## 2 Understanding the Jargon: Equipment and Data Products
### Understanding ALS Technology
The mirror in an oscillating mirror scanner must be decelerated, stopped and then accelerated to change direction at the edge of the swath, and because of this it produces a non-uniform distribution of laser shots along the scan line, especially close to the edge of the swath. Figure 2 exemplifies the ground track of an oscillating mirror scanner. As an alternative, to overcome the limitation of non-uniform distribution of laser shots by an oscillating mirror scanner, carefully designed rotating polygons with constant rotational speeds allow for a more uniform distribution of laser shots along the scan line, producing a ground track that looks like an array of forward or back slashes (Figure 3b). The ground track produced by a Palmer scanner is akin to a calligraphy loop (Figure 3c), where the laser shots are distributed in a constant angle cone (constant angle of incidence) that is scanned along the ground by the platform forward motion. While the Palmer scanner produces an interesting distribution pattern it might not be well suited to produce ground returns of terrain occluded by thick forest canopies as they do not distribute laser shots close to Nadir (more about this below). Independent of the scan mechanism employed the system scanning angle and flying height determines the swath width (Figure 2). The scanning frequency and PRF determine the across-track spacing of the laser pulses (Across-track resolution). The aircraft ground speed and scan frequency determine the down-track resolution. These above described and ground track patterns illustrated in Figure 3 represent the ground sampling of a single pass under ideal conditions. In reality these ground  
Figure 2: Schematic diagram illustrating the basic operational principles and main system components of ALS technology. tracks are affected by the aircraft flight dynamics. Slight variations in the aircraft's ground speed and attitude distort and modify the distribution of LASER shots over the target surface. Surveys are planned to include overlap between adjacent swaths in order to achieve a more uniform distribution of points over the ground and to illuminate the same target surface from a variety of angles (more on this below). Also this overlap can be used to achieve a better elevation agreement between adjacent swaths (see Section 3.2.2). Figure 4 depicts the path of a single laser pulse fired straight at nadir, it illustrates the way the laser pulse energy spreads in a conical fashion as it propagates through the atmosphere, similar to the pattern of a highly directive spot light. This spread is determined by the laser beam divergence and, in conjunction with the flying height, defines the size of the LASER footprint projected on the ground (or whatever target the footprint intersects). Some airborne systems, such as NASA's Laser Vegetation Imaging Sensor (LVIS) [31] and the Multiple Altimeter Beam Experimental Lidar (MABEL) [33], have footprints as large as 10-30 m in diameter and are used to simulate or validate spaceborne LiDAR sensors such as IceSat GLAS sensor (retired in 2010) and the upcoming sensor for IceSat2. However, most commercial airborne LiDAR units are characterized by beam divergences that produce footprints between 15 and 90 cm in diameter from their typical operational altitudes, and thus are considered \"small footprint\" systems. Figure 4 also illustrates a time-_versus_-intensity, or waveform plot of a laser pulse as it travels from the sensor to the target and back. When the pulse exits the sensor it generally has a nearly Gaussian profile in the time domain. As the light interacts with objects such as trees or the ground some of the energy is reflected back towards the sensor modifying the waveform shape according to the geometric properties of the target.