AIS Systems
AIS is arguably the single biggest safety improvement for collision avoidance since radar — and unlike radar, it tells you the other vessel's name, course, speed, and intentions.
How AIS Works — Classes, Transmission, and Reception
The Automatic Identification System (AIS) operates on two dedicated VHF frequencies — 161.975 MHz (Channel 87B) and 162.025 MHz (Channel 88B) — using a self-organizing time-division multiple access (SOTDMA or CSTDMA) protocol that allows thousands of vessels to share the same frequencies without interference. Each AIS-equipped vessel broadcasts a data packet containing its identity, position, course, speed, and other information at regular intervals. Every other AIS unit within VHF range receives these broadcasts, decodes them, and displays the targets on a screen. No subscription, no internet connection, no cellular signal required — it works anywhere on the ocean using the same VHF radio waves your marine radio uses.
Class A transponders are mandatory on commercial vessels over 300 gross tons, all passenger ships, and international cargo vessels. They transmit at 12.5 watts with dynamic reporting rates — every 2 seconds when underway and turning, up to every 3 minutes when at anchor. Class A units have full SOTDMA capability, meaning they actively manage their own time slots on the channel and have priority over Class B units. Recreational boats almost never need Class A, and the units cost $2,000-5,000.
Class B transponders are designed for recreational and small commercial vessels. Standard Class B units transmit at 2 watts using CSTDMA (carrier-sense time division multiple access), which means they listen for an open time slot before transmitting — they yield to Class A units. They report position every 30 seconds when moving faster than 2 knots and every 3 minutes when slower or at anchor. Class B+ (also called Class B CS) is the newer standard — it transmits at 5 watts, reports every 5 seconds at speed, and includes the vessel's name in every transmission rather than only in the less-frequent static data messages. For cruising sailboats, a Class B+ transponder is the clear recommendation — the higher power and faster reporting rate mean your vessel appears more prominently and reliably on other vessels' AIS displays.
Receive-only AIS units (sometimes called AIS receivers) listen to AIS broadcasts from other vessels but do not transmit your own vessel's information. They cost $100-300 and are better than nothing — you can see other traffic — but the safety benefit of AIS is bidirectional. A ship's officer watching an AIS display at 3 AM will see your transponding vessel and can take early action to avoid you; a receive-only unit makes you invisible to them. For coastal cruising or any passage that intersects commercial shipping lanes, a transponder is essential, not optional.
If budget forces a choice between a receive-only AIS unit and a Class B transponder, choose the transponder. Being visible to commercial traffic at 3 AM is more valuable than seeing them on your screen — you are already keeping watch, but the container ship's officer may be relying entirely on AIS and radar to detect you. A transponder makes you a named target on their display with course, speed, and CPA calculations.
AIS Data — What You Transmit and What You Receive
AIS messages carry three categories of data: dynamic data that updates continuously, static data that is programmed once, and voyage-related data that changes per trip. Understanding what each category contains helps you interpret the targets on your screen and ensures your own vessel is broadcasting accurate information that other vessels can trust and act upon.
Dynamic data includes your vessel's position (from the GPS built into or connected to your AIS transponder), speed over ground (SOG), course over ground (COG), heading (if a heading sensor is connected), rate of turn (Class A only), and navigational status (underway using engine, under sail, at anchor, not under command, etc.). This data updates with every transmission — every 5-30 seconds depending on your AIS class and speed. The GPS position is the most critical element; if your AIS transponder's GPS antenna has a poor sky view or is using position data from an external GPS that has failed, you will be broadcasting an incorrect position. Other vessels will see you where you are not.
Static data is programmed into the transponder during installation and includes your MMSI number, vessel name, call sign, IMO number (commercial vessels), vessel type (sailing vessel, pleasure craft, etc.), and vessel dimensions including the position of the GPS antenna relative to bow, stern, port, and starboard. Programming the vessel dimensions correctly is important — it determines where the AIS target symbol is placed relative to the actual position. If you enter the GPS antenna offset incorrectly, your vessel's plotted position may be displaced by the length of your boat, which matters in close-quarters situations.
Voyage-related data on Class A includes destination and ETA, but on Class B this is not transmitted. What you receive from other vessels is the same data in reverse — their name, MMSI, position, course, speed, vessel type, and dimensions. Your AIS display or chartplotter processes this data to calculate CPA (Closest Point of Approach) — the minimum distance the target will pass from you if neither vessel alters course — and TCPA (Time to Closest Point of Approach) — how many minutes until that closest point occurs. These two numbers are the core of collision avoidance. A CPA of 0.1 nautical miles with a TCPA of 15 minutes means you have fifteen minutes to take action before a vessel passes within 600 feet — or closer.
