Instrument Landing System (ILS)

• Prior to World War II, landing in poor weather was dangerous
• Early navigation systems only gave lateral position info
• 1930s pilots calculated their position relative to radio stations in low visibility
• All final approaches were visual because there was no instrument guidance
• ILS commands allow non-visual approaches to runways
• With ILS, outside visibility is not needed until touchdown

ILS Lesson topics

• Ground based radio transmitting equipment

• Airborne receiving equipment

• Instruments pilots use to navigate along the route

• ILS remains the main landing aid worldwide

• ILS is a precision approach system for zero decision height and visibility using modern equipment

• ILS gives:

  • Glide path
  • Localiser information
  • Distance to the runway threshold

ILS Components

• Guides aircraft from kilometers out to the runway threshold

• Lateral guidance is called the localiser

• Vertical guidance is called the glideslope

• Marker transmitters indicate position on the approach path

• The entire system uses ground and airborne equipment

• Aircraft use VHF localiser signals for lateral guidance

• UHF glideslope signals are used for vertical guidance

• Tuning to localiser frequency tunes the glideslope receiver automatically

• FAA has more to ILS than localiser and glideslope signals

  • Guidance information: localiser and glideslope
  • Range information: OM and MM beacons
  • Visual information: approach, touchdown, centreline, and runway lights

• An ILS system uses horizontal and vertical needles to indicate the aircraft position relative to the glide path

  • If horizontal needle is above the center, the aircraft is below, and vice versa

• Aircraft have a cockpit with computerized instrument landing equipment that interprets ground signals

  • Localiser beam width: 3° to 6°
  • Glideslope beam gives vertical information
  • Glide path width: 3° with a 1.4° height

• Three marker beacons indicate the distance from the runway threshold

  • Outer Marker (OM) is approximately 8 km from the runway
  • Middle Marker (MM) is approximately 1 km from the threshold
  • Inner Marker (IM) is between the middle marker and runway threshold(aircraft is 100 feet above)

• Most Australian airports lack an inner marker due to good weather conditions

• The system is fixed for one runway

• Airports may have multiple ILS for each runway

Ground-based equipment

• Localiser (LOC) transmitter and antenna are at the far end of the runway
• Glideslope (GS) transmitter and antenna are located near the runway threshold
• Outer, middle, and inner marker antennas

Glideslope

• Glideslope gives vertical guidance for the ILS approach
• Standard glideslope path is 3º downhill to the runway end
• Glideslope antenna operates between 328 and 335 MHz (UHF) and a frequency of 329.15 MHz
• Radiated glideslope signals produce two lobes:
  • The upper lobe is modulated by 90 Hz
  • The lower lobe by 150 Hz
• Equal audio signals occur at approximately 3° above the horizontal on the centre line
  • Defines the glideslope approach path
• 90 Hz signal predominates if the aircraft is too high
• 150 Hz signal predominates if the aircraft is too low
• Glideslope projection angle: 3° above horizontal and intersects:
  • Inner marker 100ft
  • MM at 200 feet
  • OM at 1,400 feet above the runway elevation
• Glideslope is typically usable to 10 NM

Glideslope System Characteristics

• Frequency Range: 329.15 to 335.0 MHz
• Number of Channels: 40 channels
• Channel increments: 150 kHz
• Modulation: Carrier is amplitude modulated to a depth of 40 percent by both the 90
• Signal Polarization: Carrier is horizontally polarized and 150 Hz tones

Localiser

• Localiser gives pilots lateral/azimuth information to guide to runway centreline
• Operates between 108 and 112 MHz, with a lowest frequency of 108.1 MHz
• Radiated lobes are side by side on the extended runway line
  • Line of equal modulation defines the runway approach path's centre line
• The 90 Hz signal predominates if the aircraft is left of the centreline
• 150 Hz signal predominates if the aircraft is right of the centreline
• Sensitivity: About 500 ft. from the centreline near the Outer Marker and 150 ft. near the Middle Marker
• It also carries audio identification
• Antenna is located at the far end of the runway
• Signal is usable 18 NM from the field
• Morse code Identification is a three-letter identifier preceded by the letter I
• Localiser and glideslope frequencies are paired
• The carrier is amplitude modulated to a depth of 20 percent by both the 90 and 150 Hz tones
• Carrier is horizontally polarized

