LORAN CLORAN C

By Farhan SaeedBy Farhan Saeed

Loran CLoran C Loran-C is a hyperbolic radio navigation system. Loran-C is a hyperbolic radio navigation system. The systems operate on the principle that the difference The systems operate on the principle that the difference

in the time of arrival of signals from two or more stations, in the time of arrival of signals from two or more stations, observed at a point in the coverage area, is a measure observed at a point in the coverage area, is a measure of the difference in distance from the point of observation of the difference in distance from the point of observation to each of the stations. to each of the stations.

Loran-C employs time difference measurements of Loran-C employs time difference measurements of signals received from at least three fixed transmitting signals received from at least three fixed transmitting stations. The stations are grouped to form a 'chain' of stations. The stations are grouped to form a 'chain' of which one stations is labelled the master (designated M) which one stations is labelled the master (designated M) and the others are called secondary stations (designated and the others are called secondary stations (designated W, X, Y, or Z).W, X, Y, or Z).

Loran CLoran C

For a given master-secondary pair of For a given master-secondary pair of stations, a constant difference in the time stations, a constant difference in the time of arrival of signals defines a hyperbolic of arrival of signals defines a hyperbolic Line Of Position (LOP). Line Of Position (LOP).

Second master-secondary pair results in a Second master-secondary pair results in a second LOP. second LOP.

The position fix is achieved by observing The position fix is achieved by observing the intersections of the two LOPs on the intersections of the two LOPs on specially latticed Loran-C charts.specially latticed Loran-C charts.

Master / SlaveMaster / Slave

MM is the is the master master station.station.

W, X, YW, X, Y and and ZZ are are known asknown as secondary secondary stationsstations (or slaves). (or slaves).

M

W

X

Y

Z

Master / SlaveMaster / Slave The master transmits a set of The master transmits a set of 8 plus 1 pulses8 plus 1 pulses.. The pulses are received at the ship and at The pulses are received at the ship and at W, X, YW, X, Y and and

ZZ.. When the ship receives the first master pulse, it starts a When the ship receives the first master pulse, it starts a

timing clock.timing clock. When the secondary stations receive the first master When the secondary stations receive the first master

pulse, they wait for a short time known as a pulse, they wait for a short time known as a coding coding delaydelay and then each transmits a similar set of and then each transmits a similar set of 8 pulses8 pulses..

The ship receives the pulses from The ship receives the pulses from W, X, YW, X, Y and and ZZ and and times the interval between receiving the master pulse times the interval between receiving the master pulse and receiving each of the four secondary pulses.and receiving each of the four secondary pulses.

Coding DelayCoding Delay

The coding delay is such that the ship will The coding delay is such that the ship will always receive the master station pulse first, always receive the master station pulse first, then W pulse, then X pulse then Y pulse and then W pulse, then X pulse then Y pulse and finally Z pulse. finally Z pulse.

The coding delay also is such that the pulses do The coding delay also is such that the pulses do not overlap as they are received. not overlap as they are received.

After a short interval of between one twentieth to After a short interval of between one twentieth to one tenth of a second, the master station one tenth of a second, the master station transmits another set of pulses and the cycle transmits another set of pulses and the cycle repeats.repeats.

Time DifferenceTime Difference

The position of the ship determines the The position of the ship determines the time differences time differences

If we know the time differences, we know If we know the time differences, we know the ship’s position. the ship’s position.

Ninth PulseNinth Pulse

It enables the Loran receiver to identify the It enables the Loran receiver to identify the master station.master station.

It is used to transmit warnings if any It is used to transmit warnings if any station is not transmitting correctly. The station is not transmitting correctly. The warnings trigger alarms in the Loran warnings trigger alarms in the Loran receiver.receiver.

Time Difference MeasurementTime Difference Measurement

Pulse matchingPulse matchingCycle matchingCycle matching

Group Repetition Interval Group Repetition Interval

Each chain sends its pulses at a specifiedEach chain sends its pulses at a specified Group Repetition Interval (GRI). Group Repetition Interval (GRI).

There are several different intervals. Each There are several different intervals. Each is a few hundreds of microseconds less is a few hundreds of microseconds less than 50,000, 60,000, 80,000, 90,000 or than 50,000, 60,000, 80,000, 90,000 or 100,000 100,000 seconds. seconds.

