Bringing special order accuracy to small diameter AUVs

L3Harris has armed its Iver3 AUV with Special Order survey navigation capability – thanks to our Mini-Ranger 2 Ultra-Short BaseLine (USBL) system and AvTrak 6. Find out how – and how it will help cut offshore wind farm survey costs.

The challenge

Demand for underwater surveys in coastal waters as part of the offshore wind life cycle is at an all-time high. There’s little sign of that demand falling, thanks to a global shift to renewable energy. But surveys come at a cost.

To keep costs down, surveyors are looking at innovative ways to survey more seabed more efficiently using smaller AUVs that reduce operational costs by requiring smaller vessels and fewer people and reducing their overall carbon footprint.

This includes International Hydrographic Organization (IHO) classified Special Order standard surveys, which are often specified for pre- and post-surveys underwater power cables and UXO surveys.

To meet navigation requirements, Special Order surveys in shallower waters tend to require ship-mounted sensors, to achieve the absolute positional accuracy required (using the vessel’s RTK/GNSS position).

In deeper waters, heavier, larger diameter AUVs are used. Because they rely on a DVL and an inertial navigation system (INS) for their navigation, they also must be supported by large vessels.

But what if you could perform Special Order surveys in shallower coastal waters with smaller low-cost AUVs, reducing your reliance on large vessels, reducing logistics, emissions and operational costs?

This was the challenge set by customers of L3Harris who were keen to find ways to use their Iver3 AUV for Special Order surveys.

Specifically, they set out to test if USBL-aiding at high enough accuracy for Special Order surveys could be achieved within the size, weight and power limitations of a smaller, light weight AUV platform, such as the 5.8 in-diameter Iver3.

If it could, it would allow underwater run-times of more than eight hours, and a new paradigm in lower operational cost coastal surveys.

The solution

L3Harris had already used our Mini-Ranger 2 for USBL aiding an iXblue Phins INS, via one of our AvTrak 6 OEM Nanos, on one of its Iver3s on a separate customer project. Read more here.

Mini-Ranger 2 is the ideal USBL system for coastal operations, supporting high-elevation tracking of up to 10 targets simultaneously down to 995 m water depth (extendable to 4,000 m), as well as data harvesting. It offers performance without the cost and complexity of a larger deep water USBL system.

At the heart of the system our HPT 3000, a highly capable surface deployed USBL transceiver which is optimized for performance in shallow water, high elevation and long lay back operating scenarios, as well data telemetry.

To provide INS aiding, L3Harris’ customer chose to integrate our AvTrak 6 OEM Nano, with a remote transducer, into their Iver3.

AvTrak 6 OEM Nano is the smallest variant of our AvTrak transceiver, designed for ease of integration on smaller underwater vehicles. It combines the functions of a transponder, transceiver and telemetry link, enabling communications, tracking and USBL aiding for subsea robotics. It’s also based on our 6G hardware platform, which means it’s interoperable with all our USBL, LBL and INS systems. This provides flexibility, but also performance enhancement, for example when combined with a Sonardyne INS.

As in the earlier project, the AvTrak 6 was programmed to speak the right language to the third-party IXBLUE Phins INS, by sending out an APOS PSIM SSB telegram, instead of our proprietary SPOS. The AvTrak 6 transducer was also supplied on a special Iver3 mounting post design, to make it easy to plug and play into the AUV – and any other Iver3, for that matter.

Mini Ranger 2 was pole-mounted to the support vessel with hemisphere GPS for topside absolute positioning.

The results

L3Harris ran a <5 km-long survey, with six 800 m legs, in challenging <20 m water depth in Narraganset Bay on the north side of Rhode Island Sound.

Real-time USBL-aiding improved the survey accuracy by upwards of 7 m on far end of survey legs through the turns. During the survey, real-time survey performance ranges up to 800m were tested with USBL position standard deviations ranging from 1 – 3 m.

In addition, using Mini-Ranger 2’s robotics pack, the L3Harris team were able to provide the vehicle with information of the acoustic environment, so its autonomy could make decisions to adjust the AvTrak 6 OEM Nano’s power and gain settings to improve acoustics without a human in the loop.

“The standard deviation on the USBL position fixes were accurate allowing real-time position updates to the INS. The ease of operation using the Mini-Ranger 2 topside to track and monitor environmental influences throughout the survey also added a layer of safety for monitoring the UUV.”

“This set up allows surveyors to meet Special Order navigation standards, which is a big deal for small unmanned underwater vehicles (UUVs),” says John Sloat, Application Engineering Lead, L3Harris. “The problem before has been that there’s not a lot of real estate inside smaller UUVs to house the onboard technology required for Special Order navigation capability in. But now they can.

“This capability adds a high level of survey efficiency by reducing the need to frequently surface for a position fix which increases the vehicles endurance, reduces the risk of vehicle entanglement or loss, and allows for larger areas to be surveyed. It also means they have real-time data vehicle positioning, which, when post-processed, could be improved even more. That’s really valuable to survey customers and we do see this as the cusp of a new UUV survey era,” adds John.

What was also hugely valuable to the L3Harris team was the support from Sonardyne’s experts.

“The level of support and service from Sonardyne has been exceptional,” says John. “We are very pleased. It’s been a fantastic experience at every phase of the integration we have done and that’s a big de-risk for us, having a partnership like this.”