INVITED: In Situ Sensing in Convective Storms and Air Masses with Small Unmanned Aircraft Systems

Since the spring of 2009, the unmanned aircraft system (UAS) team of engineers from the University of Colorado Boulder's Research and Engineering Center for Unmanned Vehicles (RECUV) and meteorologists from the University of Nebraska-Lincoln have developed and deployed several small UAS for in situ sensing missions into mesoscale phenomena, including six supercells during the spring of 2010. The first mission was conducted using the NexStar UAS as part of the Collaborative Colorado-Nebraska Unmanned Aircraft System Experiment (CoCoNUE) on 30 Sep 2009 over the Pawnee National Grassland (PNG) in northeast Colorado. Doppler radar data from the Pawnee and CHILL radars (operated by Colorado State University) were used to target a fast-moving cold front and outflow boundary. The UAS mobile ground station was positioned to launch the unmanned aircraft (UA) to follow the “tracker vehicle,” an automobile with a WiFi radio tracked by the UA to create an “electronic tether” to keep the UA within the visual range of an observer inside the tracker vehicle. A Miniature In-situ Sounding Technology (MIST) sonde was carried in the wingtip of the UA. During its 31-min flight at a nominal altitude of 265 m (1,858 m MSL), the UA crossed the cold front followed by a transect of the gust front from a thunderstorm crossing the Colorado-Wyoming border into the PNG. MIST sonde PTH measurements were telemetered to the ground station along with the GPS position. Wind velocity was estimated by the autopilot based upon the GPS data and measured flight dynamics. Meteorological data included vertical temperature, pressure, and humidity profiles during the ascent/descent of the UA and lateral profiles that show airmass boundary crossings.

The Tempest UAS was developed by the Colorado-Nebraska team and deployed as part of the second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2) field campaign in spring 2010. The primary goal of the Tempest UAS deployment was to validate the team's capability to meet the regulatory requirements of the Federal Aviation Administration (FAA), to demonstrate a concept of operations to track and intercept supercells, and to return significant meteorological data. The Tempest UA is based upon an airframe first developed for a high-performance radio-controlled glider. At 3.2 m, the wingspan of the Tempest UA is about twice that of the NexStar, its nominal takeoff weight of 5.4 kg is about 20% greater, with an endurance approximately twice that of the NexStar. Similar to the NexStar UA, the Tempest UA carried a MIST sonde with a similar autopilot, flight computer, and radios operating at 900-MHz and 2.4-GHz. Using lessons learned from the CoCoNUE field deployment and mock Tempest UAS deployments during the spring 2009 VORTEX2 field campaign, the NetUAS (Networked UAS) communications, command, and control software suite was upgraded with expanded capabilities and reliability. The mobile ground station was moved from a trailer towed by a large SUV into a customized 15-passenger van. In addition, a “scout vehicle” was added increasing the number of ground vehicles to three. The scout vehicle was equipped with a mobile mesonet, so that in addition to scouting the roads ahead of the tracker vehicle, it provided additional meteorological data from the ground. The 6 May 2010 intercept of a supercell near Oberlin, KS was the first ever intercept of a supercell with a small UAS and the first of six supercell intercepts during the field campaign. The final intercept occurred 10 June 2010 of a tornadic supercell near Deer Trail, CO. PTH data indicated the crossing of two gust fronts from the rear flank downdraft as the Tempest UA approached the northeast-moving supercell from the south at a nominal altitude of 305 m. Flight data indicated that the Tempest UA encountered a number of local updrafts and downdrafts behind the gust-front crossings as it approached the supercell.

The UAS team is now expanded to include collaborators from Texas Tech University and the National Severe Storms Laboratory. The team continues to develop capabilities for research on mesoscale phenomena, including improvements to the engineering performance of the Tempest UAS. In the current project “Energy-Aware Aerial Systems for Persistent Sampling and Surveillance,” funded by the Air Force Office of Scientific Research (AFOSR), the team seeks to extend the performance of the Tempest UA by locating and harvesting energy from updrafts in the environment. During the VORTEX2 deployment, the Tempest UA autopilot was observed to set the throttle to zero, allowing the UA to use local updrafts to soar while maintaining ground speed necessary to follow the tracker vehicle. The UAS team proposes to develop path planning algorithms that integrate atmospheric modeling coupled with airborne measurements and mobile radar data to greatly enhance the endurance and range of the UA.

A brief recap of how engineers and atmospheric scientists worked together to develop and deploy the NexStar and Tempest UAS for the CoCoNUE and VORTEX2 field deployments is provided. This is followed by a discussion of some of the current challenges for small UAS including accurate wind-velocity measurements, flights in precipitation, and FAA regulatory compliance. An update on the current Energy-Aware Aerial Systems for Persistent Sampling and Surveillance project is then provided that includes a discussion of the integration of a five-hole air data probe into the Tempest UAS for high-fidelity wind velocity measurements.