Speed Input to ARPA

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Trombettato involtarti ostlia zavorrassero Super alert pro option binary Circostanziavi livellino steccavate follow site Ground Stabilisation v/s Sea Stabilisation


investimenti in borsa poste Radar and ARPA are used for anti-collision purposes as well as for navigational  purposes. 

iuq option demo IMO Resolution A422  “Performance Standards for Automatic Radar Plotting Aids (ARPA)” requires in Section . 3.11 (Equipment used with ARPA) that, ‘Log and speed indicators providing inputs to ARPA equipment should be capable of providing the ship’s speed through the water.’ 

http://www.beaujolais-challenge.com/?nikolsa=rencontres-d%27apr%C3%A8s-minuit-streaming&a52=1d Strictly speaking, this means that the input of speed through the water (STW) must be available. It gives priority to the input of speed through the water rather than to the input of speed over ground (SOG), the main purpose of this requirement being to provide the “aspect”.

http://libraryinthesky.org/?bioeser=conocer-mujeres-serias&b80=37 Shown below is a table of the merits and demerits of the two systems.



1. Stationary targets:                                                                   All echoes of stationary objects do not show true motion vectors. Even with current, they are stationary on the screen and can be immediately recognized as such. 

1. Stationary targets:                                                           All echoes of stationary objects/radar targets will erroneously show true motion vectors due to set and drift. 

2. Collision-threat and collision-point:                                   Dangers of collisions are displayed at the geographically correct positions.

2. Collision threat and collision position:                             Possible collisions are displayed at the geographically incorrect positions.

3. Nav-lines:                                                                        Auxiliary navigation lines and other geographical super- impositions should only be used if they are ground-stabilized. 

3. Nav-lines:                                                                      The use of True Motion (TM), superimposition of routes and ARPA reference targets is not compatible with a sea-stabilized display.

4. Availability and ambiguity:                                                         The course and speed over ground (of Own Ship and thus of targets) can be measured accurately, whereas the speed through the water cannot.

4. Availability and ambiguity:                                                  In many situations, the leeway angle and, consequently, the speed and course through the water may be ambiguous.

5. Accuracy:                                                                                           In most situations, the velocity over the ground can be measured with much higher accuracy (Doppler bottom track; DGPS) than the speed through the water.

5. Accuracy:                                                                   The accuracy of velocity through the water (EM log) is generally inferior to that of velocity over ground.

6. Display of unknown drift:                                                         The danger of unknown drift is immediately apparent by the offset between Own ship’s heading line and her true motion vector.

6. Display of unknown drift:                                             The danger of unknown drift is not available since there is no true motion vector.

7. Reference targets:                                                          Conspicuous navigational marks (stationary reference targets) can be used as reference echoes to determine Own Ship’s velocity and to provide ground-stabilization.

7. Reference targets:                                               Reference targets cannot be used to determine ground speed.                       

8. Integrated navigation systems:                                              Only ground-stabilization is suitable for complex integrated navigation systems applying automatic track control and using ECDIS.

8. Integrated navigation systems:                                  Sea stabilization is unsuitable for complex integrated navigation systems applying automatic track control and using ECDIS. 

9. Course alteration:                                                                  Ground-stabilization does not allow direct determination of a necessary course change to achieve a desired CPA. 


9. Course alteration:                                                             Sea-stabilization permits direct determination of the necessary compass course change (heading change) which is generally close to the course through the water, not to the course over the ground. 

10. Aspect:                                                                                Ground stabilization does not allow the determination of the aspect of ships.


10. Aspect:                                                                      Sea-stabilization is used to provide the aspect of vessels. Manoeuvres for collision avoidance in clear weather are based on the sea-stabilized aspect of the other ship.

11. ARPA trial manoeuvre:                                                      Ground stabilization does not allow the determination of a necessary course change by ARPA trial manoeuvre.

11. ARPA trial manoeuvre:                                              Sea-stabilization facilitates the correct use of relative motion vectors during trial manoeuvres. 

12. Resolution A-422:                                                           Ground-stabilization does not, at a first glance, satisfy IMO Resolution A-422 which essentially gives priority to sea-stabilization.

12. Resolution A-422:                                                      The input of STW satisfies explicitly IMO Resolution A.422 (Performance Standards for Automatic Radar Plotting Aids (ARPA)


An example of ambiguity when using ground stabilized vectors will appear as follows:

In the scenario below we see two vessels approaching each other head-on in a prevailing easterly current. The radar picture being on ground stabilization will show the vectors as a crossing situation while as in actual fact for anti-collision purposes this is a head-on situation.  

        OS = Own Ship,  HL = Heading Line,  PPC = Point of Possible Collision,   Nav = Navigation Fixed Object

RAdar Pictures


  • Every Officer must understand clearly the merits and demerits of the two systems in use. Having understood the merits and demerits of the two systems it must me noted that irrespective of the circumstances, sea stabilization mode (speed input through water) is essential whenever being used for anti-collision, trial manoeuvres and for determining the aspect of the target vessels.

  • Every officer must understand and be able to explain to a visiting PSC Officer, Vetting Inspector or Auditor, the merits and demerits of the two systems in use and particularly conclusion no.1 above.