After installing your AIS transponder, verify your vessel appears correctly on MarineTraffic.com or VesselFinder.com — both display live AIS data. Check that your vessel name, type, dimensions, and position are accurate. If your vessel type shows as "cargo ship" or your dimensions are wildly wrong, reprogram the static data before your next passage. Incorrect static data erodes other mariners' trust in your AIS target.
Display Options — Dedicated, Chartplotter Overlay, and Tablet Apps
AIS data is only useful if you can see it clearly and act on it quickly. You have three main display options: a dedicated AIS display, an overlay on your existing chartplotter, or a tablet or laptop running navigation software that receives AIS data via a multiplexer or wireless gateway. Each approach has trade-offs in cost, integration quality, and reliability, and most well-equipped cruising boats end up using at least two of these simultaneously for redundancy.
Chartplotter overlay is the most common and most practical approach for cruising sailboats. Your multifunction display (Garmin, Raymarine, B&G, Simrad, Furuno) connects to the AIS transponder via NMEA 2000 or NMEA 0183, and AIS targets appear directly on the chart alongside your own vessel. Each target shows as a triangle indicating course and speed, with the vessel name, MMSI, CPA, and TCPA available by selecting the target. This integration means you see traffic in geographic context — you can tell at a glance whether a target will cross your path, pass ahead or astern, and whether the CPA is comfortable or requires action. Set your CPA alarm to trigger at 0.5-1.0 nautical miles and your TCPA alarm at 10-15 minutes. These alarms are your sleeping crew's best friend on night passages.
Dedicated AIS displays are standalone units that show AIS data independently of your chartplotter. They range from simple text-based units that list nearby targets in a table to full graphic plotters. The primary argument for a dedicated display is redundancy — if your chartplotter fails, you still have AIS situational awareness. Some transponders (like the Vesper Cortex or em-trak B954) include built-in displays or integrate with smartphones via Wi-Fi, providing a secondary display without a separate hardware purchase.
Tablet and laptop displays using apps like iNavX, Navionics, OpenCPN, or SEAiq can receive AIS data from a wireless multiplexer (like a Vesper WatchMate, Digital Yacht iAISTX, or Yacht Devices YDWG-02) and display it on a chart. This is an excellent secondary display and a powerful planning tool — a 10-inch tablet shows more chart area and more target information than most fixed chartplotters. However, tablets are consumer electronics, not marine-hardened devices. Screen brightness may be insufficient in direct sunlight, touchscreens fail when wet, batteries die at inconvenient times, and Wi-Fi connections drop. Use a tablet as a supplement, not a primary navigation display, and protect it in a waterproof case with an external power connection.
Set your chartplotter's CPA alarm to 0.5 NM and TCPA alarm to 15 minutes as a starting point, then adjust based on your sailing area. In open ocean, 0.5 NM may be fine; in busy coastal waters or shipping channels, you may want 1.0 NM. The alarm should wake the off-watch crew in time to assess the situation and alter course if needed — not announce an imminent collision.
Antenna Configuration — Separate vs Shared with Splitter
An AIS transponder needs a VHF antenna because it operates on VHF frequencies. You have two options: install a separate dedicated AIS antenna, or share your existing VHF radio antenna using an antenna splitter. Both approaches work, but each has implications for performance, installation complexity, and failure modes that you need to understand before choosing.
A separate dedicated antenna is the simplest and most reliable approach. Install a second VHF antenna — typically a 3 dB whip on the stern rail or pushpit — and run its own coax directly to the AIS transponder. This gives the AIS system a completely independent antenna path with no shared components that could fail. The transponder can transmit and receive simultaneously without any coordination with the VHF radio. The downside is the second antenna: it adds windage, costs $80-150 plus the coax run, and on a sailboat already crowded with a masthead VHF antenna, radar, wind instruments, and Windex, finding a mounting location that does not interfere with other equipment or rigging requires some thought.
An antenna splitter allows your AIS transponder and VHF radio to share the same masthead antenna. Modern splitters (like the Vesper SP160 or Shakespeare AIS-3) use solid-state switching to route the antenna connection to whichever device is actively transmitting and passively split the received signal between both devices. Quality splitters introduce less than 1 dB of insertion loss — barely noticeable in practice. The advantage is that your AIS transponder uses the masthead antenna, which gives it the best possible range due to the antenna's height. The risk is that the splitter is now a single point of failure — if it dies, you lose both VHF radio and AIS antenna connections.
The practical recommendation for cruising sailboats is to use a splitter for the masthead antenna as the primary AIS antenna path and keep a separate stern-rail antenna as a backup for the VHF radio. If the splitter fails, you can reconnect the VHF radio to the backup antenna and retain voice communication capability. Wire the backup antenna coax to a convenient junction point near the nav station so the swap takes minutes, not hours. Label all coax cables clearly at both ends — in a crisis, you do not want to guess which cable goes where.
