How to check if AIS is transmitting??

[Brewgyver]

Originally Posted by David Sandowich

(snip) Also most Class Bs operate at 2.5 watts verses 25 watts on Class C ... a big difference. FWIW ...Ray Marines new AIS700 transmits at 5 watts.Bottom line.... AIS was meant for ship to ship not ship to Marine traffic.

"Class C"???
I think you mean Class A, and I believe the standard is 12.5 watts, not 25.
Em-Trak also makes a higher output Class B, their 350 and 360 models are both 5 watts.

While the AIS system was originally designed to be ship to ship, advances is satellites over the last 20 years or so, some of which were developed for the purpose, mean that Class A and B AIS signals transmitted over most of the globe are being routinely received by satellites.

[Seagirt]

That is more info than I can currently digest fully, thanks! I was under the assumption that information was transferred via satellites just as often as ship to ship and so had assumed that I wasn't transmitting at all. But I guess it is still possible that I am transmitting but not within line of site and near enough to a land station that the information is getting picked up by marinetraffic/shipfinder/etc...places that I can search via internet?

I took a look at how far away I was picking up other AIS signals and it was at least 20 nm away. Most of these seemed to be class A but there were some class b mixed in. I will wait till I pull away from land before I contact any ships about whether or not they are picking up my signal. VHF transmission where I am is limited and now that I've learned more about AIS I'm surprised I'm seeing as many targets as I am.

It is a little disconcerting that my signal could be as limited at to only display to another ship within 4 miles. That is hardly enough time for larger vessels to react. I would definitely look into getting a designated antennae if that were the case. But, first thing is to confirm that I am actually transmitting.

Thanks again to all who have offered up info.

Originally Posted by Cpt Pat

Ann is correct on all points. I'll add some details...

While AIS is line of sight, there are conditions that can allow AIS data packets to propagate incredible distances. Since I've operated a Marine Traffic volunteer receiving station for years (details and stats here: https://www.marinetraffic.com/ais/details/stations/595), and I'm logging reception distances, my record "DX" (distant operation) is 12,000 miles - essentially the other side of the planet. These single packets, which are only 20 milliseconds in length, can travel hundreds of miles during periods of atmospheric ducting caused by thermal inversion layers that act as a mirror, much like thermoclines in the ocean reflect sound back at the bottom (an effect well known to submariners to evade sonar detection). Single packets thousands of miles distant can also reflect off ionized meteor trails. So if you see a vessel beyond line of sight that appears only intermittently: nothing is wrong. Its signal may have well been refracted to you by the atmosphere - much like during a sunset you can often see a distorted image of the sun minutes after it has actually set. If the atmosphere can do that to light, it can certainly do the same to VHF. The effect is only temporary.

Only by asking for an AIS check with several vessels can you get a feel for your transmission range. With an old protocol (CSTDMA) 2.5 watt Class B transmitter, I wouldn't be disappointed with a range of 4 miles, and 6 miles with a newer 5 watt SODTMA transponder. Doubling the power does not double the range. At best, it only increases the range by a factor of 1.41 (the squared-root of 2). Your range is mostly constrained by your (and the receiver's) antenna height above the water. You'll get the greatest benefit by raising the antenna - not the power.

If your vessel appears on Marine Traffic, you only have confirmation that it is transmitting - but nothing else about your transmission effectiveness (antenna, coax, etc.). You need to examine where that receiving station is located, its antenna height above MSL, and the receiver type - some receivers are just cheap insensitive scanners with very basic antennas (only receiving strong signals) while others are actual AIS transponders that will receive very weak signals. If its a cheap receiver - that's good. You have to be pounding on that receiver to be heard at all.

Don't be disappointed if Marine Traffic never reports your vessel via satellite reception. Far fewer Class B transponders can be heard by satellite than the Class A transponders. The Class As tend to drown them out at satellite altitudes through contentioning over the available time slots.

