Search and Rescue (SAR) is receiving a lot of attention recently due to several high-profile incidents on land, in the air and at sea. Flight MH370 left Kuala Lumpur was bound for Beijing in March 2014 when it disappeared, with 239 people on board. Even after the largest and most expensive search in aviation history, to date, neither any confirmed debris from the aircraft nor any survivors have been found.
After that incident, because of potential features of the new Cospas-Sarsat system, the International Civil Aviation Organization asked Cospas-Sarsat to work on the Emergency Locator Transmitters (ELT) capabilities, particularly during a distress event in order to avoid future incident like the one of MH370. Cospas-Sarsat, an international, humanitarian satellite based search and rescue system, has helped saved more than 44,000 lives since its inception in 1982. Cospas-Sarsat is a non-profit satellite-based search and rescue distress alert detection and information distribution system. It provides accurate, timely, and reliable distress alert and location data to SAR authorities, increasing the survival chances for people in distress by reducing the time it takes to locate them and relay this information to responders.
Established in 1979 by Canada, France, the USA and the former Soviet Union, the Cospas-Sarsat Programme currently has 45 countries and organisations that maintain, co-ordinate and operate the interoperable ground and space segments in line with Cospas-Sarsat specifications and performance standards. Today, about 1.4 million beacons are currently estimated to be in use worldwide, some 500,000 ships and 150,000 aircraft are equipped with COSPAS/SARSAT distress beacons.
Todate such signals have been tracked using hosted payloads on a combination of polar-orbiting Low Earth Orbit (LEO) satellites and Geostationary Orbit (GEO) satellites, but the international community is now implementing such capability on Global Navigation Satellite Systems (GNSS) in Medium Earth Orbit (MEO). The system is now undergoing a profound evolution, the future Cospas-Sarsat will rely on Satellites in Medium Earth orbit, MEOSAR and GEO, replacing the LEO/GEO design, and lead to improved performance, precision and responsiveness.
The MEOSAR system offer the advantages of both the LEOSAR and GEOSAR systems without their limitations by providing transmission of the distress message and independent location of the beacon, with near-real-time worldwide coverage. Users benefit from Global coverage; Single burst detection and location capability; Reduced detection time from hours to just minutes after the distress beacon is activated; Improved independent GNSS localisation of the distress alert under 5 km or better 95% of the time; Improved availability with increased satellite redundancy; Better signal detection in difficult terrain and weather conditions; and Automatic acknowledgment to the person in distress thanks to SAR/Galileo RLS. The MEO segment for SAR is also referred to MEOSAR. MEOSAR, when fully implemented, will be hosted on GNSS constellations including the U.S. Global Positioning System (GPS), Europe’s Galileo, Russia’s GLONASS, and possibly China’s BeiDou.
A particular area of interest to NASA is the use of MEOSAR for search and rescue of astronauts in the event of a launch-abort and during landing. This capability was demonstrated on Oct. 11, 2018, during the launch-abort of the Soyuz MS-10 carrying a new U.S.-Russia crew to the International Space Station. During this event the MEOSAR Local User Terminal (MEOLUT) at GSFC was the only ground station that received the Soyuz emergency beacon transmissions relayed from one Galileo and one GPS Distress Alerting Satellite System (DASS) -capable spacecraft. Initial location was determined upon parachute deployment, and final position was determined upon landing with a 1.6 km location error.
Canada’s Department of National Defence (DND) has a requirement for the procurement of the Space System for the Search of Vessels in Distress (COSPAS), Search and Rescue Satellite Aided Tracking (SARSAT) programme. It is an international initiative for the development of a coordinated satellite system for Search and Rescue (SAR). The objectives of the Canada Medium Earth Orbit Search and Rescue (MEOSAR) project are to implement and commission the Canadian contributions to the international COSPAS-SARSAT MEOSAR system in collaboration with the United States of America (USA). The MEOSAR Project is composed of 2 segments: a space segment and a ground segment. The work required under this procurement applies only to the space segment.
The mandate of the space segment of the MEOSAR project is to deliver up to 22 flight ready Search and Rescue/Global Positioning System (SAR/GPS) repeaters with a single flight ready spare repeater. These repeaters and associated test equipment will be delivered to the US Government for integration in the next generation of United States Air Force GPS-III satellites. The delivery of these SAR/GPS repeaters will fulfill Canada’s obligations with respect to the International COSPAS-SARSAT Programme Agreement (ICSPA). The expected duration of the project is fifteen years.
