LORAN C

Long RAnge Navigation version C

Originally a marine navigation system

Became feasible for aircraft navigation with the introduction

of microprocessors

Frequency of Operation: 100kHz (all stations)

LORAN C

• A HYPERBOLIC SYSTEM

i.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.

LORAN C

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

LORAN C

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

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)

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

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 chain’s signals

LORAN CHow 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

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.

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)

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!!

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%)

LORAN C Antennas

“Top loaded” antenna with ground plane

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

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

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

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

Phase Locked Loops (PLLs)

LORAN C Accuracy

• Absolute Accuracy depends on geometry

•0.1 to 0.25NM

• Repeatability

•20 to 100m

Error Sources

Variation in propagation speed (land vs water, type of terrain)

Changes in signal strength

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