I'm sure this is not a new thing, but I just found out about it, and I think it's pretty neat!
this post was submitted on 08 Aug 2025
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Trivia time!
Yes, over a certain size. AIS class A transponders are mandatory on ships over 24meters. Smaller than that, and many people use them anyway because it's convenient. Many recreational boats use class B with lower transmit power.
Anyone can operate an AIS transponder in receive-only, enabling you to receive the data yourself. This is how sites like that operate - Many volunteers who receive and forward the NMEA string to the site. Hell, if you have one of those cheap USB-SDRs you can roll your own at minimal cost. Decoding the relevant NMEA strings (AIVDO and AIVDM, readable at 38400 baud) is a bit of a pain in the ass, as it's built around a 6-bit ASCII table. But I managed to do it with a perl hack once upon a time, so it's highly doable.
NB: While not legal, it's very easy to disable an AIS transponder temporarily. War ships often do this.
Tumbleweed
I'm glad you asked; A more reliable method for establishing position is by sending a position request via DSC (VHF channel 70 or MF 2187.5kHz), as that's a lot harder to disable. You're gonna need an ROC or GOC to do this legally, though. As well as a MMSI, come to think of it.
Source: I used to work with maritime comm systems, including installation of both VHF radios, MF radios, and AIS transponder. I have a GOC.
Oooh, I love this kind of comment, packed with fascinating information I don't really understand but I'm sure someone does!
As I no longer work in that field, I'm happy to answer any questions you (or anyone else, for that matter) may have on the subject. To me it's remnant information I no longer need, but to others it might come in handy. Or at least interesting.
I know next to nothing about ships and laws that pertain to them, but on the topic of pirates, do ships have the right/authority to defend themselves? In another comment of yours, you mentioned war canoes, if a ship sees or detects a craft like this and it doesn’t respond to communications, what’s to prevent crew on board from blasting them out of the water?
My understanding is basically that it's up to what local laws allow, combined with ship owners policies.
We saw what was most likely pirates a few times, but we had armed escorts so they were never openly hostile. You can't just shoot at any that vaguely resembles pirates, as there are fishing boats in the area as well, and they don't look that different.
Story time!
I remember chilling on the back deck, doing my hour of lookout in case someone were trying to board us. One of our chase boats (armed escorts) contacted me via the portable VHF I was carrying to let me know there was a small boat looking like it was trying to approach. I found this odd, as the only boats I saw in the vicinity were boats that were supposed to be there. I looked over the railing, and knew what the chase boat was talking about: Yup, there was a boat there, alright. But it was one of ours, they were just normally much further away. Turned out they were almost out of water after being in an open boat for an entire day that was abnormally hot even by Nigerian standards. I gave them a tray full of ice cold coke cans and a few bags of doritos from the workshop next to me, and they grinningly went back to their job of towing away fishing nets that had drifted into our survey area.
Is it true that when larger ships pass by an area they suck up internet bandwidth from satellites like StarLink? Someone told me that and it sounded made up. Wondering if that could cause a safety issue for a smaller ship.
Is that method of disabling AIS used by pirates? I don't know if piracy is even common.
One of my fears of sea travel is basically being locked out of communications...
No-ish. There's something to it, tho: The more users on a carrier, the less bandwidth available per uplink. With VSAT (which pretty much every large ship has), this isn't really a problem, as there's plenty of bandwidth available. Bandwidth allocation is pretty expensive, so most ships cap their bandwidth relatively low. With the ships I work with these days it's usually between 0.5mbit/s to 2mbit/s. I've never seen this degrade when entering high traffic areas.
With starlink there's a bit more to it - I installed one of the first offshore starlink antennae, and back then it was insanely fast. Nowadays there are a lot more starlink terminals around, so it's not that fast anymore. So in general, bandwidth is a lot lower per terminal due to the large amount of customers, so a large ship or two nearby probably won't make much difference. Large ships don't get more bandwidth just because they're large. But it's common that they have several terminals (I usually installed two per ship), but that's mostly for redundancy and avoid that the ships superstructure comes between the dish and the satellite.
As for pirates disabling AIS, I doubt they have AIS to begin with. I used to work in nigerian waters, and the pirates in question usually use very small boats loaded with fuel for endurance. We're talking something that is marginally larger than a hollowed out log with an engine strapped on it. We used to call them "war-canoes".
Locked out of comms: Depending on what you call comms, this doesn't really happen. Sure, internet can go out (and it sometimes did when I was offshore myself), but anyone who might need to reach you in case of emergency know how - Usually by calling your employer. There's a myriad of ways of talking to someone onboard a ship. Normal internet is the most common nowadays. If that doesn't work, they still usually have phone service. And if that doesn't work, it's actually possible to route phonecalls via MF or VHF radios (expensive as fuck, though). And if that doesn't work, there are things such as Telex that enable you to send text. The available hardware depends on the ship and which areas it's rated for, but it basically boils down to ships being required to have comms equipment that enable voice AND text for the rated area. Near the polar regions this basically means Telex and HF radio. For anywhere else, this usually means MF radio and Inmarsat.
Fun anecdote: A coworker of mine, during a period of no internet being available, was instructed that he needed to head to the bridge ASAP due to urgent communications from home. I was his shift leader at the time, and it was not a fun message to relay, as we knew this often meant serious illness in close family. We were all relieved to learn that it was just his dad who had come across a really good offer for a quad bike, and my guy onboard needed to let him know yes or no within the day.
Just to clarify, as this overlaps with several jobs I've had, including my current one:
2008-2012: Offshore seismic survey tech.
2015-2019: Maritime comm and navigation tech 2020-Now: Support/IT dude for offshore seismic survey hardware.
This guy transponds!
I just got a hackrf and it has an AIS tool built in. It's real neat, I've got to play with it a while. It plots them on a map, but it's extremely low res. My city is like 4 pixels. I'd really like to be able to fix that somehow.
I've also noticed a lot of the information it reports seems wrong. Like the same boat can be reporting anchor down and 5 knots. And even more are under way with engine but 0.0 knots. I'm not a boat captain but that doesn't seem right.
It's still endless fun just reading the names of the boats as they pass
The ship state (at anchor, etc) is a manual setting that the bridge crew will have to change. Same goes for destination. And bridge crew has enough on their plates already, so updating it isn't exactly a priority. That's why you often see "Ch 16" set as destination, as it's basically shorthand for "If you really need to know, ask us on VHF Ch 16".
As for receiving AIS data, if you're able to decode the data, it's easy to plot it yourself. AIS messages (AIVDO is the one you're interested in) consists of several messages, and most of them contains the ships position. Every so often, the ships name is transmitted (The MMSI number is the primary identifier).
Well that's a bit of confusion cleared up, thanks! I did actually notice I'd get some information and then a little later the name would show up too. I figured it was just bad reception missing the full message, but you're saying they're sent separately? I guess I'll stop fiddling with my antenna
The fact they just put their contact info for destination is hilarious to me. I should get a radio license.
Yeah, your antenna is fine. AIS contains more information than can be fit into a single packet, so it's spread out over several. Every packet contains the MMSI of the ship, which you can use to link every packet together. If I remember correctly, there are three or so packet types. Position and speed is sent relatively often. And static info such as name and other info that doesn't change a whole lot is sent much more infrequently.
Maritime radio license is easy to get. An ROC which covers the basics is a day or two. It's basically just a course on how to properly operate a VHF. GOC covers a lot more (telex, inmarsat, EPIRBs, etc) and basically certifies you as a radio operator for any ship. It takes about a week.