Help understanding AIS systems

[JebLostInSpace]

I've read several posts here about various AIS systems that seem like they probably answer my question, but seem to assume a degree of basic understanding that I don't have...

Ultimately what I want to understand is, what will it take for me to add the ability to transmit AIS (other boats can see me on their chartplotters)?

My current setup already allows me to see other boats broadcasting on AIS. I have a SIMRAD suite of electronics (chartplotter, radar, depth, wind...) which displays other boats who are broadcasting AIS. I can also see a basic display of nearby AIS signals directly on my Standard Horizon VHF.

After some initial googling, I thought the only reason that my position was not displayed for other boaters was that I did not have an MMSI number, so I got one. I input this number on both the SIMRAD system and directly into the Standard Horizon VHF. However, I've now come to realize that I also need additional equipment in order to broadcast AIS, which leads to several questions.

1. What equipment do I need? I've seen the SIMRAD NAIS-500 Class B unit. Do I also need an antenna, or can I use the existing antenna? (Or, can I use the antenna on my SSB that I also haven't figured out how to use yet... ) Is there a cheaper alternative I can use?

2. How do I install all this new equipment so that stuff works together? I'm assuming that the current receiving capability is all run through the Standard Horizon VHF, and the SIMRAD chartplotter is somehow communicating with the VHF to give me the nice integrated display, but maybe both units are receiving independently?

3. Why is there such a distinction between systems that only receive vs. those that do both? What is the use case for receiving only? I would think anyone serious enough to feel the need to see other vessels from a distance would also want to be seen??

4. I now have the option to place DSC calls via the Stnadard Horizon VHF, now that I have input my MMSI number. (At least I think I do, I haven't tried it). If I can make DSC calls, how is this different from broadcasting my AIS position?

As a final note, please explain things to me like I'm 5... I have no idea how any of this actually works

[CatNewBee]

AIS is a different frequency than DSC, but it uses the same boat identification number MMSI. You cannot send or receive AIS messages with a normal VHF unit, but there are combo units, that can.

What you have is an AIS receiver, for a 5yo, it is a radio or tv set, you can listen to a program or watch, but you cannot broadcast a program.

If you want to be seen, you need a sender. A class B AIS transceiver is like your VHF radio, a transmitter and receiver in one unit. As such it needs an antenna. it can be a separate one or with a splitter, you can use the existing VHF antenna. The splitter makes sure the aerial is switched to your AIS transmitter when it wants to send your position and then back to receive signals to both AIS receiver and VHF receiver. Same if the VHF transmits, the antenna is used then exclusively by the VHF transmitter. This is done to protect the receivers from the high output energy and prevent transmission errors.

Your AIS receiver receives all the data and creates digital messages on the NMEA backbone, that are processed by the other devices. You enter your mmsi number to prevent your plotter to fire up DANGEROUS AIS TARGET warnings for your own vessel.

SSB antennas and gear cannot be used for AIS, total different frequency range, AIS uses a dedicated frequency near the VHF band, so you can use VHF antenas.

AIS operates principally on two dedicated frequencies or VHF channels: AIS 1: Works on 161.975*MHz- Channel 87B (Simplex, for ship to ship) AIS 2: 162.025*MHz- Channel 88B (Duplex for ship to shore), it is a digital transmission, you would not hear it.

If you want the grown-ups version, read the article here:

https://www.marineinsight.com/marine...-channels/amp/

[Dsanduril]

I'll give it a go.

Why receive only? - Myself I can't think of a good reason. But, it is far less expensive. And it will show you the big boats/ships required to carry AIS. So, if your concern is making sure you don't miss any big ships, and your eyes will take care of the small stuff, then receive only works for you. And maybe that's because it can be hard to estimate the movement of bigger ships. AIS computes that for you, and tells you their speed. Use it right and it may even tell you their destination (if they entered it correctly). That may help you guess as to whether their next turn will be to port or starboard. Do remember that just because big ships are required to have AIS doesn't mean that they do, or that it is working properly. A watch is still required.

