International Cospas-Sarsat Programme

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 (ELT). One designed for use aboard a marine vessel is called an emergency position-indicating radio beacon (EPIRB). And one that is designed to be carried by an individual is known as a personal locator beacon (PLB). Sometimes PLBs are carried aboard aircraft or vessels, but whether this satisfies safety requirements depends on local regulations.[6] A Cospas-Sarsat beacon does not transmit until it is activated in an emergency (or when certain testing features are activated by the user). 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 (e.g., ELTs may be automatically activated by a physical shock, such as in a crash, and EPIRBs may be automatically activated by contact with water). There are no subscription or other costs imposed by Cospas-Sarsat for beacon ownership or use. (Some countries may impose licensing and/or registration charges for beacon ownership, and some jurisdictions may assess costs for rescue operations.)[16] See below for recent beacon innovations.

The Cospas-Sarsat system operational space segment consists of SARR and/or SARP instruments aboard:[17]

• Five satellites in polar low-altitude Earth orbit with LEOSAR (low-altitude Earth orbit search-and-rescue) payloads (one other in preparation for use),
• Twelve satellites in geostationary Earth orbit with GEOSAR (geostationary Earth orbit search-and-rescue) payloads (two others in preparation for use),
• 48 satellites in medium-altitude Earth orbit with MEOSAR (medium-altitude Earth orbit search-and-rescue) payloads (seven others in preparation for use).

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.

The satellites are monitored by receiving ground stations (LUTs) equipped to track (point at and follow) 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.

Example of LEOSAR signal footprint.

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 (and therefore, relatively small "footprint" of visibility of any particular part of the Earth at any given time), 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 (that typically are located in less remote areas). The five 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 latitudes.

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.

Because their geostationary orbit does not provide a relative motion between a distress beacon and a GEOSAR satellite, there is no opportunity to use the Doppler effect to calculate the location of a beacon. Therefore, the GEOSAR satellites only can relay a beacon's distress message. If the beacon is a model with a feature to report its location (e.g., from an on-board GPS receiver) then that location is relayed to SAR authorities. While the inability to independently locate a beacon is a drawback of GEOSAR satellites, those satellites have an advantage in that the present constellation well covers the entire Earth in real time, except for the polar regions.

The most recent space segment augmentation for Cospas-Sarsat is MEOSAR. MEOSAR blends the advantages of the LEOSAR and GEOSAR systems, while avoiding their drawbacks. The MEOSAR system is becoming the dominant capability of Cospas-Sarsat. In addition to the large number of satellites, the MEOSAR system benefits from relatively large satellite footprints and sufficient satellite motion relative to a point on the ground to allow the use of Doppler measurements as part of the method of calculating a distress beacon's location. MEOSAR consists of SARR transponders aboard the following navigation-satellite constellations: the European Union's Galileo, Russia's Glonass, and the United States' Global Positioning System (GPS).[18][19][20][21] In November 2022, China became the newest MEOSAR space-segment provider, with Cospas-Sarsat SAR payloads aboard six of its BeiDou (BDS) navigation spacecraft. The first SAR-equipped BDS spacecraft was launched on 19 September 2018, and the last on 23 November 2019.

Operational distribution of MEOSAR alert data began at 1300 UTC on 13 December 2016. Following continued testing and adjustment, a declaration of initial operational capability (IOC) was made by the Cospas-Sarsat Council effective from 25 April 2023. The MEOSAR system advances the ability to provide near-instantaneous detection, identification, and location-determination of 406-MHz beacons. Prior to the operational introduction of MEOSAR, MEOSAR data was successfully used to assist in determining the crash location of EgyptAir flight 804 in the Mediterranean Sea.[22] The location of a distress beacon is calculated by the receiving LUT by analyzing the frequency-difference-of-arrival (related to Doppler-induced variations), and/or the time-difference-of-arrival of a beacon's radio signal due to the differences in distance between the beacon and each MEOSAR satellite that may be in view.

