added underwater acoustic positioning
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@ -236,9 +236,9 @@ The following section outlines how the use of an unmanned underwater vehicle
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(UUV) can make mission operations more efficient and precise.
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The state of current UUV usage throughout cable repair operations is outlined
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in order to identify the critical capabilities and requirements.
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The future of the domain is then explored and the challenges identified
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before exploring how these can be overcome in order to meet the determined
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requirements.
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The future of the domain is then explored and the challenges in applying
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these developments to sub-sea cable repair are identified before exploring
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how these can be overcome in order to meet the determined requirements.
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Prior to this, the domain of UUVs as a whole is described in order to outline
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the scope of available vehicles.
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\begin_inset Flex TODO Note (Margin)
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@ -962,14 +962,14 @@ Current AUV Usage
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\begin_layout Standard
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Autonomous underwater vehicles are well suited to survey and research operations
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, without human intervention they sweep a given area collecting data for
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; without human intervention they sweep a given area collecting data for
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analysis.
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This can include bathymetry
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\begin_inset Foot
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status open
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\begin_layout Plain Layout
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The measurement of the depth of a body of water
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Measurement of the depth of a body of water
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\end_layout
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\end_inset
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@ -1025,6 +1025,36 @@ literal "false"
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\end_layout
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\begin_layout Subsubsection
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Advantages
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\end_layout
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\begin_layout Standard
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An advantage of using an autonomous vehicle would be the lack of need for
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the surface vessel to maintain position directly above the ROV and fault;
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instead the surface vessel would stay within a larger area only to maintain
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contact with the UUV.
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This could reduce the required power directed to dynamic positioning which
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in higher sea states can become a significant draw.
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Additionally as the UUV can move independently, the surface vehicle would
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not need to directly track the vehicles movement; for example when the
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UUV is re-burying the repaired cable in shallower waters.
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This would, again, lower the required propulsion power used by the surface
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vessel.
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\end_layout
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\begin_layout Standard
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Another advantage could be a reduction in risk during mission operations.
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With a traditional tethered ROV, should the umbilical cable be broken the
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vehicle would likely lose functionality and require specialist recovery.
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This break could occur as a result of a fault in the tether management
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system, high storm activity causing too much tension on the system, or
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in less likely scenarios, animal intervention.
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An autonomous vehicle has no tether to break and a hybrid ROV/AUV could
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likely be instructed to take control and return home should the tether
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break during missions involving direct human control.
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\end_layout
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\begin_layout Subsubsection
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Domain Challenges
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\end_layout
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@ -1034,14 +1064,25 @@ Navigation
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\end_layout
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\begin_layout Standard
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One of the main advantages of using an autonomous vehicle for sub-sea cable
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repairs would be the physical de-coupling of the vehicles, however this
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also poses the most significant challenge.
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As mentioned, one of the main advantages of using an autonomous vehicle
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for sub-sea cable repairs would be the physical de-coupling of the vehicles,
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however this also poses the most significant challenge.
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In typical ROV operations, the operator has knowledge of the location of
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the ROV relative to the surface vessel.
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As the surface vessel is GNSS-enabled (Likely GPS) it has knowledge of
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its position in world co-ordinates and the operator can use this to reduce
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the ROV's cable search space.
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As the surface vessel is GNSS
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\begin_inset Foot
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status open
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\begin_layout Plain Layout
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Global Navigation Satellite System, the generic term for satellite aided
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global navigation of which the American GPS, Russian GLONASS and European
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Galileo systems are examples
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\end_layout
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\end_inset
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-enabled (Likely GPS) it has knowledge of its position in world co-ordinates
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and the operator can use this to reduce the ROV's cable search space.
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\begin_inset Flex TODO Note (Margin)
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status open
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@ -1121,8 +1162,8 @@ The vehicle will be designed for hybrid ROV/AUV operations.
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vessel with the ability to operate in a similar fashion to existing ROVs
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(human controller, tethered power and data connection).
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This will have a number of benefits, primarily that the vehicle should
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be able to benefit from autonomous operation with the ability for direct
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human control in missions deemed to complex for autonomous control.
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be able to benefit from autonomous operation where possible with the ability
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for direct human control in missions deemed to complex for autonomous control.
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\end_layout
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@ -1141,7 +1182,7 @@ As previously described, box frame UUVs are well suited to sub-sea cable
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As such a box frame of similar specifications to those currently used,
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\begin_inset CommandInset citation
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LatexCommand cite
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LatexCommand citep
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key "global-marine-atlas-data-sheet,rov-hector-7-datasheet"
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literal "false"
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@ -1153,6 +1194,26 @@ literal "false"
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a mission without a constant power supply from the surface vessel.
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\end_layout
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\begin_layout Subsubsection
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Communication
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\end_layout
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\begin_layout Standard
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As the UUV is now expected to operate independently of the surface vessel,
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it should have the ability to bi-directionally wirelessly communicate with
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the surface vessel.
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Uses for such a communications channel include the UUV reporting it's mission
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status and the surface vessel providing high-level instructions such as
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\emph on
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return home
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\emph default
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orders.
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When operating underwater, acoustic signals are the primary medium for
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wireless communication.
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\end_layout
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\begin_layout Subsubsection
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Navigation
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\end_layout
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@ -1200,24 +1261,156 @@ However, despite the use of a Kalman filter allowing more precise approximations
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means that the overall uncertainty will still continually increase over
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time.
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In land-based robotics this is mitigated through the use of periodic GPS
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measurements which have low uncertainty and help to place an upper bound
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on the overall error.
