ecoSUB navigation
Network localisation is an ideal solution to enhance the positional accuracy of AUVs.
ecoSUB AUVs are designed to be smart, low cost platforms. Enhancing navigational accuracy typically involves ustiling expensive and often high power navigational sensors, such as DVL and/or INS, in some cases these sensors can be upwards of GBP 100k, between 5-10 times the price of an ecoSUB AUV.
Early design concepts for ecoSUB accepted they would use dead reckoning for navigation, and that accuracy would be limited to a percentage over distance travelled, typically +/- 10%. Precise navigational accuracy is not essential to all applications (ocean gliders have very limited navigation and provide extremely useful data) and the value to users of a low cost AUV platform suggested this trade-off was welcome.
ecoSUB AUVs are designed to be smart, low cost platforms. Enhancing navigational accuracy typically involves ustiling expensive and often high power navigational sensors, such as DVL and/or INS, in some cases these sensors can be upwards of GBP 100k, between 5-10 times the price of an ecoSUB AUV.
Early design concepts for ecoSUB accepted they would use dead reckoning for navigation, and that accuracy would be limited to a percentage over distance travelled, typically +/- 10%. Precise navigational accuracy is not essential to all applications (ocean gliders have very limited navigation and provide extremely useful data) and the value to users of a low cost AUV platform suggested this trade-off was welcome.
Illustration of dead reckoning - error is a percentage of distance travelled - the further the vehicle travels without GPS, the greater the uncertainty is
We quickly realised that the interest in ecoSUB AUVs was substantial and many potential users were keen to see better navigation to enable applications like imaging, that require good standards of geo-referencing. During 2018 we received funding from Innovate UK, along with National Oceanography Centre and University of Newcastle, to develop a solution which enabled vehicles to work together, supported by low cost nano-modems to provide a method of localisation within a smart network to enable better navigation.
Network localisation
Network localisation involves nodes which maintain position on the surface receiving GPS, and when pinged by underwater vehicles, broadcast their position using an underwater acoustic modem. The underwater vehicles calculate the range from the broadcast node and triangulate their position. We suggest working with a minimum of three surface nodes, but the vehicles can use as few as one (just with higher potential error). The surface nodes can be ecoSUB AUVs, and this allows for a dynamic network with potentially interchanging roles, alternatively our Hermes C3 box or Hermes C3 OEM with acoustics can be used either from a fixed structure (jetty, shore, buoy, etc), deployed from a vessel, or as a payload on a Unmanned Surface Vehicle (USV). The network is unlimited in scale and users can cover a large area with many surface nodes, with nodes dropping in and out of the network as required.
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On board the underwater vehicle, position information is fused by an algorithm with the vehicles own dead reckoner information to treat uncertainty and provide the most accurate estimation of real position, the algorithm then enhances the vehicles navigation, correcting course accordingly. An enhanced awareness of position underwater, effectively using underwater GPS, enables the vehicle to be more situational aware and able to respond to currents, for example, which the dead reckoner would be largely unaware of.
Network localisation has been shown to increase navigational accuracy to +/- 5-10m regardless of distance travelled, which marks a major advance in low cost AUV operation and the opportunity to engage in many more interesting applications. |
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Cost of using network localisation
The cost of operating Network Localisation is low. Typically we use our ecoSUBu5 vehicles to act as surface nodes, three of these vehicles with nano-modems are likely to cost under GBP 45,000 ex VAT EXW UK, and then multiple underwater vehicles can be deployed to complete work quickly. Deploying three surface nodes and three ecoSUBm5's with nano-modems has a total equipment cost of less than GBP 100,000 ex VAT EXW UK - substantially less than a single light class AUV with equivalent navigation sensors.
ecoSUB LBL surface nodes can also be deployed on fixed platforms. Users operating regularly in offshore wind farms or ports and harbours can use special versions of HERMES C3, with solar power packs installed on turbines or even navigation buoys to provide a semi permanently deployed LBL network, so that ecoSUB can be deployed routinely in a regularly surveyed location .
ecoSUB LBL surface nodes can also be deployed on fixed platforms. Users operating regularly in offshore wind farms or ports and harbours can use special versions of HERMES C3, with solar power packs installed on turbines or even navigation buoys to provide a semi permanently deployed LBL network, so that ecoSUB can be deployed routinely in a regularly surveyed location .
Network Localisation paper published
The work completed in the development of Network Localisation has been published in Field Robotics Journal - click here to download the paper
ecoSUBs can operate in a network...
The network layer developed has a lot more scope than just providing localisation. Our network can provide a medium for vehicle-to-vehicle communication, mission re-tasking, data sharing, adaptive strategies and sampling techniques, gateway communications and many more potential applications.
