Communication Satellites, Orbits, Characteristics, Systems, Uses

INTRODUCTION

In general terms, a satellite is a smaller object that revolves around a larger object in space. For example, moon is a natural satellite of earth.

We know that Communication refers to the exchange (sharing) of information between two or more entities, through any medium or channel. In other words, it is nothing but sending, receiving and processing information.

If the communication takes place between any two earth stations through a satellite, then it is called as satellite communication.

The following two kinds of propagation are used earlier for communication up to some distance.

Ground wave propagation− Ground wave propagation is suitable for frequencies up to 30MHz. This method of communication makes use of the troposphere conditions of the earth.
Sky wave propagation− The suitable bandwidth for this type of communication is broadly between 30–40 MHz and it makes use of the ionosphere properties of the earth.

The maximum hop or the station distance is limited to 1500KM only in both ground wave propagation and sky wave propagation. Satellite communication overcomes this limitation. In this method, satellites provide communication for long distances, which is well beyond the line of sight.

Working of Satellite:

A satellite is a body that moves around another body in a particular path. A communication satellite is nothing but a microwave repeater station in space. It is helpful in telecommunications, radio and television along with internet applications.

A repeater is a circuit, which increases the strength of the received signal and then transmits it. But, this repeater works as a transponder. That means, it changes the frequency band of the transmitted signal from the received one.

The frequency with which, the signal is sent into the space is called as Uplink frequency. Similarly, the frequency with which, the signal is sent by the transponder is called as Downlink frequency. The following figure illustrates this concept

The transmission of signal from first earth station to satellite through a channel is called as uplink. Similarly, the transmission of signal from satellite to second earth station through a channel is called as downlink.

Uplink frequency is the frequency at which, the first earth station is communicating with satellite. The satellite transponder converts this signal into another frequency and sends it down to the second earth station. This frequency is called as Downlink frequency. In similar way, second earth station can also communicate with the first one.

The process of satellite communication begins at an earth station. Here, an installation is designed to transmit and receive signals from a satellite in an orbit around the earth. Earth stations send the information to satellites in the form of high powered, high frequency (GHz range) signals.

The satellites receive and retransmit the signals back to earth where they are received by other earth stations in the coverage area of the satellite. Satellite's footprint is the area which receives a signal of useful strength from the satellite.

Applications of Satellite Communication

Satellite communication plays a vital role in our daily life. Following are the applications of satellite communication −

Radio broadcasting and voice communications
TV broadcasting such as Direct to Home (DTH)
Internet applications such as providing Internet connection for data transfer, GPS applications, Internet surfing, etc.
Military applications and navigations
Remote sensing applications
Weather condition monitoring & Forecasting

Forces acting on Satellite:

A satellite, when it revolves around the earth, it undergoes a pulling force from the earth due to earth’s gravitational force. This force is known as Centripetal force (F₁) because this force tends the satellite towards it.

Mathematically, the Centripetal force (F₁) acting on satellite due to earth can be written as

Where,

G= Universal gravitational constant and it equal to 6.673 X 10^-11 N-m²/kg²

M= Mass of the earth = 5.98 X 10²⁴kg

m= Mass of the satellite

R= Distance from satellite to centre of the earth

A satellite, when it revolves around the earth, it undergoes a pulling force from the sun and the moon due to their gravitational forces. This force is known as Centrifugal force (F₂) because this force tends the satellite away from earth.

Mathematically, the Centrifugal force (F₂) acting on satellite can be written as

Where, v is the orbital velocity of satellite.

Orbital Velocity

Orbital velocity of satellite is the velocity at which, the satellite revolves around earth. Satellite doesn’t deviate from its orbit and moves with certain velocity in that orbit, when both Centripetal and Centrifugal forces are balance each other.

So, equate Centripetal force (F₁) and Centrifugal force (F₂).

Where,

G= Universal gravitational constant and it equal to 6.673 X 10^-11 N-m²/kg²

M= Mass of the earth = 5.98 X 10²⁴kg

R= Distance from satellite to centre of the earth

So, the orbital velocity mainly depends on the distance from satellite to center of the Earth (R), since G & M are constants.

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 Orbit Satellites

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.

Geosynchronous satellites orbit Earth above the equator with the same angular velocity as Earth.

