Maritime Radio
GMDSS, DSC, VHF channels, EPIRB, NAVTEX — the complete technical guide to marine radio communication.
GMDSS: The Global Maritime Distress and Safety System
The Global Maritime Distress and Safety System (GMDSS), adopted by the International Maritime Organization (IMO) in 1988 and fully implemented in 1999, replaced the century-old Morse-code-based distress system with an integrated network of satellite and terrestrial radio technologies. Every SOLAS vessel — passenger ships, tankers, and cargo ships over 300 GT — must carry GMDSS equipment. The system guarantees that a distress alert from any ocean reaches a Rescue Coordination Centre (RCC) within seconds.
GMDSS is not a single technology; it is a framework of complementary systems selected for each vessel based on where it operates. The IMO divides the world's oceans into four "sea areas," each served by specific radio technologies. A vessel transiting from port to port automatically meets its GMDSS obligations by carrying the equipment designated for the sea areas on its route.
GMDSS Sea Areas
GMDSS defines four operational sea areas based on the range of radio technologies available. The sea area determines which equipment a vessel must carry. A ship operating in Sea Area A1, for example, needs only VHF DSC and EPIRB — but a ship on a transatlantic route crossing through A3 must also carry Inmarsat terminals and HF equipment.
┌─────────┬─────────────────────────────────────────────────────────────────┐ │ Sea Area│ Coverage │ ├─────────┼─────────────────────────────────────────────────────────────────┤ │ A1 │ VHF DSC range from a coast station or MF relay station │ │ │ ~20-30 nautical miles (~37-56 km) │ │ │ Equipment: VHF DSC, EPIRB, SART, NAVTEX │ ├─────────┼─────────────────────────────────────────────────────────────────┤ │ A2 │ MF DSC range from a coast station │ │ │ ~100-150 nautical miles (~185-278 km) │ │ │ Equipment: VHF DSC, MF DSC, EPIRB, SART, NAVTEX │ ├─────────┼─────────────────────────────────────────────────────────────────┤ │ A3 │ Inmarsat geostationary satellite coverage │ │ │ ~200-600 nautical miles from coast (between 76°N & 76°S) │ │ │ Equipment: VHF/MF DSC, HF DSC, Inmarsat-C, EPIRB, SART │ ├─────────┼─────────────────────────────────────────────────────────────────┤ │ A4 │ Polar regions beyond A3 — HF only (no Inmarsat) │ │ │ Equipment: VHF/MF DSC, HF DSC, EPIRB, SART, NAVTEX │ └─────────┴─────────────────────────────────────────────────────────────────┘
Sea Area A1 extends to the range of VHF DSC from a coast station, typically 20–30 nautical miles. Sea Area A2 extends to MF DSC range, roughly 100–150 nautical miles. Sea Area A3 covers the Inmarsat geostationary satellite footprints between approximately 76°N and 76°S latitude. Sea Area A4 includes the polar regions beyond A3, where only HF radio provides coverage — no Inmarsat satellites reach these latitudes.
VHF Marine Channels
The VHF marine band (156–174 MHz) provides the primary short-range voice and data communication for ships. Channel 16 is the international distress and calling frequency — every vessel must monitor it continuously when at sea. Channel 70 carries DSC digital alerts and is never used for voice. The duplex channels use separate transmit/receive frequencies for ship-to-shore communication through coast stations.
