BERGS to resemble ships, and may suddenly reveal 

 one's ovm craft to a distant observer. (25) 



LOOPS. 1. In standing-wave patterns, points at 

 which the acoustic pressure, or other measure of 

 acoustic energy, exhibit maximum values are known 

 as loops. (4) 



2. That part of a STANDING WAVE or 

 CLAPOTIS where the vertical motion is greatest and 

 the horizontal velocities are least. Loops (some- 

 times called antinodes) are associated with 

 clapotis, and with SEICHE action resulting from 

 resonant wave reflections in a harbor or bay. (11) 



LOPHOPHORE . A ridge about the mouth region bear- 

 ing tentacles in some invertebrates. (19) 



LORAC . Lorac is a continuous wave, phase compar- 

 ison, hyperbolic positioning system that uses three 

 shore stations and permits an unlimited number of 

 users. It uses frequencies in the vicinity of two 

 megacycles. The useful range is about 200 nautical 

 miles, and accuracies range from about 15 feet on 

 the base line to about 400 feet at the outer limits 

 of the usable area. The shore equipment consists 

 of four transmitters and two receivers and es- 

 tablishes the hyperbolic lattice by using two pairs 

 of frequencies with an audio "beat" frequency of 

 315 cycles between the two frequencies of the red 

 pair and 135 cycles between the frequencies of the 

 green pair. The red, center, and green stations 

 should be established at about 60 to 120 miles 

 apart, with' the angle between the two base lines 

 less than 135- and preferably with the triad con- 

 cave toward the area of interest. A position is 

 read from the phase meters of the shipboard 

 receiver in the red and green "lanes". A lane is 

 a half-wave length which is roughly 230 feet wide 

 (depending upon the frequency being used) on the 

 base line. (29) 



LORAN (LONG RANGE NAVIGATION) . An electronic 

 navigational system developed by the United States 

 during World War 11. Hyperbolic lines of position 

 are determined by measuring the difference in the 

 time of reception of synchronized pulse signals 

 from two fixed transmitters. See also LORAN A, 

 (29) 



LORAN-A . A radio navigation system in which the 

 transmitting stations operate in pairs to provide 

 the navigator a line of position. The operation 

 of a loran system can be summarized as follows: 

 A master ground station transmits signals consist- 

 ing of short pulses of radio frequency energy on 

 a channel in the 1,800 to 2,000 kc . band. A slave 

 station receives these pulses and uses them to syn- 

 chronize its transmitter, which in turn transmits 

 similar short pulses of radio frequency energy. 

 The two signals are received aboard a ship or air- 

 craft on a specially designed radio receiver. The 

 difference in time of arrival between the master 

 and slave signals, measured in microseconds, is 

 shown on the receiver. This time difference deter- 

 mines a unique hyperbolic line of position on the 

 earth's surface. The same procedure with another 

 pair of signals provides an additional line of 

 position which is crossed with the first line to 

 obtain a Loran fix. (34) 



LORAN -B . Loran B is similar to standard LORAN 

 (LORAN A) in that it is a pulse and hyperbolic sys- 

 tem. The envelopes are matched as in standard 

 Loran to determine proper lane identification. 

 For greater accuracy, the cycles of radio 

 frequency energy inside the envelopes (or pulses) 

 are compared in phase, the same as any phase com- 

 parison system. The only difference being that 

 the information presented on the dials is given in 

 nanoseconds instead of fractional parts of a lane. 

 (29) 



LORAN-C ■ Loran C is similar to standard LORAN 



(LORAN A) in that it is a pulse and hyperbolic 

 positioning system. In Loran C, the envelopes (or 

 the shape of the pulse) are used to determine the 

 proper lane identification. This is accomplished 

 by matching the pulses as is now done in standard 

 Loran. 



Loran C has the advantage of being a very long 

 range hi-precision system capable of giving a 

 position with a high degree of accuracy, without 

 the problem of lane count identification. It has 

 frequencies in the vicinity of 100 kilocycles. It 

 has a further advantage over other phase comparison 

 systems in that there is no sky wave contamination. 

 Regular phase systems radiate continuously, and the 

 sky waves can cause phase cancellation or distor- 

 tion of the received signal. Since LORAN C is a 

 pulse system, the sky waves arrive too late to 

 cause interference with the ground wave. Only the 

 beginning or the front of the pulse is used, and 

 any signals which arrive earlier or later are 

 automatically rejected by the receiver. (29) 



LOST TIME . The period in an FM sonar, just after 

 FLYBACK, during which the sound field must be 

 reestablished. Its duration equals travel time 

 of the signal to and from the target. (5) 



LOWER HIGH WATER (LHW) . The lower of the two high 

 waters of any tidal day. (11) 



LOWER LOW WATER (LLW) . The lower of the two low 

 waters of any tidal day. The single low water oc- 

 curring daily during periods when the tide is di- 

 urnal is considered to be a lower low water. (11) 



LOW-PASS FILTER . A low-pass filter is a wave filter 

 having a single transmission band extending from 

 zero up to some critical or cutoff frequency which 

 is not infinite. (9) 



LOW WATER (LW) . The minimum height reached by a 

 falling tide. The height may be due solely to the 

 periodic tidal forces or it may have superimposed 

 upon it the effects of meteorological conditions. 

 (14) 



LOW WATER DATUM . An approximation to the plane of 

 mean low water that has been adopted as a standard 

 reference plane for a limited area and is retained 

 for an indefinite period regardless of the fact 

 that it may differ slightly from a better deter- 

 mination of MEAN LOW WATER from a subsequent series 

 of observations. (14) 



LOW WATER EQUINOCTIAL SPRINGS . Low water springs 

 near the times of the equinoxes. Expressed in the 

 terms of the harmonic constants, it is a plane 

 depressed below MEAN SEA LEVEL by an amount equal 

 to the sum of the amplitudes of the constants M2, 

 S2, and K2. (14) 



LOW WATER INEQUALITY . See DIURNAL INEQUALITY. 

 LOW WATER INTERVAL . See LUNITIDAL INTERVAL. 



LOW WATER LINE . The Intersection of any standard 

 low tide datum plane with the shore, (11) 



LOW WATER OF ORDINARY SPRING TIDES (LWOST) . A 

 tidal datum appearing in some British publications, 

 based on low water of ordinary spring tides. (11) 



LOW WATER SPRINGS. See MEAN LOW WATER SPRINGS. 



LPD . U. S. Navy ship designation, for an Amphi- 

 bious Transport, Dock. 



LPH. U. S. Navy designation for an Amphibious 

 Assault Ship. 



LS. Light-Ship. 



70 