Existing Cospas-Sarsat System
Global Search and Rescue (SAR) operations quickly locate and help people in distress. Cospas-Sarsat Participants implement, maintain, co-ordinate and operate a satellite system capable of detecting distress alert transmission from radio beacons and of determining their position anywhere on the globe. The system is available to maritime and aviation users and to individual persons in distress situations on a non-discriminatory basis. It is free of charge for the end-user. The main components of the system:
- Distress beacons operating at 406 MHz (users);
- SAR payloads on satellites in low-altitude, medium and geostationary Earth orbit;
- Ground receiving stations (LUTs) spread around the world; and
- A network of Mission Control Centres (MCCs) to distribute distress alert and location information to SAR authorities, worldwide.
In simple terms these emergency transmitters send a signal that is picked up by one of the geostationary satellites (GEOSAR) which record the identity of the beacon and its position using the beacon’s built-in GPS, or by one of the polar-orbiting satellites (LEOSAR). The signal is relayed to a ground station which in turn is connected to professional search and rescue services. The system operates 24 hours a day, 365 days a year, providing accurate, timely and reliable distress alert and location data to help search and rescue authorities assist persons in distress. The alerts are provided to the SAR operation centers using the space and ground segments to detect, process and relay transmissions of the emergency beacons carried by the users
The Existing system, based on satellite constellation in both low Earth orbit (LEO) called Low Earth Orbiting Search And Rescue (LEOSAR) and geostationary Earth orbit (GEO) called Geostationary Orbiting Search And Rescue (GEOSAR), will be reaching end-of-life towards 2020. One drawback of the present system is that the person who has activated their emergency beacon has no confirmation that their signal has been received. It would obviously be a great comfort to that person to receive some sort of feedback and know that help was on its way.
System Description and Operation
Cospas-Sarsat system consists of emergency radio beacons, equipment on satellites in low-Earth polar or in geosynchronous orbits, ground receiving stations also called Local User Terminals (LUTs), Mission Control Centers (MCCs), and Rescue Coordination Centers (RCCs).The ground station, called a local user terminal or LUT, processes the signal (or the onboard processor telemetry), and calculates the position from which beacon signal originated. This position is transmitted to a mission control center, MCC, where it is combined with identification data and other information about that beacon.
The mission control center then transmits an alert message to the appropriate rescue coordination center, RCC, 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 center. The RCC then begins the actual search and rescue operation. User Segment Three primary types of beacons are used to transmit the distress signals 1) Emergency Position Indicating Radio Beacons (EPIRBs) for maritime applications, 2) Emergency Locator Transmitters (ELTs) for aviation applications, and 3) Personal Locator Beacons (PLBs) for individuals in distress.Rescue beacons transmit on 121.5, 243.0 and 406 MHz. The 121.5MHz is used as an homing signal internationally. The beacons may be manually or automatically activated, in the latter case by hydrostatic or gravity (G)-switch systems.
Space Segment The system uses two different types of satellites: polar-orbiting satellites in low-Earth orbit (LEO) and satellites in geosynchronous orbit.
Space Segment — LEO Component. The LEOSAR constellation consists of five satellites in three orbital planes. Their altitude is around 850 kilometers, with an inclination of 99 degrees from the equator. LEO-SATs complete an orbit in about 100 minutes, with each providing global coverage for 406 MHz distress signals about twice a day (twice a day at equator but every 100 minutes at the poles). LEO satellites may locate beacons almost anywhere thanks to Doppler Effect, however with a limited instantaneous coverage.
The Cospas-Sarsat LEO system uses two modes of operation. In the Bent-Pipe or repeater mode, the Search and Rescue Repeater, or SARR, immediately retransmits received beacon signals to any LUT in the satellite’s footprint. This mode is possible when the spacecraft is visible to both the beacon and the ground station simultaneously, an area approximately 2,500 miles (4,000 km) in diameter.
