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Technical Discussions
ARPA using water or ground speed MarineTalk Discussion Forums
All dates are given in mm/dd/yyyy format. Technical Discussions
  ARPA using water or ground speed
    Post Reply

  Member: Andrew Fairgrieve Post Date: 6/28/2000  
Water speed into ARPA is best for collision avoidance I am told, not ground
speed. It is an IMO rule. It is supposed to allow "aspect" to be discovered,
wheras ground speed will not. Unfortunately I can not understand the
reasoning, and words do not help me. Can someone produce two worst-case
radar screen plots showing side by side what the OOW will see if speed input
is water speed and ground speed, in the direction of the bow.
It may be interesting to see what happens if speed over ground in the
direction of course over ground is used, because several GPS manufacturers
provide this and it is being used into ARPA. There are ships using my dual
axis ground speed system but that gives longitudinal speed. Thanks
Andrew Fairgrieve

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  Member: Captain Post Date: 7/8/2000  
well, i think when you dive a little bit deeper into the subject, you will discover things.
when you think of the "water speed", it means that you tell ARPA to calculate speed of the vessel over the ground (SOG) + wind + current + tide current (which applicable) and all the relevant details affecting the vessel's speed.
when you put "ground speed" then you have to calculate all the details manually.
the IMO regulation might have been to allow the OOW's to see the "actual view" of all couse, speed and relevant information.
but me myself, would not trust in ARPA that much. because, there are so many incidents that the vessels are grounded or collided because of "electronic equipment failure".
i hope that would help ,but if it didn't please do not hesitate to write me, so i'll keep tham as my questions and seek the answer. and it will be much more better if you address the problem more clearly.


Capt.Tolga BALCI
DPA & IT Manager

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  Member: Jac.A. Spaans Post Date: 7/13/2000  
The answer of Capt. Tolga (not rely on ARPA) is not very helpful. In dense fog one has to rely on ARPA for a great deal.
Waterspeed is normaly taken from the waterspeed log, interfaced with the ARPA. For collision avoidance all ships around 'own ship' experience the same current, so it is actually the waterspeed that determines the aspect. Groundspeed could give misleading information.

Own ship sees other ship in bearing North at say 6 nmi. Heading of own ship is 020, of other ship 160; both waterspeeds are say 6 kn. Other ship is burdened vessel (own ship approaching over SB).
Now suppose that a westerly tide runs at 3 kn. The vector of the groundpeed of own vessel points 350 and of other vessel 190. Looking at groundvectors, own ship is now burdened vessel (other ship approaching over SB)

Discussion on this issue has slowed down in the last years. Looking to the ColReg waterspeed is the correct way. However, many ships prefer groundstabilisation, because fix objects are fix on the PPI. With waterspeed fix objects move with current on PPI.
Best way would be: input of both water and ground speed; PPI ground stabilised, ARPA has spring dial to show operator waterpeed vectors on request.

With the high speed vessels of today and the mostly (relatively) low speed tidal currents the difference between groundspeedvector and waterspeedvector is not so large as in the above example. For this reason most masters prefer ground stabilised. I do not know what the position of IMO (and shipping inspection) is on this issue.
For extensive treatment of this issue, see The Journal of Navigation Vol 48 Number 1 (January 1995), page 29 to 52.

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  Member: Andrew Fairgrieve Post Date: 7/13/2000  
Dear Capt Spaans
Thank you for your excellent reply.
From your scenario I calculate the effect on own ship is..
Movement to port over ground 2.82 knots
Slow the longitudinal speed ahead by 1.03 knots.
The longitudinal speed over ground is therefore (6-1.03) 4.97 knots.
(This is the speed given by a ground speed log not a GPS.)

If 4.97 knots is fed in to own ship arpa in place of 6 knots
with the same heading 020 I still can not see how things will
change. I think the vectors will be the same and would appreciate your further advice.

