avionics and aircraft electrical systems chapter 3 navigation systems

Avionics and Aircraft

Electrical SystemsChapter 3

Navigation Systems

Navigation Systems

Learning Objectives

Understand the different Radio Navigation Aids andtheir principles of operation.

Understand aircraft radar systems and their principlesof operation.

Understand the principle of operation of Inertial Navigation System (INS).

Understand the principle of operation of the Global Positioning System (GPS).

Navigation Systems

Since the early days of flight navigation has improvedimmensely.

The first flights were done using road maps, but now wehave 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.

Navigation SystemsADF(NDB)

A non-directional (radio) beacon (NDB) is a radio transmitter at a known location, used as an aviation or marine navigational aid.

The aircraft equipment that receives the radio signal is anADF.

NDBs operate on a frequency between190 kHz and 1750 kHz

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.

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.

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

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.

Navigation Systems

The red needle is the ADF pointer and will always point directly to the stationin relation to the nose of the aircraft.

The compass card is driven by the compass system and the bearing thatthe station is from the aircraft can beread off the pointer i.e. 021º.

Depending on the power of the transmitter, the signal can be received up to 300nm for navigational NDBs or for an airfieldNDB it could be as little as 35nm.

A compass card is not essential as the aircraft can be flownin the direction of the needle.

Navigation Systems

Aerials

The NDB aerial is vertically polarised and has the flat array on top to improveit’s radiating efficiency.

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 117.95 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.

Navigation Systems

A VOR Ground Station

The VOR signal is displayed on an instrument on the aircraftwhich 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.

Each ground station sendsan identifying signal in MorseCode.

Navigation Systems

335

GS

N MILES COURSE

011

The VOR information can also be displayed on the HSI toshow lateral displacement.

Required bearing to VOR

To/From arrowpointing to the VOR.

Required course pointer

Course deviationbar.

Shows aircraft isleft of requiredcourse.

Navigation Systems TACANIs 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 960-1215 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.

Navigation Systems

A radio signal takes approximately 12.36 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.

TACANantenna

http://en.wikipedia.org/wiki/File:TACAN_Antenna.jpg

http://en.wikipedia.org/wiki/File:TACAN_Antenna.jpg

Navigation Systems

The frequency is selected on a controlbox and the distance can be displayedon a separate digital display or on the HSI.

The bearing is displayed on needleson a compass card repeater.

To display correctly the TACAN needsa compass feed.

Navigation SystemsVOR/DME

This is the civilian version of the TACAN.

The distance part of a TACAN is co-located with a VORand works on a paired frequency, so when the VOR is selected the DME is automatically selected.

A VOR/DME ground station.

http://en.wikipedia.org/wiki/File:VORTAC_TGO_Aichtal_Germany_01.JPG

ILS is a runway approach aid:

Instrument Landing System (ILS)

Navigation Systems

ILS is used to guide the aircraft to the runway threshold in poor visibility.

The pathway then leads to the touch-down point on the runway

These define a radio beam that is like a pathway in the sky

Fixed transmitters on the ground send out a special pattern of radio signals

LocalizerTransmitter

GlidepathTransmitter

Runway

90Hz

150Hz

Glidepath

ILS transmits 2 frequencies

Glidepath

90Hz signal is strongest, so aircraft is above the Glidepath

ILS

Navigation Systems



Runway

90Hz

150Hz

Glidepath

Glidepath


150Hz signal is strongest, so aircraft is below the Glidepath

ILS

Navigation Systems



Runway

90Hz

150Hz

Glidepath

Glidepath


Both signals are equal, so aircraft is on the Glidepath

ILS

Navigation Systems


GlideslopeTransmitter

RunwayRunway Centre Line

90Hz

150Hz

Localizer

ILS transmits 2 frequencies90Hz signal is strongest, so aircraft is right of the Localizer

ILS

Navigation Systems




90Hz

150Hz

Localizer

ILS transmits 2 frequencies150Hz signal is strongest, so aircraft is left of the Localizer

ILS

Navigation Systems




90Hz

150Hz

Localizer

ILS transmits 2 frequenciesBoth signals are equal, so aircraft is on the Localizer

ILS

Navigation Systems




90Hz

150Hz



Runway

90Hz

150Hz

Glidepath

ILSNavigation Systems

Navigation SystemsINS

INS is based on measuring acceleration in all planes andcalculating distance and speed from the acceleration.

To initiate the INS system the unit must be powered upfrom 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.


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.


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.

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 signalwhich is coded in time pulses.

Navigation Systems

Satellite 1

Satellite 3

Satellite 2

3

3

1

1

2 2

Each satellite has an atomic clock which is synchronised with each other to ensure best accuracy for the system.

