There's no GPS underwater. Position is obtained with acoustic systems —USBL and LBL— that measure distances using sound, backed up by inertial sensors and imaging sonar to see the surroundings. The diver doesn't operate this equipment, but understanding how it works improves communication with the surface and safety.
One of the first mental jolts when moving from recreational to commercial diving is discovering just how much a professional underwater operation is an exercise in positioning. On a work vessel there are people whose only job is to know, at every moment and to within centimetres, where everything is underwater: the vessel, the bell, the ROV, the structure and the diver.
The diver doesn't operate these systems, but lives alongside them every day and their work depends on them functioning. Understanding the basics changes how you communicate with the surface.
The underlying problem: there's no GPS underwater
Satellite radio signals die out within the first few centimetres of salt water. A GPS receiver underwater sees nothing. So all underwater positioning relies on the one thing that propagates well in that medium: sonido.
The basic idea is always the same. A transmitter sends an acoustic pulse, a receiver sends it back, the round-trip time is measured and, knowing the speed of sound in water, the distance is calculated. From there, with geometry, a position is obtained.
That detail of "knowing the speed of sound" is trickier than it sounds: the speed varies with temperature, salinity and pressure, and changes with depth, forming layers. That's why, before a precision operation, the sound velocity profile is measured through the water column.
USBL: the vessel's reference system
USBL —ultra short baseline— is the most widespread system. A transducer mounted on the vessel's hull, with several elements packed closely together, emits a pulse. A transponder fitted to the object being located responds. The transducer measures the travel time —which gives the distance— and the phase difference between its elements —which gives the direction. With distance and direction you get the position relative to the vessel, and combining that with the vessel's GPS and attitude sensors, the absolute position.
Its big advantage is that nothing needs to be deployed on the seabed: a transponder is fitted to the object and that's it. Its limitation is that accuracy degrades with depth, because a small angular error is amplified over distance.
On a diving operation, it's common to have transponders on the bell, on the ROV and sometimes on the diver themselves.
LBL: precision within a specific area
LBL —long baseline— works the other way round. Several beacons are moored on the seabed around the work area, their positions carefully calibrated, and from there any object can determine its position by measuring distances to those beacons. It's the same principle as GPS, but with beacons on the seabed instead of satellites.
It gives very high, stable accuracy within the area covered, and it's used when the job demands extreme precision, for example in metrology or in structure installation. The cost is time: the beacons have to be deployed, calibrated and recovered afterwards.
Inertial sensors and aids
Acoustic systems give a position at set intervals and with some noise. To smooth this out and fill in gaps they're combined with inertial sensors, which measure accelerations and rotations, and with sensors measuring speed relative to the seabed. The combination gives a continuous, precise track, much better than any of the systems on their own.
This same principle is what lets a vessel hold position without anchoring, which is the basis of dynamic positioning , which any diving operation from a vessel depends on.
Imaging sonar: seeing without seeing
Positioning is one thing; seeing the surroundings is another. That's what imaging sonar is for: it sweeps the surroundings with acoustic pulses and builds up a picture of what's around — the structure, the seabed, obstacles, other equipment.
On a bell operation, the sonar mounted on it is a direct safety tool. In zero visibility, it lets the supervisor place the bell relative to the structure, control distances during descent and guide the diver as they describe what they're feeling. On a day aboard a DSV, that's one of the screens people watch most.
There are also other types of sonar in the working environment: side-scan for producing images of the seabed, multibeam for bathymetry and sub-bottom profilers for seeing beneath the sediment.
What this means for the diver
A diver doesn't calibrate an LBL system or interpret a point cloud. But there are four things that affect them directly.
The surface knows where you are, but not as precisely as you imagine. An acoustic system has a margin of error, and that margin grows with depth and with conditions. If the surface tells you the structure is ten metres to the north, that's a good estimate, not an absolute certainty. The physical reference —the guide line, the structure itself, the umbilical— remains your anchor.
