250+ TOP MCQs on Microwave Landing System and Answers

Avionics Multiple Choice Questions on “Microwave Landing System”.

1. What is the main weakness of the ILS system?
a) Less power
b) Less range
c) More noise
d) Environment sensitivity
Answer: d
Clarification: The main weakness of the ILS system was its sensitivity towards the environmental factors. Since the frequency used by the system is in MHz, it was more susceptible to atmospheric and weather interference.

2. What does the MLS use to overcome the weakness of ILS system?
a) Narrow beam width antennas
b) More power
c) Filers for separating noise
d) Array of antennas
Answer: a
Clarification: The main weakness of the ILS system is eliminated by using narrow beam width antennas which are physically small since the frequency is high. As frequency increases antenna size decreases.

3. Which of the following is not one of the basic components of the MLS system?
a) Azimuth ground station
b) Elevation ground station
c) DME
d) Radar
Answer: d
Clarification: A basic MLS consists of azimuth and elevation ground stations and a conventional DME for 3D positioning on approach course to 40° on either side of center line and to 15° elevation above the runway.

4. Why is a back azimuth station used in some MLS?
a) Better range
b) Less noise
c) Missed approach
d) Back up for main azimuth station
Answer: c
Clarification: An expanded MLS system uses back azimuth stations. It is generally used for departure and for missed approaches. It provides lateral guidance to 40° on either side of the center line.

5. What is the number of channels in the MLS system?
a) 10
b) 50
c) 4000
d) 200
Answer: d
Clarification: The MLS station transmit both angle and data functions on one of 200 frequencies between 5031.0 and 5190.7 MHz. The relatively high number of channels allows the use of multiple MLS in metropolitan cities.

6. Which of the following determines the required transmission power in MLS?
a) Noise
b) Bit error rate
c) Runway length
d) Aircraft size
Answer: b
Clarification: The acceptable bit error rate of the differential phase shift keying (DPSK) transmissions at the 20-nmi limit determines the transmitter power needed in the ground stations.

7. Using an audio tone to encode angles is an efficient method.
a) True
b) False
Answer: b
Clarification: Early MLS with mechanical scanned array used a varying audio tone to encode the pointing angle on the scanning beam pattern. The FAA adopted the time interval between successive passages of the unmodulated beam as an efficient means of angle encoding.

8. What is the typical scanning rate of the MLS antenna?
a) 20,000°/sec
b) 10000°/sec
c) 500°/sec
d) 37°/sec
Answer: a
Clarification: The very high scanning rate of 20,000°/sec provides about 40 samples per second of the angle data, a rate ten times higher than that needed to control the aircraft.

9. MLS ESA works on the principle of phase shifting.
a) True
b) False
Answer: a
Clarification: An Electronically scanned array antenna is an array of radiating elements with a feed network incorporating variable propagation delays. These arrays cause the antenna pattern to rotate by “phase shifting” the RF signal.