Autonomous Underwater Vehicles

Inertial Labs MRU-ES and Autonomous Underwater Vehicles

Overview

Exploring our planet’s oceans is a tradition that goes back centuries. Humans have an intrinsic curiosity fueled by a desire to learn about our surroundings. Motivated by this desire, we have continued to advance marine navigation technology, enabling us to explore the unknown reaches of the planet’s waters. To accomplish this feat, our ancestors used the natural tools given to them, like the stars. From there, celestial navigation developed, and devices such as the gnomon, kamal, sea astrolabe, quadrant, cross-staff, and sextant were created. By increasing our ability to go further out into the unknown waters, these ancient navigation devices paved the way for marine navigation technology to develop. These developments, in turn, fostered inventions thought to be mere fantasy and allowed humans to grow in their understanding of the Earth. That unsatisfied curiosity continued to live in our hearts and minds until 1957, when the applied physics laboratory developed the first autonomous underwater vehicle (AUV) at the University of Washington by Stan Murphy, Bob Francois, and Terry Ewart. The early AUVs were used for research purposes such as studying underwater diffusion, acoustic transmission, and submarine wake. 

What is an AUV?

An Autonomous Underwater Vehicle (AUV) is a marine vessel that can dive deep underwater and autonomously navigate. Autonomous vehicles are independent and require no service vessel or control station attachment. An AUV can navigate underwater with a preprogrammed plan or, in some cases, get remotely controlled by a user on a nearby vessel or onshore. Autonomous Underwater Vehicles are our extension into the ocean. Whether near the surface or underwater, AUVs can go where our human physical limitations would have historically prevented us from venturing. 

Applications

AUVs have various capabilities that lend them to various commercial, government, and military uses. AUVs have greatly improved our ability to survey the Earth’s oceans and ocean floor, which has allowed us to grow in our understanding of the world we live.

Given the independent nature of AUVs, they can perform tasks such as searching for planes, ships, or submerged cars after accidents. They are capable of ocean floor mapping, marine wildlife maintenance, debris field detection, fishery operations, and valuable scientific research without real-time human input. Given the challenging nature of exploring the Earth’s oceans, AUVs have greatly expanded man’s ability to survey the part of our world that makes up most of Earth. Humans will continue pushing the impossible’s boundaries with Autonomous Underwater Vehicles by exploring the sea below.     

Hardware and Navigation Techniques of an AUV

AUVs can dive deep into the ocean, under ice, and into other areas where typical forms of communication cannot reach. They have various uses and are invaluable resources when researching the oceans. But how do AUVs do this? What hardware do they possess, and what navigation techniques do they employ when navigating rugged terrain like the ocean? 

Hardware and Supporting Technology


Passive Sonar

In some cases, AUVs utilize a variation of sonar called passive sonar.  The passive sonar, juxtaposed against the active sonar, is a way to measure acoustic waves that naturally occur around the AUV.

This use of naturally occurring acoustic waves is effective because it eliminates the need for the vessel to create sound waves, which get done by an active sonar system but could disrupt the environment an AUV is monitoring. Passive sonar is desirable because it decreases mechanical and interfacing complexities for the design, thus reducing operators’ cost and risk of failure. Click here to learn more about the difference between active and passive sonar.

Navigation Sensors

  • Motion Reference Unit (MRU)

The Inertial Labs MRU utilizes advanced Kalman filtering methods to fuse pressure sensors, 3-axes each calibrated in full operational temperature range precision fluxgate magnetometers, accelerometers, and gyroscopes to provide accurate position, velocity, heading, pitch, and roll for marine applications like AUVs.

 

  • Doppler Velocity Logs (DVL)

A DVL is used in AUVs to measure the velocity of the underwater craft relative to the ocean floor. The DVL is critical in correcting real-time position, navigation, and timing data on the AUV when conducting subsea operations.

 

  • Global Navigation Satellite System (GNSS) Receiver

A (GNSS) receiver is a satellite-based navigation system that uses signals from satellite constellations to find the location of a vehicle. What is important to note here is that not all areas of the Earth have GNSS accessibility, which is known as GNSS-denied territories. Places like the deep ocean are particularly likely to be GNSS-denied territories. That is why it is essential to have a variety of sensors available to systems like AUVs. Many AUVs take occasional voyages to the surface mid-mission and utilize a GNSS receiver to correct position, velocity, heading, and timing errors that may have built up while the vessel was deep underwater, out of reach from GNSS connectivity.

