Question 1

Which ionospheric layer is primarily responsible for daytime refraction of HF signals?

Accepted Answer

The E layer, the thickest and most active during daytime, efficiently refracts HF signals back to Earth’s surface.

Answer

The D layer, which remains active at night, allows skywaves to refract off Earth’s magnetic field instead.

Answer

The D layer, being the least ionized, absorbs very little energy, letting HF signals pass cleanly to higher layers.

Answer

The F layer, containing the majority of ions, becomes dominant only at night and affects higher frequencies.

Question 2

What is the most likely reason for complete failure of NDB direction detection using a loop antenna?

Accepted Answer

The loop antenna is set exactly perpendicular (90°) to the incoming wave, creating no phase difference and no output.

Answer

The NDB is transmitting in the VHF band, which cannot produce directional information with an available loop antenna.

Answer

The aircraft’s receiver suffers from FM capture, where a stronger signal suppresses the NDB’s directional data.

Answer

The loop antenna length equals half the wavelength (½λ), causing a constant 180° phase error in its designed detection capabilities.

Question 3

Which form of Amplitude Modulation maximizes range while minimizing bandwidth?

Accepted Answer

Single Sideband AM – removes the duplicate sideband, using less bandwidth and power for a greater range.

Answer

Double Sideband AM – transmits two identical sidebands, ensuring full signal clarity but wasting bandwidth.

Answer

Frequency Modulation (FM) – adjusts frequency while keeping amplitude constant, preserving signal strength.

Answer

Pulse Modulation – sends on-off pulses that minimize bandwidth but cannot send a pure carrier wave.

Question 4

What is the key functional difference between a parabolic dish and a planar array radar antenna?

Accepted Answer

The dish must physically rotate for full coverage, while the array digitally sweeps the beam electronically.

Answer

The parabolic dish receives signals in all directions, while the planar array transmits a focused beam.

Answer

The planar array combines signals from many small elements, while the dish uses a single receiver point.

Answer

The dish transmits directionally only, while the array is omnidirectional and receives all the reflected signals.

Question 5

What effect does the F layer have on HF signals, and why not on VHF signals?

Accepted Answer

The F layer refracts HF waves strongly back to Earth, while VHF refracts slightly and escapes to space.

Answer

The F layer allows HF to pass through but reflects VHF due to the shorter wavelength of the given VHF signals.

Answer

The F layer turns HF into a ground wave, while VHF travels as a space wave unaffected by refraction.

Answer

The F layer absorbs HF but not VHF because the higher frequency travels with lower amplitude.

Question 6

What must be true about radio waves from an omnidirectional NDB antenna?

Accepted Answer

The waves must radiate uniformly in all directions, so aircraft can detect the signal from any bearing.

Answer

The frequency must be constantly shifted, preventing Doppler effects from incoming aircraft.

Answer

The amplitude must be continuously modulated by audio, ensuring audibility in every direction.

Answer

The waves must be vertically polarized, matching the orientation of the standard civil and military aircraft antennas.

Question 7

Which antenna type and principle best amplify weak signals from great distances?

Accepted Answer

A Parabolic Antenna, whose dish focuses weak incoming waves onto a receiver at the focal point.

Answer

A Loop Antenna, which produces voltage from phase differences across its opposite sides.

Answer

A Helical Antenna, which shortens wavelength to concentrate the energy into a narrow area.

Answer

A Slotted Planar Array, which amplifies signals by combining multiple synchronized slot radiators.

Question 8

What happens to the carrier wave at the peak of the modulating audio wave in AM?

Accepted Answer

The carrier’s amplitude increases to a maximum, mirroring the peak of the modulating audio wave.

Answer

The carrier’s frequency increases, corresponding directly to the audio wave’s highest amplitude.

Answer

The carrier’s frequency decreases, since amplitude and frequency vary inversely in FM systems.

Answer

The carrier’s phase shifts by 180°, ensuring and maintain minimal distortion during the transmission process.

Question 9

How can a low-frequency NDB signal reach a flight crew over mountainous terrain?

Accepted Answer

The low frequency’s long wavelength causes bending around Earth’s curvature as a ground wave.

Answer

The low frequency creates longer modulation pulses, increasing the transmission energy and the range.

Answer

The low frequency resists Doppler shifts, preventing deflection and maintaining the steady reception.

