The Complete Guide to AIS: Classes, Range, Receivers & Setup
AIS is one of the few upgrades that makes you safer just by existing on the boat — but "AIS" covers a handful of very different products, transmit powers, and installation choices. Here's everything that actually matters when you're picking one.
The Automatic Identification System (AIS) works by having equipped vessels broadcast their position, speed, heading, and identity over dedicated VHF frequencies, so anyone else with AIS — another boat, a shore station, a chartplotter — can see them on a screen, day or night, in fog, well beyond visual or radar range. Where it gets confusing is that "AIS" isn't one product. It's a class (A, B, or B+), a capability (receive-only or transmit-and-receive), and a form factor (built into your VHF or standalone) — and those choices combine into setups with very different real-world performance.
How AIS Actually Works
Every AIS unit talks on two internationally reserved marine VHF channels: AIS 1 at 161.975 MHz (channel 87B) and AIS 2 at 162.025 MHz (channel 88B). Thousands of vessels share those two channels at once using time-division access — each minute is sliced into 2,250 individual time slots, and transponders self-organize which slot they transmit in (SOTDMA) or contend for an open slot (CSTDMA), so messages don't collide even in busy harbors.
What actually gets sent falls into three categories:
- Static data — MMSI, vessel name, call sign, type, and dimensions. Programmed once at setup and rarely changed.
- Dynamic data — position, course over ground, speed, heading, and rate of turn. Pulled continuously from GPS and compass and re-sent every few seconds to half a minute, depending on class.
- Voyage data — draft, destination, and ETA. Entered manually, only available on Class A, and the least reliable field since it depends on someone keeping it updated.
Range: Why Antenna Height Beats Wattage
AIS travels by line-of-sight VHF propagation, the same physics as your radio — which means antenna height matters as much as transmit power, sometimes more. A Class A station on a ship with a masthead antenna 100+ feet up can be seen 20–40 nautical miles out. A Class B unit on a sailboat with a 6-foot antenna above the water might only reach 5–6 miles, even though the gap in transmit power (2W vs. 12.5W) looks larger than that on paper. Mounting your AIS antenna as high as practical, with clean low-loss cable and solid connections, does more for your real-world range than upgrading class alone.
Class A vs. Class B vs. Class B+
AIS transponder class determines transmit power and how often your position updates — which in turn determines how reliably other vessels see you.
Class A
The SOLAS-mandated class for commercial ships and large vessels. Fastest update rate, a dedicated type-approved display, and the only class that broadcasts voyage data like destination and ETA. More capable, more expensive, and more than most recreational boats need.
Class B (CSTDMA)
Fixed 30-second update rate regardless of speed, relies on a connected chartplotter rather than its own display, and can't broadcast destination data. The most common, most affordable recreational choice.
Class B+ (SOTDMA)
2.5x the power of standard Class B, with a dynamic update rate that speeds up as you go faster — similar behavior to Class A. Reserves a guaranteed transmission time slot, making it far more reliable in crowded, signal-dense waters.
| Class A | Class B (CSTDMA) | Class B+ (SOTDMA) | |
|---|---|---|---|
| Transmit power | 12.5W | 2W | 5W |
| Update rate | 2–10 sec (speed-based) | Fixed 30 sec | Dynamic (speed-based) |
| Typical range* | 20–40 nm | 5–6 nm | 8–15 nm |
| Display | Dedicated, type-approved | Connected chartplotter | Connected chartplotter |
| Typical vessel | Commercial / SOLAS | Recreational | Recreational, offshore/fast |
*Range is heavily dependent on antenna height and installation — see the range section above.
Receiver vs. Transceiver (Transponder)
Independent of class, every AIS device falls into one of two functional categories:
- AIS Receiver (receive-only): listens for AIS signals from other vessels and displays them on your chartplotter. You gain full situational awareness of AIS-equipped traffic around you — but you're invisible to them. Simpler, cheaper, no MMSI transmit setup required.
- AIS Transceiver / Transponder: does everything a receiver does, plus broadcasts your own vessel's position, speed, heading, and identity (tied to your MMSI) so other AIS-equipped vessels and shore stations can see you too. This is what actually puts you on a commercial ship's watch officer's screen.
The short version: a receiver lets you see others; a transponder lets others see you too. In a busy shipping channel, fog, or any offshore passage where being seen by a freighter matters as much as seeing it, a transponder is the meaningfully safer choice — a receiver alone only tells half the story.
