International Cospas-Sarsat Programme

The International Cospas-Sarsat Programme is a satellite-aided search and rescue initiative. It is organized as a treaty-based, nonprofit, intergovernmental, humanitarian cooperative of 45 nations and agencies. It is dedicated to detecting and locating emergency locator radio beacons activated by persons, aircraft or vessels in distress, and forwarding this alert information to authorities that can take action for rescue. Member countries support the distribution of distress alerts using a constellation of around 65 satellites orbiting the Earth which carry transponders and signal processors capable of locating an emergency beacon anywhere on Earth transmitting on the Cospas-Sarsat frequency of 406 MHz.
Distress alerts are detected, located and forwarded to over 200 countries and territories at no cost to beacon owners or the receiving government agencies. Cospas-Sarsat was conceived and initiated by Canada, France, the United States, and the former Soviet Union in 1979. The first rescue using the technology of Cospas-Sarsat occurred on. The definitive agreement of the organization was signed by those four states as the "parties" to the agreement on 1 July 1988.
The term Cospas-Sarsat derives from COSPAS, an acronym from the transliterated Russian "Космическая Система Поиска Аварийных Судов", meaning "Space-based System for Locating Vessels in Distress", and SARSAT, an acronym for "Search and Rescue Satellite-Aided Tracking".

Background

Cospas-Sarsat is best known as the system that detects and locates emergency beacons activated by aircraft, ships and people engaged in recreational activities in remote areas, and then sends these distress alerts to search-and-rescue authorities. Distress beacons capable of being detected by the Cospas-Sarsat System are available from several manufacturers and vendor chains. Cospas-Sarsat does not make or sell beacons.
Between September 1982 and December 2023 the Cospas-Sarsat System provided assistance in rescuing at least 63,745 people in 19,883 SAR events. In 2023 Cospas-Sarsat on average assisted in the rescue of almost nine persons each day. In 2021, 2022 and 2023, Cospas-Sarsat assistance included the following:

Year	People rescued	SAR events	Aviation	Land	Maritime
2023	3,109	1,076	20%	44%	36%
2022	3,223	1,144	20%	39%	41%
2021	3,623	1,149	18%	45%	37%

These statistics under-count the number of events where Cospas-Sarsat assisted, because they only include cases when an accurate report from SAR personnel is provided back through reporting channels to the Cospas-Sarsat Secretariat.
Cospas-Sarsat does not undertake search-and-rescue operations. This is the responsibility of national administrations that have accepted responsibility for SAR in various geographic regions of the world. Cospas-Sarsat provides alert data to those authorities.
Cospas-Sarsat cooperates with United Nations-affiliated agencies, such as the International Civil Aviation Organization, the International Maritime Organization, and the International Telecommunication Union, among other international organizations, to ensure the compatibility of the Cospas-Sarsat distress alerting services with the needs, the standards and the applicable recommendations of the global community. Cospas-Sarsat is an element of the IMO's Global Maritime Distress Safety System, and is a component of ICAO's Global Aeronautical Distress and Safety System. The IMO requires automatic-activating Cospas-Sarsat beacons on all vessels subject to requirements of the International Convention for the Safety of Life at Sea, commercial fishing vessels, and all passenger ships in international waters. Similarly, ICAO requires Cospas-Sarsat beacons aboard aircraft on international flights, as well as the ability to track such aircraft when in distress. National administrations often impose requirements in addition to the international requirements of those agencies.
Cospas-Sarsat only monitors for alerts from digital distress beacons that transmit on 406 MHz. Older beacons that transmit using the legacy analog signal on 121.5 MHz or 243 MHz rely on being received only by nearby aircraft or rescue personnel. For satellite reception of alerts by Cospas-Sarsat the beacon must be a model that transmits at 406 MHz.
Cospas-Sarsat has received many honors for its humanitarian work, including induction into the Space Foundation's Space Technology Hall of Fame for space technologies improving the quality of life for all humanity.