Answer

The low frequency penetrates the F layer, providing a strong and reliable skywave reflection path.

Question 10

What is the change observed in the carrier wave at the point of maximum audio amplitude in AM?

Accepted Answer

The carrier wave's amplitude is edited to reach a maximum value corresponding to the audio wave's peak.

Answer

The carrier wave's phase is shifted by 180°, which prevents signal distortion during transmission.

Answer

The carrier wave's frequency is edited to reach a minimum value, as the amplitude and frequency are inversely proportional.

Answer

The carrier wave's frequency is edited to reach a maximum value corresponding to the audio wave's peak.

Question 11

An early aviation engineer proposes using a Low Frequency (LF) signal of 300 kHz for long-range aircraft communication. The proposal is rejected for practical reasons tied to antenna physics.

Accepted Answer

The 1 km wavelength requires an antenna of about 500 m, half the wavelength—physically impossible to mount on an aircraft.

Answer

The LF band is reserved mainly for maritime navigation, and such frequencies do not appear on normal aviation charts.

Answer

The 300 kHz signal struggles to refract around Earth and is easily disrupted by static, reducing the overall reliability in long-range operations.

Answer

The energy demand for maintaining a stable LF carrier is too high for standard airborne power systems, reducing efficiency.

Question 12

An avionics technician installs antennas on two aircraft: one for long-range HF and another for local VHF communication. Which statement best describes their relative sizes and efficiencies?

Accepted Answer

The HF antenna accommodates wavelengths from 150 m to 13.6 m, often built at 1/8 or 1/16 of ideal size for practical mounting.

Answer

The VHF antenna is a quarter-wavelength (≈1.25 m), while the HF antenna is a half-wavelength (≈7 m) at its lowest frequency.

Answer

The VHF antenna measures about 7 m, while the HF antenna measures 1.25 m, making installation on smaller aircraft more difficult.

Answer

The HF antenna operates most efficiently at 13.6 m, while VHF’s shorter antenna makes its range less reliable over distance.

Question 13

An aircraft approaches a VDF station after flying over a large body of water. The controller’s QDR appears inconsistent with the map. What explains this error?

Accepted Answer

Electromagnetic waves slow over water, bending slightly and shifting the apparent direction of arrival at the VDF receiver.

Answer

Multipath reflection and water resonance amplify the error so significantly that the controller must request radio silence.

Answer

Surface-wave propagation and coastline reflection cause the station to detect a false reciprocal bearing from the signal path.

Answer

Refraction due to cooler air over water makes the signal skip or bounce, producing an apparent displacement of direction.

Question 14

A controller receives a distress call from a pilot who is unsure of their location and reports being “in the clouds west of the airport.” What is the most appropriate use of VDF in this case?

Accepted Answer

To use the magnetic QDM, enabling the pilot to plot a line on the chart, identify their position, and plan a safe route.

Answer

To initiate a VDF approach procedure, overriding typical air traffic priorities due to emergency status.

Answer

To issue a QDR from the station, allowing the pilot to fly outward until visual ground contact is reestablished.

Answer

To immediately provide a Class A or B bearing, allowing the pilot to follow it directly back even without an available map reference.

Question 15

A pilot tunes an NDB frequency and sees the ADF needle respond but is unsure it’s the correct beacon. Why is positive identification necessary?

Accepted Answer

The N0N wave is an unmodulated carrier, and any nearby electrical source may produce a similar signal without identification.

Answer

The low-frequency nature of NDB signals makes them prone to long-range overlap, complicating source distinction.

Answer

The vertically polarized LF signal suffers from night effect, often distorting the ADF needle beyond accurate use.

Answer

The A1A identification code interruptions cause erratic movement of the needle, requiring necessary cross-checks before the flight.

Question 16

An air traffic controller reports that the VDF system determined the bearing from the aircraft to be 090° magnetic (QDR). What fundamental property of the radio wave makes this bearing magnetic by default?

Accepted Answer

The radio waves are electromagnetic, and the VDF ground reference is based on magnetic orientation unless corrected for variation.

Answer

The system outputs QDR and QDM as reciprocal values, which are always defined magnetically as per the aviation convention.

Answer

The Adcock antenna array is inherently aligned to the Earth’s magnetic field rather than True North.

Answer

The VDF receiver automatically integrates the aircraft’s magnetic heading when computing the displayed bearing.

