Subsea Positioning
Omnitrack USBL USBL Positioning System
Render - tethered buoy, with its coiled tether
Omnitrack is Cerulean's flagship subsea positioning system: a ring buoy carrying the ROV Locator together with two GNSS receivers, so the topside knows both where it is and which way it is pointing.
Dual GNSS, a battery and the acoustic head on one ring buoy
The Wi-Fi configuration, topside only: the subsea unit rides on the ROV and is not pictured. Hover or tap a number to trace a figure to the hardware.
- 01Battery / Wi-Fi enclosureAbout 10 hOn a 4,000 mAh 14.8 V pack. The Wi-Fi module draws about 6 W (400 mA at 14.8 V).
- 02Dual GNSS3–5 m CEPGNSS position error. The two antennas also give heading, to 0.5° probable error.
- 03Wi-Fi antenna7 dBi, 2.4 GHzAbout 250 m to a similarly equipped base station, 20–30 m to a bare laptop.
- 04Ring buoy7.7 kg / 17 lbBuoy weight, Wi-Fi version. The tethered version is 8.6 kg / 19 lb.
- 05ROV Locator, topside50 m depth exposureA Mk III transceiver on this configuration, acoustic sync at 25 kHz. The subsea unit on the ROV is rated to 300 m.
Every Omnitrack USBL Configuration
The same USBL positioning system. What changes is how the topside connects to it, which topside unit it carries, and what comes in the box.
What differs between the configurations▾
| Specification | Wired - 25 m PoE tether + Network Hub, Mk V Rx fitted | Wired - 25 m PoE tether + Network Hub | Wireless - Wi-Fi module + Network Hub |
|---|---|---|---|
| Topside link | Wired - 25 m PoE tether | Wired - 25 m PoE tether | Wireless - Wi-Fi module |
| Topside unit | Mk V receiver - syncs over the ROV's Ethernet; needs a switch port on the vehicle | Mk III transceiver - acoustic sync, no network link to the ROV needed | Mk III transceiver - acoustic sync, no network link to the ROV needed |
| In the box | Network Hub, Mk V receiver fitted | Network Hub | Network Hub |
| Choose it when | The buoy can stay tethered to the boat | The buoy can stay tethered to the boat | The buoy must drift free of the boat |
Everything else on this page is common to all 3 configurations; which vehicle each one is cut for is on the cards above.
Key specifications21 rows▾
| Maximum range | 300 m |
| Update rate | 1 Hz (up to 5 Hz on the Mk V configuration, at shorter range) |
| Maximum depth exposure, subsea unit (minimum crush depth) | 300 m |
| Maximum depth exposure, topside unit | 50 m |
| Buoy weight, tethered version | 8.6 kg / 19 lb |
| Buoy weight, Wi-Fi version | 7.7 kg / 17 lb |
| Topside link | 25 m PoE tether, or the Wi-Fi module |
| Ping frequency (acoustic sync, Mk III topside) | 25 kHz |
| Transmit power, re 1 µPa at 1 m | 185 dB |
| Wi-Fi band | 2.4 GHz |
| Wi-Fi antenna supplied | 7 dBi |
| Wi-Fi range, to a similarly equipped base station | about 250 m |
| Wi-Fi range, to a bare laptop | 20–30 m |
| Wi-Fi module power draw | about 6 W (400 mA at 14.8 V) |
| Battery endurance | about 10 h on a 4,000 mAh 14.8 V pack |
| GNSS position error | 3–5 m CEP |
| GNSS compass (heading) error | 0.5° probable |
| Yaw/azimuth resolution (not accuracy) | 0.1° |
| Slant range resolution (not accuracy) | 0.1 m |
| IMU angular accuracy, typical | 2° |
| Combined angular error | 2.1° probable |
Range, update rate and depth exposure are the three figures Cerulean publish on the Omnitrack product page. The rest come from the Omnitrack reference documentation and the ROV Locator specifications.
