Contents I. Introduction
Airborne laser scanning (ALS) has become a leading method for collecting 3-D range data in remote sensing. In addition to the 3-D coordinates (), ALS systems record the intensity values for laser returns. Those intensity values can be used for several applications, e.g., change detection, forest measurements, and object classification [1]–[10]. The intensity of received laser return depends on several system and target properties and also on flight parameters. The received intensity can be well described by radar equation [11] P_{r} = {P_{t}D_{r}^{2} \over 4\pi R^{4}\beta_{t}^{2}}\eta_{{\rm sys}}\eta_{{\rm atm}}\sigma\eqno{\hbox{(1)}}
where is the received signal power (watts), is the transmitted signal power (watts), is the diameter of receiver aperture (meters), is the range from the sensor to the target (meters), is the laser beamwidth (radians), is the system transmission factor, is the atmospheric transmission factor, and is the target cross section (square meters). Equation (1) shows that the received power depends on the properties of the ALS system, range from the sensor to the object, atmospheric conditions during the measurements, and the laser footprint size on the target. Thus, the intensity values have to be corrected for atmospheric conditions, range, pulse energy, and incidence angle, particularly if the survey has included the use of several field-of-view angles or pulse-repetition frequencies (PRFs).
