A Laser Hygrometer Payload for the ScanEagle UAS

The rate of climate change in the Arctic is larger than elsewhere on Earth. Our degree of understanding of this acceleration remains incomplete because 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. One example is the retreat of the summer ice edge, which may be driven by an increase in the downwelling long wave radiative flux in the Spring. This increase may be linked to an increase in atmospheric water vapor and low level clouds. Mixed phase clouds also affect the radiative balance. Improved understanding of the formation and stability of mixed phase clouds is a research priority and requires measurement of the supersaturation conditions within them. There is a need for a miniature airborne sensor payload for profiling the thermodynamic state of the atmosphere that measures water vapor, temperature, and pressure with sufficient precision to derive supersaturation.

Physical Sciences Inc. and Princeton University together are developing a laser hygrometer payload for measurements of water vapor, temperature and pressure that is compatible with the payload resources of small Unmanned Aircraft Systems (UASs) (ScanEagle or ArcticShark). Drawing on prior experience acquired during development of airborne laser hygrometers for manned aircraft, our payload is an in-situ, open path design in which the optical absorption path is created using a pylon placed in the free airstream. Judicious choice of the diode laser wavelength of 2.7 μm provides sensitivity sufficient to enable measurements of supersaturations in the 100-120% RH(ice) range with a very small optical pathlength. Our sensor design has been adapted to operation in icing conditions, by virtue of a short optical path, pylon heating, and use of hydrophobic coatings. We will review the engineering design of the payload, sensor characterization and plans to flight demonstrate it on a ScanEagle UAS in collaboration with University of North Dakota.