Improve ROV navigation and avoid obstacles underwater with sonars

What is a sonar and how do they work?

Sonar is an acronym for SOund Navigation And Ranging. Sonar technology has been used in most marine and aquatic industries for decades. This tech is integrated into a wide range of products. Some sonars are as simple as the echo sounders used on fishing vessels to locate fish. However, some are highly advanced and are used in highly specialized navigation or even threat detection by military vessels.

Most sonars, though complex, can be explained simply; you make a noise, or a sound wave, going out from your position, and depending on the time and strength of echoes returning to your position, you can determine how far away you are located from something. For some types of sonars, one can also determine how big, thick, or what kind of items are around you since some materials reflect sound more than others.

What are underwater sonars used for?

There are limitless use-cases for sonar, but we will focus on the (3) we see most:

• Collect data such as seabed mapping or imaging of items underwater
• Underwater navigation in less than ideal visibility or at long distances
• Locate or search for items or missing persons

Scientists and researchers typically use sonars to map the seabed. They collect data to provide information on protecting and managing life and assets below the surface. The ROV is often essential to understanding the collected sonar data since its camera records real-time video and images. These images and sonar data give a more complete picture.

Underwater navigation is vastly different than navigation above the surface. The ocean space can be dark, affected by particles in the water creating low visibility, and is generally an unknown space for humans. Sonars make it easier for ROV operators to understand where the ROV is located relative to the items discovered. Properly integrated sonars can extend your vision dramatically, provide detailed location information, and make navigation much simpler.

Closely related to underwater navigation is the use-case of search and rescue. Sonars are often used to locate or search for items or missing persons. As sonars reflect echoes from objects far beyond what a camera can capture, a tool like this on an ROV naturally helps search operations.

Two main categories for underwater sonars: active and passive

Sonars are generally categorized as either active or passive. Passive sonar uses listening only to detect noise from marine objects such as ships and marine animals. These types of sonars are popular for military vessels and scientist missions that concentrate on “listening” to the ocean. Active sonars send out an acoustic signal and then listen for a reflection (echo) of that signal. It’s the active type of sonars that are mounted on ROVs as they can be used to provide depth indications, object imaging, and aid in navigation.

Active sonar systems make use of beam-forming, frequency shifting, and backscatter analysis to characterize targets insonified. - Christ and Wernli Sr (2007)

Active Sonar Systems: Single- versus multi-beam

According to Christ and Werli Sr (2007, p. 401), active sonar systems used on ROVs generally fall into three categories:

• Multibeam
• Mechanically or side-scanning sonar (single-beam)
• Single-beam directional sonar

Single-beam sonars use one pulse with one reception on a single receiving element. These systems are popular on the market today because of their simplicity and low cost and are popular for mapping larger seafloor areas. On the other hand, Multibeam transmits one wide pulse in a fan-shaped pattern and receives the backscatter (sound energy bouncing or reflecting off items or surfaces) on a large number of receiving elements on a detailed level that provides a quick image generation. It collects data on both seafloor depth and backscatter. Multibeam is especially useful for imaging details on structures below the surface and acts as the ROV operator viewing tool.

The difference between imaging- and profiling sonar systems

Imaging sonars have a fan-shaped sonar beam that scans a given area by either rotating or moving in a straight line. An ROV operator using an imaging sonar should be able to recognize sizes, shapes, and surfaces reflecting characteristics of the chosen target. The primary purpose of the imaging sonar is to act as a viewing tool (Christ and Wernli Sr, 2007, p. 404).

Profiling sonars use a narrow pencil-shaped sonar beam to scan across the surface of a given area, generating a single profile line on the display monitor. This line, consisting of a few thousand points, accurately describes the cross-section of the targeted area (Christ and Wernli Sr, 2007, p. 405). The primary purpose of profiling sonars is often to measure the depth or for creating sea bottom characterization. One can utilize both imaging and profiling sonars on one ROV to accomplish both imaging and altitude measurements (distance from bottom).

What type of sonar should I choose?

There are quite a few factors to consider when choosing the correct sonar for your ROV. When selecting, remember to keep in mind your use case, your prior knowledge, and your budget. The differences in sonar models can be compared to the differences in cameras. Simple technology can be quite cost-effective and intuitive to operate, while advanced imaging will require a more significant investment and a higher degree of operational competence.

Operating a point and shoot camera is easy but may not produce the desired outcome. Using a DSLR is more complex, but with the right training, will produce vastly superior pictures.The barrier to utilizing any advanced system’s full potential can be high, and in many situations, extensive knowledge on use and technology is needed. Remember to consider the learning curve associated with the sonar device you choose for your ROV.

Before deciding on a sonar system, it's important to properly review your use case(s) and not just go for the “best available” equipment. If you need a sonar to map or scan a larger area, to locate an object, or do seabed mappings, a 360-degree scanning sonar would likely fit your needs. However, a multibeam is probably the best option for use cases where image details are extremely important, or for navigation in real time.

The list of sonars is long, their cost varies greatly, and the technology ranges from simple to very complex. It is your job as the client/operator to find the device that best fits your needs. We at Blueye have a proven track record of helping our clients select the right tools to get the job done, and we would like to help you too. Please get in touch with us if you would like to know more about how sonar technology can assist your business. We are here to help.

Applications for ROV sonars

• Create 3D renderings of structures and inspect material integrity in detail
• Geophysical research
• Bathymetry and item location
• Mapping seabed
• Locate lost or missing objects
• Increase the visual range for search and rescue missions
• Easier maneuvering of ROV in murky waters
• Material identification

Sonars available with the Blueye X3 ROV

360 degree mechanical sonars

ISS360 Imaging Sonar from Impact Subsea

The ISS360 Imaging Sonar provides excellent image clarity with a range capability of up to 90 meters. As a very compact imaging sonar, the ISS360 is ideal for navigation, obstacle avoidance, and target identification sonar.

Ping360 Scanning sonar from Blue Robotics

With a Ping360 scanning sonar mounted on a Blueye X3 ROV, you'll get a top-down view of the ROV's surroundings. See echoes from objects such as ropes, walls, dock pilings, rocks, boats, fish, and any other structures or objects that reflect sound waves. With that, you have reference points to navigate from, regardless of water visibility, and you can locate important features in the water quickly.

Ping Sonar Altimeter and Echosounder from Blue Robotics

The Ping Echosounder and Altimeter is a low-cost underwater sonar that measures the distance to objects underwater. It has a 30-meter range, 30-degree beamwidth, and 300-meter depth rating. This also allows for the new control mode auto altitude. This mode can be useful when inspecting a pipeline for instance.

Multibeam sonars

Oculus M750d from Blueprint Subsea

The benchmark in multibeam imaging sonars. The Oculus M-Series offers single and dual-frequency models, unparalleled image quality in a compact, rugged form factor, and a depth rating of 500 m.

Micron Gemini from Tritech

The Micron Gemini is probably the world’s smallest multibeam imaging sonar. It has a 90° horizontal field of view and a 50m range. With an update rate up to 20Hz, the Micron Gemini brings real-time imaging to places where multibeam was never possible previously. A major benefit for this sonar is its compact size and light weight which maintains the agile maneuvering capabilities of the drone.