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)

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

LORAN CA useful property of the hyperbola is that its tangent at any point bisects the angle subtended by the line joining the two fociExercise: Use this property to determine where the best geometry occurs (LOP at 90)

LORAN CHow 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 CEach station transmits a series of eight of these pulsesPulse separation is 1000s (1ms)

Note: In most chains the master transmits a ninth pulse after 2000s. This can be used to indicate the status or integrity of the chains 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 ChainsA group consisting of a Master and up to four slaves is called a chainEach chain is identified by a Group Repetition Rate (GRI) which is the time between transmissions from the master.

LORAN C ChainsEach 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 TransmittersDue 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 polarizedAntenna therefore is a vertical mast (ideally a quarter wavelength long (3km) (10,000 ft.)Not very practical!!

LORAN C AntennasAntennas are typically about 400m highTo improve the current flow, many are top loadedThey 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 stationsThe 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 30s, 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 receiverSignals strength may vary by 120dBLarge dynamic range requiredNoise 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 GRIPLL locks on to carrier to generate master clockLocks on to slave pulsesMeasures 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 geometry0.1 to 0.25NM Repeatability20 to 100m

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

LORAN C

IntegrityMonitors are installed throughout the LORAN C coverage areaThese monitors adjust the transmitter timing to compensate for changing propagation conditionsIf excessive errors are detected, the master transmitter is commanded to blink the ninth pulse off and on to indicate which station is unreliableFor airborne use, this can be done within 10 seconds of detection

LORAN C Coverage