┌──────────┬──────────────┬──────────────┬──────────────────────────────────┐ │ Channel │ TX (MHz) │ RX (MHz) │ Use / Notes │ ├──────────┼──────────────┼──────────────┼──────────────────────────────────┤ │ 01 │ 156.050 │ 156.050 │ Simplex │ │ 06 │ 156.300 │ 156.300 │ Bridge-to-bridge, port ops │ │ 07 │ 156.350 │ 156.350 │ Port operations │ │ 08 │ 156.400 │ 156.400 │ Fleet communications │ │ 09 │ 156.450 │ 161.050 │ Calling / Duplex │ │ 10 │ 156.500 │ 161.100 │ Duplex │ │ 11 │ 156.550 │ 161.150 │ Duplex │ │ 12 │ 156.600 │ 161.200 │ Duplex │ │ 13 │ 156.650 │ 156.650 │ Bridge-to-bridge safety │ │ 14 │ 156.700 │ 161.300 │ Port operations │ │ 15 │ 156.750 │ 161.350 │ USCG liaison │ │ 16 │ 156.800 │ 156.800 │ DISTRESS / SAFETY / CALLING │ │ 17 │ 156.850 │ 156.850 │ USCG operations │ │ 18/19 │ 156.900/950 │ 156.900/950 │ USCG (low power / high power) │ │ 20 │ 157.000 │ 161.600 │ Duplex │ │ 22A │ 157.100 │ 161.700 │ USCG public correspondence │ │ 67 │ 156.375 │ 156.375 │ Port operations (low power) │ │ 70 │ 156.525 │ 156.525 │ DSC — Digital Selective Calling │ └──────────┴──────────────┴──────────────┴──────────────────────────────────┘
Channel 16 (156.800 MHz) is monitored by all vessels and coast stations at all times. Initial distress, urgency, and safety calls are made on Ch 16, then traffic is moved to a working channel. Channel 13 (156.650 MHz) is the international bridge-to-bridge channel for navigational safety — collision avoidance, passage intentions, and visibility warnings. Channel 70 (156.525 MHz) carries exclusively DSC digital alerts and is never used for voice.
Digital Selective Calling (DSC)
DSC is the automated calling system at the heart of GMDSS. It allows a ship to transmit a digitally encoded distress alert containing its MMSI (Maritime Mobile Service Identity), position, and the nature of the emergency. Coast stations and other vessels with DSC-capable radios automatically receive and decode the alert. DSC eliminates the need for a human operator to continuously listen for voice distress calls.
┌──────────────────────────────────────────────────────────────────────┐ │ DSC Technical Specifications │ ├──────────────────────────────────────────────────────────────────────┤ │ Modulation │ │ ───────── │ │ MF/HF: F1B/J2B — 170 Hz frequency shift, 100 baud │ │ VHF: G2B — FSK 1300-2100 Hz, 1200 baud, modulation index 2.0 │ │ │ │ MMSI Format (Maritime Mobile Service Identity) │ │ ───────────────────────────────────────────── │ │ 9 digits: MID (3-digit Maritime Identification Digit) + ship ID │ │ Example: 367001234 → MID 367 (USA), vessel #001234 │ │ │ │ Distress Frequencies (DSC Alerting) │ │ ──────────────────────────────────── │ │ VHF Ch 70: 156.525 MHz │ │ MF: 2187.5 kHz │ │ HF 4: 4207.5 kHz │ │ HF 6: 6312.0 kHz │ │ HF 8: 8414.5 kHz │ │ HF 12: 12577.0 kHz │ │ HF 16: 16804.5 kHz │ │ │ │ Time Diversity (Error Protection) │ │ ────────────────────────────────── │ │ MF/HF: Each character transmitted twice, 400 ms delay │ │ VHF: Each character transmitted twice, 33 ms delay │ │ │ │ DSC Categories │ │ ────────────── │ │ 108: Distress │ │ 110: Urgency │ │ 112: Safety │ │ 114: 3x Distress (relayed by another vessel) │ │ 116: 3x Urgency │ │ 118: 3x Safety │ │ 120-123: Individual calls (routine) │ │ 124-127: Group calls │ │ 128-129: All ships │ └──────────────────────────────────────────────────────────────────────┘
DSC operates on a simple but robust protocol. A ship's DSC controller generates a digital burst containing the MMSI, the category of the call (distress, urgency, safety, or routine), and the working frequency to follow. This burst is transmitted on the designated DSC frequency (e.g., VHF Ch 70 or MF 2187.5 kHz). The alerting vessel then switches to Channel 16 or a designated working channel to follow up with a voice mayday.
On MF and HF, DSC uses time diversity to combat fading: each character is transmitted twice with a 400 ms delay between transmissions. On VHF, the delay is only 33 ms — much shorter because VHF line-of-sight paths have minimal fading.
