3.1. Position reference systems

A DP systems rely on more than one position monitoring systems in order to get an accurate and reliable input for the current position of the vessel. For DP Class 2 or Class 3, it is necessary to use three different position monitoring systems. Two systems are not enough because if one of them system malfunctions and does not give corrent data the DP control system is not able to identify which system is wrong. Thus, it is necessay to have at least 3 reference systems active to provide a two-out-of-three voting and identify the wrong set of data. Below are the 5 most frequently used systems:

PSV-DP-Systems1. DGPS

2. Hydroacoustic position reference (HPR)

  • Long Baseline systems, LBL
  • Short Baseline systems, SBL 
  • Ultra-short baseline systems, USBL

3. Taut wire

4. Laser-based systems

5. Artemis

 

Thialf dynamic positioningFor example, one of world's largest heavy lift vessel, the Thialf can use 5 different  monitoring systems for Dynamic Positioning:

  • 2 x satellite DGPS
  • 1 x mechanical taut wire (300 m)
  • 1 x Artemis
  • 2 x acoustic SSBL/LBL
  • 1 x Fan-beam laser

 

3.2. DGPS

A GPS receiver returns the position of the system in terms of global coordinates by comparing the distance of the GPS receiver from at least 4 satellites. The position of the satellites is know and the distance between the the GPS receiver and the satellite is determine from the time required for the signal to travel from the satellite to the GPS receiver.
GPS accuracy: 10 meters (with a clear sky view and enough satellites)  

The sources of error for a GPS systems are:
- Ionosphere 4.0 meters
- Satelite clock errors 2.1 meters
- Ephemeris 2.1 meters
- Troposphere 0.7 meters
- Receiver: 0.5 meters
- Multipath distortion: 1.0 meter
Total error: 10 m
 

DGPS (Differential GPS) is essentially a GPS that uses wireless correction data to correct the error and enhance the accuracy of the GPS. For operating DGPS system we need a nearby, fix GPS station (base station). The fixed GPS station, receives the signal from the satellites and calculates the distance of the station to each satellite. At the same time the station knows the exact location of the satellites and the true distance of the station to each satellite. By comparing the two data set (GPS distance and true distance) the station calculates the error for each satellite. Those values are instantly transmitted to the GPS system of the vessel which adds or subtracts the values to correct its own GPS measurements. 

DGPS accuracy: 3 meters

RTK GPS (Real Time Kinematic GPS) satellite navigation is a technique used to enhance the precision of position data derived from satellite-based positioning systems, being usable in conjunction with GPS, GLONASS and/or Galileo. It uses measurements of the phase of the signal′s carrier wave, rather than the information content of the signal, and relies on a single reference station to provide real-time corrections, providing up to centimetre-level accuracy. With reference to GPS in particular, the system is commonly referred to as Carrier-Phase Enhancement, or CPGPS.[citation needed] It has application in land survey and in hydrographic survey.

RTK GPS accuracy:  1 cm ± 2 cm/10 km from the base station

Short videos on GPS systems:
GPS Video 1 - GPS Basics
GPS Video 2 - RTK

 

Other references
http://what-isthe.blogspot.com/2010/04/what-is-accuracy-of-dgps.html
http://en.wikipedia.org/wiki/Differential_GPS

 

3.3. Hydro-acoustic position reference systems

The system uses one or more transponders located at a predetermined position on the seabed and transducers mount at hull of the vessel. The transducer sends a pulse to the transponders. When the transponder receives the pulse it sends a new pulse that it is received at the transducer. The transmit/receive time is proportional to the distance between the transponder and the transducer. Using the distance from 4 transponder we can determine the position of the vessel.

Accuracy: Subsea target tracking: cm accuracy independent of depth (0-3 cm)

3.4. Taut wire

 

3.5. Laser

The system is primarily used as a dynamic positioning (DP) reference sensor measuring the position of an offshore support vessel (OSV) relative to an offshore structure such as a platform. Using the position data from the Fanbeam and other sensors, the DP system automatically holds the vessel on station allowing operations to take place. The system is regularly used as the primary position reference during critical short-range operations such as cargo container lifts from platform supply vessels.