A growing market of small, hand-portable AUVs and ROVs is enabled by advancements in sensor technology, such as the Nortek Nucleus 1000 navigation sensor package.

 

By Rachel McAlpine • Lila Johnson • Asim Azad

Established in 2021 with a commitment to innovation, collaboration and a pioneering vision, the General Oceans group has rapidly become a driving force in the underwater technology industry. Comprising six operating companies—Nortek, Tritech International Ltd., Klein Marine Systems, Reach Robotics, Strategic Robotic Systems and, most recently, RS Aqua—General Oceans is at the forefront of developing cutting-edge solutions that push the boundaries of subsea exploration and sensing.

In this article, we highlight the latest technologies from Nortek and Tritech that are shaping the future of the industry.

Growing Low-Cost Vehicle Market

The world of subsea vehicles has changed rapidly over the last few years. While large work-class ROVs still dominate larger and more specialized subsea operations, a new market of small, easy-to-operate AUVs and ROVs has exploded.

The growth in availability of these smaller and ever-more-capable vehicles would not be possible without advancements in sensor technology. One of the key sensor elements that has previously limited these small vehicles from performing at a high level was the lack of a compact and affordable navigation system option. Now, advanced navigation sensor technology makes it possible for even small vehicles to be equipped with a single sensor that collects all the necessary data to navigate underwater.

 

The Nucleus 1000 is about the size of a can of tuna, ideal for very small AUVs or ROVs.

 

Subsea Navigation Systems

In many cases, a subsea navigation system comprises a Doppler velocity log (DVL) for speed over ground and an inertial navigation system (INS) for directional information, or a similar combination. However, these sensors have historically been prohibitively expensive and too big to be considered for smaller vehicles, not to mention the complex integration requirements associated with this type of navigation system.

These limitations meant that navigation systems on small AUVs or ROVs were either foregone or limited. This in turn meant that small vehicles were limited in the types of jobs they could do; without a reliable navigation system, certain tasks were out of the question, especially autonomous tasks without input from an operator. Even station-keeping is a challenge without a reliable positioning sensor.

The advent of newer and, importantly, smaller DVL technology has now opened doors for these types of vehicles to perform missions that they could not do previously. These vehicles typically don’t require the same level of survey-grade accuracy required by work-class ROVs but still greatly benefit from navigational sensors.

Nortek’s Nucleus 1000 is one example of a sensor suite that provides navigational capabilities to these small AUVs and ROVs. The Nucleus offers the additional benefit of being a combined sensor suite that offers DVL and INS (or attitude and heading reference system, AHRS) information in one small package.

 

This diagram shows some of the key steps associated with creating a subsea navigation system. The Nucleus sensor package brings together multiple data streams, removing some of the integration work typically required.

 

Benefits of a Combined DVL/INS Sensor Suite

While, on its own, DVL technology getting smaller creates possibilities for small vehicles, a DVL alone does not enable these vehicles to navigate. The Nortek Nucleus 1000 is unique in that it offers both DVL functionality and a pre-synchronized AHRS or INS. This alleviates the need for multiple sensors and reduces the amount of integration required to achieve navigation underwater.

In addition to providing a dedicated INS data stream, the Nucleus software shows a vehicle’s location in real time on a map using inputs from the sensors and navigation algorithms.

Users can now add a source of basic positioning to their vehicle without substantial time, cost and payload demands. The Nucleus only needs to be provided with a GPS position to begin navigating using absolute positioning.

Providing Earth-referenced position to a vehicle control system means that users who need to know where they are, where they’ve been and where they’re going now have a preconfigured, pre-synchronized navigation package.

 

The Nucleus 1000 shows a vehicle’s location on a map in real time, requiring only an initial GPS input.

 

Additional Sensor Capabilities

Payload demands are one of the biggest limiting factors on adding sensors to small ROVs and AUVs, so combined sensor suites such as the Nucleus aim to provide as much information as possible without requiring additional space or power from the vehicle.

The ability to collect current profiles, for example, is beneficial for several applications. If the environment where the vehicle is being deployed has currents whose strength exceed the capacity of the vehicle, then an operator can deploy the vehicle at the surface to take a current profile. Scientific applications often require current profile information, which can be collected from the same sensor that the vehicle uses to navigate. Collecting current profiles is also beneficial as a safety check before sending divers into the water.

The Nucleus also has a dedicated altimeter beam, which can take a direct measurement of the distance to the seafloor, and has dedicated temperature and pressure data streams as well.

Reliable Sensors for Autonomy 

As the subsea vehicle industry continues to grow, many vehicle manufacturers and users are pushing the limits of what vehicles can do without intervention from a pilot or other operator. This increase in autonomous operations means that onboard sensors must become increasingly reliable.

While small sensors such as the Nucleus may not have the same long-term accuracy required by larger work-class vehicles, having a compact, reliable and multifunctional navigation sensor suite on board small vehicles makes a world of difference in the ability to perform autonomous functions.

Even if vehicle autonomy is not the end-goal of a user, having reliable and easy-to-use equipment enables crew members of any experience level to launch and operate small vehicles for inspections or other applications. Station-keeping, operating at a constant depth, or navigating to waypoints in unfamiliar terrain all require data from sensors such as the Nucleus.

