- Satellite Communication Tutorial
- Satellite Communication - Home
- Orbital Mechanics
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- Earth Orbit Satellites
- Look Angles & Orbital Perturbations
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- Earth Segment Subsystems
- Examples of Earth Stations
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Earth Orbit Satellites
Satellite should be properly placed in the corresponding orbit after leaving it in the space. It revolves in a particular way and serves its purpose for scientific, military or commercial. The orbits, which are assigned to satellites with respect to earth are called as Earth Orbits. The satellites present in those orbits are called as Earth Orbit Satellites.
We should choose an orbit properly for a satellite based on the requirement. For example, if the satellite is placed in lower orbit, then it takes less time to travel around the earth and there will be better resolution in an onboard camera. Similarly, if the satellite is placed in higher orbit, then it takes more time to travel around the earth and it covers more earth’s surface at one time.
Following are the three important types of Earth Orbit satellites −
- Geosynchronous Earth Orbit Satellites
- Medium Earth Orbit Satellites
- Low Earth Orbit Satellites
Now, let us discuss about each type of earth orbit satellites one by one.
Geosynchronous Earth OrbitSatellites
A Geo-synchronous Earth Orbit (GEO) Satellite is one, which is placed at an altitude of 22,300 miles above the Earth. This orbit is synchronized with a side real day (i.e., 23 hours 56 minutes). This orbit can have inclination and eccentricity.
It may not be circular. This orbit can be tilted at the poles of the earth. But, it appears stationary when observed from the Earth. These satellites are used for satellite Television.
The same geo-synchronous orbit, if it is circular and in the plane of equator, then it is called as Geostationary orbit. These Satellites are placed at 35,900kms (same as Geosynchronous) above the Earth’s Equator and they keep on rotating with respect to earth’s direction (west to east).
The satellites present in these orbits have the angular velocity same as that of earth. Hence, these satellites are considered as stationary with respect to earth since, these are in synchronous with the Earth’s rotation.
The advantage of Geostationary orbit is that no need to track the antennas in order to find the position of satellites.
Geostationary Earth Orbit Satellites are used for weather forecasting, satellite TV, satellite radio and other types of global communications.
The following figure shows the difference between Geo-synchronous and Geo-stationary orbits. The axis of rotation indicates the movement of Earth.
Note − Every Geostationary orbit is a Geo-synchronous orbit. But, the converse need not be true.
Medium Earth Orbit Satellites
Medium Earth Orbit (MEO) satellites will orbit at distances of about 8000 miles from earth's surface. Signals transmitted from a MEO satellite travel a shorter distance. Due to this, the signal strength at the receiving end gets improved. This shows that smaller and light weight receiving terminals can be used at the receiving end.
Transmission delay can be defined as the time it takes for a signal to travel up to a satellite and back down to a receiving station. In this case, there is less transmission delay. Because, the signal travels for a shorter distance to and from the MEO satellite.
For real-time communications, the shorter the transmission delay, the better will be the communication system. As an example, if a GEO satellite requires 0.25 seconds for a round trip, then MEO satellite requires less than 0.1 seconds to complete the same trip. MEOs operate in the frequency range of 2 GHz and above.
These satellites are used for High speed telephone signals. Ten or more MEO satellites are required in order to cover entire earth.
Low Earth Orbit Satellites
Low Earth Orbit LEO) satellites are mainly classified into three categories. Those are little LEOs, big LEOs, and Mega-LEOs. LEOs will orbit at a distance of 500 to 1000 miles above the earth's surface. These satellites are used for satellite phones and GPS.
This relatively short distance reduces transmission delay to only 0.05 seconds. This further reduces the need for sensitive and bulky receiving equipment. Twenty or more LEO satellites are required to cover entire earth.
Little LEOs will operate in the 800 MHz (0.8 GHz) range. Big LEOs will operate in the 2 GHz or above range, and Mega-LEOs operates in the 20-30 GHz range.
The higher frequencies associated with Mega-LEOs translates into more information carrying capacity and yields to the capability of real-time, low delay video transmission scheme.
The following figure depicts the paths of LEO, MEO and GEO
Here, a question may arise that with more than 200 satellites that are in geosynchronous orbit, how do we keep them from running into each other or from attempting to use the same location in space?
To answer this problem (question), international regulatory bodies like the International Telecommunications Union (ITU) and national government organizations like the Federal Communications Commission (FCC) designate the locations on the geosynchronous orbit, where the communications satellites can be located.
These locations are specified in degrees of longitude and are called as orbital slots. The FCC and ITU have progressively reduced the required spacing down to only 2 degrees for C-band and Ku-band satellites due to the huge demand for orbital slots.