Curved AIS Position Projections?

[richwest3]

Can anyone explain why the course projection I saw on an AIS vessel was curved? I've never seen this before. There's a screenshot here and attached. The curve is in the upper right quadrant of the screen.

The curve would vary and even be a straight line at times. Sometimes it would curve to the right and sometimes to the left.

My chartplotter is a Simrad NSS9 EVO3 with a Simrad AIS as well. The projection lines are set for 30 minutes. Any ideas? Mr. Google provided no answers.

[stevead]

Rate Of Turn (ROT), usually expressed as degrees per minute. (in this case a minute being a minute of time, not a minute of angle !)

AIS Class A vessels (aka big ships), include a ROT value in their AIS transmissions. In this example, the ship is obviously turning and thus the projected course appears as a curve.

[JPA Cate]

Yes. The projected moment of intersection between the two projected courses will probably determine what the two vessels actually do to avoid each other. The one turning now, also has that rock to avoid if he continues his turn.

Ann

[Jim Cate]

I agree that the curve is most likely some form of projection based on the rate of turn info in the AIS packet. However, none of the AIS displays that I've encountered do that projection, and I think for a good reason: there is no info about how long the ROT will be maintained, and hence the projected curve is nonsense.

IMO, far better to project from the known heading/COG and rely upon human extrapolation of the ROT data to estimate the likely course over the next few minutes.

Jim

[HeywoodJ]

The SEAiq system has an option for this (and a couple of other nifty features for tracking large vessels in tight quarters). Note that using straight vectors is the option that must be selected:

Use Straight Course Vectors (Only SEAiq Pilot) : When enabled, course vectors for all vessels are straight (no turning). Note that this setting only affects course vectors and not other similar item such as swept path, docking paths, etc. This is intended to be used in situations where incorporating rate-of-turn into course vectors may be confusing, such as when there is strong drift and slow vessel speed.
Show Course from Bow & Stern (Only SEAiq Pilot) : When enabled, course vectors for all vessels are shown from both bow and stern, instead of center or conning position. The dual vectors are only shown if sufficient information is available, such as as ROT and HDG, for the two vectors to be useful. This does not affect Trial ROT, which is only shown from center/conning.

I think the EVO3 has an option to display the predicted vector using either heading or COG? It isn't addressed in the manual, but would be interesting to see if this curved projection is based on heading (which makes maybe a modicum of sense) and if it would only show a straight line based on COG?

[Martkimwat]

That is interesting. I have not seen that problem on my AIS, any vessel turning just shows the projected course and interception zone moving across the screen rather like a clock hand (ie a straight line sweeping across the screen) I have had AIS alerts even though I know the vessel is not going to hit me because there is a piece of land in between, never seen a curved AIS plot, and that includes very large modern vessels of all sorts (VLCCs, freighters, tankers, Ferries and military vessels). Is it function of certain brands of equipment? I have Raymarine electronics apart from my AIS which is a McMurdo class B transponder/ receiver.
Is it something I can select? might be interesting to check it out.

[richwest3]

Jim Cate suggested that this projected course that includes ROT is nonsense. I definitely agree. This curved projection changed from one side, then the other, then straight. The amount of curve changed with every data update. It was useless for determining the future position of the vessel.

I went looking this morning on my chartplotter (Simrad NSS9 EVO3 with pretty current software) to see if there was any setting for this "feature" but couldn't find one. Under settings/vessels, extension lines can be adjusted somewhat. The only options are length (30 minutes in my case) and a check box next to "Course Over Ground" with no indication of what changes when checked or unchecked.

[HeywoodJ]

Originally Posted by richwest3

... a check box next to "Course Over Ground" with no indication of what changes when checked or unchecked.

Only from reading between the lines, because the manual is not that good, but if that COG box is not checked it will use the heading information for the predictor line, if it is checked it will use the COG. Class A targets transmit both heading and COG as they can be significantly different, particularly when maneuvering.

As mentioned, I'd be curious to see if COG only displays a straight line? But then you have to play with settings while you have a curved projection displayed.