ADF/NDB Navigation

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ADF/NDB Navigation

• AST 241
• Professor Barnhart

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NDB Background

• Oldest form of radio navigation still in use
• Operate in the LF/MF range (190 535 kHz- or
  between 100 and 10,000 meters)
• Still in use because of its simplicity and low
  cost to set up and operate decent accuracy
• Little used for enroute navigation in the US but
  still used for approaches
• Used for enroute nav. In much of the world

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NDB background

• NDBs are also useful at ranges up to 600 N.M.
• Ground equipment consists of a simple a.m. radio
  station
• Aircraft receiver is little more complicated than
  an AM radio.

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Principles of NDB Navigation

• Transmitters vary in power from 25 watts for
  Locator Outer Markers (LOMs) on an ILS approach
  to as much as 2000 watts.
• The range of the most powerful transmitters is
  listed as 75 miles but is usually much greater.
• 4 types of NDBs Compas locators, Approach aids
  (25 NM), Enroute beacon, High power beacons- used
  in some costal areas- largely obsolete

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Aircraft Equipment

• The Automatic Direction Finder (ADF) consists of
• AM receiver
• Sense Antenna
• Loop Antenna (directional antenna)
• Indicator (fixed or movable card)

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Aircraft Equipment Ctn.

• The Automatic in ADF comes from the fact that the
  directional antenna is rotated electronically
  rather than manually by rotating the antenna or
  turning the aircraft to make signal strength
  determinations.

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ADF Operation

• There is a switch on the receiver marked
• OFF/ADF/ANT/BFO
• ANT- gives maximum receiver sensitivity and
  should be used for tuning and identifying (3
  letter coded I.D. except for LOMs which have 2
  letter I.D.s)
• BFO (ContinuousWave) position is used for better
  identifying unmodulated signals which are used in
  other parts of the world.

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ADF Operation Ctn.

• After identifying station be sure to place
  selector switch in the ADF mode as the
  indicator will not display any bearing
  information in the ANT mode
• Leave volume up continuously when using the ADF
  as NDB frequencies can shift
• Most units have a Test function which swings
  the needle dont use if the needle doesnt
  swing. This verifies proper signal reception.

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ADF indicators

• Fixed card
• With a fixed card there are two ways to determine
  magnetic bearing to the station
• Turn the aircraft toward the station and note
  heading
• MH RB MB (to the station)
• MH aircraft magnetic heading
• RB Relative bearing
• MB Magnetic Bearing (to the station)
• (If the result is more than 360 then subtract 360

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ADF Indicators

• Movable Card Indicators
• Set aircraft heading at top of the indicator and
  read the relative bearing to the station.

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NDB Navigation Techniques

• Homing
• Turing the aircraft such that the needle always
  points directly toward the station- easy but
  inefficient as it does not compensate for
  crosswinds
• Results in a curved path to the station in a
  crosswind situation

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NDB Navigation Techniques

• Tracking
• Establishing a wind correction angle that negates
  the drift caused by the crosswind
• Principle WHEN THE ANGLE OF DEFLECTION THE
  ANGLE OF INTERCEPTION YOURE ON COURSE
• Or when the angle formed by the aircraft heading
  and the desired course is the same as the angle
  between either the 0 or 180 mark on the indicator
  and the pointer, the aircraft is on course.

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Tracking ctn.

• Tracking tip- When tracking inbound on an NDB
  bearing and you note youre off course
• Note the angle of deflection, double it, and use
  that as your intercept heading. When that angle
  of deflection is then noted on the indicator
  (from the nose or tail) the aircraft is back on
  course
• Establish an initial track by taking half of that
  intercept angle out

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Tracking Ctn.

• Example
• You want to track inbound on the 360 bearing to
  the station. There is a wind from the west.
• While tracking (heading north) unaware of the
  wind you note a left needle deflection of 10
  degrees.
• Double that to 20 degrees for your intercept
  heading and turn the aircraft left to 340 degrees.

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Tracking Ctn.

• As you turn to 340 you notice the needle move
  toward the top of the indicator.
• As you continue on 340 the needle will drift
  toward the right. When it indicates 20 degrees,
  the aircraft is back on course.
• Take half your calculated intercept heading and
  use that as your initial track heading (350
  degrees). This results in a 010 degree indication
  on the indicator.
• Repeat the process until track correction and
  needle deflection are equal.

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NDB/ADF Limitations

• Night Effect-
• NDBs emit three signals, among these are the
  ground wave and the sky wave. Normally the
  ground wave is the strongest which is the one use
  for navigation.
• The skip zone is the difference or gap between
  the ground wave and the reflected sky wave.
• Light passing through the ionosphere causes
  height fluctuations within it varying the angle
  of the reflected sky wave and possibly causing
  signal confusion during this time between the two
  signals. The needle may wander. Resolves
  shortly after sunrise or sunset.

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Fading

• Usually occurs at night when ground wave and sky
  wave interact going in and out of phase causing
  the signals to be either canceled or reinforced
  as the atmosphere changes.
• Pilots will notice a rythmic swinging of the
  needle and a volume fluctuation of the
  identifier.
• Average the fluctuations and note the bearing
  when the signal seems strong.

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Shoreline Effect

• Ground waves change direction as they pass from
  land to water and visa versa they are bent
  slightly.
• Pilots should note potential bearing indication
  errors when flying in the vicinity of costal
  areas. NDBs used primarily for oceanic
  navigation have been designed to minimize this
  error.

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Other sources of Error

• Terrain- mountains, areas with high
  concentrations of iron
• Interference- between two stations may cause the
  ADF to oscillate between one station and the
  other- indicated by receiving two simultaneous
  I.D. codes. Usually resolves itself with
  proximity.

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Thunderstorms

• Lightening can create signal disturbances causing
  the needle to momentarily swing in the direction
  of the storm. As such ADF is sometimes referred
  to as a Poor mans Storm Scope.
• Pilots should not rely on ADF indications in the
  vicinity of thunderstorms.

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ADF Accuracy

• Variable- not possible to determine. As such,
  back up ADF indications whenever possible.

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The End

• Questions?