Hence, geosynchronous (sometimes called stationary or geostationary) satellites appear to remain in a fixed location above one spot on Earth’s surface.
Since a geosynchronous satellite appears to remain in a fixed location, no special antenna tracking equipment is necessary—earth station antennas are simply pointed at the satellite.
Geosynchronous orbits are circular; therefore, the speed of rotation is constant throughout the orbit. There is only one geosynchronous earth orbit; however, it is occupied by a large number of satellites.
Ideally, geosynchronous satellites should remain stationary above a chosen location over the equator in an Equatorial orbit; however, the sun and the moon exert gravitational forces, solar winds sweep past Earth, and Earth is not perfectly spherical.
Therefore, these unbalanced forces cause geosynchronous satellites to drift slowly away from their assigned locations in a figure-eight excursion with a 24-hour period that follows a wandering path slightly above and below the equatorial plane. This occurs in a special type of inclined orbit sometimes called a stationary inclined orbit.
Ground controllers must periodically adjust satellite positions to counteract these forces. The process of maneuvering a satellite within a preassigned window is called station keeping.

SATELLITE LAUNCHING

Satellites stay in space for most of their life time. We know that the environment of weightlessness is present in the space. That’s why satellites don’t require additional strong frames in space. But, those are required during launching process. Because in that process satellite shakes violently, till the satellite has been placed in a proper orbit.

The design of satellites should be compatible with one or more launch vehicles in order to place the satellite in an orbit.

We know that the period of revolution will be more for higher apogee altitude according to Kepler’s second law. The period of geostationary transfer orbit is nearly equal to 16 hours.

If perigee is increased to GEO altitude (around 36,000 km), then the period of revolution will increase to 24 hours.

Launching of Satellites

The process of placing the satellite in a proper orbit is known as launching process. During this process, from earth stations we can control the operation of satellite. Mainly, there are four stages in launching a satellite.

First Stage− The first stage of launch vehicle contains rockets and fuel for lifting the satellite along with launch vehicle from ground.
Second Stage− The second stage of launch vehicle contains smaller rockets. These are ignited after completion of first stage. They have their own fuel tanks in order to send the satellite into space.
Third Stage− The third (upper) stage of the launch vehicle is connected to the satellite fairing. This fairing is a metal shield, which contains the satellite and it protects the satellite.
Fourth Stage− Satellite gets separated from the upper stage of launch vehicle, when it has been reached to out of Earth's atmosphere. Then, the satellite will go to a “transfer orbit”. This orbit sends the satellite higher into space.

When the satellite reached to the desired height of the orbit, its subsystems like solar panels and communication antennas gets unfurled. Then the satellite takes its position in the orbit with other satellites. Now, the satellite is ready to provide services to the public.

Satellite Antennas

The antennas, which are used in satellite are known as satellite antennas. There are mainly four types of Antennas. They are:

Wire Antennas
Horn Antennas
Array Antennas
Reflector Antennas

Wire Antennas are the basic antennas. Eg: Mono pole and dipole antennas. These are used in very high frequencies in order to provide the communication for TTCM subsystem. Wire antennas are Omni-directional antennas suitable for covering the range of access and to provide signal strength in all directions.

Horn Antenna is an example of aperture antenna. It is used in satellites in order to cover more area on earth. Horn antennas are used in microwave frequency range. The same feed horn can be used for both transmitting and receiving the signals. A device named duplexer, which separates these two signals.

Array Antennas are used in satellites to form multiple beams from single aperture. Beam shaping can be achieved fusing an array of basic elements. The elements are arranged so that their radiation patterns provide mutual reinforcement in certain directions and cancellation in others

Reflector Antennas provide a focusing mechanism which concentrates the energy in a given direction. These are highly directional antennas. So, Parabolic reflectors increase the gain of antennas in satellite communication system.

TRANSPONDERS

The subsystem, which provides the connecting link between transmitting and receiving antennas of a satellite is known as Transponder. It is one of the most important subsystem of space segment subsystems.

Transponder performs the functions of both transmitter and receiver (Responder) in a satellite. Hence, the word ‘Transponder’ is obtained by the combining few letters of two words, Transmitter (Trans) and Responder (ponder).

Candidates can practice 150+Mock Tests with BYJU'S Exam Prep Test Series for exams like GATE, ESE, NIELIT from the following link: