Also known as: COSPAS-SARSAT, Cospas-Sarsat
COSPAS-SARSAT is the international, humanitarian satellite system that detects and locates activated emergency distress beacons and relays the alert to search-and-rescue authorities.1 It listens on the worldwide distress frequency of 406 MHz for the short digital bursts sent by an EPIRB, PLB, or ELT, then pinpoints the beacon using a combination of satellite multilateration, Doppler processing, and any position the beacon itself encodes from GNSS.
Overview
COSPAS-SARSAT is not a navigation constellation but a distress-alerting overlay that rides on several satellite systems. A beacon in an emergency transmits a five-watt burst on 406 MHz roughly every 50 seconds. Instruments aboard low-Earth-orbit (LEOSAR), geostationary (GEOSAR), and — since the 2010s — GNSS medium-Earth-orbit satellites (MEOSAR) receive the burst and forward it to ground stations called Local User Terminals, which decode the beacon’s identity and compute its location before passing the alert to national rescue coordination centres.
Technical characteristics
| Property | Value |
|---|---|
| Uplink | 406.0–406.1 MHz |
| Burst | ~0.5 s digital message, ~5 W, every ~50 s |
| Modulation | Phase-modulated biphase-L (BPSK-family) |
| Space segment | LEOSAR + GEOSAR + MEOSAR |
| Location methods | Doppler, multilateration (MEOSAR), encoded GNSS position |
| Legacy analog | 121.5 MHz homing (no longer satellite-monitored) |
The digital 406 MHz message carries a unique 15-hex-character beacon identity, the country of registration, and often a GNSS position embedded by the beacon. LEOSAR satellites locate a beacon by measuring the Doppler shift of the 406 MHz carrier as they pass overhead; the modern MEOSAR payloads on GPS, Galileo, and GLONASS satellites instead locate it almost instantly by multilateration from time-of-arrival differences across many satellites. The older analog 121.5 MHz beacons are no longer satellite-monitored and remain useful only for short-range homing.
History
COSPAS-SARSAT was created by the United States, Soviet Union, Canada, and France, with the first satellite launched and the first lives saved in 1982. It has since been credited with tens of thousands of rescues. The transition from analog 121.5 MHz to digital 406 MHz beacons (completed for satellite monitoring in 2009) dramatically cut false alerts and improved location accuracy.
Deployment
The system is operated by an intergovernmental organization with dozens of member states and covers the entire globe. Its beacons — EPIRBs on ships, ELTs in aircraft, and PLBs carried by individuals — are mandated for many maritime and aviation uses.
Decoding it with GopherTrunk
GopherTrunk does not decode COSPAS-SARSAT. Although the 406 MHz uplink sits in the UHF range a scanner can tune, decoding distress beacons is a specialized, regulated task and is out of scope for GopherTrunk’s land-mobile trunking focus. Because the beacon signal is a terrestrial 406 MHz burst rather than a weak L-band satellite signal, it is technically far more accessible to a general software-defined radio than GNSS is — hobby decoders for the beacon message format exist — but deliberately transmitting or interfering with 406 MHz is illegal, and GopherTrunk provides no support for it.
Sources
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Cospas-Sarsat — Wikipedia, for the system’s founding, the 406 MHz beacon burst, the LEOSAR/GEOSAR/MEOSAR segments, Doppler and multilateration location, and the retirement of 121.5 MHz satellite monitoring. ↩