## 2 Challenges and Possibilities in the Context of Autonomy Levels for Unmanned Systems
### The ALFUS Framework for Snake Robot Perception-Driven Obstacle-Aided Locomotion  
In this work, the focus is on snake robots designed as unmanned ground vehicle (UGV) systems with the aim of achieving perception-driven obstacle-aided locomotion. To the best of our knowledge, little research has been done in the past concerning this topic. When designing such systems, different levels of autonomy can be identified from an operational point of view. In Section 2.1, we have already briefly touched upon the concepts of environmental complexity (EC) and mission complexity (MC) to better categorise existing snake robots. By additionally considering the external system independence (ESI) metric, which represents the independence of snake robots from other external systems or from human operators, the so-called ALFUS framework [6] can be adopted and applied to provide a more in-depth overview for the design of snake robot perception-driven obstacle-aided locomotion, as shown in Figure 3.  
The levels of autonomy for a system can be defined for various aspects of the system and categorised according to several taxonomies [20]. Regarding the external system independence for snake robots, we introduce the following autonomy levels (AL) according to gradually increased complexity:  
Figure 3: The autonomy levels for unmanned systems (ALFUS) framework [6] applied to snake robot perception-driven obstacle-aided locomotion. * All guidance performed by external systems. In order to successfully accomplish the assigned mission within a defined scope, the snake robot requires full guidance and interaction with either a human operator or other external systems;
* Completely predetermined guidance functions. All planning, guidance, and navigation actions are predetermined in advance based on perception. The snake robot is capable of very low adaptation to environmental changes;
* Situational awareness [21]. The snake robot has a higher level of perception and autonomy with high adaptation to environmental changes. The system is not only capable of comprehending and understanding the current situation, but it can also make an extrapolation or projection of the actual information forward in time to determine how it will affect future states of the operational environment;
* Cognition and decision making. The snake robot has higher levels of prehension, intrinsically safe cognition, and decision-making capacity for reacting to unknown environmental changes;
* Autonomous operation. The snake robot is capable of fully autonomous capabilities. The system can achieve its assigned mission successfully without any intervention from human or any other external system while adapting to different environmental conditions. Concerning the environmental complexity (EC), the following autonomy levels (AL) can be identified according to gradually increased complexity:  
* No external perception. The snake robot executes a set of preprogrammed or planned actions in an open loop manner;
* Reactive--no representation. The snake robot does not generate an explicit environment representation, but the motion planner is able to react to sensor input feedback;
* Geometrical information (2D, 3D). Starting from sensor data, the snake robot can generate a geometric representation of the environment which is used for planning--typically for obstacle avoidance;
* Structural interpretation. The environment representation includes structural relationships between objects in the environment;
* Environmental affordance and dynamics. Higher-level entities and properties can be derived from the environment perception, including separate treatment for static and dynamic elements; different properties from the objects which the snake robot is interacting with might be of interest according to the specific task being performed. With reference to mission complexity (MC), the following autonomy levels (AL) can be identified according to gradually increased complexity:  
* No adaptation to mission changes.