In the store and forward mode, the on-board processor, or SARP, receives, demodulates and measure FOA (Frequency of Arrival) of the received signals. All the data is stored in the internal memory until the visibility of Ground station where data can be downloaded. This mode is possible only with 406-MHz beacons. It provides true global coverage.
Space Segment — GEO Component. The GEO satellites have a very wide field of view (typical coverage from 70°N to 70°S), which offers real-time detection but no possibility of independent location as the Doppler Effect is negligible in GEOs, but for beacons equipped with a GNSS receiver, the encoded position can be retrieved from the alert message.
The LEOSAR Cospas-Sarsat system is able to locate a distress beacon independently by measuring successive transmissions (called “bursts”) of a beacon received at a satellite. The signals received by LEO satellites are relayed to a network of LUTs that locate the beacon by measuring the Doppler shift caused by the motion of the satellite with respect to the beacon. This process can locate beacons within an accuracy of 12.4 miles (20 km) for 121.5-MHz beacons and 3.1 miles (5 km) for 406-MHz beacons.
Beacons that transmit between 406.0 and 406.1 MHz send digitally encoded information which includes a beacon ID for accessing a user registration database and can also include location data derived from the Global Positioning System (GPS). Encoded location is of great value when using a geostationary (GEO) satellite for relaying beacon signals because a GEO satellite provides immediate alerting.
Existing microwave remote sensing instruments used for Earth observation face a clear tradeoff between spatial resolution and revisit times at global scales. The typical imaging capabilities of current systems range from daily observations at kilometer-scale resolutions provided by scatterometers to meter-scale resolutions at lower temporal rates (more than ten days) typical of synthetic aperture radars (SARs). A natural way to fill the gap between these two extremes is to use medium-Earth-orbit SAR (MEO-SAR) systems. MEO satellites are deployed at altitudes above the region of low Earth orbits (LEOs), ending at around 2000 km and below the geosynchronous orbits (GEOs) near 35 786 km. MEO SAR shows a clear potential to provide advantages in terms of spatial coverage, downlink visibility, and global temporal revisit times, e.g., providing moderate resolution images (some tens of meters) at daily rates.
MEOSAR includes global satellite coverage and near-instantaneous distress beacon detection (72 MEOSAR satellites vs. 12 today), more accurate beacon location calculations (by using 6 MEOSAR ground station antennas) and a unique Return Link Service feature that confirms receipt of the distress signal. With MEOSAR, a distress beacon can be located within 100 meters (328 feet), 95% of the time, and within 5 minutes instead of taking up to several hours today. Several countries are already using or implementing MEOSAR systems including two of the world’s most active SAR regions – the U.S. and, as announced recently, the southern Asia Pacific region of Australia/New Zealand.
The MEOSAR constellation is much bigger than the LEO and GEO systems. A beacon in the MEOSAR system can be located by lots of satellites at once who can then measure its transmission and triangulate the location. They will then be able to relay it to several ground antennas. This will improve the likelihood of detection and the accuracy of location determination.
One example of how much quicker this system is; in Australia MEO satellites were able to calculate the position of a non-GPS-equipped EPIRB in just 10 minutes, compared with 2-4 hours for the LEO system. The MEOSAR system benefits other too; for the SAR teams less time will be spent searching and more time rescuing, with the highly accurate search area coordinates. The new system design calls for equipping 14 GPS satellites with S-band repeaters, and 2 Galileo and 1 GLONASS spacecraft with L-band repeaters. When a distress signal is transmitted, all satellites in view of the beacon repeat the message, which is received by MEOLUT ground station. These stations, typically equipped with four or six directional antennas, are continuously tracking a subset of MEOSAR-capable GNSS satellites overhead. As numerous satellites will be visible above each Distress beacons will be detected and located more quickly and accurately than today, in as little as one beacon burst, that is, about 50 seconds.
The localization method changes from FOA-only using successive bursts of a distress beacon to a combination of TOA and FOA measurements based on one burst. (Two-dimensional position determination is possible when at least two TOA/FOA measurements are received correctly, but a minimum of three measurements is generally required to provide sufficient accuracy). Multiple bursts can still be used to refine the position of a beacon. With numerous satellites, each with Earth coverage or footprint significantly larger than the LEO satellites (about seven times larger), the MEOSAR constellations will enable an instantaneous and worldwide coverage.