Examples;Examples;49900 49900 sec sec known as Station 4990known as Station 499059300 59300 sec sec known as Station 5930known as Station 5930

Time Difference MeasurementTime Difference Measurement

Uses the third cycle of the received pulse Uses the third cycle of the received pulse because;because; The start of the received pulse may be too weak to be The start of the received pulse may be too weak to be

heard heard The master and secondary signals may not be The master and secondary signals may not be

received at the same strength.received at the same strength. It is possible to accurately identify the time when the It is possible to accurately identify the time when the

third cycle ends and time this point. third cycle ends and time this point. This part of the pulse arrives at the ship before there This part of the pulse arrives at the ship before there

can be any sky wave interference. can be any sky wave interference.

AccuracyAccuracy

The accuracy of the Loran system The accuracy of the Loran system depends upon: depends upon: The accuracy of measuring the timing delays The accuracy of measuring the timing delays

(0.1 (0.1 sec). sec).The angle between the Loran lines of position The angle between the Loran lines of position

(LOP).(LOP).The position of the ship in the Loran coverage The position of the ship in the Loran coverage

area, that is whether the position is near the area, that is whether the position is near the base line or the base line extension.base line or the base line extension.

Additional Secondary Factor (ASF) Additional Secondary Factor (ASF)

The Latitude/Longitude computation in many The Latitude/Longitude computation in many receivers is based upon a pure seawater receivers is based upon a pure seawater propagation path. propagation path.

Over land distances signals travels at a slower Over land distances signals travels at a slower speed. speed.

For those receivers that accommodate the For those receivers that accommodate the correction it is called an correction it is called an Additional Secondary Additional Secondary Factor (ASF) Factor (ASF) correction, and this is applied correction, and this is applied automatically when the receiver computes the automatically when the receiver computes the latitude and longitude.latitude and longitude.

eLoraneLoran Enhanced Loran, or eLoran, is independent of Enhanced Loran, or eLoran, is independent of

GPS but fully compatible in its positioning and GPS but fully compatible in its positioning and timing information, and its failure modes are very timing information, and its failure modes are very different. different.

eLoran is based on the existing low frequency eLoran is based on the existing low frequency Loran-C infrastructures that exist today in the Loran-C infrastructures that exist today in the United States, Europe, and Far East, and in fact United States, Europe, and Far East, and in fact throughout much of the northern hemisphere. throughout much of the northern hemisphere.

It is an internationally recognized positioning and It is an internationally recognized positioning and timing service, the latest evolution of the low timing service, the latest evolution of the low frequency long-range navigation (Loran-C) frequency long-range navigation (Loran-C) radionavigation system. radionavigation system.

eLoraneLoran

Why eLoran? Why eLoran? GPS is vulnerable to disruption, and it doesn't GPS is vulnerable to disruption, and it doesn't

work everywhere - entering a tunnel or parking work everywhere - entering a tunnel or parking garage or even traveling down a narrow city garage or even traveling down a narrow city street: the navigation system generally alerts to street: the navigation system generally alerts to "loss of satellite reception." "loss of satellite reception."

Some interruptions of cell-phone operations or Some interruptions of cell-phone operations or losses of other services for no apparent reason losses of other services for no apparent reason have been the result of GNSS interference. have been the result of GNSS interference.

eLoraneLoran Perhaps the most exciting changes from Loran-C to Perhaps the most exciting changes from Loran-C to

eLoran are the new operating concepts. eLoran are the new operating concepts. All transmitters are timed directly to UTC, so that a user All transmitters are timed directly to UTC, so that a user

may use all eLoran signals in view and may combine may use all eLoran signals in view and may combine them with GNSS signals for robust position and time them with GNSS signals for robust position and time solutions. solutions.

Each transmitter includes a messaging channel; this is Each transmitter includes a messaging channel; this is an in-band signaling channel that allows the eLoran an in-band signaling channel that allows the eLoran signal to also carry information to improve the user's signal to also carry information to improve the user's solution. solution.

Very much like GPS this messaging channel provides Very much like GPS this messaging channel provides transmitter identification, time of transmission, differential transmitter identification, time of transmission, differential corrections, and authentication and integrity signals.corrections, and authentication and integrity signals.

Any Questions ?Any Questions ?