System accuracy is not a single number. Cerulean give CEP as square_root( 25 + ( sine(2.1°) × slant range in metres )² ) - so the closer the ROV is to the buoy the better it gets, with the floor set by GNSS position accuracy.
Resolution is not accuracy, and Cerulean say so on their own table. The 0.1° and 0.1 m figures are the smallest increments reported. The usable angular figure is the 2° IMU accuracy, twenty times coarser.
The two depth ratings are for different parts. The subsea unit is rated to 300 m. The topside unit in the buoy is rated to 50 m - it is a surface component, and the 300 m headline does not apply to it.
The 300 m range is a firmware limit Cerulean set to stop adjacent systems interfering with each other, not a signal that fades out. It stops at 300 m.
The buoy is supplied with either the 25 m tether or the Wi-Fi module, never both.
Published by Cerulean - specification page. The acoustic, angular and topside depth figures are published by Cerulean for the ROV Locator inside it - ROV Locator specifications
Applications▾
- Search and recovery - mark where a target was found and come back to it. Relative repeatability is the useful property here, and it is better than the absolute figures imply.
- Geo-referenced sonar and video - every frame carries a position, so a side-scan mosaic or an inspection recording becomes something you hand to a client rather than something you narrate.
- Structure inspection - quay walls, piles and hulls, which is exactly where a magnetic compass is least trustworthy and dual-GNSS heading earns its price.
- Autonomous survey lines - position hold and waypoint navigation on ArduSub. Read the integration note before promising anyone a mission plan.
- Pier and small-boat work - the buoy deploys by hand. No over-the-side pole mount, no vessel modification, nothing bolted to anything.
What's in the box▾
Cerulean’s own packing list for this product.
What’s in the box
- Ring-buoy float with its frame and brace
- Sting in upper and lower segments - hinged on current units - with the retention bolt and thumbscrew
- ROV Locator transceiver with its grip and hose clamp
- Two folding GNSS antenna arms, assembly bolts and an antenna stowage clip
- Battery / Wi-Fi enclosure with its mounting hose clamps, and the battery
- Either a 25 m PoE tether or the Wi-Fi module with its 7 dBi antenna - one or the other, never both. The Network Hub is included in the wired PoE bundle.
Downloads▾
What accuracy actually means here
Cerulean publish the formula instead of a flattering number. Here is what it works out to.
Cerulean publish the formula rather than a headline number, which is unusual and worth repeating in full. System accuracy is:
CEP ≈ √( 25 + ( sin(2.1°) × slant range in metres )² )
Two errors drive it. The topside GNSS knows where the buoy is to 3–5 m. The acoustics know the bearing to the vehicle to about 2.1°, combining the 2° angular figure with 0.5° of GNSS compass error. Bearing error turns into distance error as the vehicle moves away.
| Slant range | System CEP |
|---|---|
| 25 m | about 5.1 m |
| 50 m | about 5.3 m |
| 100 m | about 6.2 m |
| 200 m | about 8.9 m |
| 300 m | about 12.1 m |
Work through it and something useful falls out: the 25 in that formula is the GNSS error squared. Out to roughly 136 m, your absolute position is limited by the GPS on the buoy, not by the sonar. A more accurate USBL would not help you inside that radius. RTK on the topside would.
Two things this does not mean. It is absolute accuracy in world coordinates, and relative tracking - watching the vehicle move, holding station, returning to a marked point - is considerably tighter than these numbers suggest. And it assumes the GNSS compass is not shaded by your boat or gear, which Cerulean are explicit about.
The table is our arithmetic on Cerulean's published formula, using their 5 m GNSS figure. It is not a measurement.
The same arithmetic is worked through in full in the knowledge centre, with the field-test results and a comparison against a short-baseline system.
How accurate is USBL positioning on a small ROV?Resolution is not accuracy
Cerulean's specification table carries a warning that gets stripped out of most reseller listings: resolution is not accuracy.
The ROV Locator resolves bearing to 0.1° and slant range to 0.1 m. Those are the smallest increments it reports. The IMU's actual angular accuracy is 2°, twenty times coarser. Any listing quoting 0.1° under a heading about precision has confused the two.
