GPS on Big Ships

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BlueSkyNick

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To the professionals around here, this might sound like a stupid question, but that hasn't stopped me asking them in the past.

GPS doesn't indicate the position of the vessel, but more accurately the position of its antenna. On my 10.5m boat, the antenna is on the stern rail. If I was to navigate by electronics alone, in zero vis, sat at the chart table, in a confined area I know that my worst margin of error is only 10m. As I know my heading, I have a reasonably good idea where the bow is.

If I was on a big ship in a confined area, eg an estuary leading into a dockyard, again in zero vis, I would want to be pretty sure that I know the location of both ends of the vessel.

Do they have seperate GPS systems at the bow and stern, or is that too simple?
 
It is quite possible that they do on very long ships (and on specialised work boats), but the ECS/ECDIS systems I have seen (even on small vessels including our own) all the sensors eg depth, including GPS you set up their position in a coordinate frame based on the conning position of the ship.

That is, if the GPS antenna is 20m behind the conning position and 5m to starboard, you enter those and the ECS then corrects (and displays) the GPS position to that of the conning (or other selected) position. From that one then knows where the rest of the ship is. {Edit: I should add that the ECS draws the vessel in plan to scale on the chart from the vessel's beam and length which you enter into the ECS}

For conning in pilotage one sets up the vessels advance and transfer for turns etc in the ECS and it takes those things into account when announcing a turn for a waypoint so that you don't end up with the bow on the bank ie it will put rudder over 200m, or whatever, before the waypoint so that the turn is made correctly onto the next course (remembering that this results in "cutting the corner").

John
 
Berthin was abandoned in Fog in my day - and listening to Southampton VTS on foggy days they still stop/anchor if its peasouper.
 
Good question. Sorry to disillusion you but your GPS is incapable of giving you the accurate position at your antenna, as it has its own variable errors. Even with EGNOS /WAAS working correctly you will only know your position to about half the length of your vessel. There are also errors in your chart, and can you plot the length of your vessel accurately on your chart anyway?

Big ships have the same problems with GPS accuracy. But they will also use radar and gyro compass. As an example look at some big ferries and you will see radar scanners mounted fairly low down on superstructure at the stern - these are used when berthing stern to. In ideal situations Radar is mathematically less accurate than GPS but has the advantage that the display shows contacts relative to the ship, not as calculated from military hardware in a high orbit then displayed relative to various surveyors work. Generally the navigation officers will manouvre their vessel allowing for the limitations of the nav aids available. Zero vis rarely exists in reality, but if visibility is below acceptable ranges port authorities will limit or even stop ship movements. When berthing very large vessels local berthing aids are often used, such as laser range meters to give distance off the jetty and speed of approach.

Relying on only one nav aid in any conditions, but especially reduced vis, is asking for trouble. As a navigation lecturer (Fishing and MN deck officers) we watch students during ship bridge simulator exercises and if they neglect to check the accuracy of GPS we slowly degrade their position information until they either notice and take appropriate action or suffer the consequences. Better to learn the hard way on a computer than at sea.

When working on survey vessels we had to make an allowance from positioning antenna to transducers and allowed for heading and transducer offset. This becomes critical when trying to position divers or equipment over a fixed point on the seabed and at times we used a plotting sheet to allow for the offsets.
 
Thanks for your informative response. I am aware of GPS accuracy errors and no I can't plot down to 10m - just wanted to illustrate my point, really. As I said, it was somewhat hypothetical - if vis was that bad, I wouldn't be sat at the chart table too long, myself!!
 
On bigger ships, the GPS is used in conjunction with an input from the gyro and a plan model of the ship based on x,y co-ordinates to build a reasonably accurate birds-eye picture for close manuoevering. The gps antenna position is programmed into the co-ordinate system, and the gyro input is used to rotate the wole coordinate frame (ship) around the position provided by gps. Slight simplification, the gps position is itself offset to the turning point of the vessel.

