AIS Transponder Product

Overview

Automatic Identification System (AIS) is a maritime traffic surveillance system that enables vessels to broadcast their position, course, speed, ship identification, and cargo information to nearby ships and shore-based receiving stations. The system works continuously during a voyage and is mandatory on all large commercial vessels, tankers, and passenger ships under IMO SOLAS regulations. Every 10–30 seconds, depending on the ship's speed and maneuverability, the AIS transponder automatically encodes the vessel's data into a digital packet and transmits it via VHF radio on dedicated channels (161.975 MHz and 162.025 MHz, labeled 87A and 87B). All other AIS-equipped ships within line-of-sight receive and decode the packet, displaying nearby vessel positions on their navigation displays and allowing early collision avoidance.

The system comprises several integrated modules: [[ais-transponder-receiver-module|dual independent VHF receivers]] on channels 87A and 87B that continuously listen for incoming AIS packets from other ships, a [[ais-transponder-transmitter-module|time-slotted VHF transmitter]] that broadcasts the vessel's own position and identity, a [[ais-transponder-gps-module|GPS receiver and heading sensor]] that supply the transponder with real-time position, course, and speed data, a [[ais-transponder-processor|message processor]] running the AIS protocol (ITU-R M.1371), and a [[ais-transponder-display-unit|bridge-mounted display]] showing the own ship's data and all received target vessels. Unlike traditional radar, which detects objects by reflection, AIS works by explicit transmission—every ship tells every other ship where it is, eliminating blind spots and surprise encounters in congested shipping lanes.

How it works

The [[ais-transponder-processor|message processor]] is the heart of the system. It runs a finite state machine that samples the [[ais-transponder-gps-module|GPS receiver]] every second to get the vessel's current latitude, longitude, speed over ground (SOG), and course over ground (COG). The processor also reads the ship's heading from an external [[ais-transponder-heading-sensor|fluxgate compass]] or from the autopilot system (if available) to compute true heading. It then assembles this data into an AIS Type 1 message (Position Report Class A), a 168-bit packet that includes the vessel's MMSI (Maritime Mobile Service Identifier, a unique 9-digit number assigned by the flag state), call sign, ship type, dimensions, position, SOG, COG, heading, rate of turn, and navigational status (underway, moored, aground, etc.).

The ITU-R M.1371 standard defines self-organizing time-slot transmission: instead of all ships transmitting randomly and colliding, they synchronize to GPS-derived time slots. The shore infrastructure and AIS base stations broadcast a time reference, and the transponder's GPS receiver locks to this by extracting the pulse-per-second (PPS) output. Divided into ~2250 reserved transmit slots per minute, the processor assigns itself a slot and transmits only during that window. The [[ais-transponder-gmsk-modulator|GMSK modulator]] encodes the 168-bit message into a 9.6 kbaud digital signal, with 6 bits of Hamming-encoded parity for error correction. This modulated signal is boosted by the [[ais-transponder-power-amp|1 W Class A power amplifier]] and radiated by the [[ais-transponder-antenna|dedicated VHF antenna]].

The system uses frequency-hopping transmission: in one time slot, the [[ais-transponder-tx-oscillator|transmitter frequency synthesizer]] hops to channel 87A (161.975 MHz), transmits the packet in ~26 milliseconds, and then hops to channel 87B (162.025 MHz) in the next assigned slot. This dual-frequency approach ensures that a single blocked channel does not prevent transmission or reception. Meanwhile, the [[ais-transponder-receiver-module|dual receivers]] operate continuously on both 87A and 87B, each capturing any incoming AIS packets from nearby ships. When a packet arrives with sufficient signal-to-noise ratio, the [[ais-transponder-rf-front-end|low-noise preamplifier]] boosts the signal, the [[ais-transponder-if-filter|IF bandpass filter]] rejects adjacent-channel interference, and the [[ais-transponder-gmsk-demod|GMSK demodulator]] recovers the bit stream. The [[ais-transponder-fec-decoder|FEC decoder]] applies Hamming error correction, restoring any single-bit errors introduced by fading or noise.

Once decoded, the processor extracts the sender's MMSI, position, heading, and speed from the received message and adds this vessel as a target to the [[ais-transponder-display-unit|display]]. The display renders the target list as a map or tabular format, showing the range, bearing, and closest point of approach (CPA) and time to CPA (TCPA) for each vessel. The officer at the helm can instantly identify which ships are nearby, their vector movements, and any risk of collision. This is far superior to radar in congested harbors or shipping lanes where dozens of vessels may be within the detection range.

Message types and protocol

AIS defines several message types beyond Type 1 (Position Report Class A). Type 5 is the Static and Voyage Data message, transmitted every 6 minutes, containing the ship's name, International Maritime Organization (IMO) number, call sign, vessel type, dimensions (length, beam, draught), and destination port. Type 19 is the Extended Class B Position Report, used by smaller vessels (fishing boats, pleasure yachts) that don't require full Class A certification. Type 24 is the Static Data message for Class B transponders. Type 27 is a position-only message for low-power or low-bandwidth operation. The [[ais-transponder-processor|processor]] automatically generates and transmits these messages on a periodic schedule defined by the ITU-R M.1371 reporting rules.

Reporting frequency depends on ship state: a vessel underway and maneuvering reports every 10 seconds, a vessel underway at constant course reports every 30 seconds, and a moored vessel reports every 3 minutes. This adaptive reporting prevents network congestion and conserves transponder power when the ship is stationary. If the [[ais-transponder-gps-module|GPS receiver]] indicates a large position change or heading change since the last report, the processor may increase the reporting rate (SOG/COG changes >0.5 m/s or >5.1° trigger a report in the next slot). The processor's [[ais-transponder-protocol-memory|EEPROM]] stores the vessel's static data (name, MMSI, call sign, dimensions) and all user-configured settings; this data survives power-loss and is transmitted in every cycle.

Reception, display, and integration

The [[ais-transponder-display-unit|display unit]] receives target data from the processor via a serial interface and renders it in multiple formats. A map view shows own ship at the center with received targets plotted as colored circles or vectors indicating their course and speed relative to the vessel. A tabular list sorts targets by range or CPA time, allowing the officer to quickly identify any vessels approaching on a collision course. When the Closest Point of Approach (CPA) falls below a user-set threshold (typically 2 nm) and the time to CPA is less than 15 minutes, the display issues an audible alarm to alert the bridge.

The transponder integrates with the ship's [[ais-transponder-external-interface|Bridge Integrated Navigation System (BINS)]]: the processor outputs target data and own-ship messages via NMEA 0183 sentences to the [[ais-transponder-external-interface|UART gateway]], which relays this to the Electronic Chart Display and Information System (ECDIS). The ECDIS can then display AIS targets overlaid on the electronic chart, merging with radar data and passage plan waypoints. The VHF radio (if equipped with an AIS interface) can also receive target information and trigger radio contact on the appropriate voice channel.

Regulatory and safety

AIS became mandatory for all vessels over 300 GT (gross tonnage) and all passenger vessels on 1 July 2002 under IMO SOLAS regulations (Chapter V: Safety of Navigation, Regulation 19). Class A AIS (the standard transponder for merchant and naval vessels) uses 1 W transmission power and dual-frequency time-slotted access, ensuring all ships in the same area can receive each other's broadcasts. Reception range is typically 20–40 nautical miles from horizon limitations, but coastlines with elevated receiving antennas extend this to 50+ nm. Modern AIS networks are so ubiquitous that merchant ships track one another with AIS rather than radar, providing a transparent traffic picture that reduces collision risk and improves search-and-rescue operations.