1. Avionics and Aircraft Electrical Systems
Chapter 3
Navigation Systems

2. Understand the different Radio Navigation Aids and
Navigation Systems
Learning Objectives
Understand the different Radio Navigation Aids and
their principles of operation.
Understand the principle of operation of Inertial Navigation
System (INS).
Understand the principle of operation of the Global
Positioning System (GPS).
Understand aircraft radar systems and their principles
of operation.

3. Since the early days of flight navigation has improved immensely.
Navigation Systems
Since the early days of flight navigation has improved
immensely.
The first flights were done using road maps, but now we
have progressed to GPS and INS.
To start we will look at the various radio navigation aids.
Automatic Direction Finding – ADF(NDB).
Omnidirectional Beacon –VOR.
Tactical Air Navigation System –TACAN.
Instrument Landing System –ILS.

4. A non-directional (radio) beacon (NDB) is a radio
Navigation Systems
ADF(NDB)
A non-directional (radio) beacon (NDB) is a radio
transmitter at a known location, used as an aviation or
marine navigational aid.
As the name implies the signal transmitted does not include 
inherent directional information,
The signal is in Morse Code incorporating the station's
identifier which is used to confirm the station and its
operational status.
NDBs operate on a frequency between190 kHz and 1750 kHz
The aircraft equipment that receives the radio signal is an
ADF.

5. An automatic direction finder (ADF) is an aircraft
Navigation Systems
An automatic direction finder (ADF) is an aircraft
radio-navigation instrument that automatically and
continuously displays the relative bearing from the aircraft
to the transmitter.
ADF receivers are normally tuned to aviation NDBs
operating in the LW band between 190 – 535 kHz.
Most ADF receivers can also receive medium wave (AM)
broadcast stations, although these are less reliable for
navigational purposes.

6. The operator tunes the ADF receiver to the correct frequency
Navigation Systems
The operator tunes the ADF receiver to the correct frequency
and verifies the identity of the beacon by listening to the 
morse code signal transmitted by the NDB

7. ADFs contain a small array of fixed aerials which use electronic
Navigation Systems
ADFs contain a small array of fixed aerials which use electronic
sensors to deduce the direction using the strength and phase 
of the signals from each aerial.
In flight, the ADF's RMI(Radio Magnetic Indicator) or direction
indicator will always point to the broadcast station regardless
of aircraft heading.
However a banked attitude can have a slight effect on the
reading.
The needle will still generally indicate towards the beacon,
however it suffers from DIP error where the needle dips down
in the direction of the turn.

8. The red needle is the ADF pointer and
Navigation Systems
The red needle is the ADF pointer and
will always point directly to the station
in relation to the nose of the aircraft.
The compass card is driven by the
compass system and the bearing that
the station is from the aircraft can be
read off the pointer i.e. 021º.
A compass card is not essential as the aircraft can be flown
in the direction of the needle.
Depending on the power of the transmitter, the signal can be
received up to 300nm for navigational NDBs or for an airfield
NDB it could be as little as 35nm.

9. The NDB aerial is vertically polarised
Navigation Systems
Aerials
The NDB aerial is vertically polarised
and has the flat array on top to improve
it’s radiating efficiency.

10. A VOR is a type of short-range radio navigation system for
Navigation Systems
VOR
A VOR is a type of short-range radio navigation system for 
aircraft, enabling aircraft to determine their position and stay
on course by receiving radio signals transmitted by a
network of fixed ground radio beacons using a receiver unit.
It operates in the VHF band from 108 to  MHz.
A VOR ground station sends out a master signal, and a
highly directional second signal that varies in phase 30 times
a second compared to the master.
By comparing the phase of the secondary signal to the
master, the angle (bearing) to the station can be determined.
The VOR has a range of about 200 nm.

11. Each ground station sends an identifying signal in Morse Code.
Navigation Systems
A VOR Ground Station
Each ground station sends
an identifying signal in Morse
Code.
The VOR signal is displayed on an instrument on the aircraft
which requires a compass input to display the correct data.
Red needle No 1 VOR (Bearing 031º)
Green needle No 2 VOR (Bearing 012º)
The frequency is selected
on a control unit.

12. The VOR information can also be displayed on the HSI to
Navigation Systems
The VOR information can also be displayed on the HSI to
show lateral displacement.
Required course pointer
Required bearing to VOR
335 GS
N MILES
COURSE 1
Course deviation
bar.
To/From arrow
pointing to the
VOR.
Shows aircraft is
left of required
course.

