Operational Considerations

(1) Reflectance. Reflectance is an important property that affects LiDAR performance. The amount of energy that arrives back at the LIDAR receiver is directly proportional to the percentage of energy that reflects off the object, or in other words the object’s reflectance. The reflectance of the object is wavelength-dependent, and because LiDAR systems are monochromatic, the reflectance at that particular wavelength determines how detectable an object is given the laser power. Figure 1-6 shows the relative spectral reflectance of various common landscape materials.

(2) Operating Altitude. The operating altitude for an airborne LiDAR system is largely dependent on the required point density of data and the ability of the laser to reliably detect returned energy and determine the elevation of a target at varying reflectivities. Some LiDAR systems are specifically designed as low-altitude sensors with relatively low pulse energy. These systems have typically high PRFs that enable the acquisition of 20-50 points per square meter at operating altitude of 500-3000 ft. Other systems are designed to be used at much higher operating altitudes (3000-8000 ft). These systems are designed for wide-area mapping with swath widths that can extend to 1500 meters. Until early 2006, high-altitude sensors were limited by the inability to track multiple pulses in air (MPiA). For these sensors without MPiA capability, an emitted laser pulse had to bounce off the target and be received by the sensor before the next pulse could be emitted. As a result, the PRF and operating altitude had to be limited in order to have only 1 pulse in the air at any instant of time. Recent developments in firmware now allow the tracking of MPiA, also known as Multiple Time Around (MTA), and some sensors can track up to 8 pulses in the air. MPiA technology has enabled LiDAR sensors to use 2 laser sources simultaneously (dual-channel lasers), thereby producing 800 KHz PRF and the ability to operate at altitudes of over 7000 ft.

(3) Return Type. Early versions of airborne LiDAR systems were capable of recording only one pulse at low pulse repetition rates. However, more advanced LiDAR systems can record simultaneously multiple returns for each transmitted pulse, and the reflected intensity for each return. Multiple return LiDAR systems are very useful in forestry applications or even to derive bare Earth topography under vegetation. When the laser beam from a multi-return system interacts with a tree canopy, then the first return is usually assumed to arrive from the top of the tree (or where the transmitted laser beam first interacts with the tree canopy). The last return may interact with the ground underneath the tree, although the ability to map the ground is largely dependent on the density of the vegetated canopy. Intermediate returns, perhaps 2nd, 3rd, and 4th, are expected to be caused by tree branches and understory vegetation between the top of the canopy and the ground.