We live our life knowing that many satellite orbit our planet everyday and that they are helping us in several ways, you might be surprised to know that there are almost 4900 satellites orbiting the earth. The most obvious questions that come to mind are 

  • why are these satellite in totally different orbits?
  • How does the  satellite carry out all of its function? And what are the components inside them, which help them to accomplish all of their alloted task? Let's explore the answer to all these questions in details?
It's well known fact that a satellite stay in orbit because of the balanced between gravitational pull & Centrifugal force.The angular velocity of satellite is decided by the force balance equation that balanced gravitational and centrifugal forces. 
              GMm/R^2 = mRw^2............
                    Where,
                       G=Gravitational Constant
                       M & m=masses of objects interacting
                        R= Distance between these interacting object
                        w=Angular speed of satellite
when the satellite is deployed it is given sufficient speed to balance these two forces.A satellite near to the earth required more speed to resist the gravitational force then once located further from the earth due to the negligible resistance in space. Satellite never loose speed.These means a satellite will continue their circular motion around the earth without any external energy source.

Satellite are placed either in low earth orbit,Medium earth orbit or Geosynchronous earth orbit.These three orbit are illustrated here we will get into more details of later.There is an interesting region in space called that Van Allen Belt.A region full of highly energetic charge particles which could serious damage the electronic section of satellite.Generally it is preferred not to park satellite in the Van Allen Belt.The decision on what orbit is to be chosen for placing the satellite depends upon application and purpose of the satellite.If the satellite is to be built for earth observation , Weather forecasting,Geographic area surveying, Satellite phone call etc. then orbit closer to earth are Chosen. LEO is the closest to the earth at an altitude of between 160 km and 2000 km and it's orbital period is approx. 1.5 hours but these types of satellite covers less area of earth so many satellite are required to obtained global coverage. That's why in case of broadcasting a high orbit such as GEO is chosen.Satellite in Geosynchronous orbit are a height of 35,786 km.And rotates at the same angular speed as earth.it means the satellite takes exactly 23 hrs 56 min. And 4 sec to complete one rotation within the Geosynchronous orbits.

  • There is special category of orbit called Geostationary orbit,which is concentric to equator of the earth.These satellite remains stationary with respect to earth due to this, Geostationary satellite are the ideal choice for the television broadcasting since it's mean you don't have to adjust the angle of your satellite dish again and again. This is the reason Why the Geostationary belt is so crowed with satellite  and it's managed by an international organization called ITU ( International Telecommunication Union) is Established in 17 may 1865 & its Headquarter is in Geneva, Switzerland.

Geosynchronous orbit are occupied by a few navigation satellite also Geo satellite can cover one third of the earth's surface. So three satellite are sufficient to cover the entire earth.for navigation application such as GPS. MEO is the wise option even though the Leo is closest to the earth.
Satellite in this orbits revolve at a very high speed due to this, receiver on the earth fail to carry out the navigation calculation accurately. moreover, LEO needs a lots more satellite to cover the entire earth, thus GPS Satellite uses MEO.

Components of Satellite 
 
In a typical GPS system,24 satellite can cover the entire earth and the orbital period 12 hours. Satellite have the different main components of a communication satellite along with their function. At heart of communication satellite are the transponder.The main task of transponder is to change the frequency of received signal remove any noise signal and amplifying the signal power. On Kv Band satellite, the transponder convert from 14GHz to 12GHz.and the satellite can have 20 or more transponders.
It is obvious that transponder requires a great deal of electrical power to handle all of these functions.for power supply a satellite has the option of batteries and solar panel.The solar panel is used to power the electronic equipment but during an eclipse time batteries are used.

  • The sun sensor helps to angle the solar panel in the right direction so that the maximum power can be extracted from the sun.Now lets see how the transponder received the input signal from the antennas.The most common antenna fixed to satellite are reflector antenna.A satellite is supposed to follows it's intended smooth orbit.However the gravitational field around the satellite is not uniform.due to the unequal ass distribution of the earth and the presence of moon and the sun because of this sometimes the satellite gets displaced from its intended orbital position.
This is a dangerous situation, since it will lead to a complete loss of signal. To avoid such a situation, satellite make use a upthruster. The thrusters are fired and keep the satellite in the right position. These also helps satellites to avoid space junk. The fuel needed for the thruster is saved in the tank in the satellite body. The position of the satellite and control of the thruster are continuously monitored from an earth station. Apart from the position controls, the earth station also monitors the satellite health and speed. This is done through tracking telemetry and control system. These continuously send the signal to earth stations and maintain the contact between earth and satellite.

Generally, these signals are exchanged at different frequency to distinguish from other communication signals. Have you ever thought what happen to a satellite when it is no longer functional, or its lifespan is nearing to the end. These satellite could harm other operational satellite or spacecraft.  To deal with this situation inactive satellites are transferred to the graveyard orbit by activating the thruster just by increasing the rotational speed of satellite. We will be able to transfer it to a higher radius orbit.For this operation, the thruster consumes same amount of fuel as a satellite needs for about three months of station keeping, the satellite we have discussed so far are communication satellite.

For the satellite, the must important components are an atomic clock and the antenna. The L Band navigation antenna used in these kind of satellite are illustrated here. The earth observation satellites which are mostly in LEO, carry various types sensors, images etc. Depending on the mission, now for some interesting information. Have you ever think why satellite is folded with gold colored foil. What is purpose of foil? In-fact, it is not foil as it appears to be at first sight. If you take a cross-section of it, you can see it has a multi-layer structure. Satellite face huge temperature variation in space where temperature is varies from -150°C to 200°C. Moreover satellite face the issue of heavy solar radiation from the sun. This material actually acts as a shield which protects the satellite components from the heavy temperature variation and from solar radiation.