36 IRE TRANSACTIONS ON AERONAUTICAL AND NAVIGATIONAL ELECTRONICS March

with present test equipment." In the AN/APN-34 though the resolver is smaller, a new type of resolver(XA-4 and XA-5) equipments, space was not considered is used in the ARN-21; this resolver, used in conjunctionto be at the same premium as now considered. The dis- with a new type of circuit, has reduced the harmonictance indicator dial was larger, and the meter needle was content of the timing wave to less than 2 per cent. As adriven directly by the servo. The resolver produced only result, the ranging circuit in the ARN-21 produces an10 miles delay per revolution. The 8 kc sine wave used accuracy of + 100 ft up to 10 miles range, and + 300 ftas the timing reference was produced by a circuit which for distances greater than 10 miles.contained an oscillator coil which occupied 4 cubicinches of space, thus permitting a very good Q value Over-all TACAN DME Accuracyand a resultant large pure sine wave output. All these The situation as regards over-all TACAN DME sys-features contributed to the extremely good ranging ac- tem accuracy are summarized in Table II.curacy of these DME models.

In the ARN-21, however, the requirements were such TABLE IIthat certain of the above features had to be compromised. DME SYSTEM ACCURACYThese compromises are not inherent to the system butonly to the specifications regarding range, space, andtype of display, which had to be met by the present URN-3 Transponder Delay ±200 ft

models of equipment. For example, the Veeder-Root Pulse Rise Time, URN-3 Receiver +300 ftmodels of equipment. For example, the Veeder-Root Indicator Reading, ID-310 +600 ftcounter indicator, in comparison with a pointer and dial Thyratron and Delay Lines Drift ±300 ftindicator, has inevitable backlash; the last drum of the Airborne Ranging Circuits ± 100 ft close-inVeeder-Root counter in the ARN-21 is only half thelinear extent of the dial indicator of the APN-34; theresolver in the ARN-21 is smaller and must produce 20 The above listed individual errors will generally notrather than 10 miles of delay per revolution. add arithmetically. From the statistical point of view

However, in going to the ARN-21, a number of the root mean square error is significant; the RMS errorpartially compensating improvements are incorporated based on the individual errors listed above is 760 ft toin the fine ranging circuits. As one instance, the timing 820 ft, depending on the range. Experimental observa-reference sine wave (which has a 4 kc frequency rather tion has shown that the over-all system error has notthan the earlier 8 kc frequency used in the 100 mile exceeded +700 ft for strong signals, and +1,000 ft forsystem) is produced by a crystal controlled oscillator very weak signals. These observations substantiate therather than by a circuit depending on high Q alone for present nominally specified TACAN DME accuracy,stability. This improvement almost completely elimi- which is stated to be ± 600 ft plus 0.2 per cent of thenates the frequency (irift problem. Secondly, even distance measured.

Radio Beam Coupler System*H. HECHTt AND G. F. JUDEt

Summary-The radio beam coupler system has been recently de- INTRODUCTIONveloped as an en route navigation and approach aid for modem trans-port and bombing aircraft. It is applicable to both military and com- r¶[ HE ADVANTAGE of using automatic control tomercial operations. st guide aircraft along radio courses was recognized

The system comprises an electronic amplifier and cockpit switch- many years ago. In the late 1930's auitomaticing elements for coupling the automatic pilot to VOR and ILS radio landing work was initiated at the Sperry Gyroscopefacilities. Accuracy and efficiency of control is improved over previoussystems~~~~~~~~~~~~~~whluoai eunigadsnigsmlf h ua Company to improve the reliability of aircraft operationpilot's operating procedures. Design features such as magnIetic com- under adverse weather conditions. However, it was notponents and hermetic sealing enhance the reliability of the amlplifier until the development of high-performance electronicunit. automatic pilots during World W;ar II that means were

available for realizing many of the early concepts.* Manuscript received by the PGANE, June 14, 1955. Tlhis paper The tremendous expansion in recent years of the role

was presented to the National Conference on Aeronautical Electron- Of air transportation in our civil and military life hasiCS at Dayton, Ohio, on May 10, 1955.

