How the Cospas-Sarsat system works

Cospas-Sarsat is an international, humanitarian search and rescue system that uses satellites to detect and locate emergency beacons carried by ships, aircraft, or individuals. The system consists of a network of satellites, ground stations, mission control centres, and rescue coordination centres.

When an emergency beacon is activated, the signal is received by a satellite and relayed to the nearest available ground station. The ground station, called a Local User Terminal, processes the signal and calculates the position from which it originated. This position is transmitted to a mission control centre where it is joined with identification data and other information on that beacon.The mission control centre then transmits an alert message to the appropriate rescue coordination centre based on the geographic location of the beacon. If the location of the beacon is in another country's area of responsibility, then the alert is transmitted to that country's mission control centre.

The SARSAT system uses satellites in low-earth and geostationary orbits to detect and locate aviators, mariners, and land-based users in distress. The satellites relay distress signals from emergency beacons to Australia's two ground stations and ultimately to Australian Search and Rescue in Canberra.

The system is designed to provide distress alert and location data to assist search and rescue (SAR) operations, using spacecraft and ground facilities to detect and locate the signals of distress beacons operating on 406 Megahertz (MHz) or 121.5 MHz.

Satellites

The Cospas-Sarsat System primarily uses polar orbiting satellites that provide the ability to detect and locate 406 MHz alerts worldwide and 121.5 MHz alerts for about sixty percent of the world. In addition to the polar orbiting satellites, the system uses geostationary satellites which only carry receivers for digital 406 Mz beacons.

Most of the SARSAT satellites orbit at an altitude of 844 km and complete an orbit every 100 minutes. Their orbits are inclined 99 degrees from the equator. Each satellite carries a Search and Rescue Repeater which receives and retransmits 121.5 MHz and 243 MHz signals anytime the satellite is in view of a ground station. Also carried is a Search and Rescue Processor which receives 406 MHz transmissions, provides measurements of the frequency and time, then retransmits this data in real-time or stores it aboard for later transmission. If the satellite is in view of a ground station when a 406 MHz signal is received the data is received in real-time. The satellite also stores each signal it receives and continuously downloads this data. If the satellite was not in view of a ground station when it received a beacon signal, the next ground station that sees that satellite will receive the data. This provides global coverage for 406 MHz distress signals.

The COSPAS instrument is carried aboard the NADEZHDA navigation satellite orbiting the Earth every 105 minutes at an altitude of 992 km and an orbital inclination of 83 degrees. The COSPAS instrument was built by the former Soviet Union and continues to be operated by the Russian Federation. The only major difference between COSPAS and SARSAT is that the Russian satellites do not receive 243 MHz distress signals.

Mission Control Centres (MCCs)

MCCs have been set up in most countries operating at least one LUT (ground station). Australia has two LUTs, one at Albany and the other at Bundaberg and monitors a third LUT at Wellington in New Zealand. Their main functions are to:

collect, store and sort the data from LUTs and other MCCs ;
provide data exchange within the Cospas-Sarsat System; and
distribute alert and location data.

Australia's MCC is located at AMSA in Canberra. All MCCs in the system are interconnected through appropriate networks for the distribution of system information and alert data.