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Aviation History
1968
1968 - 0233.PDF
U7 International. 15 February 1968 227 ven for a demonstration flight during which the test and P ording facilities aboard XS235 were explained in detail and H monstrated. Although the USAF has several flying labor- ' tones (Hercules, F-108 and two helicopters) for testing naviga- H n aids (and iaertial systems in particular), it has no aircraft comprehensively equipped as the Boscombe Down Comet, which is self-contained, not only as far as test and recording ear is concerned, but in the datums which are used to fix the position of the aircraft virtually anywhere in the world without the need for ground-based aids. The USAF relies largely on the highly accurate FPS-16 radar network, which is spread across he continent, as a reference for its own flight trials, but the irange of the radars is limited and they do not extend far out over the ocean where so much of Boscombe Down's testing akes place. On the other hand, the ground testing facilities at jicdloman for inertial systems and individual components is on a scale far larger than anything in the UK. While the Comet ivas there, the trials team, under the command of Wg Cdr £ W. Hare, met their opposite numbers for talks on inertial kystems testing and were shown over the ground facilities. Two rest days followed, during which our Holloman hosts once again demonstrated the generous American hospitality which everyone aboard the Comet enjoyed at each base where we called. On the Monday morning we took off from Holloman at 1100 GMT, which was 5 a.m. local time—a very much cooler environment than on the previous take-offs in the desert. It was just as well, for we were full of fuel for the long haul across the southern states to Jacksonville, Florida, where we cleared US customs, and then across the Atlantic to Bermuda, reached after 5hr 40min flying time from Holloman. After a night-stop in Bermuda and a further night-stop in the Azores, ne returned to Boscombe after some 14,000 miles' flying. he Testing System That outline of the eight-day journey by Comet sets the background for the kind of tests which the Navigation and adio Division is conducting all the time. Much of the equip- ent which has its performance evaluated in the Comet is of ilitary interest, but there is an increasing Mintech concern th civil applications, particularly when comparatively new ound is being broken, as with inertial equipment The difference between laboratory testing, under virtually ideal conditions, and testing in the real environment is ormous and the difficulties are similarly enlarged. If those difficulties can be overcome, as they usually are at Boscombe, then the results of "real world" evaluations are more valid an any other test so far devised. The key to success in the field—and it is a key which the ericans do not yet possess—consists of a combination of ighly accurate navigation datums and a flexible system of ecording. The datums—heading, speed, position and time— ieed to be virtually one order better than the equipment on tnal. This might sound impossible until it is remembered that tnere is no size or weight restriction on datum equipment, and •hat to some extent a high rate of data sampling can com- pensate for small inaccuracies. The high rate of sampling allows the trials team to achieve a better interpolation of the true position during further processing which takes place in 'tieground laboratory after each flight. fhe basic system of testing at the moment is to use as *»ums a twin-gyro platform, a Kollsman auto-astro-tracker, ~°r*\n °/A> Doppler and Decca. The measurements of heading, peed an(i position given by these datum equipments is read' 6d at varyin8 intervals based on half-second steps. The in. lngs from the equipment on trial is recorded at similar , v^s and all the information is stored on punched-paper ground- processing °n *e ground. The computer on the head 81VCS the triak team a teleprinter read-out of errors in under"? Spee(* an<* P°s>tion as recorded by the equipment had\^ to this the Navigation and Radio Division has w devise a "quick-look" system which enables the trials t0 ™ s.am'P1e the data being recorded during the flight itself, ind w" " m^es sense. They need to see that both the datum lot flv f eq"lpment is performing satisfactorily so that they do equJDm , or six hours and then find that part of the * ient was unserviceable all the time. Perhaps a component Fit Lt E. Fonstad, RCAF, using the input/output unit for the Elliott 920 computer to gain real-time experience in computing prodedures in anticipation of the Comet having its own digital computer this year The 920 computer itself, normally used with an Elliott £5 inertial platform. Mike Harden of Elliott is reading the output tape Sid Smith of the RAE with the Sperry SGNI0 inertial platform (in front of him). The control and display panel is on his left and the electronics and power supplies are in racks on the floor has a scaling error which has to be adjusted before valid results can be obtained. At the moment, this experimental "howgozit" is carried out manually, by differencing the outputs of the datum and trials equipment. The real heart of the system is the recording equipment itself. At the moment it is a 32-word digital recorder, known as Highway, which was built to the N&R Division's specifica- tion by GEC. The recording rate is controlled by the crystal clock—accurate to one part of 106—which produces a train of 0.1 sec pulses which are gated by the recording control unit (RCU) to give a recording rate of 0.5sec upwards. Some two years ago it was beginning to be found that this gating system was not flexible enough to cope with some of the equipment under test. An example is Loran, which changes reception from slave station A to station B very approximately every two seconds. If recordings were made of this sequence every isec, a very large proportion of recorded data would be misleading. The RCU circuitry had therefore to be modified so that in the case of a Loran input, recordings would be taken
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