I85PREP
⌘ K
Brunei DCA CPL(A) Radio Navigation study materials
Brunei DCA CPL(A) Radio Navigation study resources

Brunei DCA CPL(A) Radio Navigation practice questions

One question at a time. Build your Brunei DCA CPL(A) Radio Navigation understanding.

First 50 of 200 questions. Each one lists the choices, the correct answer, and an explanation. The checker at the top of this page saves progress in this browser when it is available.

Continue in the practice tool above to load more questions, 50 at a time.

Question 1Radio wave fundamentals and propagation

Which property of a radio wave is measured in hertz?

  1. Amplitude
  2. Wavelength
  3. Polarisation
  4. Frequency Correct answer.

Explanation

Frequency is the number of complete cycles of a wave passing a fixed point each second, so it is expressed in hertz, where one hertz equals one cycle per second. Wavelength is a distance in metres, amplitude is a magnitude of field strength, and polarisation describes the plane of the electric field.

Question 2Radio wave fundamentals and propagation

As a radio wave travels from one medium into another, which quantity remains unchanged?

  1. Direction of travel
  2. Frequency Correct answer.
  3. Wavelength
  4. Speed

Explanation

Frequency is set by the source generating the wave and is unaffected by the medium it passes through. Speed changes with the refractive properties of the medium, and because speed changes while frequency stays fixed, wavelength must also change; the path may also bend, altering direction.

Question 3Radio wave fundamentals and propagation

Approximately how fast do radio waves travel in free space?

  1. 300 000 kilometres per second Correct answer.
  2. 300 metres per second
  3. 30 000 metres per second
  4. 3 000 kilometres per second

Explanation

All electromagnetic waves, including radio waves, propagate in a vacuum at the speed of light, which is approximately 300 000 kilometres per second, or 3 x 10^8 metres per second. This constant is the basis for computing wavelength from frequency.

Question 4Radio wave fundamentals and propagation

A transmitter radiates on 120 MHz. What is the approximate wavelength in free space?

  1. 0.25 metres
  2. 2.5 metres Correct answer.
  3. 250 metres
  4. 25 metres

Explanation

Wavelength equals the speed of light divided by frequency, so 300 000 000 metres per second divided by 120 000 000 hertz gives 2.5 metres. This relationship is used constantly in aviation to relate a VHF frequency to its physical wavelength.

Question 5Radio wave fundamentals and propagation

Which frequency band is used by civilian VHF aviation communications and VOR?

  1. 225 to 400 MHz
  2. 960 to 1 215 MHz
  3. 118 to 137 MHz Correct answer.
  4. 108 to 118 MHz

Explanation

The civil VHF communication band and the VOR navigation band occupy 118 to 137 MHz, with VOR channels interleaved just below at 108 to 118 MHz. This band supports line-of-sight propagation suitable for short and medium range navigation.

Question 6Radio wave fundamentals and propagation

What is the polarisation of a wave whose electric field oscillates vertically?

  1. Vertical polarisation Correct answer.
  2. Elliptical polarisation
  3. Circular polarisation
  4. Horizontal polarisation

Explanation

Polarisation is defined by the orientation of the electric field vector. If the electric field oscillates in a vertical plane, the wave is vertically polarised, and a receiving antenna should be mounted to match that plane for maximum signal pickup.

Question 7Radio wave fundamentals and propagation

Why is correct antenna polarisation alignment important for VHF reception?

  1. It increases the speed of propagation
  2. It prevents sky wave interference
  3. It maximises signal transfer and reduces loss Correct answer.
  4. It changes the transmitted frequency

Explanation

A receiving antenna responds best to a wave whose electric field matches its own orientation; a mismatch causes polarisation loss and a weaker signal. Since VHF antennas are usually vertically polarised, the receiving antenna must also be vertical to capture maximum energy.

Question 8Radio wave fundamentals and propagation

What happens to the strength of a signal as distance from the transmitter doubles in free space?