Question 17

A pilot flying an older aircraft is aware of ADF turning (dip) error and intends to turn onto a desired magnetic QDM track of 270° toward an NDB. What heading should the pilot initially turn to, and what causes this error?

Accepted Answer

Turn onto 260°, because the sense antenna temporarily interferes with the loop, producing a false bearing while banking.

Answer

Turn onto 280°, because the coastal refraction becomes more pronounced when the aircraft is in a turn near the ground.

Answer

Turn onto 280°, because the loop antenna’s time delay momentarily distorts the phase difference during the turn.

Answer

Turn onto 260°, because magnetic flux disturbances during turns overpower the ADF’s automatic compensation circuits.

Question 18

An aircraft is flying inland from the ocean toward a coastal NDB. The pilot notices that the aircraft is slightly off-track despite the ADF needle still indicating the beacon correctly. What is the operational consequence of this situation?

Accepted Answer

The pilot’s perceived track no longer matches actual position, leading to an off-course navigation error due to signal bending.

Answer

The pilot must apply the magnetic variation of the NDB to the bearing shown on the ADF to correct for the bending beam.

Answer

The error causes the ADF needle to point wildly toward the coastline, which is the exact opposite of the intended track.

Answer

The signal experiences intermittent interruption, forcing the pilot to confirm the NDB’s A1A ident before continuing.

Question 19

An aviation reference lists the NDB frequency range as 190 kHz to 1,750 kHz. Why was this specific portion of the LF/MF spectrum chosen for navigation?

Accepted Answer

The range provides ideal surface-wave propagation, ensuring long-distance coverage with stable signal strength.

Answer

The range ensures shorter wavelengths, allowing practical antenna lengths suitable for aircraft installation.

Answer

The range lies below the high-frequency band, avoiding interference with line-of-sight VHF communication systems.

Answer

The range supports older A1A identification codes, which depend on beat-frequency oscillators (BFOs) for decoding.

Question 20

A pilot uses a Relative Bearing Indicator (RBI) with a manual compass card and is tracking a QDM of 360°. The aircraft’s magnetic heading is 090°. The pilot manually aligns the compass card to match the current heading. What will the RBI needle now indicate, and what does the display represent?

Accepted Answer

The needle points to 360°, representing the true magnetic QDM to the NDB, acting effectively as an RMI.

Answer

The needle points to 090°, indicating the QDR (bearing from the station) on the corrected card.

Answer

The needle points to 270°, indicating the relative bearing to the station after alignment.

Answer

The needle points to 180°, showing the reciprocal of the current heading relative to the station’s location.

Question 21

The TO/FROM flag on an OBI is an essential navigation indicator, but it becomes unreliable at one specific point in relation to the VOR station. Where does this ambiguity occur?

Accepted Answer

In the Cone of Confusion, directly over the VOR station, where the TO/FROM flag fluctuates rapidly.

Answer

At the VOR’s maximum service volume limit, where signal strength begins to fade significantly.

Answer

When the OBS-selected course is exactly 90° offset from the aircraft’s present radial.

Answer

When the DME and VOR are used simultaneously, causing the indicator to default into a specific standby mode.

Question 22

A DME receiver measures a total elapsed time of 1,200 microseconds for a pulse round trip. Knowing that the ground station introduces a fixed 50 microsecond delay, what was the actual travel time of the radio wave between aircraft and station?

Accepted Answer

1,150 microseconds

Answer

1,250 microseconds.

Answer

50 microseconds.

Answer

600 microseconds.

Question 23

A DME system computes ground speed and time-to-station by analyzing the rate of change in slant range. Under which specific flight condition will this calculated ground speed be least accurate?

Accepted Answer

When flying tangentially to the DME station

Answer

When the aircraft is very distant from the DME station.

Answer

When the aircraft’s altitude changes rapidly during a climb or descent.

Answer

When the aircraft is flying a 90° cross-course relative to the station.

Question 24

The VOR is considered a major technological improvement over the NDB. Which key difference in their radio signal bands and propagation accounts for the VOR’s superior accuracy and resistance to interference?

Accepted Answer

The VOR operates in the VHF band, while the NDB operates in LF/MF bands susceptible to static and sky-wave distortion.

Answer

The VOR’s modulation is 30 Hz, while the NDB’s identification tone is 1,020 Hz.

