Accuracy of nasa ais engine

[crossbones]

If I can see a ship as it passes a fixed harbor transponder then I can, on the ais engine, pinpoint exactly on the ship where the transponder is fitted. The accuracy appears better than the 10 metres or so I would expect from the gps system. On thinking about this I concluded that, as my gps and the target’s gps are getting their positions from the same satelites, the relative accuracy of the gps readings is more important than their absolute accuracy. The relative accuracy being considerably better. Since the ais system transmits position with a resolution of 1/10,000 of a minute (about 6 inches) what limits the relative accuracy I can expect?

 

[tome]

Provided you can both see the same satellites (ie both antennas unobstructed and not more than a few miles apart), receiver noise is probably the main factor and relative accuracies could be as good as 1-2 metres horizontal on a decent gps set.

 

[MIKE_MCKIE]

While such accuracy is very nice to have, if you are in a situation with another vsl, where better than 10 metres accuracy is required, I would suggest that a long white stick (Boat hook, fenders, lifejackets etc) would be more appropriate!

 

[Oldhand]

I have watched many moored ships at dockside on another AIS system calibrated to charts. Some ships seem very accurately placed and some aren't or move around, i.e. big variation in the quality of GPS position being output by the AIS transcievers. Also some ships either do not give out their dimensions on AIS or have obviously entered them incorrectly, as a result they are not plotted the correct size against the dockside.

I think you are expecting too much in trying to dertermine where the AIS antenna is mounted on a vessel. It is certainly outside the designed purpose of AIS to try and use such position accuracy and if the vessel is moving there is the time lag element of the transmited data.

 

[jonjo]

Happy to be corrected by the resident merchant marine pro's, but...

Is the location of the AIS antenna significant?

The location of a GPS antenna on a long vessel might be important in AIS dependant very close quarter collision avoidance.

 

[crms]

You should not be using AIS for a close quarter collision avoidance!!! I also think it is unlikely that both your GPS and and the target's GPS are tracking the same set of satellites.

 

[jonjo]

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You should not be using AIS for a close quarter collision avoidance!!!

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I don't have one. But since you are an expert please answer the question... is the position of the AIS antenna relevant?

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I also think it is unlikely that both your GPS and and the target's GPS are tracking the same set of satellites.

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So what? Regardless of the portfolio of satellites used both positions are accurate to say 15 meters. The type of close quarter situation I was thinking about was 200 to 400 meters, at which point ambiguity re. the location of the GPS feeding the AIS on the other vessel affects the calculations.

 

[tome]

Blimey, not worth getting hot under the collar.

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is the position of the AIS antenna relevant?

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Not at all, but the position of the gps antenna is highly relevant on a large ship as this is where the AIS position is fed from.

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I also think it is unlikely that both your GPS and and the target's GPS are tracking the same set of satellites.

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On a channel crossing, you'll both see the same sats for all intents regardless of position.

I don't see why you wouldn't use AIS for close quarters avoidance if you had solid identified contacts. It's far more precise than radar ARPA.