Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

[Jim Cate]

Originally Posted by transmitterdan

Class B does not allow external GPS input. It must be integrated with the receiver.

I don't think that is so for receive only AIS. My older ICOM receiver required input from an external GPS.

Jim

[El Pinguino]

Originally Posted by transmitterdan

Class B does not allow external GPS input. It must be integrated with the receiver.

Well..... my AMEC black box Class B has no internal GPS and must have an external GPS attached.....

Some ( at least ) of the Vesper units while having internal GPS can have external GPS attached.

Maybe you mean it can't just pluck a GPS possy from an NMEA feed?

Can't be a 5V GPS USB puck..... must be 3.3V..

This is common to both AMEC and Vesper... see page 16 of this Vesper manual https://downloads.vespermarine.com/A...MX850-505F.pdf

[DotDun]

Originally Posted by El Pinguino

Well..... my AMEC black box Class B has no internal GPS and must have an external GPS attached.....

Some ( at least ) of the Vesper units while having internal GPS can have external GPS attached.

Maybe you mean it can't just pluck a GPS possy from an NMEA feed?

Can't be a 5V GPS USB puck..... must be 3.3V..

This is common to both AMEC and Vesper... see page 16 of this Vesper manual https://downloads.vespermarine.com/A...MX850-505F.pdf

You are talking about a GPS antenna, the GPS still resides in the black box.

[DotDun]

Originally Posted by Jim Cate

I don't think that is so for receive only AIS. My older ICOM receiver required input from an external GPS.

Jim

There is no such thing as a Class B receive only unit, he is referring to a Class B transponder. There are no rules pertaining to receive only AIS.

[Jim Cate]

There is no such thing as a Class B receive only unit,

This is certainly true...

he is referring to a Class B transponder.

so this must logically be true as well.

Was not clear to me, but you are correct.

Jim

[Kelkara]

Originally Posted by DotDun

There is no such thing as a Class B receive only unit, he is referring to a Class B transponder. There are no rules pertaining to receive only AIS.

Actually I had my lingo mixed up ... I was referring to a recieve-only unit.

I don't currently have any AIS, but I'm planning on experimenting with OpenCPN and an SDR dongle to receive AIS, and using my handheld GPS for position. I regularly change the datum on my handheld GPS to match local maps when travelling ... and I don't always remember to reset it to WGS84 when I get home. I was wondering if I accidentally had a strange datum set if it would produce incorrect CPA calculations.

[DotDun]

Originally Posted by Kelkara

Actually I had my lingo mixed up ... I was referring to a recieve-only unit.

I don't currently have any AIS, but I'm planning on experimenting with OpenCPN and an SDR dongle to receive AIS, and using my handheld GPS for position. I regularly change the datum on my handheld GPS to match local maps when travelling ... and I don't always remember to reset it to WGS84 when I get home. I was wondering if I accidentally had a strange datum set if it would produce incorrect CPA calculations.

Since OpenCPN is calculating the CPA/TCPA, it should convert the AIS data to whatever datum OCPN is set.

[Uricanejack]

truthfully I am no expert on what AIS can or can't do. I don't even have one. I just have questions.

A class a ships presumably AIS gets imputes from all it instruments.
some of it is input manually some automatically.
It can only be as accurate as what is input.
The name type of voyage and current condition are manually input.
One would hope they get the name and other Id right but current nav status is often out of date.

The ships position will come from the ships GPS.
The ship probably has more than 1 GPS so the primary GPS.

The ships course probably comes from its gyro compass and will only be as accurate as the gyro. A good gyro accuracy is typically about 1 degree.
The ships speed probably comes from the ships log. probably a Doppler.

Is it set to water or ground speed?
Good collision avoidance practice is to use course and speed through the water. Water speed.

So every 10 or 30 seconds it sends out it GPS position Heading or Course and Speed.

Your AIS. What is it using?

Probably has its own GPS or gets an impute from your boats GPS.

But where does your AIS get your boats course and speed from?

Your paddle wheel log and magnetic compass? No probably not.

Or does it calculate the course and speed from 1 position to the next.

My guess is your typical AIS on a small boat is calculating course and speed between positions. The calculated course and speed could only be as accurate as your steering. If the time between positions is short even though the two positions might be very accurate the course and speed might not be.

