Category: Avionics Objective Questions

Avionics Multiple Choice Questions on “Digital Signal Processing”.

1. Any digital computer can be used for DSP.
a) True
b) False
Answer: a
Clarification: DSP is the use of a fast digital computer or digital circuitry to perform processing on digital signals. Any digital computer with sufficient speed and memory can be used for DSP.

2. What happens after the signal is passed through the analog to digital converter in a DSP?
a) Changed back to analog
b) Stored in a RAM
c) Amplified
d) Attenuated
Answer: b
Clarification: When the signal is converted from analog to digital it is a sequence of binary numbers which is stored in the RAM. A user defined code that is usually stored in the ROM performs mathematical and other manipulations after which it is converted back into analog signals.

3. Who is credited with creating the stored program concept?
a) John Von Neumann
b) Larry Page
c) Alan Turing
d) Ken Thompson
Answer: a
Clarification: Physicist John Von Neumann is generally credited with creating the stored program concept that is the basis of operation of all digital computers. Binary words representing computer instructions are stored sequentially in a memory to form a program. The instructions are fetched and executed one at a time at high speed.

4. What is the accessibility limitation of only one data or instruction set at a time from the memory called?
a) Von Neumann limitation
b) Von Neumann limit
c) Von Neumann speed
d) Von Neumann bottleneck
Answer: d
Clarification: The accessibility limitation of only one data or instruction set at a time from the memory is called as Von Neumann bottleneck. This has the effect of greatly limiting the execution speed.

5. Which type of architecture uses different storage space for program code and the data?
a) Von Neumann architecture
b) Harvard architecture
c) Fragmented architecture
d) Split cell architecture
Answer: b
Clarification: In a Harvard architecture microprocessor, there are two memories, a program or instruction memory, usually a ROM, and a data memory, which is a RAM. Also, there are two data paths into and out of the CPU between the memories. Because both instructions and data can be accessed simultaneously, very high-speed operation is possible.

6. What is the reason for the need of high speed DSP?
a) Less power consumption at higher speeds
b) Better processing capabilities
c) High sampling frequency
d) Easily programmable
Answer: c
Clarification: The time taken for input/output and the processing time together must be smaller than the sampling period to ensure the continuous flow of data. Since high sampling frequencies are needed for accurately converting the analog signal to digital, high speed DSP is a must.

7. Selectivity in a DSP is better than its analog equivalent.
a) True
b) False
Answer: a
Clarification: With DSP, the filters can have characteristics far superior to those of equivalent analog filters. Selectivity can be better because of the ease of controlling binary numbers, and the passband or reject band can be customized to the application.

8. Reduction in the number of binary words required to represent an analog signal is called ________
a) Undersampling
b) Oversampling
c) Data compression
d) Data minimization
Answer: c
Clarification: Data compression is a process that reduces the number of binary words needed to represent a given analog signal. Since analog to digital conversion produces a huge amount of data, for transmission it is a necessity that data is compressed.

9. What is the process of examining the frequency content of a signal?
a) Signal decoding
b) Spectrum analysis
c) Signal analysis
d) Data analysis
Answer: b
Clarification: Spectrum analysis is the process of examining a signal to determine its frequency content. Algorithms such as discrete Fourier transform (DFT) and FPGA is used to analyze the frequency content of an input signal.

10. What is the program that is used to speed the spectrum analysis process?
a) DDFT
b) FDFT
c) FGPA
d) Fast Fourier transforms
Answer: d
Clarification: The DFT is a complex program that is long and time-consuming to run. In general, computers are not fast enough to perform DFT in real time as the signal occurs. Therefore, a special version of the algorithm has been developed to speed up the calculation. Known as the fast Fourier transform (FFT), it permits real-time signal spectrum analysis.

11. Which of the following is not possible when the signal is analog?
a) Phase shifting
b) Equalization
c) Modulation
d) Data compression
Answer: d
Clarification: Data compression is done by checking redundancy in data. Data redundancy checking is only possible when the data is in digital form and hence data compression cannot be done in analog signals.

Avionics Multiple Choice Questions on “Satellite Subsystems”.

1. Which of the following is not a satellite subsystem?
a) Ground station
b) Power system
c) Telemetry tracking
d) Communication subsystem
Answer: a
Clarification: The communication subsystem is the most important part of the satellite. It requires varies additional systems like the power system, propulsion system, telemetry system for its proper functioning. The ground system however is not one of the satellite subsystem and is independent of the satellite. It is just a transponder to monitor and command the satellite.

