punched-card controlled aircraft navigation computer

PROCEEDINGS OF THE I.R.E.

Punched-Card Controlled AircraftNavigation Computer*

E. H. FRITZEt, MEMBER, IRE

Summary-This paper presents the theory and a description of acourse-line navigation computer for use with the presently availablevhf omnirange navigation system or with the proposed completeomnirange-dme navigation system. A position fix may be obtainedwith the equipment by triangulation using line of bearing informa-tion from two omniranges. A further computation then producescourse and distance indications with respect to an arbitrary groundpoint as selected by the pilot. Alternatively, position-fix informationderived from an oange-dme ground station may be used asinput data for course-line computation. The punched-card reference-data system and pictorial course display instrumentation associatedwith the computer are described.

INTRODUCTIOND URING the last few years omnidirectional radio-

range stations operating in the vhf portion ofthe radio spectrum have been installed through-

out the United States by the Civil Aeronautics Admin-istration (CAA).1 An airborne navigation receiver tunedto a local omnirange provides continuous accurate in-formation regarding the bearing of the ground trans-mitter with respect to the aircraft. Navigation receiverinstallations have been made in military and commercialaircraft of various types, and many of the larger air-craft carry dual omnirange facilities involving the useof two navigation receivers. With -a dual installation,lines of position with respect to two separated groundtransmitter sites can be determined simultaneously anda plot of these two lines of position on a chart will deter-mine a position fix for the aircraft. The availability ofsuch bearing information suggests the use of an analogcomputer to obtain a continuous position fix for an air-craft. Using such a fix, a further computation will pro-duce course and distance indications which enable thepilot to fly the aircraft along a chosen course to an ar-bitrary destination. The equipment to be describedcomputes such arbitrary courses based on a two bearingomnirange fix in one mode of operation.The Radio Technical Commission for Aeronautics

(RTCA) with representatives from the airlines, govern-ment agencies, and industry, have outlined a plan forthe development and installation of additional aids toair navigation. Under this plan the CAA will ulti-mately install a distance-measuring equipment (dme)transponder at the site of each omnirange station. Air-craft which are equipped with airborne dme facilitieswill then have a continuous measure of distance as wellas bearing to a selected ground station. It is anticipated

* Decimal classification: R526.12. Original manuscript receivedby the Institute, February 4, 1952; revised manuscript receivedDecember 4, 1952.

t Collins Radio Company, Cedar Rapids, Iowa.1 H. C. Hurley, S. R. Anderson, and H. F. Keary, "The civil

aeronautics administration vhf omnirange," PROC. I.R.E., vol. 39,pp. 1506-1520; December, 1951.

that this rho-theta fix information will be used as inputdata for a course-line computer. The subject equipmentwill provide such computer service in a second mode ofoperation.The use of a course-line navigation computer re-

quires insertion of certain items of reference data. Themanual insertion of these data by the pilot usinghand-set knobs can become a burden in certain naviga-tion situations. This is particularly true in omnirangetriangulation operation. Circuits have been developedto insert such reference data automatically by meansof a compact punched-card system. The use of thissystem reduces the requirement for control manipula-tion by the pilot and the attendant probability of inser-tion of erroneous data.

MAGNETIC

Fig. 1-Omnirange triangulation.

TRIANGULATION SOLUTION FOR DISTANCE

The triangle involved in this computation is shownin Fig. 1. The angles 01 and 02 are the omnibearings oftwo selected omnirange ground stations referenced tolocal magnetic north. The station bearing 01 degreesfrom the aircraft is termed the "master station," andthat bearing 02 degrees is termed the "auxiliary station."The position fix is obtained by solution of this trianglefor the distance p from the aircraft to the master omni-range station. Because of the use of two omnibearinginputs (01 and 02) as primary information, the solutionfor distance is termed a theta-theta computation. Thecomputed distance and the omnibearing of the masterstation give a continuous polar co-ordinate fix for theaircraft.

Let us consider first the case where 01 and 02 are basedon the same north reference. The trigonometry of thesolution for distance p can be carried out by applicationof the law of sines in terms of the angles A and B and

734 June

Fritze: Punched-Card Controlled Aircraft Navigation Computer

Fig. 2-Functional circuit diagram of computer

the side D of the triangle formed by the aircraft and thetwo stations:

sin Bp = D-.- (1)

sin AD distance from master to auxiliary stationX = magnetic bearing of auxiliary station from master

station

A = 01- 02B = 02- X

In addition to the angles 01 and 02 which are enteredinto the computer automatically by two vhf navigationreceivers, the computation requires that information besupplied regarding the location of the auxiliary stationwith respect to the master station. These data remainfixed while the aircraft remains within the useful navi-gation area of a selected station pair.

