sUAS-based Payload Development and Testing for Quantifying Optical Turbulence

Abstract

Small UAS (sUAS) are a cost-effective and easy to use solution to fill the niche between surface-based measurement platforms (e.g. buoys, WaveRider, etc.) and measurements with manned aircraft, providing a three dimensional view of the lower atmosphere. Due to their small size, sUAS can also be used in difficult to reach areas such as within a wind farm at altitudes that are difficult for tower based measurements. Because of these reasons, there have been many developments of sUAS in the past decades for meteorological applications. One important application of the sUAS-based environmental sampling is to quantify the lower atmosphere to initialize or constrain forecast models or as an independent validation for model evaluation, and may also be used to support tactical decisions, aiding in the prediction of electromagnetic (EM) wave propagation and propagation of high energy laser (HEL), where the atmospheric refractive properties are dependent on state variables such as pressure, temperature, and humidity in the lowest 2 km of the atmosphere. Quantifying the low level gradients of these variables is critical for predicting radar and communication signal and HEL propagations through the atmosphere.

Our recent development in sUAS instrumentation focused on sensing capability to quantify optical turbulence in the lower atmosphere that has important applications in characterizing atmospheric effects on free-space optical communication and high energy laser weapon performance in the atmosphere. These electro-optical (EO) systems are mainly affected by atmospheric scintillation quantified by the structure function parameter(Cn2) of the atmospheric index of refraction. Although water vapor is one of the variables determining the index of refraction in the optical wavelength, a large component of Cn2 is the temperature structure parameter, CT2, and can be calculated with high-rate temperature perturbation measurements. Hence, our focus was on developing a payload to quantify high-rate temperature perturbations in addition to mean meteorological variables such as wind, temperature, humidity, and pressure.

The airframe we use is adapted from the Finwing Penguin built for the first-person-viewpoint (FPV) hobby market. The airframe includes a raised pusher propeller scheme that places the propulsion downstream of the met sensors, and shields the operator from the propeller during hand-launches. The basic meteorological sensors include the self-recording multi-parameter weather sensor (InterMet XQ) and modified radiosonde (iMet-1) to obtain GPS coordinates, pressure, temperature, and relative humidity data. Data from the flight controller, pitot-static tube, and IMU systems were processed to retrieve mean wind speed and direction. For fast temperature measurements, we integrated a fine-wire (0.001 in) thermocouple and small diameter (0.02 in) Pt100 RTD probe into the Penguin. The thermocouple does not provide highly accurate measurements due to its non-linear response and the need for a second temperature measurement at the cold-junction. Albeit slower responding, the more accurate Pt100 temperature sensor is deployed to measure the mean air temperature and adjust the thermocouple's mean temperature component.

In this presentation, we will introduce our effort of Penguin payload development. Results from the most recent Penguin test flights at McMillan Air Field, Camp Roberts, CA will be used to demonstrate the sampling capability. We will also show results from our ground evaluation tests at Marina Airport where the Penguin system made measurements side-by-side with proven sensors and data acquisition systems. The purpose of the ground test was to verify thermocouple amplifier provides sufficient time-resolution to resolve thermal plumes. The perturbation data from both sonic anemometer and Penguin data will be used to evaluate the capability of obtaining CT2 from a sUAS.