A Method of Determining the Impulse Response Function for an Elastic Scattering Lidar-Nephelometer with Coaxial Sounding Scheme. Part I

Abstract

A coaxial sounding scheme in a pulsed lidar and nephelometer regime used for measurements of the intensity of propagation of a beam within a layer and a given extinction coeffi cient is considered. Measurement of the impulse response function by means of perforated screens is evaluated in the nephelometer regime.Questions of calibration of the existing remote sounding instruments (lidar) in the optical range have still not been resolved. The ultimate goal of calibration of any lidar is the measurement of the base parameters of the scattering medium (atmosphere) [1]. These parameters include the backscatter factor and the extinction coeffi cient. In the course of calibration, it is necessary to measure the signal from a homogeneous atmosphere without attenuation (inpulse response function), the route dependence of the coverage of the visual fi eld with the sounding beam (geometric form factor), the angular dimensions of the visual fi eld and the sounding beam, etc.The problem of the near zone, which is where the angular characteristics of the visual fi eld and the sounding beam associated with the geometry of a partially coherent beam and integration of rapidly oscillating functions are formed, must be solved in the process of measuring the impulse response function. Since information for modeling is often insuffi cient, the impulse response function is measured experimentally in the near zone determined from the route dependence of the signal on the scattering surfaces. During the measurements, the orientation of the surface relative to the sounding beam must not vary along routes upwards of several hundred meters in length. For far distances, edge effects may appear, where a part of the beam passes out of the target plane. The angular scattering diagram, including diffuse and specular components, must be known for calibrated scattering surfaces. Note that, under laboratory conditions, it is possible to detect radiation from foreign objects. In the case of an open-air site, the experiment will depend on atmospheric conditions. Within the near zone, the signal from a scattering surface may exceed by several orders of magnitude the return signal from the atmosphere. This entails the use of attenuators to weaken the energy of the beam by several orders of magnitude. Distortions of the route dependence of the geometric form factor are possible. These and other problems are critical for miniature lidars, since for such lidars practically the entire return signal from the atmosphere is detected in the near zone [2].