Introduction
The International Cospas-Sarsat Programme initiated the development of the Medium-altitude Earth Orbiting Satellite System for Search and Rescue (MEOSAR system) in 2004. Since then, search-and-rescue (SAR) repeater-payloads have been placed on the satellites of the Global Navigation Satellite Systems (GNSS) of Europe (Galileo), Russia (Glonass), the USA (GPS) and, most recently, China (BDS, also known as “BeiDou”). Following an early operational capability (EOC) phase, the initial operational capability (IOC) of the MEOSAR system was declared in April 2023. The full operational capability (FOC) of the system is anticipated to be declared in 2024. MEOSAR complements the existing LEOSAR (satellites in low-altitude orbits) and GEOSAR (satellites in geostationary orbit) systems, and gradually is replacing the LEOSAR system as the primary satellite architecture for Cospas-Sarsat.
The Cospas-Sarsat System
The Cospas-Sarsat System is comprised of:
- 406 MHz distress radiobeacons (including ELTs for aviation use, EPIRBs for maritime use, and PLBs for personal use), which transmit signals during distress situations,
- instruments on board satellites, which detect the signals transmitted by distress radiobeacons,
- ground receiving stations, referred to as Local Users Terminals (LUTs), which receive and process the satellite downlink signal to generate distress alerts, and
- Mission Control Centers (MCCs), which receive alerts produced by LUTs and forward them to Search and Rescue Points of Contact (SPOCs).
The current operational Cospas-Sarsat System includes three types of satellites:
- satellites in low-altitude Earth orbit (LEO), which form the LEOSAR system,
- satellites in geostationary Earth orbit (GEO), which form the GEOSAR system, and
- satellites in medium-altitude Earth orbit (MEO), which form the MEOSAR system currently operating with early operational capability (EOC).
Once fully operational, the MEOSAR system will provide global coverage and near-real-time beacon detection and independent location.
The MEOSAR System
Global Navigation Satellite System (GNSS) satellites orbit the Earth at an altitude between 19,000 and 23,000 km, a range considered to be medium-altitude Earth orbit. Hence, this component of Cospas-Sarsat is known as the Medium-altitude Earth Orbit Search and Rescue system, or MEOSAR. It complements the existing LEOSAR and GEOSAR systems.
The current LEOSAR and GEOSAR systems that detect and locate distress beacons have shortcomings that MEOSAR overcomes. The GEOSAR system constantly covers the entire Earth except the high-latitude (i.e., polar) regions. Although the GEOSAR system can nearly instantaneously receive beacon distress messages across most of the globe, it cannot locate a beacon unless the location is encoded in the beacon’s message from a navigation (GNSS) receiver inside or connected to the beacon. The LEOSAR system can locate a beacon without location information being transmitted in the beacon message (or can confirm the location even if position information is transmitted in the beacon message), but the LEOSAR satellites have a view of only a small part of the Earth at any given time, which at times creates a delay in the distress signal reaching a ground station. While LEOSAR and GEOSAR still provide valuable search-and-rescue capabilities, MEOSAR is a revolution in system architecture.
The MEOSAR system architecture offers the advantages of both the LEOSAR and GEOSAR systems without their limitations by providing reliable reception of distress messages and independent localization of beacons, with near-real-time worldwide coverage. The large number of MEOSAR satellites allows each distress message to be relayed at the same time by several satellites to several ground antennas, improving the likelihood of quick detection and improving the accuracy of the location determination. The MEOSAR system also facilitates other planned enhancements for Cospas-Sarsat beacons, such as a return-link-service (RLS) transmission to a distress beacon that provides the user with a confirmation that the distress message has been received and the beacon’s location determined.
At the beginning of 2013, Cospas-Sarsat entered a Demonstration and Evaluation (D&E) phase for the MEOSAR system that showed MEOSAR’s capabilities and promise for the future. The early operational capability (EOC) phase was declared for the MEOSAR system in December 2016, which allowed operational use of MEOSAR alerts. The current phase of the MEOSAR initial operational capability (IOC) phase, where distress alerts from the MEOSAR system are provided to SAR authorities with improved performance, began in April 2023. When enough commissioned ground segment (MEOLUTs and MCCs) is available to provide worldwide, near-real-time coverage, the MEOSAR system will be declared at full operational capability (FOC), which is anticipated in 2024.
The MEOSAR system is backward-compatible with all 406-MHz Cospas-Sarsat beacons that have been supported by the LEOSAR and GEOSAR systems for decades.
The MEOSAR System Concept
