AIS

[rickchart]

I have just fitted a NASA AIS radar unit to my boat. Only problem is that you have to use a separate VHF aerial which must be mounted, according to the instructions, a mininum of 2 metres in the horizontal plane away from the radio VHF aerial to avoid interference. Not easy at the top of the mast of a Sadler 29!

I bought a Vitronix stub aerial to use with the VHF and have tested the AIS unit by just hauling the aerial up to the spreaders with the pennant halyard. It works!

I discussed the problem with NASA at the boat show and they admitted that, in their testing, they had merely used the test boat's VHF radio aerial disconnected from the radio and had no practical answer to my query.

It obviously needs to be as high as possible to get the range but need not necessarily be at the top of the mast. In fact a lower effective range of say 16 miles rather than 32 might give less overall information but more useful information on say a channel crossing.

I could mount it one one of the spreaders or on top of my blipper mounting 2/3rds the way up the mast. If the latter I assume that the mast would screen VHF transmissions from the area astern? Would there be any other adverse effects on VHF reception of AIS transmissions if it were to be mounted, say, 1 foot in front of the mast?

Any helpful advice would be appreciated - I can wire most things up but am not an electronic genius!

 

[tome]

We often fit multiple VHF or UHF antennas. We get the best results by concentrating on vertical rather than horizontal antenna separations (they are vertically polarised).

AIS antenna on the spreaders should be fine, and since it's not transmitting a lower gain rubber-duck type will probably do nicely. Space it half way along the spreader to minimise reflections from the cap shroud and mast.

 

[rickchart]

Thanks - very helpful! This should still give me sufficient range and not be too difficult to fit.

 

[fontmell]

Why not get a splitter and just use one antenna as I do for both VHF and FM radio ?

 

[Anonymous]

Splitters have an 'insertion loss' - i.e. power is reflected back into the transmitter - so if you use a 'splitter' you will lose transmitted power. Seems pointless to degrade the essential VHF transmitter for the sake of a rather non-essential AIS set. Worse still if you degrade the VHF to listen to Jimmy Young /forums/images/graemlins/laugh.gif

 

[fluffc]

AIS relies on interogating ships within a given geographical area for their information. Naturally the ships may be doing this, so you can just "receive" whats in the air already. BUT ships don't continually transmit their AIS information. IMO

 

[Anonymous]

[ QUOTE ]
BUT ships don't continually transmit their AIS information. IMO

[/ QUOTE ]Yes they do.

 

[tome]

AIS operates in several modes. When within reach of a port, the shore side control transmissions. Once out at sea, they poll each other so will still transmit own data.

 

[Anonymous]

[ QUOTE ]
AIS operates in several modes. When within reach of a port, the shore side control transmissions. Once out at sea, they poll each other so will still transmit own data.

[/ QUOTE ]No, they are autonomous. Have a look at http://www.navcen.uscg.gov/enav/ais/how_AIS_works.htm "The AIS transponder normally works in an autonomous and continuous mode, regardless of whether it is operating in the open seas or coastal or inland areas. Transmissions use 9.6 kb GMSK FM modulation over 25 or 12.5 kHz channels using HDLC packet protocols. Although only one radio channel is necessary, each station transmits and receives over two radio channels to avoid interference problems, and to allow channels to be shifted without communications loss from other ships. The system provides for automatic contention resolution between itself and other stations, and communications integrity is maintained even in overload situations."

 

[jonjo]

[ QUOTE ]
AIS relies on interogating ships within a given geographical area for their information. Naturally the ships may be doing this, so you can just "receive" whats in the air already. BUT ships don't continually transmit their AIS information. IMO

[/ QUOTE ]
I think this is the most misleading comment about AIS ever posted at ybw.com

 

[Ships_Cat]

Ditto /forums/images/graemlins/shocked.gif.

Especially troubling as in the great AIS Radar debate threads some claimed that they could not imagine yachtsmen not understanding how AIS worked.

John

 

[Oldhand]

Couldn't agree more. Currently showing Alacrtity docked in Poole harbour and Barfleur passing Old Harry outbound and that's from home.

 

[hightech]

AIS transmission rate is also related to boat speed.

At stop or slow spped they transmit about every 30 seconds. Once the speed goes above about 10 Knots then transmission rate goes to every 10 seconds. (Dont quote me on these figures as being exact as I am going on memory).

 

[tome]

They are not autonomous when within a VTS area (eg Solent) when they operate in an assigned mode with the shore station determining transmission schedules.

The USCG description is a good introduction but not comprehensive - the ITU recommendation (M1371-1) runs to 98 pages and is quite complex, especially in regard to access algorithms. There are 3 modes; Autonomous, Assigned, and Polled.

It doesn't really matter how the transmission assignments are accomplished if all you are doing is listening to AIS reports as the mode is irrelevant.

There was a previous discussion here, see A bit about AIS

 

[Oldhand]

I'm getting AIS from a docked ship every 20 seconds not 30 seconds as you suggest. Not receiving any moving ships at the moment so can't check if it changes but don't remember noticing an increased rate. However, I did read somewhere that transmissions alternated between the 2 frequencies, if that is true then a docked ship would transmit every 10 seconds.

 

[tome]

The intervals are only nominal, and there are 2 classes of equipment. One TDMA transmitter is used to alternate between the 2 channels.

Nominal intervals Class A:

Anchored or moored and <3 knots 3 mins
Anchored or moored and >3 kts 10 secs
Moving 0 -14 kts 10s
Moving 0 -14 kts and changing course 3 1/3s
Moving 14-23 kts 6s
Moving 14-23 kts and changing course 2s
Moving >23 kts 2s

Nominal intervals Class B:

< 2 kts 3 min
Moving 2-14 kts 30s
Moving 14-23 kts 15s
Moving >23 kts 5s
SAR aircraft 10s
Navaids 3 min
AIS base station 10s

Should have added that you are probably only monitoring one frequency so will only see 50% of the updates.