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COSPAS-SARSAT

Besides INMARSAT, the COSPAS-SARSAT satellite system is another part of the space segment of the Global Maritime Distress and Safety System (GMDSS). COSPAS-SARSAT is an international satellite-based Search-and-Rescue system established by Russia, Canada, USA, Denmark and France to locate emergency radio beacons transmitting on 406 MHz. In contrast to INMARSAT, the COSPAS-SARSAT system has been conceived to cover also the remote polar regions of Sea Area 4.




System Concept

The basic COSPAS-SARSAT concept is illustrated below.

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The system is composed of:

  • mobile distress radio beacons (ELTs for aviation use, EPIRBs for maritime use, and PLBs for personal use) which transmit signals during distress situations
  • satellites equipped with instruments which detect the signals transmitted by the mobile distress radio beacons
  • ground receiving stations, referred to as Local Users Terminals (LUTs), which receive and process the satellite downlink signal to generate distress alerts
  • Mission Control Centres (MCCs) which receive the alerts produced by LUTs and forward them to Rescue-Coordination Centres (RCCs), Search-and-Rescue Centres or other MCCs.



Satellite Configuration

The COSPAS-SARSAT system space segment consists of SAR signal processors (SARP) aboard of satellites, which can detect and process the distress signals transmitted by emergency beacons operating on 406 MHz. In order to provide world-wide coverage, the COSPAS-SARSAT system uses complementary satellite systems. The system currently consists of:

  • 5 low-earth polar orbit satellites composing the LEOSAR system, and
  • 4 geosynchronous satellites forming the GEOSAR system.

But it is investigated to add a new capability to the COSPAS-SARSAT system called MEOSAR (Medium Earth Orbit Search and Rescue satellites). This system will put SAR processors aboard the GPS satellite constellation and the Galileo positioning system constellation. MEOSAR satellites will be able to send information (e.g. distress acknowledgment) back to the distress radio beacon via the GPS or Galileo downlink.

The COSPAS-SARSAT system benefits from the complementary capabilities of the GEOSAR and LEOSAR system. The GEOSAR system can provide almost immediate alerting for locations in the coverage area of the GEOSAR satellite, whereas the LEOSAR system can provide full coverage of the polar stations. Since LEOSAR satellites are not stationary they can also calculate the location of distress events using Doppler processing techniques. Also because the LEOSAR satellites are continuously moving with respect to the beacon, the satellite connections are less susceptible to obstructions which may block a beacon signal in a given direction.
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The LEOSAR System

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The LEOSAR system is currently composed of 5 satellites in a low-altitude Earth orbit (LEO). These satellites are on polar orbits. The LEOSAR system thus can provide coverage of the polar regions, which are beyond the coverage of geostationary satellites. Each satellite makes a complete orbit around the poles in about 100 minutes. Their orbits are fixed in space at an altitude of 850 km. While they circle the globe, the Earth rotates underneath them, so that each pass gives a scan shifted by about 25° to the west. On an altitude of 850 to 1000 km, the LEOSAR satellites have a "footprint" that is approximately 6000 km wide.
The use of low-altitude orbiting satellites provides for a strong Doppler effect in the up-link signal thereby enabling the use of Doppler positioning techniques.

Up until mid-2007, two of the LEOSAR satellites were COSPAS satellites provided and operated by the Russian Federation. However, they were decommissioned on 6 August 2007, and currently (2008), the American NOAA Polar Operational Environmental Satellites (POES) and the EUMETSAT MetOp-A are the only LEOSAR satellites in service.

LEOSAR satellites are monitored by 44 world-wide distributed LEOLUTs (low Earth orbit local user terminals). The LEOSAR satellites operate in a store-and-forward mode for 406 MHz signals - they store distress signals and forward them to the next LEOLUT ground station they overfly. This technique results in two different coverage modes for the LEOSAR system: local coverage and global coverage.
In local coverage mode, the LEOSAR satellite has contact to at least one of the LEOLUTs so that when a beacon signal is received, the decoded information can be sent immediately to the LEOLUT(s). In global coverage mode, there the LEOSAR satellite has currently no contact to a GEOLUT, so the received an decoded beacon information must be stored until a GEOLUT appears within the footprint of the satellite. Only then the stored information can be forwarded.

