Based upon the range of the electromagnetic spectrum, there are three distinct types of active remote sensing:
1. Active Optical Remote Sensing: Active optical remote sensing involves using a laser beam upon a remote target to illuminate it, analyzing the reflected or backscattered radiation in order to acquire certain properties about the target. Generally speaking, the velocity, location, temperature and material composition of a distant target can be ascertained using this method. An example of a highly sophisticated active instrument is the LIDAR (LIght Detection and Ranging). The instrument works by using a transmitter and a receiver. The laser generates pulses which excite the specified target, causing it to absorb radiation at certain wavelengths. The target then reflects radiation in the form of photons which are detected by the LIDAR's sensors and converted to an electrical signal.
Applications of LIDAR have been numerous, ranging from archeology, geology, forestry and atmospheric physics to name a few. One example of the application of LIDAR in meteorology is for the study of aerosols and clouds. Another example is using LIDAR to determine how much carbon dioxide is in the atmosphere. In agriculture, it has been used to help farmers determine what regions of the field expensive fertilizers need to be applied to. In law enforcement, LIDAR is replacing radar guns for the measurement of vehicle speed, and LIDAR guns are proving their worth. The automobile industry has also started using it for Adaptive Cruise Control (ACC) systems.
LADAR (Laser Detection and Ranging) is the acronym that is used in military parlance for LIDAR. A limitation of this system has been the sensor range. Wider laser pulses are being employed to overcome the limited range of sensors, however, this limits the resolution of the target location. Coherent detectors using ultra-fast photodiodes for better handling of the optical signal are now being developed to improve the overall efficiency. Many are now commercially available.