Cameras are found on almost all underwater vehicles to provide feedback to the operator or information for oceanic researchers. Vision based navigation involves the use of one or more video cameras mounted on the vehicle, a video digitizer, a processor and, in general, depending on depth, a light source. By performing image processing on the received frames, the required navigation tasks can be completed or required navigation information can be calculated. The usual setup for the vision system is a single downward facing camera taking images of the sea floor at an altitude of between 1 and 5 meters (see Fig. 2). The use of optical systems, like all navigation sensors, has both advantages and disadvantages. If the challenges of underwater optical imaging, described in section 2, can be successfully addressed some of the potential advantages of vision based underwater navigation include:
• Underwater vehicles are commonly fitted with vision sensors for biological, geological and archaeological survey needs. As such, they have become standard equipment onboard submersibles. As a readily available sensor, vision can be incorporated into a navigation framework to provide alternative vehicle motion estimates when working near the seafloor in relatively clear water.
• The visual data received from optical systems can be easily interpreted by humans and thus provides an effective man-machine interface. Further processing of the visual data can be processed to perform vehicle navigation.
• Optical imaging systems are relatively inexpensive sensors and only require the camera itself, an image digitizer, a host computer and a light source dependent on conditions. The depth rating, low light sensitivity, resolution and whether the camera is zoom or non-zoom, colour or monochrome can all affect pricing.
• Cameras are relatively light weight with small form factors and low power consumption. These can be important issues for deployment on autonomous craft. Unfortunately most missions require artificial lighting which adds significantly to both the weight and power demands of the system.
• Optical imaging has a very high update rate or frame rate and thus allows for high update rate navigation data. The image digitizing hardware and the computation cost of the image processing algorithms are the constraints of the system rather than the optical imager itself.
• Optical imaging systems provide high resolution data with measurement accuracies in the order of millimetres when working near the seafloor.
• Imaging systems can provide 3D position (stereovision) and orientation information, in a fixed world coordinate frame, without requiring the deployment of artificial landmarks or transponders.
• Optical systems have been proven to be capable of providing underwater vehicle navigation without the aid of other sensors.
• Optical imaging systems are very diverse and can be implemented to perform many navigation and positioning applications including: cable tracking, mosaicking, station keeping and motion estimation.
For the purposes of the review the general setup and assumptions about the state of the vehicle and the environment conditions are described. These assumptions are adhered to by all literature and algorithms described unless specifically stated otherwise.
• The underwater vehicle carries a single down-looking calibrated camera to perform seabed imaging.
• The underwater vehicle and thus the camera is piloted at an altitude above the seafloor which allows the acquisition of satisfactory seafloor imagery. This altitude can be affected by external conditions affecting the maximum imaging range.
• The imaged underwater terrain is planar. In most underwater environments this is not the case but the affects of this assumption are reduced using robust statistics for more accurate vehicle motion recovery. This assumption can also be relaxed due the fact that the differences in depth within the imaged seabed are negligible with respect to the average distance from the camera to the seabed.
• The turbidity of the water allows for sufficient visibility for reasonable optical imaging of the working area.
• The light present in the scene is sufficient to allow the camera to obtain satisfactory seafloor imagery.
• An instrumented platform which allows for comparison of results or measurement data fusion is employed.
• Known reference frames between the vehicle and the camera and the vehicle and any other sensors utilized in the navigation technique.
Fig. 2. Camera and lights setup (red box illustrates image frame)
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