LORAN C By Farhan Saeed

Loran C Loran-C is a hyperbolic radio navigation system. The systems operate on the principle that the difference in the time of arrival of signals from two or more stations, observed at a point in the coverage area, is a measure of the difference in distance from the point of observation to each of the stations. Loran-C employs time difference measurements of signals received from at least three fixed transmitting stations. The stations are grouped to form a 'chain' of which one stations is labelled the master (designated M) and the others are called secondary stations (designated W, X, Y, or Z).

Loran C For a given master-secondary pair of stations, a constant difference in the time of arrival of signals defines a hyperbolic Line Of Position (LOP). Second master-secondary pair results in a second LOP. The position fix is achieved by observing the intersections of the two LOPs on specially latticed Loran-C charts.

Master / Slave M is the master station. W X M is the master station. W, X, Y and Z are known as secondary stations (or slaves). M Z Y

Master / Slave The master transmits a set of 8 plus 1 pulses. The pulses are received at the ship and at W, X, Y and Z. When the ship receives the first master pulse, it starts a timing clock. When the secondary stations receive the first master pulse, they wait for a short time known as a coding delay and then each transmits a similar set of 8 pulses. The ship receives the pulses from W, X, Y and Z and times the interval between receiving the master pulse and receiving each of the four secondary pulses.

Coding Delay The coding delay is such that the ship will always receive the master station pulse first, then W pulse, then X pulse then Y pulse and finally Z pulse. The coding delay also is such that the pulses do not overlap as they are received. After a short interval of between one twentieth to one tenth of a second, the master station transmits another set of pulses and the cycle repeats.

Time Difference The position of the ship determines the time differences If we know the time differences, we know the ship’s position.

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

Time Difference Measurement Pulse matching Cycle matching

Group Repetition Interval Each chain sends its pulses at a specified Group Repetition Interval (GRI). There are several different intervals. Each is a few hundreds of microseconds less than 50,000, 60,000, 80,000, 90,000 or 100,000  seconds. Examples; 49900  sec known as Station 4990 59300  sec known as Station 5930

Time Difference Measurement Uses the third cycle of the received pulse because; The start of the received pulse may be too weak to be heard The master and secondary signals may not be received at the same strength. It is possible to accurately identify the time when the third cycle ends and time this point. This part of the pulse arrives at the ship before there can be any sky wave interference.

Accuracy The accuracy of the Loran system depends upon: The accuracy of measuring the timing delays (0.1  sec). The angle between the Loran lines of position (LOP). The position of the ship in the Loran coverage area, that is whether the position is near the base line or the base line extension.

Additional Secondary Factor (ASF) The Latitude/Longitude computation in many receivers is based upon a pure seawater propagation path. Over land distances signals travels at a slower speed. For those receivers that accommodate the correction it is called an Additional Secondary Factor (ASF) correction, and this is applied automatically when the receiver computes the latitude and longitude.

eLoran Enhanced Loran, or eLoran, is independent of GPS but fully compatible in its positioning and timing information, and its failure modes are very different. eLoran is based on the existing low frequency Loran-C infrastructures that exist today in the United States, Europe, and Far East, and in fact throughout much of the northern hemisphere. It is an internationally recognized positioning and timing service, the latest evolution of the low frequency long-range navigation (Loran-C) radionavigation system.

eLoran Why eLoran? GPS is vulnerable to disruption, and it doesn't work everywhere - entering a tunnel or parking garage or even traveling down a narrow city street: the navigation system generally alerts to "loss of satellite reception." Some interruptions of cell-phone operations or losses of other services for no apparent reason have been the result of GNSS interference.

eLoran Perhaps the most exciting changes from Loran-C to eLoran are the new operating concepts. All transmitters are timed directly to UTC, so that a user may use all eLoran signals in view and may combine them with GNSS signals for robust position and time solutions. Each transmitter includes a messaging channel; this is an in-band signaling channel that allows the eLoran signal to also carry information to improve the user's solution. Very much like GPS this messaging channel provides transmitter identification, time of transmission, differential corrections, and authentication and integrity signals.

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