My own station (my actual AIS transponder) is so sensitive that it will report vessels in my harbor that may have serious defects in their antenna (I was plotting one that had lost its antenna entirely) due only to proximity. You can get the receiving station details (on a web browser - the smartphone app is different) by clicking on the vessel, looking at the displayed data block, and then clicking the link for the receiving station number at the bottom. There you will see the receiving station details in the upper right corner.

If you want to verify your transponder data was properly configured (depressingly, many are not), you can use the CG link I posted above in this thread. You only have to have come within reception range of a CG AIS base station once within the recent past to have an entry in that database. You can see the locations of CG AIS base station on Marine Traffic. Thy appear as yellow towers (provided you haven't filtered them out).

73
N8QH

[Jim Cate]

fwiw:

Equipment: Vesper Vision class B with Vesper splitter to masthead antenna, shared with VHF.

We have run tests with another similarly equipped yacht, and found detection up to around 12 nmi both ways. We see class A ships routinely at 30 to 40 nmi. We have seen class Bs up to around 20 miles,but not routinely. Ships seem to detect us at least 10 miles away, for that is where we've seen them apparently alter course a bit to open up very close CPAs.

On the other hand, some yachts with unknown installations show up very poorly, often not appearing until 3-4 nmi, sometimes less. I attribute this to poor installations, not poor AIS equipment.

Jim

[Cpt Pat]

Originally Posted by Seagirt

It is a little disconcerting that my signal could be as limited at to only display to another ship within 4 miles. That is hardly enough time for larger vessels to react. I would definitely look into getting a designated antennae if that were the case. But, first thing is to confirm that I am actually transmitting.

4 miles is not a limitation, only my expectation. With a masthead mounted antenna, I would suspect a fault if vessels that routinely receive other Class B stations at greater distances didn't see my vessel at a range of 4 miles. I have no control over the quality of the other vessel's AIS installation, and in practice, 4 miles was the best I could expect when transmitting Class B to middling-quality installations on other vessels with low antennas. Large ships reported seeing me at 15 miles.

So far as other larger vessels reacting is concerned: I don't expect really large vessels, such as container ships, to react at all. I have had large vessels alter course (I was once entangled in a crab pot line right in the middle of a traffic lane), but I don't expect it, and I thank them profusely when they do. (That incident made every cent I paid for the AIS transponder worth the cost.) I started a camera rolling so maybe my next-of-kin could learn I didn't get crunched in the traffic lane out of negligence: https://tinyurl.com/yxt5g9bj The radio traffic you will hear in the background is S.F. Vessel Traffic Service. I called them before starting the video recording.

I regard container ships as being driven by robots on inalterable courses. The utility of having AIS in this case is that it gives the helmsman your vessel name, so they can call your vessel by name to tell you to get out of the way before your boat dings their propeller when it chops your boat into toothpicks. (As a courtesy, in a close approach situation, I will call the behemoth to tell them I have them in sight and that I am giving way. Arguing with a large transport ship over who gives way is like expecting a train to alter course.) I have also had large transport ships well out to sea delay a turn that would otherwise put them in a close quarters conflict.

If you have a CSTDMA (older style Class B) transponder, one thing to watch out for is the transponder not transmitting at all due to on board radio frequency interference (RFI). The "CS" in CSTDMA stands for "carrier sense." It essentially "listens" for quiet periods in which to transmit. I had an LED deck light that, when on, produced so much RFI that my AIS transponder interpreted it as a continuous carrier -- and stopped transmitting altogether. The noisy LED also wiped out reception of other AIS vessels more than half a mile away.