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measurements which have low, constant uncertainty and help to place an
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upper bound on the overall error.
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As previously mentioned, GNSS systems do not work deep underwater and as
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such, another method for providing these external updates must be used.
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\end_layout
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\begin_layout Standard
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The following proposes methods for providing global positioning to the UUV
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without a traditional GNSS system.
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This will be completed in two stages, the first being to provide the UUV
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with the ability to measure the location of a fixed point relative to itself.
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In parallel, the global co-ordinates of this fixed point will be communicated
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to the UUV in order to infer it's own global location.
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\end_layout
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\begin_layout Paragraph
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Underwater Acoustic Positioning
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\end_layout
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\begin_layout Standard
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Alongisde the use of acoustic signals for communications it will also be
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employed for positioning.
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One application for this is underwater acoustic positioning which employs
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the use of time-of-flight measurements to beacons of a known location to
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triangulate an object's location.
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There are different configurations for such a system depending on how these
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beacons are laid out,
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\emph on
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long-baseline
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\emph default
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(LBL) systems involve beacons located on the sea floor.
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Spreading these beacons around the working area of an ROV widens the baseline
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of the system and provides higher accuracy when triangulating.
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This configuration is best suited to static areas of research such as ship
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wrecks where an initial time devoted to deploying and calibrating these
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underwater beacons is a reasonable expense to pay for the required high
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accuracy.
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This is not the case for sub-sea cable repairs where the deployment, calibratio
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n and recovery of beacons on the seabed would be prohibitively complex and
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add significant time to the duration of a mission.
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\end_layout
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\begin_layout Standard
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\emph on
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Short-baseline
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\emph default
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(SBL) systems involve a number of beacons placed at the furthest corners
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of the surface vessel, this has the benefit of requiring little set-up
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and pack-down at the cost of reduced accuracy.
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Relative to the UUV these beacons are all on a similar bearing when operating
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at a distance, as a result changes in the vehicle's location would be reflected
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in similar changes to the measurements from all of the beacons.
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Previously, with a long-baseline, the beacons are ideally surrounding the
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UUV's working area and changes in its location are reflected in different
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distance deltas for each beacon allowing tighter triangulation.
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Accuracy can be improved by extending the beacons away from the vessel
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to extend the baseline as far as possible.
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\begin_inset Flex TODO Note (Margin)
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status open
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\begin_layout Plain Layout
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such as on a boom?
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\end_layout
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\end_inset
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\end_layout
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\begin_layout Standard
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One method to mitigate the drawbacks of both described methods is by using
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GPS Intelligent Buoys (GIBs).
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This configuration, also referred to as an
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\emph on
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inverted long-baseline
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\emph default
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, allows a much wider baseline than the surface-vessel-mounted beacons by
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deploying a group of
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\emph on
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smart buoys
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\emph default
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around the expected working area of the UUV.
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The use of buoys as opposed to beacons on the sea-floor significantly decreases
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the preparation and clean-up mission phases.
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\end_layout
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\begin_layout Standard
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Of these methods it is proposed that the surface vessel be equipped with
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a short-baseline beacon array as well as a population of GIBs.
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This will allow the choice between higher accuracy or faster mission turnaround
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be decided by mission conditions as well as providing redundancy for either
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system.
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In shallower waters, the accuracy of the onboard SBL may be deemed sufficient
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however in deeper water where the UUV is operating far further from the
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surface vessel, the compactness of the SBL baseline may require the higher
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accuracy of the GIBs
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\begin_inset Foot
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status open
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\begin_layout Plain Layout
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In practice the two could be used in conjunction for efficiency.
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As the UUV is deployed it initially uses the onboard SBL array while the
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surface vessel makes a pass around the working area deploying GIBs for
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use as the UUV gets deeper
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\end_layout
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\end_inset
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.
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The GIBs would be considered additional accuracy, the SBL would be used
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alongside the GIBs and act as an extra node in the array.
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Additionally the weather and sea conditions could play a factor in the
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decision.
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In higher sea states and stormy weather, the deployment and recovery of
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GIBs may be deemed too risky and the SBL could be used alone.
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\end_layout
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\begin_layout Paragraph
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Underwater Acoustic Communications
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Global Calibration
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\end_layout
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\begin_layout Standard
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The above underwater acoustic positioning system will allow the UUV to keep
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track of it's position relative to known points at the surface, however
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this alone will not provide the UUV with its global location.
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In order for the UUV to calibrate it's local map to global co-ordinates,
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the global position of these surface points must be provided.
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This will be conducted over the previously described acoustic communication
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channel.
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As it could be expected that this channel has a low bandwidth, these updates
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need not be excessively frequent.
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\end_layout
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\begin_layout Paragraph
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Acoustic Doppler Current Profiling
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\end_layout
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\begin_layout Standard
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While accelerometers and gyroscopes would be expected components of any
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mobile dead reckoning navigation system, additional sensors well-suited
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to sub-sea localisation will allow the vessel's movement to be more precise.
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One such sensor is a
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\emph on
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Doppler velocity log
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\emph default
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which estimates the vessel's velocity by tracking the seabed.
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\end_layout
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\begin_layout Subsubsection
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Control
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\end_layout
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@ -1226,6 +1419,10 @@ Control
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Summary
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\end_layout
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\begin_layout Section
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Digitalisation
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\end_layout
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\begin_layout Standard
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\begin_inset Newpage newpage
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\end_inset
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