Background and Principles of Tacan Data Link

1959 IRE TRANSACTIONS ON AERONAUTICAL AND NAVIGATIONAL ELECTRONICS

Background and Principles of TacanData Link*

B. ALEXANDERt, R. C. RENICKt, AND J. F. SULLIVANt

Summary-The integration of Tacan with very-high-frequencyomnidirectional range (VOR) to provide one comnmon air-navigationand air-traffic-control system requires, for full usefulness, a methodof automatic air-surface communication. Such a method, called theTacan data link, has been devised, and is being flight-tested. Usingthis data link, messages can be received and sent to each of 120 air-craft every 2.67 seconds. Such messages would consist of naviga-tional, aircraft-status, and traffic-control information. The data linkemploys the Tacan Surface Beacon to carry both analog and digitally-coded messages, interpolating the coded pulse bursts, which lastapproximately 3 msec, 45 times every second. No additional trans-mitters or receivers are needed.

INTRODUCTIONA major advance was made toward the solutionA of the air-traffic problem when the United States

government decided to integrate civil and mili-tary short-range avigational systems into a commonsystem. The Tacan Avigational System' was combinedwith the existing VOR and standardized as Vortac toprovide a common civil and military avigational aid.Either Tacan or Vortac will supply avigational informa-tion to enable a pilot to travel accurately to any desti-nation.The Vortac program is now being broadly imple-

mented by the Federal Airways Administration (FAA)and the Department of Defense. Plans have been com-pleted for 1200 Vortac stations. Many of these stationsare expected to be commissioned in 1959, using equip-ment originally developed by the military. Some 328specialized equipments are currently being developedto FAA specifications.The Tacan equipment has been brought to a high

state of readiness. The surface equipment has beenordered in large quantity. Over 28,000 airborne sets havebeen delivered and an additional 9,000 are on order.Fig. 1 shows the units of a Tacan AN/ARN-21 airborneinstallation. In parallel with this production program,development has been continued on Vortac equipmentfor the commercial operator. Fig. 2 shows the recentlydeveloped Vortac equipment, which is designed inmodular form. The larger Vortac distance-measuringequipment-Tacan (DMET) unit can be used withexisting VOR equipment. Over 180 of these DMET's

* Manuscript received by the PGANE, May 27, 1957; revisedmanuscript received, January 25, 1959. Paper originally published inElectrical Commun., vol. 34; September, 1957.

t Fed. Telecommun. Lab., ITT Corp., Nutley, N. J.I Electrical Commun., vol. 33, March, 1956, was devoted entirely

to the Tacan system.

Fig. 1-Airborne Tacan equipment.

Fig. 2-Airborne Vortac equipment.

have been ordered by the airlines with deliveries startingin 1959. Fig. 2 also shows the small Tacan bearingadapter that can be added as a replacement for thepresent VOR equipment.

9

10 IRE TRANSACTIONS ON AERONAUTICAL AND NAVIGATIONAL ELECTRONICS March

COMMON SYSTEMBoth the Tacan and Vortac avigational aids were de-

signed with the common-system philosophy in mind.Rather than a multiplicity of RF channels and a corre-sponding multiplicity of antennas, transmitters, andreceivers on each aircraft, the common-system philoso-phy has envisioned the use of a single band of RF chan-nels to provide all the necessary services for routineaviation operations.The research program for integrating the instrument

landing system with Tacan was undertaken before itsdevelopment was complete. Research on the localizerwas sponsored by the U. S. Air Force; an experimentalmodel was delivered in 1955. The glide-slope researchhas been conducted under sponsorship of the Inter-national Telephone and Telegraph Corporation. Thisprogram has resulted in equipment for a 1000-mclocalizer that can be used by any Tacan-equipped air-craft with no additional equipment whatsoever, and aglide slope that will require_a minimum of additionalairborne equipment. This equipment has been evalu-ated by the military. The feasibility of providing theseservices within the 1-mc Tacan channels was thoroughlyproven. The addition of Tacan instrument landing is thefirst step in applying the common-system point of view;the data link is the second.Although either Tacan or Vortac would permit a pilot

