IAS Mantra Series : Satellite Communication -2

 

Q. What are the advantage and disadvantage of synchronous satellites?

1. It is apparently always stationary. Hence access to it is easier from earth stations which remain at constant distance and line of sight from the satellite. Quality of service is same for rural and urban areas.
2. Synchronous altitude is well above the high intensity inner radiation belt and above the most intense region of outer radiation belt. Thus adequate protection can be obtained for the satellite against radiation.
3. The stationary orbit is sunlit for over 99% of the time. This simplifies generation and storage of power and reduces the number of temperature cycles at the appropriate time to put the orbit of the satellite over the equator  After several revolutions a small rocket motor built into the satellite is fired to place the satellite into a circular orbit around the equator called the geosynchronous earth obit (GEO). This motor is known as apogee kick motor (AKM). This motor if fired at the apogee of the elliptical GTO and thus it is called apogee kick motor. The satellite is oriented by ground station control with its jet thrusters. The satellite drives in this orbit for several revolutions.
Then altitude stabilization is initiated. Thrusters are fired to start the spinning for spin stabilization of the satellite. Antennas are properly aligned. Solar cell panels are adjusted to produce optimum power. The satellite is allowed to operate for several days and is tracked from ground. Sometimes fine-tuning of the satellite is needed is its final designated position. This is done by additional firing of the jet thrusters. Finally it starts operation of communicating with the earth stations. GTO and GEO orbits are shown in the diagram below.

In any earth satellite launch, the largest fraction of the energy expended by the rocket is used to accelerate the rocket from rest until it is about 32 Kms above the surface of the earth. To make the most efficient use of the fuel, it is customary to shed excess mass from the launcher as it move upward on launch. This is called staging.

Satellite Launch Vehicles

Satellite are launched into its orbit in space by using satellite launch vehicles. These consist of multi-stage rockets. Geosynchronous satellite are launched using geosynchronous satellite launch vehicle or GSLV. In case of inclined or polar orbit satellite the launch vehicle is termed as polar satellite launched vehicle or PSLV.
As the multistage rocket moves up onto the sky, when each stage of the rocket is completed, that portion of the launcher is expended. This goes on until the final stage places the satellite into the desired orbit. Launch vehicles can be classified into two types, namely, (1) expendable lunch vehicles can be classified into two types, namely, (1) expendable launch vehicle (ELV) and reusable launch vehicle (RLV). Example of the ELV type launch vehicles are Ariane, Delta etc. ELV type launch vehicle itself gets destroyed in space. These vehicles sometimes carry more than one satellite. In case of RLV type launch vehicles like the space shuttle the vehicle returns to the earth after leaving the satellite in space. Such launch vehicles are also called space transporation system (STS). RLV types after returning from space are refurbished and used once again fro launching satellites. More advanced launched vehicles are being developed that would provide both single state to orbit (SSTO) and RLV capabilities. The launch vehicle is a complex system. It consists of propulsion systems, auto piloting system, aerodynamic structure, interactive steering subsystems etc. For the launch vehicle, navigation and guidance are important. This is achieved y proper mechanical design and use of software control. Auto-piloting system is used for altitude error computation and stabilization of the vehicle. Launch vehicles have on board computers for this purpose. Simulation software such as NISA, MARC, NASTRAN, COSMOC etc. are used.

Writing Skill Test

1. What do you understand by term  period of a satellite.(50 words)
2.What are the requirements of Geostationary Earth Orbit satellites?(50 words)
3. What is the geosynchronous satellite?(50 words)
4. What is the relation between true anomaly and eccentric anomaly?(50 words)
5. Define the terms (i) inclination and (ii) argument of perigee.(50 words)
6. What is sub-satellite points? Explain .(50 words)

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