DSC vs AIS - AIS is automatic and happens somewhere between every 2 seconds (big, high speed vessels) and 30 seconds (Class B little guys like us). DSC only happens when you tell it to. And AIS includes course, speed, and heading. All automatically. DSC really isn't intended to broadcast your position to everyone around you (except when you hit the distress button). AIS is, it is purpose-built to do that.

If you don't already have an AIS transceiver then you just buy one. There are many out there. The come in three flavors; Class A (meant for big ships) higher power, broadcasts more frequently, gets a reserved time slot of broadcasting, Class B (meant for smaller vessels), lower power, broadcasts less frequently, broadcasts when the channel is free (so in the unlikely case that all time slots are reserved it won't broadcast), Class B+ (technically Class B SOTDMA) a hybrid, meant for smaller vessels, but gets a reserved time slot like Class A. It may be that you have an existing device that comes in another version that broadcasts AIS - you'd need to list specific models to know. Then you would just swap out the old for the new. Otherwise you're adding another device to the system.

AIS uses the VHF frequencies (the very top end) so uses a VHF antenna or a special AIS antenna that is pretty much the same but tuned for that top end of the band. There are two options when installing an AIS transceiver; install a second "VHF" antenna, or install an active (powered) splitter that switches your single VHF antenna back-and-forth between your VHF and your AIS transmitter. There's a huge thread here currently about the evils of the splitter, and another one about how minor the evil is. Pick your poison.

How does everything work together? That depends on what you already have. If your instruments are networked together (sounds like they are) using NMEA 2000 (guessing they are) then you add the AIS to the network. If you are using older NMEA 0183 then it gets a bit more complicated. It can be done, but the specifics depend on the current installation.

[tkeithlu]

Personally, I'm in favor of banning AIS receivers. We should be willing to let others know that we are there if we want to know that they are there.

On the other hand, what happened to the Matsutec HP33? It's no longer for sale in the US. I was lucky to get one at under $400 when they were on the market. Now all the transponders either only feed into a glass cockpit arrangement or cost enough that cruisers may forgo them.

[Pizzazz]

My attempt at the simplified version. AIS is an automatic identification system that broadcasts your position and information around you (up to 20-30 nm). In order to transmit your position you need a special device that takes your position and heading, adds the MMSI number and sends it out. Such devices cost $500-1,000 and are difficult to install properly. They are mandatory for large ships. Receiving AIS information is very easy and this function is incorporated into most modern VHF radios. You can also use a cheap $19 RF dongle to receive the information but there is no need for such complexity because most VHF radios have the capability. The radio feeds the data to all devices on board.

DSC uses exactly the same messaging system as AIS but is called selective calling which means that you manually send the call request, along with your GPS position. It is broadcast on channel 70 (AIS on channels 87B/88B).

Your most interesting question is what is the difference between AIS and DSC. On a certain technical level they are exactly the same. However, the maritime standards bodies seem to design systems in a very specific way, so that you can get predictable results at sea. Therefore, they set one channel (70) for DSC calls only and marked this a safety frequency. Maritime radios cannot access it for voice communications and you can only send specific pre-configured messages. That being said, if you are at sea, and there is a strong, safety driven need to be seen by other ships you can use the DSC system as a poor man’s AIS. Just broadcast an ALL SHIPS SAFETY call, selecting one of the unused voice channels for follow up, every 30 min or so. You will annoy the hell out of neighboring ships but you will show up on their displays. I can only imagine doing this in dense fog or really adverse conditions. Given the limitations of DSC (you can only select a callback channel), it would be really cool if an unofficial proxy was developed for what the callback channels could mean (something like 27 could be test), 69 could be no reply needed, etc.). Then a lot more people will have access to digital alerting for safety (manual AIS). The Coastguard May object though...

[paullmaloney3]

This website milltechmarine.com had lots of useful info and the owner was willing to talk to me.
I ended up with a vesper splitter, and a unit for sending AIS. AMEC-B600-PA02 AMEC WideLink B600 Class B SOTDMA AIS Transponder. the install was pretty straightforward and I am getting good results. you could give the specs on your present equipment for your chart plotter and vhf and he would tell you if compatible.

