## 2 Understanding the Jargon: Equipment and Data Products
### Understanding ALS Technology  
To introduce and explain the taxonomy of ALS systems (Figure 1) we begin with the single function that all LiDAR units have in common: to measure the two-way travel time of light from the sensor to the target (Earth's surface, vegetation, manmade structures, _etc._). Today most, if not all, ALS systems employ the Time-of-Flight (ToF) ranging principle, which means that they use short laser pulses and electronic systems to measure the time it takes for each laser pulse to make a round trip to and from a reflective surface. For more information on this and other ranging principles see [24]. From the ToF and knowledge of the speed of light in the medium between the sensor and the target (e.g., air, water, or sections of each) the range for each LASER pulse is computed. The number of pulses emitted by a LASER each second is called the pulse repetition frequency or PRF. Generally the higher the PRF the higher the spatial-sampling capabilities of an ALS system (number of laser shots fired/unit of surface area). It is becoming increasingly common for ALS systems to have more than one LASER that operate simultaneously to achieve a higher effective PRF at a single optical wavelength (color), or at different optical wavelengths to facilitate different applications. For example, some ALS systems transmit near-IR and green laser pulses, which enablesthem to efficiently map dry terrain and terrain covered with water, including lakes, streams and shallow coastal waters. In addition ALS systems may be classified as single pulse systems if they must detect the return signal from a pulse before emitting a second pulse; or as multi-pulse systems if they can fire a second or third pulse before receiving returns from the first pulse. At higher elevations (longer range), the round trip travel time of the LASER pulses can severely restrict the PRF of single pulse systems. The second aspect that ALS systems share (however they may differ in the way they accomplish it) is that they all sample a finite number of points distributed over an extended surface. One way this is done is to illuminate an extended area with a single LASER pulse, and detect multiple individual returns using a 2D spatial detector array, analogous to taking a photo with a digital camera using a flash to illuminate the scene. Such ALS units are referred to as Flash LiDAR/LADAR systems. This technology is common in the military arena but very limited in the commercial sector [26, 27]. The approach most commonly used currently in commercial LiDAR units, is to illuminate a relatively small area (spot) with each LASER pulse and detect returns using a single channel detector. The pointing of the LASER is then changed, using an optical scanner, and another sample of the surface is collected. Several different types of optical scanners are used, among the most common being oscillating mirror, rotating polygon (a more proper name would be rotating prism as it is a 3D solid rather than a 2D polygon that rotates), Palmer scanner (nutating mirror) and the Risley prisms scanner [22, 28]. In addition to the above mentioned mechanical scanners, electro-optical scanning mechanisms such as the scanning optical-fiber bundle [21] and liquid-crystal waveguides may be more common in the future. Each of these scanners has advantages and disadvantages, depending on the specific application and characteristics of the project area. It is also possible to build LiDAR units that simultaneously emit multiple LASER pulses, of the same or different wavelengths (colors), share a common scanner or use multiple scanners,  
Figure 1: Technical classification of Airborne Laser Scanning Systems. and have multi-channel detectors. This approach is becoming more common as researchers seek to extract additional information from LiDAR observations, by analyzing the reflections of the surface at different wavelengths (colors) [29].