13. Is a navigation system used by military aircraft. It provides
Navigation Systems
TACAN
Is a navigation system used by military aircraft. It provides
the user with bearing and distance (slant-range) to a ground
or ship-borne station (aircraft carrier).
The bearing unit of TACAN works on the same principle as
the VOR but is more accurate since it makes use of a two
frequency principle, with 15 Hz and 135 Hz components.
It operates in the frequency band  MHz.
The distance system is a transponder-based radio navigation
technology that measures slant range distance by timing the 
propagation delay of VHF radio signals.
The aircraft sends and receives a pulsed pairs signal– two
pulses of fixed duration and separation.
It’s range can be up to 240nm.

14. A radio signal takes approximately 12.36 microseconds to
Navigation Systems
A radio signal takes approximately microseconds to
travel 1 nautical mile to the target and back.
The time difference between interrogation and reply is
measured by the interrogator's timing circuitry and converted
to a distance measurement (slant range), in nautical miles,
then displayed in the cockpit.
TACAN antenna

15. The frequency is selected on a control
Navigation Systems
The frequency is selected on a control
box and the distance can be displayed
on a separate digital display or on the
HSI.
The bearing is displayed on needles
on a compass card repeater.
To display correctly the TACAN needs
a compass feed.

16. This is the civilian version of the TACAN.
Navigation Systems
VOR/DME
This is the civilian version of the TACAN.
The distance part of a TACAN is co-located with a VOR
and works on a paired frequency, so when the VOR is
selected the DME is automatically selected.
A VOR/DME ground station.

17. Instrument Landing System (ILS)
Navigation Systems
Instrument Landing System (ILS)
ILS is a runway approach aid:
ILS is used to guide the aircraft to the runway threshold in
poor visibility.
Fixed transmitters on the ground send out a special pattern
of radio signals
These define a radio beam that is like a pathway in the sky
The pathway then leads to the touch-down point on the runway

18. ILS transmits 2 frequencies
Navigation Systems
ILS
Glidepath
ILS transmits 2 frequencies
90Hz signal is strongest, so aircraft is above the Glidepath
Localizer
Transmitter
Glidepath
Runway
90Hz
150Hz

19. ILS transmits 2 frequencies
Navigation Systems
ILS
Glidepath
ILS transmits 2 frequencies
150Hz signal is strongest, so aircraft is below the Glidepath
Localizer
Transmitter
Glidepath
Runway
90Hz
150Hz

20. ILS transmits 2 frequencies
Navigation Systems
ILS
Glidepath
ILS transmits 2 frequencies
Both signals are equal, so aircraft is on the Glidepath
Localizer
Transmitter
Glidepath
Runway
90Hz
150Hz

21. ILS transmits 2 frequencies
Navigation Systems
ILS
Localizer
ILS transmits 2 frequencies
90Hz signal is strongest, so aircraft is right of the Localizer
Localizer
Transmitter
Glideslope
Runway
Runway Centre Line
90Hz
150Hz

22. ILS transmits 2 frequencies
Navigation Systems
ILS
Localizer
ILS transmits 2 frequencies
150Hz signal is strongest, so aircraft is left of the Localizer
Localizer
Transmitter
Glideslope
Runway
Runway Centre Line
90Hz
150Hz

23. ILS transmits 2 frequencies
Navigation Systems
ILS
Localizer
ILS transmits 2 frequencies
Both signals are equal, so aircraft is on the Localizer
Localizer
Transmitter
Glideslope
Runway
Runway Centre Line
90Hz
150Hz

24. ILS Navigation Systems 90Hz 150Hz 90Hz 150Hz Localizer Transmitter
Glidepath
Runway
90Hz
150Hz
Localizer
Transmitter
Glideslope
Runway
Runway Centre Line
90Hz
150Hz

25. INS is based on measuring acceleration in all planes and
Navigation Systems
INS
INS is based on measuring acceleration in all planes and
calculating distance and speed from the acceleration.
To initiate the INS system the unit must be powered up
from a completely stationary position.
It also needs a very accurate present position entered. This
information can be found on parking area TAPs or from an
en-route supplement.
The accelerometers are mounted on three axis, Vertical,
North facing and East facing.
When the unit is powered up it runs through a set up
procedure which aligns these axis, by measuring the earths
rate of rotation.