System operation

The system consists of a ground segment and a space segment that include:

Distress radio-beacons to be activated in a life-threatening emergency
SAR signal repeaters and SAR signal processors aboard satellites
Satellite downlink receiving and signal processing ground stations called LUTs
Mission control centres that distribute to rescue coordination centres distress alert data generated by the LUTs
Rescue coordination centres that facilitate coordination of the SAR agency and personnel response to a distress situation.
Beacons

A Cospas-Sarsat distress beacon is a digital 406-MHz radio transmitter that can be activated in a life-threatening emergency to summon assistance from government authorities. Beacons are manufactured and sold by dozens of vendors. They are classified in three main types. A beacon designed for use aboard an aircraft is known as an emergency locator transmitter. One designed for use aboard a marine vessel is called an emergency position-indicating radio beacon. And one that is designed to be carried by an individual is known as a personal locator beacon. Sometimes PLBs are carried aboard aircraft or vessels, but whether this satisfies safety requirements depends on local regulations. A Cospas-Sarsat beacon does not transmit until it is activated in an emergency. Some beacons are designed to be manually activated by a person pressing a button, and some others are designed for automatic activation in certain circumstances. There are no subscription or other costs imposed by Cospas-Sarsat for beacon ownership or use. See below for recent beacon innovations.

Space segment

The Cospas-Sarsat system operational space segment consists of SARR and/or SARP instruments aboard:

Three satellites in polar low-altitude Earth orbit with LEOSAR payloads,
13 satellites in geostationary Earth orbit with GEOSAR payloads,
49 satellites in medium-altitude Earth orbit with MEOSAR payloads.

A SARR or SARP instrument is a secondary payload and associated antennas attached to those satellites as an adjunct to the primary satellite mission. A SARR instrument retransmits a beacon distress signal to a satellite ground station in real time. A SARP instrument records the data from the distress signal so that the information can later be gathered by a ground station when the satellite passes overhead.

Ground segment

The satellites are monitored by receiving ground stations equipped to track the satellites using satellite dishes or phased antenna arrays. LUTs are installed by individual national administrations or agencies. The distress messages received by a LUT are transferred to an associated mission control centre which uses a detailed set of computer algorithms to route the messages to rescue coordination centres worldwide.

System architecture

See: Mission control centre
When a distress beacon is activated, the Cospas-Sarsat system:

decodes the binary coded message of the beacon, which contains information such as the identity of the vessel/aircraft and, for beacons equipped with the feature, the location of the beacon derived from a local navigation source, and
performs a mathematical analysis of the signal to calculate the location of the beacon, even if the beacon's location is not reported in the distress message.

The Cospas-Sarsat system is the only satellite distress alerting system that is capable of this dual, redundant means of locating an activated distress beacon.
The SARR and/or SARP instrument typically is attached to a satellite that is being launched primarily for another purpose. The primary mission of all of the LEOSAR and GEOSAR satellites is meteorological. The primary mission of all of the MEOSAR satellites is navigation.

LEOSAR

LEOSAR was the original Cospas-Sarsat space segment architecture. The complementary LEOSAR-satellite orbits provide periodic coverage of the entire Earth. Because of their relatively low altitude, there are intervals of time when a LEOSAR satellite may not be over a particular geographic location. So there can be a delay in receiving an alert signal, and a delay in relaying that signal to the ground. For this reason, LEOSAR satellites are equipped with the "store-and-forward" SARP modules in addition to "real-time" SARR modules. The satellite can pass over a remote area of the Earth and receive a distress message, and then forward that data later when it passes into view of a ground station. The three satellites in the LEOSAR constellation have approximately 100 minute orbits. Because of their polar orbits the latency between satellite passes overhead is smallest at the poles and higher at the equator. Reliance on the LEOSAR system is gradually being reduced in favor of the GEOSAR and MEOSAR systems described below.
The Cospas-Sarsat LEOSAR system was made possible by Doppler processing. LUTs detecting distress signals relayed by LEOSAR satellites perform mathematical calculations based on the Doppler-induced frequency shift received by the satellites as they pass over a beacon transmitting at a fixed frequency. From the mathematical calculations, it is possible to determine both bearing and range with respect to the satellite. The range and bearing are measured from the rate of change of the received frequency, which varies both according to the path of the satellite in space and the rotation of the Earth. This allows a computer algorithm to trilaterate the position of the beacon. A faster change in the received frequency indicates that the beacon is closer to the satellite's ground track. When the beacon is moving toward or away from the satellite track due to the Earth's rotation, the Doppler shift induced by that motion also can be used in the calculation.