It has proved more convenient in practice to intro-duce the station separation reference data in terms ofrectangular rather than polar co-ordinates. The east-west and north-south separations are denoted by x8and ys in Fig. 1, based on magnetic north. To use thisreference data, equation (1) may be expanded inrectangular co-ordinates:

p sin (01- 02) = D sin (02-X)-X1 COS 02 + yj sin 02 = ya sin 02- X COS 02

(y, - y,) sin02 - (X1-7 X.) COS 02 = 0, (2)where

x1 = -p sin0 01 in terms of conventional omnibearings=--p cos ofJ from aircraft to station.

Fig. 2 is a diagram showing primary circuits of thecomputer as used with input bearing data from twonavigation receivers. The equipment employs a voltage-distance analog and a computation scheme based onaddition of rectangular components of the vectors in-volved in the navigation problem. Distance magnitudesare represented by proportional 400-cycle voltages withsign determined by in phase or out of phase relation toa reference voltage. Electromechanical resolvers areemployed for the trigonometric operations required inthe computation.The master and auxiliary omnibearings are intro-

duced by means of two omnibearing indicator servo-mechanisms which connect electrically with conven-tional circuits of the navigation receivers. The distanceto station p is obtained by an. implicit solution of (2)when 0, and 02 have a common reference. A 400-cyclevoltage proportional to a trial distance p is applied tothe input windings of the two aircraft position re-solvers under control of a variable autotransformerdriven by the distance motor. Corresponding rectangu-lar co-ordinate voltages x1 and y, describing the positionof the aircraft with respect to the master station areobtained from the output windings of these resolvers.Voltages proportional to the station separation co-ordinates x8 and y8 are obtained from the punched-cardsystem, to be described later. Voltages representingxl-x, and yi-Y. are obtained by subtraction of theseparation co-ordinates from the aircraft position co-ordinates at the output of one of the aircraft positionresolvers. These proportional signals are applied to theinput of the auxiliary-bearing resolver, as shown inFig. 2, and resolution based on 02 produces at the out-

1953 735


put of this resolver the two signals shown as E3 and E4.E3 is termed the "primary-error voltage" and E4 the"correction-error voltage." Ignoring the correction errorfor the present, the primary-error voltage will be

E3 = (yi -Ye) sin62 - (X1 -Xs) COS 02. (3)If this error voltage is applied as input to the distance-servo amplifier, a solution for p obtains when the erroris driven to zero by servo adjustment of the variableautotransformer to satisfy (2). The distance servo mustbe phased in accordance with the sign of 61-62 as canbe shown by examination of the expression for error.For an error Ap

E3 = Ay1 sin62 - AX1 COS 02

E3 = Ap sin (01 -02). (4)

The sense of the error signal thus varies with thesign of sin (01-02). The phase sense of the servo isdetermined by the phasing amplifier shown in Fig. 2.The phasing amplifier receives an input signal cor-responding to sin (01-02) from the synchro link associ-ated with the two omnibearing shafts. The phasingamplifier (which is essentially a 400-cycle phase detec-tor) controls the phase sense of the distance servothrough a relay.

In the foregoing discussion it has been assumed thatbearings received from the two stations are based onthe same north reference. Actually, each station iszeroed to local magnetic north, and therefore there isusually a difference in the angular reference of any twolines of bearing. The relatively short distances involvedin the navigation triangle make possible a satisfactorysolution by plane trigonometry provided that a correc-tion is introduced for this north-reference difference.In practice, magnetic north at the master station isused as angular reference for the computation and thecorrection is applied to the auxiliary omnibearing. Therequired correction varies in range from zero to plusor minus several degrees depending on geographicallocation of the stations and their relative position.

2

Fig. 3-Auxiliary bearing correction circuit.

