## 6 Simulation Environment and Results
### VTT Ship Simulator Environment  
The simulator environment at VTT is depicted in Figure 6. A flexible research simulator allows fast modification of its different parts [52]. It enables research on autonomous ships due to realistic modelling of different ship types, maneuvering in different environments, and modelling of sea traffic. Simulation scenarios can be conducted in specific regions, applying the desired environmental conditions. The _simulator bridge_ is made of real consoles and control devices. It includes conning displays and real instrumentation. An essential part is the electronic chart display and information (ECDIS) system for nautical navigation, fulfilling IMO regulations.  
The ship handling simulator solves the vessel's equation of motion in the time domain at the horizontal plane. That is, the motions are evaluated for surge, yaw and sway directions, as shown in Figure 7. The hydrodynamic forces are determined for hull, propulsion and control devices. Furthermore, the environmental conditions can be taken into account by wind, current and wave effects.  
The collision avoidance system in the simulator consists of a situation awareness module providing information to the decision-making system. The module makes decisions based on implemented COIEGs-based rules and path planning. The autopilot steers the vessel to the desired location along the defined path.  
A wide-speed-range autopilot is utilized to maneuver the vessel through the desired course. The autopilot consists of the heading, track and docking modes. The heading mode tries to keep the given heading angle at the given speed, that is, it is a Proportional Integral Derivative (PID)-basedheading controller [54]. Moreover, the track mode keeps the vessel in the given track, as shown in Figure 8.  
The track also includes so-called track limits, which define the safe water zone. That is, the vessel's location will remain inside the track limits.  
Finally, the docking mode navigates the ship to the dock by controlling the main propulsion units together with the stern and bow thrusters. With the docking mode, the vessel can be set to translational and rotational motions so that complex docking maneuvers can be carried out.  
The current autonomous collision avoidance system can still be characterized as a reactive system. The decision-making module evaluates the evasive actions according to state vectors of own ship and traffic vessels. During an evasive action, the autopilot mode is changed from the track mode to the heading mode. However, the location of the ship related to the track and track limits is also monitored in the heading mode. For harbor maneuvering, the function of the track mode is extended to slow speeds.  
In this work, the situation awareness system is not modelled at the sensor level. The 'own ship' receives the state vectors of the surrounding simulated traffic vessels directly according to the simulation condition, that is, it emulates a transponder-type data transfer system. This type of investigation can also be conducted by applying reactive collision avoidance systems. In this case, the data delays have an impact on the instantaneous traffic condition considered in the decision-making procedure.  
Figure 6: VTT ship simulator for connectivity research.  
Figure 7: Ship motion directions.  
That is, the contributions due to the delays in the situation awareness can be evaluated in the simulation environment.