Editing Dynamic positioning (section)

==Positioning systems==
{{further|Geopositioning}}

There are several means to determine a ship's position at sea. Most traditional methods used for ships navigation are not accurate enough for some modern requirements. For that reason, several [[positioning system]]s have been developed during the past decades. Producers of DP systems are: Marine Technologies LLC, [[Kongsberg Maritime]], [[Navis Engineering Oy]], [[GE]], [[SIREHNA (company)|SIREHNA]], [[Wärtsilä]] (ex L-3), MT-div. Chouest,{{typo help inline|date=March 2019}} [[Rolls-Royce plc]], [[Praxis Automation Technology]], Brunvoll AS. The term '''digital anchor''' has been used to describe such dynamic positioning systems.<ref>{{Cite web |url=https://mercurymarine-gsdesign1.netdna-ssl.com/media/filer_public/eb/69/eb69ed99-8eec-4a3b-8fff-1b564fef577a/skyhook_digital_anchor_sell_sheet.pdf |title=Advertisement from Mercury Marine |access-date=2015-05-22 |archive-date=2015-05-22 |archive-url=https://web.archive.org/web/20150522111249/https://mercurymarine-gsdesign1.netdna-ssl.com/media/filer_public/eb/69/eb69ed99-8eec-4a3b-8fff-1b564fef577a/skyhook_digital_anchor_sell_sheet.pdf |url-status=dead }}</ref>
. The applications and availability depends on the type of work and water depth. The most common position reference systems (PRS) and position measuring systems (PME) are:

[[Image:GPS Satellite NASA art-iif.jpg|thumb|[[GPS]] satellite in orbit]]
*'''[[DGPS]], Differential [[Global Positioning System|GPS]]'''. The position obtained by GPS is not accurate enough for use by DP. The position is improved by use of a fixed ground-based reference station (differential station) that compares the GPS position to the known position of the station. The correction is sent to the DGPS receiver by long wave radio frequency. For use in DP an even higher accuracy and reliability is needed. Companies such as Veripos, [[Fugro]] or C-Nav supply differential signals via satellite, enabling the combination of several differential stations. The advantage of DGPS is that it is almost always available. Disadvantages include degradation of the signal by ionospheric or atmospheric disturbances, blockage of satellites by cranes or structures and deterioration of the signal at high altitudes.<ref name="IMCA M141" /> There are also systems installed on vessels that use various [[GNSS Augmentation|Augmentation]] systems, as well as combining GPS position with [[GLONASS]].<ref name="Veripos" />
*'''Acoustics'''. This system consists of one or more [[transponders]] placed on the seabed and a [[transducer]] placed in the ship's hull. The transducer sends an acoustic signal (by means of [[Piezoelectricity|piezoelectric]] elements) to the transponder, which is triggered to reply. As the velocity of sound through water is known (preferably a soundprofile is taken regularly), the distance is known. Because there are many elements on the transducer, the direction of the signal from the transponder can be determined. Now the position of the ship relative to the transponder can be calculated. Disadvantages are the vulnerability to noise by thrusters or other acoustic systems. The use is limited in shallow waters because of ray bending that occurs when sound travels through water horizontally. Three types of HPR systems are commonly used:
**'''Ultra- or super- short base line, USBL or SSBL'''. This works as described above. Because the angle to the transponder is measured, a correction needs to be made for the ship's roll and pitch. These are determined by Motion Reference Units. Because of the nature of [[angle]] measurement, the accuracy deteriorates with increasing water depth.
**'''Long base line, LBL'''. This consists of an array of at least three transponders. The initial position of the transponders is determined by USBL and/ or by measuring the baselines between the transponders. Once that is done, only the ranges to the transponders need to be measured to determine a relative position. The position should theoretically be located at the intersection of imaginary spheres, one around each transponder, with a radius equal to the time between transmission and reception multiplied by the speed of sound through water. Because angle measurement is not necessary, the accuracy in large water depths is better than USBL.
**'''Short baseline, SBL'''. This works with an array of transducers in the ship's hull. These determine their position to a transponder, so a solution is found in the same way as with LBL. As the array is located on the ship, it needs to be corrected for roll and pitch.<ref name="IMCA M151" />
*'''Riser Angle Monitoring'''. On drillships, riser angle monitoring can be fed into the DP system. It may be an electrical [[inclinometer]] or based on USBL, where a riser angle monitoring transponder is fitted to the riser and a remote inclinometer unit is installed on the Blow Out Preventer (BOP) and interrogated through the ship's HPR.