[ArmyDaveNY]

AIS class B transceivers have come down a lot in price these past few years. As others have said, you can integrate them into your chart plotter or in many cases, your radio.


There are a number of reasons for wanting the ability to not transmit or to turn it off. First, you have to remember that AIS receivers will NOT tell you if if there are no other boats around, or tell you all of the other boats/ships that are around. It will ONLY tell you what other boats/shops are around that have AIS.


In crowded ports it can easily create information overload situations. Here in New York for example, I can easily receive notifications of three to ten boats/ships per minute! Or more on some days. You will want different notification parameters in a busy harbor than you do when cruising on the open ocean.


In many parts of the world you will NOT want to transmit your location. I have a number of customers who are very concerned with attacks in certain parts of the world. It is also true that citizens/flags of certain countries are more at risk in some parts of the world than others.


Good luck with your AIS!

[Tugwit]

Other replies have addressed AIS; below is an excerpt from an article I wrote for my local YC newsletter (DSC-MFD-AIS-BFD?)to encourage wider use of the DSC features of modern VHF radios.

DSC offers many benefits in safety and convenience. However, there is a learning curve which most boaters fail to climb:

In 2011, the USCG reported that the vast majority of DSC emergency broadcasts received to date were effectively useless: 6 of 10 (60%) of calls failed to include a valid MMSI, and 9 of 10 (90%) of calls failed to report a position because a GPS had not been connected to the radio.

Frankly, I doubt the situation has improved much since 2011. So much for Safety. But DSC also offers increased convenience in day-to-day use of a VHF radio, and perhaps that might motivate increased utilization.

"VHF/DSC
Marine VHF radios have been used for voice communication among vessels and shore-stations for decades. Radios have become much more affordable over the years--in 1978, my first marine VHF radio cost $669, the equivalent of $2,695 in 2018 dollars! But today you can get a marine VHF radio with far more capabilities for a couple hundred bucks, and there is really no excuse for not having one--and knowing how to use it.

For convenience, the allocated VHF spectrum is divided into channels. Channel 16 is designated as the International Distress, Safety, and Calling channel. While most recreational vessels are not legally required to have a VHF radio, it is a law that all boaters with VHF radios (voluntarily equipped or not) monitor channel16 while underway. And though I doubt anyone has been thrown in the brig for violating this law, it does have its consequences--ignorance and stupidity have a way of creating their own punishments.

Anyone who has monitored CH16 for long has undoubtedly heard some clown gleefully rambling on about his mother-in-law’s colonoscopy until a clearly irritated Coast Guard officer breaks in and shuts him up with a terse, “…Channel 16 is a Distress and Calling channel ONLY—please take your traffic to a working channel.” Working channels (e.g. 68, 69, 72and 78) are assigned to be used by recreational boaters for routine intership conversations.

Still other VHF channels are designated for specific purposes, e.g. 22 for the USCG, 14 for Southern Puget Sound Vessel Traffic Services, 13 for Bridge-to-Bridge. Monitoring some of these channels as well as 16 (using the scanning feature found on most VHF radios) often provides very relevant and actionable information for mariners--particularly in the more congested portions of the Salish Sea. See http://www.navcen.uscg.gov/?pageName=mtvhf for the official list and http://goo.gl/A23McI for an abbreviated version.

Traditional VHF radios rely solely on voice communication. While effective, there are several limitations. First of all, in congested areas, there is often a lot of chatter on 16 with multiple boats trying to make contact with multiple other boats--it can be tough to break in and hail your buddy. (To address this congestion, Channel 9 is designated an alternate hailing frequency--another good use for channel scanning.)

In a real emergency, you can clear channel 16 by hailing “Mayday”. The first thing the CG will want to know is the nature of your emergency, then your position (lat/lon), the number of persons on board, available PFDs, and so forth. It takes a while to find and communicate all this information clearly and accurately, and meanwhile you may have better things to do—your boat may be filling up with water, for instance, with no one to work the pump. Or it might be useful to locate and get your spouse back on board.

