The Cospas-Sarsat 406 MHz System

The Cospas-Sarsat system detects and locates distress beacons operating at 121.5 MHz and 406 MHz.

The 406 MHz system is composed of:

406 MHz radiobeacons carried aboard ships (EPIRBs), aircraft (ELTs), or used as personal locator beacons (PLBs);
polar-orbiting satellites in low Earth orbit from the LEOSAR system and geostationary satellites from the GEOSAR system; and
the associated LUTs for the respective satellite systems (referred to as LEOLUTs for the LEOSAR system and GEOLUTs for the GEOSAR system).

406 MHz Beacons

Frequencies in the 406.0 - 406.1 MHz band have been exclusively reserved for distress beacons operating with satellite systems. 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.

Second generation 406 MHz beacons have been introduced since 1997 which allow for the transmission in the 406 MHz message of encoded position data acquired by the beacons from global satellite navigation systems, using internal or external navigation receivers. This feature is of particular interest for GEOSAR alerts which, otherwise, would not be able to provide position information.

406 MHz LEOSAR System

The Cospas-Sarsat 406 MHz LEOSAR system uses the same polar-orbiting satellites as the 121.5 MHz system and, therefore, operates with the same basic constraints which result from non-continuous coverage provided by LEOSAR satellites, although with significantly improved performance resulting from the improved beacon technical characteristics. 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. The Cospas-Sarsat 406 MHz LEOSAR system operates in two coverage modes, namely local and global coverage.

406 MHz LEOSAR Local Mode

When the satellite receives 406 MHz beacon signals, the on-board Search and Rescue Processor (SARP) recovers the digital data from the beacon signal, measures the Doppler frequency shift and time-tags the information. The result of this processing is formatted as digital data which is transferred to the satellite downlink for transmission to any LEOLUT in view. This data is also simultaneously stored on the spacecraft for later transmission and ground processing in the global coverage mode.

The diagram to the left depicts a LEOSAR satellite orbiting the Earth and its instantaneous field of view is indicated by the red circle. In this example the beacon located in the Northern Atlantic is within the local coverage area of the LEOLUT located on the north west coast of Africa whereas the beacon located in Antarctica is not.

In addition to the 406 MHz local mode provided by the 406 MHz SARP instrument, a 406 MHz repeater, on Sarsat satellites only, can also provide a 406 MHz local mode of operation. The difference between the SARP and the repeater is that the SARP performs some of the processing onboard the satellite, whereas the repeater simply reflects the beacon signal to the Earth, thereby requiring additional processing on the ground.

406 MHz LEOSAR Global Mode

The 406 MHz SARP system provides global coverage by storing data derived from onboard processing of beacons signals, in the spacecraft memory unit. The content of the memory is continuously broadcast on the satellite downlink. Therefore, each beacon can be located by all LEOLUTs which track the satellite (even for LEOLUTs which were not in the footprint of the satellite at the time the beacon was detected by the satellite). This provides the 406 MHz global coverage and introduces ground segment processing redundancy.

The diagram to the right depicts a LEOSAR satellite orbiting the Earth in the direction of the north pole. The blue circle represents the satellite field of view at a point in the recent past when the satellite was over the southern Atlantic Ocean. At that point in time the satellite detected the 406 MHz beacon in Antarctica, however, since there were no LEOLUTs in its field of view, a distress alert could not be generated at that time. Nevertheless, the satellite continued to transmit the processed data associated with this distress beacon. When the LEOLUT located on the north west coast of Africa came into the view of the satellite, this LEOLUT received the beacon information and generated a distress alert.

The 406 MHz global mode may also offer an additional advantage over the local mode in respect of alerting time. As the beacon message is recorded 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 could be considerably reduced.

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

406 MHz 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 36,000 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 degrees North and 70 degrees South latitude.

Since GEOSAR satellites remain fixed relative to the Earth, there is no Doppler effect on the received frequency and, therefore, the Doppler positioning technique cannot be used to locate distress beacons. To provide rescuers with position information, the beacon location must be either:

acquired by the beacon though an internal or an external navigation receiver and encoded in the beacon message, or
derived from the LEOSAR system Doppler processing.

Cospas-Sarsat has demonstrated that the GEOSAR and LEOSAR system search and rescue capabilities are complementary. For example the GEOSAR system can provide almost immediate alerting in the footprint of the GEOSAR satellite, whereas the LEOSAR system:

provides excellent coverage of the polar regions (which are beyond the coverage of geostationary satellites);
can calculate the location of distress events using Doppler processing techniques; and
is less susceptible to obstructions which may block a beacon signal in a given direction because the satellite is continuously moving with respect to the beacon.