Avionics Multiple Choice Questions on “Satellite Communication Systems”.
1. The satellite that is used as a relay to extend communication distance is called as __________
a) Relay satellites
b) Communication satellites
c) Repeater satellites
d) Geosynchronous satellites
Answer: b
Clarification: Communication satellites are not originators of information to be transmitted. If a transmitting station cannot communicate directly with one or more receiving stations because of line-of-sight restrictions, a satellite can be used. The transmitting station sends the information to the satellite, which in turn re-transmits it to the receiving stations.
2. The transmitter-receiver combination in the satellite is known as a _______
a) Relay
b) Repeater
c) Transponder
d) Duplexer
Answer: c
Clarification: The transmitter-receiver combination in the satellite is known as a transponder. The basic functions of a transponder are amplification and frequency translation. The reason for frequency translation is that the transponder cannot transmit and receive on the same frequency.
3. The downlink frequency is lower than the uplink frequency.
a) True
b) False
Answer: a
Clarification: The original signal being transmitted from the earth station to the satellite is called the uplink, and the re-transmitted signal from the satellite to the receiving stations is called the downlink. Usually, the downlink frequency is lower than the uplink frequency. A typical uplink frequency is 6 GHz, and a common downlink frequency is 4 GHz.
4. What is the reason for carrying multiple transponders in a satellite?
a) More number of operating channel
b) Better reception
c) More gain
d) Redundancy
Answer: a
Clarification: To be economically feasible, a satellite must be capable of handling several channels. As a result, most satellites contain multiple transponders, each operating at a different frequency. Each transponder represents an individual communication channel.
5. Why are VHF, UHF, and microwave signals used in satellite communication?
a) More bandwidth
b) More spectrum space
c) Are not diffracted by the ionosphere
d) Economically viable
Answer: c
Clarification: VHF, UHF, and microwave signals penetrate the ionosphere with little or no attenuation and are not refracted to earth. Lower frequencies undergo total internal refraction and get reflected back to earth.
6. What is the reason for shifting from c band to ku band in satellite communication?
a) Lesser attenuation
b) Less power requirements
c) More bandwidth
d) Overcrowding
Answer: d
Clarification: Most new communication satellites will operate in the Ku band. This upward shift in frequency is happening because the C band is overcrowded. Many communication satellites are in orbit now, most of them operating in the C band. However, there is some difficulty with interference because of the heavy usage. The only way this interference will be minimized is to shift all future satellite communication to higher frequencies.
7. Which of the following bands cannot be used for satellite communication?
a) MF
b) Ku
c) X
d) C
Answer: a
Clarification: MF is a lower frequency band than Ku, C and X bands and does not lie in the microwave spectrum. Microwaves are used for satellite communication since the lower bands get reflected by the ionosphere.
8. What is the maximum theoretical data rate if a transponder is used for binary transmission and has a bandwidth of 36MHz?
a) 32Mpbs
b) 72Mpbs
c) 36Mpbs
d) 12Mpbs
Answer: b
Clarification: For binary transmission, the maximum theoretical data rate or channel capacity C for a given bandwidth B is C = 2B = 2(36) =72Mpbs.
9. Why are techniques like frequency reuse and spatial isolation carried out?
a) Reduce traffic load
b) More gain
c) High speed
d) Error detection
Answer: a
Clarification: at times there is more traffic than there are transponders to handle it. For that reason, numerous techniques have been developed to effectively increase the bandwidth and signal-carrying capacity of the satellite. Two of these techniques are known as frequency reuse and spatial isolation.
10. Which technique uses two different antennas to reduce traffic on the same frequency?
a) Spatial isolation
b) Frequency reuse
c) Multiplexing
d) Modulation
Answer: b
Clarification: In the frequency reuse technique two systems use the same frequency, although operating on exactly the same frequencies, they are isolated from each other by the use of special antenna techniques. For example, a vertically polarized antenna will not respond to a horizontally polarized signal and vice versa. Or a left-hand circularly polarized (LHCP) antenna will not respond to a right-hand circularly polarized (RHCP) signal and vice versa.
11. Which technique uses spot beam antennas to divide the area covered by the satellite into smaller segments?
a) Spatial isolation
b) Frequency reuse
c) Multiplexing
d) Modulation
Answer: a
Clarification: By using narrow beam or spot beam antennas, the area on the earth covered by the satellite can be divided up into smaller segments. Earth stations in each segment may actually use the same frequency, but because of the very narrow beam widths of the antennas, there is no interference between adjacent segments.
12. Spatial-division multiple access (SDMA) depends on satellite location and not frequency.
a) True
b) False
Answer: a
Clarification: Spatial-division multiple access uses spatial isolation technique. Earth stations in each segment may actually use the same frequency, but because of the very narrow beam widths of the antennas, there is no interference between adjacent segments. This technique is referred to a spatial-division multiple access (SDMA) in that access to the satellite depends on location and not frequency.