Many regions on Earth are covered in local mode, but this depends on the current satellite positions and the local coverage is not continuous. Therefore, global coverage mode may often result in a shorter alerting time. As the beacon message is recorded and stored in the satellite memory by the first satellite pass which detected the beacon, the waiting time is not dependent upon the satellite achieving simultaneous visibility with the LEOLUT and the beacon. Consequently, the time required to produce alerts is considerably reduced. Once a 406-MHz distress signal is detected and stored by a satellite, the satellite will send this data to all LEOLUTs it may contact for 24 hours.

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The animated graphic above depicts two beacons: the yellow beacon is detected in global mode only whereas the red beacon is detected in both local and global modes.



The GEOSAR System

COSPAS-SARSAT has demonstrated that the current generation of COSPAS-SARSAT 406-MHz beacons could be detected using search and rescue instruments on board geostationary satellites. 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.

Geostationary satellites orbit the Earth at an altitude of 36000 km, with an orbit period of 24 hours, thus appearing fixed relative to the Earth at approximately 0 degrees latitude (i.e. over the equator). A single geostationary satellite provides GEOSAR uplink coverage of about one third of the globe, except for polar regions. Therefore, three geostationary satellites equally spaced in longitude can provide continuous coverage of all areas of the globe between approximately 70° North and 70° South latitude.

SARP are currently (2010) installed on the following 5 geostationary satellites:

  • The GOES (USA) geostationary satellites GOES-East at 75° W and GOES-West at 135° W
  • The INSAT-3A (India) geostationary satellite at 94° E
  • The Meteosat (EU) geostationary satellites MSG-1 at 10° E and MSG-2 fixed at 00° (Prime Meridian)

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The GEOSAR satellites are currently (2010) monitored by about 17 GEOLUTs (Geostationary Earth Orbit Local User Terminals). 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 GEOSAR 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 following Table summarizes the LEOSAR and GEOSAR features and capabilities:

Channel Processing LEOSAR GEOSAR
406.- MHz Beacon identification provided,
Beacon location information calculated by measurement of Doppler shift
Beacon identification provided,
Beacon location information available if encoded in beacon message
Global coverage,
but not instantaneous
Near instantaneous alerting,
but only in the GEOSAR coverage area
121.5 MHz Beacon identification not provided
Beacon location information available
Local mode coverage only
Not supported

Important Notice: Processing of 121.5 MHz beacons was terminated on 1 February 2009

This change, was brought about by the unreliability of the 121.5/243 MHz beacons in an emergency situation. Data reveals that with a 121.5 MHz beacon, only one alert out of every 50 is a genuine distress situation. This has a significant effect on expending the limited resources of Search-and-Rescue personnel and platforms.
When a 406 MHz beacon signal is received, search and rescue personnel can retrieve information from a registration database. This includes the beacon owner's contact information, emergency contact information, and vessel/aircraft identifying characteristics. With 406 MHz beacons, false alerts have been reduced significantly, and, when properly registered, can usually be resolved with a telephone call to the beacon owner. Consequently, real alerts can receive the attention they deserve.




Emergency Position Indicating Radio Beacon (EPIRB)

EPIRBS are for use in maritime applications and are designed to operate with the COSPAS-SARSAT system. From 2009 on, the COSPAS-SARSAT system will detect and locate only distress beacons operating at 406 MHz. Service for the older beacons operating at 121.5 MHz will terminate on 1. February 2009. But 406-MHz EPIRBs will continue to transmit a 121.5-MHz "homing" signal after activation, allowing overflying aircrafts and rescue crafts to locate the vessel in distress.

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The COSPAS-SARSAT 406-MHz beacons have been specifically designed for use with the LEOSAR system to provide improved performance in comparison to the older 121.5-MHz beacons. They are more sophisticated because of the specific requirements on the stability of the transmitted frequency, and the inclusion of a digital message which allows the transmission of encoded data such as unique beacon identification. The COSPAS-SARSAT 406-MHz beacons also allow for the transmission of encoded position data acquired by the beacons from global satellite navigation systems, using internal or external navigation receivers (GPS). This feature is of particular interest for GEOSAR alerts which, otherwise, would not be able to provide position information.