The Coast Guard has issued an alert on this problem: https://www.dco.uscg.mil/Portals/9/DCO%20Documents/5p/CG-5PC/INV/Alerts/1318.pdf?ver=2018-08-16-091109-630. If you have LEDs and a CSTDMA transponder, I would test by turning on every LED on my vessel and confirming the transponder still transmits and the receiver range isn't reduced (in addition to running the test recommended in the CG Alert above). In my case, I retired my CSTDMA transponder and replaced it with a SO(self-organizing) TDMA unit, which didn't exhibit the same noise-induced non-transmission symptoms. I also killed that LED with extreme prejudice (in case some hapless person pulled it out of the dumpster to use it).

If you are curious about the potential maximum range of sporadic AIS reception, you can look at the log kept on received AIS stations at my station here: https://aprs.fi/info/i/N8QH. Look at the "(rx => tx)" column under the "Stations heard directly by N8QH" heading. There really is no "maximum" range for occasional AIS reception. I just never expect my transmission or reception range to ever be "solid" past 20 miles. My antenna height is 11 meters above the surface.

[Cpt Pat]

If you want to calculate VHF line-of-sight (which is slightly greater than visible line-of-sight due to atmospheric refraction) here is a simple calculator: https://www.qsl.net/w4sat/horizon.htm

BTW, the Coast Guard has its antennas on towers, atop mountains when available, with repeaters to cover wide areas. That's why you will often hear only their side of a communication.

[Seagirt]

Again, above and beyond in the information you provided, thanks! Interesting about the interference issue, I'll look into that. FYI, My AIS unit is an Emtrak B100, WestMarine antennae splitter, SHG2000vhf, and older Lowrance plotter. Things are hooked up via nmea0183 (Thanks alot SH for not adding nmea2000 to the vhf!).

And just to clarify, I by no means meant that I expected the larger ships to be aware of my position and obligated to avoid me. But being as my speed is sometimes quite slow I am hoping that the hassle of what takes place after a collision, regardless of who's fault it is, would be enough for them to consider avoiding me if I am unable to entirely get out of the way. And I thank them profusely in advance, even if not entirely altruistic on their part. I can't imagine being hung up on crab pots in a traffic lane! More fuel to the fire to figure this AIS issue out.

Originally Posted by Cpt Pat

4 miles is not a limitation, only my expectation. With a masthead mounted antenna, I would .................. reception range to ever be "solid" past 20 miles. My antenna height is 11 meters above the surface.

[GILow]

Am I the only one wondering what a crab pot is doing in a traffic lane?

[Cpt Pat]

Originally Posted by GILow

Am I the only one wondering what a crab pot is doing in a traffic lane?

I asked that question many times myself.. while I was fouled on that line. In my sailing grounds, they are like mines, and during crab season the sea becomes a slalom course. The local crab fishermen say boats should pass harmlessly over them - but the salmon fishermen complain bitterly about the constant entanglements with their props.

In my case, my engine wasn't running when I got entangled. I was just sailing merrily along on a close reach, when my boat stopped and turned down wind. The crab pot line got caught between my outboard's anti-cavitation plate and the prop, and then went "ziiiip" until the foam float was impaled on a prop blade. I couldn't pull the engine out against the hundred pound pull of the line combined with astern swell hits. I'm guessing the pot was buried, making it a very effective anchor.

[GILow]

Originally Posted by Cpt Pat

In my case, my engine wasn't running when I got entangled. I was just sailing merrily along on a close reach, when my boat stopped and turned down wind. The crab pot line got caught between my outboard's anti-cavitation plate and the prop, and then went "ziiiip" until the foam float was impaled on a prop blade. I couldn't pull the engine out against the hundred pound pull of the line combined with astern swell hits. I'm guessing the pot was buried, making it a very effective anchor.

Well, credit where it is due, it was a very business-like and calm bit of work you performed there. (Though I found myself flinching and ducking every time the boom swung back and forth.). You’d be a valuable member of any boat’s crew.

[Seagirt]

Closure: I got away from the dock and was picked up on Vesselfinder and a few days later showed up on Marinetraffic so at least I don't have my AIS wires crossed.

Thanks all!