Compact Laser Hygrometer for In Situ Measurement of Water Vapor from Small Unmanned Aerial Vehicles

The rate of climate change in the Arctic is larger than elsewhere on Earth. The Arctic has unique and complex couplings and feedbacks between the surface and the atmosphere that in turn modify the radiative balance there differently than elsewhere. Current understanding holds that an increase in downwelling long wave radiative flux, driven by increased water vapor and mixed phase clouds, may be accelerating climate change. There is a need to measure the thermodynamic state (water vapor, temperature and pressure) of the Arctic troposphere in order to better understand the development and impact of these clouds. A new compact sensor payload deployed on a small Unmanned Aircraft System is an efficient route to providing the data needed to advance our understanding. The overall objective of the Phase I and II project is to demonstrate the feasibility of a compact laser hygrometer payload to make high precision measurements of water vapor from a small unmanned aircraft. The payload is based on a diode laser optical absorption sensor and sensitive detection technology. The Phase I program demonstrated the feasibility of the laser hygrometer payload design through modeling and laboratory experiments. The design for a flight- worthy, compact sensor was developed for the precision and accuracy required for the target measurements. Laboratory experiments demonstrated the required measurement precision, accuracy, and sensitivity. System size, weight, and power estimates were developed. The design is compatible with the payload resources of the target small unmanned aircraft system. In the Phase II program, the design for an engineering prototype will be completed. The prototype will be fabricated, undergo calibration verification, environmental testing, and other preflight qualification testing. The engineering prototype will then be integrated into a demonstration small unmanned aircraft and demonstration missions flown. Commercial Applications and Other Benefits: The proposed laser hygrometer will enable measurements of water vapor from low altitude long endurance unmanned air systems to support studies of mixed phase clouds and their role in climate change in the Arctic. As larger unmanned aircraft are integrated into the national airspace, they will be used to collect meteorological data for numerical weather predictions. This is a logical extension of the Aircraft Meteorological Data Relay program that collects meteorological data worldwide from commercial aircraft. The proposed laser hygrometer will be a key component of that meteorological payload.