Avionics Multiple Choice Questions on “Atmospheric Effects on Satellite Signals”.
1. How does troposphere affect the satellite signals?
a) Reduces velocity
b) Reflects the signals
c) Refracts the signal
d) Bit inversion occurs
Answer: a
Clarification: The major sources of errors are when the signal pass through the atmosphere. The troposphere causes the propagation velocity of the signal to be slowed, compressing the signal wavelength.
2. Which of the following makes the existence of ionosphere possible?
a) Rotation of the Earth
b) Ultraviolet radiation from sun
c) Solar flares
d) Radiation from distant stars
Answer: b
Clarification: The ionosphere owes its existence to the ultraviolet radiation from the sun. The photons possess a certain amount of energy which is enough to break down electrons from their parent atom. The result is a large number of free, negatively charged, electrons and positively charged atoms and molecules called ions.
3. Satellite signals are refracted by the ionosphere.
a) True
b) False
Answer: b
Clarification: Below 30MHz frequency the ionosphere almost acts like a mirror reflecting the signal back to Earth. At higher frequencies, such as those used in satellite communication, radio waves pass through the atmosphere.
4. What happens to the satellite signals as the density of the ionosphere is high?
a) Velocity increases
b) Velocity decreases
c) Signal strength increases
d) Frequency reduces
Answer: a
Clarification: The velocity of the carrier, the pure sinusoidal wave, is increased in the presence of electrons. Thus the greater the density of electrons in the ionosphere greater the velocity of the signal.
5. What is the increase in velocity of the signal by the ionosphere termed as?
a) Differential velocity
b) Velocity advance
c) Phase advance
d) Signal advance
Answer: c
Clarification: The result of satellite signal travelling through ionosphere is that a particular phase of the signal arrives at the receiver earlier than it would have had if the signal travelled in a complete vacuum. This early arrival is termed as phase advance.
6. TEC in the ionosphere is a function of solar radiation.
a) True
b) False
Answer: a
Clarification: TEC or the total electron content in the ionosphere is a function of solar radiation. As more radiation falls on Earth, more number of molecules are ionized and hence TEC is more in the daytime than at the night.
7. Which of the following is true with respect to ionospheric scintillation effects?
a) It causes signal fading
b) Occurs at the equatorial and polar regions
c) Occurs mostly at day
d) Occurs when high solar activity is present
Answer: c
Clarification: Ionospheric scintillation causes signal fading and effects in the equatorial and the Polar Regions. Strong scintillation effects are rare localized at certain times during the night, and usually occurs during periods of high solar activity.
8. The refractivity of the troposphere is not a function of ________
a) Frequency
b) Temperature
c) Pressure
d) Partial pressure of water vapour
Answer: a
Clarification: Unlike the refractivity of the ionosphere, the refractivity of the troposphere is not the function of carrier frequency. At a given altitude it is determined from where P is total pressure, T is the absolute temperature, and e is the partial pressure of water vapour.