Q46) What is Launch window ?
Answer: The launch window is a term used to describe a time period in which a particular mission must be launched.
Q47) Explain Space-segment.
Answer: The space segment of an artificial satellite system is one of its three operational components (the others being the user and ground segments). It comprises the satellite or satellite constellation and the uplink and downlink satellite links.
Q48) Define Elevation angle.
Answer: The angle between vertical plane and line pointing to satellite.
Q49) Define Azimuth angle.
Answer: The angle between local horizontal plane and the plane passing through earth station, satellite and center of earth is called as azimuth angle.
Q50) Will the velocity of satellite in orbit remains same or it changes ?
Answer:
- In circular orbit: velocity of satellite remains same.
But in elliptical orbit, As satellite comes near to planet(ie. Earth) speed of satellite increases and when it goes farthest from planet speed is decreases. As per the kepler's second law(ie. Kepler's Law of Equal Areas.
Q51) What is Polar orbit ?
Answer: Polar orbit is one in which satellite is orbiting in such a way as to cover the north and south polar regions.
Q52) Conditions for launching pad (Launchnig station) ?
Answer: Conditions for Launching station:
- Near the sea:
- Once a rocket ignites and lifts off, there's not much control over it even if it deviates from its set path or does not follow its trajectory.
- If such a situation however occurs, a destruct command is given out. This command destroys or completely disintegrates the rocket and makes it fall into the sea.
- If there is no such provision, the rocket can fall on land causing massive destruction. So, places near the sea or desert, with no habitation, are chosen as space ports as a measure of protection.
- Near to the Equator:
- If the launch location is near to the Equator, a lot of fuel can be saved.
- Stable geographical platform:
- The landmass available should be should be solid enough to withstand the intense vibrations produced during the launch. The soil should be strong, with hard rock below it, just like in Sriharikota.
Q53) What are the types of satellite stabilization ?
Answer: Spin stabilization and three-axis stabilization are two methods that are used to orient satellites.
Q54) What is spin stabilization ?
Answer: In spin stabilization, the entire spacecraft rotates around its own vertical axis, spinning like a top. This keeps the spacecraft's orientation in space under control. The advantage of spin stabilization is that it is a very simple way to keep the spacecraft pointed in a certain direction. The spinning spacecraft resists perturbing forces, which tend to be small in space, just like a gyroscope or a top.
Q55) What three-axis stabilization stabilization ?
Answer:
- The three axis stabilisation as the name suggest use three axis called as pitch, roll and yaw to achieve attitude control.
- Yaw axis: Directed towards the earth’s centre.
- Pitch axis: Normal to the orbital plane.
- Roll axis: Tangent to the orbit.
- In three-axis stabilization, satellites have small spinning wheels, called reaction wheels or momentum wheels, that rotate so as to keep the satellite in the desired orientation in relation to the Earth and the Sun. If satellite sensors detect that the satellite is moving away from the proper orientation, the spinning wheels speed up or slow down to return the satellite to its correct position.
Q56) What is the Laser satellite communications ?
Answer: The basic principles of laser communications and the designing of laser systems are analyzed. Consideration is given to the link equation, the transmitter terminal, antennas, the optical detector, optical modulation formats, deriving error rate data, and signal requirements for acquisition and tracking.
Q57) What is the Isotropic antenna ?
Answer: An isotropic antenna is an ideal antenna that radiates its power uniformly in all directions. There is no actual physical isotropic antenna. However, an isotropic antenna is often used as a reference antenna for the antenna gain.
Q58) Define Frequency-Reuse.
Answer: Frequency re-use is a technique used for utilizing a specific range of frequencies more than once within the same satellite sytem so that the total capacity of the system is increased without increasing its allocated bandwidth.
Q59) Define Antenna Gain.
Answer: The term Antenna Gain describes how much power is transmitted in the direction of peak radiation to that of an isotropic source. Antenna gain is more commonly quoted than directivity in an antenna's specification sheet because it takes into account the actual losses that occur.
Q60) Define Inclination.
Answer: The angle between the plane of the orbit of a satellite an the equatorial plane. A orbit of a perfectly geostationary satellite has an inclination of 0 degrees.
Q61) Define Inclined Orbit.
Answer: An orbit that approximates the geostationary orbit but whose plane is tilted sloghtly with respect to the equatorial plane with the consequence that the satellite appears to move about its nominal position in a daily motion "figure of eigth" when viewed from the ground.
Q62) Explain Satellite attitude.
Answer: The attitude of a satellite is its position in space - its orientation. Attitude determines what a satellite looks at - which way its cameras are facing, and the angle the satellite makes with the object it is orbiting.
Q63) Define Line of Apsides.
Answer: Line of Apsides is the line joining the perigee and apogee through the center of the earth.
Q64) What is the difference between C/N and G/T ratios ?
Answer: The C/N ratio is the ratio of carrier power to noise power at the receiver input. Therefore it essentially tells us about the performance of the "Satellite Link" and is expressed in dB.
Whereas, the G/T ratio is the ratio of Receiver gain to the system noise temperature. It determines the quality of the "Receiving Earth Station" and is expressed in dB/k.
Q65) What are the functions of satellite TTC (Telemetry, Tracking and Control) ?
Answer: The telemetry system is used to report status of on-board subsystems to the ground station. Tracking is used to know the position of the satellite all the time. A command and control system permits ground station to control the satellite position.
Q66) Define "Circular Orbit" in terms of satellite communication ?
Answer: A satellite orbit in which the distance between the centres of mass of the satellite and of the primary body [the earth] is constant.
Q67) What is Intermodulation Noise ?
Answer: When a number of carriers pass through a device having non-linear characteristics, intermodulation products are generated due to the amplitude and phase non-linearities. These intermodulation products are not seaparable but appear as a type of noise in the system which is known as INTERMODULATION NOISE.
Q68) Exlain "DBS" in terms of satellite communication ?
Answer: DBS stands for Direct Broadcast Satellite.
A general term that is commonly used to describe satellites and satelliet systems that broadcast information directly to individual end user.
Q69) Explain "DVB" ?
Answer: DVB stands for Digital Video Braodcasting.
It is a coherent set of European standards for transmission and reception of digital television signals via satellite, cable or terrestrial means, developed under the auspices of the digital Video broadcasting project and formalized by the European Telecommunication Standards Institute (ETSI).
Q70) Define "Antenna Alignment" ?
Answer: Antenna alignment is defined as the process of optimising the orientation of a satellite antenna's main direction of sensitivity towards the satellite to maximise the received signal level and to minimize the chance of receiving un-wanted interference from other satellite systems.
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