3.1.1. Information Perception Technology
The information perception system includes perception sensors and information fusion techniques. It obtains the information about the ship’s own state and the environment that the ship is in. By identifying the ship’s situation, the perception system provides a data basis for collision avoidance and path planning, which is expected to lead to safer and more reliable navigation. The information on the ship’s own state includes the position, speed, course, and other navigational information, as well as the information on the operational state of the equipment systems and the state of the carried cargo. The environmental information refers to the meteorological and hydrological conditions, and the status of surrounding vessels and obstacles. Depending on the type of equipped sensors, the intelligent ship information perception technologies can be distinguished in the following four categories.
Radar is an essential sensor in the maritime field. The perception is mostly in the form of processing of radar images and signals. This category of equipment includes the ultrasonic radar, millimeter-wave radar, continuous-wave radar, etc. Radar has high resolution and accuracy, and can be used in all-weather and wide-area detection. However, there are also certain limitations: the detection of radar is susceptible to weather conditions, and the radar images often suffer from problems such as noise interference, uneven brightness, target loss, etc. [
19].
- 2.
Lidar-based information perception technology
Light Detection and Ranging (Lidar) works in a similar way to radar. It is mainly used for purposes of target detection and obstacle avoidance. Achtert et al. combined Doppler radar with a steady-motion platform to measure the atmospheric wind profiles [
20]. The results show that the data coverage of this method is comparable to that of land-based measurements. However, this method provides a more detailed and higher temporal resolution view of atmospheric boundary variability, compared to the radiosonde measurements. Compared to radar, Lidar has a higher range accuracy and stronger anti-interference capability, but the detection range is smaller.
- 3.
AIS-based information perception technology
AIS, as an important tool for the perception of waterborne traffic information, provides a complementary source of data to radar, since it has no dead zone. However, this method only applies to vessels equipped with AIS transponders. Thus, AIS cannot be taken as the only form of information perception for intelligent ships. Prasad et al. utilized AIS data and multi-sensor information to augment the data from weather sensors, which can be applicable for the control and navigation of ships in foggy weather or other restricted-visibility conditions [
21]. Zhou et al. developed a regression model using AIS data to quantify the impacts of wind and current on ship behavior without the input of specific ship maneuvering details [
15]. Regarding the data quality, the current AIS data mostly contain errors and data loss, which may lead to wrong or at least incomplete information acquisition.
- 4.
Vision-based information perception technology
Vision sensors acquire the image information of the surrounding environment by machine vision and process the captured images to achieve environmental perception. Wang et al. proposed a framework for automatic detection and localization of USV real-time targets based on binocular vision [
22]. It extracts and matches the features within a target area determined by a deep convolution network. Then, the target is localized using the calibrated binocular camera parameters in the triangulation measurement principle. The experimental results proved the delivery of both accurate detection and high-precision positioning results in real-time applications. Currently, vision sensors would be the cutting-edge method to perceive the surrounding information. However, to achieve a full overview of the surroundings, a sophisticated sensor system is needed, which possibly implies a high risk of machine failure.
3.1.2. Risk Perception Method
Intelligent ships rely on sensors to obtain basic information, apply information fusion techniques to judge the ship’s situation, and estimate the collision risk. There have been a number of studies on the risk analysis or assessment of ship collision. Some researchers tend to indicate the risk as a numerical index, such as the Minimum Safety Passing Distance (MSPD) [
23,
24] and the Collision Risk Index (CRI) [
25,
26,
27,
28,
29], which is deemed as a numerical form of risk. Whereas, the other way to reflect risk is in two-dimensional graphics, such as Ship Domain (SD) [
30,
31,
32,
33], dangerous region (DR) [
34,
35,
36], and action lines (AL) [
37,
38].
The MSPD method usually provides a deterministic result of a collision event in the given scenario, i.e., occurrence or non-occurrence. The premise is that when both the own ship (OS) and the target ship (TS) keep their course and speed, if the Distance at the Closest Point of Approach (DCPA) is smaller than the MSPD, a collision occurs. On the contrary, OS can safely pass TS. This method has been widely adopted for manned and unmanned ships [
23,
24]. In addition, the MSPD is also an essential risk indicator in CRI calculation [
39].
