Monday, January 15, 2018

AIS - Should you have it?

AIS - Automatic Identification System - is a system whereby ships (boats, yachts) broadcast information about their location, speed, course, and much more on marine VHF channel 87 and 88 as digital data.  (Tune your VHF there if you want to hear what digital data sounds like.)

There are two classes of AIS devices, Class A and Class B.  Class A devices are intended for commercial vessels, and transmit with more power and more frequently than the Class B devices, which are primarily intended for recreational craft.

For either class, there are two kinds of devices, transceivers (transmitter/receivers, also incorrectly called 'transponders' - a term meaning transmitter/responder, a device that provides data when interrogated by an incoming signal), and receivers.  As you might suppose, transceivers both broadcast their ship's data and also receive data from other transceivers.  The other type of device, the receiver, collects data but does not broadcast it.  In practical terms, vessels with transceivers can see each other, but tho vessels with receivers can see those with transceivers, they are invisible to other vessels.

In order to provide position, speed, heading, etc data, transceivers need GPS information.  Many have a built-in GPS receiver, a few can use an external GPS via NMEA sentences.  Receivers do not need access to a GPS.  Typically, both transmitters and receivers make the data they collect available via NMEA 0183 and/or NMEA 2000.  Some also provide the data via RS-232 or even USB.

How is the data made available to a human?  Typically, a chartplotter will be connected to the AIS device via a NMEA bus and will display detected ships as icons on the display in their correct position, usually with a speed/direction vector.  The chartplotter will also note Point of Closest Approach, and if that is within a settable danger radius, will sound an alarm.  Tho few computers (laptops, tablets) are equipped to receive NMEA signals, adapters are available.  And of course almost all computers have RS-232 and USB capabilities.  This means that if you are running a navigation application on your computer, and it is one which is capable of receiving and interpreting NMEA sentences, then your computer will display nearby ships just like a chartplotter.

But... there are web applications which show AIS ship data world-wide...  why would you buy an AIS unit yourself?  Why not just use one of these apps on your phone?

The problem with the web-based AIS apps is one of reliability and delay.  Imagine how many AIS receivers are needed to provide world-wide coverage.  Now imagine the internet network that is required to collect and process all this data.  It is a huge system.  And there are processing delays - significant delays.  And there are system and network failures.  Further, making use of the web app that results from all of this collection and processing requires that you are not in a cell phone dead spot (there are plenty of dead spots in the San Juan islands, for example).  All of this combines to mean that the web display will be at best a picture of the situation from some time back.  I used a web app on my iPhone before we got our AIS receiver.  There were times when the display showed a Washington State ferry more than five miles away, when in fact that ferry was right next to us.  Kind of funny in bright sunlight, but not so funny in dense fog.  If you have your own AIS receiver, you won't be depending on someone else's system with all of its inherent delays and dependability issues, or your local ability to connect to the Internet.  Complexity is the Enemy of Reliability.

Coming also is the use of synthetic AToN (Aids To Navigation - buoys, etc.).  For this, a VHF station on shore broadcasts AIS data as if it were an AToN at a particular spot.  There is at least one of tow in Puget Sound, and you can expect more, particularly for situations where a buoy has gone missing or has drifted out of place.  The synthetic AToN will show up on your chartplotter, even if you can't see it with your own eyes.

The case of the out-of-place bouy is an interesting one...  suppose that a buoy has drifted 100 yeards into shallow water.  Would you believe your eyes, or would you believe the display on the chartplotter?

If you never are out and about in less than good visibility, then you won't need RADAR or AIS.  But if you find that you need to navigate at night or in poor visibility, then you should have both RADAR and, at the least, an AIS receiver on board.

Are they expensive?  Receivers are much less expensive than transceivers.  As a personal example, I traded an old iPhone for a used AIS receiver.  Watch eBay and craigslist - many boats that originally had just a receiver are now upgrading to a transceiver.

A number of VHF radios now are available with built-in AIS receivers, seemingly a natural blending of capabilities.  However as a standalone AIS receiver their displays are so small as to be almost ludicrous, and they provide so little information that they do little more than raise the anxiety level on board.  But connected to a chartplotter, this is a viable solution.

How about the transceivers?  Well, I just checked eBay, and found a new one, with a built-in GPS for $400.  There are a lot more available in the $500 range.  You can also spend thousands for a completely self-contained unit that includes its own display capability.  I find these to be ill-conceived, since if you have a chartplotter aboard, you won't be using the display.  And if you're investing this kind of money, you almost certainly have a first class chartplotter on board.

As for us aboard Eolian, we have that AIS receiver I got in trade for the old iPhone, and it is hooked up to our chartplotter.  I will strongly consider getting a transceiver when the prices fall more, or when some start appearing on the used market at a lower price.  In the mean time, it is wonderful to be able to see a ferry approaching Thatcher pass from the inside when I am approaching from the outside with no sightline.

(A tip of the hat to Jason for prompting me to write this!)



SV Pelagia said...

I'd say "yes, one should have AIS;you never know when visibilty becomes an issue".

On warning: don't assume all "commercial" ships have AIS. We've seen large US Fishing trawlers/seiners (?) off the west coast without AIS, many tugs without, and, in Mexico, most boats without. One can not be complacent (thinking everyone has AIS).

I have to disagree on one point. Stand-alone systems with their own display ARE very useful. Not "thousands of dollars"; some, such as Vesper Marine units cost $850-$1250 and use very little power (3-6 watts). So no problem running them for days on end while doing a long passage. Something you cannot say about running a chartplotter.