26. When the unit is aligned only the East axis will measure any
Navigation Systems
INS
When the unit is aligned only the East axis will measure any
movement as the earth rotates.
It normally takes 8 mins to align the unit but the longer it is
left aligning the more accurate it becomes.
Once aligned it is selected into Navigation Mode and the
aircraft can now move.
From this point onwards the unit works out how far it has
moved in any direction and plots a new position in three
dimensions.
To be able to give full Navigation information it also needs a
TAS feed to work out ETAs and wind.

27. Due to unit drift error rates of 3nm/hr are normal.
Navigation Systems
INS
Due to unit drift error rates of 3nm/hr are normal.
The replacement of mechanical accelerometers by Ring
Laser Systems has reduced this to below 1nm/hr.
This system measures the bend of a laser beam and equates
the amount of bend to an acceleration.

28. The GPS system is based on radio signals from satellites
Navigation Systems
GPS
The GPS system is based on radio signals from satellites
in space which then give a 3 dimensional fix to the receiver
unit.
The unit can be initiated at any time, even while flying, and
requires 4 satellite signals to operate fully and accurately.
It can operate with 3 satellites but accuracy is degraded.
To initiate quickly it requires an approximate position entered
but can initiate by itself. This will take a lot longer time.
There are 24 satellites positioned in orbit around the earth
and they are positioned to ensure that at any place on earth
4 or more satellites can be received.
Each satellite can be identified by it’s individual radio signal
which is coded in time pulses.

29. Each satellite has an atomic clock which is synchronised
Navigation Systems
Satellite 2
Satellite 1
Satellite 3 3 1 2
Each satellite has an atomic clock which is synchronised
with each other to ensure best accuracy for the system.

30. A stand alone GPS is a present position system only and to
Navigation Systems
GPS
A stand alone GPS is a present position system only and to
calculate all navigational information the GPS requires a
TAS and compass feed.
GPS accuracy is now down to 1 or 2 metres and can be
used for precision approaches to approved airfields.
The system was initially administered by the US Air Force,
but in the mid 2000s it was handed to civilian administration.
When run by the military accuracy was kept to a lesser
level for non-military users and only friendly allies given the
‘P’ codes to allow the GPS to operate to full accuracy.
This practice was stopped when it became civilian.

31. Flight Management System (FMS)
Navigation Systems
Flight Management System (FMS)
In modern nav systems a GPS/INS mixed together is used
and gains the best parts of each system to provide a very
accurate navigational system.
Control and
Display Unit
The FMS can be tied into the
navigational displays and coupled
to the auto-pilot, so that the whole
flight can be programmed while on
the ground before engine start.
Most FMS systems can be
programmed on a computer in
Flight Planning and taken to the
aircraft in a loading unit and loaded
in seconds on arrival at the aircraft.

32. This is an aircraft mounted radar system which can detect
Navigation Systems
Weather Radar
This is an aircraft mounted radar system which can detect
cloud and thunderstorms and can also be used for ground
mapping.
The returns come from the water droplets in the cloud.
The more water content the larger the return.
Modern weather radars are mostly pulse-Doppler radars,
capable of detecting the motion of rain droplets in addition
to the intensity of the precipitation.
Weather radars send directional pulses of microwave 
radiation, in the order of a microsecond long, using a 
cavity magnetron or klystron tube connected by a 
waveguide to a parabolic antenna.

33. The transmitted radar beam is like a torch beam.
Navigation Systems
Weather Radar
The transmitted radar beam is like a torch beam.
The antenna sweeps from side to side over a 160º arc.
This produces a continuously updating picture of the weather.
The elapse rate of the display can be adjusted to give either
a constant picture or a rapidly fading picture.

34. Navigation Systems
Weather Radar
How the scan works

35. The beam scan is normally automatically stabilized to ensure
Navigation Systems
Weather Radar
The beam scan is normally automatically stabilized to ensure
the beam scans horizontally to the earth’s surface.
This is done by feeding signals from an INS or Vertical Gyro
to keep the antenna level no matter the attitude of the aircraft.
The antenna has a tilt mechanism which allows the operator
to move the beam up or down, usually to a maximum of 15º.
This can be useful when determining the top or bottom of a
cloud structure.
If the tilt is increased or decreased the return will disappear
from the screen.
If the range at which this happens is multiplied by the tilt
angle and then by 100, the result is the height above or
below your present altitude.
i.e. 40nm x 2º x100 = 8,000ft

36. Weather returns show as colours starting at light green for
Navigation Systems
Weather Radar
Weather returns show as colours starting at light green for
light rain, working through yellow and red to purple for
thunderstorms.
The weather picture can be displayed on a purpose designed
scope or under-layed on the navigation display of a flat screen
flight instrument system.