The MMSI is a unique 9-digit identifier permanently programmed into each vessel's DSC equipment. The first three digits are the Maritime Identification Digit (MID), assigned by the ITU to flag states. For example, MID 219 = Denmark, MID 235 = United Kingdom, MID 367 = United States. The MMSI is also stored in the EPIRB so that the COSPAS-SARSAT system can identify the vessel in distress.
Distress Frequencies
Maritime distress communication operates on specific frequencies allocated internationally by the ITU for distress, safety, and calling. These frequencies form a layered safety net: VHF for short-range voice, MF for medium range, HF for global coverage, and satellite frequencies for alerting from anywhere on Earth.
┌──────────────────────────────────────────────────────────────────────┐ │ Maritime Distress Frequencies │ ├──────────────┬────────────────┬──────────────────────────────────────┤ │ Band │ Frequency │ Purpose │ ├──────────────┼────────────────┼──────────────────────────────────────┤ │ VHF Ch 16 │ 156.800 MHz │ International distress voice │ │ VHF Ch 70 │ 156.525 MHz │ International distress DSC │ │ MF │ 2182 kHz │ Medium-range distress voice │ │ MF │ 2187.5 kHz │ Medium-range distress DSC │ │ HF 4 MHz │ 4125 kHz │ Long-range distress voice │ │ HF 6 MHz │ 6215 kHz │ Long-range distress voice │ │ HF 8 MHz │ 8291 kHz │ Long-range distress voice │ │ HF 12 MHz │ 12290 kHz │ Long-range distress voice │ │ HF 16 MHz │ 16420 kHz │ Long-range distress voice │ │ MF │ 500 kHz │ Historical CW distress (deprecated) │ │ Satellite │ 406.025 MHz │ EPIRB satellite alert (COSPAS) │ │ Homing │ 121.5 MHz │ EPIRB homing beacon for SAR │ └──────────────┴────────────────┴──────────────────────────────────────┘
The legacy CW (Morse) distress frequency of 500 kHz was phased out after GMDSS implementation. The voice distress frequency of 2182 kHzremains as a backup for MF-capable vessels. On VHF, Channel 16 (voice) and Channel 70 (DSC) together form the primary distress system for ships within 30 nautical miles of shore.
EPIRB: Emergency Position-Indicating Radio Beacon
The EPIRB is the ultimate distress beacon. When a ship sinks, the EPIRB is either manually activated by the crew or automatically deployed by a hydrostatic release unit (HRU) that triggers at 1.5–4 meters of water depth. The EPIRB transmits on 406.025–406.050 MHz to the COSPAS-SARSAT satellite constellation and simultaneously on 121.5 MHz for SAR aircraft homing.
┌──────────────────────────────────────────────────────────────────────┐ │ EPIRB Specifications │ ├──────────────────────────────────────────────────────────────────────┤ │ 406 MHz Satellite EPIRB │ │ ───────────────────────── │ │ Frequency: 406.025 - 406.050 MHz │ │ Output Power: 5 W ERP (Effective Radiated Power) │ │ Battery Life: 48 hours minimum (at -20°C) │ │ Modulation: AM/PM with 40 bps data │ │ Position Error: ≤ 100 m (with GPS) / ≤ 2 km (Doppler-only) │ │ Transmission: Every 50 seconds (406 MHz burst) │ │ 121.5 MHz Homing: Continuous FM sweep, for SAR aircraft direction │ │ │ │ Satellite Systems (COSPAS-SARSAT) │ │ ────────────────────────────────── │ │ LEOSAR: Low Earth Orbit (800-1000 km) │ │ Doppler-based position fix; global coverage │ │ Delay: 30-90 minutes for position │ │ MEOSAR: Medium Earth Orbit (~20,200 km) │ │ Multi-satellite ranging; fast position fix │ │ Delay: minutes (near-real-time) │ │ GEOSAR: Geostationary (~35,786 km) │ │ Instant alert relay; no position from Doppler alone │ │ Limited to satellite footprint coverage │ │ │ │ DSC VHF EPIRB (Short-Range) │ │ ───────────────────────────── │ │ Frequency: 156.525 MHz (VHF Ch 70) │ │ Output Power: 100 mW │ │ Battery Life: 48 hours │ │ Use: Overboard man-overboard; alerts nearby vessels │ └──────────────────────────────────────────────────────────────────────┘
The 406 MHz EPIRB is a marvel of engineering. It contains a GPS receiver that encodes the vessel's precise latitude and longitude into each transmission, allowing search and rescue to reach the position within 100 meters. Without GPS, the COSPAS-SARSAT system calculates the position using Doppler shift analysis — the apparent frequency change caused by the relative motion between the satellite and the beacon — but this method is less accurate and requires satellite overpass time.
