Geostationary Satellites

Satellites are the heavenly bodies or objects that orbit around larger celestial bodies such as planets or bigger stars. Satellites can be natural or artificial. A typical example of a natural satellite is the moon, which orbits the earth. An artificial satellite is man made and launched for observation, navigation or communication purposes. Artificial satellites can be geostationary satellites and polar satellites. While geostationary satellites appear to be fixed with respect to the observer on earth, polar satellites seem to revolve in polar orbits around the earth.

What are geostationary satellites?

Geostationary satellites are a type of artificial satellite. Geo means ‘earth,’ while stationary means ‘at rest’. Geostationary satellites appear to be stationary or fixed at a certain height, approximately 22,236 miles above sea level, as seen by an observer from the earth’s surface. 

For a satellite to appear motionless from any point on earth:-

• It should rotate in a time period of 24 hours.

• Its circular orbit should coincide with the equatorial plane of the earth.

• Its direction of rotation should be the same as that of the earth around its polar axis.

Geostationary satellites have a circular orbit in the equatorial plane of the earth and a time period equal to the time of rotation of the earth, i.e. 24 hrs. Hence, when it circles about the axis of the earth at a speed of 1.91 miles per second, moving from West to East, it appears to be stationary with reference to any point at the earth’s surface. This happens because the earth is rotating with the same angular velocity as the geostationary satellites.

Height of geostationary satellites from the earth’s surface

The centripetal force required to keep the satellite in a circular orbit is given by,

Fc = mv2/ R

Where m is the mass of the satellite

v is its orbital velocity = 2R / T (i)

R is the distance of the satellite from the earth.

The gravitational force exerted by the earth on the satellite is given by,

Fg = GMem / R2

Where G is the gravitational constant 

Me is the mass of the earth.

For a stable orbital motion,

Fc = Fg

mv2/ R = GMem / R2

v2 = GMe / R (ii)

Equating (i) and (ii)

42R2 / T2 = GMe / R

R3 = GM (T2 / 42) (iii)

We know that acceleration due to gravity is given by g = GM / Re2

Putting it in equation (iii),

R = (g Re2 T2 / 42) ⅓

Where Re is the radius of the earth.

T = time period of rotation = 24 hrs = 86400 s

R = 42147 Km

If h is the height of the geostationary satellite from the earth’s surface,

We know that R = Re + h

Hence, h = R – Re

 h = 42147 – 6.37 x 103

 h = 35777 Km

Uses of geostationary satellites

Most commonly, geostationary satellites work as

• Communication satellites

• Weather satellites

• Television satellites

• Spying satellites

Advantages of geostationary satellites

The ionosphere can only reflect electromagnetic waves beyond a particular frequency. An example of such an EM wave is the Radio Waves having a frequency of 2 MHz to 10MHz, which are reflected by the ionosphere. These waves are further broadcasted from an antenna to points far away from where direct waves cannot reach due to the earth’s curvature.

But the waves used for television and other communications are much higher in frequency and cannot be reflected by the ionosphere. Hence, using geostationary satellites fixed on broadcasting stations makes it possible to receive the communication signals and broadcast them to a wide area.

Geostationary satellites have the ability to make repeated observations over a definite area, making them extremely useful for spying. These satellites offer a high and sharp resolution making it possible to observe environmental conditions and track down upcoming storms or hurricanes.

Disadvantages of geostationary satellites

Geostationary satellites offer a poor spatial resolution near the poles as it easily becomes obscured by high peaks. As the orbits in which geostationary satellites circle are higher, they offer a poorer resolution when compared with polar satellites. Hence, the overall setup of launching geostationary satellites is costlier than other satellites.

Conclusion

Geostationary satellites are the type of artificial satellites that can be fixed from any point of the earth’s surface, thus getting the name. This occurs because the time period of these satellites is equal to that of the earth’s rotation, and their circular orbit is fixed at the equatorial plane. Geostationary satellites are commonly used for communication, spying and broadcasting television signals as they offer a high temporal resolution and can make repeated observations over a definite area. The downfalls of geostationary satellites are their high cost of launching and the poor spatial resolution near the poles.