Love for AIS

[jezjez]

Just come across channel in a charter boat with AIS. Night crossing. Of course there's the real world to be eyeballing at all times, but AIS really helped us to identify the biggest dangers and we could see the "motorway" traffic coming from beyond the horizon - brilliant and very reassuring help.
For those who don't know it - AIS is a ship idetification system which shows BGR and speed and looks like space invaders! Beware the similarity - I didn't see any explode on the screen in a fireball but i could watch them marching across the screen towards us or not. From what i've seen, I think for £500ish the electronic chart and AIS can be yours.

 

[shmoo]

Its a cheaper than that.

About 150 GBP for the nasa engine, retired laptop, free SeaClear plotter software, scan your own charts in. Really useful in traffic lanes.

Health Warning: not a substitute for lookout, not all ships have AIS, not all that do work.

 

[Talbot]

even better, buy the cheap AIS software from www.shipplotter.com and get CPA reports and a warning zone. Makes AIS really really useful. Enables you to identify the visual contacts, and you can then play a game of guess the CPA from range nd ship aspect - this really allows you to hone your skills, and pass a boring watch!

 

[shmoo]

[ QUOTE ]

buy the cheap AIS software from www.shipplotter.com


[/ QUOTE ]

The free software from SeaClear gives CPA, TCPA and allows setup of collision alarm on both.

I would welcome seeing a comparison between shipplotter and seaclear if anyone has done one. I use the later a good deal and rate it highly, but shipplotter does seem to be mentioned on the forums a lot.

 

[jerryat]

Very interesting thread for me as I've been thinking about an AIS engine (probably Nasa) to use with Seaclear on my laptop. However, I could use some advice on three questions:

1. Do you use your existing VHF aerial with a splitter to feed both vhf and AIS, or do you need a separate, dedicated aerial?

2, If the same aerial is used, is there any risk that the VHF transmitting would damage the AIS engine.

3. Does anybody know if the Nasa/Seaclear set-up will run with the old Tsumnai 99 Charts I have loaded?

 

[Talbot]

2 options for getting the gps info to both:

1. extract nmea data via serial link and split one to laptop for chart system, the other to the dedicated gps in line for AIS receiver (on NASA engine) - disadvantage is that this may reduce the amount of AIS data shown if in a very busy environment.

2. feed gps into laptop, then split the signal within the laptop using FransonGPSGate, and supply to bothe programms simultaneously - this is what I do, and reckon its worth the cash.

 

[jerryat]

Thanks Talbot,

Should have mentioned that I've got a USB Haicom GPS mouse which is excellent, and shows my boat position on the charts to a high degree of accuracy. I assumed that the gps info would automatically by 'picked up' by the Shiplotter or Seaclear s/w. Is this not the case or must I have the Fransongpsgate s/w you recommend?

 

[shmoo]

I think jerryat wanted to get VHF signal to both AIS and radio not split the NMEA.

I use a seperate antenna. A regular VHF one but mounted on the pushpit - you don't need lots of range for AIS. If you use the same sort of plugs everywhere you have a very straightforward emergency antenna in case the mast falls down.

I couldn't bring myself to believe that a splitter wouldn't toast the AIS when I put the VHF on transmit!

(edit) just checked on seaclear website www.sping.com and they say it supports USB - I have no experience of this myself: gps too old!

 

[Talbot]

If you are using Seaclear for both nav and AIS, its not a problem. Personally I prefer to be able to look at the AIS without too much nav to cloud the picture (I use a calibrated satellite imagerather than a chart - by preference) and thus can swap between the AIS and the chart plotter (Maxsea) and not lose contacts due to a cluttered picture. To be able to do this means that you need two seperate GPS feeds, as the first programme to use GPS locks out any other programme. Fransongpsgate gets over that problem by creating virtual comm ports (as many as you want).

I also have a seperate AIS antenna on the stern gantry, which can be swapped over with the VHF antenna.

 

[jerryat]

Hi smhoo and Talbot,

Thanks for the advice. I carry an emergency VHF aerial, so will try that temporarily on the pushpit before fitting a permanent aerial there.

Yep, I figured 25 watts down the aerial lead wouldn't do the AIS engine a lot of good!!

 

[jonjo]

My AIS decoder is connected to a separate pushpit mounted aeriel (lower horizontal on the pushpit). Maximum AIS target range is a disappointing 6-8 miles.

