LORAN C

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LORAN C
Long RAnge Navigation version C Originally a
marine navigation systemBecame feasible for
aircraft navigation with the introduction of
microprocessors Frequency of Operation 100kHz
(all stations)
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LORAN C

• A HYPERBOLIC SYSTEMi.e. lines of position are
  hyperbolas
• This results from the fact that the lines of
  position are determined by measuring the
  DIFFERENCE in distance from two points.

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LORAN C

One station is referred to as the Master and the
others as Slaves
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LORAN C

• At least two lines of position are required for a
  position fix thus more than one slave is required

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LORAN C

• A useful property of the hyperbola is that its
  tangent at any point bisects the angle subtended
  by the line joining the two foci
• Exercise Use this property to determine where
  the best geometry occurs (LOP at 90º)

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LORAN C

• How do we determine the time difference?
• Each station, starting with the Master, transmits
  a series of pulses with the following shape

This pulse has a bandwidth of about 20kHz
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LORAN C

• Each station transmits a series of eight of these
  pulses
• Pulse separation is 1000µs (1ms)
• Note In most chains the master transmits a ninth
  pulse after 2000µs. This can be used to indicate
  the status or integrity of the chains signals

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LORAN C

• How do we identify the pulses from each station?
• The stations transmit their signals in sequence.
  The delay between signals from each station is
  such that the signal from the previous
  transmission is out of the coverage area before
  the next is sent.
• Thus they always appear in the same order

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LORAN C Chains

• A group consisting of a Master and up to four
  slaves is called a chain
• Each chain is identified by a Group Repetition
  Rate (GRI) which is the time between
  transmissions from the master.

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LORAN C Chains

• Each slave transmits its pulse train at a
  specified interval after the master has
  transmitted.
• This is called the emission delay (ED) and is
  made up of the master-slave time (MS) and a
  coding delay (CD)

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LORAN C Transmitters

• Due to the long distances covered by each LORAN C
  chain, the power transmitted must be high (0.5
  to 4 MW)
• Propagation is by ground wave and thus has to be
  vertically polarized
• Antenna therefore is a vertical mast (ideally a
  quarter wavelength long (3km) (10,000 ft.)
• Not very practical!!

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LORAN C Antennas

• Antennas are typically about 400m high
• To improve the current flow, many are top
  loaded
• They are still not very efficient (10)

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LORAN C Antennas

Top loaded antenna with ground plane
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LORAN C Receivers

• Receivers require a data base which provides
• the location (Lat/Lon) of the Master and Slave
  stations
• the GRI of the chains to be used
• the Time Delays for the individual stations
• The LORAN C signal travels both by ground wave
  and sky wave
• ground wave gives stable, reliable timing
• sky wave does not due to the variable nature of
  the ionosphere
• ground wave is attenuated more and hence is
  weaker and can be contaminated by the sky wave

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LORAN C Receivers

• Since sky wave is always delayed by a minimum of
  30µs, the positive-going zero crossover of the
  third cycle of the ground wave is used for timing

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LORAN C Receivers

• Problems to be solved by receiver
• Signals strength may vary by 120dB
• Large dynamic range required
• Noise at LF can be very high due to long range
  propagation of interference (e.g. lightning in
  tropics)
• Signal to noise ratio can be 20 dB

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LORAN C Receivers

• Receiver Operation
• Searches for Master pulses using known GRI
• PLL locks on to carrier to generate master clock
• Locks on to slave pulses
• Measures Master/slave time interval and subtracts
  the Emission Delay (ED)
• Calculates the distances and position

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Phase Locked Loops (PLLs)

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LORAN C Accuracy

Error Sources Variation in propagation speed
(land vs water, type of terrain) Changes in
signal strength

• Absolute Accuracy depends on geometry
• 0.1 to 0.25NM
• Repeatability
• 20 to 100m

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LORAN C

• Integrity
• Monitors are installed throughout the LORAN C
  coverage area
• These monitors adjust the transmitter timing to
  compensate for changing propagation conditions
• If excessive errors are detected, the master
  transmitter is commanded to blink the ninth
  pulse off and on to indicate which station is
  unreliable
• For airborne use, this can be done within 10
  seconds of detection

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LORAN C Coverage