NASA AIS - any good?

Philiz said:
so may be a bit limited compared to other (possibly more expensive) units. But I find it good enough for the areas we frequent.
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Hey!
So they work in P.S.P's then.
If yer think YMT is taskin, work that one out Philiz:D

Sorry a bit of Fred Dwift folks, kinda
But I couldn't resist;)
 
PaulGooch said:
The reason i asked, was to see if it was possible for you to use an AIS engine and overlay the AIS data on the plotter. This is a vastly better way of doing things. You can see at a glance where the targets are in relation to your position and the plotter will monitor targets and provide closest point of approach alarms etc.

Unfortunately, it doesn't seem that your plotters support AIS, so unless you fancy a plotter upgrade you don't have many choices. It's either upgrade a plotter (lots of benefits here, but apart from the expense), install software on a laptop and use an AIS engine (not a very elegant solution) or use the NASA unit.

The best solution, IMO, is a new plotter, dual channel receiver (such as a Comar engine) and a second antenna. This might be out of your current budget plans though.

It's important to note :

1) The NASA AIS is not waterproof.
2) You will need a second VHF antenna, don't use a splitter.
3) You will need to be able to supply the unit with an NMEA feed from your GPS.

Two reasons why i said it isn't the best. Firstly, it has limited functionality compared to plotter overlay. Secondly, it's a single channel receiver, as are all NASA AIS engines too. Here's why single channel AIS is inferior, although given your choices you may have to live with it, unless you upgrade a plotter and use a dual channel receiver :


AIS uses channel A and B (not to be confused with Class A and B AIS), these are channels the same as your VHF channels (161.975 MHz and 162.025 MHz). Vessels required to transmit AIS data alternate between the two channels. A single channel receiver, such as the NASA one, will monitor both channels in much the same way your VHF does "dual watch". When there is some data transmitted to channel A, it will switch to channel A, decode the data and pass it to your plotter, all good stuff. The problem is, whilst it was dealing with the incoming data on channel A, any data incoming on channel B will be lost.

Data such as speed, course etc are transmitted every few seconds to every three minutes, depending on how quickly the ships heading etc are changing. Other data, such as the ships name, LOA, draft, etc are only transmitted every 6 minutes, miss this one and you won't get a chance to obtain the ships name for another 6 minutes.

You could technically miss a transmission any number of times. So it is technically possible that a vessel would never be "seen" by your AIS. It's probably very unlikely that this would happen, but you will definitely miss some transmissions, which will reduce the accuracy of your AIS. If a vessel made a single, sudden change of course it would transmit that data within seconds, if not immediately. If you missed the transmission and the vessel then held it's course and speed it wouldn't be transmitted again for another three minutes, hopefully you don't miss this one.

The busier the area, the more transmissions, the more likely you are to miss data.
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Thanks for the very comprehensive explanation. Makes sense now!
 
Hi guys,
I have a Nasa AIS engine2 which I have upgraded to the engine3 with a replacement chip which NASA send me.
Its now connected to my Raymarine A50 poltter and a laptop (not always) and uses an antenna splitter( Easysplit) together with my DSC VHF and one entenna.
Its nonsense that a decent splitter cannot be used.
Everything works excellent. I used it this summer during a 2 month cruise to Norway and Sweden and it was a big help!
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YELLOW BOAT said:
Hi guys,
I have a Nasa AIS engine2 which I have upgraded to the engine3 with a replacement chip which NASA send me.
Its now connected to my Raymarine A50 poltter and a laptop (not always) and uses an antenna splitter( Easysplit) together with my DSC VHF and one entenna.
Its nonsense that a decent splitter cannot be used.
Everything works excellent. I used it this summer during a 2 month cruise to Norway and Sweden and it was a big help!
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Click to expand...

The AIS engine works "part time" per channel, as explained previously, THEN you add a splitter, this means that any time you are transmitting on the VHF, you have absolutely NO AIS reception at all, nada, zilch zip.

For less than the cost of the splitter you could have purchased a second VHF antenna and connected it to the AIS using a PL259 on the coax and an adapter to fit the AIS engine. You would have mounted the AIS engine in such a position as the antenna cable could have been unplugged from the adapter and fitted to the VHF set as a backup antenna.

So, you spent more money for a system that doesn't work as well and doesn't give you a backup VHF antenna as a free of charge bonus.

Nonsense ?
 