With respect to GPS-hosted payloads, experimental S-band payloads aboard 18 GPS Block IIR and GPS Block IIF satellites, and four payloads aboard GPS Block IIIA satellites are used operationally by the Cospas-Sarsat System. GPS Block IIIF satellites are planned to have dedicated, operational L-band SAR payloads provided by Canada, with launches beginning around 2026. The GPS SAR system is known as the Distress Alerting Satellite System (DASS) by NASA.[23][24][25]

Additionally, the Galileo component of the MEOSAR system is able to download information back to the distress radio-beacon by encoding "Return Link Service" messages into the Galileo navigation data stream. It can be used to activate an indicator on the beacon to confirm receipt of the distress message.[26][27][28]

As at December 2019 the LEOSAR satellites are tracked and monitored by 60 commissioned LEOLUT (low-altitude Earth-orbit local user terminals) antennas, the GEOSAR satellites by 25 commissioned GEOLUT antennas and the MEOSAR satellites by 24 commissioned MEOLUT stations, each having multiple antennas. The data from these earth stations is transferred to and distributed by 32 MCCs established globally, of which 3 remain under development.[29][30] (See infobox for the countries and agencies that are ground-segment providers.)

Most Cospas-Sarsat-compatible 406-MHz beacons also transmit distress or tracking signals on additional frequencies. Most commonly, Cospas-Sarsat beacons have a 121.5-MHz transmitter to provide a signal that can be received by local search crews (airborne, on ground or at sea) using direction-finding equipment. Additionally, the latest EPIRBs include an automatic identification system (AIS) transmitter in the marine VHF band that allows the beacon to be easily tracked from nearby vessels. Recent PLB models designed for attachment to marine life vests transmit an AIS signal to act as a maritime survivor locating system, also known as a man overboard (MOB) system, that activates alarms on nearby vessels and allows the beacon to be tracked by properly equipped vessels.

Beacons with such combinations of signals simultaneously allow for global alerting through the 406-MHz transmission to satellites and the swiftest local response from the 121.5-MHz and AIS transmissions (particularly in the maritime environment by nearby vessels).

In response to recent commercial aviation disasters and subsequent ICAO requirements for autonomous tracking of aircraft in distress,[31][32] Cospas-Sarsat established specifications for ELTs for distress tracking (ELT(DT)s) to meet the ICAO requirements (amended Annex 6, Part I of the Convention on International Civil Aviation). Whereas conventional ELTs are designed to activate on impact or by manual activation by the flight crew, ELT(DT)s activate autonomously when an aircraft enters threatening flight configurations that have been predetermined by expert agencies. In this way, ELT(DT)s allow a plane in distress to be tracked in-flight, prior to any crash, without human intervention aboard the aircraft. ELT(DT)s have been specified using both the existing beacon transmission method (narrowband BPSK) and the second-generation (spread-spectrum QPSK) modulation schemes (see transmission technologies below). Cospas-Sarsat capability for receiving and processing distress messages from ELT(DT)s using the narrowband BPSK transmission method was declared operational effective 1 January 2023. Declaration for ELT(DT)s using the spread-spectrum QPSK modulation method is expected in late 2023 or early 2024.

There has been one transmission modulation method used by Cospas-Sarsat 406-MHz digital beacons since their inception more than 30 years ago, binary phase-shift keying (BPSK), with two allowed bit-string lengths: 112 (with 87 bits of message information) and 144 (with 119 bits of message information). Several message protocols are allowed in the available message-bit string to accommodate different kinds of beacons (ELTs, EPIRBs and PLBs), different vessel/aircraft identifiers, and different national requirements. The time length of these transmissions is approximately one-half second. These narrowband transmissions occupy approximately 3 kHz of bandwidth in a channelized scheme across the assigned 406.0 to 406.1 MHz band.[33]

Cospas-Sarsat is in the process of specifying a new, additional beacon modulation and message scheme based on spread-spectrum technology with quadrature phase-shift keying (QPSK). Presently beacons that will use this scheme are termed "second generation" beacons. This technology will allow the use of battery-saving lower-power transmissions, improve the accuracy of the determination of the beacon location by the Cospas-Sarsat System, and avoid the need for discrete channelization in the assigned 406.0 to 406.1 MHz band (e.g., avoiding the need for periodic closing and opening of channels by Cospas-Sarsat for use by beacon manufacturers based on narrowband channel loading). Second-generation beacons will have a longer transmission period of one second, with 250 transmitted bits, 202 of those being message bits. Additionally, the information sent in the message bits from one transmission to the next can be changed on a rotating transmission schedule ("rotating message fields") to allow significantly more information to be communicated over the course of a series of transmission bursts.[34]

On 23 November 1979, a "memorandum of understanding concerning cooperation in a joint experimental satellite-aided search and rescue project" was signed in Leningrad, USSR, among the U.S. National Aeronautics and Space Administration, the USSR Ministry of Merchant Marine, the Centre National d'Etudes Spatiales of France, and the Department of Communications of Canada. Under Article 3 of the memorandum, it was stated that:[35]

"Cooperation will be achieved through effecting interoperability between the SARSAT project and the COSPAS project at 121.5MHz, 243MHz and in the 406.0 – 406.1 MHz band and conducting of tests, mutual exchange of test results and preparation of a joint report. The objective of this cooperation is to demonstrate that equipment carried on low-altitude, near polar-orbiting satellites can facilitate the detection and location of distress signals by relaying information from aircraft and ships in distress to ground stations, where the information processing is completed and passed to rescue services."

"This joint Project will permit the Parties to make recommendations on follow-on global applications."

The first system satellite, "COSPAS-1" (Kosmos 1383), was launched from Plesetsk Cosmodrome on June 29, 1982.[36][37][38] Cospas-Sarsat began tracking the two original types of distress beacons, EPIRBs and ELTs, in September, 1982. The first persons were rescued with the assistance of Cospas-Sarsat when the distress signal from a small plane was relayed by the COSPAS-1 satellite to a then-experimental ground station in Ottawa, Ontario, Canada. The story has been related by the plane's pilot, Jonathan Ziegelheim, who rescue authorities judged would probably have died of his injuries if it were not for Cospas-Sarsat.[39][40][41]

Prior to the founding of Cospas-Sarsat, the civilian aviation community had already been using the 121.5 MHz frequency for distress, while the military aviation community utilized 243.0 MHz as the primary distress frequency with the 121.5 MHz frequency as an alternate. In each case, detection of the distress signal relied on reception by aircraft passing nearby, and localization of the signal was done with Earth-based direction finding equipment. Satellites made it possible to expand this "local" search paradigm into a global capability.

Each of the four founding Party States took responsibility for one of the major tasks in the project. The United States (with project leadership from NASA's Goddard Space Flight Center in Greenbelt, MD, USA) directed Datron Systems in Chatsworth, CA, USA to design and build LUT ground stations to receive the downlink from the satellites. At Datron, a team designed a LUT with five horn antennas, and Jeffrey Pawlan designed the downconverter and the specialized monopulse receiver capable of locking onto the downlink from the satellites. France and Canada were responsible for the data generation and decoding. They designed the computer that determined the approximate position of the beacon from the Doppler shift of the beacon's signal caused by the relative motion of the beacon and the receiving satellite. The former Soviet Union was responsible for the design and construction of the first satellite to be launched. Engineers from all four countries met in Moscow in February 1982 to successfully test the operational functionality of all of the equipment together in the same laboratory.

The Party States led development of the 406-MHz marine EPIRB, that used a digital messaging scheme, for detection by the system. The EPIRB was seen as a key advancement in SAR technology in the perilous maritime environment. The digital message allowed the beacon and its associated vessel to be uniquely identified. Early in its history, the Cospas-Sarsat system was engineered to detect beacon-alerts transmitted at 406 MHz, 121.5 MHz and 243.0 MHz. Because of a large number of false alerts, and the inability to uniquely identify such beacons because of their old, analogue technology (that provided no message, only a tone indicating distress), the Cospas-Sarsat system beginning in 2009 stopped receiving alerts from beacons operating at 121.5 MHz and 243.0 MHz, and now only receives and processes alerts from modern, digital 406-MHz beacons.

In the early 2000s (in 2003 in the USA) a new type of distress beacon, the personal locator beacon (PLB), became available[42] for use by individuals who cannot contact emergency services through normal telephone-originated services, such as 1-1-2 or 9-1-1. Typically PLBs are used by people engaged in recreational activities in remote areas, and by small-aircraft pilots and mariners as an adjunct to (or, when permitted, a substitute for) an ELT or EPIRB.

The design of distress beacons as a whole has evolved significantly since 1982. The newest 406-MHz beacons often incorporate global navigation satellite system (GNSS) receivers (such as those using GPS). Such beacons determine their location using the internal GNSS receiver (or a connection to an external navigation source) and transmit in their distress message highly accurate position reports. This provides a second method for Cospas-Sarsat to know the location of the distress, in addition to the calculations independently done by Cospas-Sarsat LUTs to determine the location. The distress alert received by the satellites and the beacon location contained in the message and/or calculated from the distress signal are forwarded almost instantly to SAR agencies by Cospas-Sarsat's extensive international data-distribution network. This two-tiered reliability and global coverage of the system has inspired the current motto of SAR agencies: "Taking the 'Search' out of Search and Rescue."[43]