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28 Navigation and comms The required navigation accuracy is entirely a function of the kind of missions customers want their Sabertooths to undertake, Siesjo stresses, adding that in ROV mode navigation is done by recognising things on the sonar display. With more sophisticated AUVs, however, the core sensor mix is a combination of an inertial measurement unit and a Doppler velocity log (DVL). These are available at various price points and levels of accuracy, and are usually specified by the customer. Among its standard options for navigation sensors, for example, is the Syrinx DVL from Sonardyne, or a high- end inertial sensor such as a fibre-optic gyro from a supplier such as iXblue. It is also possible to specify a system that supplements onboard sensors with references to external networks of acoustic beacons and transponders. “Suppliers such as Sonardyne, Kongsberg and Evologics all supply complete or partial systems,” Siesjo says. “For example, Sonardyne’s SprintNav has built-in support for navigating relative to Sonardyne transponders. Some customers prefer Sonardyne, others prefer Kongsberg. On the commercial side it is very much customer-driven.” While such high-end components are very reliable, failures can cause serious problems, as the kind of graceful degradation required in safety-critical manned systems is not part of the AUV culture yet. “If you are carrying out an autonomous mission and your DVL stops working, there isn’t much you can do other than bring the vehicle back to the surface. In discussing future vehicles that could be deployed on the sea floor for long periods, redundancy could be a good solution,” Siesjo says. “If you have some degree of manual control, it is up to the supervisor or pilot to see if something can still be achieved, but it gets quite complicated quite quickly when you weigh degrees of degradation against requirements for getting data back.” On the surface, the vehicle can use UHF, satcom and wi-fi to communicate, all through a single cylindrical antenna that also receives GPS signals. High- bandwidth underwater comms are via an optical system from BlueComm (now part of Sonardyne), which provides bandwidth, data rates and range very similar to those that wi-fi achieves through the air. Acoustic systems are also an option, but these offer much lower data rates of a few kilobits per second over short ranges, falling to rates measured in bits per second over greater distances. Acoustic comms are also vulnerable to interference from sources of noise aboard the vehicle, so Saab has put a lot of engineering effort into identifying, eliminating and filtering out problem frequencies. The same applies in principle to EMI, the suppression of which is also very important if the multiple sensors aboard any vehicle, including the Sabertooth, are to work successfully. Control and autonomy In creating the overall control scheme, Saab took what Siesjo describes as a layered approach, with the lowest layer managing simple things such as stability and station-keeping. “On top of that, you have a more or less direct link to an interface over which an operator could send control signals, or it would go to February/March 2021 | Unmanned Systems Technology The Sprint-Nav is a compact all-in-one subsea navigation instrument combining Sonardyne’s Sprint INS, a Syrinx DVL 600 kHz doppler velocity log and a high- accuracy intelligent pressure sensor (Courtesy of Sonardyne) As the vehicle gets closer to an object it needs to avoid, it gets more and more negative input from the obstacle avoidance system