASE wavemeter device, a classical Fabry-Perot interferometer approach, pro-vides wavelength control to form an autonomous system to calculate the wavelength centroid of each laser pulse. This real-time information is used to accurately tune the lasers to the required wavelengths of the atmospheric species. The difficulty in implementing this wavemeter is providing the performance requirements in the ER-2 environment. The temperature can vary from 10°C to 40°C and the pressure from 14.7 psi to 3.5 psi. To solve the thermal problem, the wavemeter optical components are enclosed in a housing that is controlled above the 40°C highest expected temperature at a tempera-ture of 43Ã‚Â°C. The interferometer is mounted in a thermal-vacuum chamber that is controlled to 45°C. The interferometer in vacuum provides an optical path with no refractive index variation. With the temperature control implemented, the next prob-lem was how to mount the interferometer in the thermal-vacuum chamber with little mechanical stress from the mounting fixture. After several configurations were investigated, a three fingered flexure provides a mounting to the stability of the laser source and of the electronic readout system used for the measurement. Fabry-Perot interferometers have been constructed of low thermal expansivity materials to form a three stage instrument. Two stages operating in a "bootstrap" mode provide the wavelength centroid measurement. The third stage provides data to retrieve the wavelength profile. Several classical wavelength reduction algorithms have been implemented that routinely exhibit an "end-to-end" system random noise of less than ±0.06 picometer.