to fly safely to his destination were there only one air-craft in the air, neither system by itself is of much helpin avoiding collisions or maintaining orderly traffic inthe vicinity of crowded terminals. To accomplish theseends, air-traffic-control centers must have accurate in-formation on the identity, position, altitude, and direc-tion of travel of all aircraft in the controlled air space.Under sponsorship of the U. S. Navy, a method of

automatic air-surface communication has been de-veloped that makes use of the Tacan Vortac channelswithout deteriorating the avigational service providedby this system, or diminishing the value of the Tacanor Vortac sets already produced. This new communi-cation service, designated the Tacan data link, hasgone through both experimental and service-test engi-neering phases and is now undergoing evaluation by theNavy. Preliminary results of this evaluation indicate en-tirely satisfactory technical performance, and availa-bility wherever signal is present for the distance-meas-uring function.Tacan was initially developed for the avigation of

carrier-based aircraft. Its use as a general-purpose aidwas soon recognized, and incorporation in the commonsystem resulted. Similarly, Tacan data link was ini-tially developed for automatic two-way transmission ofcontrol data for Naval air operations. This developmentwas predicated on the full use of the Tacan system andmade fully compatible with Tacan. Thus, now thatTacan is incorporated in the common system, it is pos-sible to provide integrated data-link service, as origi-nally conceived in the common-system master plan.

Just as the distance-measuring portion of the Tacansystem will be of the most immediate use to aerial navi-gation, it is envisioned that the reporting feature of thedata link will be of immediate concern in air-traffic con-trol operations. On this basis, ITT has sponsored de-velopment of a light-weight transistorized airborne setwhich incorporates only the essential reporting featuresof the Tacan data link. This is expected to be flight-tested in 1959.

DATA LINK

An aircraft with a data-link unit added to its Tacanor Vortac set automatically reports position, altitude,heading and speed to the Tacan data-link terminal towhich it is tuned. Reports from the instruments in theaircraft are transmitted automatically at regular inter-vals and at a high rate. Consequently, the informationreceived by the controller on the position and status ofof the aircraft is up to date and accurate. By providingthese services to the controller, the data link frees thepilot of the responsibility for making position reports byvoice radio. It also frees the over-crowded radio-tele-phone channels from the heavy load of routine report-ing, leaving them free for emergency traffic.

In addition to furnishing automatic reports, the datalink may be used for transmitting clearances or com-mands from the air-traffic-control center to individu-ally addressed aircraft. These messages have the sameaccuracy and up-to-date character as the automaticposition reports. By using this feature, a controller canautomatically transmit and have displayed to the pilota full set of avigational orders, including ordered posi-tion, altitude, course and speed. Moreover, routinetraffic-control procedural messages such as LET DOWN1000, HOLD, etc., can be transmitted and displayed tothe pilot in his own language.

Thus, without the use of radio-telephone channels,without more crowding of the radio spectrum, and with-out the language problems encountered in internationalflight, substantially all the communications necessaryfor orderly traffic control can be accomplished.The services provided by the data link are illustrated

in Fig. 3. In less than 3 seconds on any of the 126Tacan channels, 120 aircraft can be serviced. To eachaircraft, individual orders of bearing, distance, heading,altitude, and speed can be transmitted, in addition to31 ready-made messages such as HOLD, PROCEED,etc. This information can be generated either auto-matically, by computers, or manually via suitable con-trol positions. At the same time, each of the 120 air-craft automatically encodes a full report including itsbearing and distance from the Vortac beacon, its head-ing, altitude and speed, and any of 31 predeterminedpilot-originated messages. These data are passed to

computers for automatic processing and to displays formonitoring, manual control, or both.Table I shows in detail the message content of the

data link as presently instrumented. Only minor equip-

Alexander, Renick, and Sullivan: Background and Principles of Tacan Data Link

120 AIRCRAFT

00 +5e>0 Q\\\\I/ / ossO

O*RIRG HI I - A°flEO °'SZ "\\\\1//I

IIAIC-ITS-HD"-^TTD -e1R &;P>9V-\V ///P S/

MESSAGSEA

Fig. 3-Tacan data-link services.

