Communication satellites are human-made satellites that work in such a way that the satellite receives a signal from one point on earth, amplifies the signal and send it back to another point earth using transponders. Thus, it creates the communication network between the source that transmitted the signal and receiver.
Communication satellite or the artificial satellite can be considered as the repeater in the sky that repeats the signals in order to extend the communication network and cover a long distance.
You must be thinking why don’t we use the natural satellite such as Moon for establishing the communication between sender and receiver. Well, the first constraint is the distance, the natural satellites are so far that they can create a long delay in transmitting the signal which will ultimately cause a delay in communication.
We also prefer artificial satellite over the natural satellite as we can install electronic equipment on the artificial satellite to amplify the signal that regenerates the signal weakened during travelling.
What is Transponder?
A Transponder receives a signal from the source transmitter, amplifies it and reemits it in another frequency thereby avoiding interference between incoming and outgoing signal.
What is Footprint?
As we all know that the satellites propagate signal wave in the line of sight. The satellite receives the signal from source transmitter, amplifies it and rebroadcast it to the receiver. The rebroadcasted signal aims at a particular area on the earth which we term as a footprint. The signal power at the centre of the footprint is maximum and it decreases gradually as we move out from the footprint.
Now, every communication satellite needs a path to travel around the earth which is termed as ‘orbit’. Orbit can be equatorial, inclined and polar. To determine the orbital period i.e. time required by the satellite to complete one trip around the earth we use Kepler’s law:
Here, C is constant i.e. (1/100), unit of distance is in kilometres, the unit of the period is in seconds.
The orbital period of a satellite is important while deciding where to locate the satellite. But there is one more constraint i.e. Van Allen belts. This belt has a layer of highly charged particles that are trapped due to the gravity of the earth. If you place any satellite in this belt, then it would be destroyed into particles as it travels through the belt.
Considering the factors above three regions have been identified where the satellite can be placed safely. The figure below shows you the location of each kind of communication satellite.
Depending upon the orbit location satellites can be classified into three types.
Types of Communication Satellite
1. Geostationary Earth Orbit (Geostationary Satellites)
Geostationary satellite according to its name appears stationary to a viewer observing it from the earth. This is because the geostationary satellite travels with the same angular velocity as that of the earth.
The geostationary satellite lies in the geostationary orbit that is above 35,786 km above the earth’s equator. The satellite appears to be stationary due to factors such as its orbital speed, it’s the distance from the earth and the earth gravitational force. And thus, there is an only one geostationary orbit.
But having only one satellite in the geostationary orbit doesn’t cover the view of the entire earth as satellite propagates signal in line of sight and due to earth’s curve structure, it is impossible to cover the entire globe. Therefore, we require three geostationary satellite to cover the entire earth.
2. Medium Earth Orbit (MEO Satellites)
The MEO satellite lies in the orbit below the geostationary orbit. You can observe the figure above as the MEO satellites lies between the two Van Allen belts. The speed of MEO satellites is faster as compared to geostationary satellites and complete the entire trip around the earth in just 6-7 hours.
As it is closer to the earth compared to GEO satellites it emits a smaller footprint. Comparatively, it also requires less powerful transmitters due to the reduced distance between the satellite and earth.
Due to their speed, they don’t appear stationary in the sky and hence they must be tracked. The most popular example of a MEO satellite is GPS (Global Positioning System). GPS is owned by the government of the United States and is operated by their space force. The system includes 24 satellites that are used for navigation purpose over land, sea and air. MEO satellites are not used for the telecommunication.
3. Low Earth Orbit (LEO Satellites)
LEO satellites lie in the lower altitude as compared to MEO. It travels in the polar orbit at an altitude of 500 to 2000 km. As the LEO satellites much closer to earth they have a smaller footprint compared to GEO and MEO, i.e. about 8000 km.
The LEO satellites are much faster than GEO’s and MEO’s they complete their trip around the earth in 90 to 12 minutes. With this information, if we calculate the speed of the LEO satellites it approximately 20,000 km to 25,000 km/hr. Their round-trip delay is a few milliseconds which is acceptable for audio communication.
To form a complete network, the LEO satellites are organized in a constellation pattern. The adjacent satellites in the network are connected to each other through the inter-satellite link (ISLs), each satellite in the network act as a switch.
A mobile device communicates with the LEO satellite with the help of user mobile link (UML) and the satellite communicates with an earth station using gateway link (GWL).
Further, we can divide the LEO system into three categories
- Little LEO: Used for low data rate messaging and it operates on 1GHz.
- Big LEO: It operates between 1 to 3 GHz its examples are Globalstar and Iridium that provides services such as voice, data, navigation, paging and fax on land, air and sea.
- Broad LEO: Its services are similar to fibre optics its example is Teledesic, that provides worldwide broadband internet service.
Advantage of Communication Satellite
The main advantage of communication satellite is that it provides communication on the remote areas of the earth which are even undeveloped without investing on the ground infrastructure.
So, this is how communication satellite works. We have also discussed three types of communication satellite GEO, MEO and LEO. Communication satellite services attract many applications.