Laser Positioning Systems.
Simple laser positioning systems explained.
Laser based positioning for Dynamic Positioned vessel systems have been around or several decades and used extensively offshore. Today some of the well-known systems are Fan-beam, Spot-Track and CyScan
All these systems are laser based positioning systems designed for repetitive high accuracy relative range and bearing measurements from offshore support vessels and other marine structures. The laser has two lenses, one for transmitting and one for receiving.
The Laser based system uses position data to automatically hold vessels on station, and is often the primary position reference employed during critical close proximity operations, by offshore supply vessels, however the use on OSV operations is not exclusive and fan beams are used in a wide range of applications particularly in the offshore sector.
The Laser based system utilities a very high-frequency laser pulse with a vertically fanned output, over a 360° angular range and passive retro-reflective targets attached to moving or stationary platforms or vessels, return the laser pulse back to the sensor despite relative movement of the vessel. These passive retro-reflective targets may be simple retro-reflective tubes for close proximity operations or an engineered prism cluster which it is claimed can offer operational ranges up to 2000 meters
The laser is mounted on a motorized base allowing for horizontal rotation in both directions. The laser can also be tilted in the vertical plane using an integral Auto tilt mechanism allowing the laser to lock on targets at a different height to the sensor or compensate for differences or changes in height due to change of draft etc.
Typically the reflectors will already be deployed before the vessel moves into position. Initially the laser will perform a sweep to determine what targets are available. The DP operator can then select the preferred target and set the system to track it. Once the sensor locks on to the desired target it uses the returned pulse to measure the range and bearing between the laser unit and the targets, software then calculates relative position and heading providing accurate time-of-flight information and communicates this to the DP system.
The DPO can then set up operational limits such as range gates etc. As with any system there are always errors and constraints which include, poor selection of target, following a false target i.e. someone in reflective PPE, failing to clean the laser lenses, and obscuration by personnel working , snow, fog, sandstorm, rain or bulk discharge.
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Laser PRS Fan-beam Cyscan Spotrack