Long-Path Spectroscopy to an Airborne Retroreflector on a Quadcopter

The use of small unmanned aerial systems (sUAS) for atmospheric measurements is growing rapidly because of the ability to provide both horizontal and vertical spatial resolution. However, the payload restrictions of small systems limit the available instrumentation. In particular, high-sensitivity trace gas measurement systems for use at ambient concentrations are currently too heavy and large to fly on most sUASs. Here, we present a new measurement system, based on the technique of dual frequency comb spectroscopy, that addresses this difficulty by measuring the path-integrated concentrations of CO₂, CH₄, H₂O, and HDO between a ground station and a retroreflector mounted on a small quadcopter with high precision. The eye-safe frequency comb light is launched from a telescope on a fast azimuth, elevation gimbal to a lightweight retroreflector on a small quadcopter, which also carries a high-precision real-time kinematic GPS receiver for differential path length measurements and pressure, temperature, and humidity sensors. We track the motion of the quadcopter using an image-processing-based feedback system. Here, we will describe this system and discuss tests of applications such as quantifying small area emissions sources as well measuring vertical mixing within the boundary layer.