The correction is introduced in a rather simple man-

ner through use of the second output winding of theauxiliary bearing resolver. Fig. 3 shows the elements ofthis circuit. R1 and R2 form a low-impedance voltagedivider while R3 and R4 act as a high-impedance-summing network for the two resultant signals. Sup-

pressing the distance-voltage proportionality constants,

El xi - x

E2= y1- Y

E3= (y -y8) sin62 - (X1 -X8) COS 02

E4= K(yi -ys) COS 02 + K(xi - Xe) sin 62,

where

R2K

R1 + R2

These voltages may be summed to give

ET = (y' - ye)(sin 62 + K COS 62)-(Xl -XS)(COS 62- K sin 62)

since

sin 62 + K cos 02 = \/1 + K2 sin (62 + 4,)

and

cos 02- K sin 62 = V + K2 COS (62 + 4)

ET = (yl - y)VI1 + K2 sin (02 + 4)- (xi -xe)\l + K2 cos (02 + 4),

where , = tan-' K is the desired correction angle. Com-parison of (5) with (3) shows that the effect of addingthe correction signal is the same as would result froma mechanical rotation of the auxiliary omnibearingshaft by an angle 4,. The factor /I +K2, which affectserror loop gain, is approximately unity for the relativelysmall correction angles involved in the distance com-putation. A solution for p is obtained including thenorth-reference correction when the combined errorsignal of (5) is driven to zero by the computer. With theauxiliary bearing resolver zeroed with a 4-degree equiva-lent signal from the correction winding, corrections inthe range of plus or minus 4 degrees can be obtained bythe choice of a proper value for R2.

PUNCHED-CARD SYSTEM

A consideration of the number of items of referencedata which must be entered into the computer to makepossible the computation of distance p, using a givenomnirange pair, shows that there are five such items.These consist of two omnirange frequency selections,two co-ordinates defining relative station position, andthe north-reference correction angle. A computer whichrequires manual insertion of each of these data be-comes burdensome in use because of the number ofknob settings required. It is to be noted that the fiveitems of reference data are completely determined bythe selection of a given station pair. It would be desirableif the associated reference data could be introducedautomatically as part of the station-selection operation.A system has been developed which accomplishes this

using punched cards as data-storing means. Fig. 4 isa photograph of a typical punched card containing com-

736 June

(5)


P"T

LAI'APIEP

cCT

puter~~~f dat fo fiv stto pairs CG (CiaoHihs

*X:000ff 0:X :0O*ES00 00-* 0

*002U'S 0 0: *X * -

possible chic of .PN (Pntac,llnoi),BD

* * * 0 00

0** 0C 000-!1** 0..*'

0 0X::*. 0 0 ,0

00: 00 0 , 0'

Fig. 4-Typical punched card.

puter data for five station pairs. CGT (Chicago Heights,

Illinois) serves as master station for the entire card with

possible choice of PNT (Pontiac, Illinois), BDF

(Bradford, Illinois), LAF (Lafayette, Indiana), API(Naperville, Illinois), and MEP (Millersburg, Indiana)as auxiliary stations. Data for the two omnirange fre-quency selections and the three computer parameters are

punched in the card. Five pairs with no common stationcan be set up if desired.

Since the solution for distance depends on triangula-tion, there is a region within which no usable distancecomputation is possible adjacent to and including thebaseline connecting any station pair. Master and auxil-iary stations must be selected so that the projected lineof flight avoids this area. The use of punched-card datainsertion aids in rapid selection of suitable station pairsand associated reference data as a flight progresses.The reference information stored in the punched card

is read out for computer use by a punched-card reader.This unit is pictured in Fig. 5 mounted in the overheadpanel of a Beech D18S aircraft. Selection of a stationpair is accomplished by positioning the card in accord-ance with the station-identification letters printed at thetop. The punched cards for this service are made of adurable plastic material. The card reader operates bymeans of a bank of miniature switches which read outrelevant information from the card for each of thefive station settings provided. The switches are SPDTpossible various binary circuit combinations.The navigation receivers are remotely tuned from

the cockpit by a manually operated frequency-controlunit in conventional operation. The frequency-controlsystem permits the selection of any one of 280 channelsusing nine control wires in a re-entrant circuit. Thecontrol circuit includes nine SPDT switches which areclosed in various combinations to select a desired fre-quency. Tuning operations in the receiver are ac-complished by motor-driven autopositioners employing

Fig. 5-Card-reader installation.