2. Which of the following is not a part of the propulsion subsystem of a satellite?
a) Gyroscope
b) Jet thruster
c) AKM
d) Fuel control system
Answer: a
Clarification: The propulsion subsystem consists of the AKM(Apogee kick motor), jet thruster and the fuel control system. Gyroscopes and other attitude systems fall under the attitude control subsystem.

3. Which of the following are common baseband signals transmitted from the earth ground station?
a) Navigational data, computer data, video
b) Computer data, navigational data, voice
c) Voice, video, computer data
d) Computer data
Answer: c
Clarification: An earth station takes the signals to be transmitted, known as baseband signals, and modulates a microwave carrier. The three most common baseband signals are voice, video, and computer data.

4. Which of the following components receives, translates the signal frequency and re-transmits the signal in a satellite?
a) Repeater
b) Relay
c) Transponder
d) Transducer
Answer: c
Clarification: The uplink signals from earth are amplified, translated in frequency, and re-transmitted on the downlink to one or more earth stations. The component that performs this function is known as a transponder.

5. Why is there a huge spectrum space between the transmitted and received signal in satellite communication?
a) Reduce interference
b) Maximum efficiency
c) Less attenuation
d) To reduce space occupied by filters
Answer: a
Clarification: Because of the close proximity of the transmitter and the receiver in the satellite, the high transmitter output power for the downlink is picked up by that satellite receiver. Naturally, the uplink signal is totally obliterated. Furthermore, the transmitter output fed back into the receiver input causes oscillation. To avoid this problem, the receiver and transmitter in the satellite transponder are designed to operate at separate frequencies. In this way, they will not interfere with each other.

6. Which of the following transponders convert the uplink signal to downlink signal using two mixers
a) Single conversion transponders
b) Dual conversion transponders
c) Regenerative transponders
d) Dual mixer transponder
Answer: b
Clarification: A dual-conversion transponder makes the frequency translation in two steps with two mixers. No demodulation occurs.

7. In a regenerative transponder, the signal is demodulated and modulated again before transmission.
a) True
b) False
Answer: a
Clarification: A regenerative repeater demodulates the uplink signal after the frequency is translated to some lower intermediate frequency. The recovered baseband signal is then used to modulate the downlink signal.

8. What is the number of transponders if the satellite uses 12 channels of frequency and frequency reuse is implemented?
a) 12
b) 6
c) 24
d) 3
Answer: c
Clarification: Since in frequency reuse each channel can be used twice the numbers of transponders are also doubled. 12 x 2 = 24 transponders, two for each frequency.

9. Why is it not possible to provide transmit function by wideband amplifier and mixer circuits?
a) Heavy attenuation
b) High power output over wideband is not possible
c) Economically not profitable
d) Weight of the system increases five fold
Answer: b
Clarification: it is generally not possible to generate very high output power over such wide bandwidth. The fact is that no components and circuits can do this well. The high-power amplifiers in most transponders are traveling-wave tubes that inherently have limited bandwidth. They operate well over a small range but cannot deal with the entire 500-MHz bandwidth allocated to a satellite.

10. Which of the following is not true?
a) Battery is only used as a back up
b) When in orbit, solar power is always available
c) Battery is used for initial satellite orientation and stabilization
d) The batteries are charged using solar power
Answer: b
Clarification: When a satellite goes into an eclipse or when the solar panels are not properly positioned, there is a temporary cut in solar power supply. In situations like this the batteries take over temporarily and keep the satellite operating. The batteries are not large enough to power the satellite for a long time; they are used as a backup system for eclipses, initial satellite orientation and stabilization, or emergency conditions.

11. Telemetry, command, and control (TC&C) subsystem allow a ground station to monitor and control conditions in the satellite.
a) True
b) False
Answer: a
Clarification: The telemetry system is used to report the status of the onboard subsystems to the ground station. The telemetry system typically consists of various electronic sensors whose data are selected by a multiplexer and then converted to a digital signal, which then modulates an internal transmitter. This transmitter sends the telemetry information back to the earth station, where it is recorded and monitored.

Avionics online test on “Gyroscopes – 2”.