Cerulean also publish measured results, which is rarer still. On a controlled test across an ice-covered lake - no wind, no current, nothing moving - a Mk III system gave a position scatter band about 10 m long at 110 m range, and about 35 m long at 295 m. Bearing error ran about ±2° and ±2.5° respectively. Cerulean note the site was not ideal for sonar.
Those are the numbers to plan a job around.
Field-test figures published by Cerulean: 110 m test, 295 m test. Resolution and accuracy figures from the ROV Locator specifications.
Omnitrack USBL, and what it produces
Renders of the hardware, and what it looks like in SonarView.
How the buoy turns a ping into a latitude
The buoy sits on the surface with a transducer on a pole below it. The vehicle carries a matching subsea unit.
1. The buoy establishes itself. Two GNSS antennas on a fixed baseline give both position and heading. Two antennas rather than one is what makes the heading trustworthy - a magnetic compass this close to a boat, a quay wall or a set of thrusters is guessing.
2. The acoustics measure the vehicle. A 25 kHz ping at 185 dB re 1 µPa at 1 m gives slant range and relative bearing, once per second. Enabling the ROV's own depth reading - Cerulean recommend it - removes one unknown and tightens the solution.
3. The two combine. Buoy position, plus buoy heading, plus range and bearing to the vehicle, equals the vehicle's latitude and longitude.
4. The result goes to the autopilot. Fed to ArduSub over MAVLink, it becomes the position estimate that waypoint navigation and position hold need.
If GNSS is lost - under a bridge, inside a dock, against a large hull - an onboard magnetometer takes over as fallback. Heading accuracy degrades; the system does not stop.
The topside does not have to be the ring buoy. Cerulean also support mounting it on a BlueBoat, a Blue Buoy, or a user-supplied float with a rigid mount.
Ping frequency is the acoustic-sync figure, which is the Mk III topside. Mounting options: Cerulean's floatation and mounting notes.
The 1 Hz question, and the DVL
One position per second is a position, not a control loop. Between fixes the vehicle keeps moving and the display keeps showing where it was.
Cerulean's own answer is the Tracker 650 DVL, which supplies velocity at a much higher rate. SonarView fuses the two into a continuous track; on MAVLink and ArduSub vehicles the autopilot does the fusion itself, so it works without SonarView in the loop. USBL supplies the absolute reference that stops long-term drift, the DVL supplies the smooth motion between fixes, and each covers the other's weakness.
The Mk V configuration narrows the gap on its own, reaching up to 5 Hz at shorter ranges over network sync.
Whether you need the DVL depends on the job. Logging where things are: Omnitrack alone is fine. Holding station against current, flying a survey line, anything where the pilot is reacting to the position display: add the DVL.
Tracker 650 DVL, from €2,625 - what it adds and when it is worth it DVL or USBL: which does your ROV need?What it takes to make it fly the vehicle
The supported path. A Blue Robotics vehicle running ArduSub and BlueOS. Omnitrack hands the autopilot geolocation and odometry through mavlink2rest, and that is what makes waypoint navigation and position hold possible. SonarView is free, runs on Windows, macOS, Linux and in-browser through BlueOS, and can push the required ArduPilot parameters for you.
Two things worth knowing before you commit.
Do not let NMEA GPS reach ArduSub. Cerulean document this one plainly: if NMEA-formatted GPS position gets to ArduSub, it will thereafter disable position hold and DVL dead reckoning. The stream can arrive from any GPS on the same network, or from leaving NMEA output switched on. It is a five-minute fix if you know, and a lost day if you do not.
Position hold is a parameter set, not a switch. Roughly nineteen ArduPilot parameters govern whether the vehicle will accept an external position source - GPS_TYPE, the EK3 source selectors, VISO_TYPE and the rest. SonarView sets them. When it goes wrong it presents as QGroundControl refusing to change mode, citing missing position, and a BlueROV operator running a Mk III with a DVL reported exactly that on the Blue Robotics forum in October 2025 after applying the recommended parameters. One report, not a general specification - but it is the kind of thing worth having our number for.