Answer

The VOR employs Doppler shift techniques, whereas the NDB relies solely on amplitude modulation.

Answer

The VOR transmits dual 30 Hz reference and variable signals, while the NDB transmits a single amplitude-modulated signal.

Question 25

VOR navigation is based on comparing two 30 Hz signals — one reference and one variable. What property of the rotating (variable) signal allows the system to identify an aircraft’s radial from the station?

Accepted Answer

The variable signal’s phase shifts 1° for every 1° change in magnetic azimuth around the station.

Answer

The variable signal’s amplitude doubles at the 360° (North) radial.

Answer

The variable signal is frequency modulated, while the reference signal is in a specific amplitude modulated signal.

Answer

The variable signal carries the station’s Morse code identification.

Question 26

If a VOR is designated as a High-Altitude VOR, what primary benefit does this provide to aircraft within line-of-sight?

Accepted Answer

It transmits at higher power, extending the usable range.

Answer

It is always paired with a dedicated, high-precision DME transmitter.

Answer

It is fully immune to adverse weather or ionospheric interference.

Answer

It guarantees operation as a Doppler VOR, improving bearing accuracy.

Question 27

An aircraft maintains constant altitude while flying exactly perpendicular (90°) to a DME station radial. What will the DME-derived ground speed indication show?

Accepted Answer

Nearly zero ground speed, since the slant range is not changing.

Answer

A negative speed value, implying the aircraft is moving away rapidly.

Answer

A fluctuating, unreliable speed reading.

Answer

The true ground speed of the aircraft.

Question 28

DME operates between 960 MHz and 1,215 MHz, transmitting and receiving pulse pairs. Which other common aircraft system also uses this same frequency band?

Accepted Answer

ATC Transponder

Answer

VOR.

Answer

ILS.

Answer

Marker Beacons.

Question 29

The aircraft’s magnetic radial is determined by the phase difference (Δϕ) between the reference and variable signals. If the measured phase difference is 90°, which radial is the aircraft on?

Accepted Answer

090° (East).

Answer

270° (West).

Answer

000° (North).

Answer

180° (South).

Question 30

During a brief signal interruption, the DME continues displaying distance information. How does this memory function operate internally?

Accepted Answer

It extrapolates the distance using the last known ground speed for a time interval.

Answer

It freezes the last valid distance and displays it until signal reacquisition.

Answer

It temporarily switches to the collocated VOR’s range data to maintain the readout.

Answer

It transmits at high pulse rates (150 pps) until the signal returns.

Question 31

A pilot flying an aircraft notices the horizontal needle on the OBI is deflected downward during an ILS approach. Which immediate action and corresponding aircraft position best reflect this indication?

Accepted Answer

Increase power to climb, as the aircraft is below the glide path and the signal is above it.

Answer

Maintain pitch and bank left, as the aircraft is right of the localizer and needs lateral correction.

Answer

Reduce power to descend, as the aircraft is above the 3° glide path and the signal is too low.

Answer

Increase power to climb, as the aircraft is above the 3° glide path and the signal is too high.

Question 32

While conducting a Category I ILS approach, a pilot hears a 1,300 Hz alternating dot–dash tone and sees an amber light illuminate. What is the purpose of this indication?

Accepted Answer

Indicates proximity to Decision Altitude, 0.6–1 NM from the runway threshold.

Answer

Signals a glide slope failure, requiring transition to a non-precision approach.

Answer

Marks the start of lateral guidance, 4–7 NM from the runway threshold.

Answer

Confirms arrival near the runway threshold, used in CAT II/III approaches.

Question 33

An aircraft’s NAV receiver is tuned to 108.75 MHz. Based solely on this frequency, what automatic action occurs?

Accepted Answer

The system auto-pairs a UHF glide slope channel between 329.30–335.00 MHz.

Answer

The system identifies it as a VOR frequency and decodes a radial signal.

Answer

The system prioritizes the 150 Hz tone for lateral guidance due to the odd decimal.

Answer

The pilot must manually tune the UHF glide slope as pairing only works on even MHz.

Question 34

A ground crew works near the runway end during arrivals. ATC advises a landing 787 is now past the Outer Marker. What must ATC ensure regarding the ground crew?

Accepted Answer

Order them to stop and exit the Critical Area

Answer

Tell them to exit the Sensitive Area to prevent signal bending.