1. Which of the following systems do not use MEMS gyro?
a) Space launch vehicles
b) Flight control systems
c) Missile guidance
d) Car stability systems
Answer: a
Clarification: MEMS gyros are being exploited not only in avionic applications such as flight control, standby attitude and heading reference systems, and missile mid-course guidance but also in the automobile industry in car stability enhancement systems. Space launch vehicles use ring laser gyros which are more accurate.

2. Why is a tuning fork gyro better than the vibrating cylinder type gyro?
a) Reduce base motion
b) Low power consumption
c) Easy to manufacture
d) Low run up time
Answer: a
Clarification: The vibrating mass rate gyro is a simple instrument suffers from the unacceptable characteristic that the smallest linear motions applied to its base cause unacceptably large errors. To overcome the effect of base motion, it is necessary to use balanced oscillations in which the oscillations of one mass are counter-balanced by equal and opposite motion of a second equal mass as in the case of tuning fork gyro.

3. Which type of gyros works by calculating the difference in time taken by light to travel in a closed circle?
a) Ring laser gyro
b) MEMS gyro
c) Floated gyro
d) Dry tuned gyro
Answer: a
Clarification: Optical gyroscopes such as the ring laser gyro and the fibre optic gyro measure angular rate of rotation by sensing the resulting difference in the transit times for laser light waves travelling around a closed path in opposite directions.

4. What is the basic principle by which laser gyros work?
a) Coriolis effect
b) Conservation of momentum
c) Sagnac effect
d) Mass conservation
Answer: c
Clarification: The Sagnac effect is a relativistic phenomena of light in a rotating reference system. When laser beams circulate in a closed path that is rotating in an inertial space, the optical length seen by the co-rotating beam appears longer than that seen by counter rotating beam.

5. Which of the following type of gyro works by counting the nodes of standing waves of laser beams?
a) Ring laser gyro
b) Optical fiber gyro
c) Dry tuned gyro
d) Floated gyro
Answer: a
Clarification: In a Ring laser gyro, the counterpropagating beams form resonant nodes within the gyro cavity and create an electromagnetic standing wave that remains fixed with inertial space. When the housing of the gyro rotates, a detector can count nodes of the standing waves, each of which represents an angle.

6. A fiber optic gyro measures rotation using the _______ of the two beams?
a) Intensity difference
b) Resonance
c) Interference intensity
d) Wavelength difference
Answer: c
Clarification: In a fiber optic gyro, The counterpropagating beams are launched into an optical path and are recombined as they exit. The interference generated by the recombination depends on the optical phase difference between the two beams and therefore provides a measure of rotation.

7. In two mode RLGs, only linearly polarized modes can be resonant in the cavity.
a) True
b) False
Answer: a
Clarification: Two mode RLGs are planar by design so that only linearly polarized modes can be resonant in the cavity. Suppression of one of the two modes of polarization ensures stable operation.

8. The emission of photons by the gas medium that fills the RLG cavity is called as?
a) Laser discharge
b) Spontaneous discharge
c) Spontaneous emission
d) Laser emission
Answer: c
Clarification: The gas medium that supplies the gain for the laser also occasionally emits photons which are unrelated to the laser signal. This is known as spontaneous emissions are leads to noise and random walk in the RLG angle output.

9. What is the cause of lock-in phenomena in RLG?
a) Backscatter
b) Low intensity
c) High intensity
d) Low standing wave ratio
Answer: a
Clarification: Lock-in phenomenon leads makes the RLG insensitive to low angular rates. The cause of the lock-in phenomenon is backscatter within the cavity, usually resulting from imperfections in or particulates in the mirror surfaces.

10. Which method uses a turntable in RLG to reduce lock-in errors?
a) Intensity control
b) Rate biasing
c) Using mechanical dither
d) Random rate fixing
Answer: b
Clarification: The method of rate biasing employs a turntable that applies a constant rotation to the gyro. An angular encoder measures the relative angle between the instrument and its base.

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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.

Avionics Multiple Choice Questions on “Fly-By-Wire Technology – 1”.

1. Why is the BFW system used?
a) To control aerodynamically efficient but unstable body
b) For communications in aircraft
c) Better navigation
d) Reduced chances of enemy detection
Answer: a
Clarification: The literal meaning of Fly By wire system is flying by wires(I.e; electrically). One of the main advantages of using FBW is to exploit increased aerodynamic shapes at the cost of stability. The FBW system provides high integrity automatic stabilization to compensate for the loss of stability and provide overall better performance.