Marker Beacons

• Beacons operate on 75 MHz in a narrow fan shape
• Pilots use audio and visual cues for distance
• Beacons are at various distances
  • Position is obtained from air navigation charts
• OM approx. 8 KM from the runway threshold:
  • Modulated with 400 Hz
  • Emits continuous dashes at two per second
• MM is approx. 1 KM from runway threshold
  • Modulated with 1300 Hz and emits dots and dashes
  • Pilots must decide whether to proceed with the landing or not
• IM is located where the flight path is 100 feet above the ground (approx. 150 metres from runway threshold)
  • Modulated with 3000 Hz
  • Emits dots continuously at a rate of 6 per second
  • Used only for Category II operations
• OM beacon is blue
• MM is amber
• IM is white
• The sound gets 'quicker' and the tone 'higher' moving towards the airport
• Beacons use narrow fan shaped vertical pattern beams
• OM indicates where an aircraft intercepts the glide path
• MM is 200 feet above the runway on the glide path
• IM is the Decision Height point
• Modulation: The carrier amplitude modulates to 95 percent

ILS Ground Installation

• Glideslope transmitter: radio beam for vertical guidance to runway threshold
• Localiser transmitter: radio beam for lateral guidance to runway threshold
• Marker beacons: vertical, fan-shaped radio beams indicate position on glideslope

Runway Visual Range (RVR)

• The range an aircraft pilot can see runway surface markings, lights that delineate, or centreline to land

Decision Height (DH)

• An altitude/ height for precision flying at which a pilot must begin a missed approach if visual guidance is unavailable

Categories of Precision Approach and Landing Operations:

• Category I: DH 200 feet and RVR 550 metres
• Category II: DH 100 feet and RVR 300 metres
• Category Illa: No DH/DH below 100 feet RVR not less than 175 metres
• Category IIIb: No DH/DH below 50 feet/RVR less than 75 metres
• Category IIIc: No DH and no RVR limitations
• Special authorization and equipment is needed for categories II and III operations

ILS Airborne Equipment

• Receivers with localiser, glideslope and marker beacon
• Three antennas
• Control unit
• Indicator displaying localiser and glideslope deviation
• Cockpit marker beacon lights
• Beacon receiver system is separate from the localiser and glideslope receiver system

Airborne Receivers

• VHF localiser receiver

• UHF glideslope receiver

• Marker receiver

• Localiser and glideslope receivers are in one radio unit with circuits for receiving, decoding, and computing

  • Contains self-testing and monitoring circuits to ensure reliability

• Beacon receiver has circuits to receive a modulated carrier signal and convert it to audio/visual output

• Controller is the same used for VOR: selects a VHF localiser frequency and the paired UHF glideslope frequency

• Tuning for glideslope and localiser is controlled by the VHF

• Some control units have VHF communication control in addition to ILS and VOR controls

• Marker beacon indicator consists of three annunciators to indicate marker beacon over which the aircraft is passing

  • Blue: outer marker (O)
  • Amber: middle marker (M)
  • White: inner marker (I)

• Beacons give distance by identifying points on the approach

• Beacons are low-power transmitters that operate at 75 MHz with 3 W or less

• Elliptical beams radiate upward from the ground

  • 2,400 ft long and 4,200 ft wide at 1,000 ft altitude

• Outer marker is located 8KM from the threshold within 100M of the extended runway centreline

  • intersects the glideslope vertically at 1,400 ft above runway
  • Marks the point at which aircraft normally intercept the glide path
  • Designates the beginning of the final approach segment

• 400 Hz is an audible low tone with continuous Morse code dashes at two per second

• Pilot hears a tone and sees a blue light that flashes in sync

• Where geographic conditions prevent outer marker positioning, the DME unit to provide position fix on the localizer

  • Most ILS installations replace the OM with an NDB

• Middle markers have been removed from all ILS facilities in Canada

  • Located approximately 1KM from the threshold on the extended runway centreline
  • Crosses the glideslope at approximately 200 to 250 ft above runway
  • Near the missed approach point for the ILS Category I approach.

• Australian airports don't often have an inner marker because conditions don't necessitate Category III ILS

• Sensitivity can be increased by selecting a switch for airway beacon use, or lower sensitivity for pinpointing beacons

• The receive frequency is set at 75 MHz and has a low sensitivity for the correct beacons

• Lights associated with the Outer and Middle Markers flash automatically

• Inner marker is sometimes labelled FM/Z, which refers to airways beacon

• Select speaker phones tones via speakers or headphones

• Multi-indicator systems need a Load Resistors so that the receiver output remains constant, and must be fitted into the circuit as per manufacturer's instructions'

• A deviation indicator movement impedance of 1000 ohms and needs 150 μΑ for Full-Scale Deflection

• The voltage with a deviation output of the receiver should be 150 mV for a DDM of 0.155.