1953 737


seeking switches which position tuning elements in ac-cordance with the cockpit control setting.2 As a resultof the nature of these circuits, they are readily adaptedto punched-card operation. Control wires are connectedto switches in the punched-card reader in place of thosein the frequency-control unit. Switching is provided inthe card reader to automatically return the receivers tomanual frequency control when the punched-card sys-tem is not in use.

FREQUENCY CONTROLWIRES TO RECEIVER

20 0

0

3 4 5 6 7 8 9 10 I 1120 0

0

400 CYCLE LINE

60Q0000oH

ARBITRARY-COURSE COMPUTATION

In either theta-theta or rho-theta operation of thecomputer, the analog-position co-ordinates of the air-craft with respect to the reference ground station serveas inputs to a second section of the equipment. This isan arbitrary-course computer which has the basic func-tion of translating station bearing and distance datainto information fixing the location of the aircraft withrespect to a course line through a destination chosen bythe pilot. The chosen destination is referred to as awaypoint. It may be either an intermediate check pointduring the flight or the final destination of the aircraft.This portion of the equipment is also concerned withthe display of output information to the pilot.

In theta-theta operation the position co-ordinatesx1 and yi are available at the output of the aircraftposition resolvers as a result of omnirange triangulation.In rho-theta operation a reconnection of the distanceservo amplifier (Fig. 2) sets up a voltage proportionalto dme distance at the tap of the variable autotrans-former associated with the aircraft position resolvers.This voltage is resolved into components xi and yi, asbefore, on basis of omnibearing of ground station.

MAGNETICNORTH

I-o

COORDINATE VOLTAGE TOCOMPUTING CIRCUIT

Fig. 6 Card-reading circuits.

In omnirange triangulation operation, ac voltages rep-resenting rectangular distance co-ordinates x8 and ya of(5) are obtained from accurately wound tapped trans-formers under punched-card control. Fig. 6 showstypical circuits for frequency selection and for obtainingco-ordinate voltages. Proportional voltages can be se-lected in 0.5-mile steps over a range from 0 to 127.5 milesby operation of eight card-actuated switches in thebinary addition arrangement shown. Sign is determinedby two additional switches in a reversing connection. Thenorth-reference correction of the auxiliary bearing isaccomplished by punched-card control of the resistanceR2 of Fig. 3. R2 is composed of a number of fixed re-sistors bridged by card-controlled switches. The desiredcorrection is set up by shorting out appropriate re-sistors in accordance with the correction punching.

Airborne dme equipments also employ a switch-controlled frequency selection circuit and the punched-card reader can be arranged to tune a dme and an omni-range receiver automatically to a selected omnirange-distance ground station in rho-theta operation. Positionco-ordinates required for arbitrary-course computationcan also be set in by punched card as will be shown.

2 H. M. Schweighofer and A. H. Wulfsberg, 'Multichannel re-mote control system," Electronics, vol. 25, pp. 202-210; February,1952.

Fig. 7-Arbitrary course computation.

Fig. 7 is a diagram showing the addition of rectangu-lar co-ordinates and resolution of vectors involved inthe arbitrary-course computation. This computation re-quires that the position co-ordinates of the desired way-point with respect to the reference ground station beset into the computer. These co-ordinates may be intro-duced either manually or automatically. In manualoperation, the pilot sets in the magnetic bearing (azi-muth) of the waypoint with respect to the station andthe distance separating waypoint and station by meansof a waypoint-selector instrument. These polar data arethen resolved into corresponding rectangular co-or-dinates X2 and Y2. The circuit involved is shown in Fig.2. A 400-cycle voltage is obtained from an accuratelytapped transformer in accordance with the distance

738 June


COMPUTER CHASSIS INSTALLED WAYPOINT SELECTOR COURSE INDICATOR DISTANCE NDCTORIN RADIO COMPARTMENT

Fig. 8-Pictorial block diagram.

co-ordinate setting. This voltage is resolved into com-ponents X2 and Y2 by the waypoint resolver based on thesetting of the azimuth co-ordinate. In automatic opera-tionl the co-ordinates X2 and Y2 are obtained directlyfrom the punched-card reader in accordance with pre-coded data. The circuit required is the same as thatshown in Fig. 6 for generation of x8 and ya.Manual waypoint selection has the advantage of ex-

tremne flexibility in choice of waypoint, but requires twoco-ordinate settings. Automatic waypoint selection hasthe advantage of requiring no co-ordinate settings, butdoes require a predetermined flight route. All frequencyselection and co-ordinate data for five waypoints canbe coded in a single card to provide for several hundredmiles of flight along a predetermined route. Manual andautomatic operation may be used interchangeably witha switching arrangement.The rectangular co-ordinate voltages describing the

position of waypoint with respect to the aircraft are ob-tained by addition of waypoint co-ordinate voltages tooutputs of the second aircraft position resolver.

