## 2 Challenges and Possibilities in the Context of Autonomy Levels for Unmanned Systems
### Classification of Snake Robots as Unmanned Vehicle Systems  
An uncrewed or unmanned vehicle (UV) is a mobile system not having or needing a person, a crew, or staff operator on board [9]. UV systems can either be remote-controlled or remote-guided vehicles, or they can be autonomous vehicles capable of sensing their environment and navigating on their own. UV systems can be categorised according to their operational environment as follows: unmanned ground vehicle (UGV; e.g., autonomous cars or legged robots); unmanned surface vehicle (USV; i.e., unmanned systems used for operation on the surface of water); autonomous underwater vehicle (AUV) or unmanned undersea/underwater vehicle (UUV) for the operation underwater; unmanned aerial vehicle (UAV), such as unmanned aircraft generally known as \"drones\"; unmanned spacecraft, both remote-controlled (\"unmanned space mission\") and autonomous (\"robotic spacecraft\" or \"space probe\"). According to this terminology, snake robots can be classified as uncrewed vehicle (UV) systems. In particular, a snake robot constitutes a highly adaptable UV system due to its potential ability to perform a large variety of tasks in different operational environments. Following the standard nomenclature for UV systems, snake robots can be classified as shown in Figure 2, where some of the most significant systems are reported based on the literature for the sake of illustration. Several snake robots are implemented as unmanned ground vehicle (UGV) systems that are capable of performing the following tasks:* locomotion on flat or slightly rough surfaces, such as the _ACM III_ snake robot [10], which was the world's first snake robot, or the _toroidal skin drive_ (TSD) snake robot [11], which is equipped with a skin drive propulsion system;
* climbing slopes, pipes, or trees, such as the _Creeping snake Robot_[12], which is capable of obtaining an environmentally-adaptable body shape to climb slopes, or the _PIKo_ snake robot [13], which is equipped with a mechanism for navigating complex pipe structures, or the _Uncle Sam_ snake robot [14], which is provided with a strong and compact joint mechanism for climbing trees;
* locomoting in the presence of obstacles, such as the _Aiko_ snake robot [3], which is capable of pushing against external obstacles apart from a flat ground, or the _Kulko_ snake robot [15], which is provided with a contact force measurement system for obstacle-aided locomotion. To better assess the different working environments in which these robotic systems operate, we may consider various metrics. For instance, the environmental complexity (EC)--a measure of entropy and the compressibility of the environment as seen by the robot's sensors [16]--may be used. Another useful metric is the mission complexity (MC), which is an estimation of the complicatedness of the environment as seen by the robot's perception system [6]. Both environmental complexity EC and MC gradually increase when moving from the former examples to the last ones. With a slight additional increase in terms of EC and MC, other snake robot systems are designed as unmanned surface vehicle (USV), autonomous underwater vehicle (AUV), or unmanned undersea/underwater vehicle (UUV) systems. An example of such systems is the _Lambda_ snake robot [17], which is capable of performing underwater locomotion. Another notable example of such systems is the _ACM-R5_ snake robot [18], which is an amphibious snake-like robot characterised by its hermetic dust and waterproof body structure. With an additional increase in terms of both EC and MC, flying or gliding snake robots may be designed as unmanned aerial vehicle (UAV) systems. These systems can be inspired by the study and analysis of the gliding capabilities in flying snakes, such as the _Chrysopelea_, more commonly known as the flying snake or gliding snake [19]. This category of snake robot still does not exist, but perhaps one day it will be realised when the necessary technological advances become a reality.