Comparative Unsteady Atmospheric Boundary Layer Observations Using Active And Passive Systems

Measuring of atmospheric boundary conditions is normally reserved for balloon mounted sensors that are considered disposable. These sensors are high cost and are at the mercy of wind, weather and atmospheric conditions. This creates a unique problem when high accuracy, localized measurements are required. Small Unmanned Aircraft Systems (sUAS) are one option to replace radiosondes with a system for low to medium altitude measurements across micro and meso scales. These DroneSondes may offer greater capability with the benefit of repeated, albeit with additional complexity based on control requirements. This paper presents a discussion on comparison between active and passive control of DroneSonde systems for unsteady boundary layer measurements.

Oklahoma State University is developing a low-cost all-in-one solution to this problem (Sonde v3.0). Using high accuracy integrated sensors, high speed microcontrollers, onboard EEPROM and radio transmission, boundary layer conditions can be accurately measured using active (drone mounted) or passive control (balloon, rocket or dropping). Sensors contained include GPS, barometer, inertial measurement unit, humidity, temperature, light, and wind. The accuracy and frequency of these sensors meet or exceed requirements laid out by the national weather service and report data in NetCDF format to easily integrate into forecast models.

Sensor packages that are drone mounted can be deployed to a specific location and/or preprogrammed to profile an area. All data is reported to a ground control station via 900 MHz radio links and is capable of multiple sensors at once. High speed data is recorded to the onboard EEPROM for later off loading and lower speed data is reported through radio links to give a real-time view of atmospheric conditions.

When mounted to a drone, the sensor can be run independently for up to one hour or it can use onboard power to extend life if necessary. When under passive control, battery life is restricted to typically one hour depending on the battery size nad power draw. After a data collection mission is completed the sensors broadcast their location and make an audible cue for easy recovery when the sensors are dropped from an aircraft. After all data is collected, the ground control station fuses all data streams and produces two files to the user. The first being in NetCDF format for uploading to the National Weather Service data servers and the second in CSV format for easy post-processing of data.

In order to prove accuracy and precision of these instruments, we conducted calibration and validation tests in unsteady flows using the OSU gust tunnel for each sensor system to ensure that the results accurately represent the actual environment. Throughout these validation tests, the sensors will be analyzed for their static scenarios as well as their response time to highly dynamic environments. Results from tests in unsteady boundary layers of actively controlled swarms are compared with passive control, i.e., Lagrangian drifters, up to 3,000 ft during varying atmospheric conditions.