  1. It stays the same
  2. It halves
  3. It doubles
  4. It falls to about one quarter Correct answer.

Explanation

In free space the power density of a radiated wave falls with the square of distance, the inverse square law. Doubling the range therefore reduces the received power density to about one quarter, which is why range is limited without amplification or favourable propagation.

Question 9Radio wave fundamentals and propagation

Which propagation mode allows VHF signals to travel beyond the radio horizon?

  1. Tropospheric bending and ducting Correct answer.
  2. Magnetic induction
  3. Ground wave
  4. Sky wave via the ionosphere

Explanation

VHF normally travels line of sight, but variations in the lower atmosphere can refract or trap the signal, extending range beyond the geometric horizon through tropospheric bending and ducting. The ionosphere does not normally reflect VHF, and ground waves are limited at these frequencies.

Question 10Radio wave fundamentals and propagation

Which layer of the atmosphere is chiefly responsible for reflecting HF sky waves?

  1. Troposphere
  2. Ionosphere Correct answer.
  3. Mesosphere
  4. Stratosphere

Explanation

The ionosphere contains ionised layers that refract and reflect HF radio waves back toward the earth, enabling long distance communication. The troposphere and stratosphere are neutral regions where bending is much weaker and limited to higher frequencies.

Question 11Radio wave fundamentals and propagation

What is the primary cause of radio wave refraction in the atmosphere?

  1. Absorption by water vapour
  2. Doppler shift of the carrier
  3. Change in refractive index with height Correct answer.
  4. Reflection from the ground

Explanation

The refractive index of air varies with temperature, pressure and humidity, and this variation with height causes the wave path to bend gradually. This is why radio horizon extends slightly beyond the optical horizon and why ducting can occur.

Question 12Radio wave fundamentals and propagation

What does the term 'radio horizon' describe?

  1. The maximum range of the transmitter's power supply
  2. The distance at which direct line of sight to the antenna is lost Correct answer.
  3. The boundary of the antenna's near field
  4. The altitude of the ionosphere

Explanation

The radio horizon is the effective limit of direct line of sight, slightly beyond the optical horizon because of atmospheric refraction. Beyond it, VHF and UHF signals weaken rapidly unless diffraction, ducting or other effects extend coverage.

Question 13Radio wave fundamentals and propagation

Which factor most affects the line-of-sight range of a VHF ground station?

  1. The frequency stability of the transmitter
  2. The height of the transmitting and receiving antennas Correct answer.
  3. The colour of the antenna mast
  4. The length of the antenna cable

Explanation

Because VHF propagation is essentially line of sight, increasing antenna height raises the radio horizon and extends usable range. Frequency stability and cable length affect signal quality but not the geometric reach of the path.

Question 14Radio wave fundamentals and propagation

What is diffraction in radio propagation?

  1. Absorption of energy by the ground
  2. Bending of waves around obstacles Correct answer.
  3. Rotation of the polarisation plane
  4. Generation of harmonics in the transmitter

Explanation

Diffraction occurs when a wave encounters an obstacle and bends around its edges, allowing some signal to reach areas shadowed from direct line of sight. It provides weak but useful coverage behind hills and buildings, especially at lower frequencies.

Question 15Radio wave fundamentals and propagation

What is the main effect of multipath propagation on a VOR receiver?

  1. It changes the frequency band
  2. It improves the signal-to-noise ratio
  3. It causes bearing errors and fluctuation Correct answer.
  4. It increases the transmitter power

Explanation

Multipath occurs when signals arrive by more than one path with different delays, causing them to add or cancel at the receiver. In navigation systems such as VOR this produces bearing errors and fluctuation, especially at low altitudes or near terrain.

Question 16Radio wave fundamentals and propagation

Which statement best describes modulation?

  1. It is the process of amplifying the antenna current
  2. It is the process of converting AC to DC
  3. It is the process of filtering out unwanted frequencies
  4. It is the process of varying a carrier wave to carry information Correct answer.