Built-In vs. Standalone
AIS capability increasingly shows up baked into VHF radios, not just as a separate box. Both approaches work, but they come with real tradeoffs worth understanding before you buy.
Built Into the VHF Radio
Convenient and space-saving — one unit handles both DSC calling and AIS. Works well when it's the only AIS receiver aboard. The complication is the antenna: a shared VHF/AIS antenna runs through a splitter, and splitters briefly interrupt VHF reception while the AIS side transmits — a workable tradeoff, just not perfectly transparent.
Standalone AIS Unit
A dedicated transponder or receiver, wired into your NMEA 0183 or NMEA 2000 network to feed any connected chartplotter, MFD, or app. Typically paired with its own dedicated antenna (and often its own small GPS antenna) rather than a splitter, avoiding the reception interruption entirely — generally the more robust setup for serious offshore use.
Collision Avoidance: CPA and TCPA Alarms
Most AIS-capable chartplotters and displays don't just show targets — they calculate risk. Two numbers drive that: CPA (Closest Point of Approach), the closest distance a target will pass from you if both vessels hold their current course and speed, and TCPA (Time to Closest Point of Approach), how long until that closest pass actually happens. Set a CPA distance and TCPA time threshold in your display, and the unit sounds an alarm any time a target is projected to cross inside both.
Who Actually Needs What
How It Connects to the Rest of Your Electronics
Whether built-in or standalone, most modern AIS units connect over NMEA 2000 (increasingly the standard for plug-and-play chartplotter integration) or the older NMEA 0183 standard. Many also offer Wi-Fi or Bluetooth output, streaming AIS targets straight to navigation apps like Navionics or TimeZero on a phone or tablet — useful for monitoring traffic from the cockpit, helm, or anywhere else on the boat without being tied to a fixed display.
Frequently Asked Questions
Do I need AIS if I already have radar?
They solve overlapping but different problems. Radar sees anything reflective — including boats without AIS, buoys, and land — but tells you nothing about identity, course, or intentions. AIS gives you exact identity, course, speed, and often a name and callsign to hail, but only for equipped vessels. Most serious offshore setups run both; AIS doesn't replace radar and radar doesn't replace AIS.
Is a Class B transponder legal for offshore passages?
Yes — Class B is fully legal for recreational vessels, including offshore use. It's not the SOLAS-mandated class commercial ships must carry, but there's no restriction on a private vessel using Class B (or B+) anywhere recreational boats are permitted to operate.
Can I see AIS targets on my phone without a chartplotter?
Yes, if your AIS unit has Wi-Fi or Bluetooth output. It streams NMEA data to navigation apps like Navionics or TimeZero on a phone or tablet, which is a common way to get AIS on a boat that doesn't have (or doesn't need) a dedicated MFD at the helm.
Do I need to register anything before I can transmit?
Yes. Any transmitting AIS unit needs a valid MMSI (Maritime Mobile Service Identity) programmed in before it broadcasts. In the U.S., recreational boaters can get one free through BoatUS or Sea Tow; it's what ties your AIS broadcasts, and your DSC-equipped VHF radio, to your specific vessel.
Quick Buying Checklist
- Decide if you need to be seen, not just see others — that's the receiver-vs-transponder decision, and for offshore or high-traffic use, a transponder is the safer call.
- For recreational boats, Class B is the standard choice; step up to Class B+ (SOTDMA) if you run offshore, run fast, or spend time in crowded shipping traffic.
- Mount the antenna as high as practically possible — it does more for real-world range than transmit power alone.
- Decide built-in vs. standalone based on whether your VHF already handles AIS well and whether you're comfortable running a splitter versus a dedicated antenna.
- If adding a standalone transponder to a boat with an AIS-capable VHF, avoid running two active receivers on the same network.
- Confirm your MMSI is registered and correctly programmed before transmitting.
- Set CPA/TCPA alarm thresholds intentionally, and tune them from real use instead of leaving factory defaults.
- Check connectivity (NMEA 2000, NMEA 0183, Wi-Fi/Bluetooth) against what your chartplotter or navigation app actually supports.
Most recreational boats end up in the same place: a Class B or Class B+ transponder, either built into a capable VHF or run as a clean standalone unit with its own antenna mounted high, feeding a chartplotter over NMEA 2000. The details matter less than getting the fundamentals right — transmitting your own position, not just watching others, and avoiding the setup mistakes that quietly undercut what AIS is supposed to do for you.
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