 

Acoustic Modems

Acoustic modems are physical devices placed underwater or on the surface that transmit position data to an AUV. The AUV can then use that data to triangulate its position. Acoustic modems transmit data underwater by transforming digital signals into low-frequency sound waves that a receiving modem can understand. The receiving acoustic modem then converts the sound wave back to a digital signal to be read by the onboard systems used by a surface buoy or other receiving entity. This two-way communication allows for data transmission in places where conventional methods of communication don’t work (GPS, high-frequency RF transmissions). Additionally, these digital signals will enable the AUV to determine its position using a dead reckoning technique. 

Navigation Techniques


Dead Reckoning

Dead reckoning is finding the vehicle’s current position using the previously known position, estimated speed, direction, and heading over time. Acoustic modems aid in this dead reckoning process by transmitting valuable position, navigation, timing (PNT), and orientation data that allows the AUV to know its position about its previous location.

 

Long Base-Line Tracking

 

Long-baseline (LBL) tracking systems use hardware like acoustic modems and other underwater tools that the AUV detects and turn into location data. Using that data, the AUV can accurately navigate the ocean floor. This navigation technique can also get classified as an acoustic positioning system (APS). The downside to this method is that the AUV depends on these tools to find its position and heading.  

 

Simultaneous Localization and Mapping (SLAM)

 

SLAM is when an unmanned vehicle autonomously builds an image of its surroundings. Using SLAM techniques, the AUV can localize itself in that environment and gain important orientation and navigation data.

 

The Inertial Labs MRU-ES with Aiding Data

The Inertial Labs MRU-ES is critical in an AUV’s ability to navigate autonomously by providing valuable position, velocity, and timing data. Furthermore, the MRU-ES can measure a vehicle’s heave, surge, and sway, which grants the AUV the ability to navigate in GPS-denied territory. Additionally, while operating in subsea environments, it’s always beneficial to input aiding data from other sensors to increase the accuracy of position, orientation, timing, and velocity. During extensive maneuvers or testing during harsh weather conditions, increased accuracy from external input into the Inertial Labs MRU provides an extra layer of comfort. The Inertial Labs MRU-ES can receive data from external sensors using industry-wide accepted sentence formats, where it directly passes it to internal algorithms while operating. This aiding data from the AUV supplies significant heading corrections, particularly during GPS outages. These instruments do not rely on satellite Line of Sight (LoS). Additionally, you can input data into Inertial Labs MRUs from an external GNSS receiver (if the AUV rises to the water’s surface).

Inertial Labs MRU-ES and AUVS


Components of Inertial Labs MRU-ES

  • Inertial Measurement Unit (IMU)

The Inertial Labs IMU provides high-accuracy and performance applications, like AUVs. The Inertial Labs, tactical grade IMU, has a shallow in-run bias (1 deg/hr) which results in a low offset bias over some time and minimizes drift in the application. The Inertial Labs IMU also provides valuable orientation data for the AUV, allowing the vehicle to better understand its heading and orientation while on a mission.

 

  • Subsea Enclosure

The Inertial Labs MRU can be submerged underwater inside a stainless-steel enclosure. It also protects the hardware of the MRU unit while submerged underwater. Our Inertial Labs subsea enclosures can dive up to 1000m, granting a vessel greater mobility and range in its diving missions.

 

  • Embedded Fluxgate Magnetic Compass

The embedded fluxgate magnetic compass gets utilized for accurate heading calculations based on magnetic or true north. The fluxgate magnetic compass also aids navigation calculation through its advanced Kalman filter, capable of maintaining a precise PNT solution even in a GPS-denied environment.

 

Conclusion

The Marine industry is known for its price tag. Current AUV navigation and orientation technology market solutions often push reliability and professionalism before product performance and functionality. Inertial Labs is committed to meeting all those needs, not just a couple. By analyzing what sensors are available on AUVs, Inertial Labs supplies end-users with functional, reliable, high-performing solutions while maintaining professionalism. The Inertial Labs MRU-ES is engineered to accept aiding data from many sensors. Finally, after decades of research, the MRU-ES has evolved to be configurable with the most critical sensing components like a DVL or an IMU, which are both commonly used by marine vessels like AUVs. Attitude is Everything, and with the Inertial Labs MRU-ES, autonomous underwater vehicles gain access to accurate GPS-denied navigation, attitude, and timing data that allow end-users and developers to advance and develop technologies needed to support the exploration of the hardest-to-reach spots on Earth. 

Autonomous Underwater Vehicles and the Inertial Labs MRU-ES

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