Answer

Confirm they monitor the 75 MHz marker beacon channel.

Answer

Clear them from the runway to prepare for aircraft landing.

Question 35

On final approach, a pilot observes the vertical CDI needle deflected to the right. Which combination of signals and corrections explains this?

Accepted Answer

90 Hz signal stronger → aircraft left of centerline → fly right.

Answer

150 Hz signal stronger → aircraft right of centerline → fly left.

Answer

90 Hz signal stronger → aircraft right of centerline → fly left.

Answer

150 Hz signal stronger → aircraft left of centerline → fly right.

Question 36

A pilot attempts a high-precision approach in mountainous terrain, but ILS is unreliable due to beam bending. Which MLS characteristic solves this problem?

Accepted Answer

MLS operates in the SHF band, allowing immunity to ILS interference.

Answer

MLS provides a back-azimuth function for missed approach guidance.

Answer

MLS uses a fixed-angle beam that simplifies siting in hilly terrain.

Answer

MLS operates in the VHF band, less affected by terrain.

Question 37

What is the fundamental difference between ILS localizer and MLS azimuth cockpit guidance?

Accepted Answer

ILS shows deviation from runway centerline, while MLS shows even offset.

Answer

ILS uses a magenta diamond, while MLS uses a green steering arrow.

Answer

ILS provides a curved approach; MLS forces a straight-in final.

Answer

ILS uses a 1,020 Hz tone, while MLS uses time-referenced scanning beams.

Question 38

A pilot hears a 400 Hz continuous dash tone with a blue light illuminated during approach. What standard action should the pilot take?

Accepted Answer

Check altitude for glide path intercept at the IAF.

Answer

Execute a missed approach, as this indicates the Inner Marker.

Answer

Reduce speed to maintain 0.75° full-scale glide slope sensitivity.

Answer

Begin the flare maneuver near the runway threshold.

Question 39

The full-scale deflection of an ILS localizer needle represents 2.5°, while a VOR needle represents 10°. What is the operational significance of this difference?

Accepted Answer

ILS is more sensitive, demanding corrections during final approach.

Answer

VOR uses a vertical needle; ILS uses a horizontal needle for lateral guidance.

Answer

VOR is non-precision; ILS is a precision approach.

Answer

ILS is reliable up to 10 NM; VOR up to 18 NM.

Question 40

What key capability of MLS allows flexible operations for varied aircraft types?

Accepted Answer

MLS offers 200 channels and supports wide vertical and lateral guidance angles

Answer

MLS operates within the VHF band for stability.

Answer

MLS includes built-in DME capability, unlike ILS.

Answer

MLS transmits runway surface data unavailable through ILS.

Question 41

A navigator is flying a long-range oceanic route at night, relying on a system that measures the time difference (TD) between distinct, short radio bursts from synchronized ground stations. The major challenge this system faces that could compromise its positional accuracy is:

Accepted Answer

Contamination of the primary signal by a delayed reflection from the ionosphere, particularly during darkness.

Answer

The inherent ambiguity of a phase difference measurement repeating every wavelength.

Answer

Positional distortion resulting from radio waves traveling significantly slower over dry land than over seawater.

Answer

Reduced ground speed measurement accuracy due to altitude dependency over calm water.

Question 42

An engineer is designing a new civil navigation system that must provide meter-level accuracy for short-range harbor approaches while utilizing continuous wave signals. To mitigate the system's primary weakness—lane ambiguity—the engineer should prioritize which technical solution?

Accepted Answer

Implementing Multipulse/Orange techniques to keep track of the specific hyperbolic zone (lane) the receiver is in.

Answer

Operating in the Low Frequency (LF) range to maximize groundwave propagation range and signal stability.

Answer

Directing two radar beams fore and aft to compare frequency shifts and cancel vertical motion errors.

Answer

Incorporating a pulse envelope measurement alongside the carrier phase for a coarse, unambiguous fix.

Question 43

During a transoceanic flight in the 1960s, a pilot notes a discrepancy between the aircraft's calculated position and its chart-plotted LORAN-C coordinates while flying over an island chain. The ground stations are perfectly synchronized. Which fundamental radio propagation characteristic is the most likely cause of this error?

Accepted Answer

The difference in surface conductivity causing radio waves to travel slower over the landmass than over the surrounding sea.