Before buying the Nasa AIS box you might want to research whether it can listen to both AIS radio frequencies. My understanding is that it misses 50% of the AIS squawks.

 

[pappaecho]

Mounted the second aerial below the first on the mizzen. My Nasa works well with up to 16 vessels simultaneously, and most can be discarded as being below immediate danger. Well worth the money.

 

[jb2006]

[ QUOTE ]
Yep, I figured 25 watts down the aerial lead wouldn't do the AIS engine a lot of good!!

[/ QUOTE ]

That's where the splitter comes in - as I understand, it is hardwired from the aerial to the vhf, but disconnects the AIS receiver from the aerial when you transmit. If it fails you could toast the AIS I suppose, but the hardwiring should maintain the aerial for the vhf.

When we had a Legend, we had a separate stub aerial on the cockpit arch and the detection range was up to 32 miles

 

[jerryat]

It says the following on their website:

The unit can receive ships on either the A or B AIS channels. In default setting it alternates between the two channels.

Operating frequencies - 162.025 and 161.975 mHz.


Is this what you suggest it doesn't do?

 

[jerryat]

Yep, I think the separate aeriel is the sensible and safer way to go.

 

[jb2006]

OK - when I sort out what I need to display AIS on the C80 plotter on the new boat I was thinking of going for a splitter

 

[jonjo]

The bit that worries me is the "either" word. I think the more expensive units can receive and process both frequencies concurrently.

This factoid lodged in my brain a year or so ago, I cannot recall the source but is has concerned me since. We need a pro like Tome to adjudicate because I am no expert in this area.

My specific concern is that in a close encounter "has she stopped turning" situation, waiting 20 seconds instead of 10 for an update squawk from the vessel could be critical.

 

[jonjo]

Did some more Goggling on the dual channel, not any wiser but more info at these links:

http://www.panbo.com/yae/archives/001472.html

http://www.easyais.de/en_index.php

Conclusion is even if 50% of AIS info is missed the additional positional uncertainty is 51 meters at most for a class A merchant ship. Class B is much worse but not much of that about just now.

 

[jerryat]

[ QUOTE ]
Did some more Goggling on the dual channel, not any wiser but more info at these links:

http://www.panbo.com/yae/archives/001472.html

http://www.easyais.de/en_index.php

Conclusion is even if 50% of AIS info is missed the additional positional uncertainty is 51 meters at most for a class A merchant ship. Class B is much worse but not much of that about just now.

[/ QUOTE ]

Hi jonjo,

Thanks for those very interesting links. The 'easyais' one is particularly informative and I think sets my mind at rest re the difference in reporting time. The distances involved in the delay time due to switching is pretty small even for the 20 knot vessels, probably the most 'important/usual' ship speed range when crossing the channel/shipping lanes for example.

I'm going to look at makes other than just Nasa now to see if there are other advantages that make the extra pennies worthwhile.

 

[fireball]

this bit from the easyAIS.de website was interesting:

[ QUOTE ]

- fast 20kts class A target assumed:
* 2 channel update rate:2 seconds
* position change within reporting interval: 20,6 m, within GPS uncertainty (10 to 50 m)
* 2 channel multiplex update rate: 4 seconds
* position change within reporting interval: 41,2 m, within GPS uncertainty (10 to 50 m)

- slow 10kts class A target assumed:
* 2 channel update rate: 10 seconds
* position change within reporting interval: 51,4 m
* 2 channel multiplex update rate: 20 seconds
* position change within reporting interval: 103 m

- slow 8kts class B target assumed:
* 2 channel update rate: 30 sec
* position change within reporting interval: 123 m
* 2 channel multiplex update rate: 60 seconds
* position change within reporting interval: 248m

- fast 20kts class B target assumed:
* 2 channel update rate: 30 sec
* position change within reporting interval: 308 m
* 2 channel multiplex update rate: 60 seconds
* position change within reporting interval: 616 m


[/ QUOTE ]

So for class A transmissions with the Nasa engine the most you would expect the data to be out is 103m - I don't think I'd want to be that close anyway! Halved for the easyAIS engine (and others that run full duplex)
The class B targets are a little more concerning ... up to 600m out .... and even with a full duplex they can still be 300m out ... less than reliable in poor vis!