The ship/aircraft carries an emergency transmitter (beacon), which could be activated either manually or automatically in case of a distress situation, such as accident, crash, sinking, fire, explosion, etc. There are three types of uplink units, known as airborne Emergency Locator Terminals (ELTs), maritime Emergency Position Indicator Radio Beacons (EPIRBs) and personal Locator Beacons (PLBs). These units transmit signals that are detected by COSPAS-SARSAT polar orbiting satellites and are relayed to a ground receiving station termed as Local User Terminal (LUT), which processes the signals to determine the beacon location. Alerts are then relayed, together with the location data, via a Mission Control Centre (MCC), either to another MCC or to an appropriate Search & rescue point of contact (SPOC) to initiate search and rescue operations.  The location of beacons is determined by Doppler principle using the relative motion between the satellite and the beacon. The carrier frequency transmitted by the beacon is reasonably stable during the short period when the beacon is within a satellite’s visibility.  The international distress frequencies used are 121.5 MHz (and 243 MHz) and the 406.0/406.1 MHz band. The 406 MHz units are more sophisticated than the 121.5 MHz beacons because of the inclusion of identification codes in the message.

The SARSAT satellites carry; in addition to the 121.5 MHz and 406 MHz receive equipment, the 243 MHz receivers on-board while the COSPAS spacecraft carry only the 121.5 MHz and 406 MHz receivers.

The COSPAS-SARSAT system provides two coverage modes for the detection and location of beacons, the real-time mode and the global coverage mode. Both the 121.5 MHz and 406 MHz systems operate in the real-time mode while the 406 MHz system operates in the global coverage mode also. A repeater on-board the spacecraft relays the 121.5 MHz (and 243 MHz in the case of SARSAT) signals directly to the ground. If a LUT is in view of the spacecraft, the signal can be received and processed at the LUT.   In case of 406 MHz beacons signals received by the satellite, the Doppler shift is measured on-board and the beacon digital data recovered from the beacon signal. This information is then time-tagged, formatted as digital data, and transferred to the repeater downlink for real-time transmission to any LUT in view. The data is also simultaneously stored on the spacecraft for later transmission. This stored data is continuously dumped for reception by other LUTs, thereby making it possible for all LUTs to locate each 406 MHz beacons.

The Mission Control Centre (MCC) collects, stores and sorts the data from LUTs and other MCCs, and provides data exchange within the COSPAS-SARSAT system and to Search and Rescue networks.