AIS Indicated Speed

[Buck Turgidson]

I am not familiar with the airline industry but the current training and advice in the maritime world is that a GNSS should not be relied as a sole means of position fixing or for collision avoidance.

I agree completely. That is not the same as saying it's prone to errors and not to be relied on. You included "sole means" which applies to any single source of nav.

I think the current advice is:

The prudent navigator should never rely totally on GNSS navigation and should regularly crosscheck the ship’s position using other means particularly in areas where the charts are based on old surveys.

The maritime industry could help itself by using technology developed in the airline industry for the integrity monitoring and alerting of GNSS

 

[jackh]

And if it was speed through the water one would have to know the current at their position and their leeway to work out CPA!

Ah but I only go sailing for the trigonometry...

 

[Tomahawk]

Not sure which?

But there is a huge great grey battleship in Plymouth Sound right now.... Well looking at the shape of it I am assuming it is a battleship?

But there is no AIS plot form anything in the sound? Perhaps it is only there if you can eyeball it?

 

[Giblets]

But there is a huge great grey battleship in Plymouth Sound right now.... Well looking at the shape of it I am assuming it is a battleship?

But there is no AIS plot form anything in the sound? Perhaps it is only there if you can eyeball it?

Looks like HMS Kent on marinetraffic.com.

 

[Mistroma]

Surely as the target data is relative (to your position) it is also derived from GPS? It's SOG and COG also take the affects of wind and tide into account.
Allan

I think it is likely to be due to the following difference (Simplified):

1) AIS bases CPA on data supplied by all parties involved (i.e. extracted from your GPS and GPS based data transmitted by other vessels). Doesn't mean that all the "external" data is always accurate, just usually more reliable than MARPA (see below).

2) MARPA uses data from your vessel (same as AIS) but calculates speed, course and relative position of others from their radar echoes. Unfortunately, your boat is likely to be rolling & yawing plus the radar beam width and target's shape (plus other factors) combine to produce "fuzzy" information. So the information used to calculate CPA is less reliable at any given time.

So MARPA will tend to give a poorer estimate of a ships speed & course than that based on AIS.

Others have already pointed out that AIS data is not 100% either (But generally better than MARPA). I spotted a target earlier this summer that appeared to be well over 1000nm away. I think I was somewhere in Biscay and spotted it on the AIS list. It obviously was not visible on the plotter display but I did monitor it for an hour or so out of interest. I'd need to check my notes but remember seeing the same vessel in harbour the next morning. So the AIS data was miles out for quite a long time.