TABLE ITACAN DATA-LINK MESSAGE CONTENTS

Aircraft to Surface Surface to Aircraft

General GeneralIdentity-automatic by time Address-any of 1008 identities

separationMode-automatic feedback fea- Mode-such as traffic control,

ture automatic, approach, etc.31 Ready-made messages-pilot 31 Ready-made messages-

initiated, such as HOLDING, such as HOLD, PROCEED,LANDING, etc. WHEELS DOWN, etc.

5 Acknowledgments-pilot initi- 5 Commands-indication ofated reply to new orders from major change in controlsurface orders

Status Reports Control OrdersDistance-automatically-encoded Distance-on 20 or 200-nauti-

from Tacan distance equip- cal miles (37 or 370 km)ment using 20- or 200-nauti- scalescal-mile (37 or 370 km) scales

Bearing-automatically encoded Bearing-on 0-3600 scalefrom Tacan-bearing equip-ment, 0-360°

Altitude-automatically-encoded Altitude-on 5000 or 50,000from barometric or radio al- foot (1520 or 15,200 meter)timeter using 5000 or 50,000 scalesfoot (1520 or 15,200 meter)scales

Heading-automatically-decoded Heading-on 0-360° scale, from radio-magnetic indica-

tor, 0-360°Air speed-automatically-encod- Air speed-on 0-650-knot (0-

ed, 0-650 knots (0-1200 km 1200 km) scaleper hour)

significance of the data. Thus, bearing signals are pre-cise to within 0.5° and heading messages to within 2°.Comparable accuracies are employed for the other or-ders and reports.To guarantee thoroughly up-to-date information,

service rates have been established for data-link trans-missions that are sufficient to have negligible effect ontraffic-control dynamics. The data-link equipment lo-cated at each Vortac site is capable of handling 90messages per second (45 surface-to-air orders and 45air-to-surface automatic reports). Since each Vortac siteis capable of servicing 120 aircraft simultaneously, every2.67 seconds each aircraft can receive an individually-addressed order and transmit back its automatic report.It has been estimated that voice transmission of thesedata for the 120 aircraft would require one hour. Flexi-bility has been provided to permit some aircraft to beaddressed even more often. It should be noted that thiscapacity is far more than sufficient for the foreseeablefuture.

Considerable attention has been paid to the problemsof integrating the data link with an air-traffic-controlsystem. Fig. 4 illustrates how this might be done.

AT VORTACSITE

AT AIR TRAFFICCONTROL CENTER

t TO OTHERVORTAC SITES

Fig. 4-Data link or air-traffic control.

Only a limited amount of code and speed translationneed be added at each Vortac site. This is substantiallyless than the Vortac equipment presently located ateach site. Communication between the Vortac-site,data-link terminals and the computers and displays atair-traffic-control centers requires merely conventionaltelephone lines as illustrated in Fig. 4.

ment modifications would be required to change thedata transmitted. For example, if for some reason a

speed report was not required but a fuel-reserve mess-

sage was, the encoding pick-off device for generatingreports would have to be put in the fuel-gauge unitrather than in the air-speed instrument, as at present.The accuracy with which data are transmitted is com-

mensurate with the accuracy of the basic data them-selves. The discrete orders and discrete reports are

transmitted with protective coding features so thaterrors are practically impossible. The control orders andstatus reports have tapered accuracy depending on the

PRINCIPLES OF TACANFrom the earliest days of Tacan, the integration of a

compatible data link was envisioned. The selection ofthe Tacan frequencies, bandwidth, and modulation wasguided by the objective of ultimately adding data-linkservice. Therefore an explanation of Tacan must precedea description of the data link.Tacan is a polar-coordinate avigational system by

means of which each aircraft measures its bearing anddistance with respect to a surface beacon to which it istuned. Transmission to and from the beacon are onseparate channels, each 1 mc wide, in the band from

1959 11


(a) DISTANCE INDICATION

)TIME MEASUREMENT

INTERROGATORL_

I NTERROGATIONPULSES

GROUNDTRANSPONDER IREC(BEACON) _

GROUND-AIRCHANNEL

REPLY PULSES

RQTATIO1 OF AITEONAIf. OECIEES

_ 6K*67 SICROSECOND S-

Fig. 5-Typical Tacan pulses.