Xr - XI + X2, YT = YI + Y2.

The pilot chooses his desired magnetic course to thewaypoint with the same course selector used to selectomnirange courses. The chosen course is specified as

Os in Fig. 7. The voltages describing XT and YT are re-

solved by the course resolver to a new set of axes as

determined by the selected course to obtain a measure

of thie distance to go from present position along thecourse to the waypoint and the lateral deviation of the

aircraft from the course. Distance to go is described asF and course deviation as G in Fig. 7.

F = XT sin 03 + YT COS 03

G = XT COS 03 - YT sin 03.(6)

(7)The distance indicated is from the aircraft to a planethrough the waypoint perpendicular to the selectedcourse line. When the aircraft is on course, the directdistance to the waypoint is obtained. The voltages pro-portional to distance and course deviation as obtainedfrom the output windings of the course resolver providesignals for the cockpit instrumentation circuits.

INSTRUMENTATION

Fig. 8 is a pictorial diagram of the computer andassociated instruments. Output indications of the coursecomputation are displayed in the cockpit by a courseindicator and a distance indicator.The course indicator is a multipurpose pictorial in-

strument which is intended for use in conventionalomnirange flying and during Instrument-Landing-System (ILS) approach in addition to its applicationwith the navigation computer.3 This indicator combinesinformation regarding aircraft heading and deviationfrom course in a manner which gives a pictorial displayof the navigation situation. In use with the navigationcomputer, a course to the waypoint is selected by meansof the course selector control of this instrument. The

3 W. G. Anderson and E. H. Fritze, 'Instrument approach sys-tem steering computer," PROC. I.R.E., vol. 41, pp. 219-228; Febru-ary, 1953.

195S3 739


course resolver of the computer is located in the courseindicator and the shaft of this resolver is positioned bythe course selector control to accomplish the co-or-dinate resolution of (6) and (7).The nature of the course display can best be ex-

plained by reference to Fig. 8. For the situation shown,the heading of the aircraft is 30-degrees magnetic asindicated by the angular position of the azimuth ringwith respect to the stationary lubber line at the top ofthe indicator and the symbolic aircraft in the center.A computer course of 60 degrees has been chosen bymeans of the course selector which rotates the positionindicating course bar within the azimuth ring. Once acourse has been selected, the course bar rotates withthe azimuth ring in response to heading changes. Thecourse bar is moved across the face of the indicator ina direction perpendicular to the course in accordancewith the lateral deviation of the aircraft from the se-lected course line. In the situation shown the selectedcourse line is ahead of the aircraft. If the present head-ing is maintained, the course bar will move downwardand to the right as the aircraft approaches the courseline. An appropriate change in heading at the propertime will bring the aircraft on course. The course indi-cator presents a picture which is equivalent to whatwould be seen visually ahead of the aircraft if the se-lected course line were marked on the terrain below.The directional (to-from) indicator pointing in the 60-degree direction above the aircraft symbol shows thedirection to the selected waypoint along the course line.

Circuits associated with the course indication areshown in block diagram form in Fig. 2. The 400-cyclevoltage obtained from the "G" output winding of thecourse resolver is phase detected to produce a cor-

responding dc signal of proper polarity and in proportionto the course deviation. This signal is applied to themeter movement associated with the course deviationbar. Voltage from "F" output winding controls a similarphase detector which provides a dc signal to the metermovement associated with the to-from indicator.The servo-driven distance indicator gives a position

indication against a scale graduated in miles. It includesa linear potentiometer supplied from the 400-cycle lineas a follow-up transducer. A voltage proportional toindicator-shaft position is obtained from the tap of thispotentiometer. The distance output is produced bvservo comparison of the course resolver distancevoltage "F" with this shaft position voltage. The indi-cation is unaffected by normal changes in supply voltagein this bridge arrangement.The distance indicator also includes flag-alarm meters

which indicate failure of certain signals from the naviga-tion receivers, dme, or computer. In omnirange tri-angulation operation a flag indication is produced atany time the aircraft enters a region in which the dif-ference in direction of the lines of bearing to the twostations in use is less than a preset angle. This flagindication is provided to prevent use of the triangulationfix in those regions along the station baseline and farout from a given station pair where satisfactory ac-curacy cannot be obtained. The signal for this purposeis obtained from the output of the phasing amplifierin accordance with the input signal, which is propor-tional to sin (01-02).

