SSPI Satellite Communications Technologies & Systems 2 — Questions and Answers
Question 1: What is the primary advantage of a Low Earth Orbit (LEO) satellite constellation compared to a single GEO satellite for broadband internet?
- Lower latency due to shorter signal path (Correct answer)
- Higher power output per satellite
- Wider coverage from a single satellite
- Simpler ground station tracking requirements
Correct answer: Lower latency due to shorter signal path
LEO satellites orbit at 500–2,000 km altitude, resulting in round-trip latency of 20–40 ms versus ~600 ms for GEO.
Question 2: Which modulation scheme is commonly used in DVB-S2 to maximize spectral efficiency under good signal conditions?
- BPSK
- QPSK
- 16APSK (Correct answer)
- GMSK
Correct answer: 16APSK
DVB-S2 supports adaptive coding and modulation up to 32APSK, with 16APSK used under moderately favorable link conditions for high spectral efficiency.
Question 3: A satellite's Effective Isotropic Radiated Power (EIRP) is defined as:
- Transmitter power divided by antenna gain
- Transmitter power multiplied by antenna gain (Correct answer)
- Received signal power at the ground terminal
- Noise power spectral density at the receiver
Correct answer: Transmitter power multiplied by antenna gain
EIRP = transmit power × antenna gain (or equivalently, Pt + Gt in dB), representing the total effective power radiated toward the target.
Question 4: In satellite communications, what does the term 'rain fade' describe?
- Signal loss caused by satellite orbital decay
- Attenuation of Ka- or Ku-band signals by precipitation (Correct answer)
- Interference from adjacent satellites during rainstorms
- Increased thermal noise during humid weather
Correct answer: Attenuation of Ka- or Ku-band signals by precipitation
Rain fade is the absorption and scattering of microwave signals above ~10 GHz by raindrops, significantly impacting Ka-band (26.5–40 GHz) links.
Question 5: Which multiple access technique assigns a unique frequency slot to each earth station for the duration of a call, regardless of whether data is being transmitted?
- TDMA
- FDMA (Correct answer)
- CDMA
- OFDMA
Correct answer: FDMA
FDMA (Frequency Division Multiple Access) reserves a dedicated frequency channel per user for the call duration, making it inefficient for bursty traffic.
Question 6: What is the purpose of a satellite's apogee kick motor (AKM)?
- To maintain station-keeping at GEO
- To transfer the satellite from a transfer orbit to its operational orbit (Correct answer)
- To despin the satellite after separation from the launch vehicle
- To power the attitude control thrusters during eclipse
Correct answer: To transfer the satellite from a transfer orbit to its operational orbit
The AKM fires at the apogee of the geostationary transfer orbit (GTO) to circularize the orbit at GEO altitude (~35,786 km).
Question 7: Polarization reuse in satellite communications allows a transponder to:
- Operate at two different frequencies simultaneously
- Double spectrum capacity by transmitting two signals on orthogonal polarizations (Correct answer)
- Reduce rain fade by switching polarization during precipitation
- Increase antenna gain by combining both polarizations
Correct answer: Double spectrum capacity by transmitting two signals on orthogonal polarizations
Frequency reuse via orthogonal polarizations (e.g., RHCP/LHCP or V/H) doubles the usable bandwidth within the same frequency band.
SSPI Satellite Communications Technologies & Systems 2
What is the primary advantage of a Low Earth Orbit (LEO) satellite constellation compared to a single GEO satellite for broadband internet?