I have the engine 3 with a seperate aerial and linked into my Garmin 4008. It does alternate between channels A & B but so far it seems okay for the price. I might be wrong but I think the units which scan both channnels constantly are another 100 quid or so.

Does anyone know what determines if a vessel transmits on A or B?
 
Philiz said:
I have the engine 3 with a seperate aerial and linked into my Garmin 4008. It does alternate between channels A & B but so far it seems okay for the price. I might be wrong but I think the units which scan both channnels constantly are another 100 quid or so.
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I'm pretty sure you've worked out that i'm not saying it isn't OK for the money :)

I'm not saying anyone should change theirs, not even saying that on-one should buy one. Juts saying how they work.

As for the antenna, it really seems a no brainer to me. Why pay £170 for a splitter (which reduces performance) when you can fit a second antenna (acting as a backup VHF antenna) for £60. I know which case i'd say was nonsense :p

Yes, about £100 or so Phil.

Does anyone know what determines if a vessel transmits on A or B?
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How and when the transmission is made is complex Phil. Read the section "Detailed description: Class A units" here : http://en.wikipedia.org/wiki/Automatic_Identification_System
 
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The reason for a splitter is not the cost of a second antenna but the problem where to put it.
If the AIS is e.g. on the pushpit the maximum range is much smaller than on the masttop.
A second antenna on the masttop is impossible because of the interference between the two antennas. They need at least one meter distance!
Off courze I could install a pole on the stern but that would have to be quite long to have a benefit.
As for the difference A and B.
A is the syatem which is required for vessels over 300 Tons and send much more info than B such as destination , size , load etc.
A updates much faster than B . A every 10 or 20 seconds ( don't pin me on it)
and B only every 2-3 minutes.
B ist the more simple system for non requirement vessels such as fishing tugs and pleasure crafts. In mainland Europe its also used by several cargo ships on rivers and canals.
B sends only things like name mmsi speed and position.
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YELLOW BOAT said:
The reason for a splitter is not the cost of a second antenna but the problem where to put it.
If the AIS is e.g. on the pushpit the maximum range is much smaller than on the masttop.
A second antenna on the masttop is impossible because of the interference between the two antennas. They need at least one meter distance!
Off courze I could install a pole on the stern but that would have to be quite long to have a benefit.
As for the difference A and B.
A is the syatem which is required for vessels over 300 Tons and send much more info than B such as destination , size , load etc.
A updates much faster than B . A every 10 or 20 seconds ( don't pin me on it)
and B only every 2-3 minutes.
B ist the more simple system for non requirement vessels such as fishing tugs and pleasure crafts. In mainland Europe its also used by several cargo ships on rivers and canals.
B sends only things like name mmsi speed and position.
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Click to expand...

We don't have mast tops here, it's a mobo forum :)

You don't need the two metre clearance between two antennas when using one for AIS, as AIS is passive (only receives). Sure you may get some bleed when transmitting, but that's still better than a splitter, IMO.

As for your description "A and B", sorry, but you have no idea how the system works. You have as i keep saying, confused type A and b with channel A and B.

You need to go back and read what i originally typed, or follow the link i just posted to Philiz.
 
first of all ,sorry about the motorboat thread. I'm new here and simply forgot to look.
The question was "what does it mean whan a ship transmit A or B".
My answer about the two types of AIS transponders is right!
The expensive A type for > 300 ton shops and the cheap class B for smaller norn requirement ships .
Of course there is also the 2 channels the AIS is sending on. That's the difference between an alternating channel reciever like the Nasa AIS and the more expensive dual channel recievers that actually have two recievers in it.
About the distance between two antenna's.
Indeed the distance for an AIS transponder antenna and a VHF antenna is 3meter horizontally and 2 meter vertically ( in Holland) . But for a VHF and an AIS reciver antenna "some" distance is adviced which ist mostly adviced as > 1 meter.
By the way even a simple metal stick next to and parallel to the VHF antenna will give a disturbance!.
That's why you cannot place the VHF antenna on the spreader of a sailing yacht ( its not even allowed by Dutch authorities).
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YELLOW BOAT said:
first of all ,sorry about the motorboat thread. I'm new here and simply forgot to look.
The question was "what does it mean whan a ship transmit A or B".
My answer about the two types of AIS transponders is right!
The expensive A type for > 300 ton shops and the cheap class B for smaller norn requirement ships .
Of course there is also the 2 channels the AIS is sending on. That's the difference between an alternating channel reciever like the Nasa AIS and the more expensive dual channel recievers that actually have two recievers in it.
About the distance between two antenna's.
Indeed the distance for an AIS transponder antenna and a VHF antenna is 3meter horizontally and 2 meter vertically ( in Holland) . But for a VHF and an AIS reciver antenna "some" distance is adviced which ist mostly adviced as > 1 meter.
By the way even a simple metal stick next to and parallel to the VHF antenna will give a disturbance!.
That's why you cannot place the VHF antenna on the spreader of a sailing yacht ( its not even allowed by Dutch authorities).
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Click to expand...