Your description feeds the system. When you confirm by voice that you've found a specific item, you're giving the surface team a real control point that validates or corrects what the screen shows. Being precise when describing things isn't courtesy: it's data.
Acoustic noise matters. Tools, thrusters and cavitation generate noise that degrades these systems. Sometimes you're asked to stop a tool to take a reading, and knowing why avoids frustration.
It's a professional door-opener. Someone who understands positioning and survey has an advantage moving into inspection, project coordination or supervision. It's knowledge you don't pick up on the diving course, and it sets a candidate apart.
How to learn enough
You don't need a survey course. It's enough to do three things systematically on board: ask for the positioning plan to be explained at the briefing, go up to the survey cabin now and then to see what your dive looks like from there, and ask when something doesn't add up between what you see underwater and what the surface is saying.
That curiosity, kept up over a few campaigns, builds a practical understanding that shows later. And it fits the general logic of a career in this trade: the diver who lasts and progresses isn't the one who can take the most punishment, it's the one who understands the system they're working in best. You can see that idea developed in the chain of command on a diving operation and on the page for equipment.
The speed of sound, the detail that governs everything
It's worth dwelling on this point because it's the one that causes the most errors. Sound in seawater travels at a speed that depends on temperature, salinity and pressure. It isn't constant: it changes with depth and forms layers, and in some areas those layers even bend acoustic rays the way a lens would.
All acoustic positioning systems calculate distances from travel times, so if the assumed speed doesn't match the real one, the position comes out shifted. That's why, before a precision operation, a probe is lowered to measure the sound velocity profile through the whole water column and that profile is loaded into the system.
For the diver, the practical consequence is understanding that an acoustic position is an estimate with a margin, not an exact coordinate, and that margin can grow if conditions change during the day.
When the surface and the diver don't agree
It's a real and fairly common situation: the system says you're in one place and what you're describing doesn't match what should be there. It could be a positioning error, the structure not being where the drawing said, or you misreading what you're touching.
The right way to resolve it isn't for either side to insist they're right, but to verify. Going back to a known physical reference and retracing the route from there usually untangles it within a few minutes. And if the discrepancy persists, you stop and reassess.
This connects to something more general about the trade: safety depends on information flowing in both directions. A diver who stays quiet because they assume the surface knows better is removing a layer of control. Honestly describing what you see, and what you don't understand, is part of the job.
Where the technology is heading
The clear trend is towards sensor fusion: combining acoustics, inertial, optical imaging and sonar into a single system that gives a continuous position and a far richer picture of the surroundings than any of them alone. That translates into more precise, better-documented operations with more data to review afterwards.
For the diver it means the work is recorded in ever more detail and that the quality of their communication is part of a technical record. It isn't surveillance: it's that a structure inspection is worth what data it produces, and the diver is one of the sources.
Frequently asked questions
Why doesn't GPS work underwater?
Because satellite radio signals are attenuated within a few centimetres of salt water. Below the surface you have to rely on sound, which does propagate well, which is why all underwater positioning systems are acoustic.
What's the difference between USBL and LBL?
USBL calculates position from a single transducer on the vessel by measuring distance and angle. LBL uses several moored beacons and triangulates distances between them, which gives more precision within a specific area in exchange for having to deploy and calibrate them.
Does the diver wear a transponder?
In many operations, yes, especially on bell work or where recording position matters. It's a small item fitted to the equipment that lets the surface know where the diver is relative to the structure.
What's sonar used for on a diving bell?
So the supervisor can see the structure and seabed around the bell in zero visibility, help guide the diver and control the distance to obstacles during descent and work.
Does a diver need sonar training?
It isn't mandatory, but understanding the basic concepts greatly improves the quality of communication with the supervisor and the survey team, and it's a clear plus for anyone looking to move into inspection or supervision.
Published 3 September 2026 · By Arthur More, professional saturation diver
Photo: NashvilleCorps · CC BY-SA via Openverse