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Aviation History
1948
1948 - 0464.PDF
392 FLIGHT APRIL 8TH, 1948 SPEED RECORDING New American System for the Precise Measurement of Speed by Radio UNTIL fairly recently, the measurement of aircraft speedsdid not involve much difficulty. Certainly, speed is arelative quantity, but the precision of surveying methods and the rapidity with which time-recording mechanisms can bsoperated has, up till now, been enough for flight speeds to be measured with an adequate degree of accuracy. Jet and rocketpropulsion have, however, lifted aircraft maxima, both of speed and altitude, to such an extent that the precise measurementof speed by orthodox means has become not only physically more difficult, but also subject to greater relative error. The U.S. Army Air Forces have evolved a method wherebyspeed can be measured at all altitudes up to about 60,000ft to within cumulative errors of less than 0.5 per cent. A com-prehensive description of the system appeared in an article by Charles S. Franklin, published in the February issue of ourU.S. contemporary. Electronics. Basically, the system com- prises two modified instrument-landing beams through whichthe aircraft flies, airborne radio equipment sending a signal at the instadt of passage through eachbeam, to a ground receiver incorporating an elec- tronic timing mechanism. The official title of thesystem is " all-altitude speed course." The ground equipment consists of two radiotransmitting sets, similar to instrument-landing system (I.L.S.) beam transmitters, which weiedesigned originally to serve as multi-frequency instrument-landing localizers, Each set is essen-tially a radio transmitter and a radio frequency radiating system, the desired frequency beingobtained by inserting an appropriate crystal unit and retuning the transmitter circuits. These twobeam transmitters provide the " sheets " of signals through which the aircraft flies on a course per-pendicular to them. A course monitor unit is-placed on the surveyedazimuth line at a distance of 400ft from the trans- mitter, and this method of directional control issufficiently accurate, to maintain the beam posi- tion to within ±8ft at a distance of twelve milesfrom the transmitter. It is, of course, equally important in an " all-altitude " system that, in addition to directional parallelism, accuracy shall be maintained in thevertical plane. The initial azimuth adjustment-is made by lining-up each of the beam transmitterswith their respective course monitors. While this xnethod alone results in a good approximation, itis not sufficiently accurate, nor is there any proof of exact course alignment in the region where itis to be employed. A satisfactory method of checking both azimuthalignment and vertical lean is by means of airborne flash-tube equipment and a zenith camera locatedat surveyed ground positions. The flash-tube can be triggered on and off, depending upon the position relatk'eto the course of the aircraft in which it is carried. The zenith camera provides an accurate method of recording the triggeredlight flash and computing deviation from surveyed ground positions. The flash-tube provides a high-intensity flash havinga duration of approximately 200 micro-seconds. \ The Zenith Camera A zenith camera, as its name implies, is one designed tophotograph objects directly overhead. It employs a 24m f/6 camera objective, and records the image on a 4 X5H1 plate. Thecamera can be levelled precisely to an accuracy of the order of one second of arc by means of a mercury-pool artificial horizonreflecting an image of a cross-wire in the levelling eye-piece formed by an optical collimating system. On each photographicplate an image of these cross-wires is impressed by the collimat- ing system to define the exact optical centre of the plate. Thisoptical centre is stationed directly over the Coast and Geodetic surveyed point, located exactly on the course and approxi-mately 12.5 miles from the beam transmitter. The accuracy of measurement of position of the airborne flash-tube at an alti-tude of 10,000ft is of the order of two feet. The aircraft carrying the flash-tube unit is equipped withan automatic coupling unit between the beam receiver and the automatic pilot, thus making it possible for the aircraft to be kept exactly on course or slowly to oscillate about this posi-tion. The test aircraft is flown at night, and the zenith camera shutter is kept open during each flight. Since the flash-tubeunit operates only when the aircraft is exceedingly close to thi; on-course position, it follows, that a centre-line drawn throughthe recorded dots on the photographic plate represents thi- actual position of the on-co'urse signal in space. Azimuth alignment is accomplished by determining the trui;position of the beam signal in the manner described, then shift- ing the course until its position coincides with the surveyedposition. Zenith-camera photographic plates are developed in the field, course deviation is quickly computed, and this infor-mation transmitted by radio to the beam transmitter, wh«£3& the course is shifted the desired direction and amount. '4§jpcourse monitor aerial, located 400ft from the beam transmitter,* is then moved to this established line-of-position and is hence-forth used as an electronic bench-mark to maintain azimuth START - CONTROL BOX TIMING STAT (AIRBORNE) *ERIAI I [MODULATOR TUNJNG U AKD TIME RECORDING SET RADIO SET (STATION RADIO TRANSMITTING SET (STATION 1) Layout diagram shewing relationship, of ground units and airborne equipment employed for the All-altitude Speed Course. alignment. Once the azimuth alignment is determined at theinitial altitnde of 5,000ft, the test aircraft makes passes at suc- cessive altitudes up to its ceiling. Zenith-camera records ofthese flights, plotted against altitude, determine course lean. Making a test run on the speed course is a comparativelysimple operation. The pilot first sets the channel selector on the control box to correspond with the frequency of the firstbeam to be crossed. After the test aircraft has reached the desired altitude, the pilot communicates with the ground oper-ator at station 2, announcing the first run. He then switches over from Communication to Speed Course; as the aircraftcrosses the first beam on a heading of 90 deg to that of the beams, the ground chronograph will be automatically triggeredand both the airborne and ground visual indicators will move from one side to the other, indicating that the first beam hasbeen crossed. The pilot then changes the channel selector to the second beam which, when crossed, automatically keys therecording chronograph a second time. Two passes are made in opposite directions in order to compensate for wind velocity.The elapsed time is read from the chronograph tape. Although several possible sources of error exist in the presentall-altitude speed course, the total errors add up to considerably less than 0.5 per cent. The heading of the aircraft over thecourse will not be exactly as desired, but it is reasonable to assume that its direction may be held with less than a 3 deg
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