SSPI Satellite Communications Technologies & Systems 3 — Questions and Answers
Question 1: In a satellite link budget, the figure of merit G/T represents:
- Gain of the transmit antenna divided by system temperature
- Receive antenna gain divided by system noise temperature (Correct answer)
- Ground terminal output power relative to thermal noise
- Transponder gain-to-temperature ratio at GEO
Correct answer: Receive antenna gain divided by system noise temperature
G/T is the receive antenna gain (dBi) minus the system noise temperature (dBK), a key metric for evaluating earth station receive performance.
Question 2: Which orbital slot phenomenon requires geostationary satellites to perform regular north-south station-keeping maneuvers?
- Solar radiation pressure drift
- Lunar and solar gravitational perturbations causing inclination changes (Correct answer)
- Earth's oblateness causing east-west drift
- Atmospheric drag at GEO altitude
Correct answer: Lunar and solar gravitational perturbations causing inclination changes
Lunar and solar gravity cause the orbital plane to tilt (inclination), requiring north-south station-keeping burns typically consuming most of the satellite's propellant budget.
Question 3: A bent-pipe transponder in a communications satellite differs from an on-board processing (OBP) transponder in that it:
- Demodulates, processes, and re-modulates signals on board
- Simply amplifies and frequency-shifts the signal without demodulation (Correct answer)
- Connects directly to an IP router on the satellite
- Performs forward error correction before retransmission
Correct answer: Simply amplifies and frequency-shifts the signal without demodulation
A bent-pipe transponder is a frequency translator and amplifier only; OBP transponders demodulate, process, and re-modulate, enabling more flexible routing.
Question 4: The Nyquist bandwidth of a satellite carrier is most closely related to:
- The carrier frequency in the uplink band
- The symbol rate of the modulated signal (Correct answer)
- The transponder's saturated output power
- The orbital altitude of the satellite
Correct answer: The symbol rate of the modulated signal
The minimum (Nyquist) bandwidth equals the symbol rate (baud rate), so a 36 Msps signal requires at least 36 MHz of transponder bandwidth.
Question 5: What is the function of a Low-Noise Block downconverter (LNB) in a satellite receive system?
- Converts DC power to the satellite's downlink frequency
- Amplifies and downconverts the satellite signal with minimal added noise (Correct answer)
- Splits the signal to multiple receivers without amplification
- Tracks the satellite's position and adjusts antenna pointing
Correct answer: Amplifies and downconverts the satellite signal with minimal added noise
The LNB amplifies the weak satellite signal and converts it from Ku/Ka band to a lower IF frequency (950–2,150 MHz) for transmission over coaxial cable.
Question 6: Doppler shift is most significant for which type of satellite orbit when designing the ground segment?
- Geostationary orbit (GEO)
- Highly Elliptical Orbit (HEO)
- Low Earth Orbit (LEO) (Correct answer)
- Medium Earth Orbit (MEO)
Correct answer: Low Earth Orbit (LEO)
LEO satellites move rapidly relative to ground stations (~7.8 km/s), causing Doppler shifts of ±40 kHz at Ku-band that must be compensated in modem designs.
Question 7: In satellite communications, what does 'link margin' represent?
- The difference between available C/N and the minimum required C/N for reliable demodulation (Correct answer)
- The ratio of uplink power to downlink power
- The maximum EIRP the satellite is licensed to transmit
- The spare transponder capacity reserved for backup traffic
Correct answer: The difference between available C/N and the minimum required C/N for reliable demodulation
Link margin is the dB difference between the actual carrier-to-noise ratio and the threshold C/N required to achieve the target BER, providing a buffer against fading.
SSPI Satellite Communications Technologies & Systems 3
In a satellite link budget, the figure of merit G/T represents: