SBCA Satellite Theory & Signal Transmission 4 — Questions and Answers
Question 1: In satellite forward error correction (FEC), what does a code rate of 3/4 mean?
- For every 4 bits transmitted, 3 are information bits and 1 is redundancy (Correct answer)
- Three data streams are combined into four redundant channels
- The encoder processes data at 75% of the transponder clock rate
- Three-quarters of the available bandwidth is reserved for error correction overhead
Correct answer: For every 4 bits transmitted, 3 are information bits and 1 is redundancy
A 3/4 code rate means 3 information bits are transmitted for every 4 bits on the channel; the additional bit is redundancy used for error correction at the receiver.
Question 2: What is the effect of increasing the symbol rate (baud rate) of a satellite carrier while keeping the transponder bandwidth constant?
- The carrier eventually exceeds the transponder bandwidth, causing adjacent channel interference (Correct answer)
- The signal power increases proportionally without bandwidth impact
- The noise floor decreases, improving link margin
- The satellite can accept more simultaneous users on the same transponder
Correct answer: The carrier eventually exceeds the transponder bandwidth, causing adjacent channel interference
Symbol rate and bandwidth are directly related; increasing symbol rate beyond the transponder bandwidth causes spectral spillover and interference to adjacent carriers.
Question 3: Which type of satellite orbit is used by GPS, Galileo, and Glonass navigation systems?
- Medium Earth Orbit (MEO) (Correct answer)
- Low Earth Orbit (LEO)
- Geostationary Orbit (GEO)
- Highly Elliptical Orbit (HEO)
Correct answer: Medium Earth Orbit (MEO)
Navigation constellations use MEO at approximately 19,000–24,000 km altitude, providing global coverage with relatively few satellites compared to LEO.
Question 4: In satellite link budgets, free-space path loss (FSPL) increases with what two parameters?
- Frequency and distance (Correct answer)
- Satellite EIRP and earth station G/T
- Transponder bandwidth and modulation order
- Antenna aperture and noise temperature
Correct answer: Frequency and distance
FSPL (dB) = 20log(d) + 20log(f) + 92.44 (for km and GHz), showing that higher frequency and greater distance both increase path loss.
Question 5: What is the purpose of a low-noise block downconverter (LNB) in a satellite receive system?
- Amplify the weak satellite signal and convert it to a lower intermediate frequency with minimal added noise (Correct answer)
- Decode the digital signal and remove forward error correction overhead
- Control the dish azimuth and elevation to track the satellite
- Filter adjacent satellite interference before signal demodulation
Correct answer: Amplify the weak satellite signal and convert it to a lower intermediate frequency with minimal added noise
The LNB amplifies the downlink signal at the dish focal point and frequency-converts it to an L-band IF (950–2150 MHz) for transmission over coaxial cable to the indoor receiver.
Question 6: Frequency reuse in satellite systems allows the same frequency band to be used multiple times by exploiting which technique(s)?
- Orthogonal polarizations and/or spatially separated spot beams (Correct answer)
- Higher satellite transmit power and wider transponder bandwidth
- Advanced error correction codes and adaptive modulation
- Spreading codes in CDMA and wider channel spacing
Correct answer: Orthogonal polarizations and/or spatially separated spot beams
Satellites reuse spectrum by separating signals with orthogonal polarizations (horizontal/vertical or left/right circular) and with geographically isolated spot beams that limit interference.
Question 7: What is a geosynchronous satellite, and how does it differ from a geostationary satellite?
- Geosynchronous matches Earth's rotation period but may have non-zero inclination; geostationary is also equatorial (Correct answer)
- Geosynchronous is in LEO orbit; geostationary is in MEO orbit
- Geosynchronous uses elliptical orbits only; geostationary uses circular orbits only
- Geosynchronous and geostationary are interchangeable terms with no distinction
Correct answer: Geosynchronous matches Earth's rotation period but may have non-zero inclination; geostationary is also equatorial
All geostationary satellites are geosynchronous, but geosynchronous satellites with non-zero inclination trace a figure-8 ground track (analemma) instead of remaining fixed over one point.
SBCA Satellite Theory & Signal Transmission 4
In satellite forward error correction (FEC), what does a code rate of 3/4 mean?