The assessment of ship collision is influenced by multiple factors, such as ship speed, course, distance to TS, speed ratio, and meteorological and hydrological conditions, etc. Besides, the presence of sensor errors also leads to the uncertainty of the collision process. The CRI measurement provides an exact value of the threat level, which is an intuitive indicator of the collision risk. The current main CRI measurement methods include the DCPA and the Time to the Closest Point of Approach (TCPA) weighting methods [
25,
26], fuzzy logic algorithms [
27], and neural networks [
28]. When using the weighting method to measure CRI, the different dimensions of DCPA and TCPA are usually ignored, which makes the calculation result inaccurate. In addition, in the multi-ship situation, it is impossible to objectively reflect the threat level of each ship. Fuzzy logic methods are quite subjective when calculating CRI, and can only be applied in certain specific scenarios. Neural network algorithms require a substantial storage of expert experience and knowledge in advance and plenty of sample learning. Thus, this algorithm cannot satisfactorily fulfill the instantaneity requirement of CRI.
SD is a graphic depiction of the ship collision risk, which is usually a group of areas around OS to visualize the risk. When TS enters or is about to enter the area, a collision alert is triggered. Szlapczynski et al. defined two SD-based safety parameters: Degree of Domain Violation (DDV) and Time to Domain Violation (TDV) [
30]. The results show that the accuracy of DDV/TDV is higher than that of the DCPA/TCPA. Qiao et al. developed a quadratic ship domain model considering the uncertainty of ship position and proposed a method to calculate the spatial collision risk, which had been improved in further applications [
31]. Some researchers assess the collision risk by developing new SD models. Bakdi et al. developed an adaptive SD model for risk identification through a spatial risk function based on the type of encounter situation and collision hazard [
32]. The results showed competitive advantages in terms of intuitiveness and computational efficiency. Aiming at the shortage of including single factors in SD in previous studies, Guan et al. established an SD model based on fuzzy logic considering multiple variables [
33]. The obtained results support the decision-making of collision avoidance and early prediction of collision risk. However, the calculation of this method is complex, and not suitable for risk identification in the case of multi-ship encounters.
The DR is designed to collect a set of OS’s speed or course that leads to a conflict with TS and display this set of speed or course to the Officer on Watch (OOW) in a graphical form. Velocity Obstacle (VO) is a typical algorithm in this category. It is capable of seeking out the optimal collision-free solution in two-ship and multi-ship encounter scenarios [
34], considering ship dynamics [
35] and maneuverability [
36].
The method of AL focuses on identifying a line of action around OS in geographic space, which indicates the final timing of OS to complete collision avoidance through a series of actions. AL is usually obtained by simulations. Szlapczynski et al. determined AL by a series of simulations of various types of ship encounters under different conditions using a hydrodynamic model of ship movement [
37]. Namgung et al. established an adaptive neuro fuzzy inference system to judge the CRI of the optimal position and timing [
38]. The system ensures that the OOW has sufficient time to make decisions and take the necessary actions of collision avoidance.
Based on the manifestations of the risk perception methods presented above, the models referred by literature research are summarized in
Table 4.
Among the risk perception methods, the number of models adopting CRI and SD is the highest. Only a few models apply the method of MSPD, DR, and AL. MSPD can be deemed as a basis for the calculation of CRI, while DR and AL can provide decision-making support for ship collision avoidance on the basis of SD display. In the future, more models adopting DR and AL can be developed for collision risk perception purposes.
When the collision risk is indicated in a numerical form, it can be intuitively compared. A higher value indicates higher risk, and vice versa. For intelligent ships, risks in the numerical form are easier to involve in the system control link, and provide an informative basis for intelligent decision-making. In the multi-ship encounter situation, the graphic expression of risk allows to intuitively divide TSs into several groups defined by graphical indicator, SD, DR, or AL. However, in such a form, the risks of TSs in each group still cannot be further compared. The graphic form is indeed more intuitive for the operator, which can be integrated into the map to support the OOW in obtaining an overview of the surrounding situation. To make full use of the advantages of both methods, a risk perception platform with integrated collision risk digitalization and visualization can be considered in the future. It is expected to more efficiently and accurately perceive the risks.
In current navigation practice, the collision risk can be assessed with the assistance of some techniques and systems on board. However, for manned ships, the OOW mostly tends to judge the risk by good seamanship and situational awareness, considering the perceived information from the assistance system, instead of directly adopting the indicated risk result. Thus, the risk perception result still largely depends on the experience, knowledge, and skills of the crew. However, it is difficult for OOWs to maintain good situational awareness and precautions when simultaneously monitoring multiple ships. For intelligent ships, the collision risk can be monitored in real-time via reliable and timely identification of obstacles by sensors and information fusion techniques.
Most of the risk perception models are based on AIS data. Generally, the uncertainty of trajectory data is not considered. Besides, there could be some special circumstances when the AIS equipment is off, or the signal transmission fails. In such a situation, AIS data are no longer available, let alone the data accuracy. These facts lead to the error between the theoretical research and the reality. In the future, it is necessary to introduce parameters of uncertainty when developing risk perception models to consider the corresponding impacts and integrate alternative methods without AIS data as mandatory input.