LORAN-Long Range Navigation basics
LORAN-C avionic system is a long range navigational aids and it stands for Long Range Navigation revision C. The system provides A/C position fix in 2-dimentional. In fact, this technology is known as a hyperbolic system since it determines NAV fix using hyperboles or parabolic lines generated from the intersection of signals radiated by ground stations.
To obtain the Aircraft position fix at least 2 hyperbolic grids are required. This means that at least two Line of Position have to be generated from a single master station and at least two slave ground stations. The same has been depicted in the figure-1.
On the ground station following are the specifications of LORAN:
• Master and slave LORAN -C Transmit systems.
• Frequency of operation is low frequency from 90 to 110KHz.
• LORAN-C signal pulses propagate using Ground waves.
• Possible Tx topologies of LORAN system are as follows:
Trial consisting of 1 master and 2 slave stations
Star consisting of 1 master and 3 slave stations
Square consisting of 1 master and 4 slave stations
• The master station transmits a sequence of 9 pulses.
• After a delay, the slave station will emit a sequence of 8 pulses.
• In the Aircraft, LORAN-C receive system works at the LF frequency.
• Highly selective receiver in order to avoid interference from the other signals.
• When the 9 pulses from the master station reaches the Aircraft, the receiver will start counting the time it takes for the slave 8 pulses to reach the airborne receiver.
• To calculate the aircraft position fix, we need to know the position of the master and slave stations, the station separation distance and δt.
• There exist receiver with dual LORAN-C and GPS usage. The processor within this receiver will calculate and output following parameters.
• Aircraft position with latitude and longitude
• Wind drift
• WPT information
• Airport data
• LORAN-C does not suffer from LOS since it operates at LF band.
• Hyperbolic grid NAV provides a more direct route similar to WPT NAV.
• Signal propagation is based on GND waves; therefore large travel ranges are possible.
• Error will be about +/- 150 m.
• Ground wave propagation error, the velocity at which the signal travels may vary, and hence some offset may be introduced in calculating the aircraft position fix.
• There are not enough LORAN-C ground stations in the world to cover all possible air routes.
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