Thursday, January 12, 2012

Loran phrase long range navigation


Loran, abbreviation of the phrase long range navigation, used to designate a radio navigation system developed during World War II. Loran is one of several systems that enable navigators to establish the position of their ships or aircraft by finding the difference in the time that it takes radio signals to reach them from two synchronized transmitters spaced some distance apart.
The loran transmitter system consists of one so-called master and one slave station. The master station emits a short pulse or signal at regular intervals, and this pulse is repeated by the slave station, which is controlled from the master station by radio. Both of these signals are received aboard the ship or aircraft, are amplified, and are recorded as small waves or irregularities on the screen of a cathode-ray tube. The circuits of the receiver are so arranged that the distance between the waves corresponds to the difference in time between the arrivals of the signals from the two stations. The receiver is also equipped with an electronic timing device so that this difference in time can be measured in microseconds (millionths of a second). Because radio waves travel at a uniform speed of about 300,000 km (about 186,000 mi) per sec, the location of all points where the signals from the two stations are separated by a given time interval can be represented by a definite curve, which is a hyperbola. The navigator is equipped with a map giving a series of these curves, called loran lines of position, and, after finding the time interval, for example, of 3 microseconds, knows that the position of the craft is somewhere on the 3-microsecond curve on the chart. By switching to another pair of loran transmitters and repeating the procedure, the navigator can find another curve representing the craft's position, and can then determine the craft's actual position, which is at the intersection of the two loran lines of position. Loran has a useful range of about 2250 km (about 1400 mi) by night and about 1200 km (about 750 mi) by day. The signal is usually transmitted in the frequency band of 1.8 to 2.0 MHz. Loran can be used for setting and holding a course as well as for determining position, and it has the advantage of being independent of weather conditions. The accuracy ranges from a few hundred meters to a few kilometers, depending on the equipment used and the distance of the craft from the transmitter. See Navigation; Radio.

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