LOng RAnge Navigation- LORAN, (Class II navigation)

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LOng RAnge Navigation- LORAN, (Class II
navigation)

• AST 241
• Dr. Barnhart

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LORAN Overview

• 1st Modern practical area navigation system for
  use in general aviation.
• Developed for maritime navigation. Early LORAN
  As were exclusively for marine use
• Still in use today although its use beyond 6 8
  years from now is uncertain

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LORAN Overview

• Certain LORAN-C receivers are approved for IFR
  Enroute and Terminal navigation but not for
  approaches.
• Most LORAN-C recievers are used for VFR
  navigation
• For all practical purposes (for the pilot)
  LORAN-C provides the same type of nav. Data to
  the pilot ( ground track, desired track,
  groundspeed, ETE)

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LORAN-C Theory

• Operates in the LOW frequency electromagnetic
  energy radio bandwidth which follows the
  curvature of the earth therefore making it
  suitable for longer ranges

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LORAN Theory

• The LORAN-C concept is based on groups or
  chains of stations around the world.
• Worldwide there are 28 chains.
• Each chain consists of one master station and 4
  6 secondary stations in a geographic area (ie.
  Northeast U.S.chain, Great Lakes U.S. chain, etc.)

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LORAN Theory

• The master station of each chain broadcasts a
  continuous string of Low Frequency pulses
  (measures in microseconds).
• The unique time between the start and stop of
  each pulse identifies the particular chain.

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LORAN Theory

• This time is known as the Group Repetition
  Interval or GRI.
• For instance the time between the start and stop
  of each pulse in the northeast U.S. chain is
  99,600 microseconds or 9960 for short. 9960 is
  identified as the great lakes chain

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LORAN Theory

• The GRI is entered into the LORAN-C unit in the
  aircraft upon startup along with the approximate
  LAT./Long. Position during the initialization
  process. This helps the unit find its position
  more quickly.

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LORAN Theory

• The LORAN-C unit then searches for that master
  station and the two strongest secondary signals.
• The master station pulse triggers a response from
  the secondary stations. The receiver corrects for
  any time lag in pulse reply.

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LORAN Theory

• The receiver then calculates the time it takes
  these signals to reach the aircraft and
  essentially generates hyperbolic Lines of
  Position (LOP) from each station- known as a
  hyperbolic lines of position since the lines look
  like a parabola.

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LORAN Theory

• Two intersecting hyperbolas produce two possible
  points for the aircraft position.
• Using a second slave (or secondary) station the
  receiver then pinpoints its position. Entering
  the Lat./long. During initialization also aids in
  this process.

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LORAN Limitations

• Area of coverage limited to chains which dont
  give global coverage
• The sky wave component of the signal bounces
  off the ionosphere causing navigational errors
  when traveling greater than 1,000 NM from the
  farthest master or secondary station- causes
  errors in position of up to three miles (greater
  in some cases).

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LORAN Limitations- sky waves

• 1,000 miles is significant as at ranges less than
  this, the ground wave is significantly greater in
  strength than the sky waves therefore the
  receiver is programmed to reject the weaker sky
  waves.

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LORAN Limitations- sky waves

• At distances of between 1,000 and 1,400 NM the
  ground and sky waves are approximately the same
  strength making the signals seem the same to the
  receiver.
• Since the sky waves take longer to reach the
  receiver than the ground waves, this leads to
  position errors.

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LORAN Limitations- sky waves

• Manufacturers deal with in one of two ways
• Programming the receivers to reject all sky waves
  thereby reducing the effective range of the unit
  (but maximizing the accuracy)
• Informing the user that nav. Data may be
  inaccurate between 1,000 and 1,400 NM
• At ranges greater than 1,400 NM, the receivers
  can be programmed to use the sky wave

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LORAN Limitations

• As the aircraft moves through precipitation, it
  generates static electricity which generates
  electromagnetic noise around the aircraft
  thereby interfering with the LORAN signal-
  causing the Signal Strength to Noise ratio (SNR
  ratio) to go to unacceptable limits.
• SNR ratio is a function of precip. Intensity and
  distance from the stations.

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LORAN- Accuracy

• LORAN accuracy is also best over water during the
  day.
• Conversely it is least accurate over land masses
  at night (sky waves more intense at night).

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LORAN Accuracy

• Repeatable Accuracy- ability to return to a
  pre-set position for LORAN-C is .01 NM or 60ft.
• Absolute Accuracy- ability of the receiver to
  determine its position independently is generally
  within at least 3 NM for distances up to 2,800 NM.