Answer

The continuous, harmonically related frequencies causing the receiver to synthesize an incorrect common difference frequency.

Answer

The symmetrical sampling of fore and aft beams failing to account for significant terrain slope variations.

Answer

Small, uncorrected velocity errors accumulating into a large position error over the extended duration of the flight.

Question 44

A navigation chain consists of a Master station and three secondary stations (Red, Green, Purple), all transmitting continuous wave signals at harmonically related frequencies. This system's core design philosophy prioritizes:

Accepted Answer

High local precision for maneuvering and guidance during coastal and terminal aerial approaches.

Answer

Autonomous operation to continuously determine motion relative to the Earth without relying on fixed ground stations.

Answer

Mitigation of skywave interference by measuring the time difference on the leading edge of the groundwave pulse.

Answer

Long-range timing reliability to support transoceanic and military air navigation over vast, unpopulated areas.

Question 45

A pilot receives an ATC message: "LORAN monitor is in alarm, state your intentions." This phraseology alerts the pilot to a critical safety concern directly related to the system's reliance on external synchronization. What is the implied threat to the flight's current navigation?

Accepted Answer

The synchronization precision between the Master and Secondary stations has degraded below a safety threshold.

Answer

The system has failed to integrate ground speed over time, leading to cumulative positional drift.

Answer

The receiver is experiencing a mirror effect over calm water, resulting in weaker radar signal returns.

Answer

The frequency shift of the ground-reflected radar beams is no longer accurately measurable due to low altitude.

Question 46

Why was a system that provided the ground track and velocity (Doppler) considered a "crucial link" leading to Area Navigation (RNAV) and GNSS, rather than the systems that provided a direct position fix (Hyperbolic)?

Accepted Answer

RNAV and GNSS require an aircraft to know its exact motion relative to the Earth to define a flexible, direct flight path.

Answer

Doppler systems could better mitigate Skywave interference using the Groundwave Sampling Technique.

Answer

The Doppler shift principle is based on time-difference measurement, which directly translates to a hyperbolic line of position.

Answer

Hyperbolic systems only operated effectively at low altitudes over calm water, limiting their global utility.

Question 47

A major operational drawback of the early LORAN-A system was its labor-intensive nature, requiring a trained operator to interpret the hyperbolic grids. This difficulty stemmed from:

Accepted Answer

The need to manually match the Master and Secondary radio pulses on a Cathode Ray Tube (CRT) oscilloscope.

Answer

The necessity of continuously integrating ground speed and drift angle to calculate a relative position.

Answer

The complexity of applying Secondary Phase Correction (SPC) tables for every new segment of inland flight.

Answer

The requirement to synthesize a common difference frequency from the harmonically related signals to resolve lane ambiguity.

Question 48

The shift from celestial navigation to radio navigation was not merely a change in technology, but a change in operational philosophy. Radio navigation replaced the dependence on the stars with:

Accepted Answer

The use of engineered, all-weather electronic signals to manage precision flight paths.

Answer

The reliance on the Earth's magnetic field to provide continuous heading and attitude reference data.

Answer

The autonomous measurement of an aircraft's acceleration tied only to its internal motion.

Answer

The principle of dead reckoning that only required an estimate of wind and true airspeed.

Question 49

During World War II, the U.S. developed LORAN (Long Range Navigation) specifically because the existing radio aids were insufficient for transoceanic operations. LORAN's purpose was primarily driven by the need to provide:

Accepted Answer

Accurate position fixing for both ships and aircraft beyond the limited range of visual or celestial references.

Answer

High-precision, meter-level accuracy for close-range coastal maneuvering.

Answer

A fully self-contained solution that eliminated dependency on vulnerable ground infrastructure.

Answer

Real-time, continuous velocity data that was independent of wind or air mass motion.

Question 50

Which of the following modern technologies still employs the Doppler velocity sensor concept for its core function, prioritizing local, instantaneous velocity over absolute global position?

Accepted Answer

Autonomous UAVs and missile guidance systems requiring real-time motion and track data.

Answer

The Master Station in a LORAN chain that initiates the timing sequence.

Answer

The Group Repetition Interval (GRI) used to define the structured sequence of LORAN pulse groups.

Answer

The Groundwave Sampling Technique used to eliminate skywave interference.

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