SSPI Satellite Communications Technologies & Systems 5 — Questions and Answers
Question 1: What is the primary purpose of Forward Error Correction (FEC) in a satellite communications system?
- Retransmit lost packets from the ground station
- Add redundant bits to allow the receiver to detect and correct errors without retransmission (Correct answer)
- Encrypt data before uplink transmission
- Reduce the carrier frequency to improve propagation
Correct answer: Add redundant bits to allow the receiver to detect and correct errors without retransmission
FEC codes (e.g., Turbo codes, LDPC) add calculated redundancy so the receiver can reconstruct corrupted bits, crucial for one-way broadcast links where retransmission is impractical.
Question 2: A satellite in a Molniya orbit is specifically designed to provide extended coverage over which geographic regions?
- Equatorial regions with highest population density
- High-latitude polar and sub-polar regions (Correct answer)
- Maritime routes in the tropics
- Mid-latitude regions in the continental US
Correct answer: High-latitude polar and sub-polar regions
Molniya orbits (63.4° inclination, ~12-hour period) spend most of their time at high apogee over the northern latitudes, providing long dwell times over Arctic and high-latitude areas.
Question 3: What is 'intermodulation distortion' in a satellite transponder, and when does it become most severe?
- Frequency drift caused by thermal cycling; worst during eclipse
- Nonlinear mixing of multiple carriers in an amplifier; worst when operating near saturation (Correct answer)
- Signal reflection from the satellite bus structure; worst at specific aspect angles
- Timing errors between TDMA bursts; worst at high traffic loads
Correct answer: Nonlinear mixing of multiple carriers in an amplifier; worst when operating near saturation
When multiple carriers share a nonlinear amplifier near saturation, their mixing products create intermodulation (IM) products that interfere with adjacent carriers; output back-off reduces but doesn't eliminate IM.
Question 4: In the context of satellite communications, what does 'orbital debris mitigation' require of GEO satellites at end of life?
- Deorbit to a 200 km altitude graveyard orbit within 5 years
- Boost to a graveyard orbit at least 300 km above GEO (Correct answer)
- Maintain station-keeping indefinitely using electric propulsion
- Transfer to MEO to reduce collision risk with LEO constellations
Correct answer: Boost to a graveyard orbit at least 300 km above GEO
ITU and national regulations require end-of-life GEO satellites to be boosted to a graveyard orbit ≥300 km above GEO (≥36,086 km) to protect the operational GEO belt.
Question 5: What advantage does Direct Broadcast Satellite (DBS) service have over traditional FSS (Fixed Satellite Service) in the context of consumer reception?
- DBS operates at lower frequencies with less rain fade
- DBS uses higher EIRP spot beams enabling small (45–90 cm) consumer dishes (Correct answer)
- DBS satellites are in LEO for lower latency reception
- DBS uses unencrypted signals for universal access
Correct answer: DBS uses higher EIRP spot beams enabling small (45–90 cm) consumer dishes
DBS transponders transmit at high power in concentrated spot beams, allowing households to receive signals with small, inexpensive fixed dishes rather than large C-band dishes.
Question 6: Which parameter most directly determines the maximum data throughput achievable through a satellite transponder with a fixed bandwidth?
- Carrier frequency (uplink vs. downlink)
- Spectral efficiency of the modulation and FEC coding scheme (Correct answer)
- Satellite orbital altitude
- Number of earth stations sharing the transponder
Correct answer: Spectral efficiency of the modulation and FEC coding scheme
Spectral efficiency (bits/s/Hz), determined by the modulation order and FEC rate, directly sets the data rate achievable within a given transponder bandwidth.
Question 7: In satellite network design, what is the role of a Network Control System (NCS) or Network Management System (NMS)?
- Provides RF amplification redundancy for the satellite payload
- Monitors, configures, and manages bandwidth allocation and terminal parameters across the network (Correct answer)
- Performs physical layer encryption of all uplink carriers
- Acts as a backup earth station during primary hub failures
Correct answer: Monitors, configures, and manages bandwidth allocation and terminal parameters across the network
The NCS/NMS centrally controls bandwidth assignment, monitors link quality, configures terminal parameters, and manages faults across all nodes in the satellite network.
SSPI Satellite Communications Technologies & Systems 5
What is the primary purpose of Forward Error Correction (FEC) in a satellite communications system?