I've just re-read your posts and it seems clear that you have some confusion over class A and B, channels A and B and the frequency of transmissions.

Class A is for commercial shipping and class B is leisure, as you say.

Channels A and B is used by class A systems. Class B systems transmit less information, on a lower power setting and in a different fashion to class A systems.

As for the frequency of transmission times you mentioned, that isn't for class A versus Class B systems. That is for the different types of information the class A transponders broadcast, some data is broadcast every 2-10 seconds, some every 6 minutes.

I really don't have the time to go over it all again, it's all in my earlier posts, or the link posted to Philiz.

Oh, welcome to the forum.
 
Thepipdoc said:
Are these units any good?

http://www.nasamarine.com/proddetail.php?prod=AIS_radar

Or are there any other AIS recievers that do a similar job?
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I have had the Nasa unit for about 5 years, and the basic design been around far longer than that.
It has been reliable. Like a lot of Nasa stuff it does a basic job with no frills - not even an off switch. As already noted it is North up, so you have to get your head around your course and that of the approaching traffic; in practice this is fairly easy. My aerial height is less than 2mts, but even so I should get better than the 4miles range that I manage. I think I have some continuity problem there, but the 4 miles is ok for me, so I have not got around to sorting it out. I normally seem to nail most targets in the end on my saily boat but if you are fast moving you will certainly need the aerial high up.
The minus points of the thing have been covered but it is alone in its price bracket and I have had cause to bless it.
Would I buy one again? At this point maybe not. I would consider putting the 200quid towards a Digital Yacht Plotter/AIS, though this would be in the region of: c£500 + charts
 
I can't resist giving one comment.

What you say about the A transponders , that's right.
Its send freq. depends on the speed , about 10 sec at 14 knots and 2 sec at >23 knots. Its 3 min for anchored ships.

For B transponders its 3 min if < 2knots and 30 sec if > 2 knots.
Static figures are send every 6 minutes for both A and B.

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YELLOW BOAT said:
I can't resist giving one comment.

What you say about the A transponders , that's right.
Its send freq. depends on the speed , about 10 sec at 14 knots and 2 sec at >23 knots. Its 3 min for anchored ships.

For B transponders its 3 min if < 2knots and 30 sec if > 2 knots.
Static figures are send every 6 minutes for both A and B.

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Click to expand...

It's the important stuff that is sent every few seconds, speed/course etc

Less important stuff, like ship name is what is sent every 6 minutes.



But, like i said, there is nothing wrong with the NASA stuff, as such. It is cheap and works within the limitations of it's design and construction.

I have a NASA wind instrument on my boat. Compared to a Garmin/Raymarine wind instrument, it isn't as good. But, at not much over £100 it's cheap and gives me a good enough idea of wind speed and direction.
 
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PaulGooch said:
By the way, the dual channel Comar AIS multi (combined AIS engine with integrated splitter) costs less money than the cheap single channel NASA AIS and an Easysplit splitter.

Still not as good as separate antennas though.
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Yes, that's the one I should have bought. The NASA unit was an impulse buy as it was on special offer.

The link doesn't seem to work Paul, just takes me back to the message reply panel.

Pssst, anybody want to buy a Nasa engine3 unit, good as new ;)
 
doug748 said:
My aerial height is less than 2mts, but even so I should get better than the 4miles range that I manage. I think I have some continuity problem there, but the 4 miles is ok for me, so I have not got around to sorting it out.
Click to expand...

The formula for calculating vhf range (n.m.) is 1.4 x root of the combined height of the sending and receiving antennas (in feet). If your antenna is 5' above sea level and the transmitting ships antenna is, say, 100' above sea level you should have a range of about 15 nm. For an antenna on the masthead of a yacht, say 60', range to the same ship would be around 22 miles.

For AIS a range of 10 miles or so is fine because it gives you 1/2 hour so to react to a ship doing 20 knots.
 
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