Which Imaging Sonar for a Small ROV? A Practical Guide

The three types, plainly

Mechanical scanning sonar

A motor swings a single transducer through an arc, building a circular picture around the vehicle. The Blue Robotics Ping360 is the best-known example in this class.

It works, it is simple, and the top-down view is intuitive for pilots. The catch is time. A full sweep takes several seconds, so the picture is always slightly out of date, and if you want a second look at something you wait for the head to come back round. Practical range is usually in the 30–50 m band.

Fan-beam scanning sonar

The transducer is fixed. It fires one beam that is very narrow horizontally and tall vertically, and software stitches thousands of those returns into a continuous image as the vehicle turns and moves. The Cerulean Omniscan family works this way.

Because nothing rotates mechanically, the range is far longer and the horizontal resolution is far finer. The trade-off is that the pilot is the scanner - you turn the vehicle to look around. Most pilots adapt within a dive.

Multibeam imaging sonar

Many beams fire at once, producing a live video-rate image of everything in the forward sector. This is what you want for close-in inspection, moving through structure, or flying fast.

It is also where the money goes. Entry-level units in this class start around four times the price of the other two.

The comparison that actually decides it

Indicative EUR, excluding VAT and shipping. Mechanical scanning is the Blue Robotics Ping360 (BR-100399); multibeam is the Sonoptix ECHO (BR-102221). All four are products SepcoTech carries, so all four figures are checkable against our own list.

The line that matters most is the first one. For roughly the same outlay, a fan-beam unit sees two to four times further than a mechanically scanned one - four, at the top of the range, with the Omniscan 450 FS. If your work is finding things - lost gear, debris, a wreck, a body, a pipeline - that difference decides how many hours you spend on site.

If your work is flying through a jacket structure or inspecting a quay wall from two metres off, the refresh rate matters more than the range, and multibeam earns its price.

Two fan-beam options, and which one

Both are 450 kHz, both are rated to 300 m, and both are driven by the same free software. What separates them is not performance so much as what they ask of the vehicle.

The Omniscan 450 FS is the one to take if you have the room. It reaches 120 m against 100 m and its beam is 0.8° against 1.1°, so it both sees further and separates two objects lying closer together. It wants a port on the vehicle and a bracket of its own.

The Omniscan 450 Compact is the one to take if you have not. Transducer, electronics and a three-port Ethernet switch are in a single 124 × 121 × 56 mm housing, and 10° of downward tilt is built in. On a small ROV with one spare Ethernet port that switch is frequently the difference between a second sensor fitting and not fitting - it frees a port rather than taking one. The reach and the beam are the price you pay.

There is no Blue Robotics edition of either, and none is needed: the standard unit is what goes on a BlueROV2 or a BlueBoat. The factory editions exist for the vehicles that use a different connector.

What 120 metres of range really buys you

Detection is not identification. At 120 m the Omniscan 450 FS will show you that something is there. You then drive toward it and the picture sharpens as you close.

That two-stage workflow is the point. Instead of mowing the lawn at 20 m range hoping to pass over the target, you park, rotate, find the contact at distance, and go straight to it. On a harbour search that can be the difference between one dive and four.

Two specifications govern how well this works.

Beam width, 0.8°. This is what separates two objects lying close together. Narrower is sharper. It is also a far-field figure - beyond about 5 m from the transducer. Closer in, the effective beam widens.

Beam height, 50°. The vertical spread. A tall beam means you see both the distant seabed ahead and objects directly below the vehicle, without needing to fly low. If you are working ten metres off the bottom, this is the number that keeps the seabed in the picture.

The honest caveat: ping rate falls with range

You will see 20 Hz quoted as the maximum ping rate. That figure holds out to about 30 m. Beyond that, the speed of sound sets the limit - the sonar has to wait for the echo to come back before it can fire again.

In practice:

• 30 m range: around 20 pings per second
• 50 m range: around 10 pings per second
• 140 m range: around 5 pings per second

This is physics, not a product limitation - every sonar faces it. But it changes how you fly. At long range you turn the vehicle slowly, or the mosaic develops gaps between beams. Nobody’s marketing page tells you this, and it is the single most useful thing to know before your first dive.

Do you need a DVL as well?

This is the second question everyone asks, and the answer is usually no.

To build a continuous image, the software needs to know how the vehicle has moved. On a surface vessel, GPS handles it. Underwater, the traditional answer is a Doppler Velocity Log or a USBL - both of which add several thousand euros.

The Omniscan works around this by interleaving two kinds of ping. Imaging pings build the picture. Doppler pings are processed separately to estimate velocity along the beam direction. A single forward-facing unit therefore renders your approach toward a contact correctly, with no DVL fitted.

Add a second unit at 90° to one side and you get two velocity axes - full 2D motion tracking, plus side-scan imagery from the same hardware. That second axis is also what enables position hold on BlueOS and ArduSub vehicles.

If you already run a DVL, the sonar will use it and the result is better still.

Software, and the thing to check before you buy

Cerulean sonars run on SonarView, which is included free and runs on Windows, macOS, Linux, and in a browser via BlueOS on the vehicle’s onboard computer. There is no licence fee and no annual subscription - worth checking against any competing quote, where software is often a separate line item.

One thing to be clear about: SonarView is the integration point, not the autopilot. There is no direct ArduPilot link. On a BlueROV2 or BlueBoat this is a non-issue, because BlueOS handles it. On a custom AUV or an unusual platform, budget some integration time. Anyone telling you otherwise has not built one.

Which vehicle are you flying?

Mounting is the practical constraint that catches people out. Current factory-configured options cover:

• Blue Robotics BlueROV2 - purpose-made bracket, top or bottom mount. The FS needs an Ethernet switch on the vehicle; the Compact has one inside it
• Blue Robotics BlueBoat - surface variants for side-scan survey work
• QYSEA FIFISH E-Go and W6 - plug-and-play editions
• Chasing M2 Pro and Pro Max - sliding platform mount on the support rod
• Blueye - Blue Trail connector edition
• Custom builds - standard form factors, 100BaseT Ethernet and 10–30 V power

The FS should sit horizontally and be angled roughly 20° downward. Depth rating comes in two flavours: 100 m, which is lighter and actually buoyant in water, and 300 m, which weighs 825 g in air. Pick on working depth first, payload budget second.

A decision in three lines

• Finding things across open ground, in bad visibility, on a budget → fan beam. This is the largest use case and where the Omniscan 450 FS sits.
• The same job, but the vehicle has no room and no spare port → fan beam again, in the Omniscan 450 Compact’s all-in-one housing with its own Ethernet switch.
• Flying close to structure, needing a live picture → multibeam.
• Basic obstacle awareness, short range, simplest possible setup → mechanical scanning.

If you are still unsure, tell us what you need to do and what you are flying.

It names the exact model and part number, and shows you what it ruled out and why.

FAQ