The basic Cospas-Sarsat concept is illustrated in the adjacent figure. The System is composed of:
distress radiobeacons (ELTs for aviation use, EPIRBs for maritime use, and PLBs for personal use) which transmit signals during distress situations;
instruments on board satellites in geostationary and low-altitude Earth orbits 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 Rescue Coordination Centers (RCCs), Search and Rescue Points Of Contacts (SPOCs) or other MCCs.
The Cospas-Sarsat System includes two 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
The future Cospas-Sarsat System will include a new type of satellite in the medium-altitude Earth orbit (MEO) which will form the MEOSAR System.
Additional information on the three satellite systems, the LUTs, and the MCCs is provided in the tabs below.
- Local User Terminals (LUTs)
- Mission Control Centres (MCCs)
Cospas-Sarsat has demonstrated that the detection and location of 406 MHz distress beacon signals can be greatly facilitated by global monitoring based on low-altitude spacecraft in near-polar orbits. Complete, yet non continuous coverage of the Earth is achieved using simple emergency beacons operating on 406 MHz to signal a distress. The coverage is not continuous because polar orbiting satellites can only view a portion of the Earth at any given time (see figure at left). Consequently the System cannot produce distress alerts until the satellite is in a position where it can "see" the distress beacon. However, since the satellite onboard 406 MHz processor includes a memory module, the satellite is able to store distress beacon information and rebroadcast it when the satellite comes within view of a LUT, thereby providing global coverage.
As described above, a single satellite, circling the Earth around the poles, eventually views the entire Earth surface. The "orbital plane", or path of the satellite, remains fixed, while the Earth rotates underneath it. At most, it takes only one half rotation of the Earth (i.e. 12 hours) for any location to pass under the orbital plane. With a second satellite, having an orbital plane at right angles to the first, only one quarter of a rotation is required, or 6 hours maximum. Similarly, as more satellites orbit the Earth in different planes, the waiting time is further reduced. The Cospas-Sarsat System design constellation is four satellites which provide a typical waiting time of less than one hour at mid-latitudes.
The LEOSAR system calculates the location of distress events using Doppler processing techniques. Doppler processing is based upon the principle that the frequency of the distress beacon, as "heard" by the satellite instrument, is affected by the relative velocity of the satellite with respect to the beacon. By monitoring the change of the beacon frequency of the received beacon signal and knowing the exact position of the satellite, the LUT is able to calculate the location of the beacon.
The GEOSAR system consists of 406 MHz repeaters carried on board various geostationary satellites, and the associated ground facilities called GEOLUTs which process the satellite signal.
As a GEOSAR satellite remains fixed relative to the Earth, there is no Doppler effect on the received frequency and Doppler radio location positioning techniques cannot be used to locate distress beacons. To provide rescuers with beacon position information, such information must be either:
acquired by the beacon through an internal or an external navigation receiver and encoded in the beacon message, or
derived, with possible delays, from the LEOSAR System.
Cospas-Sarsat is in the process of upgrading its satellite system by placing search-and-rescue receivers (i.e., repeaters or transponders) on new GPS satellites operated by the United States, navigation satellites of Russia (GLONASS) that began deployment last year, and European GALILEO navigation satellites that began launching 12 October 2012. Once qualified as operational, this system augmentation will dramatically improve both the speed and location-accuracy for detecting beacons.
Those satellites orbit the Earth at an altitude between 19,000 and 24,000 km, a range considered as 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 will complement the existing LEOSAR and GEOSAR systems.
The current LEOSAR and GEOSAR systems contribute respective advantages to detection and location of distress beacons that have been activated. The GEOSAR system constantly covers the entire Earth except the high-latitude (e.g., polar) regions. While the GEOSAR system can receive beacons distress messages across most of the globe, it cannot locate the beacon unless the location is encoded in the beacon’s message from a local navigation receiver. The LEOSAR system can locate a beacon without the aid of a GPS or other navigation signal to the beacon, but the LEOSAR satellites have a view of only a small part of the Earth at any given time, so there may be a delay in receiving the distress signal over LEOSAR.
Once fully operational, the MEOSAR system will offer the advantages of both LEOSAR and GEOSAR systems without their current limitations by providing transmission of the distress message, and independent location of the beacon, with a near real time worldwide coverage.
The MEOSAR system also will facilitate otherplanned enhancementsfor Cospas-Sarsat beacons, such as a return link transmission that will allow the beacon to provide to the user a confirmation that the distress message has been received.
The large number of MEOSAR satellites that will be in orbit when the system is fully operational will allow each distress message to be relayed at the same time by several satellites to several ground antennas, improving the likelihood of detection and the accuracy of the location determination.
At the beginning of 2013, Cospas-Sarsat will enter a Demonstration and Evaluation (D&E) phase for the MEOSAR system aiming to show that the MEOSAR performance meets the foreseen expectations, and that distress alerts received by SAR authorities from the MEOSAR system have the required reliability and accuracy.
