02-01-2013, 01:06 PM
Introduction to Satellite Communications
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Important Milestones (1970’s) GEO applications development
1972 First domestic satellite system operational (Canada). INTERSPUTNIK founded.
1975 First successful direct broadcast experiment (one year duration; USA-India).
1977 A plan for direct-to-home satellite broadcasting assigned by the ITU in regions 1 and 3 (most of the world except the Americas).
1979 International Mobile Satellite Organization (Inmarsat) established.
1990-95:
- Several organizations propose the use of non-geostationary (NGSO) satellite systems for mobile communications.
- Continuing growth of VSATs around the world.
- Spectrum allocation for non-GEO systems.
- Continuing growth of direct broadcast systems. DirectTV created.
1997:
- Launch of first batch of LEO for hand-held terminals (Iridium).
- Voice service telephone-sized desktop and paging service pocket size mobile terminals launched (Inmarsat).
1998: Iridium initiates services.
1999: Globalstar Initiates Service.
2000: ICO initiates Service. Iridium fails and system is sold to Boeing.
A Highly Elliptical Orbit (HEO)
A satellite in HEO typically has a perigee at about 500 km above the surface of the Earth and an apogee as high as 50,000 km. The orbit is usually inclined at 63.4 deg to provide communications services to locations at high northern latitudes. This inclination value is selected to avoid rotation of the apses; thus, a line from the Earth's center to the apogee always intersects the Earth's surface at a latitude of 63.4 deg North. Orbit period varies from eight to 24 hours. Owing to the high eccentricity of the orbit, a satellite spends about two-thirds of the orbital period near apogee, during which time it appears to be almost stationary to an observer on the Earth (a phenomenon known as `apogee dwell'). During the brief time the satellite is below the local horizon, a hand-off to another satellite in the same orbit is required in order to avoid loss of communications. Free space loss and propagation delay for this type of orbit are comparable to that of geosynchronous satellites. However, due to the comparatively great movement of a satellite in HEO relative to an observer on the Earth, satellite systems using this type of orbit must cope with large Doppler shifts.
A Medium-Earth Orbit (MEO)
By setting the altitude parameters at 10,000 km, you generated a medium-Earth orbit (MEO). This one happens to be an Intermediate Circular Orbit (ICO), since the apogee and perigee are equal. Its orbit period measures about seven hours. The maximum time during which a satellite in MEO orbit is above the local horizon for an observer on the Earth is a few hours. A global communications system using this type of orbit requires relatively few satellites in two to three orbital planes to achieve global coverage. MEO systems operate similarly to LEO systems. In MEO systems, however, hand-over is less frequent, and propagation delay and free space loss are greater. Examples of MEO (specifically ICO) systems are Inmarsat-P (10 satellites in 2 inclined planes at 10,355 km), and Odyssey (12 satellites in 3 inclined planes, also at 10,355 km).
[attachment=46151]
Important Milestones (1970’s) GEO applications development
1972 First domestic satellite system operational (Canada). INTERSPUTNIK founded.
1975 First successful direct broadcast experiment (one year duration; USA-India).
1977 A plan for direct-to-home satellite broadcasting assigned by the ITU in regions 1 and 3 (most of the world except the Americas).
1979 International Mobile Satellite Organization (Inmarsat) established.
1990-95:
- Several organizations propose the use of non-geostationary (NGSO) satellite systems for mobile communications.
- Continuing growth of VSATs around the world.
- Spectrum allocation for non-GEO systems.
- Continuing growth of direct broadcast systems. DirectTV created.
1997:
- Launch of first batch of LEO for hand-held terminals (Iridium).
- Voice service telephone-sized desktop and paging service pocket size mobile terminals launched (Inmarsat).
1998: Iridium initiates services.
1999: Globalstar Initiates Service.
2000: ICO initiates Service. Iridium fails and system is sold to Boeing.
A Highly Elliptical Orbit (HEO)
A satellite in HEO typically has a perigee at about 500 km above the surface of the Earth and an apogee as high as 50,000 km. The orbit is usually inclined at 63.4 deg to provide communications services to locations at high northern latitudes. This inclination value is selected to avoid rotation of the apses; thus, a line from the Earth's center to the apogee always intersects the Earth's surface at a latitude of 63.4 deg North. Orbit period varies from eight to 24 hours. Owing to the high eccentricity of the orbit, a satellite spends about two-thirds of the orbital period near apogee, during which time it appears to be almost stationary to an observer on the Earth (a phenomenon known as `apogee dwell'). During the brief time the satellite is below the local horizon, a hand-off to another satellite in the same orbit is required in order to avoid loss of communications. Free space loss and propagation delay for this type of orbit are comparable to that of geosynchronous satellites. However, due to the comparatively great movement of a satellite in HEO relative to an observer on the Earth, satellite systems using this type of orbit must cope with large Doppler shifts.
A Medium-Earth Orbit (MEO)
By setting the altitude parameters at 10,000 km, you generated a medium-Earth orbit (MEO). This one happens to be an Intermediate Circular Orbit (ICO), since the apogee and perigee are equal. Its orbit period measures about seven hours. The maximum time during which a satellite in MEO orbit is above the local horizon for an observer on the Earth is a few hours. A global communications system using this type of orbit requires relatively few satellites in two to three orbital planes to achieve global coverage. MEO systems operate similarly to LEO systems. In MEO systems, however, hand-over is less frequent, and propagation delay and free space loss are greater. Examples of MEO (specifically ICO) systems are Inmarsat-P (10 satellites in 2 inclined planes at 10,355 km), and Odyssey (12 satellites in 3 inclined planes, also at 10,355 km).