Question 1

Q.1. Write the four advantages of satellite communication.

Accepted Answer

Four advantages of satellite communication are: wide-area/global coverage from a single satellite, distance-independent cost, high bandwidth capacity for broadcast and broadband services, and rapid deployability for disaster/remote-area communication.

Question 2

Q.2. Calculate the time required for signal transmit from earth station to GE…

Accepted Answer

Using the GEO altitude of about 35,786 km and the speed of light, the one-way signal propagation time from an earth station to a geostationary satellite is approximately 119.3 milliseconds.

Question 3

Q.3. Describe the application of MEO satellite.

Accepted Answer

MEO satellites (orbital altitude ~2,000-35,786 km) are primarily used for global navigation satellite systems (GPS, GLONASS, Galileo, NavIC) and some regional communication/telephony systems, offering a balance between coverage area and signal latency.

Question 4

Q.4. Why Uplink frequency is greater than downlink frequency in satellite com…

Accepted Answer

The uplink frequency is kept higher than the downlink frequency because higher-frequency signals suffer greater atmospheric attenuation, and the earth station (uplink transmitter) can more easily supply the extra transmit power needed to compensate for this loss than the power-limited satellite transponder (downlink transmitter) can.

Question 5

Q.5. Name the three axis used for satellite communication expressions.

Accepted Answer

The three axes used to express satellite attitude are roll, pitch and yaw — rotations about the satellite's velocity-direction (roll), orbit-normal (pitch), and nadir-pointing (yaw) axes respectively.

Question 6

Q.6. What is the importance of solar eclipse in satellite link design?

Accepted Answer

Solar eclipse (when the Earth blocks sunlight from reaching a GEO satellite, near the equinoxes) is important in satellite link design because during eclipse the satellite must rely entirely on its onboard batteries for power, requiring adequate battery capacity to be designed into the satellite and potential link/service interruption to be anticipated.

Question 7

Q.7. What is the importance of LNA in satellite link design?

Accepted Answer

The Low Noise Amplifier (LNA) is the first active stage in an earth station receiver and is critical because it dominantly determines the overall system noise temperature (per Friis' formula), directly setting the achievable link's carrier-to-noise ratio and hence the smallest usable satellite signal that can be reliably received.

Question 8

Q.8. List the components used in earth station of Satellite communication.

Accepted Answer

The main components used in an earth station are the antenna, Low Noise Amplifier (LNA), High Power Amplifier (HPA), up-converter and down-converter, modem (modulator/demodulator), and tracking/control system.

Question 9

Q.9. Which device is used in satellite to maintain the communication frequenc…

Accepted Answer

An onboard atomic/crystal reference oscillator (frequency synthesizer/local oscillator) is used within the satellite transponder to maintain the communication frequency of the signal accurately and stably.

Question 10

Q.10. Define the types of gain and losses that occur in satellite link design.

Accepted Answer

Gains in a satellite link include antenna gain (transmit and receive) and low-noise amplifier gain, while losses include free-space path loss, atmospheric/rain attenuation, feeder/branching losses, pointing loss, and polarization mismatch loss, all combined in the overall link budget equation.

Question 11

Q.1. What are the different orbits employed in satellite communication and wh…

Accepted Answer

Satellite orbits are classified as LEO (160-2000km, low latency, used for imaging/broadband constellations), MEO (2000-35786km, used for GNSS navigation), GEO (35786km, fixed position, used for broadcast/communication/weather), and highly elliptical orbits like Molniya (used for high-latitude coverage).

Question 12

Q.2. For an earth station transmitter with an output power of 40 dBW (10000 W…

Accepted Answer

For an earth station transmitter with 40 dBW output power, 3 dB back-off loss, 3 dB branching/feeder loss and 40 dB antenna gain, the EIRP is calculated as 40-3-3+40 = 74 dBW.

Question 13

Q.3. What are the methods of stabilizing a satellite in the orbit? What are t…

Accepted Answer

Satellites are stabilized in orbit using spin stabilization (rotating the satellite body for gyroscopic stability, simple but limiting continuous antenna pointing) or three-axis (momentum-bias/reaction-wheel) stabilization (independently controlling roll/pitch/yaw for continuous, precise antenna and solar-panel pointing, at the cost of greater system complexity).

