SBCA Satellite Theory & Signal Transmission 3 — Questions and Answers
Question 1: Rain fade affects satellite signals primarily in which frequency band and above?
- Above 10 GHz (Ku-band) (Correct answer)
- Above 1 GHz (L-band)
- Above 4 GHz (C-band)
- Above 30 MHz (VHF)
Correct answer: Above 10 GHz (Ku-band)
Rain attenuation becomes significant above approximately 10 GHz because raindrops are comparable in size to the wavelength, causing scattering and absorption.
Question 2: What is the purpose of uplink power control (UPC) in satellite earth stations?
- To compensate for rain fade and maintain a constant signal level at the satellite (Correct answer)
- To reduce the cost of transmission by lowering power during clear weather
- To prevent interference with adjacent satellites
- To synchronize the carrier with the satellite transponder clock
Correct answer: To compensate for rain fade and maintain a constant signal level at the satellite
UPC dynamically increases transmit power during rain events to maintain a stable received signal level at the satellite, counteracting uplink attenuation.
Question 3: Polarization isolation between cross-polarized transponders is measured by which parameter?
- Cross-Polarization Discrimination (XPD) (Correct answer)
- Carrier-to-Noise Ratio (C/N)
- Figure of Merit (G/T)
- Noise Power Density (N0)
Correct answer: Cross-Polarization Discrimination (XPD)
XPD quantifies how well a system isolates signals on orthogonal polarizations; rain can degrade XPD, causing interference between co-frequency cross-pol transponders.
Question 4: In DVB-S2 (Digital Video Broadcasting – Satellite, 2nd generation), which modulation scheme offers the highest spectral efficiency under good signal conditions?
- 32APSK (Correct answer)
- QPSK
- 8PSK
- BPSK
Correct answer: 32APSK
32APSK carries 5 bits per symbol and is the most spectrally efficient DVB-S2 modulation, used when the link margin is high enough to support it.
Question 5: Which orbital region is designated for geostationary satellites, and at what altitude above the equator does it exist?
- Clarke Belt, approximately 35,786 km (Correct answer)
- Van Allen Belt, approximately 20,000 km
- Low Earth Orbit, approximately 400–1,200 km
- Medium Earth Orbit, approximately 8,000 km
Correct answer: Clarke Belt, approximately 35,786 km
The Clarke Belt (named after Arthur C. Clarke) at ~35,786 km is where satellite orbital period equals Earth's rotation period, making the satellite appear stationary.
Question 6: What is intermodulation distortion (IMD) in satellite transponders, and when does it become most problematic?
- Unwanted mixing products created when multiple carriers share a nonlinear amplifier (Correct answer)
- Doppler-induced frequency shifts when a satellite moves relative to earth stations
- Phase noise added by the local oscillator in the frequency conversion chain
- Thermal noise generated by the transponder low-noise amplifier
Correct answer: Unwanted mixing products created when multiple carriers share a nonlinear amplifier
IMD occurs when two or more carriers share a nonlinear device (like a TWTA near saturation), generating spurious frequencies that interfere with adjacent carriers.
Question 7: A satellite beacon signal is typically used by earth stations for which primary purpose?
- Antenna pointing and tracking the satellite (Correct answer)
- Encrypting the primary payload signal
- Providing a timing reference for TDMA bursts
- Monitoring rain fade in the downlink only
Correct answer: Antenna pointing and tracking the satellite
Earth stations lock onto the continuous beacon signal to accurately point and track the satellite, and also use it to measure propagation conditions.
SBCA Satellite Theory & Signal Transmission 3
Rain fade affects satellite signals primarily in which frequency band and above?