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Aviation History
1940
1940 - 0096.PDF
JANUARY II, 1940. Chart Making The graph must be corrected before the true least depth below low water is determined. Allowance must be made for the state of the tide at the time of sounding. The depth of the oscillators below the waterline must be ascer- tained and the accuracy of the instrument checked. This strip of sounded sea floor would be useless for chart making unless the launch's position everywhere during the run were known. This is an elementary exercise in sur- veying arid is done by " transit " or" angle-fix " methods. In the first the launch position is determined by lining it up with two different pairs of posts erected on shore, while in the '' angle-fix'' method the two subtended angles between three landmarks are observed simultaneously by two navigators and provide data from which position sub- sequently may be calculated. So hydrographic charts may be prepared with "spot depths" shown all over them or with contour lines drawn to show depths. Such a contoured chart would be very useful if the echo sounder record is to the same scale. For then a mariner wanting to know his position in a thick fog can detach the last mile of sounding record from the instrument and move it about the chart until its depth intervals exactly match the contours. This would give him his position with great certainty. As well as its application for chart making, described above, the echo-sounder is used ori ships to give a con- tinuous and instantaneous measure of the depth of water beneath the ship's keel, and so is very valuable assistance to navigation, giving immediate indication of the approach to shallow waters. Has sonic depth sounding an equally effective applica- tion in the air? Sonic and Radio Methods Every experienced pilot immediately appreciates what a tremendous life saver, not to mention nerve-sparer, such a device would prove in the air. An aeroplane pilot's wireless may "pack up," or he cannot receive signals from ground stations ; perhaps he has no wireless, or simply gets lost. He is flying blind in thick cloud or fog. He cannot remain in the air indefinitely ; he must come down and make contact with the ground somehow. Is he over a hill or a valley? Is the cloud, mist or fog down to ground level? He may have to fly on until he hits some- thing before he can find out. What wouldn't he give for a height sounder in such a predicament? And if it should happen to be night time as well, he would probably pay double! Two methods of height sounding have been developed. An American system has a transmitter on the aeroplane which emits radio waves whose frequency varies rhyth- mically. The waves are directed downward to the earth and reflected upward back to the aeroplane. The time taken by the waves to travel to the earth and back gives a measure of the height of the plane above the earth. As radio waves travel at the same speed as light, 186,000 miles per second, it might seem impossible to measure the The echo sounding system has beensuccessfully used in the Norwegian fisheries for locating large shoals.This record was instrumental in directing a fishing fleet to a goodcatch. time taken for them to travel such a short distance as, say, 50ft. But this is achieved by measuring the dif- ference of frequency between reflected and outgoing waves at the instant of reception of the reflected wave. The longer the wave has taken to be reflec- ted up from the earth, the greater is the frequency difference to be measured, since the frequency of the- emission is varying at a uniform rate all the time. In this way can height above ground be determined. But Great Britain, France and Germany have developed sonic height sounders or, as they may be called, echo altimeters. These operate like the marine echo sounder, emitting a short pulse or sound and measuring the time it takes to reach the earth and return as an echo. The British instrument, developed by the same firm which makes the marine echo sounders, was ready for trial a year before the war started. Unfortunately, the Air Ministry has not yet seen fit to install it in a plans for testing. However, the makers claim that it will record height above the ground once every second to an accuracy of 5ft. Height is recorded on a moving strip of paper in the same manner as with the marine echo sounder. A section of the ground beneath the aeroplane will therefore be recorded for the pilot to glance at whenever he likes ; and when coming in to land, even if " blind," the angle of glide and height above the aerodrome will be available for the pilot's information till the plane is actually on the ground. The instrument has a limitation. Sound travels through air at only about one-quarter of its speed through water, and air is a more difficult medium for it to traverse. The result is that an echo altimeter's vertical range is 500ft. over land and 1,000ft. over water. On the other hand, the echo will be positive and accurate to 5ft. The limited vertical range is not thought to be serious as it is near the ground that height determination becomes so vital; at higher altitudes the aneroid altimeter would still be in use. Limitations of Radio Altimeter For flying boats it will be of great value. For instance^ on the east-west Atlantic crossing they often wish to fly as low as possible to escape the strong westerlies high up. What a relief to be able to fly within 500 or 1,000ft. of the water, whether at night or in cloud or fog, knowing all the time the exact height above the surface! And, at the end of the flight, to know the instant of leaving the sea and passing over land, would be valuable not only from the point of view of being aware of the reduction of available height, but also as a check on navigation. The radio altimeter has the advantage of being able to determine heights up to 30,000ft. This, however, is not so important as accurate determination of heights from ground level to 1,000ft.; and the radio altimeter, or terrain clearance indicator as it is called, will probably have one serious defect. When over strata of certain substances having different conductivity and refractive indices it will record falsely as, for example, when over black soil over lying clay or a dry layer overlying a wet. Dry sand may not reflect radio waves at all, so they may be totally trans- mitted through it and reflected from a different stratum below. Furthermore, if there is a stratum of sand 30ft. deep, the altimeter recording will not only be 30ft. in error but five times that amount, owing to the slower speed of radio waves through sand as compared with through air. However, the research into air sounding is yet yoirig. and no doubt the perfect instrument will be produced a.s soon as information can be obtained from air tests and experiments.
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