SBCA System Troubleshooting & Maintenance 5 — Questions and Answers
Question 1: A satellite technician notices that a dish's signal peaks at an azimuth 3 degrees off from the calculated value. What is the most likely reason?
- The satellite has drifted from its assigned orbital slot
- Local magnetic declination was not applied when using a compass for alignment (Correct answer)
- The dish reflector surface has deformed
- The LNB skew is set incorrectly
Correct answer: Local magnetic declination was not applied when using a compass for alignment
Magnetic compasses show magnetic north, which differs from true north by the local declination angle; ignoring this offset causes azimuth pointing errors.
Question 2: What is the function of a waveguide pressurization system in a satellite uplink antenna system?
- To increase the signal power transmitted through the waveguide
- To prevent moisture ingress and maintain consistent dielectric properties inside the waveguide (Correct answer)
- To cool the HPA by forcing air through the transmission path
- To reduce standing wave ratio in the waveguide
Correct answer: To prevent moisture ingress and maintain consistent dielectric properties inside the waveguide
Pressurization with dry nitrogen or dehumidified air prevents condensation inside waveguides, which would cause arcing, corrosion, and signal attenuation.
Question 3: When a satellite modem reports 'ACM mode: no coding found,' what does this indicate in a DVB-S2X adaptive link?
- The modem has successfully adapted to current link conditions
- The uplink controller cannot find a valid modulation and coding scheme for current link quality (Correct answer)
- ACM mode has been disabled by the network operator
- The symbol rate is incompatible with ACM operation
Correct answer: The uplink controller cannot find a valid modulation and coding scheme for current link quality
ACM (Adaptive Coding and Modulation) requires a valid coding scheme selection; 'no coding found' means link conditions are below the minimum threshold for any available MODCOD.
Question 4: A headend technician observes that signal quality on C-band drops every evening between 6-8 PM but not on Ku-band systems. What is the most plausible cause?
- Evening peak grid power causing C-band interference
- Terrestrial microwave interference from C-band frequency reuse links increasing at peak traffic hours (Correct answer)
- Solar interference affecting only C-band frequencies
- Evening atmospheric inversion reducing C-band propagation
Correct answer: Terrestrial microwave interference from C-band frequency reuse links increasing at peak traffic hours
C-band (3.7-4.2 GHz) shares spectrum with terrestrial microwave links that may experience higher traffic and interference during business peak hours.
Question 5: After replacing a faulty LNB, a technician confirms signal strength returns to normal but BER remains high. What should be investigated next?
- The new LNB may have incorrect gain specifications
- The LNB connector or cable between LNB and IRD may have damage causing phase noise or signal distortion (Correct answer)
- The satellite has changed its transponder plan
- The IRD firmware needs updating to support the new LNB
Correct answer: The LNB connector or cable between LNB and IRD may have damage causing phase noise or signal distortion
Good signal strength with poor BER after an LNB swap often points to connector or cable issues that degrade signal quality without significantly reducing amplitude.
Question 6: What is the primary purpose of monitoring a satellite transponder's power spectral density (PSD) at a receive site?
- To calculate the uplink earth station EIRP
- To detect interference, carrier power imbalances, and unauthorized transmissions within the transponder (Correct answer)
- To measure the satellite's EIRP directly
- To determine the transponder's available bandwidth
Correct answer: To detect interference, carrier power imbalances, and unauthorized transmissions within the transponder
PSD monitoring allows operators to identify unexpected carriers, interference sources, and anomalies that would degrade legitimate services sharing the transponder.
Question 7: A VSAT hub operator receives reports that multiple remote sites lost connectivity simultaneously at the same time. What should be checked FIRST?
- Individual remote site equipment at each location
- Hub-side common equipment: HPA, modem, network routing, and uplink signal integrity (Correct answer)
- Satellite operator for transponder status
- ISP connectivity at each remote location
Correct answer: Hub-side common equipment: HPA, modem, network routing, and uplink signal integrity
Simultaneous failure at multiple remotes points to a common shared component — the hub-side equipment is the single point of failure that should be investigated first.
SBCA System Troubleshooting & Maintenance 5
A satellite technician notices that a dish's signal peaks at an azimuth 3 degrees off from the calculated value.
What is the most likely reason?