962-1213 mc, thereby providing 126 clear two-waychannels.The bearing portion is something like that of a con-

ventional VOR. The surface beacon transmits a direc-tive pattern that is rotated at a 15-rps rate. Each timea characteristic portion of this pattern passes throughnorth, an omnidirectional north signal is sent to all air-craft tuned to the frequency of the beacon. By measur-

ing the time between receipt of the north signal andreceipt of the characteristic north portion of the pattern,the relative bearing between the station and the aircraftcan be determined.The antenna revolves 15 times per second. Thus, to

an observer in the air, the antenna pattern gives theappearance of a fundamental component due to cardioidantenna pattern at 15 cps and a ninth-harmonic com-

ponent due to the cogwheel effect that is at 135 cps. Ineach antenna revolution, 1 north pulse and 8 referencepulses are generated. The time between successive ref-erence pulses is 1/135th second.The rotating directional pattern of the surface-beacon

antenna modulates constant-amplitude signals pre-

sented to it from the transmitter. The fine-grain struc-ture of the signal is composed of pulses transmitted atan average rate of 5400 per second. The exact positionsof the pulses are used to carry distance and data-linkinformation. Regarding bearing, however, these pulsesare sufficiently closely-spaced so that the envelope can

be extracted. Fig. 5 shows typical pulses and the ampli-tude modulation caused by the rotating antenna. It is

Fig. 6-Principles of distance measurement.

apparent that the pulses act as a carrier frequency forthe envelope.

Transmission is by pulse pairs with 12-j,sec spacing toreduce the effects of interference from single extraneouspulses. The pattern of pulses received due south of asurface beacon is shown. The 15 and 135-cycle modula-tions for bearing indication are evident as are the refer-ence bursts at 400 intervals. By limiting, the un-modulated central section is extracted for distancemeasurement. The additional bursts starting at 40°,1600, and 2800 are data-link transmissions.For distance measurement, each aircraft transmits

interrogating pulses as shown in Fig. 6. The surfacebeacon receives these pulses and transmits replies auto-matically over the ground-to-air channel. The airbornereceiver is tuned to this channel and by measuring thetime elapsed between its transmitted pulse and the re-ceived reply, it computes the radio travel time andhence the line-of-sight distance between itself and thesurface equipment.

PRINCIPLES OF DATA LINK

The precise location in time of roughly 50 per cent ofthe pulses transmitted from the surface beacon does notaffect bearing and distance measurement. Thus, a smallpercentage of the surface-beacon pulses may be ar-ranged into code groups to provide additional infor-mation.

Techniques for arranging pulses into code groupswere carefully chosen so that both air and surface Tacanequipment could be adapted to data-link service with-out significant modification. Furthermore, the amountof airborne equipment that must be added to a basicTacan or Vortac installation to provide full data-linkservice is surprisingly small.As shown in Fig. 5, the basic Tacan system contains

a synchronizing signal or reference burst that occurs ata 135-cycle rate. The data-link service utilizes this same


REFERENCE INFORMATIONINSTRUMENT PULSES PULSES

READING

VOLTAGE27G 9g TIME

PULSE POSITION MODULATION PRINCIPLE

Fig. 7-Ptulse-position modulation for transmissionof analog information.

synchronizing signal for the coding and decoding of itsinformation. The surface equipment interrupts thetransmission of its standard pulses 45 times a second andinserts a burst of pulses with a special configurationthat conveys the entire surface-to-air transmission to asingle aircraft. This transmission includes the identityof the aircraft, the discrete data transmitted to thataircraft, and all telemetered command information forthat aircraft. The duration of this pulse-burst trans-mission is approximately 3 milliseconds, a relativelyshort period of time compared with the standard Tacanwaveform patterns. Immediately following the receiptof a message, the particular aircraft addressed transmitsback to the surface a similar pulse-burst transmissionthat conveys its entire reply, consisting of both discretedata and telemetered report data concerning its status.