Beacon technical information

There are two categories of 406-MHz EPIRBs:

  • Category I EPIRBs are activated either manually or automatically. The automatic activation is triggered when the EPIRB is released from its bracket. Category I EPIRBs are housed in a special bracket equipped with a hydrostatic release. This mechanism releases the EPIRB at a water depth of 3-10 feet. The buoyant EPIRB then floats to the surface and begins transmitting.
  • Category II EPIRBs are manual activation only units. These should be stored in the most accessible location on board where it can be quickly accessed in an emergency.
Typical rescue beacon radios transmit a 5 watt signal for 0.5 second once every 50 seconds. According to the SARSAT system specifications, the carrier frequency of the beacons must very stable. This improves the Doppler location accuracy in the LEOSAR satellites. Nevertheless, most of the EPIRB beacons sold today include a GPS receiver so they can report their precise GPS-derived location.
The 406-MHz data channel is 170 kHz wide and centred at 406.05 MHz. On this channel, a burst of digital data is transmitted with a bit rate of 400 bps using phase modulation. Each transmission consists of a 160 ms CW preamble followed by a data message. The data message is either a 112-bit "short" message (280 ms) or a 144-bit "long" message (360 ms) both including full identification and position information. The operating life time of the beacons will be at least 24 hours in activated mode at a temperature of -20°C.
An auxiliary transmitter (homing transmitter) can be included in the 406-MHz beacon to enable SAR forces to home on the distress beacon.


Testing EPIRBs

406 MHz EPIRBs can be tested through its self-test function, which is an integral part of the device. 406-MHz EPIRBs can also be tested inside a container designed to prevent its reception by the satellite. Testing a 406-MHz EPIRB by allowing it to radiate outside such a container is illegal.


Battery Replacement

406-MHz EPIRBs use a special type of lithium battery designed for long-term low-power consumption operation. Batteries must be replaced by the date indicated on the EPIRB label using the model specified by the manufacturer. It should be replaced by a dealer approved by the manufacturer. If the replacement battery is not the proper type, the EPIRB will not operate for the duration specified in a distress.


Disposal of EPIRBs

Guidelines for disposal of EPIRBs have been developed by the International Maritime Organisation (IMO) in conjunction with COSPAS-SARSAT. This guideline includes the request that retailers and boat-owners should ensure that when an EPIRB needs to be disposed of, the satellite EPIRB should be made inoperable, by removing its battery and returning it to the manufacturer. If the EPIRB is returned to the manufacturer with the battery still installed, it should be wrapped in tin foil to prevent inadvertent transmission of signals during shipment.
Note that EPIRBS are normally registered and that upon disposal of the device also the registration should be cancelled.


Registration of 406 MHz EPIRBs

Proper registration of 406-MHz satellite emergency position-indicating radio beacon (EPIRB) is mandatory and necessary in order for the beacon distress to be forwarded to the appropriate Maritime Rescue Coordination Centres (MRCCs). Registered emergency information will be used by the RCCs to verify that a distress situation exists, and in arranging appropriate rescue efforts.


EPIRB alerts picked up by the GEOSAR satellite system will be relayed to the MRCCs well before the international COSPAS-SARSAT satellite can provide location information. If the EPIRB is properly registered, the MRCC will be able to use the registration information to immediately begin action on the case. If the EPIRB is unregistered, a distress alert may take as much as two hours longer to reach the MRCC over the COSPAS-SARSAT satellite system. If an unregistered EPIRB transmission is abbreviated for any reason, the satellite will be unable to determine the EPIRB's location, and the MRCC will be unable to respond to the distress alert.


The registration information is entered into the Beacon Registration Database maintained by NOAA/NESDIS. If your EPIRB is activated, your registration information will be sent automatically to the appropriate SAR Rescue Coordination Center (RCC) for response. One of the first things the RCC watch standers do is attempt to contact the owner/operator at the phone number listed in the database to determine if the vessel is en-route (thus ruling out the possibility of a false alarm due to accidental activation or EPIRB malfunction), the intended route of the vessel if underway, the number of people on board, etc., from a family member. If there is no answer at this number, or no information, the other numbers listed in the database will be called to attempt to get the information described above needed to assist the RCC in responding appropriately to the EPIRB alert.

Obviously, it is very important to keep the registered data in the EPIRB database up to date. If any of the registered EPIRB data (owner, vessel, address, emergency phone numbers, ...) is changed, the responsible owner MUST re-register the EPIRB. If an EPIRB is sold, the former owner should make sure that the purchaser re-registers the EPIRB, or he may be made responsible if it later becomes activated.




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