37. Because the water droplets in a thunderstorm absorb and
Navigation Systems
Weather Radar
Because the water droplets in a thunderstorm absorb and
scatter some of the radar pulse the area behind a storm does
not always appear on the screen.
This shows as a dark shadow behind the bright thunderstorm.

38. In the ground mapping mode the radar beam is turned through 90º.
Navigation Systems
Weather Radar
In the ground mapping mode the radar beam is turned
through 90º.
This gives a better ground mapping picture.

39. Secondary Surveillance Radar (IFF/SSR)
Navigation Systems
Secondary Surveillance Radar (IFF/SSR)
SSR is a radar system used in air traffic control, that not only
detects and measures the position of aircraft i.e. range and
bearing, but also requests additional information from the
aircraft itself such as its identity and altitude
The ground station interrogates the aircraft and the aircraft
transponder replies.
The transponder can reply in 4 different modes which give
various information to the ground station.

40. Mode 1 – Identifies the role of the aircraft.
Navigation Systems
Mode 1 – Identifies the role of the aircraft.
Mode 2 – Is only used on operations
and identifies a specific
airframe. This code would
normally change every
6 hrs.
Mode 3 – Identifies the specific flight
and the code is allocated
by ATC.
Mode C – Identifies the aircraft altitude to the nearest 100ft.

41. SIFF is an upgraded IFF system which incorporates modes ‘S’ and 4.
Navigation Systems
Successor IFF (SIFF)
SIFF is an upgraded IFF system which incorporates modes ‘S’
and 4.
With the introduction of Digital Air Data Units more information
is available to be used electronically.
All aircraft will provide Mode S basic DAPs. The following parameters are downlinked to the interrogating system:
DATA Input
(a) Flight Identification - flight crew.
(b) Transponder Capability - automatic.
(c) Altitude – Digital Altitude Unit (DAU).
(d) Flight Status automatic.

42. Mode S Level 2 Enhanced Surveillance
Navigation Systems
Successor IFF (SIFF)
Mode S Level 2 Enhanced Surveillance
Platforms that require frequent access to international civil Air Traffic Management (ATM) systems are required to conform to the requirements for enhanced surveillance. In these cases, the following parameters are down-linked to the interrogating system in addition to the basic parameters:
(a) Magnetic heading.
(f) True track angle.
(b) True airspeed.
(g) Track angle rate.
(c) Indicated airspeed.
(h) Ground speed.
(d) Mach number.
(i) Vertical rate.
(e) Roll angle.

43. Navigation Systems
Successor IFF (SIFF)
Mode Selectors

44. Mode 4 is a military only system which automatically identifies
Navigation Systems
Successor IFF (SIFF)
Mode 4 is a military only system which automatically identifies
an aircraft as part of an operation.
The identification is encrypted and automatically changes the
code to a pre-loaded pattern.
This should ensure that no friendly fire incidents occur.
Also the Mode 4 data can be programmed into Ground
Defence Missile Systems (Patriot) which will disable lock on
to a target squawking Mode 4.
Mode 5 is pre-positioned to facilitate a planned upgrade.

45. Traffic Collision and Avoidance System (TCAS)
Navigation Systems
Traffic Collision and Avoidance System (TCAS)
This system can only be used if SIFF has been installed.
It uses the information in Mode S to anticipate possible
collision courses between aircraft and gives commands to
keep them apart.
The information can be displayed on an electronic VSI or
over-layed on a flat screen display.
Each aircrafts transponder interrogates the others to gain
the required information.
The corrective command is programmed to ensure the aircraft
carry out different manoeuvres to keep them apart.

46. TCAS communicates with transponder equipped aircraft
Navigation Systems
TCAS
TCAS communicates with transponder equipped aircraft
that are within its Surveillance Volume.
+ 10,000 ft TCAS II
- 10,000 ft TCAS II
40NM
15NM TCAS II 46

47. Navigation Systems
TCAS
RNG 1
Surveillance Volume
40NM
25NM
15NM 47

48. The TCAS system uses two types of display to present
Navigation Systems
TCAS
The TCAS system uses two types of display to present
information to the aircrew.
Traffic Advisory display (TA)
This is given when an aircraft is within certain parameters of
your aircraft but is not deemed a threat yet.
There are various levels of Traffic Advisory which increase as
the intruder gets closer.
Resolution Advisory display (RA)
This is given when an aircraft is deemed a threat to your aircraft.
The time to closest point of approach with the intruder is now
between 15 and 35 seconds.