Other vehicles. Omnitrack works standalone through SonarView on any platform. What it will not do out of the box is drive an autopilot that is not ArduSub. Cerulean's engineers will assist with other autopilots; that is an integration project, not a purchase.
Network housekeeping. Running the Wi-Fi configuration means your laptop moves between the Omnitrack network and everything else. A second Wi-Fi dongle solves it cheaply. A BaseStation Plus solves it properly, combining access point, tether interface and bridging.
Tell us what you are flying and we will say plainly whether it is a drop-in or a project.
The NMEA behaviour is documented by Cerulean. The parameter-set difficulty is one operator's report on the Blue Robotics forum, not a general specification. Network bridging: Cerulean's hints and tips.
Common questions
Answers Cerulean publish, each linked back to the page it came from.
What is Omnitrack, exactly?
A family, not a single box. It is a set of hardware configured to suit different positioning needs - the tethered buoy and the Wi-Fi buoy are the two minimum recommended configurations. Source
What does it give the ROV?
Geolocation and odometry handed straight to the autopilot, which is what makes waypoint navigation and position hold possible. It is built to drop onto Blue Robotics systems. Source
How is it better than a bare ROV Locator?
Cerulean say it directly: the ROVL is low-cost, medium-accuracy localisation, and better performance comes from using it as part of an Omnitrack system. Omnitrack is the complete topside assembly - buoy, frame, transducer pole, dual-GNSS compass and network hub, ready to deploy. With an ROVL bundle, heading is your problem to solve. Source
How accurate is Omnitrack, in metres?
About 5 m close to the buoy, about 6 m at 100 m slant range and about 12 m at 300 m. Cerulean publish the formula rather than a single number: CEP ≈ square_root(25 + (sine(2.1°) × slant range)²). Relative tracking and repeatability are better than those absolute figures. Source
Why is accuracy no better than 5 m even when the ROV is right below the buoy?
Because the limit close in is the buoy's own GPS, not the acoustics. Standard GNSS gives 3–5 m CEP, and the acoustic error only becomes the larger term beyond about 136 m slant range. Inside that radius, better absolute accuracy means RTK on the topside, not a better USBL. Source
Can Omnitrack go to 300 m depth?
The subsea unit can. The topside unit in the buoy is rated to 50 m depth exposure - it is a surface component. One product, two ratings, and the 300 m headline refers to the part that goes down. Source
Do I need a DVL as well?
Not for knowing where the vehicle is and logging it. Yes, in practice, for holding station or flying survey lines, because 1 Hz is too slow to fly on. The Tracker 650 fills the gaps and the two fuse in SonarView or in the autopilot. Source
Does it work with a BlueROV2 out of the box?
Yes - that is the platform it was designed around. It feeds ArduSub through mavlink2rest and SonarView can push the ArduPilot parameters. Two configuration traps are worth reading first: NMEA GPS reaching ArduSub will disable position hold, and the parameter set has to be right before the vehicle will accept an external position. Source
Does the Wi-Fi version come with a battery charger?
The battery, yes. The charger, no. You need a balance charger for a 4S 14.8 V pack, which most ROV operators already have. Budget about 10 hours per charge and plan on about five in practice. Source
What happens to heading if GNSS drops out?
An onboard magnetometer takes over. Heading accuracy falls away from the 0.5° GNSS figure, and position accuracy with it, but the system keeps working. This is the case under bridges, inside docks and alongside large steel structures - worth planning for if that is your work. Source
Which units ship in each configuration?
Cerulean's current selection guide pairs a Mk III transceiver topside with a Mk V transmitter deepside, with acoustic sync always available and network sync available when there is a network. The Mk V bundle is a Mk V receiver topside with the same Mk V transmitter, network sync only. Source
Omnitrack USBL on video
Published by Cerulean Sonar on their own channel.
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