Best regards
Andrew Fairgrieve

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  Member: Jac.A. Spaans Post Date: 7/14/2000  
Dear Andrew,
I received your answer by both Marinetalk and e-mail. I am not a captain although I am a (special) member of the Neth. Capt. Ass. I am a retired professor in Nautical Sciences.
Now back to the problem.
Your calculations are right on the example I used. Please realise that the ships will collide without action 0.5 nmi West of the North-South bearing (bearing remains unchanged) after 32 min; both ships will be still on the same heading.
Indeed, longitudinal speed over the ground is 5.0 kn and lateral speed (290 for own ship) is 2.8 kn.
With radar in relative motion and input from gyro and waterspeed log, other ship will approach on the PPI from North with speed 11.3 kn. The vector of the vessel will show 160/6kn. Everything OK; fix objects will move on PPI East 3kn however.
With input from gyro and longitudinal groundspeed (5.0 kn) things change a little in the example (other examples will have greater changes). The approach speed will remain N-S/11.3 kn, however the speedvector of the other vessel will show 165.4/6.8 kn. Fix objects will move on the screen 250/2.8 kn. I do not think that there is any advantage in providing the ARPA with longitudinal groundspeed instead of longitudinal waterspeed (with gyro heading in both cases). The aspect is not correct and we have no ground based PPI.
On many radars there is the possibility to stabilise the PPI on a fix object. I do not know how the vector of the other ship is calculated in that case; it should be same way as with GPS input (groundspeed and groundcourse). As shown in my example this does not give the right information on aspect.
The real issue is: the speedvectors shown on ARPA are the result of radarinformation (successive bearings and distances, giving approach speed and course) and the own ship speedvector info (either heading and longitudinal speed or groundspeed and groundcourse)
Anyway, with Automatic Identification Systems (AIS) becoming mandatory between 2002 and 2008, own ship will receive from other ships in the VHF coverage the real time ground speed and course from the other ship's GPS !! Will this information be used for the calculation of the speedvectors of other ships ?? Suppose the information of the GPS of the other ship is not correct; will this lead to AIS assisted collisions ???
Maybe we should ask radar manufacturers to participate in this discussion !!
The article to which I referred in The Journal of Navigation can be found in any library of maritime University or College. The Institute of Navigation can be reached through the director at

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  Member: Andrew Fairgrieve Post Date: 7/16/2000  
Dear Jac
I have drawn out your plots and understand your explanation.
In the example apparant aspect is little changed by using longitudinal
ground speed, but there may be worst cases where it does change.
CPA and TCPA will remain the same by the look of it.

However, if GPS SOG had been used (5.7 knots being close to 6.0)
as a substitute to water speed the discovery of aspect would have
been hardly affected at all. What does ruin it is if GPS COG is used as
a vector.

No doubt there are other situations where the opposite is true. The final
advice must be use all the aids you can and navigate with caution. Speeds
available are dual/single axis water and ground, plus GPS cog and sog.

Regarding AIS I have a summary via info page. Used in
addition to ARPA it must be a great help, perhaps better displayed on ECDIS
rather than on ARPA. As target heading will be transmitted it should make aspect certain.
(Until the first AIS assisted collision as you say.) Presently AIS does not require
water speed to be transmitted.