Fig. 9 is a photograph of the computer and instru-ments. The computing circuits are housed in a standard1/2 ATR aircraft case and involve a total of twelvevacuum tubes. The chassis is intended for installation

Fig. 9-Computer chassis and panel instruments.

740 June


in the aircraft radio compartment. Front-panel indi-cators on the computer chassis present input bearingreadings for checking and calibration purposes.

USE WITH OMNIBEARING-DISTANCE INPUT DATAAt the present time airborne and ground dme facili-

ties are available only on a limited basis; however, ex-perimental installations have been made of airbornedme equipments and developmental models of severaltypes of computers designed specifically for use withomnibearing-distance input data.4 The airborne dmeequipment includes a linear potentiometer as a dataoutput device. When the dme is tracking, the positionof the movable tap of this potentiometer is caused to beproportional to the distance from the aircraft to theground station. One means which has been employedfor introducing distance data into a course-line com-puter involves the use of a servomechanism. In this ar-rangement a voltage proportional to distance from the'dme output potentiometer is matched by a like voltageobtained from a servo-driven low-impedance trans-ducer in the computer. The transducer in the computerthen serves as a suitable driving source for the computerresolver circuits.

Fig. 10-Introduction of dme data.

Fig. 10 is a functional circuit diagram showing themanner in which the circuit elements which are associ-ated with the triangulation solution for distance (Fig. 2)are reconnected for use with omni-dme input data. Thephasing amplifier and the auxiliary-bearing servo sys-tem are not required for this mode of operation. Theomnibearing of the reference ground station is intro-duced by the master-bearing servo system exactly as

in triangulation. The distance servo amplifier drives thedistance motor and variable autotransformer as before.The output potentiometer of the dme is excited fromthe same 400-cycle supply source as the variable auto-transformer. The voltage at the tap of the auto-transformer is continuously matched to that from thetap of the dme output potentiometer by the distanceservo to provide an input system of the type mentioned

4 L. E. Setzer, 'The TDEC Course Line and Pictorial ComputerPrograms," Technical Development Report No. 138, Technical De-velopment and Evaluation Center, Civil Aeronautics Administration.

in the preceding paragraph. The motor-driven auto-transformer serves as a remote repeater and an im-pedance changer in this arrangement to supply the air-craft position resolver with an input distance voltagefrom a low-impedance source. The arbitrary course com-putation and instrumentation circuits of the computerremain as shown in Fig. 2. The use of the punched-cardsystem to tune the airborne radio equipment and intro-duce precoded waypoint coordinates is optional.

PERFORMANCE

The navigation computer has been installed in aBeech D18S aircraft for more than a year during whichtime it has been used in omnirange triangulation opera-tion. The equipment has been regularly utilized in nor-mal operations of this aircraft which involve flights tovarious sections of the country. Flight tests have beenconducted to gather data regarding the accuracy of thecomputer.The ground-position accuracy of the computed course

lines depends directly, of course, on the accuracy of theomnibearing data received. The subject of omnibearingaccuracy is a complicated one and exhaustive investiga-tions have been made by the CAA and others to evalu-ate the omnirange system.'"5 Results of these investiga-tions indicate that for ground stations where polariza-tion errors have been minimized and which are free ofserious reflecting objects, an accuracy of better than±2 degrees can generally be expected. The better sta-tions provide bearing information accurate to within1 degree, while some stations which suffer from greaterthan ordinary reflection effects exhibit errors in certainquadrants of more than 2 degrees. Navigation receivererrors (for airline type equipment) can be held toless than 1 degree. Spurious reflections from ground ob-jects and other propagation disturbances cause a cer-tain amount of low-frequency fluctuation "noise" to besuperimposed on the omnibearing data.The nature of the trigonometric solution for distance

in the computer is such that position errors cauised byinaccuracy in the omnibearing input data are accentu-ated as the difference in direction of the two lines ofbearing becomes small. Flight tests have shown thatsatisfactory computer courses can be obtained providedthat the aircraft is within an area where signals from twoomniranges can be received and where the lines of bear-ing to the two stations differ in direction by more thana minimum angle in the range of 15 to 20 degrees. Theflag circuit associated with the phasing amplifier is ad-justed so as to disable the computer and cause a flagalarm to appear in the distance indicator whenever thedirections of the lines of bearing in use differ by lessthan a preset angle in this range. The pilot is thus in-formed of the need for choice of a different auxiliarystation in such an area.

