B07. Navigation Aids & Radar Systems Quiz

Q1. The BFO Beat Frequency Oscillator) function is used to:

Measure signal phase difference

Make the IDENT audible

Amplify the NDB signal

Points to Remember – The BFO makes the NDB’s Morse code identification audible
– NDB stations transmit idents as unmodulated continuous wave signals with no audio
– The BFO creates a local signal that mixes with the carrier to produce an audible beat frequency
– Pilots must switch the BFO on to verify the NDB station identification

Q2. In pulsed radar systems, the minimum range problem arises because:

Transmission and reception cannot occur simultaneously

The system transmits continuously

The antenna beam is too wide

Points to Remember – Transmitters and receivers share a single antenna using a T/R switch
– The receiver is switched off during pulse transmission to prevent damage
– Close targets cannot be detected if their echoes return while the transmitter is still active
– Longer pulse durations result in a larger minimum detection range

Q3. What is the primary function of the sense antenna in an Automatic Direction Finder ADF) system?

To allow for voice communication with the ground station

To increase the overall signal reception range

To resolve the 180-degree ambiguity inherent in the loop antenna

Points to Remember – The loop antenna has a figure-of-eight pattern with two nulls, causing 180 degree ambiguity
– The sense antenna is an omnidirectional antenna
– Combining both antennas creates a cardioid pattern with only one null
– This process resolves ambiguity so the needle points directly to the station

Q4. What is the primary cause of the Geometric Dilution of Precision GDOP) error in GPS?

Delay of the signal as it passes through the ionosphere

Poor angular separation between the satellites being used by the receiver

Signal reflection from the aircraft's structure

Points to Remember – GDOP occurs when GPS satellites are poorly positioned relative to each other
– Poor angular separation multiplies the position error from individual satellite measurements
– Best geometry is achieved when satellites are spread widely across the sky
– Worst geometry occurs when all satellites are clustered together overhead
– A low GDOP number indicates good geometry and a more accurate position

Q5. The polar diagram of a loop antenna is shaped like:

Figure of eight

Circle

Limacon

Points to Remember – A loop antenna has a figure-of-eight polar diagram with two equal-sized lobes
– Two sharp nulls exist at the sides where the antenna is least sensitive
– The antenna is most sensitive to signals coming from the front and back
– The two nulls cause 180° ambiguity which is resolved by a sense antenna

Q6. The combined polar diagram of the loop and sense antennas in an ADF system is a:

Limacon pattern

Cardioid pattern

Figure-of-eight pattern

Points to Remember – The loop antenna creates a figure-of-eight pattern
– The sense antenna produces a circular omnidirectional pattern
– Combining these antennas results in a heart-shaped cardioid pattern
– This cardioid pattern provides a single null to eliminate 180 degree ambiguity
– The system allows the ADF to indicate the correct direction toward the NDB station

Q7. The GPS L1 frequency transmits which type of code?

Navigation Message only

C/A Coarse Acquisition) code

PY) code only

Points to Remember – GPS L1 operates at a frequency of 1575.42 MHz
– It transmits the C/A (Coarse Acquisition) code, which is the civilian signal for standard position determination
– It also transmits the encrypted P(Y) code for military use
– L1 carries the Navigation Message, including satellite orbital data and clock corrections
– The newer L5 frequency offers enhanced accuracy specifically for aviation applications

Q8. All marker beacons operate on which frequency?

75 MHz

108 MHz

190 kHz

Points to Remember – All ILS marker beacons worldwide operate on a single fixed frequency of 75 MHz
– No tuning is required as the receiver always monitors this specific frequency
– Markers are distinguished by modulation: Outer Marker (400 Hz), Middle Marker (1300 Hz), and Inner Marker (3000 Hz)
– Each marker activates a specific light: Outer is blue, Middle is amber, and Inner is white

Q9. The frequency range used by an ILS localizer is:

108 - 112 MHz

329 - 335 MHz

112 - 117.95 MHz

Points to Remember – ILS localizer frequencies range from 108.10 to 111.95 MHz within the VHF band
– Localizer frequencies are identified by an odd first decimal place
– Glide slope frequencies operate in the 329 to 335 MHz range
– Localizer and glide slope pairs are tuned automatically when the localizer frequency is selected

Q10. ADS-B differs from traditional radar because it:

Uses pulse timing to measure range

Broadcasts aircraft position and velocity automatically without interrogation

Depends on ground interrogation

Points to Remember – ADS-B requires no ground interrogation, unlike traditional radar
– Aircraft automatically broadcast GPS-derived position, altitude, and velocity
– Broadcasts occur at regular intervals of approximately every 0.5 seconds
– Data can be received by any ground station or other nearby aircraft

Q11. If an aircraft’s glide slope needle is above the center reference, the pilot should:

Reduce power to descend toward the needle

Lower the aircraft's nose to descend toward the needle

Increase power to climb and intercept the needle

Points to Remember – A needle above the center indicates the aircraft is below the glide path
– Increase power to climb and intercept the glide slope
– Avoid reducing power as it causes the aircraft to descend further away from the path

Q12. The NAV flag appearing on a VOR indicator signifies that:

The OBS has been set to an incorrect frequency

The aircraft is perfectly centered on the selected radial

Inadequate or no signal is being received from the station

Points to Remember – The NAV or OFF flag appears when the VOR signal is inadequate or absent
– This occurs when the aircraft is out of range, the station is off-air, or terrain blocks the signal
– A visible NAV flag indicates the displayed navigation information is unreliable and should not be used

Q13. When a pilot selects the ADF or COMP mode on the function switch, the system:

