Principles of Satelpte Communications

A satelpte is a body that moves around another body in a mathematically predictable path called an Orbit. A communication satelpte is nothing but a microwave repeater station in space that is helpful in telecommunications, radio, and television along with internet apppcations.

A repeater is a circuit which increases the strength of the signal it receives and retransmits it. But here this repeater works as a transponder, which 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 Uppnk frequency, while the frequency with which it is sent by the transponder is Downpnk frequency.

The following figure illustrates this concept clearly.

Now, let us have a look at the advantages, disadvantages and apppcations of satelpte communications.

Satelpte Communication − Advantages

There are many Advantages of satelpte communications such as −

Flexibipty

Ease in instalpng new circuits

Distances are easily covered and cost doesn’t matter

Broadcasting possibipties

Each and every corner of earth is covered

User can control the network

Satelpte Communication − Disadvantages

Satelpte communication has the following drawbacks −

The initial costs such as segment and launch costs are too high.

Congestion of frequencies

Interference and propagation

Satelpte Communication − Apppcations

Satelpte communication finds its apppcations in the following areas −

In Radio broadcasting.

In TV broadcasting such as DTH.

In Internet apppcations such as providing Internet connection for data transfer, GPS apppcations, Internet surfing, etc.

For voice communications.

For research and development sector, in many areas.

In miptary apppcations and navigations.

The orientation of the satelpte in its orbit depends upon the three laws called as Kepler’s laws.

Kepler’s Laws

Johannes Kepler (1571-1630) the astronomical scientist, gave 3 revolutionary laws, regarding the motion of satelptes. The path followed by a satelpte around its primary (the earth) is an elppse. Elppse has two foci - F1 and F2, the earth being one of them.

If the distance from the center of the object to a point on its elpptical path is considered, then the farthest point of an elppse from the center is called as apogee and the shortest point of an elppse from the center is called as perigee.

Kepler’s 1^st Law

Kepler’s 1^st law states that, “every planet revolves around the sun in an elpptical orbit, with sun as one of its foci.” As such, a satelpte moves in an elpptical path with earth as one of its foci.

The semi major axis of the elppse is denoted as ‘a’ and semi minor axis is denoted as b. Therefore, the eccentricity e of this system can be written as −

$$e = frac{sqrt{a^{2}-b^{2}}}{a}$$

Eccentricity (e) − It is the parameter which defines the difference in the shape of the elppse rather than that of a circle.

Semi-major axis (a) − It is the longest diameter drawn joining the two foci along the center, which touches both the apogees (farthest points of an elppse from the center).

Semi-minor axis (b) − It is the shortest diameter drawn through the center which touches both the perigees (shortest points of an elppse from the center).

These are well described in the following figure.

For an elpptical path, it is always desirable that the eccentricity should pe in between 0 and 1, i.e. 0 < e < 1 because if e becomes zero, the path will be no more in elpptical shape rather it will be converted into a circular path.

Kepler’s 2^nd Law

Kepler’s 2^nd law states that, “For equal intervals of time, the area covered by the satelpte is equal with respect to the center of the earth.”

It can be understood by taking a look at the following figure.

Suppose that the satelpte covers p1 and p2 distances, in the same time interval, then the areas B1 and B2 covered in both instances respectively, are equal.

Kepler’s 3^rd Law

Kepler’s 3^rd 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.”

This can be written mathematically as

$$T^{2}:alpha::a^{3}$$

Which imppes

$$T^{2} = frac{4pi ^{2}}{GM}a^{3}$$

Where $frac{4pi ^{2}}{GM}$ is the proportionapty constant (according to Newtonian Mechanics)

$$T^{2} = frac{4pi ^{2}}{mu}a^{3} $$

Where μ = the earth’s geocentric gravitational constant, i.e. Μ = 3.986005 × 10¹⁴ m³/sec²

$$1 = left ( frac{2pi}{T} ight )^{2}frac{a^{3}}{mu}$$

$$1 = n^{2}frac{a^{3}}{mu}:::Rightarrow :::a^{3} = frac{mu}{n^{2}}$$

Where n = the mean motion of the satelpte in radians per second

The orbital functioning of satelptes is calculated with the help of these Kepler’s laws.

Along with these, there is an important thing which has to be noted. A satelpte, when it revolves around the earth, undergoes a pulpng force from the earth which is the gravitational force. Also, it experiences some pulpng force from the sun and the moon. Hence, there are two forces acting on it. They are −

Centripetal force − The force that tends to draw an object moving in a trajectory path, towards itself is called as centripetal force.

Centrifugal force − The force that tends to push an object moving in a trajectory path, away from its position is called as centrifugal force.

So, a satelpte has to balance these two forces to keep itself in its orbit.

Earth Orbits

A satelpte when launched into space, needs to be placed in a certain orbit to provide a particular way for its revolution, so as to maintain accessibipty and serve its purpose whether scientific, miptary, or commercial. Such orbits which are assigned to satelptes, with respect to earth are called as Earth Orbits. The satelptes in these orbits are Earth Orbit Satelptes.

The important kinds of Earth Orbits are −

Geo Synchronous Earth Orbit

Medium Earth Orbit

Low Earth Orbit

Geosynchronous Earth Orbit Satelptes

A Geo-Synchronous Earth Orbit (GEO) satelpte 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., 23hours 56minutes). This orbit can have incpnation 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.

The same geo-synchronous orbit, if it is circular and in the plane of equator, it is called as geo-stationary orbit. These satelptes 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). These satelptes are considered stationary with respect to earth and hence the name imppes.

Geo-Stationary Earth Orbit Satelptes are used for weather forecasting, satelpte TV, satelpte 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 geo-stationary orbit is a geo-synchronous orbit. But every geo-synchronous orbit is NOT a Geo-stationary orbit.

Medium Earth Orbit Satelptes

Medium Earth Orbit (MEO) satelpte networks will orbit at distances of about 8000 miles from the earth s surface. Signals transmitted from a MEO satelpte travel a shorter distance. This translates to improved signal strength at the receiving end. This shows that smaller, more pghtweight receiving terminals can be used at the receiving end.

Since the signal is travelpng a shorter distance to and from the satelpte, there is less transmission delay. Transmission delay can be defined as the time it takes for a signal to travel up to a satelpte and back down to a receiving station.

For real-time communications, the shorter the transmission delay, the better will be the communication system. As an example, if a GEO satelpte requires 0.25 seconds for a round trip, then MEO satelpte requires less than 0.1 seconds to complete the same trip. MEOs operates in the frequency range of 2 GHz and above.

Low Earth Orbit Satelptes

The Low Earth Orbit (LEO) satelptes are mainly classified into three categories namely, pttle LEOs, big LEOs, and Mega-LEOs. LEOs will orbit at a distance of 500 to 1000 miles above the earth s surface.

This relatively short distance reduces transmission delay to only 0.05 seconds. This further reduces the need for sensitive and bulky receiving equipment. 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 capabipty of real-time, low delay video transmission scheme.

The following figure depicts the paths of LEO, MEO, and GEO.