Tracking UUVs in the Deep

L3Harris Ivers AUV
UUVs present a set of significant communication and navigation challenges as they evolve in a permanently GNSS-denied environment. Here is L3Harris Ivers AUV at sunset.

Operating vessels underwater has always had its challenges, but techniques are surfacing on how to manage this with the new shoal of unmanned craft.

Mesh networking, Joint All-Domain Command and Control (JADC2), Mine Countermeasures (MCM), Intelligence Surveillance and Reconnaissance (ISR)… these are but a few examples of the arsenal of terms currently being deployed to describe the Concept of Operations (CONOPS) potential for unmanned systems. Whether in the air, on the surface or underwater, these vehicles have gained popularity for extending ranges and/or keeping humans out of harm’s way.

Yet while in many domains CONOPS help define future technological trends, Unmanned Underwater Vehicles (UUV) are different. The environment in which they operate consistently presents several challenges that continue to determine the boundaries of future CONOPS. How do communication and navigation challenges – two sides of the same coin – affect UUVs deployment? Can the limitations of physics actually be turned to navies’ creative advantage?

Physics Refresher

“One of the biggest challenges in communicating with UUVs is that you have to reach them as they travel through the water column,” Dr Marc P. Olivieri, senior fellow at L3Harris Technologies, told Armada International. The water column is the concept used to describe seawater’s variations in physical characteristics – temperature, salinity, light penetration and chemical composition (pH, oxygen, etc) at a given geographical point. These variations affect how acoustics, light and radio frequencies travel in water.

Yet acoustics and light are precisely what UUVs rely on to communicate and navigate underwater. In fact, communication and navigation are very closely interrelated, since navigation is based on communication with a known reference to help determining location. “Accuracy of navigation is theoretically related to the amount of accurate information exchanged between systems,” Dr Olivieri continued.

As such, communicating underwater is subject to a constant trade-off. Radio Frequencies (RF), commonly used to communicate above water, are absorbed and refracted as they enter the water column. The only frequencies that propagate well underwater are Very Low Frequencies (VLF) and Extremely Low Frequencies (ELF), which significantly reduces the bandwidth available to transmit data at long ranges.

Acoustics propagate better underwater, but they are subject to latency and loss of signal as communication distances increase. At around 5,000 feet (1500 metre) depths, for instance, it takes one second for the signal to reach the target and one second for the target to send a signal back; the result is three seconds of latency to obtain positioning. At around 19,700ft (6000m) depths, latency can reach 10 to 15 seconds. “In practice, this means that the further underwater a UUV navigates, the harder it becomes to locate it using acoustics,” Stéphane Meyer, division manager for Subsea Positioning and Communications, told Armada.

Finally, the same trade-offs apply to lasers. Vertically, water absorbs over 50 percent of the visible light energy within the first 32ft (10m). Horizontally, violet light has a range of 240ft (75m) whereas blue light has a range of around 500ft (150m). However, the longer the range, the smaller the bandwidth.

“Within current thinking and procurement paradigms all these trade-offs could be perceived as obstacles,” Dr Olivieri continued. However, one could also flip the script: “How can these trade-offs be leveraged to design and deploy solutions differently and reduce detection/interception probability?”

Autonomy in Challenging Environments

“What is keeping us awake is thinking how, with what we do, we can improve the autonomy of unmanned vehicles so that we can accommodate more CONOPS,” Joe Tena, commercial director, Forcys, told Armada. Even systems fitted with very efficient batteries only have limited autonomy if they are unable to navigate autonomously in GNSS-denied environments. “And the underwater domain is essentially a GNSS-denied environment,” Tena continued.

One of the key technologies for enabling navigation in GNSS-denied environments is the Inertial Navigation System (INS). INS are dead reckoning systems that use accelerometers, gyroscopes and a computer to continuously monitor the location, orientation and velocity of a moving object. INS allow navigation without GPS; however they present one challenge: over time, unaided gyroscopes and accelerometers accumulate small errors that make positioning drift. This means that over a certain distance, they will start providing the wrong positioning.