The COSPAS-SARSAT system has saved over 42,000 lives since its inception. The original constellation of LEO satellites (LEOSAR) has been supplemented and largely replaced by MEOSAR (Medium Earth Orbit Search and Rescue) satellites, which provide near-real-time position fixes rather than the 30–90 minute delay of LEOSAR Doppler processing.
NAVTEX: Navigational Telex
NAVTEX is an automated direct-printing system for receiving Maritime Safety Information (MSI) — navigational warnings, meteorological forecasts, search and rescue notices, and ice reports. NAVTEX operates on two dedicated frequencies and requires no operator intervention: the receiver decodes and prints (or displays) messages as they are broadcast by coast stations.
┌──────────────────────────────────────────────────────────────────────┐ │ NAVTEX Specifications │ ├──────────────────────────────────────────────────────────────────────┤ │ Frequencies: │ │ 518 kHz — International English MSI │ │ 490 kHz — Local language MSI (national NAVTEX service) │ │ │ │ Modulation: SITOR-B (Simplex Teletext over Radio) — FEC mode │ │ Baud Rate: 100 baud (50 baud per channel of SITOR-B) │ │ Character: 7-bit ITA-2 with start/stop bits, + error detection │ │ │ │ Transmission Cycle: │ │ Each station transmits for 2 minutes, then remains silent │ │ Multiple stations share the frequency using time-division │ │ A station may transmit up to 64 256-character messages │ │ │ │ Receiver: Automatically selects messages by area code (A-Z) │ │ Filters unwanted transmissions │ │ Stores messages for 72 hours │ │ │ │ MSI Content: │ │ A: Navigational warnings │ │ B: Meteorological warnings │ │ C: Ice reports │ │ D: Search and rescue notices │ │ E: Meteorological forecasts │ │ F: Navigational warnings (other) │ │ L: Urgency traffic │ │ N: DECCA Loran-C warnings │ │ T: Test transmissions │ │ U: Upgrades (non-urgent) │ │ V: General notices to mariners │ └──────────────────────────────────────────────────────────────────────┘
NAVTEX receivers decode SITOR-B (Frequency Shift Telegraphy, FEC mode) at 100 baud. Each message is prefixed with a standardized header: a 4-character station identifier (B1B2), a message type letter (B3), and a serial number (B4). The receiver uses these characters to filter out duplicate messages and only display messages designated for the ship's area — identified by the B1 field, which uses letters A–Z to represent geographic regions.
SART: Search and Rescue Transponder
The SART is a portable radar transponder that marks the position of survivors for search and rescue vessels and aircraft. When triggered by an incoming X-band (9 GHz) radar signal, the SART responds with a distinctive pattern of 12 dots on the radar display — unmistakable as a distress indication. The SART also produces an audible "beeping" tone inside the survival craft when the searching vessel's radar sweeps over it, helping survivors confirm that help is near.
