Nav aids

Question 1

What does VOR mean ?

Answer 1

Very high frequency Omni-directional Range

Question 2

Whar is the frequency range of the VOR stations ?

On which frequency the in board system transmits ?

Answer 2

VOR station frequency range : 108.1 to 117.95 MHz

For air-ground communication via VOR : 122.2 MHz or 126.7 MHz

Question 3

What are the components of the VOR system ?

Answer 3

On the ground : VOR transmitter and Antenna

Onboard : a reception antenna, a radio receiver and an Omni Bearing Indicator ( OBI )

Question 4

What is the Course Deviation Indicator ( CDI ) ?

Answer 4

It is the indicator that we find on an Omni Bearing Indicator. The CDI deviation is 10° at full scale and 2° at each dot.

Question 5

What are the 4 ways of checkin a VOR ?

Answer 5

1__ Using a VOT on ground
2__ In flight on a radial ( using 1 VOR , max allowable error 4°)
3__In flight on a radial with 2 VOR ( max allowable error 4° )
4__ Visual check on a radial ( max allowable error 6° )

Question 6

What is the Horizontal Situation Indicator ( HSI ) ?

Answer 6

It’s an instrument that combines the VOR with the heading indicator.

Question 7

What is the Formula to calculate the position from a VOR ?

Answer 7

(Time between two radials ( sec ) / Number of degree between two radials ) = Time to reach the station ( min )

Groundspeed × Time in min/60 = Distance to the station

Question 8

What is a DME ?

Answer 8

A Distance Measuring Equipment.
It measures the time interval between two sets of signals.
An interrogating signal and a matching ground signal sent as the reply, with the two signals having different frequencies.
The precision is around ±0.5NM ( ±3% of the distance whichever is greater )
If the plane is flying FROM or TO the station it can provide the speed as well.

Question 9

What are the frequencies used by the DME ?

Answer 9

UHF frequencies :
An airplane’s DME interrogator uses frequencies from 1025 to 1150 MHz. DME transponders transmit on a channel in the 962 to 1213 MHz range and receive on a corresponding channel between 1025 and 1150 MHz. The band is divided into 126 channels for interrogation and 126 channels for reply. The interrogation and reply frequencies always differ by 63 MHz. The spacing of all channels is 1 MHz with a signal spectrum width of 100 kHz.

Question 10

What happens when a DME station is paired with a VOR station ?

Answer 10

DME frequencies are paired to VHF omnidirectional range (VOR) frequencies and a DME interrogator is designed to automatically tune to the corresponding DME frequency when the associated VOR frequency is selected.

Question 11

What is an ADF ?

Answer 11

Automatic Direction Finder.
An ADF on board station receives the AM radio stations ( 540 to 1620 kHz ) and NDB stations ( Non Directional Beacon station which transmits in the 190 to 1750 kHz LM/MF range.

Question 12

What is the reception distance for an ADF onboard station ?

Answer 12

Since the ADF waves follow the earth curvature, the reception distance depends on the power of the transmitting station :
o low power < 50kHz
o 50 Hz < medium power < 2000 kHz
o High power > 2000 kHz

Question 13

What are the 3 types of ADF ?

Answer 13

The Fix Compass Card : needle indicate the angle between the aircraft nose and the station

The Rotable Card :
the card can be rotated to the magnetic heading and the needle indicate the magnetic heading to the station.

The single needle RMI : Compass and ADF in one

Question 14

What are the components of an ADF system on the aircraft ?

Answer 14

On the plane :
• A loop antenna
• A second antenna
• An ADF receiver

Question 15

How does the ADF works ?

Answer 15

When the pilot selects the desired frequency and mode, the signal is amplified and converted into voice or Morse code. The receiver provides information to the relative bearing indicator. The needle on the bearing indicator points toward the station.

Question 16

What are the main errors observed using the ADF system ?

Answer 16

• Quadrantal : refraction or bending of the incoming radiowaves by the metal structure of the airplane
• Night effect : Radio waves reflected back by the ionosphere can cause fluctuations 30 to 60 nautical miles from the transmitter, especially just before sunrise and just after sunset.
• Electrical storms : Electrical interference from a ground-based source or within the aircraft cand deflect the ADF needle towards the electrical source.
• Station overlap : When you pass over a NDB station.
• Terrain effect : The plane get a an erroneous signal due to the presence of a mountain which reflects the ground wave NDB signal.

• Coastline effect : Due to a change of environment there is a refraction phenomena that occurs between the two environments which generate an erroneous signal.

Question 17

What is the formula for a fixed card ADF ?

Answer 17

Magnetic Bearing = Magnetic heading + Relative bearing ( MB = MH + RB )

Question 18

What is a Radio Magnetic Indicator ?

Answer 18

Radio Magnetic Indicator is an instrument that combines radio and magnetic information to provide continuous heading, bearing and radial information ( what is the radial to the VOR station ).

Question 19

What is the special feature of the RMI compass card ?

Answer 19

The compass card is a DG that rotates automatically as the aircraft turns. The RMI compass card does not need to be periodically readjusted, it always giving the magnetic heading thanks to an electro magnetic sensor located away from aircraft main body (the flux gate compass).

Question 20

What is an ILS ?