Explanation

Modulation superimposes information on a carrier wave by varying its amplitude, frequency or phase. The carrier itself carries no information; the modulation imposed upon it conveys the audio, data or navigation signal.

Question 17Radio wave fundamentals and propagation

In amplitude modulation, which characteristic of the carrier is varied?

  1. Frequency
  2. Phase
  3. Polarisation
  4. Amplitude Correct answer.

Explanation

Amplitude modulation varies the strength of the carrier in step with the modulating signal while the frequency stays constant. Frequency modulation, by contrast, varies the carrier frequency while the amplitude remains essentially constant.

Question 18Radio wave fundamentals and propagation

What is the bandwidth of a signal?

  1. The total power radiated by the antenna
  2. The time taken for one complete cycle
  3. The physical length of the antenna
  4. The difference between the highest and lowest frequencies it occupies Correct answer.

Explanation

Bandwidth is the range of frequencies a signal occupies, expressed as the difference between its upper and lower limits. It determines how much information a channel can carry and how closely channels may be spaced without interference.

Question 19Radio wave fundamentals and propagation

Why is single sideband used in some HF aviation and maritime communications?

  1. It eliminates the need for an antenna
  2. It transmits on two frequencies at once
  3. It works only at VHF
  4. It uses less bandwidth and concentrates power in one sideband Correct answer.

Explanation

Single sideband suppresses the carrier and one sideband, so the same information occupies roughly half the bandwidth and the available power is concentrated in the remaining sideband. This improves efficiency and range for HF voice communications.

Question 20Radio wave fundamentals and propagation

What is the purpose of the squelch circuit in a VHF receiver?

  1. To increase the transmitter output power
  2. To convert the carrier to an intermediate frequency
  3. To select the antenna polarisation
  4. To mute the audio when no usable signal is present Correct answer.

Explanation

The squelch circuit monitors the received signal level and mutes the audio output when the signal falls below a set threshold, suppressing background noise between transmissions. It does not affect transmitter power or frequency conversion.

Question 21Radio navigation aids: VOR, DME and TACAN

In a conventional VOR, what is the phase relationship between the 30 Hz reference signal and the 30 Hz variable signal when the aircraft is on the 090° radial?

  1. The variable signal lags the reference by 90°
  2. The reference and variable signals are in phase
  3. The variable signal leads the reference by 90° Correct answer.
  4. The reference signal is suppressed entirely

Explanation

The variable signal phase shifts with magnetic bearing while the reference stays fixed, and the phase difference equals the bearing from the station. On the 090° radial the aircraft is due east, so the variable signal leads the reference by 90°. Phase difference and radial angle are numerically identical in this space-modulated design.

Question 22Radio navigation aids: VOR, DME and TACAN

Which frequency band does a standard VOR ground station use?

  1. 108.00 to 117.95 MHz Correct answer.
  2. 962 to 1213 MHz
  3. 118.00 to 136.975 MHz
  4. 328.6 to 335.4 MHz

Explanation

VOR operates in the very high frequency band between 108.00 and 117.95 MHz, sharing the lower portion with ILS localisers. The 118 to 137 MHz range is reserved for VHF communications. Higher bands listed belong to ILS glide path and DME respectively.

Question 23Radio navigation aids: VOR, DME and TACAN

What information does DME provide to the flight crew?

  1. Geometric altitude above the ground
  2. Slant range from the aircraft to the ground transponder Correct answer.
  3. Magnetic bearing from the aircraft to the station
  4. Ground speed referenced to the air mass

Explanation

DME measures the round-trip time of pulse pairs and converts it to slant range, which is line-of-sight distance rather than track distance along the ground. Bearing comes from VOR or TACAN azimuth, not from the DME element. Slant range slightly exceeds ground distance at high altitude.

Question 24Radio navigation aids: VOR, DME and TACAN

A TACAN station provides which navigation outputs?

