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path planner is reactive and therefore only close-range or pop-up obstacles will be
dealt with here. Observations are made, stored and analysed for a period of time in
each sector periodically according to predefined parameters or according to the
USV's speed. However, if the risk assessment unit identifies any threat in one of the
sector, it can advise the sensor to focus on that particular region until the threat is
cleared and the situation is resolved.
Additionally, the tilt part of the device works as a stabiliser for the system. Tilt
control is carried out by using feedback from a wave detection system which will
minimise the effect of wave movements on the detection system and hence keep the
camera and laser sensor stable.
2.2
Obstacle Detection/Identification Subsystems
The camera and the laser sensor provide the obstacle detection subsystem with a
complete picture of the surrounding. For video feedback, the obstacle detection
subsystem employs the well-known background subtraction technique to extract the
information regarding any obstacle in the vicinity of the USV. The basic algorithm
has been modified to ignore the sea waves which are also dynamic in nature. In
addition to the camera system which is used to detect the presence of the obstacle, the
laser sensor can provide accurate range measurements. By using the camera and the
laser sensor this subsystem will extract the location, heading angle (if dynamic) and
the size of the obstacle and transmit this information to the motion planner.
2.3
Direction/Trajectory Generation Module
Based on the information provided by the detection/identification system, the path
planner module will provide the USV with the recommended route until the mission
is accomplished and the USV reached the target. This module is capable of
generating paths that are compliant with the standard collision regulations
(COLREGs) on prevention of collision at sea [8] defined by the International
Maritime Organisation (IMO) [2]. Since the aim of this paper is to focus on detecting
and identifying the risk of static and dynamic obstacles. Therefore the implementing
of this module is not included in this paper.
2.4
Risk Assessment Unit
In addition to the above modules, the ODA system carries out its own risk
assessment. Due to the fact that the detected obstacles may be dynamic, the ODA
needs to assess the threat of these obstacles in a periodical manner. To accomplish
this task the risk assessment unit is linked to the direction/trajectory module to advise
a change in direction if there is any approaching threat. Additionally, it provides
feedback to the pan and tilt subsystem if it needs further scanning of a specific region
where a threat is believed to be approaching from. In order to evaluate the
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