We have the more basic Vesper Watchmate 850 and love it (plus its great anchor watch):

Robert Salnick said...

Pelagia -

Thanks for the benefit of your offshore experience. I can say that within the Salish Sea, we have yet to see a commercial vessel without AIS. But the world is a big place...

Jason said...

Thanks Bob, great article! Lot's to digest.


SV Pelagia said...

Not all tugs here in BC have AIS

Howard Tucker said...

This is not worded accurately:
'For either class, there are two kinds of devices, transceivers (transmitter/receivers, also incorrectly called 'transponders' - a term meaning transmitter/responder, a device that provides data when interrogated by an incoming signal), and receivers. '

There are 3 types of devices, Class A, Class B and Receivers, I would call Class A & B Devices TRANSPONDERS. The manufacturer of the devices do:

Those statements aside, here is a recap of my morning
For the naysayers and doubters of the safety of AIS..... I had a really good experience today.

We were setting up to downrigger troll for Blackmouth salmon in Puget Sound today, just south of Whidbey Island on Possession Bar. The current was flowing pretty good and I had a complete greenhorn on the boat that I was trying to teach how to rig, and set the rod with flasher and spoon and get it down without tangling and without ending up in the motors. I misjudged and we ended up with the downrigger braid wrapped around the skeg, but not in the props thank goodness. I shut off the motors and worked on the swim step for 15-20 minutes to get it free. The weather was awesome and I kept a visual eye out for other boats, especially since we were trolling adjacent to the vessel traffic lanes and close to 20+ other boats trolling.

Just as a cleared the line, got the riggers set and started heading back to my desired troll spot I got called on the VHF radio by my vessel name AND my FCC license call sign. They asked me to switch the VTS channel here (14) It was Puget Sound Vessel Traffic (VTS) asking about my status and intent. They inquired about my vessel length (in a sort of disbelief that I am only 9meters/28 feet). I let them know that we had trouble hence are crazy track. We had clearly drifted in to the VTS lanes, but there was not traffic for a couple of miles. They were pleased to know that I was getting out of the lane, gave me info on the incoming commercial vessels (fast ferry, tug and barge etc).

This would not have be possible without and AIS transponder. I have a Class A AIS, hence they were able to get some extra data (Call sign, vessel length, vessel status, heading, ROT and destination which I had entered in as a nearby port)

So I messed up by ending up in the traffic lanes, BUT someone was able to watch over me, find out my status and if need be steer traffic around me.

Just a real life good reason to have AIS transmit.

Robert Salnick said...

I am sorry Howard, but I must beg to differ. First, there are FOUR types of devices: Class A receivers, Class B receivers, Class A transceivers, and Class B transceivers. Tho all but the oldest receivers receive both Class A and Class B transmissions.

And again, I beg to differ... Tho some manufacturers in an abundance of ignorance use the term "transpondeer" to refer to their devices, that does not make it correct. A transponder is a device that responds ONLY WHEN INTERROGATED by an incoming radio signal - it DOES NOT transmit continuously or intermittently without interrogation. Commercial airliners are equipped with transponders that report the plane's altitude (and other data) when interrogated by a radio signal from air traffic control. More close to home, Puget Sound contains many buoys that are called "RACON"s - these are radar transponders. When a radar signal sweeps over the buoy, it RESPONDS by TRANSMITTING a radar signal that appears on the receiving radar screen as a morse code letter. It only transmits upon receiving an incoming radar transmission.

This is in great contrast to AIS transceivers, which transmit digital data, not in response to an incoming signal, but rather intermittently at a rate that depends on whether they are Class A or Class B transceivers. AIS transceivers cannot and do not respond to incoming interrogation signals.

Here are some of the differences between Class A and Class B transceivers:

Class A Class B
Transmitted power 12.5 watts 2 watts
Transmission interval Every few seconds 30 seconds
Channel acquisition protocol TDMA CSTDMA
Integrated display Required Not required
Information provided Extensive Basic

I am happy to hear that your AIS transceiver kept Seattle VTS (and every AIS-equipped vessel for miles!) abreast of your situation! It's a good story and indeed serves as reinforcement that every vessel should be equipped with at least with AIS receive capabilities. But your situation shows how much more important AIS transmit capabilities are!


Doug Miller said...

Thanks for the informative article.
One point on the debate about transceiver vs. transponder. While your points are valid, I tend to use the term transponder since that is what the USCG uses as well as many of the vendors in the industry. It is worth noting that an AIS transponder can be controlled by authorities and commanded to change frequencies etc. So in that sense it behaves more like a transponder.
There is no Class B receiver. All AIS receivers and transponders receive all Class A and Class B transmissions.
We now have two types of Class B transponders - CSTDMA 2-watt and SOTDMA 5-watt systems. The USCG has the official list of types of AIS here:
One final point, I would caution getting a used AIS transponder since those can only be reprogrammed by a dealer and you definitely don't want to use the transponder with someone else's MMSI. So if you are going to get a used one, make sure you have a marine electronics dealer who is ready and willing to reprogram it.
FWIW, new Class B transponders start at $449 with prices going up based on functionality, integrated features, brand etc. Ones with integrated screens have the uses as mentioned above and we certainly sold hundreds to many happy customers. But if you have a good chartplotter (or mobile device) and can have it on all the time, then it might not be worth the extra cost.
Thanks for spreading the word on AIS!

Doug Miller
Milltech Marine

Robert Salnick said...

That was the last comment I will publish on the “transponder vs transceiver” controversy. For further information, the reader is directed to:

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