All data-link and Tacan transmissions employrounded pulse pairs that are tailored to the 1-mcTacan channels. Pulse pairs are employed because oftheir proved freedom from noise and pulse-type inter-ference. The basic coding techniques are matched to thetype of data being transmitted. Analog data, such asfor instrument-dial indicating, use analog codes suitedto pulse-position modulation, as shown in Fig. 7. Whena dial indication of 0° is to be sent, the pulse pair istransmitted at the beginning of its assigned time inter-val. As the dial indication increases in value, the pulsesare transmitted at a correspondingly later time. Due tothe basic simplicity of this code, no bulky analog-to-digital or digital-to-analog converters are required inthe aircraft. Digital data such as identities of aircraft,ready-made messages, and other on-off types of infor-mation, make use of digital codes.The complete message structure is shown in Fig. 8.

Each pulse pair is represented by a single vertical line.

DISCRETE DATA TELEMETERED DATA

NORTZ SPA.' - pA*ITY CHECX 2 5 -l,;tCH CECKREIEREbCE START 9 3 PNESEIECTE. 6 S'A', WEN

BURST 'VISES ADDRESS v.AN- rlESSAGES NODE C'".%E C SPEED ALTITUDE aE.0DI RE.AING DISTANCE

ANY T0.4TOF01AN ANY ANY A.Y A.1

4OUT OF I CONSIl CO.GHI- ONE CO_* CONNII- ANLD9TELE.EE ..U.tAS O TRASSITTEO SYPULSES AT.OM AT1ON PULSE AT10. AT.O

N 10BN US NECHT- NEA

I'll2 1:1*1 276 104 IA .74 J.'l 2019 69 174 1.1 60 993 46 1 9 4..1 30 1 46 1 310 .641 3T0

W ~~~~~~~~~~~~~~~2X11M-CROStCONDS

(a)DISCRETE DATA

^ IrITELEMETERED DATA~~~~~~~~~~~~~~~~~~~~~~I

5 2

STJT ^ 5 PREStLEC1E0 S-^E.SIS OCDE AC..O.EDG9.E.TS WE$5-IS C.A.ES SPEED ALT.T.DE HE.D"- -...GS DISTANCE

ALL DISClttTE ..D TELE.ETE. i.FR^10..TO T.AS.ITTED Y1 O.E .01ABLE PUILSE IN EACH TIME INTE-A^L

1F1~11 11 FlF=RFFF+tl 119F1=1 F Ffl FF11'J 1.6* |41 *D 1.6*1 93 -1 93 1|4 931.D6 9 s 1-1 93 -61 31T 4 01-631 6 *01

>-- --- ~~~~~~~~~~~~2272.MICOSECO.DS b

(b)Fig. 8-Code structure. (Rounded figures indicate time intervals.)

(a) Ground-to-air message. (b) Air-to-ground message.

The basic Tacan north-reference bursts, which are usedfor synchronizing both bearing information and data-link transmission, are followed by various combinationsof 35 pulses. These pulses convey the identity of theaircraft, ready-made messages, designation of mode ofoperation and various procedural information. Follow-ing the transmission of these discrete orders, five posi-tion-modulated pulses are transmitted to convey thetelemetered control orders to the particular aircraft.The entire pulse train in surface-to-air transmission is

modulated by the rotating-antenna array. There is nodeterioration whatsoever of bearing information.

Since the total duration of all data-link surface-to-airtransmissions occupies less than 10 per cent of the totaltransmission time of the surface beacon, even when thesystem is used at maximum capacity, there is insignifi-cant deterioration of distance service with the additionof data link. When a data link is added to an existingTacan or Vortac installation, unequipped aircraft cancontinue to receive Tacan or Vortac avigational infor-mation and equipped aircraft can receive full avigationalservice plus data-link service, all simultaneously.

DATA-LINK AIRCRAFT INSTALLATIONS

Any aircraft equipped for Tacan distance and bearingor Vortac distance measurement can be provided withdata-link service by the addition of a data unit. Thisunit, consisting entirely of pulse and video circuits, pro-vides simultaneous data-link service to the existingTacan or Vortac avigational aids on the same RF chan-nels, and without additional transmitters or receivers.Fig. 9 shows a military type of airborne installationconsisting of Tacan navigational equipment and anAN/ARN-26 Tacan data-link coding and decodingequipment. Fig. 10 shows a Vortac aircraft installationand a smaller data unit that adds data link for air-traffic-control purposes to a Vortac installation. In bothcases, it should be noted that avigational information

1959 13


a

Fig. 9-Equipment for airborne use. Tacan navigational unitson the left and data-link units on the right.

