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  • VDF
  • ADF
  • VOR
  • ILS
  • DME
  • AWR ... Airborne weather radar
  • Radar
  • GPS

Electromagnectic fields

  • electromagnetic field consist of magnetic and electric field
  • only electric field is relevant

Wave propagation

D, E and F layer of the atmosphere
Sky wave propagation
Interference between sky and surface propagation paths

Range of a 300 kHz transmitter:

  • Sea: range = 3 * sqrt(Power)
  • Land: range = 2 * sqrt(Power)

The Ionosphere

D Layer

  • forms at sunrise and disappears at sunsets
  • only present on days

E Layer

  • present throughout the 24 hours
  • sunrise: reducing altitude
  • sunset: increasing altitude

F layer

  • surise: splitted into two layers
  • sunset: reunited
  • skip distance ... distance between transmitter and first ray of the sky wave
  • usage of medium or high frequency due to higher range of sky waves
  • more interference with low frequency


  • during the day: with D layer, the ionosphere is more extended
  • during night: the D layer disappers, maybe area where already ground waves are travelling can be hit


Single Sideband Operation

  • used mainly for long-range communication

Instrument landing system

  • Instrument: CDI with also vertical deviation
  • ILS need two antennas:
    • localizer antenna
    • glide path antennas
    • and: marker beacons
  • Identifier for the localizer, not for glide slope!

Localizer Antenna (LOC)

Localizer antennas are always behind the runway > also ILS guidance during the takeoff run is possible

  • Frequency 108-111.975 MHz
  • divided into 40 sub-channels
  • ILS frequency: first digit after decimal is odd
  • two lobes, one modulated with 150 Hz (right), one with 90 Hz (left)
  • ILS measures which signal is "stronger" > DOM (... of modulation)


  • until 17 NM coverage of 35°
  • until 25 NM coverage of 10°

Side lobes

  • side lobes are also at the 180° other side
  • "back-course approach" is technically possible

Glide path antennas (GP)

principle: only gives you vertical guidance

  • also two lobes, one modulated with 150 Hz (...), one with 90 Hz (...)
  • next to the PAPIs
  • about 300 m behind the threshold
  • own frequency, UHF, but coupled with ILS system

until ~8-10 NM, ~8°


  • 1.75 * GPA
  • 0.45 * GSA

Side lobes

  • also side lobes for the glide slope
  • rue lobe is the lowest one, therefore intercept ILS glide paths always from below

usual ILS procedure

  • Standard arrival route until initial approach fix
  • then follow ILS approach


if no DME is available:

  • outer marker (3.5-6 NM from threshold), two dashes per second at 400 Hz
  • middle marker (3.000-4.000 ft), alternating dot and dash at 1200 Hz
  • inner marker (250-1.500 ft), 6 dots per second
  • most of markers are not in use anymore
  • mainly active in scandinavia and eastern europe
  • crosscheck of altitude against distance

full deflection is 2.5 degrees (remember: for VOR it is 10 degrees!) glide slope: full defletions = 0.7 degrees


  • beam bends due to atmospheric conditions
  • reflections with objects
  • beam noise due to interference
  • weather
  • FM broadcasts

ILS categorys

  • CAT I: RVR more than 550 m or visibility more than 800 m, DH more than 200 ft
  • CAT II: RVR more than 300 m, DH between 100 ft and 200 ft
  • CAT IIIa: RVR more than 175 m, DH below 100 ft
  • CAT IIIb: RVR between 50 m and 175 m, DH below 50 ft
  • CAT IIIc: 0/0 limits