Cospas-Sarsat is an international satellite system for detecting distress alerts, locating emergency beacons and forwarding alert data to search and rescue authorities. Cospas is a Russian acronym meaning “Space System for Search of Vessels in Distress”. SARSAT stands for Search and Rescue Satellite-Aided Tracking. The system is used in aviation, maritime and land emergencies; the responsible SAR authorities organise the actual response.
Which devices raise an alert
Emergency beacons transmit digital messages in short bursts in the 406 MHz band. Three important categories are ELTs, Emergency Locator Transmitters, for aircraft; EPIRBs, Emergency Position-Indicating Radio Beacons, for maritime use; and PLBs, Personal Locator Beacons, carried by individuals.
The message includes the beacon's identifier. Associated registration records can provide rescue authorities with information about the aircraft, vessel or user and emergency contacts. The full registration record does not have to be carried in the radio signal itself. Some beacons also obtain a position from a satellite navigation receiver and include those coordinates in the distress message. An ordinary navigation receiver alone does not transmit such an alert.
From the satellite signal to rescue coordination
A satellite receives the signal and relays it to a compatible ground station, known as a Local User Terminal, LUT. The station receives and processes the relayed signals. Depending on the satellite component, this can also produce a position independently of the beacon's navigation receiver. Processed alert data passes to a Mission Control Centre, MCC.
The MCC combines related messages, correlates them with additional data and routes them to the responsible rescue coordination centre or SAR point of contact. It also works with MCCs in other countries. A Rescue Coordination Centre, RCC, assesses the incident and coordinates the rescue response.
How the position is determined
The beacon may supply its position as coordinates, or the system may calculate it from received signals. For the latter method, satellite motion or geometry matters. The terms LEOSAR, GEOSAR and MEOSAR distinguish satellite components by their orbits and the location capabilities associated with them.
| Component | How it works |
|---|---|
| LEOSAR | Low Earth orbits. Satellite motion changes the received frequency of the beacon signal. Ground stations can use this Doppler shift for location; signal reception depends on a satellite pass. |
| GEOSAR | Geostationary satellites continuously view large areas. They relay alerts but do not provide independent Doppler location. A navigation position encoded by the beacon can be forwarded with the message. |
| MEOSAR | Medium Earth orbits. Several satellites relay the signal along different paths. Differences in arrival time and received frequency allow ground processing to calculate an independent position. |
SAR payloads on Galileo satellites contribute to the MEOSAR component. Their Forward Link relays distress signals to the ground segment. This is distinct from ordinary satellite navigation: a beacon may receive navigation signals to determine its position and separately transmit its distress message through the SAR payload.
What the Return Link confirms
The Galileo Return Link Service, RLS, can automatically send an acknowledgement back to a compatible beacon. The automatic system acknowledgement confirms that the alert has been detected and its position determined by the system. The responsible rescue coordination centre decides on the rescue response; the automatic system acknowledgement does not report the start of that operation. A conversation or free-text exchange with rescue coordinators requires an additional communications link.
Relevance to military personnel
Cospas-Sarsat alert data can also reach military rescue authorities. In the US Combat Survivor Evader Locator, CSEL, the unsecured SARSAT data function is separate from secure two-way over-the-horizon data communications.