The Principle Of Loran
The phenomenal accuracy of LORAN is possible because radio signals travel at a constant known speed. Each coordinated LORAN transmitter sends out a continuous succession of sharp radio pulses. If the LORAN receiver is equidistant from two transmitters, the pulses will be coincident. If the pulses from one station are received earlier than the pulses from the other station, the difference in the time of arrival of the two pulses contains information about the difference in distance to the two transmitters.
Radio signals travel a distance of almost exactly 984 ft (300 m) a microsecond. If a LORAN receiver measures a 100-microsecond time delay between pulses from two identified transmitters, the receiver is somewhere along a line corresponding to all the locations that are 9,843 ft (30,000 m) closer to the station transmitting the pulses received first than to the other transmitter site. That is, the receiver is not necessarily located 9,843 ft from the closest station, but it is 9,843 ft closer to this station than to the other station. If pulses from a different pair of stations is measured, with at least one signal source not involved in the first measurement, the difference in the distance to these new transmitter sites can be determined similarly. If this second comparison reveals that one of these transmitter sites is 16,405 ft (50,000 m) closer than the other, the LORAN receiver will be along a different line where the difference in distance equals 16,405 ft (50,000 m). The coordinates of the point where these two lines cross satisfy both measurement pairs. A third pair of signals must be measured to remove a final ambiguity.
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