Navigation SystemsGPS

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.

Navigation SystemsFlight 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 andDisplay 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 loadedin seconds on arrival at the aircraft.

Navigation SystemsWeather Radar

This is an aircraft mounted radar system which can detect cloud and thunderstorms and can also be used for ground mapping.

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.

The returns come from the water droplets in the cloud.

The more water content the larger the return.

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.


How the scan works

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 Gyroto keep the antenna level no matter the attitude of the aircraft.

The antenna has a tilt mechanism which allows the operatorto move the beam up or down, usually to a maximum of 15º.

This can be useful when determining the top or bottom of acloud structure.

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

If the tilt is increased or decreased the return will disappear from the screen.


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.


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.

In the ground mapping mode the radar beam is turned through 90º.


This gives a better ground mapping picture.

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.

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.

Navigation Systems

All aircraft will provide Mode S basic DAPs. The following parameters are downlinked to the interrogating system:

Successor IFF (SIFF)

SIFF is an upgraded IFF system which incorporates modes ‘S’ and 4.

DATA Input

(a) Flight Identification - flight crew.

(b) Transponder Capability - automatic.

(c) Altitude – Digital Altitude Unit (DAU).

(d) Flight Status - automatic.

With the introduction of Digital Air Data Units more information is available to be used electronically.

Navigation Systems Successor IFF (SIFF)

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:

Mode S Level 2 Enhanced Surveillance

(a) Magnetic heading.

(b) True airspeed.

(c) Indicated airspeed.

(d) Mach number.

(e) Roll angle.

(f) True track angle.

(g) Track angle rate.

(h) Ground speed.

(i) Vertical rate.


Mode Selectors


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.

Traffic Collision and Avoidance System (TCAS)

Navigation Systems

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

+ 10,000 ft TCAS II + 10,000 ft TCAS II

- 10,000 ft TCAS II - 10,000 ft TCAS II

40NM40NM 15NM TCAS II 15NM TCAS II

TCAS communicates with transponder equipped aircraft that are within its Surveillance Volume.

Navigation SystemsTCAS

Navigation Systems

47

RNG 1

40NM40NM

15NM15NM

25NM25NM25NM25NM

Surveillance VolumeTCAS

Resolution Advisory display (RA)

Traffic Advisory display (TA)

The TCAS system uses two types of display to present information to the aircrew.

TCASNavigation Systems

The time to closest point of approach with the intruder is now between 15 and 35 seconds.

This is given when an aircraft is deemed a threat to your aircraft.

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.


Non Threat Traffic: Range beyond 6 NM and altitude is greater than +/- 1200 feet from your aircraft. It is not yet considered a threat.

UP

DN

BRT

21 RNG 10

6

4

4

21

0

.5

.5+13

Traffic is +1300ftclimbing.

Proximity Traffic: Range within 6 NM and altitude is within +/- 1200 feet of your aircraft. Still not yet considered a threat.

UP

DN

BRT

21 RNG 10

6

4

4

21

0

.5

.5

-05


Traffic is – 500ftdescending.


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.

UP

DN

BRT

21 RNG 10

6

4

4

21

0

.5

.5

+02

Traffic is +200ftand level.


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.

UP

DN

BRT

21 RNG 10

6

4

4

21

0

.5

.5

+02

Traffic is +200ftand level very close.

53

UP

DN

BRT

21 RNG 10

6

4

4

21

0

.5

.5+13

+02 -05


+02

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.

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.


54

UP

DN

BRT

+05

6

4

4

2

2

1

0

.5

.5

1

6

4

4

2

2

1

0

.5

.5

1

-05 +13

+02

RNG 10

Resolution Advisory Symbols: Preventative Advisory

“Monitor Vertical Speed”

+026

4

4

2

2

1

0

.5

.5

1

-05 +13

+02

+02

Green Arcis a fly to area.

Red Arc is afly from area.


UP

DN

BRT

+05

6

4

4

2

2

1

0

.5

.5

1

6

4

4

2

2

1

0

.5

.5

1

-05 +13

+02

+02

RNG 10

Resolution Advisory Symbols: Corrective Advisory

DESCEND - DESCEND

Green Arcis a fly to area.

Red Arc is afly from area.

Navigation Systems

56

Resolution Advisories and Synthesized Voice AnnouncementsRESOLUTION 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)

“ADJUST VERTICAL SPEED, ADJUST”

ANY WEAKENING OR SOFTENING OF A RA “ADJUST VERTICAL SPEED, ADJUST”

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”

Head on 1000.exe


Head on 400.exe


Over take.exe


TCR_AC_32.exe


no_bearing.exe


Non_alt.exe


avionics and aircraft electrical systems chapter 3 navigation systems

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