Loran and Shoran

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  • Topic: LORAN, Radio, Radio navigation
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  • Published : January 10, 2013
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LORAN (LOng RAnge Navigation)

- is a terrestrial radio navigation system using low frequency radio transmitters in multiple deployment (multilateration) to determine the location and speed of the receiver. - The most recent version of LORAN in use is LORAN-C, which operates in the low frequency portion of the electromagnetic spectrum from 90 to 110 Kilohertz. ♦ History

-LORAN was an American development, advancing the technology of the British GEE radio navigation system that was used early in World War II. While GEE had a range of about 400 miles (644 km), initial LORAN systems had a range of 1,200 miles (1,930 km). -It originally was known as "LRN" for Loomis Radio Navigation, after Alfred Lee Loomis, who invented the longer range system and played a crucial role in military research and development during World War II, but later was renamed to the abbreviation for the more descriptive term. - LORAN systems were built during World War II after development at the Massachusetts Institute of Technology (MIT) Radiation Laboratory and were used extensively by the US Navy and Royal Navy.

♦ Principle

“ Diagram of the LORAN principle ”
The difference between the time of reception of synchronized signals from radio stations A and B is constant along each hyperbolic curve; when demarcated on a map, such curves are known as "TD lines"

The navigational method provided by LORAN is based on the principle of the time difference between the receipt of signals from a pair of radio transmitters.

A given constant time difference between the signals from the two stations can be represented by a Hyperbolic Line of Position (LOP).

If the positions of the two synchronized stations are known, then the position of the receiver can be determined as being somewhere on a particular hyperbolic curve where the time difference between the received signals is constant.

In ideal conditions, this is proportionally equivalent to the difference of the distances from the receiver to each of the two stations.

A LORAN network with only two stations cannot provide meaningful navigation information as the 2-dimensional position of the receiver cannot be fixed due to the phase ambiguities in the system and lack of an outside phase reference.

A second application of the same principle must be used, based on the time difference of a different pair of stations.

In practice, one of the stations in the second pair also may be and frequently is in the first pair.

In simple terms, this means signals must be received from at least three transmitters to pinpoint the receiver's location. By determining the intersection of the two hyperbolic curves identified by this method, a geographic fix can be determined.

♦ LORAN Pulse

♦ LORAN Method
- In the case of LORAN, one station remains constant in each application of the principle, the master, being paired up separately with two other slave, or secondary stations. - Given two secondary stations, the time difference (TD) between the master and first secondary identifies one curve, and the time difference between the master and second secondary identifies another curve, the intersections of which will determine a geographic point in relation to the position of the three stations. These curves are referred to as TD lines. - In practice, LORAN is implemented in integrated regional arrays, or chains consisting of one master station and at least two (but often more) secondary (slave) stations, with a uniform Group Repetition Interval (GRI) defined in microseconds. The master station transmits a series of pulses, then pauses for that amount of time before transmitting the next set of pulses. - The secondary stations receive this pulse signal from the master, then wait a preset amount of milliseconds, known as the Secondary Coding Delay, to transmit a response signal. -In a given chain, each secondary's coding delay is different,...
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