(

laFig. 10-Simplified airborne apparatus with Vortac equipment

on the left and data-link units on the right.

is unimpaired, and that little additional instrumenta-tion is added to the overcrowded instrument panel ofthe aircraft.An idea of the equipment provided in the aircraft can

be obtained from Fig. 11, which shows the militaryAN/ARN-26 set. This unit adds complete two-waydata-link service to a Tacan distance and bearing instal-lation. It uses vacuum tubes and is housed in an ATRrack. It weighs 50 pounds (22.7 kilograms) and occupiesa volume of 1 cubic foot (0.03 cubic meter).The flow of signals through the airborne equipment is

outlined in Fig. 12. All signals arriving at the antennaare amplified in the receiver and passed through a pulse-pair decoder. The decoder filters out interference, andits outputs starts the master time base and the addressdecoder. The master time base synchronizes an oscillatorwith the reference-pulse burst, thereby generating atiming reference for the encoding and decoding func-tions of the equipment. The address decoder scans theoutput of the pulse-pair decoder unit to find a group ofpulses that corresponds to the address configuration setinto the airborne equipment. On receipt of such a codegroup, the address decoder permits the immediately-fol-lowing series of pulses to pass through the various mes-sage decoders. These message decoders actuate the in-

Fig. 11-Military AN/ARN-26 set with dust cover removed.

Fig. 12-Functional block diagran, of data-link system.

formation display. Immediately after the informationdisplay receives signals from the decoder, it is per-mitted to energize the message encoders, which trans-late the various measured quantities in the aircraft intodata-link signals. These signals pass to the modulatorwhere they are mixed with the normal distance interro-gations of Tacan. This combined wave train then actu-ates the Tacan transmitter, generating RF signals thatenergize the antenna.One feature of the data link is the two-way transmis-

sion of ready-made procedural messages, which make upa large percentage of the present air-to-ground voicecommunication. The selector shown in Fig. 13 displayssome of the more common of these routine messages.These are grouped into categories from which individualmessages are then selected by the pilot for transmissionto the ground by pressing the corresponding button.This arrangement is similar to the selector on a jukebox, in which tunes are listed in categories, such asclassical, jazz, and hillbilly, and an individual piece isselected by pressing the corresponding button.The Tacan data link is a communication system; as

such, it can handle any information that is supplied toit in suitable form. One particular instrumentation is

1.

mommobwmm

* t *


Fig. 13-Selector for ready-made messages.

Fig. 14-Aircraft instruments-slightly larger than standardunits-modified for data-link service.

shown in Figure 14. The primary emphasis has been toprovide displays of all information, both commands andreports, without the necessity of adding instruments tothe already overcrowded control panel. Accordingly,distance, bearing, radio-magnetic indication, air speed,and altitude instruments were modified to indicate or-dered values and to encode the corresponding informa-tion for transmission to the surface. These hermetically-sealed replacement instruments are but slightly largerthan standard instruments. The only additional instru-ment is the message and mode-of-operation unit thatdisplays any one of 31 ready-made messages being sentfrom the surface and any one of 6 modes of operation,which are types of ground control. Each instrument alsocontains a color indicator in the lower right-hand corner;red indicates off or malfunction; green is for proper

operation; and yellow calls attention to a new command.By pressing the color indicator, an acknowledgment ofthis new command is sent back to the surface.Tacan data links used primarily for air-traffic con-

trol could be less complex than the full two-way mili-tary system. The number of maneuvers that must beperformed by these aircraft and the precision with whichthey have to be accomplished are much lower thansome of the military tactical requirements. Hence, theamount of surface-to-air control information to betransmitted is greatly reduced. In fact, the major use

of the data link in the air-traffic-control picture may befor automatic air-to-ground reporting of the status of theaircraft. As has been discussed, a major service of datalink is that of automatic aircraft reporting. It relievesthe pilot of having to read his instruments and relaythe readings over voice radio; it relieves the surfaceoperator of properly translating these voice reports intosymbolic form for displays and machine processing.Development work is under way on vastly simplifiedversions of the data link which weighs approximately 15pounds (6.8 kilograms), and occupies a volume of ap-proximately 500 cubic inches (8200 cubic centimeters).It uses transistors and is designed for modern high-speed aircraft.

