Navigation Systems and Aids 1

Question 1

What is ADF?

Answer 1

Automatic Direction Finding

Question 2

What is an NDB?

Answer 2

Nondirectional Radio Beacon

Question 3

Explain how an NDB operates.

Answer 3

An NDB is a ground station which emits its radio signal out evenly in all directions.

It transmits at low and medium frequencies, between 190 and 690KHz.

An ADF can be tuned in to an NDB frequency such that an instrument in the aircraft points at the beacon, usually an arrow on the HSI.

Question 4

What are the 3 common types of loop aerial?

Answer 4

Circular
Electronic
Fixed loop

Question 5

How do loop aerials work, what is the issue they face and how is this problem dealt with?

Answer 5

Loop aerials receive the wave at a different phase in each arm, inducing a different voltage and a figure 8 polar field.

As the aerial rotates, signal received is max when the loop is parallel to the received signal and zero when it’s perpendicular.

Can therefore tell which direction the beacon is in based on the orientation of the loop aerial. However, there are two nulls given at 180 degrees to each other. Solve this issue by mounting a simple dipole in the system.

Question 6

How does a simple dipole (sense) fix the loop aerial null problem.

Answer 6

It produces a cardioid field so only a single null is received during rotation.

Question 7

What ADF loop system is used on modern aircraft and what are some advantages?

Answer 7

Fixed Loop ADF system

Smaller so can be fitted onto the aircraft with greater flexibility + less weight.

Question 8

What are the two aircraft aerials used with ADF?

Answer 8

Loop ADF antenna

Sense aerial

Question 9

What are the three ways the ADF displays the transmitter bearing in the cockpit?

Answer 9

Radio Magnetic Indicator (RMI)
Horizontal Situation Indicator (HSI)
Multifunction Display (MFD)

Question 10

Explain ‘Night Effect’

Answer 10

ADF uses the surface wave for direction finding. Some of the NDB signal ends up as sky waves. During the day these LF and MF signals are absorbed by the D region of the ionosphere, but at night they are refracted.

This causes the sky wave to interfere with the signal received from the surface wave.

Question 11

How can Night Effect errors be reduced?

Answer 11

Shielding the aerial above and below.

Designing the aerial with very shot vertical elements.

Question 12

What errors can effect ADF/NDBs?

Answer 12

Night effect

Synchronous Transmission - Interference from stations on the same/similar frequencies

Bank error - Loop aerial vertical elements no longer vertical

Coastal refraction - Radio waves speed up over water due to reduced attenuation, means the Wave refracts towards the coast. Fly over coast/land or approach 90 degrees on to minimise error.

Quadrantal error - Electrical fields produced by the aircraft

Question 13

What is a VOR?

Answer 13

VHF Omnidirectional Range

Question 14

What does a VOR consist of?

Answer 14

Omnidirectional reference signal (30Hz FM)

Rotating directional signal (1800 RPM - 30Hz AM)

Question 15

What is the term for a magnetic bearing from a VOR beacon and what is the Q-Code for this?

Answer 15

Radial

QDR

Question 16

How does a VOR work?

Answer 16

Rotating signal constantly, omnidirectional reference signal fires when the rotational signal passes magnetic NORTH.

Aircraft picks this up and starts to time, stopping when it picks up the rotational signal. Knows the RPM, therefore can find mag bearing from this.

Question 17

What does VOR performance depend on?

Answer 17

Aircraft altitude
Transmitter power
Transmitter height

Question 18

What is co-frequency interference?

Answer 18

Two signals from seperate ground beacons can overlap to form an ‘interference zone’.
Hence, VOR beacons can be quoted with a maximum protected range and altitude.

Question 19

What is VOR cone of confusion/ambiguity?

Answer 19

Radials begin to converge as you approach the VOR. Instruments will read with errors when very near/overhead.

Question 20

What is DME?

Answer 20

Distance Measuring Equipment

Question 21

What does DME measure?

Answer 21

SLANT range between the beacon (transponder) and the aircraft (interrogator).

Question 22

What band are DME frequencies?

Answer 22

UHF

Question 23

How does a DME work?

Answer 23

Aircraft (interrogator) transmits 25 pairs of pulses in a unique, random spacing pattern.

The DME ground station (transponder) receives the signal, learns the unique pattern, and then re-transmits it with a slight frequency shift to prevent interference with the incoming signal.

Aircraft identifies its own unique stream of pulse pairs, accounts for the 50 micro second delay in the transponder, and then divides by two and uses speed, distance, time to figure out the range to the DME.

Question 24

How many pulse pairs does the aircraft transfer per second when interrogating DME?

Answer 24

25

Question 25

What are the main issues with DME?

Answer 25

The ground beacon can only handle 100 aircraft at once. It moves the cutoff bar on the amplitude/signal strength to maintain 100 if too many people are trying to use it. Means aircraft further away with a weaker signal may lose their connection to the DME.

Closer to the beacon = increasing difference between slant range and ground range, particularly when at higher altitudes.

LoS - High ground will block the signal

Mutual interference between two or more beacons on the same frequency (have a protected range = Designated Operational Coverage (DOC), given as a range and height for each beacon)

Reflections from Earth’s surface, buildings and terrain can cause errors.

Question 26

What is TACAN?

Answer 26

Tactical Air Navigation

Military version of VOR/DME.

Operates solely in the UHF band.