6 "Summary Report on Evaluation of the Omni-Bearing-Dis-tance System of Air Navigation," Report 540-1, Airborne Instru-ments Laboratory, Inc.

1953 741


TABLE I

CHECKPOINT MASTER AUXILIARY SELECTED SELECTED ACTUAL INDICATED DISTANCE OFF-COURSELOCATION STATION STATION COURSE WAYPOINT DISTANCE DISTANCE ERROR ERROR

South of MLI lOW 860 Rock River 78 miles 77 miles 1 mile South 1 mileMechanicsville, Iowa West of Dixon, Ill.North of Lowden MLI IOW 860 Rock River 63t 63 South 1

West of Dixon, Ill.North of DeWitt MLI lOW 860 Rock River 472 481 1 0

West of Dixon, Ill.Elvira MLI lOW 860 Rock River 371 39 1i North 1

West of Dixon, Ill.Clinton MLI IOW 860 Rock River 30 3011 North 3

West of Dixon, Ill.West of MLI BDF 860 Rock River 0 0 0 South 1Dixon, Illinois West of Dixon, Ill.South of PNT API 890 Chicago Midway 69 71 21 0Franklin Grove AirportSouth of Steward PNT API 890 Chicago Midway 571 58- 1 South 1

AirportMcGirr PNT API 890 Chicago Midway 47 48 1 South 12

AirportNorth Aurora PNT CGT 890 Chicago Midway 251 241 1 0

AirportChicago Midway PNT CGT 890 Chicago Midway 0 1 1 South 1IAirport Airport

In bench tests of the computer for aircraft positionsoutside the flag region, an accuracy exceeding 1 per centof full scale is generally obtained. The major portionof course and distance errors found in flight tests aretraceable to corresponding omnibearing errors. Since adegree of bearing error corresponds to a mile or more ofposition error at 60 miles from a ground station, positionerrors of several miles can result in this distance rangefor stations of below average accuracy. The omnirangeground stations are continually being improved and areduction in this maximum error spread is to be ex-pected. Working with a pair of good omnirange stationsand with omnirange receivers in proper calibration,course and distance errors average less than a mile overrepresentative courses. Table I shows the results of aflight test over a 200-mile course from Cedar Rapids,Iowa, to Chicago, Illinois.The computer has been used to indicate distance to

touchdown during ILS approaches using bearings froma single omnirange within the local area of the field. Inthis mode of operation the magnetic direction of therunway line is used as a fixed master-bearing input tothe computer and is set up by a synchro link actingthrough the master-bearing servomechanism. The run-way direction is the constant line of bearing of the air-craft from the touchdown point to a close degree duringa normal ILS approach. Auxiliary cress bearings arereceived from the local omnirange station located off

the runway line. Position co-ordinates of the omnirangestation from the touchdown point are set in by punchedcard. An expanded distance scale is used in the approachcomputation (15 miles full scale) to take advantage ofthe greater accuracy which can be achieved over thereduced ILS distance range. Table II lists results ofsuch a distance measurement made along the ILS ap-proach path at Des Moines, Iowa, using DSM as asource of cross bearings. Operation in this mode i5 de-pendent on the availability of a suitably placed omni-range station in the vicinity of the field.

TABLE II

ACTUALDISTANCE TO COMPUTER MILES

CHECKPOINT END OF DISTANCE ERRORRUNWAY

Railroad 7. 1 7.3 0.2Highway 5.8 5.9 0.1Outer marker 4.1 4.2 0.1Inner marker 0.5 0.6 0.1End of runway 0 0.2 0.2

ACKNOWLEDGMENTS

The writer wishes to acknowledge the assistance ofhis associates in the work described. In particular, thesuggestions of W. H. Wirkler and the efforts of C. S.Carney contributed to this computer development.

C~A5

742 June

punched-card controlled aircraft navigation computer

Documents