I Sperry Gyroscope Co., Great Neck, N. Y. placed additional emphasis on all-weather aviation. To

1956 Hecht and Jude: Radio Beam Coupler System 37

provide the pilot of modern high speed transport aircraft justs power and monitors the crosspointer meter, altim-with a useful, reliable tool, adapted to the latest naviga- eter and marker beacons in anticipation of the transi-tioln aids, the radio beam coupler system has been de- tion to visual reference. Upon sighting the runway, theveloped. Its operational concepts are based on extensive automatic control is released and the aircraft is landedexperience in both commercial and military fleet opera- manually.tions, as well as on some 500 fully instrumented, actualpoor weather approaches at very low ceilings and visi- CONTROLLER _Ubilities.The operation, system design, ancl constructional

features of the radio beam coupler are described here. SELECTORThe performance of the equipment is demonstrated byrSWITCHflight recordings in a DC-7.

OPERATING IPROCEDURE

When approaching an airport, the radio beam couplersystem steers the aircraft to follow the ILAS course de-fined by the familiar localizer and glide slope facilities. q w

En route, the coupler also operates with the VOR navi-gation facility which defines a pattern of courses radiat-ing from a transmitter station. A series of these trans-mitters aligned at about 75-mile intervals establishes an

airway. In some cases omni-range stations are alsoplaced on airports as terminal approach guides. INDICATORThe automatic approach feature of the coupler con- Fig. 2-Operating controls.

trols the path of aircraft for the last 15 miles of theflight down to altitudes of 100 feet or less. This is a For en route navigation along an airway, the pilotcritical phase of flight. The pilot is dividing his attention selects the station frequency and desired radial frombetween handling power, gear and flaps, is receiving published charts and engages the radio beam coupler.traffic control instructions, and( is anxiously anticipatinig The system then steers the aircraft to intercept anda low altitude break-out. bracket the selected bearinig (see Fig. 3). When the

(a)1- ~~10 MILES APPROX z*

Fig. 3-Omni-range plan.

transmitter is reached, the erratic radio informationi isdisregarded, and the aircraft is automatically controlledto fly a constant heading. After passing the station the

,. F coupler automatically re-engages the beam signal andGLIDER SLOPEthe aircraft proceeds on the outbound leg of the radial.

(b) TRANSMITTER 1500 FT When he desires to use the next station along the air-way, the pilot returns the switch to Gyropilot, selects thenew station and reconnects the coupler.

Fig. I--Approach profile. The automatic switching features of the coupler havebeen the subject of much discussion with operating

With the coupler, the pilot heads the aircraft to inter- groups. In the scheme finally adopted the pilot is simplycept the localizer course and turns the coupler switch to required to head the aircraft in the general direction ofthe Loc-Range position [see Figs. 1(a) and 2]. While the the desired beam and to arm the equipment by position-aircraft proceeds to the course and automatically ing the switch. Automatic sensors then initiate the con-brackets it, the pilot may arm the glide slope sensor by trol functions when appropriate signal levels areadvancing the switch to the glide slope position. On in- reached. The pilot is thus relieved of the need for closelytercepting the glide slope, the aircraft is automatically monitorinig the crosspointer meter and switching at acontrolled in pitch to follow the incline. The pilot ad- precise instant.

38 IRE TRANSACTIONS ON AERONAUTICAL AND NAVIGATIONAL ELECTRONICS March

Another feature incorporated at the suggestion of is lower and the rate term is somewhat higher than inoperating groups is the command priority given to the the close-in condition. Other factors may also be variedpedestal controller. Beam control is discontinued and at this transition point; e.g., the bank limits which arethe conventional automatic pilot functions are restored approximately 30 degrees for the initial part are nar-whenever the pilot operates the pedestal controller turn rowed to 10 degrees for the final approach. Heading doesknob. This is of particular value for traffic avoidance or not appear in the control equation; the system can thusgo-around procedlures. Normal automatic pilot opera- generate crab angles required by crosswind conditionstion is thus instantly available at all times. without deviating from the course null.