SURFACE EQUIPMENTThe military version of the data-link surface equip-

ment, designated the AN/URN-6, is kept in twotrailers. One trailer houses a standard Tacan surfaceinstallation and the data-link storage, coding, and con-trol units. This equipment is shown in Fig. 15. Thesecond trailer contains control and display equipmentdesigned for the operational evaluation of the system bythe Navy. The interior of this trailer can be seen in Fig.16. At each of the control positions, the behavior of a

reporting aircraft can be monitored and manual ordersfor precise maneuvers can be issued.The operation of the equipment can be understood

with reference to Fig. 17. Either the manual controlconsole or associated computers are identified with indi-vidual aircraft via the assignment panel, which can beseen in the far right of Fig. 15. Through drum-accesscircuits, messages intended for individual aircraft arestored in a common-language digital code in the mag-netic drum of the storage unit. Along with the messagedetails, a requested service rate is stored. Line-finderequipment, very similar to units used in automatic tele-phone exchanges, provides a connection between ordersstored on the magnetic drum and the coding unit. Acrystal oscillator with associated count-down serviceconstitutes the master clock that controls the precisetiming of the various phases of operation. Signals fromthe clock are also used for the precise control of antennarotation. When used with the data link, field modifica-tions must be made in the antenna drive and speed-control equipment of the standard AN/URN-3 Tacanset. Having received an order from storage for a singleaircraft, the coding unit under control of the clockgenerates a surface-to-air message, as described previ-ously. This message, coded as voltage pulses, is mixedwith the normal Tacan video signal and passed to thetransmitter.The airborne reports, which immediately follow the

surface-to-air message are received from the Tacan re-ceiver, subjected to filtering by pulse-pair detection, andpassed to the coding unit. Under the control of the clock,this message is put into the common-language code andstored on the proper magnetic-drum track under the

1959 15


Fig. 15-Standard Tacan beacon in the two cabinets at the leftand digital storage, coding, and control units for the data link.

influence of the line finder. As soon as the reply messagehas been properly stored, the line finder moves on tohandle the next surface-to-air message. Since the reportsare stored in the common-language code, they are avail-able almost continuously to the users, which may bemanual control consoles, other displays, or computers.

EVALUATION TESTING

The military equipment previously indicated hasbeen exhaustively flight-tested, and used in conjunctionwith testing other experimental gear by the Navy atPatuxent River, Md., for the past two years.

Fig. 16-Five control consoles and the assignmentpanel housed in trailer.

Fig. 17-Surface Tacan data link AN/URN-6functional block diagram.

The Gyrovibrator*N. D. DIAMANTIDESt

Summary-A new device for measuring angular rate is analyzedand its mechanical and electrical design features presented. Ana-lytical proofs of three important characteristics of the instrument aregiven. These characteristics are: 1) a sinusoidally modulated-carriervoltage with the envelope representing the magnitude of the usefuloutput signal; 2) zero drift as well as zero offset effects separatedfrom the true output in the form of a dc quantity, and 3) a referencesignal through which both the magnitude and the orientation of theinput angular-velocity component in the plane of the spin are deter-mined.

* Manuscript received by the PGANE, April 9, 1958; revisedmanuscript received, December 29, 1958, U. S. Patent Appl. No.489,578; February 21, 1955. The present study represents work doneindependently of the author's duties at Goodyear Aircraft Corp.

t Goodyear Aircraft Corp., Akron, Ohio.

I NTRODUCTION

TEpHE gyrovibrator is a small angular-rate gyroscopeT which has a new basic design in its rotating ele-

ment and its pickoff member. The name of thedevice denotes the fact that a gyrating element developsvibrations in quadrature to its spin axis, the amplitudeand phase-angle of which vibrations measure the magni-tude and direction of the angular speed of the instru-ment's support. Only the theory of the device is pre-sented here, along with the expected performance, sincethe developmental work is not yet at the stage whereexperimental results can be given.

Documents

Background and Principles of Tacan Data Link