The MEOSAR D&E phase is planned to end in 2015 and will be followed by the MEOSAR Initial Operational Capability (IOC) phase, in which distress alerts provided by the MEOSAR system will be operationally used by the SAR authorities. When enough MEOSAR satellites and commissioned ground stations are available to provide worldwide, real time coverage, the MEOSAR system will be declared at its Full Operational Capability (FOC).
The MEOSAR System Concept
There are two types of LUTs in the Cospas-Sarsat System: those that are designed to operate with the LEOSAR satellite constellation are referred to as LEOLUTs, and those that operate with the GEOSAR satellite constellation are referred to as GEOLUTs.
LEOLUT and GEOLUT operators are expected to provide the SAR community with reliable alert and location data, without restriction on use and distribution. The Cospas-Sarsat Parties providing the space segment, supply LEOLUT and GEOLUT operators with System data required to operate their LUTs. To ensure that data provided by LUTs are reliable and can be used by the SAR community on an operational basis, Cospas-Sarsat has developed LUT performance specifications and procedures. Copies of the LEOLUT and GEOLUT specifications (documents C/S T.002 and C/S T.009) and commissioning standards (documents C/S T.005 and C/S T.010) are available for download under the "System Documents" section of the Professionals website (Pro/Documents).
The configuration and capabilities of each LEOLUT may vary to meet the specific requirements of the participating countries, but the Cospas and Sarsat LEOSAR spacecraft downlink signal formats ensure interoperability between the various spacecraft and all LEOLUTs meeting Cospas-Sarsat specifications.
The capability of a LEOLUT is determined, for the most part, by the LEOSAR satellite channels it was designed to process. There are a possible 2 channels that may, depending upon the specific satellite being tracked, be available for processing. Some satellites support all the channels listed below, and some only support a limited set of them.
The 406-MHz Search and Rescue Processor (SARP) satellite channel transmits received 406-MHz beacon data that has already been partially processed by the satellite to determine the identification, transmit time, and received frequency for each distress beacon transmission burst. Because of the on-board memory capability of the SARP channel, this channel provides global (yet not continuous) coverage for distress beacons that operate at 406 MHz.
The 406-MHz Search and Rescue Repeater (SARR) channel receives 406-MHz beacon transmission bursts and immediately retransmits them on the satellite downlink. Since there is no memory associated with the repeater channel, this type of processing supports only local mode coverage (i.e., the distress beacon and the LEOLUT must be in simultaneous view of the satellite for a period of time). Furthermore, since the satellite does not process the data, all the processing is performed by the LEOLUT.
For 406-MHz signals received via their respective SARR channel, each transmssion is detected and the Doppler information calculated. A beacon position is then determined using this data. The LUT is also able to provide identification information associated with the beacon.
Processing the SARP channel 2400-bps data (i.e., those generated from 406-MHz transmissions) is relatively straightforward since the Doppler frequency is measured and time-tagged on-board the spacecraft. All 406-MHz data received from the satellite memory on each pass can be processed within a few minutes of pass completion.
To maintain accurate location processing, an update of the satellite ephemeris is produced each time the LUT receives a satellite signal. The downlink carrier is monitored to provide a Doppler signal using the LUT location as a reference, or highly stable 406-MHz calibration beacons at accurately known locations are used to update the ephemeris data.
A GEOLUT is a ground receiving station in the Cospas-Sarsat System that receives and processes 406-MHz distress beacon signals which have been relayed by a Cospas-Sarsat geostationary satellite. Due to the extremely large continuous coverage footprint provided by each geostationary satellite, GEOLUTs are able to produce near instantaneous alerting over extremely large areas. However, due to the fact that the satellite remains stationary with respect to distress beacons, GEOLUTs are not able to determine beacon locations using Doppler processing techniques. In view of this, 406-MHz beacons with location protocols allow for the encoding of position data in the transmitted 406-MHz message, thus providing for quasi-real time alerting with position information via the GEOSAR system.
The "GEOLUT Availability Table" provides an indication of which GEOSAR satellite is tracked by which specific GEOLUT (Pro/System/System Monitoring/Availability Tables (QMS)).
MCCs have been set up in most countries operating at least one LUT. 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 to associated RCCs or SPOCs.
Most of the data fall into two general categories : alert data and system information.
Alert data is the generic term for Cospas-Sarsat 406 MHz data derived from distress beacons. 406 MHz beacons alert data comprise the beacon location and coded information.
System information is used primarily to keep the Cospas-Sarsat System operating at peak effectiveness and to provide users with accurate and timely alert data. It consists of satellite ephemeris and time calibration data used to determine beacon locations, the current status of the space and ground segments and coordination messages required to operate the Cospas-Sarsat System.
All MCCs in the System are interconnected through appropriate networks for the distribution of System information and alert data. To ensure data distribution reliability and integrity, Cospas-Sarsat has developed MCC performance specifications (document C/S A.005) and MCC commissioning procedures (document C/S A.006). Reports on MCC operations are provided by MCC Operators on an annual basis. World-wide exercises are performed from time to time to check the operational status and performance of all LUTs and MCCs, and data exchange procedures.