DSC (Digital Selective Calling) is a technology built on existing marine radios to send text messages between stations. The pre-formatted digital messages are information-rich and sent much quicker than voice messages--on the order of a millisecond. Each character is transmitted twice, and the range for receiving intelligible text messages can be 25%greater than for voice messages. Current radios (DSC class D for recreational use) constantly monitor for DSC messages with an independent, dedicated internal receiver.

DSC is implemented in-both VHF (short range) and HF (long range) marine radios. Some years ago, VHF channel 70 was removed from use as a voice channel and devoted to digital messages. When DSC is used, the radio transmits a short text message on channel 70 which is received by all DSC-equipped radios in range. If the message is addressed to you (the “selective” in DSC), your radio responds with a “ring”. If it’s not addressed to you, you continue in blissful ignorance and silence.

Clearly, you need to have an “address” in order to receive selective messages. That’s where MMSI comes in. The MMSI (Marine Mobile Service Identity) is like a telephone number that identifies your boat. You get one, program it into your radio, and… Voila! – radio messages directed to your boat cause your radio to “ring”.

In an emergency, hit the red button on your VHF and the radio automatically transmits a DSC Distress Alert or “Mayday” message to the Coast Guard and also to All Ships within range of your radio. Your MMSI number is included in the message, and immediately the Coast Guard knows the size, color and type of vessel, ownership and so forth--data that you provide when you obtain an MMSI number assignment. The Coast Guard can also get in touch with an emergency contact person you have designated.

So far so good. But in order to transmit your position and time when a distress call is initiated, a GPS must be connected to the VHF DSC radio. Some VHF radios are now manufactured with a GPS built in. Earlier radios, however, require wires from an external GPS to be connected. It’s not difficult, but it is a task that must be done to realize the full capability of DSC radios for emergency signaling (and other position-related functions).

Not sinking? As an option, you may identify a more precise “nature of distress” from a menu of options prior to sending the emergency message. There are also optional DSC All Ships messages that act as the DSC equivalent of “Pan Pan” or “Securité Securité” in appropriate situations.

So now we have a radio that knows who and where you are. What else can be done with this ability?
Well, as noted above you can send a DSC Individual call to a specific boat you wish to communicate with. Rather than hailing by voice on Channel 16--an increasingly hit or miss proposition--simply:

1. select your buddy’s MMSI/Boat Name from your radio’s directory or “phone book”,
2. identify the channel on which you wish to talk, and
3. punch the “send” button.

Immediately, your buddy’s radio will “ring”.

When you receive a DSC message and your radio “rings”, you have the option to “Acknowledge” the call. This is equivalent to “picking up the phone”, or, by not acknowledging an incoming call, simply “letting it ring”.

You may choose to acknowledge all incoming calls automatically, or you may elect to press a button to “answer the phone”. When you acknowledge a DSC call, both your radio and the caller’s radio switch to the channel number specified by the caller. You then simply key your microphone and begin talking. No hails, no negotiating a working channel, (almost) instantaneous connection. Slick, huh?

So say you’re buddy-boating with another vessel and you’ve arrived at the rendezvous point. You’ve lost track of your buddy and want to know how close he is in order to have the beer chilled and steaks on the BBQ when he arrives.

Simply enter your buddy’s MMSI (or select it from your radio’s directory or “phone book”), send a DSC Position Request call, and his radio will receive the request and automatically reply with his LAT/LON. No intervention is needed on his part. (Of course, if he’s sitting on his secret fishing hole, he can always turn off the automatic response to position request calls, and you will get no response.)

Suppose your buddy has broken down, and he needs your help. He can report his position to you by initiating a similar DSC Position Report call. When you receive the DSC message, your radio will “ring”. You can save his LAT/LON as a waypoint, and your radio will calculate and display his range and bearing to help you navigate to his aid.

