The Cospas-Sarsat Low Earth Orbit Search and Rescue (LEOSAR) System
The detection and location of 406 MHz distress beacon signals can be greatly facilitated by global monitoring based on low-altitude satellites 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 non continuous aspect of the coverage occurs because the polar orbiting satellites can only view a portion of the earth at any given time. 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 (ground receiving stations), thereby providing global coverage.
The Cospas-Sarsat LEOSAR system provides global coverage for 406 MHz beacons. The white region indicates the area where an over-flying satellite could be seen by a LEOLUT.
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.
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 beacons located on the southern tips of South America and Africa are 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 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 on the southern tip of South America, 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 also may 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.