Do you need a DVL or a USBL for your ROV?

What a DVL actually does

A Doppler velocity log points acoustic beams at the seabed and measures the frequency shift in the returns. From that it derives speed over ground, in three axes, many times a second.

That velocity is the input a flight controller needs to hold position. Without it, an ROV in a current has no idea it is being pushed; with it, the vehicle corrects continuously and stays put. Integrate velocity over time and you get a dead-reckoned track - a running total of how far the vehicle has travelled and in which direction.

Two things follow from this that matter when you are buying.

First, a DVL’s accuracy is relative and it accumulates error. It knows how far you have moved since you started. It does not know where you started. Errors compound over time, which is why DVL specifications talk about drift per minute rather than absolute accuracy.

Second, a DVL needs a surface within range. Typical small-ROV DVLs work from around half a metre to somewhere between 20 and 50 m off the bottom. Above that, no returns, no velocity. Mid-water in deep water, a DVL gives you nothing.

What a USBL actually does

An ultra-short baseline system puts a receiver at the surface, usually on a pole, buoy or small boat, and a transponder on the ROV. It times acoustic pulses between them and calculates range and bearing. Add GNSS at the surface and a heading reference, and you get the ROV’s latitude and longitude.

That is an absolute fix. It does not drift. Leave it running for six hours and the last position is as good as the first.

The trade-off is update rate and infrastructure. A typical small USBL updates about once per second, which is a slideshow rather than a video. And you need something at the surface - a buoy to deploy, a pole to mount, a GNSS antenna with a clear sky view.

The comparison, with real numbers

The three Cerulean prices are SepcoTech EUR list, September 2026, read from our own price list. The Water Linked figure is our list price from the Marine Robotics add-ons page; the A50’s own specifications are Water Linked’s published figures. Prices move; the quote is the authority.

The headline to take from that table is not the price column. It is the update-rate column, because that is the difference nobody explains.

What the update rate actually buys you

A USBL fix once per second sounds adequate until you watch it. An ROV moving at one knot covers about half a metre per second, so the position marker jumps half a metre at a time and the track between fixes is a guess. Add a stray multipath return - a reflection off a quay wall or a hull - and one of those jumps goes somewhere silly.

A DVL running at 5 to 20 Hz produces motion between those fixes. Fuse the two and the USBL supplies truth while the DVL supplies continuity: the absolute fix stops the drift accumulating, and the high-rate velocity smooths the track and rejects fixes that disagree with where the vehicle could plausibly have gone.

This is why the two are complementary rather than competing. Cerulean document that pairing the Tracker 650 with an Omnitrack enables USBL plus DVL sensor fusion on BlueROV2 systems, with SonarView combining both streams into one track. It is also why a survey contractor will own both and a shallow-water inspection outfit will often own neither.

Fusion with the ROV Locator on its own, for absolute position output, is a different thing and Cerulean list it as not supported on the Tracker 650. If fused absolute position is what you are after, the pairing to ask about is the Omnitrack.

The honest caveat: what a cheap DVL gives up

DVLs at the bottom of the market are cheaper for identifiable reasons, and it is worth knowing which corners were cut before you find out on a job.

The bottom type changes your range more than the datasheet suggests. Cerulean publish a maximum operating height of 25–50 m for the Tracker 650, and they are unusually candid about why it is a range rather than a number. Their own testing: on a hard bottom such as Lake Superior they regularly achieve 50 m; on a muck-bottom lake such as Lake Minnetonka they sometimes struggle to get 30 m. Soft silt absorbs your ping, and weed scatters it. This is physics, not a product flaw - every DVL ever made behaves this way - but the flattering number is the one that usually reaches the datasheet.

No internal IMU means no self-contained dead reckoning. The Tracker 650 has no IMU and reports no attitude; the angle fields in its output messages are permanently zero. It leans on the vehicle’s IMU, which is genuinely good for setup - Cerulean state plainly that the unit does not require any calibration, so there is one IMU to calibrate instead of two and no pre-dive calibration manoeuvres - but it means the quality of your dead reckoning is capped by the quality of your vehicle’s own sensors. The Water Linked A50 includes an AHRS, which is a large part of what the extra €5,375 buys.

Three beams instead of four. The Tracker 650 uses three velocity sensors; the A50 uses a four-beam array. A fourth beam gives redundancy: if one beam is looking at a rock face or a trench wall, a four-beam unit can spot the outlier and drop it. With three, there is nothing to cross-check against. Over flat ground it makes no difference. Over broken terrain it does.

You have to watch the confidence value. The Tracker 650 reports a confidence figure with every measurement - Cerulean’s own scale runs from “0 (don’t use)” to “100 (as confident as possible)”, and on their published lake test it sat between roughly 50 and 99, with the low values in heavy vegetation. That number is your warning that the sensor has lost the bottom or is struggling with it, and it is visible live in SonarView. If your work takes the vehicle up off the bottom, or over weed, plan for the gap rather than discovering it in the track afterwards.

The question everyone asks next: what about the software?

Having chosen the hardware, most people lose their first afternoon to the integration rather than the sensor. On ArduSub vehicles specifically, there are documented quirks that are worth reading before the boat is booked.

ArduSub accepts one global origin per power cycle and will not hold position until it has one. Cerulean’s wording is exact: “ArduSub accepts one and only one global origin per session (a session is the time period during which the ROV is powered).” You can let the DVL send an automatic origin of (0,0) each time it connects to the MAVLink REST server, which gets you position hold - but Cerulean are equally clear about the cost, which is that it “allows relative positioning not corresponding to the actual latitude and longitude”. Supply a real origin yourself instead and you get true coordinates on the map, but nothing holds until you do.

The one that costs people the most time: sending NMEA-formatted GPS data to ArduSub disables DVL-based position hold and dead reckoning from that point on. Cerulean’s sentence is worth reading twice - “If you send NMEA-formatted GPS position information to ArduSub, it will disable the ability thereafter to use position hold and dead reckoning based on DVL inputs.” The NMEA might come from a GPS on the same network, or from a configuration utility whose output was never turned off.

None of this is difficult once you know it. All of it is invisible until you hit it.

Deciding

Buy a DVL if the problem is the vehicle not staying where you put it - inspection work in current, holding off a structure, anything where you need both hands on the task rather than the sticks.

Buy a USBL if the problem is not knowing where the vehicle is on a chart - search and recovery, surveys that need geo-referenced data, anything where a position has to go in a report.

Buy both if you are producing survey deliverables, or if your job is to find something and then be able to say precisely where it was.

Frequently asked questions

Getting the right one

We sell all four of the systems in the table above, and we have no particular interest in which one you buy. Tell us the vehicle, the depth, what the bottom is like and what has to end up in the report, and we will name the part number - and tell you what we ruled out and why.

The hardware, the warranty and the documentation are the manufacturer’s. The stock, the EUR quote with no hidden import costs, and the support in your time zone are ours.