ILS Airborne Antennas

• Electromagnetic waves are sent and received by the radio navigation transmitter and communications radio through the antenna
• Wavelength needs to be a 1/4 long, length of the antenna can be adjusted electronically
• Three antennas are required for complete ILS operation
• VHF omnidirectional antenna operates in the 108-112 MHz for localizer operation
  • Localiser uses the same antenna as the VOR navigation receiver (VHF)
• Gideslope operates in the UHF band (329-335 MHz range)
• Works with VHF navigation antenna (aircraft is seldom more than 30 KM out and glideslope is pointing at the nose)
• ''Splitters take VOR or glideslope energy and send it to the correct receiver
• Most light aircraft use a 'Cat Whisker' antenna that's mounted high on the vertical stabilizer
• Consists of two rods (¼ wavelength of the centre VOR frequency) that are a dipole antenna capable VOR and ILS transmissions
• Marker receiver uses a 75 MHz loop antenna that is directional
• The lower the frequency the longer the antenna should be
• Not efficient enough to pick-up marker beacon signals the aircraft flies over when cruising at altitude

Antenna Polarisation

• Radio electrons dance vertically for vertically polarized antennas, horizontally if antenna pointed up/down
• When a signal bounces, it is vertically polarised
• Communications are vertically polarised for maximum coverage
• Navigation signals are polarised horizontally
• General aviation uses:
  • 720 channel VHF Comms
  • 200 channel VHF Nav receiver
  • Necessary controls with digital readout of comm and nav frequencies on the front panel
  • Control box provides tuning information for the DME and glideslope

ILS Indicators

• Displayed on:

• ILS indicators

• Course Deviation Indicators (CDI)

• Horizontal Situation Indicators (HSI)

• Attitude Director Indicators (ADI)

• Head-up Guidance Systems (HGS)

• Flight Directors (FD)

• Show lateral/vertical deviation with respect to aircraft symbol.

• Pitch and roll, and steering command are also given, and warning flags are visible whenever info isn't reliable

• Localiser pointers appear to the right of the instrument centreline if the aircraft is left of it, indicating that the pilot has to fly right and vice versa

• If the aircraft on the glideslope, the pointer is below the instrument horizon, meaning the pilot has to lower the aircraft to capture the glideslope

• In bug and dot pointers there are two displays, localiser/glideslope are at the bottom/right respectively

  • If the bug is centred, the aircraft is or glideslope, or localiser
  • if the aircraft strays from, the bug will move off high/low or left/right
  • The localiser bar and glideslope pointer are moved by meter movements

• Two-dot deflection of the localiser bar means that the aircraft deviation is 2.5°

• Full-scale deflection of the glideslope pointer means that the that deviation is 0.7"

• On the outer marker, the separation of one dot may be 100 feet

• On the inner marker 1, the separation may only be 20 feet

Localiser Indication

• If the aircraft's position is to the right of the on-course path, the 150 Hz signal will dominate, so the correction is to fly left
• If the aircraft's position is to the left of the on-course path, the 90Hz signal will dominate, so the correction is to fly right
• On a flight path 1 to 1.5 degrees left or right of the on-course path, the receiver output will be 75uA
  • the DDM is 0.155 when full deflection is achieved, and that corresponds to 2.5/3 degrees off course
  • Output is 150μA at this point

Glideslope Indications

• 90 Hz dominates above the glide path so fly down
• 150 Hz dominates below the glide path so fly up
• Zero DDM indicates no deflection
• Shows a half scale deflection at a On a flying path 0.35 above/below path for 75⎵A
  • Full scale is 0.175 DDM and 0.7 degrees with 150⎵A

Back switching

• Some localiser transmit both the front to back courses
  • Needle indicates a reverse on back bear approach without heading ref
• In back be flying reverse on correction from needle. Turn away rather then at
• When performing back make sure not to use glide scope at all
• When engaging in back course approach hit the switch reversing Localiser indicator making flying easier

Diagram Operation

• Receiver is UHF or VHF for glidescope and does not need the same audio
• Since vor loc the same thing there is one navigation that does both
• The filter consist of signal split between audio and is with both 90/150hz
• The converter splits 90 htz and 150

Marker Beacon Diagram

• Marker is set to 75mz using TRF or superheterodyne
• The audio detected goes to separation which feeds to AIS
• Lamp switch cuts DC power to lamp.
• the high switch reacts to air waves and responds.

ILS system has

• Audio, station identifier
• High flag with localiser
• Low lever five parallel
• Glide is 2v and 150 m and must stay consistent
• Autopilot is both glides scope, elevons, And has the safe singal for the machine
• The remote VOR
• Shows both nav system
• Some can be build in the units

Testing operational

• Apply power
• Switch to some frq
• Check that it is not flag

Ramp

• Test box that says it's on line
• If the function is set up correctly for ISL.
• Some has antenna and are not be be used
• Self test are set up too