Is turned off and draws no power

Is in a manual tuning mode for identifying the station

Is in automatic mode and will provide bearing information

Points to Remember – The ADF or COMP mode operates the system in automatic mode
– The ADF needle automatically rotates to point toward the NDB station
– This mode provides continuous bearing information to the pilot
– The ANT mode is used for station identification without bearing information

Q14. Each dot on the VOR Course Deviation Indicator represents:

2° deviation

5° deviation

1° deviation

Points to Remember – Each dot on the VOR CDI scale represents 2 degrees of angular deviation
– A standard indicator has 5 dots on each side, totaling a 10 degree full-scale deflection
– At 60 NM from the station, 1 dot equals approximately 2 NM of physical distance
– The VOR becomes less useful very close to the station because the physical distance for angular deviation decreases

Q15. The interrogation and reply frequencies of an SSR system are respectively:

960 MHz and 1215 MHz

1030 MHz and 1090 MHz

1090 MHz and 1030 MHz

Points to Remember – Ground stations interrogate on 1030 MHz
– Aircraft transponders reply on 1090 MHz
– A 60 MHz separation prevents interference between signals
– These frequencies are internationally standardized for all SSR systems

Q16. When a VOR’s Course Deviation Indicator CDI) needle is centered and the TO/FROM indicator shows TO, it means the aircraft is:

On the selected radial and flying away from the station

On the selected course line and flying toward the station

Directly over the VOR station the cone of confusion)

Points to Remember – The aircraft is currently on the course selected on the OBS
– A TO flag indicates the station is ahead of the aircraft on that course
– Flying the heading set on the OBS will lead the aircraft toward the station
– The flag reflects the aircraft’s position relative to the course, not its current heading

Q17. ILS localizer frequencies are found in the:

329 - 335 MHz UHF band, paired automatically with the localizer

112.00 - 117.95 MHz range and have even first decimals

108.00 - 111.95 MHz range and have odd first decimals

Points to Remember – ILS localizer frequencies range from 108.10 to 111.95 MHz
– Localizer frequencies are identified by odd first decimal places
– VOR frequencies in the same band use even first decimal places
– Tuning the localizer automatically pairs the glide slope frequency (329–335 MHz)

Q18. How does Automatic Dependent Surveillance-Broadcast ADS-B) work?

A ground radar interrogates the aircraft, which replies with its position

The aircraft automatically broadcasts its position derived from onboard systems like GPS) without the need for interrogation

It uses a network of ground stations to triangulate the aircraft's position based on its transponder signals

Points to Remember – ADS-B stands for Automatic (no crew action), Dependent (uses onboard GPS), Surveillance (for ATC tracking), and Broadcast (one-way transmission)
– Aircraft continuously transmit GPS position, altitude, velocity, and ID every 0.5 seconds
– Transmissions occur on the 1090 MHz frequency
– Ground stations and other ADS-B equipped aircraft receive these signals without needing radar interrogation

Q19. The VOR frequency band is:

190 - 1750 kHz, which includes commercial AM broadcast stations

112.00 - 117.95 MHz, exclusively for VOR use

108.00 - 117.95 MHz, with even first decimals reserved for VOR

Points to Remember – VOR operates in the 108.00 to 117.95 MHz VHF band
– Frequencies with even first decimal places are reserved for VOR
– Frequencies with odd first decimal places (up to 111.9) are reserved for ILS localizers
– This alternation allows VOR and ILS to share the same frequency band without interference

Q20. What is a key difference between the information provided by a Radio Altimeter and a Barometric Altimeter?

The radio altimeter indicates height above Mean Sea Level, while the barometric altimeter indicates height above ground level

The radio altimeter indicates absolute height above the terrain directly below the aircraft, while the barometric altimeter indicates pressure altitude

The radio altimeter is used for en-route navigation, while the barometric altimeter is used only during approach and landing

Points to Remember – Radio altimeters measure absolute height (AGL) by timing radio pulses reflected from the terrain directly below
– Barometric altimeters measure pressure altitude based on atmospheric pressure relative to a specific setting like QNH or QFE
– Radio altimeters are essential for precision during low-visibility CAT II and CAT III approach operations

Q21. The fundamental operating principle of the Global Positioning System GPS) is for the receiver to:

Measure the angular bearings from multiple satellites

Synchronize its clock with a single, highly accurate satellite clock

Measure the simultaneous distance ranges from multiple satellites

Points to Remember – Measure the distance from multiple satellites simultaneously
– Calculate signal travel time and multiply by the speed of light
– Determine 3D position using measurements from four or more satellites
– Correct receiver clock errors through these calculations

Q22. Which of the following is a drawback of ILS?

Multiple glide slope provision

False glide slope

Free from multipath error

Points to Remember – False glide slopes are spurious signals occurring at multiples of the real glide slope angle
– Intercepting a false glide slope causes instruments to show a correct path while the aircraft is actually too high
– Pilots must always intercept the ILS from below to avoid these false signals

Q23. What is the primary function of the guard time in the MLS scanning sequence?

To synchronize the aircraft's transponder with the ground station

To provide a specific interval between the end of one sweep and the start of the next

To allow the system to cool down between sweeps

Points to Remember – Provides a specific time interval between the end of one beam sweep and the start of the next
– Allows the aircraft receiver to process the measurement from the previous sweep
– Prevents overlap between consecutive sweeps to avoid measurement errors
– Ensures clean separation between the TO and FRO sweeps

Q24. The ADF control panel switch position ANT or REC) is used for:

Bearing indication

Tuning

Ident tone listening

Points to Remember – The ANT or REC switch position is used for tuning and frequency identification
– Only the sense antenna is used in this mode, so no bearing is displayed
– This mode allows the pilot to listen to the Morse code identifier and adjust volume
– The pilot must switch to ADF or COMP mode to obtain bearing information once the station is identified

Q25. How does a Pulsed Radar system determine the range to a target?