This would be course and speed over the ground not through the water.

So what is your AIS or Plotter using to calculate the CPA?

So while I am not sure this is the case I suspect. your AIS is comparing your course and speed over the ground over a very short time frame to the ships course and speed though the water.

I could be completely wrong the ships AIS might be transmitting its course and speed over the ground.

So just because the AIS gives a CPA to 0.1 of a mile. It might not be quite that accurate. I would suspect it is highly likely the CPA although the reading may be given to 1 decimal place it is nowhere near that accurate.

I is Also highly likely your small boat AIS is calculating a CPA based on comparing your course and speed over the ground to the ships course and speed through the water.

This doesn't mean don't use AIS. I just mean I don't believe what AIS tells you is as accurate as it appears.

Cone of uncertainty?

If you allow for the probability the information may not be as accurate as it appears. Consider it is likely to be accurate to within a tolerance suggested earlier by the OP.
Although I might suggest a bit more if crossing ahead.

My rule of thumb. 1 cable for every knot

[DotDun]

AIS dynamic data is GPS data - position, sog, cog. No heading data.

[Kelkara]

Originally Posted by DotDun

Since OpenCPN is calculating the CPA/TCPA, it should convert the AIS data to whatever datum OCPN is set.

It turns out that, for me at least, this has become a moot issue, since over the last hour of trying I have completely failed to connect my garmin 62s to OpenCPN ... I guess I'm going to have to buy a GPS USB puck. Thanks anyway.

Although the OpenCPN instructions do say to make sure that the GPS is providing WGS84 coordinates. So using an incorrect datum maybe could be an issue with some installations.

[Paul Elliott]

Originally Posted by DotDun

AIS dynamic data is GPS data - position, sog, cog. No heading data.

The relevant AIS Class-A dynamic data is:
Navigational Status (entered by a human)
Rate of Turn
Speed Over Ground
Position Accuracy (indicates a high-accuracy or low-accuracy GPS fix)
Longitude
Latitude
Course over Ground
True Heading

There is other data, but none germane to collision avoidance. The only human-entered data is NavStat, and we already know to take that with a huge grain of salt. Heading and ROT are not always provided, and there are specific values that indicate "not available" (All data has the "not available" option) (*)

The Class-B dynamic data contains all of these except NavStat and ROT. I seldom see HDG transmitted by a Class-B transponder.

(*) I've only seen the Not Available flag on data other than HDG or ROT one time: On a USCG boat used for interdiction, where they only broadcast their lat/lon. SOG, COG, HDG, and ROT were all "unavailable". This was with a Class-A transponder. Usually Navy ships will not broadcast AIS of any sort (but sometimes they do). USCG ships will transmit AIS in my experience. I don't see any stealth advantage to what that USCG boat was doing, but I doubt it was an equipment failure.

[Dockhead]

Originally Posted by transmitterdan

Class B does not allow external GPS input. It must be integrated with the receiver.

Probably for this very reason.

[Dockhead]

Originally Posted by DotDun

AIS dynamic data is GPS data - position, sog, cog. No heading data.

Correct for Class "B". But Class "A" also sends ROT and heading besides what Class "B" sends.

https://www.navcen.uscg.gov/?pageName=AISMessagesA


This is the dynamic data from Class "B":

https://www.navcen.uscg.gov/?pageName=AISMessagesB

[Dockhead]

Originally Posted by Paul Elliott

. . . The Class-B dynamic data contains all of these except NavStat and ROT. I seldom see HDG transmitted by a Class-B transponder.. .

I didn't think Heading was part of the Class B position report data, but I looked through it again, and you are correct. Default message is "not available" but it seems to be possible to transmit heading. Next I'll have to see whether my AIS set can be set to including heading in its position reports.

[transmitterdan]

Regarding AIS questions:

Ship position, COG and SOG come from GPS. Speed is always SOG, AIS does not care about speed through water. At least in the US, for class B the GPS must be integrated with the transponder. For class A the GPS can be external or internal, the turn rate and heading typically come from on board instruments (rate gyro, etc).

There are unapproved AIS class B units listed for sale on the Internet.