Question 14

Q.4. A quasi-GEO satellite is in a circular equatorial orbit close to geosync…

Accepted Answer

A quasi-GEO satellite with an orbital period of exactly one 24-hour solar day (rather than the 23h56m sidereal day of a true GEO satellite) requires a slightly larger orbital radius of approximately 42,240 km (compared to true GEO's 42,164 km), and since its period exceeds Earth's rotation period, the satellite gradually drifts westward relative to the ground.

Question 15

Q.5. Explain the different types of antennas used in satellite communication.

Accepted Answer

Common antenna types in satellite communication include parabolic reflector antennas (prime-focus and Cassegrain), horn antennas, and phased-array antennas, each offering different gain, beamwidth and beam-steering characteristics suited to different earth-station and satellite applications.

Question 16

Q.6. Find and explain the system noise temperature of receiver used in satell…

Accepted Answer

System noise temperature is the equivalent noise temperature representing the combined thermal noise contribution of the antenna, feeder, LNA, mixer and IF stages of a receiver, referred to the receiver input, calculated using the Friis cascaded-noise formula: Ts = Tin + TRF + TM/GRF + TIF/(GRF·GM).

Question 17

Q.7. What is the need of modulation schemes in satellite communication? Expla…

Accepted Answer

Modulation schemes are needed to efficiently encode baseband information onto the satellite's RF carrier for transmission through the space channel; common schemes include FM (analog, historically for TV/voice), and digital schemes like BPSK, QPSK and higher-order QAM, chosen to balance power efficiency, bandwidth efficiency and robustness to the satellite channel's noise and nonlinearity.

Question 18

Q.1. Explain the Attitude and Orbit Control System (AOCS) sub-systems used in…

Accepted Answer

The Attitude and Orbit Control System (AOCS) maintains a satellite's correct orientation (attitude) and orbital position (station-keeping) using sensors (sun/star/earth sensors, gyroscopes), actuators (reaction wheels, magnetic torquers, thrusters), and an onboard control processor, ensuring antennas remain correctly pointed and the satellite stays within its assigned orbital slot.

Question 19

Q.2. Consider an earth station receiver at 4 GHz has following gains and nois…

Accepted Answer

Using the Friis cascaded noise formula with the given data (Tin=60K, TRF=60K, TM=600K, TIF=1200K, GRF=17dB, Gm=0dB, GIF=25dB), the referred-to-input system noise temperature works out to approximately 155.9 K.

Question 20

Q.3. What is the importance of LNA and HPA in satellite communication? Descri…

Accepted Answer

LNA and HPA are critical earth station components: the LNA minimizes receiver noise to maximize achievable C/N for the weak downlink signal, while the HPA amplifies the uplink signal to the power level required for reliable satellite reception; common LNA types include GaAsFET/HEMT/cryogenically-cooled amplifiers, while common HPA types include TWTAs and SSPAs.

Question 21

Q.4. An earth station employs a power amplifier providing an output power of …

Accepted Answer

For an earth station with 100W output power, 5m antenna at 6.25GHz (uplink) and 4.5GHz (downlink), assuming 55% aperture efficiency, the calculated transmit antenna gain is about 47.7dB giving EIRP ≈ 67.7dBW, and the receive antenna gain is about 44.85dB, giving a G/T ratio of approximately 23.4 dB/K for a 140K system noise temperature.

Question 22

Q.5. Compare FAMA, DAMA and TDMA multiple access technologies for satellite c…

Accepted Answer

FAMA (Frequency division, contention-based access) statically or contentiously assigns frequency slots, DAMA (Demand Assignment Multiple Access) dynamically allocates channels on request for more efficient utilization of limited transponder capacity, and TDMA (Time Division Multiple Access) shares a single carrier frequency among users by allocating each a distinct time slot, offering the highest flexibility and efficiency for digital, bursty traffic.

Satellite Communication