Laser Remote Sensing

 INTRODUCTION 

 From Visual Perception to Lidar

 Sensual perception of remote objects by processing of radiative stimuli is a capability widely encountered in animals. Humans perceive only passively, that is, see and hear objects that either radiate themselves or scatter and re-transmit radiation from an external source. Some animals can locate objects actively as well, using optical or acoustical radiation they generate on purpose. 

Laser Remote Sensing

edited by Takashi Fujii Central Research Institute of Electrical Power Industry Tokyo, Japan 

Tetsuo Fukuchi 

Central Research Institute of Electrical Power Industry Tokyo, Japan

Several species of fish literally carry lanterns. Horses and elephants are known to utilize active acoustic means for orientation in complete darkness. Bats use it to locate and catch prey. Man has been using illumination with visible light from artificial sources for active optical detection of objects. Distance is inferred stereoscopically, that is, from the slightly different images obtained at the viewing angles of the two eyes, by mental construction, from two two-dimensional images, of a three-dimensional geometric relationship between different parts of the scene, or, for more remote objects, from the decrease of visual contrast. Fuji and Fukuchi / Laser Remote Sensing DK1181_c001 Final Proof page 2 18.5.2005 7:23pm 2 Weitkamp Except for stereoscopic viewing, which fails at longer distance, these methods yield relative values only. Distance can be determined in a quantitative way by measuring the transit time of radiation from the source to the object and back. Systems that rely on this principle require a pulsed or modulated source and a detection system with adequate time resolution, approximately a million times better in the optical case than in the acoustical case. Man’s senses do not nearly meet this requirement. Therefore, the technique became available to us only after the advent of suitable microwave, light, and sound sources and time-resolving detection systems. Depending on whether sound, radiowaves, or light is used, these systems are called SONAR (Sound Navigation Ranging) or SODAR (Sound Detection And Ranging), RADAR (Radio wave Detection And Ranging), or LIDAR (Light Detection And Ranging). Sonar works under water, the remaining techniques in the atmosphere. Lidar uses not just visible wavelengths (400 nm < l < 700 nm), but also ultraviolet (225 nm < l < 400 nm) and infrared radiation (0.7mm < l < 12mm). All these techniques are based on the same simple principle. A short pulse of radiation is transmitted into water or air, and the backscattered radiation is detected and analyzed. Clearly, radiation scattered from an object at a closer distance comes back sooner than that from an object at a longer distance. Scattering occurs not only on solid objects but also from the molecules and particulate matter in air and water. The return signal will, therefore, not be of the same length as the transmitted pulse, but extended in time, with a huge, but short peak from a solid object (if there is any) sitting on a much weaker, but temporally extended signal from air or water.

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