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Satellite Communication: Active and Passive Satellite

What is Satellite Communication?

Satellite communication is transporting information from one place to another using a communication satellite in orbit around the Earth. Watching the English Premier League every weekend with your friends would have been impossible without this. A communication satellite is an artificial satellite that transmits the signal via a transponder by creating a channel between the transmitter and the receiver at different Earth locations.

Telephone, radio, television, internet, and military applications use satellite communications. Believe it or not, more than 2000 artificial satellites are hurtling around in space above your heads.

Satellite Communication Block Diagram

Block diagram of satellite communications

Need for Satellite Communication

We know that there are different ways to communicate, and the propagation of these waves can occur in different ways. Ground wave propagation and skywave propagation are the two ways communication takes place for a certain distance. The maximum distance covered by them is 1500 km, which was overcome by the introduction of satellite communication.

How Satellite Communications Work?

The communication satellites are similar to the space mirrors that help us bounce signals such as radio, internet data, and television from one side of the earth to another. Three stages are involved, which explain the working of satellite communications. These are:

  • Uplink
  • Transponders
  • Downlink

Let’s consider an example of signals from a television. In the first stage, the signal from the television broadcast on the other side of the earth is first beamed up to the satellite from the ground station on the earth. This process is known as uplink.

The second stage involves transponders such as radio receivers, amplifiers, and transmitters. These transponders boost the incoming signal and change its frequency so that the outgoing signals are not altered. Depending on the incoming signal sources, the transponders vary.

The final stage involves a downlink in which the data is sent to the other end of the receiver on the earth. It is important to understand that usually, there is one uplink and multiple downlinks.
Working of satellite communication

Satellite Communications in India

It’s interesting to know that the Indian National Satellite (INSAT) system is one of the largest domestic communication systems that is placed in the geo-stational orbit. There are more than 200 transponders in the INSAT system and are used for various purposes such as telecommunications, weather forecasting, television broadcasting, disaster warning, search and rescue operations, and satellite newsgathering.

Below is the list of communication satellites along with their applications:

Satellite name Launch date Application 
GSAT-30 Jan 17, 2020 Communication
GSAT-31 Feb 06, 2020 Communication
GSAT-15 Nov 11, 2015 Communication and navigation
GSAT-10 Sep 29, 2012 Communication and navigation
INSAT-3A Apr 10, 2003 Communication and climate and environment
KALPANA-1 Sep 12, 2002 Communication and climate and environment

The Space Debris Consisting Of Satellites And Other Junk Revolving Around The Planet

The need for satellite communication becomes evident when we want to transmit the signal to far-off places, where the Earth’s curvature comes into play. This obstruction is overcome by putting communication satellites in space to transmit the signals across the curvature. Satellite communication uses two types of artificial satellites to transmit the signals:

  • Passive Satellites: If you put a hydrogen balloon that has a metallic coating over it up in the air, it technically becomes a passive satellite. Such a balloon can reflect microwave signals from one place to another. The passive satellites in space are similar. These satellites just reflect the signal back towards the Earth without amplification. Since the satellite orbit height can range from 2000 to 35786 km, attenuation due to the atmosphere also comes into play, and due to this, the received signal is often very weak.
  • Active Satellites:  Active Satellites, unlike passive satellites, amplify the transmitted signals before re-transmitting it back to Earth, ensuring excellent signal strength. Passive satellites were the earliest communication satellite, but now almost all the new ones are active satellites.

To avoid mixing up and interference signals, every user is allocated a specific frequency for transmitting. The International Telecommunication Union does this frequency allocation. Geosynchronous satellites are of note here. Geostationary orbit is present at 35786 km above Earth’s surface. If you can spot such a satellite with a telescope from Earth, it will appear stationary to you. The satellite’s orbital period and the Earth’s rotational rate are in sync.

Some More Information About Geostationary Orbits

These were some typical orbits. Apart from these, we also have orbits that address particular problems. The Russians faced one such issue. GEO satellites worked perfectly for the equatorial regions, but they had a very weak coverage near the Poles. The Russians designed an orbit with a very high inclination to address this problem. The inclination is the angle between the satellite’s orbit and the equator. This orbit was called the Molniya orbit. The orbit had excellent coverage of the North Pole for a short time. Molniya had a period of 24 hours,  but out of that, it would be close to Earth only for 6-9 hours. Russia launched more satellites in the same orbit and soon had uninterrupted coverage.

The Molniya Orbit

Satellite Communication Services

There are two categories in which satellite communication services can be classified:

  • One-way satellite communication
  • Two- way satellite communication

One-way Satellite Communication

In one-way satellite communication, the communication usually takes place between either one or multiple earth stations through the help of a satellite.

The communication takes place between the transmitter on the first earth satellite to the receiver which is the second earth satellite. The transmission of the signal is unidirectional. Some common one-way satellite communication is:

  • Position location services are provided by the radio
  • Tracking is a part of space operations services
  • Internet services take place with broadcasting satellites

Following is the figure which explains the one-way satellite communication:
One-way satellite communication service

Two-Way Satellite Communication

In two-way satellite communication, the information is exchanged between any two earth stations. It can be said that there is a point to point connectivity.

The signal is transmitted from the first earth station to the second earth station such that there are two uplinks and two downlinks between the earth stations and the satellite.

Following is the figure for the two-way satellite communication:
Two-way satellite communication service

Advantages of Satellite Communication

The following are the advantages of satellite communication:

  • Installments of circuits are easy.
  • The elasticity of these circuits is excellent.
  • With the help of satellite communication, every corner of the earth can be covered.
  • The user fully controls the network.

Disadvantages of Satellite Communication

The following are the disadvantages of satellite communication:

  • Initial expenditure is expensive.
  • There are chances of blockage of frequencies.
  • Propagation and interference.

Applications of Satellite Communication

  • Telephone
  • Television
  • Digital cinema
  • Radio broadcasting
  • Amateur radio
  • Internet access
  • Military
  • Disaster Management

Related Physics Links:

Frequently Asked Questions – FAQs

What are the two main components of satellite communication?

The two main components of satellite communication are:

  • The ground segment comprises either fixed or mobile transmission, reception, and ancillary equipment.
  • The space segment: The satellite is known as the space segment. There are three main units: the fuel system, the satellite, telemetry controls, and the transponder. The prime role of the space segment is to reflect electronic signals.

Name the countries having their own satellites.

There are a total of 12 countries that have their own satellites. A few of them are listed below:

  • India – Rohini D1
  • Japan – Ohsumi
  • China – Dong Fang Hong I

What is a space station?

A space station is an artificial structure designed for humans to live and work in outer space.

What are the three laws of Kepler?

Following are the three laws of Kepler:

  • Kepler’s first law states that every planet revolves around the sun in an elliptical orbit, and the sun is one of the foci.
  • Kepler’s second law states that for an equal interval of time, the area covered by the satellite is equal with respect to the earth’s centre.
  • Kepler’s third law states that the square of the periodic time of the orbit is proportional to the cube of the mean distance between the two bodies.

List the factors on which the carrier to noise ratio of a satellite depends on.

Following are the three factors on which the carrier-to-noise ratio depends for a satellite:

  • Bandwidth
  • Free space for path losses
  • Effective power radiated from the isotropic
Test your Knowledge on Satellite Communication

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