2. How is fly by wire system implemented in an aircraft?
a) By using control rods and linkages connecting stick to control surfaces
b) By using high power radio transmitters and receivers
c) By using computers and actuators to control surfaces
d) Artificial intelligence
Answer: c
Clarification: The fly by wire system uses actuators to move the control surfaces to maintain the stability of an aircraft. The signals sent by the pilot via the control stick is intercepted by the flight controller computer and signals are sent to the respective actuators to move the control surfaces.

3. Which one of the following is not true with respect to the FBW system?
a) FBW is not necessary for every aircraft
b) FBW makes an aircraft stealthy
c) FBW provides better control and carefree flying
d) FBW makes aircraft with low stability fly smoothly
Answer: b
Clarification: The FBW system need not necessarily provide stealth to an aircraft. It provides control, stability and carefree flying in aircraft with low natural stability. Typically stealthy aircraft design is mainly focused on stealth and has very less stability, here FBW provides good handling and control characteristics to fly the aircraft without FBW system.

4. Which of the following is not a component of the FBW system?
a) Flight control computer
b) Aircraft motion sensor
c) Actuators
d) Communication receivers
Answer: d
Clarification: The fly by wire system consists of control stick (input by the pilot), Flight control computer(processing unit), actuator control electronics(signal to actuators), actuator(movement of control surfaces). Communication systems are completely independent of FBW systems.

5. FBW system requires an air data system for control and stability.
a) True
b) False
Answer: a
Clarification: The air data system provides various information like airspeed, height(density of air), angle of attack and temperature. These parameters change with flight and are considered necessary to maintain optimal control and stability at all times.

6. How are FBW signals transmitted?
a) Simplex
b) Duplex
c) Time division multiplexing
d) Code division multiplexing
Answer: c
Clarification: Transmission of electric signals is a key element in a FBW system. Faster the signal transmission, quicker the response of the aircraft to the pilot stick controls. Time division multiplexing is used where signals can be transmitted along a network comprising only of two wires as only one set of data is being transmitted at any particular time.

7. Which one of the following is a military standard data bus?
a) Mil Std 1553
b) ARINC 629
c) ARINC 429
d) COMAC C919
Answer: a
Clarification: Mil Std 1553 is a US military standard data bus which came out in the 1970s in the F-16 and the following models of fighter aircraft. It has a data rate of 1Mbit/sec and a word length of 20 bits to encode clock, data, and address and so can receive or transmit up to 50,000 data words a second.

8. What type of cable is used in ARINC 629?
a) Coaxial cable
b) Screened twisted pair
c) Unscreened twisted pair
d) Parallel lines
Answer: c
Clarification: The ARINC 629 links and bus uses an unscreened twisted pair of wires with a connection to the bus through demountable current transformer couplers. Mil STD 1553 uses screened twisted pair cables. Parallel lines are rarely used today.

9. Why are actuators used in FBW systems?
a) Movement of control surfaces
b) Reduce drag
c) Increase lift
d) Safety equipment
Answer: a
Clarification: Unlike traditional aircraft, FBW system uses electrical signals to operate the actuators which in turn move the control surface in place of mechanical linkages and control rods directly connecting the pilot’s stick to the control surfaces.

10. Why are both electrical and hydraulic systems are used in the same aircraft?
a) To generate more force
b) Quick deflections
c) As a fail safe
d) Hydraulics for more force and electric for quick deflections
Answer: c
Clarification: Most of the main actuators are electro-hydraulic, fully electric actuators are used as a fail safe for hydraulic actuators. They must be able to survive any two failures and carry on operating satisfactorily in order to meet the aircraft safety and integrity requirements.

11. The reason for using quadruplex actuation system is _______
a) precise movement
b) to counteract heavy air loads on the control surfaces
c) quick reaction to the control stick movement
d) as a fail safe mechanism
Answer: d
Clarification: A quadruplex actuation system consists of four independent actuators whose output drives the power control unit. The fail-safe mechanism of the quadruplex system is that when one fails three can override it. All four actuators have totally independent power and control electronics.

12. In a typical quadruplex actuator system, how is the failure of an actuator identified?
a) By comparing it with the signal of the three good ones
b) By comparing it with a simulation model
c) Using in-lane fault detection
d) Using reflectometry
Answer: a
Clarification: A typical quadruplex system uses signal processing based FDI( Fault Detection and Isolation) system. It identifies the failed actuator by comparing its control signal with the other three good ones. A model-based FDI system is generally used in triplex architecture.