Communication Satellite
A communications
satellite or Comsat is an artificial satellite sent to space for the purpose of telecommunications.
A satellite needs to have an orbit the path in which it
travels around the Earth. The orbit can be equatorial, inclined, or polar.
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| Equatorial orbit satellite |
 |
| Inclined orbit satellite |
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| Polar orbit satellite |
Footprint
The footprint of a communications satellite is
the ground area that its transponders offer coverage, and
determines the satellite
dish diameter required to receive each transponder's signal. The signal power is maximum at the center and
decreases as we move out from the footprint center.
Categories of satellite
Based
on the location of the orbit, satellites can be divided into three categories:
geostationary Earth orbit (GEO), low-Earth-orbit (LEO), and middle-Earth-orbit
(MEO).
Orbit of the GEO satellite is at 35786 KM. MEO satellites
are placed at altitude between 5000 and 15000 KM. The orbit of LEO satellites
are below 2000 KM.
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| Satellite Orbit Altitudes |
The
period of a satellite, the time required a satellite to make a complete journey
around the Earth, is determined by Kepler's law. According to Kepler’s
law, the orbital period of a satellite varies as the radius of the orbit to the
3/2 power. The higher the satellite, the longer the period.
Van Allen belts
are layers of highly charged particles trapped by the earth’s magnetic field.
Any satellite flying within them would be destroyed quickly by the particles.
To
ensure stable communication, the satellite should move at the identical speed of
the Earth so that it appears to remain permanent above a certain spot. Such satellites
are called geostationary. At least
minimum three satellites equidistant from each other are requiring to provide
full earth surface coverage.
Band
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Uplink
|
Downlink
|
Bandwidth
|
Issues
|
L
|
1.5 GHz
|
1.6 GHz
|
15 MHz
|
Low bandwidth, crowded
|
S
|
1.9 GHz
|
2.2 GHz
|
70 MHz
|
Low bandwidth, crowded
|
C
|
4.0 GHz
|
6.0 GHz
|
500 MHz
|
Terrestrial interference
|
Ku
|
11 GHz
|
14 GHz
|
500 MHz
|
Rain
|
Ka
|
20 GHz
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30 GHz
|
3500 MHz
|
Rain, equipment cost
|
The
principal satellite bands
Medium-Earth-orbit (MEO)
MEO satellites are positioned between the two Van Allen belts. A satellite at
this orbit takes approximately 6-8 hours to circle the Earth.
GPS (Global Positioning System) satellite system is an example of MEO.
Low-Earth-orbit (LEO) satellites have altitude between
500 and 2000 km, with a rotation period of 90 to 120 min. They are usually rotate
through polar orbits. The satellite has a speed of 20,000 km/h and the footprint
has diameter 8000 km.
Some important satellite networks
- The Iridium system has 66 satellites in six LEO orbits, each at an
altitude of 750 KM.
- Globstar: 48 satellite, six polar orbits, 1400 KM
- Teledesic: 288 satellite, 12 LEO orbits, 1350
KM.
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