Introduction

Remote sensing of the atmosphere can be broadly classified into two categories, namely passive remote sensing and active remote sensing methods. In passive remote sensing, the source is beyond the control of the observer, e.g., radiometer, photometer, spectrometer etc. In active remote sensing, the source can be controlled by the observer, e.g., LIDAR, RADAR, SODAR, SONAR etc. In the field of remote sensing, lasers play an important role due to their inherent capability of generating well-collimated beams with outstanding characteristics of high coherence, monochromaticity, and directionality. Laser remote sensing techniques provide powerful tools for scientific studies of atmosphere, environmental monitoring,measurement of air quality parameters, remote sensing of oceans and rivers, remote assessment of vegetation etc. Developments in laser technology such as second harmonic generation, high power compact diode pumped solid-state lasers, and tunable solid-state lasers etc. have opened new possibilities of laser remote sensing to explore the earth's atmosphere. Laser remote sensing of the atmosphere is generally referred to as LIDAR, the acronym for Light Detection And Ranging. Similar to radar, in lidar, a laser pulse is sent into the atmosphere and is used as a spectroscopic probe of its physical state and chemical composition. The emitted laser beam interacts with the atmospheric constituents causing alterations in the intensity, polarization and wavelength of the backscattered light. From the measurements of these parameters of the received backscattered light, one can deduce the properties of the atmosphere and its constituents. Lidar methods allow range resolved measure- ment to obtain a vertical profile of the atmospheric parameters. The distance of the scattering medium can be deduced with high accuracy from the time delay of the return signal. Lidar systems can be operated in the wavelength range extending from the ultraviolet to the infrared (UV to IR) by using different types of lasers. Using the techniques of scattering, absorption, resonance and fluorescence, lidar can measure solid particulate matter such as aerosols, species of very low concentrations such as ozone, water vapour and metal atoms such as Na, K etc. Lidar measurements of the Rayleigh scattering from neutral atmosphere can be used to determine neutral density, pressure, temperature, and wind speed. Experimental measurements made by many groups all over the globe in the past decade have demonstrated the reliability of various lidar techniques[1−3].