By measuring the time taken for a transmitted pulse to travel to the target and back

By measuring the Doppler shift in the returned signal frequency

By comparing the phase difference between transmitted and received signals

Points to Remember – Range is determined by measuring the time a pulse takes to travel to a target and back
– Calculations use the speed of light, which is 300,000,000 meters per second
– The formula used is Range = (Speed of Light × Time) / 2
– Dividing by two accounts for the round-trip distance
– A round-trip time of 1 microsecond equals approximately 150 meters of range

Q26. The glide slope frequency range is:

75 MHz

329 - 335 MHz

108 - 112 MHz

Points to Remember – The ILS Glide Slope operates in the UHF band from 329 to 335 MHz
– There are 40 paired channels corresponding to ILS localizer frequencies
– Localizer and glide slope frequencies are automatically paired, requiring no separate tuning
– The UHF glide slope provides a highly directional beam for precise vertical guidance

Q27. NDB signals follow the curvature of the Earth because they propagate as:

Sky waves

Ground waves

Line-of-sight waves

Points to Remember – NDB signals at LF/MF frequencies travel primarily as ground waves
– Ground waves hug the Earth’s surface and follow its curvature
– This propagation allows reception at low altitudes and beyond the horizon
– Unlike VOR, NDB is not limited to line-of-sight propagation
– At night, signals can travel as sky waves, which may cause unreliable bearing readings due to night effect

Q28. The frequency band of NDBs is:

530 - 1750 kHz

190 - 530 kHz

108 - 112 MHz

Points to Remember – NDBs operate in the 190 to 530 kHz frequency range
– This range falls within the LF (Low Frequency) and lower MF (Medium Frequency) bands
– ADF receivers can tune a wider range of 190 to 1750 kHz to include commercial AM broadcasts
– The 190 to 530 kHz band is specifically allocated for international aviation NDBs

Q29. The main function of RAIM Receiver Autonomous Integrity Monitoring) in a GPS is to:

Correct for errors in the satellite's atomic clock

Increase the signal strength from weak satellites

Detect and exclude faulty satellite signals to ensure position integrity

Points to Remember – RAIM ensures GPS integrity by using redundant satellite signals
– It detects and excludes faulty satellite measurements to maintain accuracy
– At least 5 satellites are required to detect a faulty signal
– At least 6 satellites are required to exclude a faulty satellite from calculations
– GPS approaches cannot be performed if RAIM is unavailable

Q30. The primary purpose of the Omni Bearing Selector OBS) knob on a VOR indicator is to:

Control the volume of the station's identifier

Tune the receiver to the correct VOR frequency

Manually rotate the course card to select the desired radial

Points to Remember – The OBS knob manually rotates the course card to select a desired radial
– Setting the OBS causes the CDI needle to show lateral deviation from the course
– The OBS does not tune the station frequency; a separate selector is used for tuning

Q31. The standard glide slope angle for ILS approach is approximately:

3°

5°

7°

Points to Remember – The international standard ILS glide slope angle is approximately 3 degrees
– At a 3-degree angle, an aircraft descends about 300 feet per nautical mile
– Some airports use steeper angles up to 4.5 degrees due to terrain or obstacles

Q32. Why MLS was a non starter?

Failed to perform during trials

Cost of transition and emergence of GNSS

Out dated technology

Points to Remember – High costs were required for new ground equipment at thousands of airports
– Significant expenses were involved in equipping aircraft with new avionics
– GNSS and GPS emerged as cheaper and more flexible alternatives
– GPS-based approaches like RNAV and LPV provided precision guidance without ground infrastructure
– The economic unviability made MLS fail to achieve widespread adoption

Q33. In primary radar, the range of a target is determined by:

Time difference between transmitted and received pulses

Frequency shift of echo

Beam width of the antenna

Points to Remember – Primary radar measures range by calculating the time difference between pulse transmission and echo reception
– Radio waves travel at the speed of light, which is approximately 300,000 km/s
– The formula for distance is (Time Delay × Speed of Light) / 2
– Division by two is necessary because the signal travels to the target and back

Q34. The principle of operation for the ILS localizer and glide slope is based on:

Difference in Depth of Modulation DDM) or lobe comparison

The polar diagram of a cardioid with a well-defined null

Bearing by phase comparison, similar to a VOR

Points to Remember – ILS localizer and glide slope operate on the principle of Difference in Depth of Modulation (DDM) or lobe comparison
– Two overlapping lobes are transmitted at different frequencies: 90 Hz and 150 Hz
– The aircraft receiver compares the depth of modulation of each tone to determine position
– Equal DDM indicates the aircraft is on course or on the glide path
– The CDI needle deflects toward the side with the stronger signal lobe

Q35. A VOR Test Facility VOT) is used to check VOR accuracy by transmitting:

All 360 radials simultaneously for a full calibration

A specific set of radials used for terminal procedures

Only the 360-degree radial

Points to Remember – A VOR Test Facility (VOT) transmits only the 360 degree radial due north
– Set the OBS to 360 degrees for a centered CDI with a FROM flag
– Set the OBS to 180 degrees for a centered CDI with a TO flag
– The maximum allowable error for airborne equipment is +/- 4 degrees
– It provides a standardized check for VOR receiver accuracy

Q36. A key advantage of MLS over ILS is that it:

Uses a single, fixed glide path to simplify pilot workload

Operates in the VHF band, making it less susceptible to weather

Can provide guidance for approaches from multiple directions

Points to Remember – MLS provides guidance from multiple directions and along curved or segmented approach paths
– ILS is limited to straight-in approaches within 35 degrees of the runway heading
– MLS offers a wider azimuth coverage of 62 degrees
– MLS allows for selectable glide slopes ranging from 1 to 20 degrees
– The system enables curved approaches to avoid terrain, noise-sensitive areas, or conflicting traffic

Q37. What is the main purpose of systems like WAAS and India’s GAGAN?