Best regards

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  Member: Jac.A. Spaans Post Date: 7/16/2000  
Dear Andrew,
Agree on:
"use all aids and navigate with caution"
it would be helpful when shipmaster knows input of ARPA and therefore output on PPI of HIS ARPA.
I have my doubts on available info on board.
Again: wouldn't be helpful when ARPA manufacturers would participate in this discussion.
Agree on your remarks on AIS.
I looked in your website; AIS info is informative.
Another issue:
Does your docking system suffer from blocking by airbubbles from tugboats and owns ship during berthing ? I sent you an e-mail of a presentation I gave recently at a congress in San Diego where a.o. this issue is mentioned.
Best regards
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  Member: walkermarine Post Date: 12/13/2001  
We have an interesting email from a Rotterdam Pilot who explaines why he is more interested in speed through water than speed over ground and also provides the technical input as to why ARPAs should have STW.
From July 2002 of course the Latest SOLAS CHAPTER V carriage requirements state that all vessels of 300GRT and above shall have a speed through water "STW" speed and distance measuring equipment "SDME" and in addition all vessels of 50,000GRT and above shall also carry a "dual axis", ie. longitudinal and athwartships measuring, speed over ground "SOG" SDME.
Please email "" to receive a copy of the pilot's email.
To view WALKER MARINE'S range of type approved "STW", EM Speed logs, SDMEs and dual axis "SOG" SDME, using GPS technology requiring no through hull installation, plus our range of anemometers, meteorological systems, echosounders and autopilots, visit our web site "".
We welcome your responses.Thank you.

Adrian Yelland
Sales and Marketing Director
Tel +44(0)20 8941 7333
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  Member: Ranvir Jatar Post Date: 10/16/2003  
Undoubtedly water speed will provide the correct aspect of other vessels as opposed to ground speed. However, I have two questions on this:
1) What does one do when the doppler speed log on board automatically switches over to ground speed below a certain depth?
2) The rules of the road prohibit one from taking action based purely on Radar data and in fog, there is no such thing as the 'give-way' vessel. In other words, one cannot or should not take action based on the aspect suggested by an ARPA vector but only on visual data. So, why should it be so necessary to have a water speed input all?
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  Member: walkermarine Post Date: 10/16/2003  
It is a a SOLAS requirement, since 2002, that all (new) vessels over 300gt carry a speed through water log. A doppler log which could not give speed through water, STW, at all times would need to be replaced or the vessel fitted with another dedicated STW log.
I'm no expert on ARPA, but I do know that STW is the required speed input. At any time, for large vessels, right of way needs to be established from distances of many miles. In poor visibility it would be impossible to wait until a vessel's lights could be seen before taking a decision on a course to steer. The introduction of AIS should also help to clarify another vessels course and therefore make the decision safer.

Best regards, Adrian Yelland. Walker Marine
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  Member: Capt. Khandke Post Date: 12/13/2003  
Collision avoidance is only about relative movement. One is concerned only with what other vessels are doing in relation to you. Therefore only relative vectoring can be used.
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  Member: weser Post Date: 5/2/2006  
I tried to send e-mail to Adrian Yelland
Sales and Marketing Director .But impossible!If anyone has email from a Rotterdam Pilot who explaines why he is more interested in speed through water than speed over ground and also provides the technical input as to why ARPAs should have STW,please send me a copy!
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  Member: Danboy Post Date: 7/8/2010  
Hi to everyone.
Had the same considerations for a long time I have concluded on the following subjects :

1) IMO requires Log speed to be incorporated in the ARPA, but the only reason for that is that a Log speed is compulsory for any size and type of vessel, while GPS is not. On the other hand, does not clarify if this is the only acceptable speed indication for the ARPA calculations.

2) I have noticed that most of the argument is based on the fact that the "other vessel's" speed data, is not taken by the other vessel, but instead is calculated by the vector of "our own". This practically means that if "our speed" is based on GPS and COG then the result CPA (which is the required calculation outcome) should be the exact same like if used Log speed. The difference is that the correct way shouldn't be any other than the one that results from the most accurate instruments, and I believe that this is not the Speed Log.

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  Member: S.N.Kumar Post Date: 2/28/2012  
On most ships Speed log is not accurate and results could again be erroneous for ARPA.In this case GPS heading and speed inputs should serve the purpose especially in the open sea where influence of tidal streams would not be there.As long as we have a safety margin to allow for errors it should be ok.
But near the coast and especially in channels difference between using STW & Ground could really be seen and would play a significant part in CA.
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  Member: mulkesh Post Date: 4/14/2012  
hey every one how r yu doing..
dear all of yu. i really don't under stent what you all r talking about.
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  Member: Windjammer Post Date: 6/19/2012  
My view is that, in most cases, using ground stabilization with COG and SOG from the GPS will give the most accurate CPA and TCPA data. The SOG data is simply much more reliable that the STW data from most doppler logs.

