Showing papers by "Adolfo Comerón published in 1998"

Assessment of lidar inversion errors for homogeneous atmospheres

Abstract: The inversion of lidar returns from homogeneous atmospheres has been done customarily through the well-known slope method. The logarithmic operation over the range-corrected and system-normalized received signal used in this method introduces a bias in the statistics of the noise-affected processed signal that can severely distort the estimates of the atmospheric attenuation and backscatter coefficients under measurement. It is shown that a fitting of the theoretically expected exponential signal to the range-corrected received one, using as the initial guess the results provided by the slope method and a least-squares iterative procedure, can yield enhanced accuracy under low signal-to-noise ratios and especially in moderate-to-high extinction conditions.

Adaptive filter solution for processing lidar returns: optical parameter estimation

Abstract: Joint estimation of extinction and backscatter simulated profiles from elastic-backscatter lidar return signals is tackled by means of an extended Kalman filter (EKF). First, we introduced the issue from a theoretical point of view by using both an EKF formulation and an appropriate atmospheric stochastic model; second, it is tested through extensive simulation and under simplified conditions; and, finally, a first real application is discussed. An atmospheric model including both temporal and spatial correlation features is introduced to describe approximate fluctuation statistics in the sought-after atmospheric optical parameters and hence to include a priori information in the algorithm. Provided that reasonable models are given for the filter, inversion errors are shown to depend strongly on the atmospheric condition (i.e., the visibility) and the signal-to-noise ratio along the exploration path in spite of modeling errors in the assumed statistical properties of the atmospheric optical parameters. This is of advantage in the performance of the Kalman filter because they are often the point of most concern in identification problems. In light of the adaptive behavior of the filter and the inversion results, the EKF approach promises a successful alternative to present-day nonmemory algorithms based on exponential-curve fitting or differential equation formulations such as Klett’s method.

Confirmation of a multilayer arrangement of aerosols in the Barcelona air basin using two independent lidar systems

Abstract: In 1992 an experimental campaign took place in Barcelona whose main goal was the study of the circulatory patterns of air pollutants in the region. An elastic-backscatter lidar operating at a wavelength of 1.064 micrometers , being mainly sensitive to changes in the distribution of aerosols was used. The study of temporal series of lidar vertical scans revealed a multilayer arrangement of the aerosols above the city. A few years after that experiment, the UPC started to develop its own lidar system. The first stadium of the project was the construction of an elastic-backscatter lidar. In 1997, in the frame of a meteorological situation very similar to the one present during the first campaign, the UPC lidar acquired vertical profiles of elastic- backscatter in the Barcelona's atmosphere. The study of the temporal series also showed the development of elevated layers of aerosols at similar altitudes to the ones observed during the first campaign. The second experiment seems to confirm that the formation of elevated layers in Barcelona is a typical behavior for the region during the summer period and not just a particular situation.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Numerical simulation of long-path spherical wave propagation in three-dimensional random media

Abstract: This communication presents a new method to overcome some limitations in the simulation of the propagation of waves originating from a point source through a very long path in a turbulent medium. Existing propagation simulation algorithms suffer from either windowing or lack of resolution when applied to long paths. If Cartesian coordinates are used, the limited size of the numerical mesh originates undesired windowing errors in the long run. Casting the classical split-step Fourier algorithm in a spherically diverging coordinate system can solve this problem. In this way an angular mesh that adapts the source and the propagation algorithm to the geometry of the problem is used. But in long path propagation, this spherical divergent mesh causes a loss of resolution that can become a serious problem in the evaluation of the field statistical moments.

Inversion of atmospheric optical parameters from elastic-backscatter lidar returns using a Kalman filter

Abstract: Up to now, retrieval of the atmospheric extinction and backscatter has mainly relied on standard straightforward non-memory procedures such as slope-method, exponential- curve fitting and Klett's method Yet, their performance becomes ultimately limited by the inherent lack of adaptability as they only work with present returns and neither past estimations, nor the statistics of the signals or a prior uncertainties are taken into account In this work, a first inversion of the backscatter and extinction- to-backscatter ratio from pulsed elastic-backscatter lidar returns is tackled by means of an extended Kalman filter (EKF), which overcomes these limitations Thus, as long as different return signals income,the filter updates itself weighted by the unbalance between the a priori estimates of the optical parameters and the new ones based on a minimum variance criterion Calibration errors or initialization uncertainties can be assimilated also The study begins with the formulation of the inversion problem and an appropriate stochastic model Based on extensive simulation and realistic conditions, it is shown that the EKF approach enables to retrieve the sought-after optical parameters as time-range-dependent functions and hence, to track the atmospheric evolution, its performance being only limited by the quality and availability of the 'a priori' information and the accuracy of the atmospheric model assumed The study ends with an encouraging practical inversion of a live-scene measured with the Nd:YAG elastic-backscatter lidar station at our premises in Barcelona