To augment GPS by correcting signal errors, improving accuracy, integrity, and availability for aviation

To broadcast satellite clock and ephemeris data to the GPS satellites

To provide independent navigation without relying on foreign satellite constellations

Points to Remember – WAAS (USA) and GAGAN (India) are Satellite Based Augmentation Systems (SBAS) that enhance GPS performance
– Ground stations measure GPS errors like ionospheric delays and clock inaccuracies to create correction messages
– Correction data is broadcast via geostationary satellites to aircraft
– These systems provide ~3 metre accuracy and integrity monitoring for precision approaches (LPV) without airport ground aids

Q38. The principle of VOR operation is:

Bearing by loop theory

Bearing by phase comparison

Bearing by lobe comparison

Points to Remember – VOR operates on the principle of bearing by phase comparison
– Two 30 Hz signals are transmitted: a constant reference signal and a variable signal that changes with azimuth
– The airborne receiver calculates the bearing by measuring the phase difference between these two signals
– The phase difference directly corresponds to the magnetic radial (e.g., 0° at Magnetic North, 90° at the 090° radial)

Q39. The purpose of DME is to provide:

Slant range distance to a ground station

Lateral deviation from course

Aircraft altitude over terrain

Points to Remember – DME provides the slant-range distance between the aircraft and the ground station
– Slant-range is the straight-line diagonal distance, not the horizontal ground distance
– Accuracy of slant-range vs ground distance decreases at high altitudes near the station
– Distance is measured and displayed in nautical miles

Q40. The frequency band for Non-Directional Beacons NDBs) is primarily:

329 - 335 MHz, used for vertical guidance

190 - 530 kHz, within the LF and MF bands

108.00 - 117.95 MHz, shared with the VOR

Points to Remember – NDBs operate in the Low Frequency (LF) and Medium Frequency (MF) bands
– The frequency range is from 190 to 530 kHz
– Signals travel as ground waves following the Earth’s curvature
– Ground waves provide better low-altitude range than VHF signals
– Signals are vulnerable to static, thunderstorms, and coastal refraction errors

Q41. Secondary radar differs from primary radar because it:

Uses a rotating reflector antenna

Detects reflected pulses only

Requires a cooperative transponder in the target aircraft

Points to Remember – Secondary Surveillance Radar (SSR) uses interrogation and active replies rather than reflected signals
– SSR requires a working transponder on the aircraft to function as a cooperative target
– SSR provides specific data including identity (Mode A), altitude (Mode C), and additional information (Mode S)

Q42. The localizer component of the Instrument Landing System ILS) provides:

Range information via marker beacons

Azimuth guidance to align with the runway centerline

Vertical guidance to maintain a 3-degree glide path

Points to Remember – The ILS Localizer provides lateral or azimuth guidance
– It keeps the aircraft aligned with the runway centerline during approach
– It transmits two overlapping lobes: 90 Hz on the left and 150 Hz on the right
– Signals are equal when the aircraft is centered on the runway extended centerline
– Vertical guidance is provided by the Glide Slope, while marker beacons provide range checkpoints

Q43. In VOR, the reference signal is:

Fixed omnidirectional

Amplitude modulated at 30 Hz

Rotating

Points to Remember – The reference signal is fixed and omnidirectional
– It is transmitted with the same phase in all directions simultaneously
– The signal is frequency modulated (FM) at 30 Hz
– The phase difference between the reference and variable signals determines the aircraft’s bearing

Q44. The primary principle used by Distance Measuring Equipment DME) to determine range is based on:

The time delay between an interrogating pulse pair and the ground station's reply

The strength of the signal received from a VOR collocated with the DME

The phase difference between two UHF signals

Points to Remember – DME determines range by measuring the time delay between transmitting an interrogation pulse and receiving a ground station reply
– The ground station introduces a fixed 50-microsecond delay before replying
– The aircraft subtracts this fixed delay from the total round-trip time to calculate distance
– The final result represents the slant-range distance in nautical miles

Q45. A VHF Direction Finder VDF) station provides a pilot with a QDM, which is the:

Magnetic bearing of the station from the aircraft

Magnetic heading to steer to the station in zero wind

True bearing of the aircraft from the station

Points to Remember – QDM is the magnetic heading to fly in zero wind conditions to reach a VDF station
– The station calculates this by taking a bearing of the aircraft’s VHF transmission
– It is a vital tool for emergency navigation to help lost pilots return home

Q46. The principle of operation of Distance Measuring Equipment DME) is based on:

Frequency modulation on pulse

Random Pulse Repetition Frequency PRF)

Doppler shift measurement

Points to Remember – DME operates using Random Pulse Repetition Frequency (PRF)
– Aircraft interrogators transmit pairs of pulses at random intervals
– Ground stations identify specific aircraft by matching the unique random timing of their pulses
– Distance is calculated by measuring the round-trip time of these signals to determine slant-range distance

Q47. In the VOR indicator, the TO/FROM flag points up when:

Flying away from the station

Out of range

Flying toward the station

Points to Remember – The TO flag indicates that the selected OBS course points toward the VOR station
– Flying the selected course heading will take the aircraft to the station
– The flag is determined solely by the OBS setting relative to the station and does not show the actual direction of flight
– A NAV/OFF flag appears if the aircraft is out of range of the station

Q48. When a pilot selects the ALT mode on the transponder function switch, the system will transmit:

A special identification signal that makes the target flash on the controller's screen

The Mode A code and Mode C altitude information

Only the assigned 4-digit Mode A code

Points to Remember – ALT mode simultaneously activates Mode A and Mode C
– Mode A provides the four-digit squawk identity code
– Mode C provides pressure altitude information
– This is the standard operating mode for aircraft in controlled airspace
– Mode A alone transmits the identity code without altitude data
– The IDENT button transmits a special SPI pulse to make the aircraft’s blip flash on ATC screens

Q49. A major operational advantage of an NDB/ADF system over a VOR is:

It provides more precise course guidance with a well-defined null

Its signals follow the Earth's curvature, allowing reception at greater distances at low altitudes

Its signals are immune to atmospheric interference

Points to Remember – NDB signals travel as ground waves that follow the Earth’s curvature
– Signals can be received at much greater distances than line-of-sight VOR
– Better reception is available at low altitudes where terrain blocks VOR
– Systems are more cost-effective to install at remote aerodromes
– Accuracy is lower compared to VOR due to atmospheric interference

Q50. Which statement accurately compares the polar diagrams of ADF and VOR systems?

Both the cardioid ADF) and limacon VOR) are created at the aircraft's receiver

The limacon for VOR has a well-defined null used for guidance, unlike the cardioid for ADF

The cardioid for ADF is created at the receiver, while the limacon for VOR is created at the ground transmitter

Points to Remember – ADF uses a cardioid pattern created at the aircraft receiver
– VOR uses a limacon pattern created at the ground transmitter
– ADF combines loop and sense antenna outputs electronically
– VOR combines reference and variable signals to determine bearing via phase difference
– Location of pattern generation determines the specific error characteristics of each system

Q51. The Beat Frequency Oscillator BFO) is used in an ADF system to:

Resolve the 180-degree ambiguity of the loop antenna

Calculate the magnetic bearing from the aircraft to the station

Make the continuous wave CW) identifier of an NDB audible

Points to Remember – NDB stations transmit identifiers as unmodulated Continuous Wave signals
– These signals are inaudible without specialized equipment
– The BFO creates a local oscillation to produce an audible tone
– Pilots use the BFO to hear and verify the Morse code station identification

Q52. Mode C of a transponder transmits:

Altitude pressure altitude) information

Identification code only

Magnetic heading

Points to Remember – Mode C transmits pressure altitude based on the standard setting of 1013.25 hPa
– Pressure altitude is transmitted instead of QNH altitude
– ATC can view an aircraft’s altitude on radar without pilot reports
– Mode A is used for the 4-digit identity squawk code
– Mode S provides both identity and altitude plus additional data points

Q53. Why are at least four satellites required for a GPS receiver to determine a precise 3D position fix?

To allow the receiver to correct for its own clock error

To provide redundancy in case one satellite signal is lost

Because the orbits of the satellites require a minimum of four for global coverage

Points to Remember – Three satellites could provide a 3D position if the receiver had a perfectly accurate clock
– GPS receivers use inexpensive crystal clocks that contain small timing errors
– This timing error, known as receiver clock bias, creates a range error across all satellites
– A fourth satellite provides an additional equation to solve for the clock error alongside position coordinates
– Using four satellites ensures both a precise 3D position fix and clock synchronization

Q54. The VOR system provides navigational guidance by:

Comparing the phase difference between a fixed and a rotating signal

Using a directional loop antenna to find a null signal

Interpreting the amplitude modulation of a single, rotating signal

Points to Remember – VOR works by comparing the phase difference between two 30 Hz signals
– It uses one omnidirectional reference signal and one variable signal
– The phase of the variable signal changes based on the direction from the station
– The difference between these signals determines the aircraft’s radial or magnetic bearing

Q55. The glide slope component of the ILS operates in which frequency band?

VHF Band 108 - 117.95 MHz)

LF/MF Band 190 - 1750 kHz)

UHF Band 329 - 335 MHz)

Points to Remember – The ILS Glide Slope operates in the Ultra High Frequency (UHF) band between 329 and 335 MHz
– It utilizes two overlapping lobes: 90 Hz above the glide path and 150 Hz below
– Both signals are equal when the aircraft is on the correct glide path, which is typically 3 degrees
– Glide slope frequencies are automatically paired with the localizer frequency

Q56. The accuracy of the MLS in azimuth and elevation is approximately:

±10 ft and ±1 ft

±20 ft and ±2 ft

±100 ft and ±20 ft

Points to Remember – MLS provides approximately ±20 ft accuracy in azimuth (lateral) at the runway threshold
– MLS provides approximately ±2 ft accuracy in elevation (vertical) at the runway threshold
– MLS accuracy is superior to ILS, which typically achieves ±50 ft laterally and ±6 ft vertically for CAT I approaches
– High implementation costs prevented the widespread adoption of MLS despite its precision

Q57. The MLS provides azimuth and elevation information by:

Scanning vertical and horizontal beams respectively

Using Doppler frequency shift

Rotating antenna mechanically

Points to Remember – Azimuth function uses a beam scanning horizontally (left-right) to measure lateral position
– Elevation function uses a beam scanning vertically (up-down) to measure the glide path angle
– Angular position is determined by the time interval between the TO and FRO sweeps of each beam

Q58. The principle of ADF operation is based on:

Difference in depth of modulation

Bearing by loop theory

Bearing by phase comparison

Points to Remember – ADF operation is based on Loop Theory and the directional properties of a loop antenna
– The antenna is most sensitive to signals aligned with its plane and has nulls perpendicular to it
– Rotation of the loop is used to find the null position that points toward the NDB station
– This method serves as the fundamental principle of radio direction finding

Q59. A significant limitation of a basic Pulsed Radar system is its:

Minimum range, caused by the receiver being offline during transmission

Inability to determine the target's altitude

Requirement for the target to be equipped with a transponder

Points to Remember – The minimum range problem is a major limitation of pulsed radar systems
– The receiver is turned off during transmission to avoid damage from high-power signals
– Close targets remain invisible if their echoes return before the receiver is reactivated
– Minimum range is calculated as (pulse width × speed of light) / 2
– Using shorter pulses helps reduce the minimum range limitation

Q60. What is the primary advantage of a Secondary Surveillance Radar SSR) system like a transponder over Primary Radar?

It does not require a ground-based transmitter

Its signals are unaffected by weather or terrain

It can provide coded information like identity and altitude

Points to Remember – SSR provides coded information including aircraft identity and pressure altitude
– Primary radar only shows a blip and cannot distinguish between different aircraft
– Mode S SSR offers additional data like call signs and ADS-B information
– These features enable more sophisticated and accurate air traffic control surveillance

Q61. In a Secondary Surveillance Radar SSR) system, the ground interrogator transmits on _ MHz and the aircraft transponder replies on _ MHz.

121.5; 243.0

1090; 1030

1030; 1090

Points to Remember – Ground stations transmit interrogations on 1030 MHz
– Aircraft transponders reply on 1090 MHz
– A 60 MHz separation prevents signal interference
– ADS-B broadcasts also utilize the 1090 MHz frequency (1090ES)

Q62. The frequency range of a DME system is:

108.10 - 117.95 MHz

5030 - 5090 MHz

962 - 1213 MHz

Points to Remember – DME operates in the UHF band between 962 and 1213 MHz
– Aircraft interrogations and ground transponder replies are separated by 63 MHz
– Frequencies are automatically paired with co-located VOR or ILS localizers
– Tuning a VOR receiver automatically selects the corresponding DME UHF channel

Q63. The main cause of ionospheric propagation error in GPS is:

Multipath ground reflection

Variation in ionospheric electron density

Tropospheric humidity

Points to Remember – Free electrons in the ionosphere slow down GPS signals, leading to range measurement errors
– Ionospheric electron density varies with solar activity, time of day, and season
– Dual-frequency receivers (L1 and L5) can measure and correct these delays directly
– SBAS systems like GAGAN provide broadcast ionospheric correction data

Q64. The main operating frequency range of the Microwave Landing System MLS) is:

108.10 - 111.95 MHz

960 - 1215 MHz

5030 - 5090 MHz

Points to Remember – MLS operates in the C-band microwave frequency range from 5030 to 5090 MHz
– High frequencies enable narrow, precise beams for accurate guidance
– Azimuth coverage is wider than ILS, spanning from -62 degrees to +62 degrees
– Supports multiple glide angles ranging from 1 degree to 20 degrees
– Resistant to signal distortion caused by airport terrain or structures

Q65. What is the purpose of the sense aerial in an ADF system?

To improve sensitivity

To resolve 180° ambiguity

To provide frequency modulation

Points to Remember – Resolves the 180 degree ambiguity of the loop antenna
– Combines with the loop antenna’s figure-of-eight pattern
– Creates a cardioid pattern with only one null to determine the correct bearing

Q66. The ADF receiver operates in which frequency range?

3 - 30 MHz

108 - 117.95 MHz

190 - 1750 kHz

Points to Remember – The ADF receiver operates in the 190 to 1750 kHz range
– This range covers the Low Frequency (LF) and Medium Frequency (MF) bands
– NDBs transmit within the 190 to 530 kHz portion of this range
– Commercial AM broadcast stations operate in the 535 to 1705 kHz MF band
– AM stations can provide rough bearing checks but are not approved for navigation

Q67. Marker beacons on an ILS approach operate on what frequency and provide what type of information?

The same frequency as the localizer, providing azimuth correction

75 MHz, providing audio and visual cues at specific points along the approach

500 kHz, the international maritime distress frequency

Points to Remember – All marker beacons operate on a frequency of 75 MHz
– They provide distance-to-threshold checkpoints during an ILS approach
– Outer Marker (OM): Located ~7 NM out, indicated by a blue light and slow dots
– Middle Marker (MM): Located ~3500 ft out, indicated by an amber light and alternating dots/dashes
– Inner Marker (IM): Located ~1000 ft out, indicated by a white light and rapid dots

Q68. The total number of operational GPS satellites in orbit is:

12 operational and 2 spares

21 operational and 3 spares 24 total)

30 operational

Points to Remember – The original GPS constellation design consists of 24 total satellites
– This includes 21 operational satellites and 3 active spares
– The configuration ensures at least 4 satellites are visible from any point on Earth
– A minimum of 4 satellites is required for accurate 3D position fixing
– While the baseline standard is 24, the US typically maintains more for redundancy

Q69. The Indian regional satellite navigation system is known as:

NAVIC

GAGAN

GLONASS

Points to Remember – NavIC (Navigation with Indian Constellation) is India’s independent satellite navigation system
– It is also known as IRNSS (Indian Regional Navigation Satellite System)
– The system consists of 7 satellites covering India and the surrounding region up to 1500 km
– It provides a positioning accuracy of approximately 5 metres
– NavIC is a standalone GNSS constellation, while GAGAN is a GPS augmentation system

