AIS Buying Guide: Transponder vs. Receiver, Class A vs. Class B
Quick answer: An AIS receiver lets you see ships and other AIS-equipped boats on your chartplotter. An AIS transponder (transceiver) lets you see them and lets them see you. For most recreational boats that run offshore, at night or near shipping lanes, a Class B transponder is the right choice. Pick a 5 watt SOTDMA Class B if you want the strongest, most frequent signal, and plan for an MMSI number, a GPS source and a VHF-band antenna or splitter.
AIS (Automatic Identification System) is a VHF-based system where vessels broadcast their position, course, speed and identity. Large commercial ships are required to carry it, and their bridge watch relies on it. On your chartplotter, AIS turns a confusing radar blip into a named ship with a speed, heading and closest point of approach. This guide explains receivers versus transponders, Class A versus Class B, and what you need to install one.
Step 1: Receiver or transponder?
| AIS receiver | AIS transponder | |
|---|---|---|
| Sees other vessels | Yes | Yes |
| Other vessels see you | No | Yes |
| MMSI required | No | Yes |
| Cost | Lower | Higher |
| Best for | Coastal boats that want traffic awareness | Offshore, night running, shipping lanes, cruising |
Many VHF radios now have a built-in AIS receiver, which is the cheapest way to start. If you want ships to see you, you need a transponder.
Step 2: Class A vs. Class B
- Class A: built for commercial vessels, transmits at 12.5 watts and reports frequently. Required on many commercial vessels and overkill for most recreational boats.
- Class B (CSTDMA): the standard recreational transponder, transmitting at 2 watts and reporting about every 30 seconds when underway.
- Class B SOTDMA (often called B+): transmits at 5 watts and reports more often as speed increases, so your position is more current on other vessels' screens. A strong upgrade for fast boats and busy waters.
Step 3: What you need for a transponder
- MMSI number programmed into the unit. Some transponders must be programmed by the dealer.
- GPS position: most transponders have a GPS receiver and include or require a GPS antenna.
- VHF antenna: AIS uses VHF frequencies. Use a dedicated AIS antenna or a proper VHF/AIS splitter so your radio and AIS can share one antenna.
- A display: connect to your chartplotter by NMEA 2000, NMEA 0183, or Wi-Fi to a tablet app.
Step 4: Using AIS on the water
- Set a CPA/TCPA alarm (closest point of approach and time to it) so the system warns you of a collision course.
- Use the vessel name to call a ship directly on VHF. "Motor vessel Atlantic Star, this is..." gets a much faster response than "ship off my bow."
- Remember that not every boat carries AIS. Small boats, many fishing boats and most recreational traffic will not appear. AIS supplements radar and a lookout, it does not replace them.
- Many transponders offer a silent mode so you can stop transmitting when you want privacy, such as at a fishing spot, while still receiving.
Step 5: Plan the install
Mount the transponder in a dry spot near the helm or electronics, run the antenna cable short with low-loss coax, and keep the AIS antenna separated from your VHF antenna if they are not using a splitter. Power it through its own fuse. See our VHF antenna guide for mounting and cable tips.
Step 6: Receiver, transponder or both on your plotter
If your chartplotter and VHF are already networked, the simplest path is often a VHF with a built-in AIS receiver, then upgrading to a transponder later. If you already have a transponder, you do not need a separate receiver: every transponder receives too. For sailboats, mount the AIS antenna as high as practical on the stern rail or arch if the VHF antenna is at the masthead, so a dismasting does not take out both. Test the system after installing by checking that your boat appears on a public AIS tracking website or app, and that nearby ships show up on your plotter with names and speeds.
Our picks by setup
All in stock at Boat Supplies Direct.
See traffic on a budget: receivers
Digital Yacht AIS100 Receiver or the dual-channel SI-TEX MDA-2 AIS Receiver. Or get AIS built into your radio with the Standard Horizon GX2410GPS VHF.
See and be seen: Class B transponders
SI-TEX MDA-1 Class B Transceiver, a solid value (must be programmed with your MMSI), or the Garmin AIS 800 Blackbox Transceiver for Garmin networks.
Strongest Class B: 5 watt SOTDMA
Digital Yacht AIT2500 Class B+ 5W or the SI-TEX MDA-5 with built-in antenna splitter and Wi-Fi.
Antennas and splitters
Shakespeare 5396-AIS 4 foot antenna, the Digital Yacht SPL1500 VHF/AIS Splitter, or the all-in-one Digital Yacht GV30 VHF/AIS/GPS Combo Antenna.
Frequently Asked Questions
What is the difference between an AIS receiver and transponder?
A receiver only shows other AIS vessels. A transponder also broadcasts your own position, course and identity so other vessels and ships can see you.
Do recreational boats need AIS?
Most recreational boats in the U.S. are not required to carry AIS, but a Class B transponder greatly improves safety when running offshore, at night or near shipping traffic.
What is the difference between Class A and Class B AIS?
Class A is the commercial standard, at higher power and faster reporting. Class B is designed for recreational boats. Class B SOTDMA units transmit at 5 watts and report more often than standard 2 watt Class B.
Do I need an MMSI number for AIS?
Yes for a transponder, which must be programmed with your vessel's MMSI. Receivers do not need one.
Can AIS share my VHF antenna?
Yes, through a VHF/AIS antenna splitter built for the purpose. Otherwise, install a separate AIS antenna.
Will AIS show every boat around me?
No. Only vessels carrying AIS appear. Many small and recreational boats do not, so keep using radar and a proper lookout.
This guide is general information. Check current U.S. Coast Guard and FCC requirements for your vessel and follow your AIS manufacturer's installation instructions.
