Long base line sonar, commonly referred to as LBL, is a method of acoustic positioning

Acoustic location

Acoustic location is the science of using sound to determine the distance and direction of something. Location can be done actively or passively, and can take place in gases , liquids , and in solids .* Active acoustic location involves the creation of sound in order to produce an echo, which is...

 commonly used in deep water (water depth of greater than 3000 feet). A typical LBL positioning system consists of a transceiver and several beacons arranged into a structure called an array. The LBL transceiver pings each beacon and uses the 2-way travel time to calculate its position within the array. LBL positioning is much more accurate than ultra-short baseline

Ultra-short baseline

USBL is a method of underwater acoustic positioning. A complete USBL system consists of a transceiver, which is mounted on a pole under a ship, and a transponder/responder on the seafloor, a towfish, or on a ROV...

 (USBL) or SSBL surface-based positioning.

Advantages over surface tracking

In very deep water (1000m or more), acoustic positioning performed from the surface is subject to a large amount of noise and unusually long reply times due to the great distances the sound must travel. Also, a USBL system uses a single transceiver with multiple elements located close together and calculates range/bearings based on signal phase offsets. Because surface tracking may have an error radius of up to one-half percent of the water depth, 8000 feet (2,438.4 m) of water can produce up to 40 feet (12.2 m) of error in any direction. The beacons in an LBL array are typically less than a kilometer apart, and the noise levels near the seabed in deep water are much less than near the surface, so LBL can usually resolve positions with less than a foot of error, regardless of the water depth. LBL combines range information from multiple sources. This redundancy helps to eliminate noise.

Beacon Array

Generally between 5 and 25 beacons will be placed on the seafloor. The acoustic navigator will then use a method of surface tracking such as USBL to lock in the locations of each beacon, which involves taking tens to hundreds of range/bearing measurements from different locations and averaging them to produce a final position. Once the position of each beacon is known to a high precision, the array is ready for navigation.

Underwater "smart" beacons are able to sample 2-way travel times between each other, so even a large array of smart beacons may be calibrated with only 2 or 3 known positions. The remaining beacons' positions can be resolved by ranging between beacons, reducing the time required to calibrate the array.

Tracking

Once the array has been calibrated, an underwater vehicle or diver may equip a transceiver which will take ranges (based on 2-way acoustic travel time) to each beacon in the array, and send that data to a computer located on the surface to be processed. The computer uses the ranges from these known points to calculate a final position.