Lidar, an acronym for “
anging”, is an active remote sensing technique analogous to radar. Lidar systems use a laser as an active radiation source. The short pulse lengths produced by a laser (approximately 20 ns) and the spectral bandwidth (1 cm-1) allow for highly-resolved ranging measurements with high signal-to-noise. Also, as in radar, lidars could be either monostatic (collocated transmitter and receiver) or bistatic (separated transmitter and receiver). Figure 10.1 illustrates an operational, monostatic lidar. Laser radiation is transmitted and scattered or absorbed by atmospheric constituents, such as clouds, aerosols, or molecules. Photons scattered back to the receiver are then collected, directed to a detector whose signal is analog-to-digitally recorded or counted as a function of altitude or range. The strength of the return signal is related to the physical and optical properties of the scatterers.