The localizer control diagram, Fig. 4, shows howSYSTEM PRINCIPLES these terms are generated and combined. The follow-up

Io accomplish the foregoing functions the beam loop, consisting of a modulator amplifier and motor-coupler system incorporates two lateral modes, the generator, converts the crosspointer signal into shaftlocalizer and range guidance, and a vertical mode for position. A potentiometer geared to the motor shaftglide slope control. furnishes the beam displacement signal and the output

For the discussion of the localizer guidance problem of the generator furnishes the rate signal. Both signalsreference is made again to Fig. 1. The symbols y and I are fed to the limiter and output amplifier. A speeddenote the crosscourse and along-course coordinates of feedback term derived from the generator determines thethe aircraft with respect to the transmitter. The ratio filtering action of the follow-up.y/l defines the angular deviation of the aircraft from thecenter of the localizer beam. It is this angular deviation POSITION

which is displayed to the pilot on the crosspointermeter, and which is the input signal to the radio beamcoupler. Full scale deflection of the crosspointer meter TOis caused by 150 ,ua of signal corresponding to an aircraft I AUTOATIC

position 2.5 degrees away from the center of the beam. SIGNAL TUR

Lateral control of the aircraft is accomplished by Acommanding a bank angle through the turn control ofthe automatic pilot. The banked aircraft is accelerated SPEat right angles to its flight path. A control system com-manding bank angle proportional to the radio signal Fig. 4-Localizercontrol diagram.therefore tends to be unstable since it equates the basicquantity (displacement) to a function of its second de- Compared to the locaizer mode, range guidance in-rivative (acceleration). To provide the required damp- volves control of the aircraft over longer distances andinlg, a term proportional to crosscourse velocity is presents the additional problem of maintaining continu-rqigred ous control up to and through the zone immediately

Thequiredr over the transmitter. A signal proportional to relativeheading (y in Fig. 3) provides steering information over

{52 R D the transmitter as well as damping at regions away from( + ls +-JDy = 0 (1) the stations where the rate term is ineffective.

However, when aircraft heading is used, a correctionwhere D is the displacement sensitivity of the radio for crosswind must be supplied. This is accomplished bybeam coupler, R is the rate sensitivity, g is the gravita- adding to control equation a term proportional to inte-tional constant and s the differential operator. gral of course error. Prolonged beam deviations which

This is a second-order system and its damping ratio is might occur due to crosswind will thus be wiped out.

R The complete control equation for the range con-

-//-g figuration may be written as:2 VD

Fs I R\ D I1This indicates that less damping is obtained at long + ±+-)s +± +- Y = 0 (2)distances from the transmitter. If the constanits arechosen for optimum control near the runway, insufficient where H is the heading sensitivity, V is true airspeed,damping is provided for bracketing which occurs 12 to I is the integrator sensitivity, and the other terms used15 miles from the transmitter. A good compromise can are the same as in (1).be achieved by breaking the localizer control mode into Automatic bracketing of the range beams, particu-two phases, the transition being determined by the glide larly in high speed aircraft, may require relatively largeslope intercept. bank angles. The en route portion of the range flight, on

In the far-out condition the displacement sensitivity the other hand, requires only small bank angles since the

1956 Hecht and Jude: Radio Beam Coupler System 39

course is essentially straight. For this reason a mech- glide slope control system is directly proportional to theanism is provided for sensing the end of the bracketing course displacement gain and inversely proportional tophase. When this sensor operates the bank limit is re- the integrator gain. Since high integrator gain is neces-duced to 10 degrees and other parameter changes are sary to rapidly correct for static errors, it becomes de-made to ensure smooth flight along the beam center. sirable to maintain a high displacement gain as well.Another sensor circuit copes with the problems This could result in a short period instability at low

created by passage over the range station. Normally altitudes where the course is narrow. For this reason aerratic course signals are received near the transmitter lead network, in effect introducing a rate of glide pathand erratic course changes would be commanded unless error term, has been added to give a better margin ofsome precaution is taken. stability at the low altitudes.When these rapid beam changes are encountered, the To assist the pilot in smoothly engaging the glide

sensor slowly changes the bias on a variable gain stage slope control, a sensor is incorporated in the couplerthus cutting off the amplification of the beam terms. The which operates at the course center. Thus the pilot isaircraft in the meantime is guided by the heading and relieved of the necessity for constantly monitoring thebeam integral terms in a manner which very closely glide slope signal, and minimum pitch attitude changeduplicates the action of an experienced human pilot. is assured when the control is initiated.The method of accomplishing these functions is shown in A block diagram of the glide slope control system isthe range control diagram, Fig. 5. shown in Fig. 6. The radio signal is modulated and

amplified and then applied to both the lead network andthe integrator. The output of the lead network is furtheramplified and limited and is then combined with theintegrator output. Both signals are applied to the pitch

TO channel of the automatic pilot. Interlock circuitry makesSi |PILOT it possible to leave the integrator signal alone connected

CONTROL to the automatic pilot when the switch is returned fromthe approach to the localizer position. This feature ishelpful for go-around procedure or where conventionalautomatic pilot operation (without beam coupling) isdesired for the lower part of the glide slope.