  1. Azimuth only in the VHF band
  2. Glide path and localiser guidance
  3. Azimuth and slant range in the UHF band Correct answer.
  4. Slant range only in the UHF band

Explanation

TACAN combines a rotating UHF azimuth beacon with a DME ranging element, giving both bearing and distance. It operates in the 960 to 1215 MHz band, which is why military aircraft can use it without a VOR receiver. Civil aircraft fitted with DME can often use the ranging portion of a TACAN.

Question 25Radio navigation aids: VOR, DME and TACAN

Why does the DME display read slightly greater than the ground distance to the station?

  1. Because the transponder adds a fixed delay to each reply
  2. Because DME measures slant range including aircraft height Correct answer.
  3. Because the station altitude is subtracted incorrectly
  4. Because the display is rounded up to the next whole mile

Explanation

DME computes the straight-line distance from aircraft to transponder, so the aircraft's height forms one side of a right triangle and the displayed range is the hypotenuse. The error is small at long range and low altitude but grows when close in and high. The transponder delay is calibrated out by the receiver.

Question 26Radio navigation aids: VOR, DME and TACAN

An aircraft flies directly over a DME station at 30 000 ft. What is the approximate indicated range at the overhead point?

  1. About 0.5 NM
  2. About 0 NM
  3. About 5 NM Correct answer.
  4. About 30 NM

Explanation

At the overhead point the slant range equals the aircraft height, and 30 000 ft is roughly 4.9 NM. The display therefore shows about 5 NM rather than zero. This height-derived error is why DME should not be used as a sole obstacle-clearance reference near a station.

Question 27Radio navigation aids: VOR, DME and TACAN

What does the TO/FROM indicator show on a VOR display when the aircraft is on the 180° radial with the OBS set to 180?

  1. OFF
  2. No valid indication
  3. TO
  4. FROM Correct answer.

Explanation

When the selected OBS course matches the radial on which the aircraft sits, the aircraft is outbound from the station and the indicator shows FROM. The TO indication appears when the selected course points toward the station. A centred needle with the reciprocal selected would show TO.

Question 28Radio navigation aids: VOR, DME and TACAN

The CDI of a VOR receiver is fully deflected with the OBS set to 090 and the TO flag showing. Where is the aircraft relative to the selected course?

  1. It is on the 270° radial
  2. It is more than about 10° off the 090° course, on a radial well away from it Correct answer.
  3. It is exactly on the 090° radial
  4. It is beyond the station on the extended centreline

Explanation

Full-scale deflection corresponds to roughly 10° of course deviation, so a pegged needle means the aircraft is outside that band. The TO flag confirms the station lies ahead along the selected course. The exact radial must be read from a bearing source or by centring the needle.

Question 29Radio navigation aids: VOR, DME and TACAN

Approximately how wide is the course deviation indication at full scale on a standard VOR receiver?

  1. About 10° either side of the selected course Correct answer.
  2. About 35° either side of the selected course
  3. About 90° either side of the selected course
  4. About 2.5° either side of the selected course

Explanation

A conventional VOR receiver reaches full-scale deflection at roughly 10° of angular deviation from the selected course. The same angular sensitivity applies regardless of distance, so the linear width grows with range. This differs from a localiser, whose sensitivity is defined in linear width.

Question 30Radio navigation aids: VOR, DME and TACAN

What is the purpose of the ident facility on a VOR or VOR/DME station?

  1. To confirm the correct station is tuned and the aid is serviceable Correct answer.
  2. To indicate the distance to the next waypoint
  3. To provide the magnetic variation at the station
  4. To transmit the station's elevation above mean sea level

Explanation

Each VOR transmits a Morse identification, often with a voice channel, and a DME adds its own ident tone. Confirming the ident ensures the correct facility is selected and that it is operating normally. Absence of ident or a coded maintenance signal warns that the aid may be unreliable.

Question 31Radio navigation aids: VOR, DME and TACAN

What is the maximum number of pulse pairs per second transmitted by an airborne DME interrogator?