Although it is a requirement to have a speed log input into the ARPA, the mariner is usually allowed to choose between ground and sea stabilization modes based on professional judgement. (The Bahamas Maritime Authority actually requires that the ARPA be operatied in sea-stabilized mode!).

The most commonly stated justification for sea stabilization is that it allows the operator to determine the correct aspect of the target vessel. This is true, however it is also mostly irrelevant. The primary purpose of the radar in collision avoidance is to determine the risk of collision. The collision avoidance rules in the COLREGS that require knowledge of the target vessel aspect are all in Part B - Section II, which apply only to vessels in SIGHT of one another. Hence, to determine the other vessel's aspect, one should LOOK OUT THE WINDOW. Rule 19, which applies in restricted visibility, does not require knowledge of target vessel aspect.

Another advantage of ground stabilized mode is that, by comparing the own ship SOG vector with the heading marker, one can quickly determine the local set and drift.

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  Member: Post Date: 11/22/2012  
I am agreed with the above comment...
While having STW, we get correct aspect of other vsl, which is important when applying collision avoidance (COLREG Part B, Sec II). Apart from that what other advantage of having STW over SOG?
Disadvantage of having STW - vsl stopped over ground but drifting towards you.. will not show any vector and some fixed object will show vector... do't you think it might confused the OOW??

SOG will give accurate CPA and TCPA... but if you apply bit sense then you can take action even ARPA is on SOG. only few cases where you get headon or nearly head on situation, so why not toggle between STW and SOG at that time??

Personally I show all of us using SOG in ARPA.. only to justify surveyor and outsider, we posted sticker on ARPA that "STW to be used for collision avoidance"....
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  Member: crespilot Post Date: 1/4/2015  
As a pilot, I think it is a huge mistake to have these radars water stabilized for ARPA. It is my belief that a vessel is at the highest risk of collision when it is in port, near fixed objects and banks! When a radar is water stabilized, the True Vector that so many pilots use to judge speed while the vessel is rotating, is completely wrong!

Also, is the doppler speed log even accurate? 95% would be a huge failure relative to GPS! I am going to start doing a survey because I know the current in our river, and I will try to calculate a percentage error rate for a bulk of ships. GPS is extremely accurate, which means the position and speed will be accurate. If a vessel is relying on a mechanical, moving part, doppler speed log that can give erroneous information, then that in itself is the biggest justification to not use STW!

Anyway, as a pilot, I think STW has and will continue to cause accidents while vessels transit in rivers and near land. IMO/SOLAS should get on this and do it quick.
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  Member: anonymous Post Date: 3/5/2015  
Please note:
Doppler logs calculate speed in water tracking mode in deep waters, say mostly in Open Sea. However. Ground Tracking takes over while entering coastal regions.As far as use of vectors, relative or true is concerned please note it is very unreliable when vessel is in the act of turning.This is caused due to Processing delays in Processor by the everchanging course and to some extent the speed.
when vessel is set on a firm course/speed ARPA data is accurate but to carefully used by keeping safety margins of CPA. One can always set VRM and PI lines to continuously predict LOA of any target in question. Good Luck to you!!
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  Member: anonymous Post Date: 3/11/2015  
Well thank you all involved in this discussion. I'm up for my Oral examination in 2 weeks and I'm even more confused on the subject than I was before I visited this forum. Just hope I don't get asked to explain why STW is the recognized speed input for collision avoidance. Even college tutors I have asked on the subject give such conflicting views.

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  Member: pardeep punia Post Date: 4/20/2015  

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