49. Non Threat Traffic: Range beyond 6 NM and
Navigation Systems
TCAS
Non Threat Traffic: Range beyond 6 NM and
altitude is greater than +/ feet from your aircraft.
It is not yet considered a threat. 49 UP DN
BRT 2 1
RNG 10 6 4 .5
Traffic is +1300ft
climbing.
+13

50. Proximity Traffic: Range within 6 NM and altitude is
Navigation Systems
TCAS
Proximity Traffic: Range within 6 NM and altitude is
within +/ feet of your aircraft. Still not yet
considered a threat. 50 UP DN
BRT 2 1
RNG 10 6 4 .5
Traffic is – 500ft
descending.
-05

51. Traffic Advisory: The intruder is considered to be
Navigation Systems
TCAS
Traffic Advisory: The intruder is considered to be
potentially hazardous. TCAS will display a TA when
the closest point of approach is between 20 and 48
seconds. 51 UP DN
BRT 2 1
RNG 10 6 4 .5
Traffic is +200ft
and level.
+02

52. Resolution Advisory: The intruder is now
Navigation Systems
TCAS
Resolution Advisory: The intruder is now
considered to be a collision threat. The time to
closest point of approach with the intruder is now
between 15 and 35 seconds. 52 UP DN
BRT 2 1
RNG 10 6 4 .5
Traffic is +200ft
and level very close.
+02

53. A large number of contacts can be displayed at any one
Navigation Systems
TCAS 53 UP DN
BRT 2 1
RNG 10 6 4 .5
A large number of contacts
can be displayed at any one
time, but the TCAS System
will discard the lesser threats
to de-clutter the screen.
+13
-05
+02
The presence of a TA or RA
aircraft that are beyond the
selected display range is
indicated by one half of the
traffic symbol at the edge of
the screen.
+02

54. Resolution Advisory Symbols: Preventative Advisory
Navigation Systems
TCAS
Resolution Advisory Symbols: Preventative Advisory 54 UP DN
BRT
+05 6 4 2 1 .5
-05
+13
+02
RNG 10 6 4 2 1 .5
-05
+13
+02
Green Arc
is a fly to area.
Red Arc is a
fly from area.
+02
“Monitor Vertical Speed”

55. Resolution Advisory Symbols: Corrective Advisory
TCAS
Navigation Systems
Resolution Advisory Symbols: Corrective Advisory 6 4 2 1 .5 1
RNG 10 2
Green Arc
is a fly to area.
Red Arc is a
fly from area. .5
-05 4 UP
+05
+13
+02
+02 6 DN .5 4 1 2
BRT
DESCEND - DESCEND

56. Resolution Advisories and Synthesized Voice Announcements
Navigation Systems
Resolution Advisories and Synthesized Voice Announcements
RESOLUTION ADVISORY
AUDIO MESSAGE
CLIMB
“CLIMB, CLIMB”
DESCENT
“DESCEND, DESCEND”
CROSSOVER CLIMB
“CLIMB, CROSSING CLIMB.
CLIMB, CROSSING CLIMB”
CROSSOVER DESCENT
“DESCEND, CROSSING DESCEND. DESCEND, CROSSING DESCEND”
VERTICAL SPEED RESTRICTED (CLIMBING)
“ADJUST VERTICAL SPEED, ADJUST”
VERTICAL SPEED RESTRICTED (DESCENDING)
ANY WEAKENING OR SOFTENING OF A RA
PREVENTATIVE ADVISORY
“MONITOR VERTICAL SPEED”
MAINTAIN EXISTING VERTICAL SPEED
“MAINTAIN VERTICAL SPEED, MAINTAIN”
MAINTAIN EXISTING VERTICAL SPEED WHILE CROSSING THREAT’S ALTITUDE
“MAINTAIN VERTICAL SPEED, CROSSING MAINTAIN” 56

57. TCAS
Navigation Systems

58. TCAS
Navigation Systems

59. TCAS
Navigation Systems

60. TCAS
Navigation Systems

61. TCAS
Navigation Systems

62. TCAS
Navigation Systems

63. TCAS
Navigation Systems

64. TCAS
Navigation Systems