(SEM VIII) THEORY EXAMINATION 2017-18 SATELLITE COMMUNICATION

SECTION A (Brief Answers)

Types of Antennas Used in Satellite Communication
Common antennas used are parabolic reflector antennas, horn antennas, helical antennas, phased array antennas, and cassegrain antennas. Parabolic dish antennas are most widely used for high gain and directional transmission.

Block Diagram of Satellite Communication System
A satellite communication system consists of an information source, modulator, uplink transmitter, transmitting antenna, satellite transponder, receiving antenna, demodulator, and destination. The signal is transmitted to the satellite (uplink), amplified and shifted in frequency by the transponder, and sent back to Earth (downlink).

Argument of Perigee
Argument of perigee is the angle between the ascending node and the perigee of an orbit, measured in the orbital plane. It defines orientation of elliptical orbit.

Ice Effect
Ice effect occurs when ice accumulates on antennas, causing signal attenuation and distortion due to scattering and absorption.

Types of Satellite Launchers
Satellite launch vehicles include expendable launch vehicles (ELV), reusable launch vehicles (RLV), and heavy lift launchers.

Look Angles
Look angles are azimuth and elevation angles required to point an earth station antenna towards a satellite.

Sun Transit Outage
Sun transit outage occurs when the sun aligns directly behind a satellite, causing interference due to solar radiation noise.

Advantages of Satellite Communication
It provides wide coverage, reliable communication, high bandwidth, and is useful in remote areas.

TT and C Systems
TT&C stands for Telemetry, Tracking and Command systems. These systems monitor satellite health, track its position, and send control commands.

LEO vs GEO Satellites
LEO satellites orbit at low altitude with shorter delay and are used for mobile communication. GEO satellites orbit at 35,786 km and appear stationary, mainly used for broadcasting.

SECTION B (Medium Length Answers)

Look Angle Calculation Concept
Look angles are calculated using earth station latitude, longitude, and satellite longitude. Elevation angle determines how high above horizon to point antenna, while azimuth angle determines horizontal direction. These are essential for accurate antenna alignment.

Basic Principle of GPS
GPS determines position by measuring time delay of signals from satellites. Using trilateration, distance from at least four satellites is calculated. Four satellites are needed to determine three-dimensional position and correct receiver clock errors.

Working of DBS Home Receiver
In DBS television, signals from satellite are received by dish antenna, passed through low noise block converter (LNB), converted to lower frequency, then demodulated and decoded in set-top box to display audio and video signals.

Error Detection in Code Word
To verify if received code word is correct, parity check or syndrome calculation using generator matrix is performed. If syndrome is zero, code word is correct; otherwise error is detected and corrected.

Interferences in Satellite Link
Satellite links are affected by rain attenuation, atmospheric absorption, intermodulation interference, cross-polarization interference, and solar noise. These can be minimized using higher power, frequency planning, polarization isolation, and adaptive coding.

SECTION C (10-Mark Style Answers)

VSAT and LEO for Internet
VSAT (Very Small Aperture Terminal) systems provide two-way satellite communication for remote areas using small antennas. LEO satellites provide low latency internet service because of their lower orbit, but require multiple satellites for continuous coverage.

Factors Affecting Uplink and Downlink Design
Uplink design depends on transmitter power, antenna gain, free space loss, and atmospheric losses. Downlink design depends on satellite EIRP, receiving antenna gain, system noise temperature, and propagation losses. Proper link budgeting ensures required C/N ratio.

Working of DBS Television Network
DBS system transmits TV signals from uplink station to satellite, where signals are amplified and retransmitted to subscribers. At home, dish antenna collects signal, LNB converts frequency, and set-top box processes signal for viewing.

Kepler’s Laws and GEO Satellite
Kepler’s first law states orbit is elliptical. Second law states equal areas are swept in equal times. Third law relates orbital period to semi-major axis. For GEO satellite, orbital period equals 24 hours, so it remains fixed relative to Earth.

General Link Equation
The link equation calculates received power considering transmitted power, antenna gains, and path loss.
C/N = (Carrier Power) / (Noise Power).
G/T ratio = Antenna Gain / System Noise Temperature. Higher G/T improves receiver performance.

Non-Geostationary Satellites
Non-GEO satellites include LEO and MEO. They move relative to Earth, have lower delay, but require tracking antennas and multiple satellites for coverage.

Satellite Transponder
A transponder receives uplink signal, amplifies it, shifts frequency, and retransmits it back to Earth. It consists of low noise amplifier, frequency converter, and power amplifier.

Propagation Impairments
Impairments include rain fading, atmospheric loss, multipath fading, and scintillation. C-band is preferred because it suffers less rain attenuation compared to higher frequencies.