Methodology Improvements for Three-Dimensional UAV-Based Travel-Time Acoustic Atmospheric Tomography

Abstract

Abstract This paper describes a method for measuring continuous, three-dimensional temperature and wind velocity patterns in the atmospheric surface layer (ASL) using unmanned aerial vehicle–based acoustic atmospheric tomography (UBAAT). An unmanned aerial vehicle (UAV) is flown over an array of microphones on the ground. The travel time for sound rays between the UAV and each microphone is used to reconstruct 3D temperature and wind velocity fields, with the continuous motion of the UAV generating far more ray paths over much greater volumes of atmosphere than can be obtained using static speakers and microphones. Significant improvements over previous UBAAT techniques include the use of a synthetic tone rather than the natural sound generated by the UAV, use of vertical temperature and wind profiles to improve modeling of sharp changes near the ground, normalization of observations to incorporate weighted least squares techniques within Tikhonov regularization, and normalization of the model matrix to reduce bias in estimating modeling parameters when using Tikhonov regularization. This is the first case where UBAAT has been performed in three dimensions and also compared with independent temperature and wind velocity measurements. A summary of the results of simulation studies and trials results is provided, which shows that UBAAT can estimate three-dimensional temperature and wind velocity fields in the ASL with useful accuracy (approximately 1°C for temperature and 1 m s−1 for wind speed). Significance Statement This paper describes a method that is capable of providing a set of continuous 3D estimates of the temperature and wind velocity patterns in the first few hundreds of meters above the ground. It is like performing an ultrasound or MRI on the atmosphere. There is a need for good wind speed measurements to identify and examine the properties of the atmospheric surface layer (ASL). Features of interest include the nocturnal low-level jet, elevated inversions and temperature structures, wind flow over complex terrain, gravity waves, and wave propagation through a turbulent atmosphere.