Class B AIS effectiveness

[peter00]

I thought this was a pretty interesting article (written by my good friend Rob Murray) as we all wonder if deep sea ships are seeing us. I have called up many many ships in crossing situations and asked if they can see me on AIS, there is always a long pause (as they played with the gain button) and the invariable reply is, "We've got you now on radar." This didn't answer my question BUT a friend who is a coast pilot here in BC pointed out that deep sea ships get their AIS displayed on their radar. This only confused the situation further. So you might want to read this:


We did a crossing from near La Paz, Mexico (Muertos) to Mazatlan, a distance of some 195 miles. As often happens, other boats chose to take the same weather window, and we sailed in company with five others, four of whom had AIS transceivers aboard. We formed an impromptu radio net via VHF to keep in touch and amuse ourselves on the crossing, and chatted idly as we made the 36 hour crossing.

Some of those on passage never did show up on our transceiver’s target list, and others only sporadically or when very close by, so we decided to do some investigation into the causes when we got to Marina Mazatlan.

Since I thought I was particularly clever in my installation, and I did seem to have the best target list (most targets and targets at the greatest range) on my transceiver, we used Avant‘s station as a benchmark and changed out the installation of the antennas and cables on the boats that were not showing up well.


. Which only made the situation more confusing.

Class B AIS transceivers are pretty simple devices, but pretty power poor on their transmit side. They need a good antenna and clean cabling to get their digital signal clearly transmitted with their tiny 2 watts of power. When you are moving less than 2 knots, they transmit your position every 3 minutes, and when you are under way at more than 2 knots, they transmit it every 30 seconds. One of the main issues with AIS is the ‘black box’ nature of the technology. You buy it, you hook it up, and if you see targets at least some of the time, you think it’s working just fine.

Since I had a Shakespeare emergency VHF antenna, I loaned that to the two other boats, one at a time, and they used it to replace their antenna and cable setups for the test. They powered up, left the units on for 30 minutes or so, and recorded what they saw (number of targets and range), while I recorded how often I saw their output signal (expecting to see them every 3 minutes like clockwork, as they were moving less than 2 knots). Then they disconnected their antennas and cables, plugged in the replacement and then looked at the results, while I again recorded how often I saw them. As we were within a few hundred feet of each other, we expected good results.

With the existing antennas, both of which were professional installations less than 2 years old, I saw them some 2 out of 5 of the expected instances, and they picked up 2 or 3 targets. With the replacement antenna, I saw them 4 out of 5 of the expected instances. Surprising to all of us was that with my rig, I saw 5 or 6 targets, against the 2 or 3 they were picking up with their existing rigs. With the replacement antenna, they each saw 11 or 12 targets, meaning my install, while better than theirs, was still far from perfect.

If you have an AIS aboard, it’s worth taking the time to compare with other boats and even getting out your spare antenna once in a while to check that the system is working as it should. If you see another boat on AIS while on passage, ask them if they see you. The results could surprise you!

Attribution: All images used in this article are licensed under CC BY 2.5


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[Ragnar]

Hi Peter,

Rob's write up was similar to my own transponder setup on Zimaz. The install was with as splitter, and the cabling up the mast was shitty. Lo and behold, so was the range that other boats would pick me up as well as the range that I could pick up others.

I never had a receiving range problem with Slacker when I had better cabling and no splitter. During our Dan Byrne races down here in SoCal, the ability to be both heard and hear is varies greatly depending upon the cabling db loss, as evidenced by our VHF Voice Roll Calls. I've become a big believer in low loss cabling.

 

[Christian]

Can you post, or repost, the link to the Rob Murray piece. No joy using Google to find it.

Also, if there are images, I can't see them on my computer.

 

[pbryant]

Avoid antenna splitters! I experimented with four that were kindly provided by Milltech Marine, and they all decreased reception range, and presumably transmission range by an equivalent amount. I know it's nice to need only one antenna, but splitters are a bad compromise. Besides, a splitter costs more than a second dedicated antenna, and if you read the fine print on your AIS transponder's certification, it will say it is certified only with a dedicated antenna.

Two watts may sound paltry, but in the VHF band, power is much less important than antenna height line-of-sight range. As long as your signal is significantly above the noise level, adding power will not achieve anything (especially with digital AIS packet transmissions), and the noise level on the two AIS channels is normally very low. Generally, only nearby thunderstorms will degrade performance where more power would be beneficial. With an antenna on your vessel 10 meters above the surface, and on the other vessel 20 meters above the surface (about average for commercial freighters), your "solid" reception range is 19 miles (you'll see some hits farther away due to refraction and atmospheric ducting). If you increased your power to 100 watts, that range might increase to 20 miles - at best. You gain just 1 mile for all that trouble. It's cheaper to just elevate the antenna another 2 meters to gain the same effect. The line-of-sight limitation is like hull speed: you can go a little above it, but only with enormous amounts of power.

My own setup is: Nauticast B AIS-300 AIS Class B Transponder and a Digital Antenna model 578-SW mounted 12 meters above sea level atop the mainsail mast (I have two antennas at the masthead) connected by 15 meters of LMR-200 coax. Vesper Marine also makes excellent Class B transponders. Since I am a volunteer receiving station for marinetraffic.com, you can see my reception stats for the last year here: http://www.marinetraffic.com/ais/details/stations/595. (Gaps are caused whenever I was underway and had no Internet.) You will see a sudden drop in range on April 6th: that's when I lowered my tabernacled mast to the bow pulpit for a rigging change. The range is reduced dramatically because the height went from 12 meters to 2 meters. That's a good A/B comparison. My maximum reception range, averaged over 9 months, is about 80 miles. With the mast down, it's about 10 miles.