Many large ships use differential gps when within range of port which can position the antenna within 3m horizontally. This is the only time when you are likely to scale the vessel to this level and be going slow enough for heading (rather than cmg) to be relevant - on passage a cross or dot on the chart is used with a vector showing cmg and speed.

All our survey vessels use differential gps and we can get antenna position within 1m. We did some experiments with multiple gps bow/stern etc, but this was expensive and complex compared with using the gyros which are fitted by IMO requirements anyway.

As stated elsewhere, radar and well placed video cameras are the weapons of choice for the final docking manuoevres.
 
mmmm - not being picky, but feeding raw gps data through a computer using a kalman filter gives an epe (estimated position error) of about 3 - 4 meters. when the mean position is used it is a steady and reliable fix - although this is not accurate enough for d.p. (dynamic positioning) work it illustrates the raw data now available from gps can accuratly assess the position of a gps aerial.

so that coupled with a compass input and vessels characteristics will give you a pretty good idea of where the vessel is.
there have been doppler systems to interpolate velocity data remotely for decades, this can and is utalised during ship maneouvers.
intigrating that on a updated/corrected ecdis system (electronic chart display system) should give you an accurate navigational picture.

as to whether it is advisable to rely on electronic data instead of the mark one eyeball remains to be addressed.

so IMHO it is now possible to use an intigrated electronics package as an alternative to the eyeball in zero vis.

how the insurance companies and legislative authoritiies view the matter after a collision are not unknown .......... you would probably find the master would be hung, drawn and quartered.

that is probably why ppl dont use this technology in thick fog - its indefensible at an enquiry as it breaks the colregs, the tenet of good seamanship.
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Has to be a fast GPS 10 fixes/sec unlike yer Garmin leisurely 1 fix every 2 secs, and there's quite a lot of filtering and other complications. Our bare-board gps receivers cost over 1200 quid each plus another 700 per antenna. That's before we add display, processor, powers supply etc...
 
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as to whether it is advisable to rely on electronic data instead of the mark one eyeball remains to be addressed.

so IMHO it is now possible to use an intigrated electronics package as an alternative to the eyeball in zero vis.



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I am sure you are right but when you compare it to aircraft. they land on the runway at much greater speed and with far worse consequences if it goes wrong, yet it is almost a matter of routine nowadays.

So it shouldn't be unreasonable to bring a ship alongside using a combination of technology, either. All IMVHO, of course!
 
I agree
1. they dont usually destroy the tarmac for months
2. what is the alternative for an aircraft - divert ..... they cant remain at anchor can they

economics comes into it

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Nice one !!!! It affects ALL gera on ships ... Radar, Loran, old Decca, etc. etc. even down to mk1 eyeball and compass bearings ....

But many pro sets have offset set-ups to centralise the position by correcting for fore / aft / side measurements ... especially when using electronic charting systems allied to the GPS ....

Two GPS sets !!! Good one !! No - only time you have two positional indicators is when proceeding into some ports with some of the bigger ships - you have a Pilots Assistant standing on bow with radio - calling back to Pilot on bridge !!
 
You might find you're a bit out of touch here, I hesitate to use the word bolleaux again, though you do tempt me! All (even small) ships have at least 2 gps sets these days.
 
and if you count the ais and sat c's there are more than that .... each have their own gps receiver
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Dunno about big ships but my good old Navman 900 has shown me moored on the fuel berth at Ramsgate, Max zoom in the boat icon was on the right end of the berth shown on the cart chip.

Crew's confidence in GPS and plotter went up sharply! Just as well as I'd routed us through the tight bit off Margate and there were rumblings beforehand:-)
 
Sorry - me typing too quick again ...

Yes I know near all have 2 sets - redundancy provision .... what I meant was in answer to his antennae in different parts of ship to give position relative to ends of ship ... not as I incorrectly "too quick" typed GPS ....

I do apologise sir for not making that clear ...

I do in fact visit many ships wheelhouse and chart-rooms in line of my biz ... ......
 
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