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SPACE SEGMENT AND LINK DESIGN
1. Give the two segments of basic satellite communication.
a. Earth segment (or) ground segment
b. Space segment
2. Write short notes on attitude control system.
It is the system that achieves and maintains the required attitudes. The main functions of attitude control system include maintaining accurate satellite position throughout the life span of the system.
3. What is declination?
The angle of tilt is often referred to as the declination which must not be confused with the magnetic declination used in correcting compass readings.
4. What is meant by payload?
It refers to the equipment used to provide the service for which the satellite has been launched.
5. What is meant by transponder?
In a communication satellite, the equipment which provides the connecting link between the satellite’s transmit and receive antennas is referred to as the transponder.
6. Write short notes on station keeping.
It is the process of maintenance of satellite’s attitude against different factors that can cause drift with time. Satellites need to have their orbits adjusted from time to time, because the satellite is initially placed in the correct orbit, natural forces induce a progressive drift.
7. What is meant by Pitch angle?
Movement of a spacecraft about an axis which is perpendicular to its longitudinal axis. It is the degree of elevation or depression.
8. What is an propellant?
A solid or liquid substance burnt in a rocket for the purpose of producing thrust.
9. What is an Yaw?
Yaw is the rotation of a vehicle about its vertical axis.
10. What is an zero ‘g’?
Zero ‘g’ is a state when the gravitational attraction is opposed by equal and opposite inertial forces and the body experiences no mechanical stress.
11. Describe the spin stabilized satellites.
In a spin stabilized satellites, the body of the satellite spins at about 30 to 100 rpm about the axis perpendicular to the orbital plane. The satellites arem normally dual spin satellites with a spinning section and a despun section on which antennas are mounted. These are kept stationary with respect to earth by counter rotating the despun section.
12. What is meant by frequency reuse?
The carrier with opposite senses of polarization may overlap in frequency. This technique is known as frequency reuse.
13. What is meant by spot beam antenna?
A beam generated by a communication satellite antenna of sufficient size that the angular spread of sufficient size that the angular spread of the energy in the beam is very small with the result that a region that is only a few hundred km in diameter is illuminated on earth.
14. What is meant by momentum wheel stabilization?
During the spin stabilization, flywheels may be used rather than spinning the satellite. These flywheels are termed as momentum wheels.
15. What is polarization interleaving?
Overlap occurs between channels, but these are alternatively polarized left hand circular and right hand circular to reduce interference to acceptable levels. This is referred to as polarization interleaving.
16. Define S/N ratio.
The S/N introduced in the preceding section is used to refer to the ratio of signal power to noise power at the receiver output. This is known as S/N ratio.
17. What is an intermodulation noise?
Intermodulation distortion in high power amplifier can result in signal product which appear as noise and it is referred to as intermodulation noise.
18. What is an antenna loss?
It is added to noise received as radiation and the total antenna noise temperature is the sum of the equivalent noise temperature of all these sources.
19. Define sky noise.
It is a term used to describe the microwave radiation which is present throughout universe and which appears to originate from matter in any form, at finite temperature.
20. Define noise factor.
An alternative way of representing amplifier noise is by means of its noise factor. In defining the noise factor of an amplifiers, usually taken as 290 k.
21. What is TWTA?
TWTA means Traveling Wave Tube Amplifier. The TWTA is widely used in transponder to provide the final output power required to the transtube and its power supplies.
22.What is meant by thermal control and why this is necessary in a satellite?
Equipment in the satellite generates heat which has to be removed. The element used in the satellite to control thermal heat is called thermal conrol. The most important consideration is that the satellite’s equipment should operate as nearly as possible in a stable temperature environment
23.What are the functions carried out in TT&C?
Telemetry- Gathering or measure information about satellite.
Tracking- track the satellite’s movement and send correction signals as Required
Comment- send information about the satellite to earth station.
24.What is meant by redundant receiver?
A duplicate receiver is provided so that if one fails, the other is automatically switched in. The combination is referred to as a redundant receiver, meaning that although two are provided, only one is in use at a given time.
25.List out the advantages of TWT.
The advantage of the TWT over other types of tube amplifiers is that it can provide amplification over a very wide bandwidth. Input levels to the TWT must be carefully controlled, however, to minimize the effects of certain forms of distortion
26.Define input back off.
In a TWTA, the operating point must be backed off to a linear portion of the transfer characteristic to reduce the effects of intermodulation distortion.The point from the saturation point to linear region at the input is called input backoff.
27.Define diplexer & orthocoupler.
The same feed horn may be used to transmit and receive carriers with the same polarization. The transmit and receive signals are separated in a device known as a diplexer,
The polarization separation takes place in a device known as an orthocoupler, or orthogonal mode transducer (OMT). Separate horns also may be used for the transmit and receive functions, with both horns using the same reflector.
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