Answer 20

The Instrument Landing System is a precision approach system allowing landings in conditions of low visibility and ceiling.
It consists of a VHF localizer transmitter for horizontal guidance and a 3° UHF glideslope signal transmitter for vertical guidance.

Question 21

How does the ILS work ?

Answer 21

The localizer and glideslope receiver on the aircraft measures the difference in the depth of modulation of the 90Hz and 150 Hz signals.
The difference between the two signal varies depending on the deviation of the approaching aircraft from the centerline.

Question 22

What are the 3 categories of ILS ?

Answer 22

• Cat 1 Decision Height 200 feet , min visibility 2600 feet or RVR 1800 feet
• Cat 2 Decision Height 100 feet , min RVR 980 feet
• Cat 3 Decision Height 100 feet ( 50 feet Cat 3B ) , min RVR 660 feet ( 246 feet Cat 3B)

Question 23

What is the Primary Surveillance Radar ( PSR )

Answer 23

A radar that emits in the 1250-1350 MHz range frequencies and informs about the position within 80NM around it.

Question 24

What is the characteristic of a Primary Surveillance Radar ( PSR ) ?

Answer 24

It reflect parts of the electromagnetic energy sent ( Aircraft is a passive element)

Question 25

What is the Secondary Surveillance Radar ( SSR ) ?

Answer 25

A radar that interrogates on the 1030 MHz frequency and receives a transponder answer on frequency 1090 MHz.
It informs about position, altitude and identification within 250 NM around it.

Question 26

What is the characteristic of a Secondary Surveillance Radar ( SSR ) ?

Answer 26

The radar sends an interrogation signal ( Aircraft is an active element )

Question 27

What are the 3 types of transponder using a secondary radar system ?

Answer 27

• Mode A : Allow the controller to position the plane on the radar.
• Mode C : Position plus altitude provided to the controller.
• Mode S : Aircraft type, Aircraft ident, Ground speed, are further information provided to the controller.

Question 28

What does GPS stands for ?

Answer 28

Global Positioning System.

Thanks to a satellite constellation it is possible to determine a location on the earth.

Question 29

How is organized the GPS Satellite constellation ?

Answer 29

The design GPS Constellation contains 24 satellites orbiting the earth twice a day at an altitude of 20 200 KM. They are organized in 6 planes ( a minimum of 4 satellites by plane ) inclined at 55°with respect to the equator and equally spaced at 60 °.

Question 30

What are the 3 segments of the GPS system ?

Answer 30

User segment :
GPS horizontal and vertical positions are accurate 6 and 8 meters respectively for civil users ( 95% of the time )

Control Segments :
The ground control stations monitor and correct satellite orbits and timing.
The signal’s speed is closely approximated by the speed of light, with variation due to ionospheric and atmospheric effects.

Space segment :
Each Satellite has 4 atomic clocks on board with an accuracy of eight billionth of second and broadcast a PRN ( Pseudo Random Noise ) code which is repeated every millisecond.
The code transmits the satellite Identity and the Time of transmission as well as the orbit information ( ephemeris ) . It doesn’t receive any data except from the control station.

Question 31

How does the GPS actually work ?

Answer 31

The GPS compare the arrival times of each signal that’s why Satellite clocks must be synchronized to receive a correct positional report. It is then converted to a distance. It can also calculate velocity ( speed and direction )

Question 32

What are the 2 frequencies used for transmission on GPS ?

Answer 32

• 1575.42 MHz L1 for SPS Standard Positioning Service ( civil users )
• 1227.60 MHz L2 for encrypted PPS Precise Positioning Service + L1 encrypted for military users

Question 33

What are the numbers of satellites required for good geometry on a GPS system ?

Answer 33

• Position : 3
• Altitude : 4
• Approved IFR : 5
• Approved GPS approach : 6

Question 34

What are the 4 criteria of performances of a GPS system ?

Answer 34

Accuracy: the difference between a receiver’s measured and real position, speed or time;

Integrity: a system’s capacity to provide a threshold of confidence and, in the event of an anomaly in the positioning data, an alarm;

Continuity: a system’s ability to function without interruption;

Availability: the percentage of time a signal fulfils the above accuracy, integrity and continuity criteria.

Question 35

What is the RAIM ?

Answer 35

The RAIM or fault detection function of IFR approved avionics ensures Integrity. It takes a minimum of 6 satellites for RAIM to function.It will alert the pilot if the satellites do not support the applicable alert limit : 2 NM en Route, 1 NM Terminal, 0.3 NM non-precision approach.

Question 36

What is a WAAS ?

Answer 36

Wide Area Augmentation System improves GPS accuracy integrity and availability.
It uses a network of ground bases station to measure small variations in the GPS satellites’ signal.
It can improves the GPS signal accuracy from 100 meters to 7 meters.
It enables the plane to use GPS as primary means of navigation from take-off through Category precision approach.

Question 37

What are the formulas to calculate the position from an ADF ?

Answer 37

Turn the aircraft 90° from the heading to station. Measure the time required for degree change :

( Time in seconds / Amount of degree change ) = Time ( in min ) to the station

For the distance we can calculate :
Without wind -> TAS * ( Time in min to the station / 60 )

With wind -> Ground Speed * ( Time in min to the station / 60 )