EPIRB

Overview

An EPIRB (Emergency Position Indicating Radio Beacon) is a last-resort distress device that broadcasts a vessel's position and identity to satellite-based rescue networks when the ship is in grave danger and conventional radio communication has failed. Unlike a VHF radio which requires the captain to speak to a Coast Guard center or another ship that happens to be listening, an EPIRB transmits a silent digital distress signal on 406.025 MHz to the COSPAS-SARSAT network, a constellation of low-Earth-orbit satellites owned by the United States, Russia, France, Canada, and India. Within 1–2 minutes of detection, the signal is passed to rescue coordination centers around the world, and a SAR (search and rescue) operation begins.

The beacon is typically a 1–2 kg orange cylinder about 50 cm tall, fastened to the bridge wing rail or the aft davit of a ship. When activated, either manually by the captain or automatically when the beacon floats free and sinks below ~1 m after the ship sinks, it switches on, acquires the ship's position from its integrated GPS receiver, and begins transmitting a digital message containing the vessel's MMSI (Maritime Mobile Service Identity), a unique beacon ID, timestamp, and latitude/longitude. The 5 W transmitter remains active for 48 hours, and a bright xenon strobe flashes once per second, allowing rescue aircraft to visually locate the beacon when they arrive at the last-known position.

How it works

The thermal battery pack is the power source, and it has an unusual design: it is a chemical battery that generates its own heat during discharge. A primary lithium or zinc-air cell sits inside a stainless steel canister, with a nichrome heating wire embedded near the cell. When the beacon is activated (either manually via the switch or automatically by seawater pressure), a small capacitor pulse fires the heating wire, warming the thermal cell catalyst and causing a violent chemical reaction that produces the necessary voltage and current. This exothermic design allows the beacon to sit idle for years (ready for use) but deliver full power within seconds when triggered. The thermal battery supplies approximately 12 V and 1 A, enough to power the transmitter, strobe, and GPS receiver for 48 continuous hours.

The GNSS receiver is critical: without position, a distant rescue aircraft cannot locate the beacon in the vast ocean. The GPS SoC receives signals from multiple satellite constellations (GPS, GLONASS, Galileo, BeiDou) and calculates latitude and longitude. On a cold start (when the beacon has not been powered for days), the GPS receiver may take 10–15 minutes to acquire the first fix, but it stores ephemeris data in flash memory, so a warm start (when the beacon has been powered recently) is much faster at 1–3 minutes. Once a position is acquired, the receiver updates it every minute or every 10 minutes depending on the beacon model, allowing the rescue center to track the beacon's drift if it is floating in the water.

When manually activated, the captain switches the three-position switch from OFF to ARM (which lights an LED and activates the GPS receiver to begin acquisition) and then to MANUAL TX to transmit immediately. Alternatively, if the hydrostatic release mechanism is armed, the beacon will automatically transmit once it sinks to ~1 m depth, triggered by seawater pressure on a corrugated stainless steel diaphragm inside the release chamber. This automatic activation is the lifeline if the ship sinks quickly and no crew can reach the bridge-mounted beacon, the beacon floats free of a sinking ship due to its inherent buoyancy, is pulled down by an attachment lanyard, and at depth triggers itself.

The 406.025 MHz transmitter is a compact RF circuit: the frequency synthesizer is a stable crystal oscillator producing exactly 406.025 MHz (tolerance ±300 Hz), the FSK modulator encodes the distress message at 1200 bits per second (a 50-year-old data rate chosen for satellite compatibility), and the power amplifier boosts the signal to 5 W. The bandpass filter removes harmonics and sidelobe radiation, and the signal is fed to the omnidirectional whip antenna.

The strobe light is a 1 Joule xenon discharge tube flashing once per second (at night, visible from ~20 km away on a clear night). A high-voltage capacitor charges to ~500 V, a step-up transformer generates a kilovolt pulse, and this ignites the xenon. The flash lasts ~2 microseconds and resets within 1 second; the strobe continues throughout the entire 48-hour battery life, providing a homing beacon for rescue aircraft.

COSPAS-SARSAT network

The COSPAS-SARSAT system (an acronym from Russian and French: Space System for Search of Vessels in Distress) is a chain of polar-orbiting satellites operated by 45 countries and international organizations. Each satellite carries a 406 MHz receiver, and as it orbits the Earth every ~100 minutes at an altitude of 850 km, it can detect a 5 W transmitter over most of the Earth. When a satellite receives an EPIRB signal, it records the signal strength, frequency offset (Doppler shift from the satellite's motion relative to the beacon), and timestamp. As the satellite continues its orbit, Doppler shifts vary, and processing centers (called MCC, Mission Control Centers) use this variation to triangulate the beacon's position to within 1–5 km. The satellite also relays the signal to ground stations within its footprint, which uplink the data to the international rescue network within 2–5 minutes of first detection.

The EPIRB's digital message carries the vessel's MMSI and a unique 15-digit beacon identifier. The ground network cross-references this identifier with a maritime registration database, finding the ship's name, call sign, flag state, and emergency contact information. The responsible Coast Guard center is immediately notified and launches a SAR operation, dispatching helicopters, cutters, or coordinating with ships in the area.

Regulatory and certification

SOLAS Chapter II-IV (Radiocommunications) mandates that all cargo ships over 500 GT and all passenger vessels carry at least one EPIRB. The beacon must be capable of operating independently of the ship's electrical system (hence the built-in thermal battery) and must remain functional even if submerged. Certification to IEC 61097-1 (Performance Standards for EPIRBs) specifies tests for frequency accuracy, modulation quality, transmit power, GPS accuracy, battery life, strobe light brightness, and waterproofing. An EPIRB undergoes hydrostatic testing in a pressure chamber to confirm it survives 75 m submersion without water ingress, impact testing by drop and vibration, and salt-fog corrosion testing to ensure longevity in the marine environment.

A registered EPIRB receives a maritime service identifier (MMSI) and beacon ID that are linked in the COSPAS-SARSAT database. Every ship's EPIRB is registered with the flag state authority; the registration includes vessel name, hull identification number, home port, and emergency contact phone numbers. This registration is checked by Coast Guard personnel as soon as a beacon is detected, so the rescue response is matched with the right vessel within minutes. An EPIRB battery has a limited shelf life (typically 5–7 years), and maritime regulations require the beacon to be replaced every 5 years or immediately after discharge following a real distress event.