FEEDBACK

E£RARONRllFig. 5-Range control diagram.

CROSSPOltIN |i

The geometry of the glide slope control problem may TO

be examined with the aid of Fig. 1(b). Here again is anangular radio course in which the h/l ratio is indicated COTROas a crosspointer signal; in this case 150 Aa is equal tonominally 0.5 degree of deviation. This crosspointersignal is fed into the radio beam coupler and through the Aautomatic pilot causes elevator deflections which main- Fig. 6-Glide slope control diagram.tain the aircraft on the course center.The control characteristic of the glide slope guidance DESIGN FEATURES

system is given by: In implementing any system for aircraft control, theIs D I\ paramount objectives are safety and reliability. The-+ -+-) A = 0. (3) circuit design of the radio beam coupler system con-

tributes to these objectives by using conservativelyIn (3), D denotes the displacement sensitivity in degrees rated, quality components, by hermetic sealing, and byof pitch per degree of course error, and I denotes the in- employing fail-safe circuitry.tegrator sensitivity in degrees of pitch per degree of The coupler, shown in Fig. 7, is housed in a JANerror-second. Al-B case which is approximately 5 X 8 X 13 in. The unitThe integral term in this equation eliminates steady weighs 16 pounds. All electrical connections are made

errors due to initial pitch misalignment, flap changes, through a connector at the rear of the chassis. Totaland speed changes, and thus significantly simplifies the power consumption is 95 watts, resulting in cool, troubleoperating procedure. It is seen that the damping of the free operation.

40 IRE TRANSACTIONS ON AERONAUTICAL AND NAVIGATIONAL ELECTRONICS March

Except for the power supplies, all tubes in the coupler The hermetically sealed construction, shown in Fig.are type 12AT7WA. The liberal use of feedback permits 9, is tamperproof and affords protection against sand,wide variations in tube parameters and special tube dust, and humidity. The tubes are accessible at the topselection is not required. of the chassis without unsealing the units. The indi-

vidual cans for the subchassis provide a logical sparestock item.

In summary, the circuitry has been designed to minii-mize and facilitate maintenanice. The elements arestable, conservatively rated, and sealed in a suitableenivironmenit.

Fig. 7-Radio beam coupler.

The magnetic input modulators have been developedspecifically for this application. They are a full-wave,saturable transformer design. Advantages of this type Fig. 9-Interior view of coupler.are the absence of spurious ac feedbacks to the signalsource and the fact that no rectifiers are required. In the PERFORMANCEradio beam coupler the magnetic modulators are partof the feedback loop, and thus nonlinearities and gain The radio beam coupler is presently in production andchanges due to environmental and other factors are of is being installed in several military and commercialonly secondary importance. The circuits associated with aircraft. Figs. 10, 11 and 12 show the performance ob-the localizer and range modulator are shown in simpli- tained in recent tests in a DC-7 with a Sperry Gyropilot.

Fig. 10 is a reproduction of a flight record obtainedTO in a DC-7 aircraft at Los Angeles Municipal Airport.

| ~~~~CIRCUITSPOSITION FEEDBACK MILES TO

RUNWAY-10 6.0 6.0 4.0 2.0 0i 11 ~~ ~ ~~~~~~TOm°6 _. ,LDE ,SL E>,En

+TU ,9 7t II Il LL1 1 L

MAGNETIC |j E | JLOCA,LIZMER SIG,NAL | | | | | BEA I NMODULATORIS l IWII ,BNElI1 PIll1,||~45 '!1T .1 EAM DISTURBANCF::

INPUT DC-7 AIRCRAFT143 KNOTS.* 1500 FEETLOS ANGELES AIRPORT

Fig. 10-Localizer performance.FILTER FEEDBACK

The record is actually a plot against time although anFig. 8-Lateral follow-up loop. approximate distance scale is shown. One division along

the plot is about fifteen seconds or 2 mile. The upperfied form in Fig. 8. By including the modulator in the trace is aircraft roll attitude (bank angle) at a sensi-motor follow-up loop, the accuracy of conversion from tivity of 9 degrees per division. The lower trace is thedc to ac has become largely a matter of potentiometer localizer course error signal from the radio receiver at atracking and linearity. These factors can be much more sensitivity of 45 jua per division. Full scale crosspointerreadily controlled than the characteristics of a low level meter deflection falls just inside the extremes of themodulator. record.

