www.AMARAD.org mise à jour août 2012

Association des Marins Radios
LORAN
[mailto:amarad@free.fr]
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LORAN (LOng RAnge Navigation) is a terrestrial navigation system using low frequency radio transmitters that use the time interval between radio signals received from three or more stations to determine the position of a ship or aircraft. The current version of LORAN in common use is LORAN-C, which operates in the low frequency portion of the EM spectrum from 90 to 110 kHz. Many nations are users of the system, including the United States, Japan, and several European countries. Russia uses a nearly identical system in the same frequency range, called CHAYKA. LORAN use is in steep decline, with GPS being the primary replacement. However, there are current attempts to enhance and re-popularize LORAN.
Principe du LORAN 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.) By itself, with only two stations, the 2-dimensional position of the receiver cannot be fixed. A second application of the same principle must be used, based on the time difference of a different pair of stations. By determining the intersection of the two hyperbolic curves identified by the application of this method, a geographic fix can be determined.
LORAN chains (GRIs) Each LORAN chain in the world uses a unique GRI (Group Repetition Interval), which is designated by the number of microseconds divided by 10 (in practice the GRI delays are multiples of 100 microseconds). LORAN chains are often referred to by this designation, e.g. GRI 9960, the designation for the LORAN chain serving the Northeast U.S. Due to the nature of hyperbolic curves, it is possible for a particular combination of a master and 2 slave stations to result in a "grid" where the axis intersect at acute angles. For ideal positional accuracy, it is desirable to operate on a navigational grid where the axes are as Cartesian as possible -- i.e., the axes are at right angles to each other. As the receiver travels through a chain, a certain selection of secondaries whose TD lines initially formed a near-Cartesian grid can become a grid that is sharply angular. As a result, the selection of one or both secondaries should be changed so that the TD lines of the new combination are closer to right angles. To allow this, nearly all chains provide at least three, and as many as five, secondaries.
LORAN charts This nautical chart of New York Harbor includes LORAN-A TD lines. Note that the printed lines do not extend into inland waterway areas.Where available, common marine navigational charts include visible representations of TD lines at regular intervals over water areas. The TD lines representing a given master-slave pairing are printed with distinct colors, and include an indication of the specific time difference indicated by each line.
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