Long BaseLine (LBL) refers to a positioning technique that uses an array (group) of transponders that are in a known formation on the seabed. The term long baseline refers to the fact that the distance between these transponders is typically hundreds of meters, much longer than other subsea positioning methods. The position of a target within the array can then be determined by measuring the distance between itself and each transponder in the array.

An LBL system has two main elements. The first element comprises a number of acoustic transponders moored in fixed locations on the seabed. The positions of the transponders are described in a co-ordinate frame fixed to the seabed. This co-ordinate frame can be absolute (real world co-ordinates) or relative. The distances between them form the 'baselines' used by the system.

To understand why LBL is used, it is essential to appreciate the benefits. The advantages of LBL over other subsea positioning systems are the achievable accuracies, and the fact that a LBL solution is independent of water depth. The system allows for substantial QC, which might be critical for certain types of offshore operations. A disadvantage of the system however, is its time consuming installation and calibration during offshore operations, which has a commercial impact when deciding on suitable positioning systems.

Fusion 6G® is designed to position multiple subsea targets and structures with the highest possible levels of accuracy. The system can be configured to support simple tracking tasks through to complex, deepwater construction projects with multiple surface vessels and subsea vehicles working in close proximity to each other.

The Long BaseLine method provides accurate positioning over a wide area by measuring ranges from a vessel to three or more transponders deployed at known locations on the seabed or on a structure. The technique offers a high degree of positioning repeatability and with range redundancy, an estimation of the position quality can also be made.

At the heart of Fusion 6G® is Compatt 6. This is Sonardyne’s latest generation transponder that can be used to create a seabed navigation network in which targets can be positioned. Crucially, these networks can offer position accuracies of better than 5cm independent of water depth.

All positioning tasks are controlled via the Data Fusion Engine a common topside processor platform that can be used for both LBL and USBL applications.In practice this means a vessel equipped with a new Sonardyne HPT transceiver is able to undertake LBL tasks by simply switching software applications and deploying additional seabed transponders. Traditionally this would have required the use of separate and dedicated systems.

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Prospector uses a GPS calibrated network of low cost transponders to provide high accuracy absolute positioning over a wide area. Prospector can track one surface and one underwater mobile object, such as an ROV, diver, towfish or Autonomous Underwater Vehicle (AUV). This is combined with the minimum of vessel mounted hardware and easy to use software.

Some of the many tasks that Prospector can be used for include; underwater inspection, debris searches, mapping, inshore construction work, ordnance clearance, marine salvage assessment, marine biology surveys and marine archaeology.

This simplified acoustic system is extremely easy to use and yet brings the inshore diver and ROV operator the same advanced Sonardyne technology used to save time and money in most of the world's offshore oilfield developments.Four acoustic transponders are deployed on the seabed with surface or sub-surface floats to form an underwater navigation network with each transponder being deployed at one corner of a square. The positions of these four transponders are then calibrated by sailing the vessel briefly around the square (typically in under 20 minutes)

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