“We have always worked to prolong the ability to dead-reckon accurately for as long as possible,” Tena commented, and in order to do this, Forcys technology partner Sonardyne has experimented with different technologies. The use of seafloor nodes with acoustic beacons is one option. Geostationary at depth, they provide a reference that INS can use to update their position and address small errors. “Imagine an underwater satellite constellation but with acoustic beacons,” Tena added.

Sonardyne has also been working on developing hybrid acoustic inertial navigation. Key behind this type of hybrid navigation is the fusion of different sensors’ data into one single algorithm for enhanced precisioning. Normally, processed data from different sensors – e.g., latitude/longitude from the GPS, depth from depth sensors, pressure data, etc – is fed into the Doppler Velocity Log (DVL) to provide positioning to the INS, which sits separately. Sonardyne has been working on taking raw unprocessed data and feeding it directly into an algorithm integrated into the INS. “This is a subtle difference that means we can model the system better for improved accuracy and performance,” Tena noted.

iXblue has also been developing its own version of hybrid acoustic inertial navigation aimed at tracking subsea systems from a mothership: Gaps. Gaps systems are based on Ultra Short Base Line (USBL), which refers to four small antennas placed 30cm apart from each other under the mothership. The mothership emits an acoustic ping to localise the autonomous system and, based on the time it takes for the ping to return and the moment the ping reaches the four different antennas, it determines the distance and position of the system underwater. If the ping returns to all four antennas at the same time, it is located right under the ship. Gaps can track multiple UUVs at once; “in such case scenario, what is key is the acoustic modulation pattern to be able to distinguish the different systems,” Meyer specified.

“With USBL we calculate a relative acoustic position,” Meyer said. If an absolute position is desired, with latitude and longitude, iXblue offers an all-in-one Gaps system that also features an INS. “Because we know everything about the antenna’s position, we see this as bridging GPS with the underwater acoustic world.” Gaps systems include M5 (3,280ft – 1,000m range), M7 (13,123ft – 4000m) and Posedonia (32,800ft – 10,000m), and Gaps M7 was successfully demonstrated in July on ECA Group’s new R7 ROV at the Polish Naval Academy.

iXblue's Gaps M
iXblue’s Gaps M7 was successfully demonstrated in July on ECA Group’s new R7 ROV at the Polish Naval Academy.

Flipping the Script

Flipping the script about communication and navigation in the underwater domain is also about using the challenges of physics to one’s advantage. “The RF domain above sea is very busy today, and very easy to jam, whereas the underwater domain is more protected,” Hanan Marom, vice president of Business Development and Marketing at DSIT, told Armada. As peer and near-peer competition continues to grow, moving some communication underwater could prevent it from being jammed and give strategic advantage.

To enable more efficient and protected underwater communications, DSIT has developed the White Pointer, an underwater acoustic communication system that enables communication networks between surface and subsurface platforms. “The White Pointer relies on acoustic communication because it is not constant communication and does not transmit large volumes of data, so it is much harder to detect,” Marom explained. However, because acoustic ranges can be limited, White Pointer was designed to work as a network, all the systems within the network working as relays.

What is particularly interesting with White Pointer is the level of encryption. White Pointer is programmed – before but also during the mission, if there are any changes – so that platforms’ access to information is given according to clearance levels. Platforms that are not granted clearance level to access information, be they UUVs, divers, or USVs, act as simple relays between two points. “We saw that use of underwater platforms to gather and relay information at ever increasing ranges was growing, and we developed White Pointer to enable networking between capabilities so as to keep extending those ranges,” Marom concluded.

White Pointer
DSIT has developed the White Pointer, an underwater acoustic communication system that enables communication networks between surface and subsurface platforms.

