## 2 Understanding the Jargon: Equipment and Data Products
### Understanding ALS Technology
The reflected photons are registered by a photodetector, and the signal generated can be either recorded completely at a high temporal sampling rate for posterior analysis or real time range determination, as in the case of waveform digitizing systems, or analyzed on-the-fly by analog electronics such as constant fraction discriminators (CFD) that provide precise time tags of specific waveform features (discrete recording systems). The features that are usually used to determine discrete returns are the local peaks of the waveforms as marked with red ticks in parts (b) and (c) of Figure 4. From the analysis of the waveform or the time tags, the two-way flight times between the sensor and the reflective surfaces that the laser pulses encounter along its path are determined. Dividing these time intervals (time of flight) by two and multiplying by the speed of light yields the slant range to the reflective surfaces. With these slant ranges, scan angle and the aircraft  
Figure 3: Single swath ground track patterns of different scanning mechanisms (**a**) oscillating mirror; (**b**) rotating polygon; (**c**) Palmer scanner. position and attitude is possible to geolocate the discrete returns and from this the first ALS data product, the point cloud (PC) is produced. In addition to the 3D geolocation information, the PC will usually include the intensity (local peak amplitude value) for each return. The intensity is a unit-less quantity scaled digital number (dn) in either eight or 12 bits as shown in Figure 4b,c. It provides relative information on the strength of the backscattered LASER light. The intensity depends on factors such as range to target, angle of incidence, target's roughness and reflectivity (more on the use of intensity below). Of the four characteristic parameters of electromagnetic energy, amplitude, wavelength, phase and polarization, current commercial ALS only register the amplitude, however some research LIDAR systems analyze and record return polarization, amplitude at different wavelengths and phase [24]. It is foreseeable that in the near future these characteristics will be available in commercial ALS systems and will provide useful information for archaeological research. It is also important to note that, as illustrated in Figure 4, the outgoing laser pulse has a finite size in the temporal domain known as pulse width, usually defined as the temporal extent between the points that have an amplitude of half the pulse peak amplitude at either side of the peak; referred to as the full width half max (FWHM) of the LASER pulse. Depending on the type of laser used the FWHM of the pulses may be as short as several hundred picoseconds to as long as several nanoseconds. This temporal pulse width translates to a spatial pulse length of about 30 cm for each nanosecond of width. The LASER pulse width and design and speed of the recording electronics define the LiDAR range resolution. For an ALS system to separate returns from two objects aligned in the range direction the objects must be separated by a distance greater than the LiDAR range resolution (greater than the pulse  
Figure 4: Illustration of LASER beam divergence and outgoing and return signals (waveforms). **(a)** Path of the LASER pulse; **(b)** waveform digitized with a 8-bit amplitude in a single segment; **(c)** waveform digitized with a 12-bit amplitude in multiple segments. length). Typically the range resolutions for current airborne LiDAR units are several decimeters or more. This is of crucial importance for archaeology, where the structures or features of interest might be underneath short vegetation and the ALS will receive a single irresolvable mixed return from both the feature and the vegetation [34].