**3.4. Unmanned vehicles in Forensic scene**

urement accuracy, distance range, radiometry detection, angular capability, etc.). By contrast, forensic users have to assess the features and performance of the TLS so that they fit their

Given the physical basis underlying the measurement of distances TLS can be classified into

**• Flight time.** This principle is based on the accurate measurement of the time invested by the laser pulse to travel between the emitting source and the object. Knowing the flight time, and being the speed of light a constant, distances can be deduced very simply. To complete the definition of the vector two angles have to be measured simultaneously: horizontal and vertical angles. Having the distance and angles referred is easy to deduce the three Cartesian coordinates, and it is a TLS work. The laser beam sweeps the area of study really quickly (from 1.000 to 100.000 point per second), taking the measure of the coordinates of every single point according to the density set up by the user. The scanning effect is achieved using oscillating mirrors that allow small and precise angular changes. Compared to other TLS, flight time allows larger measuring distances, even kilometres. However, they are less accurate than the optical triangulation ones. Its accuracy is between a few millimetres and two or three centimetres, since the laser beam divergence increases with the distance. Similarly, the accuracy will also be determined by the minimum possible angular value between two successive points. In forensic analysis, this kind of TLS fits the requirement of

three groups: flight time, phase shift and an optical triangulation (**Figure 6**).

large scenarios in open areas, that is natural disasters or terrorist attacks.

and also in small areas outside.

archaeological remains.

**• Phase shift.** These TLS take the principle of electromagnetic distance measurement used by topographic devices, which is based on calculating the distance between the laser scanner and the object point by determining the phase shift between the transmitted and received wave. As the above method, two angles measured in perpendicular planes (horizontal and vertical) are also collected for the location of each point. Medium distances range (up to 100 m on average) and a reasonable accuracy (around a few millimetres) make that this type of TLS are the most suitable for general purposes, being broadly used in inside crime scene

**• Optical triangulation.** Unlike the above, where the distance is calculated directly, this technique is based on a simple principle of triangulation. It is intended to solve a triangle from the known value of the baseline (one side) and the measurement of the adjacent angles. The baseline is defined by the distance between the laser emitter and the camera that receive the light reflected on the object while the adjacent angles are determined during the measuring operation. By a simple mathematical operation, the position of each point can be calculated. This type of laser scanner is very accurate, below the millimetre. However, its distance range is limited to a few meters. Thus, they are the most convenient equipment for short‐range and high precision measurement, for example anthropologic studies or

requirements.

12 Forensic Analysis - From Death to Justice

In Forensic analysis, unmanned vehicles could be classified into two types: aerial or terrestrial. Basically, differences are related to the platforms but not to the payload. They share the common problems related to sensor hybridization, mission planning, electronics and telecom‐ munications. Due to its versatility and its increasing popularization, this section is devoted mainly to the aerial ones.

Since the arrival of digital photogrammetry in the early 1990s is remarkable, the development of low‐cost devices and procedures is available to all users [11]. Unmanned aerial vehicles (UAVs), also known as remotely piloted aircraft systems (RPAS), allow to obtain aerial imagery and mapping products whose applications extend to several scientific fields and therefore in forensic analysis. UAV can fly dangerous areas where forensic researchers can explore, document and reconstruct the scene safely and quickly.

Aerial imagery has the ability to display portions of the object from a different perspective. Oblique images, those that are not restricted by the verticality of the shot, open a wide range of applications in forensics, either as a supplement to the understanding of the scene, or for extracting added information to the terrestrial one. Aerial images can be integrated into more complex systems, such as GIS, by georeferencing. Surveying tools such as GNSS provide 3D information of significant points in the aerial images in order to link them to global coordinate reference systems.

There are basically two built‐on types of UAV: rotary wing and fixed wing (**Figure 7b**). The most extended are the electrical multirotors, being the most common configurations those which have 4, 6 or 8 propellers (**Figure 7a**). Multirotors improve the performance of radio control helicopters increasing manoeuvrability and stability in the air. Electrical engines also are able to reduce vibrations, an extremely important matter to achieve high‐quality images. On the other hand, the presence of multiple engines increases the security by diminishing the possibility of failure of any component.

**Figure 7.** Unmanned vehicles employed in forensic science. Multirotor (a); fixed‐wing (b) and ground vehicles (c).

Mission planning optimizes the process of data collection. Flying over complex scenarios, the operator can take advantage of the high manoeuvrability of multirotors turning off automated control of the route and turning on manual control. For this purpose, real‐time display devices are available putting virtually the operator in the point of view of the camera in the air.

UAV platforms are composed of independent devices, being possible to customize the configuration of the payload. This makes possible to have on‐board different types of cameras: SLR cameras, video, thermal, multispectral. The possibility of integrating the data coming from different sensors provides a new level of interpretation to complex and large scenarios in forensic analysis.

Regarding the terrestrial unmanned systems (**Figure 7c**), also known as unmanned ground vehicles (UGV), they can be used for inspection or target location in indoor areas or GNSS‐ denied environments. Their size can be easily scaled accordingl to the mission objectives and payload. They share the advances in the UAV field, such as the mission planner for outdoor environments. However, they take advantage of robotics, being possible add as payload a remote controlled manipulator arm, for inspection tasks. In all cases, the autonomous explo‐ ration of the scene requires a sensor hybridization of passive (photogrammetric) and active (laser) sensors for a real time mapping and localization.