  1. About 5 pairs per second
  2. About 30 pairs per second
  3. About 150 pairs per second Correct answer.
  4. About 1 500 pairs per second

Explanation

Interrogators are designed to produce a maximum of roughly 150 pulse pairs per second, which limits channel loading. The transponder reply rate is higher because it answers many aircraft, up to around 2 700 pairs per second. Search mode uses a higher rate temporarily until lock-on is achieved.

Question 32Radio navigation aids: VOR, DME and TACAN

Why does a DME interrogator reduce its pulse repetition rate after locking onto a station?

  1. To allow the transponder to change frequency
  2. To avoid interference with VOR ident transmission
  3. To increase the accuracy of the range measurement
  4. To reduce channel congestion and conserve replies for other aircraft Correct answer.

Explanation

After acquiring track, the interrogator drops to a lower rate because fewer samples are needed to maintain a stable range. This frees transponder capacity, which is limited to roughly 2 700 replies per second. Search mode uses a high rate only to find the correct reply.

Question 33Radio navigation aids: VOR, DME and TACAN

How does a DME transponder identify itself?

  1. By transmitting on a separate VHF frequency
  2. By transmitting a Morse code identifier at intervals Correct answer.
  3. By modulating the reply pulses with a voice message
  4. By varying the reply delay in a coded pattern

Explanation

The DME ground station sends its Morse ident at regular intervals, and the airborne receiver presents it to the pilot. This allows confirmation that the correct DME is being received. Some paired VOR/DME installations share a synchronised ident.

Question 34Radio navigation aids: VOR, DME and TACAN

How is a DME frequency paired with a VOR frequency?

  1. The DME channel is assigned by the ATC controller
  2. The pilot selects any DME channel independently
  3. Each VOR frequency has a fixed associated DME channel Correct answer.
  4. The DME channel is derived from the aircraft altitude

Explanation

VOR/DME pairs are assigned in a fixed table, so tuning the VOR automatically selects the associated DME channel. This ensures the range and bearing come from the same physical site. Stand-alone DME channels can also be selected directly where published.

Question 35Radio navigation aids: VOR, DME and TACAN

What is the typical range of a VOR at high altitude, and why?

  1. About 500 NM, limited only by receiver sensitivity
  2. About 20 NM, limited by frequency congestion
  3. About 200 NM, limited by line of sight to the horizon Correct answer.
  4. About 50 NM, limited by transmitter power

Explanation

VOR is a VHF line-of-sight aid, so usable range depends mainly on the radio horizon, which extends with aircraft altitude. At high level this gives roughly 200 NM, reduced by terrain and obstacles. Power and receiver sensitivity are usually adequate well beyond the horizon.

Question 36Radio navigation aids: VOR, DME and TACAN

What is the cone of confusion in VOR navigation?

  1. A region overhead the station where bearing indications become unreliable Correct answer.
  2. A zone of reduced DME accuracy at long range
  3. A sector where the ident is suppressed
  4. An area of weak signal behind a mountain

Explanation

Directly above a VOR the received signal geometry becomes ambiguous, so the CDI and TO/FROM flags may give erratic or invalid indications. The effect is most pronounced at high altitude and close to the station. Pilots should not rely on VOR bearing while passing overhead.

Question 37Radio navigation aids: VOR, DME and TACAN

An aircraft on the 360° radial at 60 NM from a VOR turns to intercept the 090° inbound course. Which initial heading is appropriate for a 45° intercept?

  1. 045°
  2. 090°
  3. 225°
  4. 135° Correct answer.

Explanation

The 090° inbound course means tracking toward the station on the 270° radial. To intercept from the north, the aircraft flies south-east, and a 45° intercept angle gives a heading of 135°. Once the CDI centres, the aircraft turns to 090°.

Question 38Radio navigation aids: VOR, DME and TACAN

What does a VOR receiver do when the selected OBS is set to the reciprocal of the radial the aircraft is on?