[[Image:Light Taut Wire.JPG|thumb|Light taut wire on the ''HOS Achiever'']]
*'''Light taut wire, LTW or LWTW'''. The oldest position reference system used for DP is still very accurate in relatively shallow water. A clumpweight is lowered to the seabed. By measuring the amount of wire paid out and the angle of the wire by a [[gimbal]] head, the relative position can be calculated. Care should be taken not to let the wire angle become too large to avoid dragging. For deeper water the system is less favourable, as current will curve the wire. There are however systems that counteract this with a gimbal head on the clumpweight. Horizontal LTW's are also used when operating close to a structure. Objects falling on the wire are a risk here.
*'''Fanbeam''' and '''CyScan'''. These are laser based position reference systems. They are very straightforward system, as only a prism cluster or tape target needs to be installed on a nearby structure or ship. Risks are the system locking on other reflecting objects and blocking of the signal. However, the Cyscan Absolute Signature which was released in 2017 was launched to address this issue. It is able to engage in an active lock with the Absolute Signature prism which reduces the chance of a wrong target being tracked. Range depends on the weather, but is typically more than 500 meters. New advancement from Guidance Marine led to the development of the SceneScan sensor which is a target-less laser PRS leveraging on the SLAM algorithm.<ref name="IMCA M170" />{{clarify|What does "leveraging on the SLAM algorithm" mean|date=March 2022}}
*'''Artemis'''. A radar-based system. A unit is placed on a fixed station (FPSO) and the unit on board the mobile station locks on to it to report the range and bearing. The operationalrange is in excess of 4 kilometers. Advantage is the reliable, all-weather performance. Disadvantage is that the unit is rather heavy and costly. Current version is the Artemis Mk6.<ref name="IMCA M174" />
*'''DARPS, Differential, Absolute and Relative Positioning System'''. Commonly used on shuttle tankers while loading from a [[Floating Production Storage and Offloading|FPSO]]. Both will have a GPS receiver. As the errors are the same for the both of them, the signal does not need to be corrected. The position from the FPSO is transmitted to the shuttle tanker, so a range and bearing can be calculated and fed into the DP system.
*'''RADius'''<ref name="RADius" /> and '''RadaScan'''. These are radar based systems; while the RADius has no moving parts, the RadaScan has a rotating antenna under the dome. Guidance Marine has improved the miniRadaScan with the RadaScan View which has an added advantage of radar back-scatter.{{clarify|reason=How does radar back-scatter provide an advantage|date=September 2019}} This enhanced the DPO's situational awareness.{{clarify|how does this enhance the DPO's situational awareness?|date=March 2022}} These systems usually have responders which are active targets that send the signal back to the sensor to report the range and bearing. The range is typically up to 600 meters.{{cn|date=September 2019}}
*'''[[Inertial navigation]]''' is used in combination with any of the above reference systems, but typically with gnss (Global Navigation Satellite System) and Hydroacoustics (USBL, LBL, or SBL).

===Heading systems===
*'''[[Gyrocompass]]es''' are normally used to determine heading.

More advanced methods are:
*'''[[Ring laser gyro|Ring-Laser gyroscopes]]'''
*'''[[Fibre optic gyroscope]]s'''
*'''Seapath''', a combination of GPS and inertial sensors.

===Other sensors===
Besides position and heading, other variables are fed into the DP system through [[sensor]]s:
*'''[[Inertial measurement unit|Motion reference units, vertical reference units or vertical reference sensors]], VRUs or MRUs or VRSs''', determine the ship's roll, pitch and heave.
*'''[[Anemometer|Wind sensors]]''' are fed into the DP system [[Feed forward (control)|feedforward]], so the system can anticipate wind gusts before the ship is blown off position.
*'''[[Draft (nautical)|Draught]] sensors''', since a change of draught influences the effect of wind and [[Ocean current|current]] on the hull.
*Other sensors depend on the kind of ship. A pipelay ship may measure the force needed to pull on the pipe, large crane vessels will have sensors to determine the cranes position, as this changes the wind model, enabling the calculation of a more accurate model (see Control systems).
*Some external forces are not directly measured. In these cases, the offset force is deduced over a period of time, allowing an average value of compensating thrust to be applied. All forces not attributable to direct measurement are labeled "current", as this is what they are assumed to be, but in reality this is a combination of current, waves, swell, and any errors in the system. As is traditional in the maritime industry, DP "current" is always recorded in the direction that it is flowing towards.