1956 Contributors 41

After the initial bracketing maneuver the aircraft trace sensitivity is 9 degrees per division and the cross-accurately followed the course and the glide slope con- pointer signal sensitivity is again 45 ,ua per division.trol engaged at about five miles from the runway. A full The initial bracket maneuver is deadbeat. The actionscale beam transient was intentionally introduced to of the over-the-station in disconnecting the controlcheck system stability at about 1,000 feet altitude above during the transition time over-the-station can bethe runway. The transient was removed after 25 seconds clearly observed.and within a minute the aircraft was again on course.

MLES TOSTATION-30 25 20 15 10

ALTITUDEIN FEET- 1200 1000 e80 600 400 t00 0

DC-? AIRCRAFT40EGE278 KNOYTS TRUE.ALT. 15000 FT. 4 EGE

DUDE SLOPE ENIGAGED DROP FULL SNAARRACLI.INTERCEPT ANGLEDC-? AIRRAFT. HALF FLAPS FLAPS1500 FEET- 13O KNOTSLOS ANGLS ARPORT

Fig. 11-Glide slope performance.

=_~~~~=MFig. 11 shows glide path error signal vs time which l0 s o 5 10 15

has been converted to distance. The time scale is 15 Fig. 12-Range performance.seconds per division and the glide slope error signaltrace sensitivity is again 45 ,ua per division. CONCLUSION

After the initial engage the aircraft stayed above the By providing the pilot with accurate, stable, flightcourse for about 45 seconds until the integrator wiped path control, the coupler system makes a significantout the initial attitude error. At a height of 400 feet step in the direction of reliable all-weather operation ofabove the runway, the landing flaps were dropped to aircraft. As air traffic density continues to increase, asfull down to check system response and stability. The new missions are undertaken and as higher performanceaircraft ballooned up about 30 feet above the course but aircraft are introduced, the economic and safety ad-wvas rapidly and accurately returned within 30 seconds. vantages obtainable by automatic path control willThe range record was obtained on the omni-range increase and provide the motivation for further de-

station at Santa Barbara, California with the aircraft velopments. New functions will be added, perhapsoperating at 15,000 feet altitude and 320 mph true among them automatic cruise control, automatic flare-airspeed. out, or control along a new long range navigation aid. ItThe approximate distance from the range transmitter is Sperry's endeavor to anticipate the operational needs

is shown at the top and bottom of the two records. The for such equipment and to develop and apply the latesttime scale is closely 10 seconds per division. The roll electronic techniques to fulfill them.

Contributors0. C. Boileau (S'49-A'51) was born in mitter Group. Upon graduation he con- Nathaniel Braverman (S'38-A'40-

Camden, N. J. on March 31, 1927. Upon tinued with RCA developing the B-47 wing SM'54) was born in New York, N. Y. on Julygraduation from high school in 1944 he en- tip antenna impedance matching unit for 4, 1915. He received the Bachelor of Electri-

tered the Navy where use with the AN/ARC-21. He returned to cal Engineering de-he served as an Elec- the University of Pennsylvania on a fel- gree in 1937 from thetronic Technician un- lowship and received his M.S.E.E. in June, School of Technologytil 1946. He was then 1953. At this time he joined the Antenna of the College of theemployed by the Ra- Development Group of Boeing Airplane City of New Yorkdio Corporation of Company where he worked on the devel- and the Master ofAmerica in television opment of hf antennas for the B-47, B-52, Science degree inproduction test until KC-135 and 707 aircraft. These hf antenna Electrical Engineer-1947when he entered studies have included the investigation of ing from the Schoolthe University of broad band impedance matching with of Engineering of Co-Pennsylvania. He re- lumped elements. In May, 1955, Mr. Boi- lumbia University inceived his B.S.E.E. in leau was assigned to the Bomber Weapons 1939.

0. C. BOILEAU, JR. 1951. During the Unit at Boeing where he heads the Thermal N. BRAVERMAN In 1939, Mr.course of his studies Radiation Devices Group. Braverman joined

he was employed by RCA in the Aviation Mr. Boileau is a member of Eta Kappa the Signal Corps (attached to the Air Force)Equipment Section, AN/ARC-21 Trans- Nu and Sigma Tau. at Wright Field as a civilian radio engineer.