Development of a UAV-Based Differential Absorption Lidar for Measuring Atmospheric Water Vapor

Differential Absorption Lidar (DIAL) is a remote sensing technique useful for making range resolved measurements of chemical species. A water vapor DIAL system was developed for use on a high altitude unmanned air vehicle (UA V), in order to study water vapor distribution in hurricanes. The system uses a diode-pumped Cr:LiSAF laser to probe water vapor absorption lines at or near 815 nm. The receiver is comprised of two detectors, an analog mode avalanche photodiode (APD), and a photon counting APD, which gives the receiver a dynamic range of 5 orders of magnitude. Data acquisition is done with a 12-bit digitizer and a separate digital counter for the photon counting APD. The data is stored locally for retrieval at the end of the mission. The system was designed for fully autonomous operation.

The DIAL system was tested on the ground in a zenith looking configuration. The system demonstrated measurements of water vapor to a range of 1800 m, with a difference from local balloon sonde data of no more than 30 - 50 % over that range. Tests showed that the system measurement uncertainty was dominated by limitations of the digitizer, which will not be a factor during airborne operation. Estimates of airborne performance suggest that, flying at an altitude of 20 km, the system is currently capable of measuring water vapor between 2 and 7.5 km with an uncertainty of < 50%. This can be reduced to < 10% uncertainty with a factor of 3 increase in return signal. Refinement of the system is therefore recommended with an emphasis on increasing laser pulse energy.

This dissertation describes the development of the DIAL system including motivation for the research, computer modeling of the return signal, selection of system components, and the testing process at the component and system levels. The system design requirements and goals are presented and used as a basis for component selection. The test results are used to extrapolate an estimate of the airborne performance of the system, and recommendations for future development of the system are made based on these estimates.