Never connect a VHF radio and an AIS transponder to the same antenna without a proper splitter. If both devices attempt to transmit simultaneously on a direct connection (even through a simple T-connector or coax switch), the transmitted power from one device feeds directly into the other's receiver front end, which will damage or destroy the input stage. A proper AIS splitter manages this switching automatically and is the only safe way to share an antenna.
MMSI Programming, AIS MOB Devices, and Practical Considerations
MMSI programming on an AIS transponder follows the same principles as VHF DSC — the MMSI must match the number registered to your vessel, and it must be entered correctly the first time because most transponders lock the MMSI after initial programming. Your AIS transponder and your VHF radio must use the same MMSI number — they represent the same vessel. If you purchased a used transponder, it may have the previous owner's MMSI still programmed, and you will need to send it to the manufacturer or an authorized dealer for reprogramming. Operating with an incorrect MMSI is illegal and creates confusion in maritime databases.
AIS MOB (Man Overboard) devices are small, personal AIS transponders designed to be worn by crew and activated if someone falls overboard. When triggered, they broadcast an AIS MOB message that appears on every AIS display within range — including your own chartplotter — as a distinctive MOB symbol with the person's GPS position. Products like the Ocean Signal rescueME MOB1, ACR AISLink MOB, and Kannad SafeLink R10 transmit on AIS frequencies at 1 watt, providing a range of 5-7 miles. They also include a 121.5 MHz homing signal that search vessels can track to within meters.
AIS SART (Search and Rescue Transponder) devices are larger units designed for life rafts. When activated, they transmit a distinctive AIS-SART signal that appears on AIS displays as a concentric circle pattern, making the life raft visible to all AIS-equipped vessels within range. An AIS-SART is an alternative to a traditional radar SART and is increasingly preferred because more vessels are equipped with AIS displays than with radar. Some GMDSS-equipped vessels are required to carry both a radar SART and an AIS-SART.
Practical installation considerations: mount the AIS transponder's GPS antenna where it has a clear view of the sky — the cabin top or a mounting plate on the pushpit works well. Avoid mounting it inside a metal enclosure or under a radar arch that blocks satellite signals. Connect the transponder to your NMEA 2000 backbone so that AIS target data flows to your chartplotter, and so the transponder can use the boat's primary GPS if its internal GPS fails. Keep the transponder accessible — you may need to update its firmware, change voyage data, or check its operational status. A unit buried behind a panel and forgotten is a unit that may fail silently, broadcasting stale data while you assume it is working.
Assign an AIS MOB device to every crew member on offshore passages. The device clips to a PFD or harness and activates automatically on water contact or manually with a pull tab. When someone goes overboard at night, the MOB alarm on your chartplotter shows exactly where they are — not where they were when someone last saw them. The $200-350 cost per device is trivial compared to the alternative of searching in the dark.
Summary
AIS transponders broadcast your vessel's identity, position, course, and speed to all equipped vessels within VHF range — a Class B+ transponder is the recommended choice for cruising sailboats, offering 5-watt transmit power and fast reporting rates.
Receive-only AIS lets you see other traffic but makes you invisible — a transponder is essential for any vessel crossing commercial shipping lanes.
CPA (Closest Point of Approach) and TCPA (Time to CPA) are the core collision avoidance calculations derived from AIS data — set alarms on your chartplotter to alert you before close encounters develop.
A quality antenna splitter allows your AIS and VHF radio to share the masthead antenna safely, but maintain a backup antenna on the stern rail for redundancy.
AIS MOB devices worn by crew provide immediate, GPS-accurate position data when someone falls overboard — assign one to every person on offshore passages.
Key Terms
- Class B+ Transponder
- The current standard AIS transponder for recreational vessels. Transmits at 5 watts using CSTDMA protocol with position reports every 5 seconds at speed, including vessel name in every message. Replaces the older 2-watt Class B standard.
- CPA (Closest Point of Approach)
- The minimum distance at which an AIS target will pass your vessel if neither vessel changes course or speed. The primary metric for collision avoidance — a small CPA requires attention and potential course alteration.
- TCPA (Time to Closest Point of Approach)
- The time in minutes until the CPA occurs. Used together with CPA to determine urgency — a CPA of 0.5 NM in 30 minutes is less urgent than the same CPA in 5 minutes.
- SOTDMA / CSTDMA
- Self-Organizing and Carrier-Sense Time Division Multiple Access — the protocols used by Class A and Class B/B+ AIS transponders respectively to share VHF channel bandwidth without interference. SOTDMA has priority; CSTDMA yields to it.
- Antenna Splitter
- An active electronic device that allows a VHF radio and an AIS transponder to safely share a single VHF antenna. Uses solid-state switching to prevent simultaneous transmission and typically introduces less than 1 dB of signal loss.
- AIS MOB Device
- A personal AIS transponder worn by crew that activates on water immersion, broadcasting the wearer's GPS position as an AIS MOB target visible on all nearby AIS displays. Range is typically 5-7 nautical miles.