┌──────────────────────────────────────────────────────────────────────┐ │ SART Specifications │ ├──────────────────────────────────────────────────────────────────────┤ │ Radar Band: 9.2-9.5 GHz (X-band) │ │ Trigger: Activated by ship/aircraft radar sweep │ │ Response: 12-pulse pattern on radar display │ │ (spaced dots: range 0.64-1.2 ms) │ │ Effective Range: ~5 nm from aircraft at 3,000 ft │ │ ~2 nm from a ship (10 kW radar) │ │ Battery: 96 hours in standby, 8 hours in active mode │ │ Activation: Manual or automatic (water switch) │ │ Mounting: Deployed on survival craft; hold above water │ │ Display: Distinctive dot pattern on all X-band radars │ │ Audible Alarm: Beeping tone when radar interrogates the SART │ └──────────────────────────────────────────────────────────────────────┘
The SART dot pattern is designed to be instantly recognizable to radar operators. The 12 dots form a distinctive arc on the radar screen, extending outward from the SART position along the line of bearing to the searching vessel. The first dot marks the SART's true position; subsequent dots represent the delay of the transponder response. This pattern cannot be confused with sea clutter, rain, or other radar returns.
Inmarsat: Maritime Satellite Communications
Inmarsat (International Maritime Satellite Organization), founded in 1979, operates a constellation of geostationary satellites at approximately 35,786 km altitude above the equator. These satellites provide continuous voice, data, and distress communication services to ships worldwide — except in the polar regions (Sea Area A4), where geostationary satellites are below the horizon.
┌──────────────────────────────────────────────────────────────────────┐ │ Inmarsat Specifications │ ├──────────────────────────────────────────────────────────────────────┤ │ Satellite Orbit: Geostationary, ~35,786 km above equator │ │ Orbital Slots: Multiple positions covering 76°N to 76°S │ │ │ │ Frequency Bands: │ │ L-band (ship): 1.5-1.6 GHz (up/down) │ │ C-band (shore): 4-6 GHz (coast earth station uplink/downlink) │ │ Ka-band (HTS): 26.5-40 GHz (Fleet Xpress high-throughput) │ │ │ │ Service Generations: │ │ ─────────────────── │ │ Inmarsat-C: │ │ Data Rate: 600 baud │ │ Mode: Store-and-forward messaging │ │ Distress: Low-speed data alert to RCC via satellite │ │ Position: GPS-fed, transmitted in each alert │ │ Voice: Not supported │ │ Latency: Seconds (real-time alert) │ │ │ │ FleetBroadband: │ │ Data Rate: Up to 432 kbps (high-priority class) │ │ Mode: IP-based voice + data simultaneously │ │ Voice: 4 kbps AMBE+ compressed │ │ Video: Possible at reduced quality │ │ Streaming: Yes (quality-of-service tiers) │ │ │ │ Fleet Xpress: │ │ Data Rate: Up to 2 Mbps (Ka-band HTS) │ │ Mode: Hybrid Ka/L-band for continuous service │ │ Fallback: Automatic switch to L-band on Ka attenuation │ │ Target: Deep-sea vessels requiring high bandwidth │ │ │ │ Mini-VSAT Broadband (Third-Party): │ │ Data Rate: Up to 2 Mbps per channel │ │ Provider: SpeedCast / SES │ └──────────────────────────────────────────────────────────────────────┘
Inmarsat-C is the most critical GMDSS satellite service. It operates in store-and-forward mode at 600 baud — slow by modern standards, but designed for reliability over voice quality. A distress alert sent via Inmarsat-C reaches a Rescue Coordination Centre within seconds, carrying the vessel's MMSI, position, course, speed, and the nature of distress. The system confirms receipt of the alert to the originating vessel, providing positive confirmation that help has been notified.
FleetBroadband and Fleet Xpress provide IP-based broadband for modern operations — email, crew welfare, voyage planning data, and vessel-to-shore communication. Fleet Xpress uses Ka-band high- throughput satellites (HTS) for speeds up to 2 Mbps, with automatic fallback to L-band when Ka signals are attenuated by rain fade.
The L-band (1.5–1.6 GHz) is used for ship earth station transmissions to the satellite because it is relatively resilient to rain attenuation compared to higher frequencies. The shore-based coast earth stations communicate with the satellite on C-band (4–6 GHz), which has greater bandwidth and less weather sensitivity. Ka-band (26.5–40 GHz) is reserved for high-throughput point-to-point links where maximum bandwidth is needed.