Q70. Continuous wave radar systems eliminate the minimum range problem because:

They use pulse compression

They transmit and receive continuously using separate antennas

They use higher pulse repetition frequency

Points to Remember – Continuous Wave (CW) radar uses separate antennas for transmitting and receiving
– The receiver is never switched off, allowing detection of targets at any range
– CW radar eliminates the minimum range problem common in pulsed systems
– A limitation of CW radar is the difficulty in determining range from a single antenna
– It is primarily used for Doppler speed measurements rather than general surveillance

Q71. The combined polar diagram of loop and sense antenna forms a:

Limacon

Circle

Cardioid

Points to Remember – Combining the figure-of-eight loop pattern with the circular sense antenna pattern creates a cardioid
– This cardioid is a heart-shaped pattern with a single null direction pointing away from the station
– The ADF uses this single null to determine the precise direction to the NDB station
– This process eliminates ambiguity and enables accurate direction finding

Q72. GAGAN is designed to:

Augment GPS accuracy and integrity for aviation in Indian airspace

Replace the GPS constellation with Indian satellites

Provide oceanic surveillance using radar

Points to Remember – GAGAN is India’s Satellite Based Augmentation System designed to improve GPS accuracy from 15 metres to 3 metres
– It provides integrity monitoring for aviation using a network of 15 Indian Reference Stations
– It broadcasts error corrections via geostationary satellites to enable precision approaches in Indian airspace

Q73. What is the function of a VOR VHF Omnidirectional Range) navigation aid?

It provides magnetic bearing from the station

It provides precision vertical guidance for landing

It provides the pilot with slant-range distance to the station

Points to Remember – VOR provides the magnetic bearing from the station
– It indicates which radial you are currently on
– VOR does not provide distance or vertical guidance
– A complete position fix requires both VOR and DME information

Q74. The ILS glide slope operates in which frequency band?

HF band

UHF band

VHF band

Points to Remember – The ILS Glide Slope operates in the Ultra High Frequency (UHF) band
– The specific frequency range used is 329 to 335 MHz
– UHF is chosen for its shorter wavelength, allowing for precise beam shaping of the 3 degree glide path
– Antenna arrays on the runway create overlapping lobes that the aircraft compares to maintain the correct path

Q75. Which statement about NDB signals is correct?

They are unaffected by weather

They are not affected by terrain conditions

They are more affected by atmospheric and terrain conditions

Points to Remember – NDB signals operate at LF/MF frequencies and are more susceptible to errors than VOR
– Thunderstorm static can cause the needle to deflect toward storms
– Night effect occurs when skywave interference happens after dark
– Coastal refraction causes signals to bend when crossing coastlines
– Terrain reflection from mountains creates multipath errors
– Precipitation static can also interfere with signal accuracy
– These errors result in NDB approaches having higher minima than ILS or VOR approaches

Q76. A Non-Precision Approach NPA) is formally classified as a Type A operation and is characterized by having:

Both lateral and vertical guidance down to a Decision Height DH)

A Decision Height DH) below 200 feet

Lateral guidance only, with a Minimum Descent Height MDH) at or above 250 feet

Points to Remember – Provides lateral guidance only without a glide slope
– Uses a Minimum Descent Height (MDH) rather than a Decision Height (DH)
– Requires an MDH at or above 250 feet
– Pilot flies level at the MDH while searching for the runway
– Requires a missed approach if the runway is not visible

Q77. The principle used in the Microwave Landing System MLS) is:

Frequency Modulation Scanning Beam

Pulse Compression Technique

Time Reference Scanning Beam TRSB)

Points to Remember – MLS operates on the Time Reference Scanning Beam (TRSB) principle
– A narrow microwave beam sweeps back and forth (TO and FRO) across the approach corridor
– The time interval between the TO and FRO sweeps determines the aircraft’s position
– This method precisely calculates both lateral azimuth and vertical elevation relative to the runway

Q78. The DME displays:

Absolute altitude above ground level

Slant range distance from the aircraft to the ground station

Horizontal distance to the ground station

Points to Remember – DME displays slant range, which is the direct diagonal distance between the aircraft and the ground station
– Slant range is different from horizontal ground distance
– Slant range is significantly higher than ground distance when the aircraft is at high altitudes close to the station
– Slant range is approximately equal to ground distance at long ranges and typical cruise altitudes

Q79. What is the fundamental operating principle of the Microwave Landing System MLS)?

Bearing by Phase Comparison

Time Reference Scanning Beam TRSB)

Difference in Depth of Modulation DDM)

Points to Remember – MLS uses the Time Reference Scanning Beam (TRSB) principle
– A narrow microwave beam scans back and forth across the approach corridor
– The aircraft measures the time interval between the TO sweep and the FRO sweep
– This time difference directly indicates the aircraft’s azimuth (centerline) or elevation (glide path) angle
– A greater time difference between sweeps signifies a greater angle from the reference path

Q80. For a Category II ILS approach, the decision height DH) and Runway Visual Range RVR) are, respectively:

DH not lower than 200 ft, and RVR not less than 550 m

DH lower than 100 ft, and RVR less than 300 m

DH lower than 200 ft but not lower than 100 ft, and RVR not less than 300 m

Points to Remember – Category II ILS Decision Height (DH) is between 100 ft and 200 ft
– Category II Runway Visual Range (RVR) is not less than 300 m
– CAT I: DH ≥ 200 ft, RVR ≥ 550 m
– CAT IIIa: DH < 100 ft, RVR ≥ 200 m
– CAT IIIb: DH < 50 ft, RVR 50-200 m
– CAT IIIc: No DH and no RVR limits (not currently in use)

Results

Q1. The BFO Beat Frequency Oscillator) function is used to:

Q2. In pulsed radar systems, the minimum range problem arises because:

Q3. What is the primary function of the sense antenna in an Automatic Direction Finder ADF) system?