  1. It shows the OFF flag
  2. It shows full-scale deflection
  3. It centres the needle and shows a TO indication Correct answer.
  4. It centres the needle and shows a FROM indication

Explanation

The OBS defines the course to be flown, and the receiver resolves the aircraft's position relative to that course. Setting the reciprocal of the current radial centres the needle but reverses the sense, giving TO. The aircraft is on the selected course line but pointing away from the station.

Question 39Radio navigation aids: VOR, DME and TACAN

How does a conventional VOR transmit its two 30 Hz signals?

  1. Both by amplitude modulation of the same carrier in phase
  2. One by pulse coding and one by phase coding
  3. Both by frequency modulation of the carrier
  4. One by amplitude modulation of the carrier and one by frequency modulation of a subcarrier Correct answer.

Explanation

The reference signal is carried as amplitude modulation of the carrier, while the variable signal is carried as frequency modulation of a 9 960 Hz subcarrier. The resulting phase difference between them encodes the aircraft bearing. This space-modulation technique allows a single carrier to convey both signals.

Question 40Radio navigation aids: VOR, DME and TACAN

What is a Doppler VOR designed to improve compared with a conventional VOR?

  1. Bearing accuracy in the presence of site and terrain reflections Correct answer.
  2. Resistance to precipitation static on the receiver
  3. Range beyond the radio horizon
  4. DME accuracy at high altitude

Explanation

Doppler VOR uses a wide-baseline antenna array and reverses the roles of the two signals so bearing is derived from a Doppler shift. This greatly reduces bearing errors caused by reflections from terrain and structures. Range remains line-of-sight limited.

Question 41Radio navigation aids: VOR, DME and TACAN

An aircraft is 40 NM from a VOR on the 270° radial and the OBS is set to 270. What does the CDI show?

  1. Centred with a TO indication Correct answer.
  2. Full-scale left deflection
  3. Centred with a FROM indication
  4. Full-scale right deflection

Explanation

The aircraft sits on the 270° radial, so the OBS set to 270 defines a course pointing toward the station. With the aircraft on that course line and the station ahead, the needle centres and the TO flag shows. Selecting 090 instead would give a centred needle with FROM.

Question 42Radio navigation aids: VOR, DME and TACAN

What is the principal advantage of TACAN over VOR for azimuth information?

  1. It provides more accurate bearing at long range in the UHF band Correct answer.
  2. It operates on the same frequency as VOR
  3. It provides glide path guidance
  4. It requires no ground station

Explanation

TACAN transmits in the UHF band, which is less affected by terrain and offers more accurate bearing information at long range. It also combines azimuth and range in a single station. These features make it attractive for military use.

Question 43Radio navigation aids: VOR, DME and TACAN

What does the airborne DME receiver do with its own transmitted pulse pairs?

  1. It uses them as a timing reference for the reply
  2. It retransmits them to the ground station
  3. It converts them into VOR bearing information
  4. It suppresses them so that only ground replies are processed Correct answer.

Explanation

The interrogator blanks its own transmissions because the reply timing is measured from the transmitted pulse, so receiving its own signal would corrupt the measurement. Only replies from the ground transponder are timed. The measured round-trip time is then converted to slant range.

Question 44Radio navigation aids: VOR, DME and TACAN

What does a DME ground transponder do when it receives more interrogations than it can answer?

  1. It changes its channel to a less congested frequency
  2. It reduces its sensitivity so that it replies only to stronger signals Correct answer.
  3. It increases its reply frequency to answer all aircraft
  4. It shuts down until the congestion clears

Explanation

A DME transponder has a finite reply capacity, so when overloaded it desensitises to serve nearby aircraft preferentially. This protects the transponder and maintains service for the most likely users. Aircraft further away may then lose lock.

Question 45Radio navigation aids: VOR, DME and TACAN

What is the standard spacing between the two pulses of a DME pulse pair?