Case Study

When the French AUV company RTsys required a sensor for its NemoSens micro-AUV that would not only contribute to vehicle navigation but also collect current profiles for a dredging application, Nortek’s compact Nucleus 1000 sensor fit the small footprint required for the application.

RTsys worked together with Nortek’s support team on the integration of the sensor. The Nucleus on board the NemoSens was used to navigate the AUV and collect current profiles in an area near the dredging operations, with the goal of characterizing the movement of the sediment plume.

 

Tritech Gemini 1200id: Air bubbles from divers escaping from a hatch on a wreck.

 

Gemini 1200id Sonar

The Gemini 1200id is the latest addition to Tritech’s multibeam sonar portfolio, targeted specifically at work-class ROVs and other applications where reliability is of the utmost importance. Based on the industry standard Gemini 720is platform, the Gemini 1200id employs a mechanical design that is tried and tested in the harshest of subsea environments and offers a degree of redundancy by design not afforded by any other platform. The sonar has been developed largely in response to requests from existing customers who regularly operate the Gemini 720is to depths as low as 4,000 m.

In addition to the 720-kHz acoustic frequency offered by the Gemini 720is, the Gemini 1200id introduces dual-frequency operation with the addition of a new 1.2-MHz acoustic operating frequency, which improves angular resolution from 1 to 0.6° and range resolution from 4 to 2.4 mm, resulting in a significant improvement in imaging resolution.

 

The Tritech Gemini 1200id multibeam sonar is ideal for work-class ROVs. It has an operating depth down to 4,000 m.

 

Gemini 1200id Design

The introduction of the 1.2-MHz acoustic operating frequency to the Gemini 1200id necessitated a redesign of the analog front-end electronics to handle the higher sampling rate requirements of the new higher-frequency mode of operation. Rather than taking the easier and cheaper route of transitioning over to single-ended analog front-end integrated circuits developed for ultrasound applications as typically used by the majority of other multibeam sonars, the Gemini 1200id continues to employ an in-house developed analog front-end solution that incorporates fully differential receiver channels consisting of discrete low-noise amplifiers, discrete variable gain amplifiers, second-order bandpass filters, and discrete high-speed, high-resolution serial ADCs (analog-to-digital converters).

This custom analog front-end solution enables the receiver to be optimized for longer-range sonar applications, rather than shorter-range medical ultrasound applications that primarily drive the development of ultrasound analog front-end integrated circuits, due to the relatively larger volumes involved. As a result of its superior analog front-end tailored for sonar applications, the Gemini 1200id has a dynamic range twice as wide as its nearest competitor.

The electronic circuits within the Gemini 1200id have also been optimized. Additional filtering is exclusively employed to minimize self-generated noise around the two acoustic operating frequencies, resulting in significant reductions in unwanted artifacts that can appear in the sonar image. The Gemini 1200id also actively attenuates external waterborne noise from other subsea equipment in the vicinity of the sonar, including from the likes of ROV thrusters, which have a tendency to generate relatively large levels of electromagnetic interference. By pushing self-generated electrical noise away from the acoustic operating frequencies, and by actively attenuating external waterborne electromagnetic interference, the Gemini 1200id offers welcomed improvements in signal-to-noise ratio (SNR).

The wide dynamic range and enhanced SNR of the Gemini 1200id enable it to produce sonar images displaying sharp and bright acoustic returns on a noise-reduced dark background, resulting in crisp sonar images of excellent clarity.

A flight-time sensor operating at 1 MHz and integrated into the side of the Gemini 1200id measures the speed of sound in the water immediately around the sonar at the time of acoustic reception to ensure that observed targets are displayed accurately for position and size.

All auxiliary port features supported on the Gemini 720is are also supported by the Gemini 1200id, and the depth rating of 4,000 m is maintained, ensuring that the Gemini 1200id offers a direct upgrade path for customers who until now have been reluctant to consider anything beyond their trusted Gemini 720is sonars for mission-critical operations.

The Gemini 1200id can be controlled using either Genesis or software development kits that support integration into third-party applications. Genesis is Tritech’s latest all-in-one software package for controlling, displaying and recording data from its portfolio of sensors. It boasts a modern, dynamic and easy-to-use interface with a comprehensive suite of integrated features.

To allow the Gemini 1200id to share bandwidth with other equipment and sensors that may be present on an existing or shared network connection, the data bandwidth consumed by the sonar can be limited by capping the ping rate, reducing resolution, or enabling data compression.

Every Gemini 1200id sonar that rolls off the production line is subjected to a strict factory acceptance testing regime to guarantee quality and performance. As part of this testing, every function of the sonar is verified and additional tests at pressure limits conducted to confirm operation under extreme conditions. The design of the sonar is qualified to ensure reliability against shock and vibration, corrosion, and for compliance against relevant electromagnetic compatibility (EMC) regulations.

The Role of Small Sensors

The subsea industry is growing quickly. Vehicle manufacturers rely on sensor innovation to bring their vehicles into harsh environments, perform complex tasks and reduce the need for divers in dangerous situations. Sensors such as the Nucleus and Gemini 1200id sonar are just one piece of a much larger puzzle that enables these innovations, and sensor technology will continue to evolve as the needs of the subsea operations market change.

Rachel McAlpine is the group marketing manager at General Oceans.

Lila Johnson is a marketing specialist at Nortek.

Asim Azad is the hardware engineering manager at Tritech International.