Flight Demonstration of a Laser Hygrometer Payload for the ScanEagle UAS

Abstract:

Both science-based goals related to aerosol and cloud processes and feedbacks of prime importance in climate change science as well as the desire to improve the accuracy of near-term weather forecasts underlies continuing development of high performance sensors for airborne deployment. Recent work has shown that, by increasing the temporal and spatial resolution of data input into Numerical Weather Prediction (NWP) codes, the accuracy of near-term forecasts can be measurably improved. One means of increasing the input data density is to equip low altitude long endurance Unmanned Aircraft Systems (UASs) with appropriate sensors so that the UAS can function as a steerable (and reusable) weather balloon. While unable to reach the 100 kft altitudes to which weather balloons can sound, UASs can operate up to altitudes of ~15 kft. However, the majority of processes impacting weather occur within the lower troposphere and boundary layer that UASs can access. The benefits of high spatial and temporal data inputs to NWP have most recently been demonstrated during the ISARRA Flight Week in Boulder, CO in Summer 2018. 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 are developing a laser hygrometer payload for measurements of water vapor, temperature and pressure that is compatible with the payload resources of small UASs (such as ScanEagle) or medium size, multisensor payload UASs (such as ArcticShark). 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, present results of successful flight demonstrations on a ScanEagle UAS in collaboration with the University of North Dakota and InSitu, and preview plans for further development.