• Because of the large number of satellites visible to a given beacon (up to 30), and thanks to position computation using FOA and TOA algorithms, the MEOSAR service can indicate the position of an endangered person in less than 10 minutes (versus 2 hours for the LEO and GEO generations). The more efficient alert notices that result will directly contribute to the efficiency of rescue operations where time is critical.
• Furthermore, the position-determination accuracy improves to just 200 meters, versus 5 kilometers previously. Another major advantage is that moving beacons (typically in aircraft) can be detected, which was not the case previously. MEOSAR services will start operating by the end of 2018, and will be able to detect the locations of aircraft in trouble during their flight.
• Robust beacon-to-satellite links provide high levels of satellite redundancy and availability, and significantly higher resilience to obstructions, such as terrain masking, for example. Indeed, the MEOSAR enhancement will benefit from the same geometry advantages (all in view and three satellite constellations) as the GNSS signals in L-band.
• The possible provision for additional (enhanced) SAR services, such as a return link to the beacon (in which the same satellite that receives a beacon burst repeats the distress signal and broadcasts the return link messages).
In light of this potential, the Cospas-Sarsat Council decided to replace the LEO space segment with a constellation of MEO satellites. GNSS satellites will carry signal repeaters to transmit distress signals to MEOLUT ground stations.
Second-Generation Beacons under MEOSAR
In parallel with the MEOSAR transition, operational requirements are under definition for a new generation of distress beacons. These second-generation beacons should ensure better system performance including detection probability, location accuracy and system capacity and allow for new purposes. The use of spread spectrum techniques for beacons in new MEOSAR system will improve dynamic location and also refine the estimation of the instantaneous speed and direction of the moving plane.
The second-generation beacon burst has three main parts:
• A preamble composed of a known PRN sequence is used for signal detection at MEOLUT level.
• A “useful message” (202 bits) contains all information needed by SAR responders, such as an identifier that gives information about the beacon and its owner available in a Cospas-Sarsat database. GNSS-encoded positions, if available, can also be transmitted in this part of the burst message to improve the accuracy of the beacon location.
• Finally, bits at the end of the burst are used for error correction, based on a BCH (250,202) code able to correct up to six bit errors.
However MEOSAR would also lead to reduced uplink communication link margins and less reliability and lack of detection expected at high elevation angles (Monopole antenna). It would also lead to more demanding Space Segment (min of 4 LEOSAR payloads vs. 24 MEOSAR payloads) resulting in higher cost to commission and operate the Space Segment infrastructure
Search and Rescue (SAR) / Galileo Service
The SAR/Galileo service, launched on 15 December 2016 as part of Galileo Initial Services, contributes to these live-saving efforts by swiftly relaying radio beacon distress signals to the relevant SAR crews by means of dedicated payloads on-board Galileo satellites, supported by three ground stations strategically deployed across Europe. On January 21 2020, the SAR/Galileo Return Link Service (RLS) was declared operational. Now, Galileo not only locates people in distress and makes their position known to the relevant authorities, the SAR/Galileo RLS provides an automatic acknowledgement message back to the user informing them that their request for help has been received.
The SAR/Galileo service is the biggest contributor to the Cospas-Sarsat MEOSAR programme in terms of ground segment and space segment assets. It provides the following two services:
- SAR/Galileo Forward Link: relay of Cospas-Sarsat 406Mhz distress signals to the ground;
- SAR/Galileo Return Link: unique return link alert that informs the sender that their distress alert has been received.
These SAR/Galileo services are fully integrated into the Cospas-Sarsat system. The SAR transponder on Galileo satellites picks up signals emitted from distress beacons in the 406 – 406.1 MHz band and broadcasts this information to dedicated ground stations (MEOLUTs) in the L-band at 1544.1 MHz. These downlink signals transmitted by the Galileo SAR payloads are used by the MEOLUTs to generate an independent location of the beacon, which is then relayed to first responders through dedicated Cospas-Sarsat Mission Control Centres.
With contributions from Galileo and other GNSS providers, Cospas-Sarsat has been able to transition from its original design based on satellites in low Earth orbits (LEOSAR), later complemented by geostationary orbit satellites (GEOSAR), towards MEOSAR – a solution based on medium orbit satellites such as Galileo.
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