Simulation of coherent Doppler LIDAR signals and their analysis with the Cohen's class : application to algorithms design for wake vortex detection and characterization

The problem of wind statistics measurements in the planetary boundary layer using a coherent Doppler LIDAR is addressed. More specifically, it focuses on the design of efficient algorithms in the time-frequency space dedicated to the detection and the characterization of atmospheric hazards, such as aircraft wake vortices or wind shears. To support this study, a simulation program has been developed which combines the numerical simulation of LASER beam propagation in a turbulent medium with state-of-the-art Large Eddy Simulation (LES) of wake vortex in atmospheric turbulence or in ground effect. This tool constitutes a complete framework for optimizing the LASER source while developing and evaluating the performance of the whole estimation process. Moreover, an analytical formulation of the coherent LIDAR Wigner-Ville spectrum has been derived along with its associated Cohen’s class of time-frequency estimators. This theoretical basis leads to explicit equations of the wind statistics that are actually retrievable, depending on the measurement conditions and the laser pulse parameters. It also yields to an adaptive spectral modeling algorithm which is used to detect the wake vortices and then estimate their wind speed probability density function as well as their extremum radial velocity profiles, related to the vortex maximum tangential velocity. Another major achievement of this thesis is the development and the validation of a fast inversion method dedicated to wake vortex characterization, e.g. the estimation of its position and intensity. In this method, a Burnham-Hallock model, transformed by a new measurement model integrating the effects of the estimation process, is directly matched to the retrieved radial velocity map. Monte Carlo simulations have been carried out on both analytical models and LES to confirm its good performance. Finally, the validation of the axial detection of vortices with a fiber-based LIDAR has been performed during a test campaign conducted by ONERA at Orly airport in April 2008 during the FIDELIO project. Wake vortex signatures have been successfully obtained with some of the algorithms developed in this thesis.(FSA 3) -- UCL, 201

Numerical simulation of a heterodyne Doppler LIDAR for wind measurement in a turbulent atmospheric boundary layer

This study concerns the modeling and the design of a monostatic heterodyne pulsed LIDAR. The heart of the system is constituted of a 1.55 mu m able to produce high pulse repetition frequency. The aim of this work is to assess its efficiency to perform accurate wind speed measurements in the low atmospheric boundary layer, from a 2-D scanning pattern, in the presence of refractive turbulence. A complete LIDAR numerical simulation technique has been developed. Its main originality is the integration of both optical and fluid dynamics numerical methods to take into account the signal coherence loss due to refractive turbulence and speckle effect as well as the fine structures of the wind field. The wind speed profiles along each line-of-sight are retrieved from the return signal using a low-order autoregressive model. An adequate averaging model is then used estimate horizontal components of the wind speed for altitudes up to 150 m.Anglai

Methodological Study of Geometric Deformation for CBCT in Proton Therapy Gantry

Cone-Beam CT systems allow in-situ imaging for guiding radiation therapy by 3D reconstruction of radiographs acquired during gantry rotation. However, deformation of gantry geometry is a primary source of reconstruction artifacts including edge blurring, loss of spatial resolution, truncation, and streaks [1]. Deviations from the ideal X-ray source and detector trajectories are caused by reproducible and random deformation. Whilst the first can be corrected by an appropriate calibration procedure, the second type shows an unpredictable pattern although its magnitude is small compared to reproducible deformation. Several methods have been proposed for correcting geometric non-idealities in tomographic imaging systems such as C-arm CT, cone-beam breast CT or LINAC-integrated cone-beam CT [2,3,4]. However, the magnitude of geometric deformation in a proton therapy gantry is typically higher than in other systems due to gantry geometry with larger source to detector distance, larger bearing structure and gravity-induced flex in the support arms. Hence, these differences limit the application of calibration methods described in the literature and justify the need for a complete simulation framework on geometric deformation in a proton therapy gantry. This study is part of the development of a calibration routine for a proton therapy gantry using a phantom with radio-opaque markers (spheres) and full sets of radiographs acquired while rotating the gantry. The calibration parameter values comprising source displacement, flat panel displacement and rotation angles, and gantry angle offset are found by using projection matrices of the object onto the detector. The method implies for each gantry angle the projection of phantom markers onto the flat panel assuming no deformation, the identification of spheres on the radiograph, and registration of nominal and measured markers positions using optimization techniques. This paper aims at developing a comprehensive test bench for assessing the geometric deformation of a proton therapy gantry, comprising a software simulator validated by in-situ measurements. A set of simulations is performed to quantify the impact of phantom geometry, marker detection rate, and incorrect estimation of marker position and choice of optimization technique on the accuracy of the calibration routine. The correlation between source and detector offsets, which is known to make the optimization unstable, is also investigated. Moreover, this test bench provides an effective tool for evaluating the performance of the calibration routine and straightforward assessment of potential improvements to the current technique. Finally, the knowledge acquired with the proposed methodological study is used to set optimal conditions for the application of the calibration routine in data sets acquired with a proton therapy gantry. Preliminary measurements of gantry geometry enable to fit models describing the gantry deformation depending on the gantry angle. These deformation models have been used to improve reconstruction in Cone-Beam CT