Satellite signals do not penetrate water: a few centimeters below the surface, GNSS ceases to exist. Anything that must be located precisely subsea — an ROV, a towfish, a rig on approach, a structure being installed — depends on acoustic positioning: measuring how long sound takes to travel between a transceiver and one or more transponders, and turning that time into position. The two dominant architectures are USBL (Ultra-Short BaseLine) and LBL (Long BaseLine).
How USBL works
In a USBL system the "brain" rides on the vessel: a transceiver with a compact hydrophone array, mounted under the hull or on a side pole. The transceiver interrogates the target's transponder and measures two things: range (from signal travel time) and angle of arrival (from the phase difference across its hydrophones). Range plus angle yields the target's position relative to the vessel, which is georeferenced by integrating GNSS, gyrocompass and a motion reference unit (MRU).
Its great virtue is mobilization: there is nothing to install on the seabed. Calibrate the system and go to work. Its limit is geometric: angular error grows with distance, so accuracy degrades in proportion to slant range — the deeper or farther the target, the larger the uncertainty.
How LBL works
LBL inverts the logic: the reference is installed on the seabed. An array of transponders is deployed at known, inter-calibrated positions, and the target locates itself by ranging to several of them and trilaterating. Because position derives from ranges only (not from angles measured off a moving vessel), accuracy is high and uniform within the array, without degrading with depth.
The cost of that accuracy is logistics: deploying, calibrating and recovering the array takes vessel time, and the array covers a defined area. That is why LBL shines on concentrated, fine-tolerance work rather than targets that roam kilometers.
Head-to-head
| USBL | LBL | |
|---|---|---|
| Infrastructure | Vessel-mounted transceiver | Seabed transponder array |
| Mobilization | Fast — no seabed equipment | Requires array deployment and calibration |
| Accuracy | Degrades with slant range | High and uniform within the array |
| Coverage | Wherever the vessel goes | The deployed array's footprint |
| Relative cost | Lower for short campaigns | Higher — justified by accuracy or duration |
| Typical use | ROV/towfish tracking, rig moves, diving | Metrology, construction, precision installation |
When to choose USBL
- Tracking ROVs, towfish or tooling during surveys and dynamic operations.
- Positioning for rig moves — platform transit and final approach, with tugs.
- Diving support and monitoring of structures during deployment.
- Short campaigns where mobilization time rules.
When to choose LBL
- Metrology for spool fabrication and subsea tie-ins.
- Installation and construction to fine tolerances (manifolds, structures, templates).
- Long campaigns concentrated on one site, where the array pays for itself.
- Deep water, where USBL error at long range stops being acceptable.
The middle ground: hybrid systems
The industry increasingly pairs acoustics with inertial navigation: an acoustic-inertial system (such as Sonardyne's SPRINT-NAV) uses acoustics to correct INS drift and the INS to smooth and hold position when acoustics degrade or drop out. For deepwater ROV operations it is now the reference configuration.
Not sure which architecture your project needs? Our team operates both USBL and LBL across the Gulf of Mexico — and as an authorized Sonardyne reseller in Mexico we also sell and rent the equipment, with local support from Ciudad del Carmen.
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