5.2 Satellite Relay
The signals transmitted by the distress beacons are relayed through the satellites of the three unique Space Segments that are used by Cospas-Sarsat:
· The Low-altitude Earth Orbit Search and Rescue (LEOSAR)[1] system consists of satellites in relatively low (800 to 1,000 km) polar orbits.
· The Medium-altitude Earth Orbit Search and Rescue (MEOSAR)[2] system consists of satellites at altitudes around twenty thousand kilometres, in orbits that are inclined to the plane of the equator.
· The Geostationary Earth Orbit Search and Rescue (GEOSAR)[3] system consists of satellites in equatorial orbits at an altitude of approximately 36,000 kilometres.
The diagram in Figure 5-2 illustrates the relative altitudes of these different constellations. The onboard instruments are all designed to receive and to relay the beacon signals automatically as each signal arrives at the satellite.
At these different altitudes, the satellites of these constellations each have different areas of visibility on the surface of the Earth. Figure 5‑3 shows the circles that are visible from each of the different types of satellite.
Figure 5‑3: Satellite Visibility Areas
This illustration shows the visibility circles for the different types of satellites used in the Cospas‑Sarsat System. The radius of a visibility circle is: about 3,000 km for a LEOSAR satellite, about 8,000 km for a MEOSAR satellite, and about 9,000 km for a GEOSAR satellite. The upper illustrations show the individual satellite visibility circles (with the percentage of the Earth’s surface that each one covers). The final image shows the three different visibility circles superimposed on the same image of the Earth.
[1] The LEOSAR spacecraft, and their SAR payloads, are described in the document C/S T.003, “Description of the 406-MHz Payloads Used in the Cospas-Sarsat LEOSAR System”. The document C/S T.004, “Cospas-Sarsat LEOSAR Space Segment Commissioning Standard”, describes the tests that are performed to characterize each spacecraft before it is approved for use in the Cospas-Sarsat system.
[2] The MEOSAR spacecraft, and their SAR payloads, are described in the document C/S T.016, “Description of the 406-MHz Payloads Used in the Cospas-Sarsat MEOSAR System”. The document C/S T.017, “Cospas-Sarsat MEOSAR Space Segment Commissioning Standard” describes the tests that are performed to characterize each spacecraft before it is approved for use in the Cospas-Sarsat system.
[3] The GEOSAR spacecraft, and their SAR payloads, are described in the document C/S T.011, “Description of the 406-MHz Payloads Used in the Cospas-Sarsat GEOSAR System”. The document C/S T.013, “Cospas-Sarsat GEOSAR Space Segment Commissioning Standard”, describes the tests that are performed to characterize each spacecraft before it is approved for use in the Cospas-Sarsat system.