In fact, encryption and multi-level security is critical when sharing data across multiple platforms within a network. That is why SAIC announced in April 2021 its acquisition of Koverse, a software company that provides a multi-level secure data management platform enabling AI and Machine Learning (ML) on complex, sensitive data. During the Sea Air Space 2022 convention, Josh Jackson, senior VP and Naval Business Unit Lead at SAIC, introduced the author to the Koverse platform as a tool designed to facilitate the application of AI and ML to data analytics. Through the zero-trust platform, different levels of data visibility are assigned to different users depending on their level of clearance. “This way, a low clearance level does not preclude anyone from carrying out their tasks; they simply do not see what they are not cleared to see,” Jackson highlighted.

Last but not least, according to Meyer, “currently underwater communication is very proprietary, however, to address navies’ need for platforms that can communicate with each other securely, open standards are indispensable.” Sonardyne has been working closely with the UK Defence Science and Technology Laboratory (DSTL) to develop a new open standard called Phorcys. Phorcys is an open acoustic standard that anyone can implement. It is a secure waveform that allows each user to encrypt their communications using private keys communicated only to those within the network. “This is a step forward from JANUS [another open acoustic communication standard] because it addresses low, medium and high frequency bands, allowing more freedom for secure communications,” Tena added.

Additionally, as UUVs are increasingly part of communications networks that also include surface assets, Blu Wireless, for instance, has developed the Millimetric Wave (MMW) communication technology. MMW technology provides wideband communication with a very narrow beam that has a very stealthy electronic signature, thus making it difficult to detect and jam. “UUVs’ main purpose is to detect underwater threats and relay that information back to the mothership,” Macy Summers, President and CEO of Blu Wireless, Inc., told Armada. “If UUVs approach USVs to transmit data and that data is then transmitted through MMW, the chances of detection are very limited.”

Enabling Swarms

“Over the past years we have seen our customer navies increasingly focusing on the challenges posed by near-peer adversaries and their unmanned systems,” Tena noted. Addressing such threat demands not only that navies procure their own unmanned systems, but that these systems be able to evolve and communicate in swarms. “These systems are essential for keeping crew out of harm’s way and extending the range of crewed vessels,” Tena added.

Sonardyne has been exploring how to support UUV swarms using acoustics, which can be challenging due to acoustics range limitations. To enable this, Sonardyne has also been working on using USBL: the mothership on which antennas are mounted constantly transmits information to UUVs to communicate both their absolute position and their position relative to one another. “We have designed a flexible architecture and protocol to enable us to exchange more information from the surface to the UUV and between UUVs,” Tena said.

Currently, most of the work being carried out by Sonardyne on UUV swarms remains in the commercial domain. This is primarily due to the fact that navies need to rely on systems that are difficult to detect and the use of acoustics challenges that specific requirement. Nevertheless, as has generally been the case for all things UUVs, a number of navies are starting to experiment with these concepts in the MCM domain. “When carrying out MCM missions the discretion imperative is not as prevalent as with other missions,” Meyer noted. As such, both iXblue’s Gaps and Sonardyne’s latest efforts are targeting these types of missions to start fielding these new systems.

Sonardyne has been exploring how to support UUV swarms using USBL.
Sonardyne has been exploring how to support UUV swarms using USBL.

Keeping It on the DL

“Underwater communication is not a new domain, technologies enabling communications and navigation in such a GNSS-denied environment have existed for quite some time,” Dr. Olivieri commented. What is changing is the way in which these technologies are being applied to develop new CONOPS for UUVs.

In fact, while the complex physics of the underwater domain continue to present certain constraints, the use of UUVs is shifting several paradigms. Key amongst those is the necessity to work in networks. This not only allows navies to extend their reach through the use of relays, but it also allows them to envisage a new type of information sharing in a domain where detection and interception are difficult. On this premise, several defence companies have been working on developing means of encrypting and protecting data. In Dr. Olivieri’s words, new paradigms are emerging around this question: “How can I use the absorption issue as an advantage to design solutions differently and reduce the probability of detection and interception?”

As the proverb goes, “necessity is the mother of invention.” Nowhere is this more relevant than in the underwater domain where networking, encryption, sensor fusion, and more, all combine to circumvent the limitations of physics.

by Dr. Alix Valenti