Q4. What is the primary cause of the Geometric Dilution of Precision GDOP) error in GPS?

Q5. The polar diagram of a loop antenna is shaped like:

Q6. The combined polar diagram of the loop and sense antennas in an ADF system is a:

Q7. The GPS L1 frequency transmits which type of code?

Q8. All marker beacons operate on which frequency?

Q9. The frequency range used by an ILS localizer is:

Q10. ADS-B differs from traditional radar because it:

Q11. If an aircraft’s glide slope needle is above the center reference, the pilot should:

Q12. The NAV flag appearing on a VOR indicator signifies that:

Q13. When a pilot selects the ADF or COMP mode on the function switch, the system:

Q14. Each dot on the VOR Course Deviation Indicator represents:

Q15. The interrogation and reply frequencies of an SSR system are respectively:

Q16. When a VOR’s Course Deviation Indicator CDI) needle is centered and the TO/FROM indicator shows TO, it means the aircraft is:

Q17. ILS localizer frequencies are found in the:

Q18. How does Automatic Dependent Surveillance-Broadcast ADS-B) work?

Q19. The VOR frequency band is:

Q20. What is a key difference between the information provided by a Radio Altimeter and a Barometric Altimeter?

Q21. The fundamental operating principle of the Global Positioning System GPS) is for the receiver to:

Q22. Which of the following is a drawback of ILS?

Q23. What is the primary function of the guard time in the MLS scanning sequence?

Q24. The ADF control panel switch position ANT or REC) is used for:

Q25. How does a Pulsed Radar system determine the range to a target?

Q26. The glide slope frequency range is:

Q27. NDB signals follow the curvature of the Earth because they propagate as:

Q28. The frequency band of NDBs is:

Q29. The main function of RAIM Receiver Autonomous Integrity Monitoring) in a GPS is to:

Q30. The primary purpose of the Omni Bearing Selector OBS) knob on a VOR indicator is to:

Q31. The standard glide slope angle for ILS approach is approximately:

Q32. Why MLS was a non starter?

Q33. In primary radar, the range of a target is determined by:

Q34. The principle of operation for the ILS localizer and glide slope is based on:

Q35. A VOR Test Facility VOT) is used to check VOR accuracy by transmitting:

Q36. A key advantage of MLS over ILS is that it:

Q37. What is the main purpose of systems like WAAS and India’s GAGAN?

Q38. The principle of VOR operation is:

Q39. The purpose of DME is to provide:

Q40. The frequency band for Non-Directional Beacons NDBs) is primarily:

Q41. Secondary radar differs from primary radar because it:

Q42. The localizer component of the Instrument Landing System ILS) provides:

Q43. In VOR, the reference signal is:

Q44. The primary principle used by Distance Measuring Equipment DME) to determine range is based on:

Q45. A VHF Direction Finder VDF) station provides a pilot with a QDM, which is the:

Q46. The principle of operation of Distance Measuring Equipment DME) is based on:

Q47. In the VOR indicator, the TO/FROM flag points up when:

Q48. When a pilot selects the ALT mode on the transponder function switch, the system will transmit:

Q49. A major operational advantage of an NDB/ADF system over a VOR is:

Q50. Which statement accurately compares the polar diagrams of ADF and VOR systems?

Q51. The Beat Frequency Oscillator BFO) is used in an ADF system to:

Q52. Mode C of a transponder transmits:

Q53. Why are at least four satellites required for a GPS receiver to determine a precise 3D position fix?

Q54. The VOR system provides navigational guidance by:

Q55. The glide slope component of the ILS operates in which frequency band?

Q56. The accuracy of the MLS in azimuth and elevation is approximately:

Q57. The MLS provides azimuth and elevation information by:

Q58. The principle of ADF operation is based on:

Q59. A significant limitation of a basic Pulsed Radar system is its:

Q60. What is the primary advantage of a Secondary Surveillance Radar SSR) system like a transponder over Primary Radar?

Q61. In a Secondary Surveillance Radar SSR) system, the ground interrogator transmits on _____ MHz and the aircraft transponder replies on _____ MHz.

Q62. The frequency range of a DME system is:

Q63. The main cause of ionospheric propagation error in GPS is:

Q64. The main operating frequency range of the Microwave Landing System MLS) is:

Q65. What is the purpose of the sense aerial in an ADF system?

Q66. The ADF receiver operates in which frequency range?

Q67. Marker beacons on an ILS approach operate on what frequency and provide what type of information?

Q68. The total number of operational GPS satellites in orbit is:

Q69. The Indian regional satellite navigation system is known as:

Q70. Continuous wave radar systems eliminate the minimum range problem because:

Q71. The combined polar diagram of loop and sense antenna forms a:

Q72. GAGAN is designed to:

Q73. What is the function of a VOR VHF Omnidirectional Range) navigation aid?

Q74. The ILS glide slope operates in which frequency band?

Q75. Which statement about NDB signals is correct?

Q76. A Non-Precision Approach NPA) is formally classified as a Type A operation and is characterized by having:

Q77. The principle used in the Microwave Landing System MLS) is:

Q78. The DME displays:

Q79. What is the fundamental operating principle of the Microwave Landing System MLS)?

Q61. In a Secondary Surveillance Radar SSR) system, the ground interrogator transmits on _ MHz and the aircraft transponder replies on _ MHz.