Laser radar or lidar systems are a direct extension of conventional microwave radars to wavelengths from the optical bands. Such systems are capable of simultaneous measurements of range, reflectivity, velocity, temperature, azimuth, and elevation angle. A monostatic system is one in which the transmitter and receiver are co-located; in this setting the transmitter/receiver combination is often referred to as a transceiver. When the transmitter and receiver are separated in space by much more than a Fresnel zone, it is called a bistatic system.
In a monostatic channel the transmitted optical wave must traverse the same atmospheric path twice in opposite directions - once in the forward direction to the target and, upon reflection, back to the receiver in the opposite direction. For double-pass propagation, wave properties are determined by turbulence-induced correlations between the incident and reflected waves that lead to a variety of optical effects such as the backscatter amplification effect and an increase in the irradiance fluctuations of the reflected waveform. Actually, two types of correlation terms arise in the complex phase perturbation of the wave - one arising from a "reciprocal path" geometry and one arising from a "folded-path" or "straight-back path" geometry of the reflected wave [1-3]. These two paths are depicted in Fig. 9.1. Put simply, the reciprocal path geometry includes the reciprocal paths ABCDA and ADCBA, which indicates that optical rays traverse along each path through exactly the same atmospheric turbulence, but in opposite directions. A folded path geometry consists of only one such path like AECFA. Effects from both types of correlation terms appear in the mean irradiance of the reflected wave, but the reciprocal path geometry creates a "positive correlation" that produces an enhancement in the mean irradiance near the optical axis at A whereas the folded path geometry creates a "negative correlation" that leads to a decrease in the mean irradiance at A. Thus, the backscatter amplification effect arises only when the magnitude of the reciprocal path terms exceeds the magnitude of the folded-path terms, which depends on the incident illumination wave, on the size and type of target (smooth, rough, etc.), and on channel conditions.
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