LORAN (Long Range Navigation) Basics

LORAN-C (Long Range Navigation revision C) is a long-range navigational aid that provides an aircraft’s position fix in two dimensions.

This technology is a hyperbolic system. It determines a navigational fix using hyperboles or parabolic lines. These lines are generated from the intersection of signals broadcast by ground stations.

To get the aircraft’s position, at least two hyperbolic grids are needed. This means two Lines of Position must be generated from a single master station and at least two slave ground stations, as shown in Figure 1.

LORAN Ground Station Specifications

Here’s a look at the specifications for LORAN ground stations:

• Master and slave LORAN-C transmit systems are used.
• The operating frequency is low, ranging from 90 to 110 KHz.
• LORAN-C signal pulses travel via ground waves.
• Possible transmission topologies include:
  • Trial: 1 master and 2 slave stations
  • Star: 1 master and 3 slave stations
  • Square: 1 master and 4 slave stations
• The master station transmits a sequence of 9 pulses.
• After a delay, the slave station emits a sequence of 8 pulses.

• In the aircraft, the LORAN-C receiver operates at a low frequency.
• It uses a highly selective receiver to avoid interference from other signals.
• When the 9 pulses from the master station reach the aircraft, the receiver starts counting the time it takes for the 8 pulses from the slave station to arrive.
• To calculate the aircraft’s position, we need to know the positions of the master and slave stations, the distance between them, and the time difference (δt).
• Some receivers can use both LORAN-C and GPS.

The processor inside the receiver will calculate and output the following parameters:

• Aircraft position (latitude and longitude)
• Ground speed (GS)
• Wind drift
• Waypoint (WPT) information
• Estimated times of arrival (ETAs)
• Airport data

Advantages of LORAN-C

• LORAN-C isn’t affected by line-of-sight issues because it operates at a low frequency.
• Hyperbolic grid navigation provides a more direct route, similar to waypoint navigation.
• Signal propagation relies on ground waves, enabling long-range travel.

Disadvantages of LORAN-C

• The error range is about +/- 150 meters.
• Ground wave propagation errors: The speed at which the signal travels can vary, potentially causing an offset in the calculated aircraft position.
• There aren’t enough LORAN-C ground stations worldwide to cover all possible air routes.