Selective messages can also be sent to a Group of boats. Each boat must enter the pre-determined “Group MMSI” into their radio’s directory. Thereafter, any DSC Group call to the group MMSI will cause all radios in the group to “ring”. Acknowledge the call and you will be switched to a channel where there is a great party-line conversation going on. This is the digital equivalent to a very efficient “Captain Oly” call.

This is a great option to keep in touch with all participants of an OYC cruise. Or a Race Committee might set one up to get in touch with all racers—say when the course is shortened, or an accident has been reported and assistance may be needed.

There are a couple of other DSC functions, as well. Calls are logged, so if someone calls you with an unrecognized MMSI, you can get back to them later if you wish (without having to write down their MMSI). You can also automatically track the positions of several vessels simultaneously.

All of these features come at a cost, of course—adding complexity to VHF operations. More problematic, however, is the fact that there must be a minimum “critical mass” of vessels in your peer group using DSC in order to provide the opportunity to practice using DSC and master its use. If none of your buddies has a telephone, it hardly pays you to get one and figure out how to use it.

Eventually, you’ll have to RTFM--or parts of it, anyway. And once you do, come out for a cruise and practice. Once you learn to use DSC to make quick and efficient calls to your buddy, I’ll bet you’ll never go back to hailing again.

Practice also provides the skill and confidence to respond well in an emergency. Ultimately, the safety benefit of being able to use a single red panic-button to send a very effective Mayday to the Coast Guard and your immediate neighbors would seem to be reason enough to make DSC work for you..."

[Telemark43]

Five year old version
AIS Receive is a radio
AIS Transmitting is a radio station...
An AIS Transceiver is both a radio for listening AND a radio station for transmitting
A radio can only see other ships
The transceiver and see other ships AND other ships can see you

[AA3JY]

AIS explained and AIS B+...

https://www.mby.com/gear/ais-b-plus-explained-104136

[jmschmidt]

United States Navy ships never activate their AIS except in high traffic areas like the Chesapeake and San Francisco Bay and a few other areas around the world. Reason: It makes them a target.

If you don't want to be a target for every pirate in a panga don't run your AIS on transmit when cruising. Yes, many of them have the ability to track you.

[Nani Kai]

Originally Posted by tkeithlu

Personally, I'm in favor of banning AIS receivers. We should be willing to let others know that we are there if we want to know that they are there..........

Wha....? Would you mind elaborating? I don't understand your perspective.

[sailingharry]

I'm a bit counter culture (second time I've posted this comment today!), but l have a problem with AIS.


I sail on the Chesapeake Bay. It's a VERY crowded area, with many many pleasure boats. There is also freighter traffic. Those big boys have an uncanny ability to appear out of no where, and they don't care about pleasure boat AIS -- we bounce around too much to track, and the big boy isn't going to change his course in the Bay -- so his only action choice is a big horn!


On my last boat, I bought an AIS reciever (new VHF). I was thrilled! I was going to set up an alarm, and get notified if a big boy was going to be close. The alarm went off several times an hour -- all the little boys. Worst case was another boat heading down the Bay with me -- we both wander a little to a collision, alarm goes off, we wander a little away, alarm resets, lather, rinse, repeat. AGH! It took me under a month to disable all AIS alarms. But it's still bad. There are so damn many targets on the screen, I can't even read the chart! So I often turn off AIS entirely.


I wish there was a user selectable way to turn off Class-B targets -- then I would use AIS.


Now, my new boat came with a transceiver. And while I usually keep it off (all reasons above -- I don't want the noise, and I don't want to contribute to the noise), we are heading next year way north, to fog, heavy shipping lanes, and minimal pleasure traffic. The AIS transceiver is indispensable there. Despite many who believe that big boys don't watch the AIS offshore, and don't broadcast, I believe the opposite -- I believe that except in very special circumstances (Somalia, perhaps?) that merchants are 100% compliant with broadcasting, and I believe that offshore they set alarms (why not? There aren't any false alarms with the system -- if it alarms offshore, there is a collision risk).



For OP -- depending on your location, AIS transceiver may well be a waste of money. Big boys won't care, and little guys won't have an alarm set, and can see and avoid you even if its 100 yards away.