FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1957
1957 - 0471.PDF
FLIGHT, 12 April 1957 473 accurate air data in the form of electrical or mechanical signalswhich can be supplied to the pilot's instruments, autopilot, navi- gation system and cameras or armament. The air data com-puters and similar equipment described below have this function. Major users of computed air data are the new navigation sys-tems. These are electro-mechanical analogue computers which integrate the recorded movement of an aircraft through the atmo-sphere with respect to time, relate it to information on wind direction and strength, and continuously display the presentground position and such other related information as may have been requested by the user. This information may include bearingand distance to a target; similar data for an alternate target or for the take-off point; ground speed; true airspeed; or windspeed.The position information is given to co-ordinates of a grid system or latitude and longitude, or in the form of bearing and distance—or in combinations of these factors. But such a computer is only as accurate as its sources of inputdata. Since true airspeed and compass or gyro heading sources are now considered reasonably accurate, and starting position canbe very accurately set in the computer before take-off, the most difficult input to provide is that concerning wind direction andstrength. Forecast alone cannot be sufficiently accurate, especially where large variations in flight altitude and long distances areinvolved. There are several makeshift methods. A series of regular surveillance radar plots of aircraft position can give a fairlyaccurate estimate of wind effect over a comparatively short flighi time. Radio aids, also, can be used to obtain a reasonably accurateposition fix from which to correct the computer's position estimate and calculate the effect of wind on the progress of the aircraft. The most accurate airborne equipment for assessing groundspeed (it is in order to convert true airspeed to ground speed that wind effects must be known) is the so-called Doppler navigationradar. Outputs from this type of equipment are intended to be used in almost all the navigational computers so far revealed,although in most cases security has restricted information on methods and accuracies of these combinations. The security pic-ture is still not at all clear, but the existence of a number of systems has been officially revealed. Major desiderata in equipment of this type are accuracy andreliability. At present, maximum errors in the region of 40 miles at the end of a transatlantic flight are being quoted. This is cer-tainly well below the requirements of any missile, but would be acceptable for bringing an aircraft into range of terminal naviga-tion aids for the landing phase. Because of security restrictions and lack of operational know-ledge, even about the equipments described below, it is impossible to estimate at present what status these computers will initiallyhave within the framework of airborne navigation aids. It is not known whether, in present military installations, they replacesome or all of the accepted aids. Though civil applications are very probably being prepared now for the big jet transports,security restricts information even on these installations. Symbolic of present navigation techniques is this navigator plotting position on an airways chart in a Britannia. The Ekco £.720 airborne weather radarscope can be seen on the left. Target and electron. multiplier Injected Output 1OO uW 9,192,631,830 c/s + FM 1OO c/s Adder 918OMc/s R.F Warning circuit \ (2,631,830 LMc/s -MOOc/s modulation Modulator Multiplier cr ,1OOMC L? MC/'' I. I synthesizer *J 5 Mc/scrystal cillator IOOC/I F.M. Modulation oscillator Magnet filter passes only Q atoms whose electron Q has flipped Electron flips if injection frequency is appro- 9.192.631,83O cps. Electron does not flip if injection frequency is incorrect Magnet filter passes only atoms whose nucleus ond valence electron have /i"j\ orientation. It prevents passage of atoms with reverse Cesium orientation chamber Automatic frequency control system The layout of the Atomichron frequency standard shows how the crystal-controlled oscillator is monitored by the cesium gas element. Such a precise frequency source will allow use of ground radio aids without interrogation transmissions from the aircraft. Atomichron Frequency Standard The National Company, Maiden, Massachusetts.A LL three American military Services have shown strong- interest in the recently revealed Atomichron, which is an atomic-resonance frequency standard accurate to one part in abillion and stable within five parts in ten billion. The accuracy attained is equivalent to that of a clock losing only three secondsin 100 years. The device is initially being applied to the distance- measuring section of the American navigation aid Navarho, wheredistance trom the ground station is found by comparing the phase of the signal received from the ground with the phase of the air-borne frequency reference. Without an Atomichron the ground and airborne frequencies would have to be synchronized by atriggering transmission from the aircraft. V.O.R., D.M.E., and other aids require such interrogation techniques. Present production versions cost about $50,000 each, but manu-facture in quantity would reduce this figure. A suitable airborne unit will weigh in the region of 601b. Conventional frequency standards must rely on die stability ofquartz crystals, but Atomichron uses a crystal oscillator continu- ously monitored and synchronized to die resonant frequency ofthe cesium atom. As the accompanying diagram shows, a given frequency generated by a normal crystal oscillator is controlledfrom an atomic beam tube (shown shaded) through a closed-loop servo network. The beam tube is maintained in vacuo, and a reservoir of cesiumchloride at die lower end is heated to produce a drift of cesium atoms up die tube. As the atoms drift away at approximately thespaed of sound they pass through a magnetic field which prevents the passage of atoms whose valence electrons have an alignmentopposite to that of the nucleus. Those of identical polarity are allowed to enter a chamber where they are subjected to die radio-frequency energy produced by the crystal oscillator, the output from which has been converted to conform with the resonancefrequency of the cesium atom, which is 9,192,631,830 c/s. If the R.F. energy input to this section of the tube is correct, the polarityof the valence electron relative to die nucleus is changed, or "flipped." If the R.F. energy is not correct the atom continueswithout change of polarity. Thereafter the atoms continue through another magnetic field which rejects diose whose polarity has notchanged. Finally the atoms strike a target, become ionized, and areattracted to the cathode of an electron multiplier which amplifies die cesium input current a million times.It can be seen that any variation in die R.F. energy input from the crystal oscillator to the central section of the tube causes areduction in the output signal from the electron multiplier and thus produces a plain unsensed error signal. The sense of dieerror which is shown to exist in the crystal oscillator is determined as follows. The R.F. signal injected into the centre of the tube isphase-modulated at 100 c/s. A motor-drive variable capacitor controls the master (5 Mc/s) crystal oscillator which generatesboth the R.F. signal injected into the tube and the Atomichron's output. One phase of this motor is energized by the electronmultiplier at the top of die tube and die other phase is excited by the 100 c/s modulation unit. The motor can thus ascertain whether the master oscillator
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