  1. 100 microseconds
  2. 50 microseconds
  3. 12 microseconds Correct answer.
  4. 2 microseconds

Explanation

DME uses pulse pairs separated by 12 microseconds for X channels and 36 microseconds for Y channels. The receiver identifies valid replies by checking this spacing, which also helps reject random interference. The paired pulse format improves detection reliability.

Question 46Radio navigation aids: VOR, DME and TACAN

Which frequency band does a standard VOR ground station use for its azimuth transmissions?

  1. Very high frequency, 108 to 118 MHz Correct answer.
  2. Ultra high frequency, 960 to 1215 MHz
  3. High frequency, 3 to 30 MHz
  4. Medium frequency, 200 to 500 kHz

Explanation

VOR operates in the very high frequency band between 108 and 118 MHz, sharing the band with the localiser portion of ILS. VHF propagation is essentially line of sight, which is why VOR range grows with aircraft altitude. The 960 to 1215 MHz band belongs to DME and TACAN, and HF or MF are not used for VOR azimuth.

Question 47Radio navigation aids: VOR, DME and TACAN

An aircraft is tuned to a VOR/DME and the DME display reads 50 NM while the aircraft is at 40 000 ft. What is the approximate slant range error?

  1. About 25 NM
  2. About 6 NM Correct answer.
  3. About 15 NM
  4. About 1 NM

Explanation

The DME measures slant range, so the indicated distance exceeds the true ground distance by an amount that grows with height. At 40 000 ft the aircraft is roughly 6.6 NM above the station, and combining that with a 50 NM indicated range gives a ground distance of about 49.6 NM, so the error is roughly 0.4 to 1 NM only if the geometry is small; the practical rule is that the error is small at long range and large when overhead. Using the geometry, the error here is on the order of 1 NM, so the closest sensible answer among the small values is about 1 NM.

Question 48Radio navigation aids: VOR, DME and TACAN

What information does a TACAN bearing transmission provide to a suitably equipped aircraft?

  1. Magnetic heading of the aircraft
  2. Magnetic bearing of the aircraft from the station Correct answer.
  3. True track of the aircraft over the ground
  4. True bearing of the station from the aircraft

Explanation

TACAN azimuth is measured at the ground station and transmitted so that the aircraft can derive its magnetic bearing from the station. Because the reference is established at the station, the indication is in magnetic terms and varies with magnetic variation at the beacon. It does not give the aircraft heading or track, which come from the aircraft's own compass and navigation systems.

Question 49Radio navigation aids: VOR, DME and TACAN

A VOR station is being received at the limit of its range and the CDI begins to wander slowly while the TO/FROM flag remains stable. What is the most likely cause?

  1. The aircraft has entered the cone of confusion
  2. The receiver is picking up weak signals with a poor signal-to-noise ratio Correct answer.
  3. The OBS has been set to the wrong radial
  4. The station has switched to voice transmissions

Explanation

At long range the received signal becomes weak and the phase comparison between the reference and variable signals is corrupted by noise, so the CDI wanders even though the flag may still show a usable indication. A wrong OBS setting produces a steady but incorrect deflection, not a slow random wander. The cone of confusion causes a brief loss of indication overhead rather than gradual instability at range.

Question 50Radio navigation aids: VOR, DME and TACAN

Why is a DME interrogator's pulse repetition frequency reduced after it locks onto a ground station?

  1. To reduce the aircraft's electrical load at high altitude
  2. To improve the accuracy of the slant range measurement
  3. To allow the receiver to listen for the station identifier between replies
  4. To keep the transponder's reply rate within its capacity so that other aircraft are not denied service Correct answer.

Explanation

A DME transponder can answer only a limited number of interrogations per second, so each aircraft searches at a high rate and then drops to a lower tracking rate once locked on. This sharing arrangement lets many aircraft use one station without saturating it. The identifier is transmitted on a separate schedule and does not require the interrogator to pause.