I have NEVER failed to be seen by the commercial vessels in the southern traffic lanes that I have hailed when crossing the lanes east and west bound. I always call as a courtesy on bridge-to-bridge (ch. 13) when any conflict exists to assure the other vessel I am giving way, and I always ask if they are "painting" me on their navigation display. I have also been hailed by USCG Sector San Francisco in relation to search and rescue operations when I was several miles offshore -- and their only way of knowing I was there was AIS.

When vessels say they "see you on radar;" they are referring to their navigation display that combines AIS and radar targets. They aren't looking at a dedicated AIS display on the bridge. Thus the confusion over radar versus AIS identification. Ask if they have a "data block" on your vessel (name, MMSI, etc.). If they do, then they are seeing your AIS transponder.

I'm an Amateur Extra operator and a licensed Global Maritime Distress and Safety System maintainer: I've been playing with this radio stuff for 40 years. With a proper installation, more power won't buy you anything, but a properly installed and dedicated AIS antenna will work just fine. If you don't have room on your masthead, consider an elevated antenna on the stern or a mounting on a spreader arm - pointed down - to avoid fouling lines (up or down orientation is electronically irrelevant).

Most of the problems I see on recreational vessels stem from the use of solderless connectors - which corrode in about a week. Yes, some connectors are "gold plated" -- but the wires aren't. Learn to solder PL-259 connectors (there are plenty of instructional demos on youtube) and then seal them with mastic tape (3M Scotch 2228 tape available at Lowes and Home Depot works well) wherever they are exposed to the weather (certainly at the antenna) because PL-259s aren't water tight. One drop of salt water inside the connector and the whole thing is ruined. Your nice copper turns to green copper chloride, and then eventually dissolves. I've seen unsealed coax where only the insulation was left and the copper dissolved a foot into the insulation.

You can get good silver-plated PL-259 connectors intended for soldering at any Ham Radio Outlet. You can also get adapters ("reducers") for whatever size of coax you are using that screw into the PL-259 base from the same source. I'm amazed at the gyrations people go through to avoid soldering - and they all fail eventually. Knowing how to solder is a vital skill if you are going to use electronics on a boat.

There is a much simpler and easier method, than described above, for troubleshooting AIS antenna issues. If you want to verify the integrity of your AIS antenna (or your VHF comms antenna), get a VSWR meter (I keep this one on board), connect your VHF transceiver to it (the very short low power bursts from your AIS transponder won't yield a usable reading), tune it to channel 88A (nearest to the AIS frequencies), transmit, and read the VSWR. A ratio below 1.2 to 1 is really good. A ratio of 1.5 to 1 is typical. Anything over 2.0 to 1 is bad, and anything over 3.0 to 1 is very bad. Any problems with the antenna, cable, or connectors will show up as high VSWR. To eliminate/confirm the antenna itself as the cause of a problem, temporarily replace the antenna with a 50 ohm dummy load (you might need a female-to-female "barrel" adapter). Remeasure the VSWR. If it improves significantly (ratio in the normal range), the problem is the antenna. If it doesn't, the problem is the coax or (much more likely) the connectors. With the dummy load in place, you can also take DC resistance measurements to verify the continuity of all the electrical junctions, and the resistance measured at the radio connector should be only slightly higher than 50 ohms. For the most common coax, RG-58, the resistance is 17 ohms per 1,000 feet (center conductor to outer conductor, end-to-end). Adjust the number proportionately for your length of coax, and add 50 ohms to get the expected value. You can troubleshoot most antenna/coax/connector problems with just a dummy load, a VSWR meter, and a multimeter. In a good radio shop, we would use a time domain reflectometer costing thousands of dollars to precisely locate the problem, but you can accomplish the same thing with a little more effort using gear you can carry on your boat costing less than $200.

 

[Wylieguy]

I agree completely. Especially the connections. They need to be soldered properly; they need to be weather-protected. You might not think that things get salty 40' above deck, but the next time you're up there, lick a fitting or two.
Most Amateur Radio folks like to "Elmer" others. I'll bet an email to your local Amateur Radio club would turn up a HAM who'd love to help you learn how to solder PL-259 connections properly. It's a little tricky to do it right. Better yet, since earning an Amateur Radio License no longer requires code, get a license and join that local Radio Club. The friends you'll make there will be more than willing to help with all kinds of electronic installations. The only caution is that they'll probably be 80+ years old, so won't be climbing the mast to check on connections up that-a-way or wriggling into the transom to check your HF antenna auto tuner.

 

[jakmang]

There are also a couple different levels of sophistication in splitter technology. One type will not work reliably with class B. I had a Smart Splitter from Miltech Marine that dropped a large percentage of my class B transmissions. I bought a more expensive one from my AIS manufacturer and the problem went away. http://www.trueheading.se/sv/klass-b-antenn-splitter

If I did it again, I might use two antennas. I do like having the AIS antenna up there 50 feet at the masthead. I can see ships over 20 miles away.