Satellite Network

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.

Equatorial orbit satellite

Inclined orbit satellite

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.

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	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	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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