What is an LBL system?
How an LBL system works
A Long BaseLine positioning system (LBL) is a subsea positioning system based on range measurements to fixed calibrated transponders that are deployed on the seabed.
The LBL transceiver is deployed on the vehicle that need to be positioned (ie. an ROV or an AUV).
The LBL transponders are autonomous (powered with either alkaline or lithium batteries) and are either deployed on tripods or with mooring lines with dead weight on the seabed.
The LBL transceiver interrogates the transponder using an acoustic interrogation signal. The transponder replies with a unique code and the LBL transceiver measures the 2-way time travel. This time travel is converted to a range by knowing the speed of sound in the water thanks to a dedicated sound velocity probe placed on the transponder and/or on the transceiver.
By interrogating several transponders (>3), it is possible to determine the position of the transducer using triangulation, same as what can be done with GNSS systems.
Range-aiding navigation and sparse-LBL
When the LBL technique is combined with Inertial Navigation Systems (INS), we talk about range-aiding navigation. Instead of computing an acoustic position, the LBL transceiver is feeding the INS with a range to a known position. The INS uses this range, together with other available external sensors (DVL, depth, USBL…), as well as its own internal sensors (accelerometers, gyroscopes) and runs a Kalman Filter to estimate the position of the subsea vehicle.
This combination of acoustics and inertial allows to reduce the number of transponders required on the seabed while achieving similar or better positioning performance: this is called sparse LBL. The acoustic update rates can be increased while keeping very high navigation performance while increasing the transponder autonomy.
Following this principle, each range measurement helps in computing a new position, as opposed to classical triangulation algorithms, for which at least three simultaneous range measurements are required to establish a position. It is therefore possible to navigate with fewer transponders without making any compromise on performance.