AIS Antenna Height

[jstevens]

Jef,
Versioni 3.18 appears to have solved my problem. I now get data to the C120. So far just the start up status messages from the engine. One of the problems of testing these things at my current location (Almerimar, ES) is the infrequency of ships transiting the area. Just coastal freighters serving the ports of Almeria, Motril and Malaga. I was very fortunate when first setting up AIS on my boat that I was in Cascais, PT right next to a busy shipping lane. I seldom had to wait more than a few seconds to start receiving target data in Cascais. Here I sometimes have to wait more than an hour.
So I haven't really seen the C120 generate a ship target display, but it at least is receiving data from the AIS engine.

Gord,
I don't understand the perplexing remark. Challenging yes, but perplexing no. All is logical and explanable . Perplexing to me is my toothbrush holder launching itself off the counter in the head, striking the flush nob on the toilet and breaking off the stem of the flush valve inside the housing. Now that is truly perplexing. I would like some physicist to explain mechanics of that incident.

John

[defjef]

John,

I am using 3.18...

Can the C80 listen to several "talkers" from its input... all at the same baud rate... and with no redundant sentences... for example.. depth, wind, heading???

Jef
sv Shiva
Contest 36s

[defjef]

Raymarine Tech Mark told me that the Baud rate of the input C80 an be set separately from the output which remains at 4800 for the NMEA port. So IF you have an AIS engine you can still talk to repeaters or autopilots at 4800 baud

Jef
sv Shiva
Contest 36s

[jstevens]

Jef,That is very good news, although not consistent with what I observed, but did not measure. I'll have to do more testing with that NMEA output.John

[jstevens]

I do need to correct my earlier post, 3.18 did not solve my problem of getting AIS data the to C120. What really solved my problem was RYOFD, otherwise known as Read Your Own Friggin' Documentation. I had the RD wire for input to the C120 clamped to the SD from the PC rather than the SD from the AIS engine. So the so-called engine initialization sentences I thought I saw on the C120 display were really coming from SOB running on my PC. I corrected my wiring and things appear to be OK. Still there is so little AIS traffic in this area I can't be sure of anything right now.

John

[defjef]

Bad news is that what Mark said is opposite of what the online tech support data base says.

This means I have to conduct some real life experiments when I get to the boat this weekend.

jef

[jstevens]

Jef,
Bummer, not the first time Raymarine has given contradictory answers to the same question. Even in their online support data base, in one tech bulletin they say the ST6000 autopilot must provide power to the SeaTalk bus, then elsewhere they say it best to have an independent power source for SeaTalk. Well, if it was easy we wouldn't get any satisfcation out of sorting it out, right.
FIW, I did finally get AIS targets to appear on my C120 display. Just not a lot of commercial shipping in the vicinity of Almerimar. I published some photos of the AIS display on my web page at http://www.svsarah.com/Sarah/Upgrade...nction_Display_
At least you can see what the C80 will display when you get all of the connections finally sorted out.

John

[aaflyer]

Oops....Someone did not have their AIS powered up.


Two oil tankers collide off British coast
No leakage reported; no indication of what caused the accident on a clear day in calm seas in the English channel.
June 9, 2006: 8:11 AM EDT

LONDON (Reuters) - Two oil tankers collided on Friday in the English channel, one of the world's most heavily used shipping lanes, but there were no injuries and no sign of any oil spill, the coast guard said.
The vessels collided off the southern English port of Dover. There was no immediate indication of what caused the collision on a clear day in calm seas.
"A helicopter ... has overflown the area. There is no sign of pollution, the damage appears to be only slight and there are no reports of any injuries," a spokesman for Dover coast guard said.
Lloyd's Casualty Reporting said it was "not a serious incident."
The coast guard said the tankers involved were the Willy and the Shakhdag. Willy is Gibraltar-flagged and Shakhdag is Malta-flagged, according to Lloyd's Casualty Rep


Lars

Sorry about the BMP that was included.

[R2boat]

One of the manufactures recommends no higher than the stern rail as at times you may be getting returns on vessels 30-40 miles away if higher and than would give you way too many targets to keep track of...

[jstevens]

R2Boat,
I'm not sure what that manufacturer is concerned about. Targets 30-40 miles away normally shouldn't present a problem as most of us are monitoring targets within 12 nm or less. The situations where I've got more targets than I can monitor is within harbors and the range is less than 3nm. That is where the AIS plotters need to be sophisticated enough to distinguish between targets underway (and real collision issues) and targets at anchor (that are not issues). A lower antenna height won't fix the problem.
That said I believe target range is more a function of VHF radio propagation than it is of antenna height, so I think a rail mount will work just fine. Of course then you will likely never see an AIS display such as I recorded a few weeks ago when sailing around the south coast of Mallorca. I heard one cargo ship call another on VHF asking them to verify their AIS output as they though it might not be working. As a naturally inquisitive sailor I checked the SOB target list on my PC. When I panned to the target I discovered it was off the coast of N. Africa, over 100nm to the south. I zoomed out in SOB and discovered dozens of ships reporting from the shipping lanes off Africa. The screen capture from SOB is attached (or should be if I've done this right).

[defjef]

My NASA AIS w/ Raymarine C80 is working now with the antenna mounted on a pole off the stern rail. The height above the water is about 12-15'. I am receiving targets over land masses so far up to 10 NM. I suspect a slightly better range if there is no land between my antenna and the target.

I am not in a very highly trafficked commercial area. There are some tows and freighters coming and going in and out of LIS and some mega yachts which are AIS equipped. When I am sailing I am not watching the C80 and since the install I have not ventured very far to test the range with my antenna height. But since it seems to work with targets up to 10 NM over land it works for me! I see no purpose in seeing vessels 30 NM away.

AIS is still quite new in the USA and it will only grow in use as the USCG mandates another 17,000 vessels to carry AIR transponders in 08, I believe. Class Bs are being offered at about $1,000 so many pleasure craft may be carrying them as well. Plotter screens could get very crowded in the future without some sort of filtering algorithms.

Jef
sv Shiva
Contest 36s

[hellosailor]

Since AIS is VHF, the range will depend on the height of both antennas--the receiver and the transmitter. VHF is line of sight, and you'd figure the range by figuring the distance to the horizon from your antenna, plus the horizon range of the other antenna.

There's a long explanation and formulas on http://www.firestik.com/Tech_Docs/dist2horizon.htm

and a simple calculator for each half at www.boatsafe.com/tools/horizon.htm

Bottom line, if you are on a small boat and the other guy is on a large ship carrying the antenna 80-100' above waterline....his height will pretty much determine the combined range, unless you REALLY push it up the mast.

[defjef]

The line of sight rule does not always appply. Depending on atmospheric conditions you can get all sorts of bending and odd effects. I have picked up targets which were not "seeable" by line of sight. I can't say why and when it will happen, but I know it does!

Jef
sv Shiva
Contest 36s

[hellosailor]

Jef-
Atmospheric skip certainly does happen, in some cases it can extend range by 500 miles or more. But "reliable" and predictable range...still follows the horizon rules. FWIW you can even get skip from the ion trails of meteors, but that will only last a fast few seconds.

[Talbot]

You are actually more likely to get a surface duct propogation if your aerial is lower!