PROCEEDINGS OF THE IRE

Direction Sensitive Doppler Device*H. P. KALMUSt, SENIOR MEMBER, IRE

Summary-A simple double-doppler device is described whichmakes it possible to determine the direction of motion in addition tomeasuring velocity. The same principle can be employed to measuredistances, temperature, or small frequency differences. In additionan application of the device for moving-target indication is described.

INTRODUCTIONTlXHE DOPPLER phenomenon has been used ex-

tensively for detecting moving targets and for de-termining the radial component of the relative

velocity between transmitter and target. Very often, itwould be advantageous to determine not only this ve-locity, but to know whether the target is "coming orgoing." This information is not furnished by a simpledoppler device because it cannot distinguish betweenpositive and negative doppler frequencies.

Theoretically, the direction of motion can be foundby determining exactly the frequency of the return sig-nal or by watching its amplitude. In practice, neithermethod can be applied. It is impossible to separate thereturn signal from the outgoing energy well enough tomeasure its frequency. Furthermore the amplitude canchange at random during motion because of a changingaspect angle so that it is very well possible that theamplitude diminishes at a given instant while the targetis approaching.A method without the above drawbacks will be de-

scribed which makes it possible to determine directionwith very simple means.

THE NEW METHODTwo signals at doppler frequency are produced. One

is the standard doppler signal. For the second one, anadditional phase-shift of 90 degrees is produced betweenthe local signal and the return voltage. This way, aphase-shift of 90 degrees exists between the two dopplersignals and it will be shown that, if this shift is positivefor increasing distance it is negative for diminishing dis-tance. Hence, a synchronous two-phase motor will turn,say, clockwise for an approaching target and counter-

A T BD - vt ---

-e- DI >

Fig. 1-Location of transmitter and target.

clockwise for a receding target. In Fig. 1, it is assumedthat a cw transmitter is located at A. A reflecting targetT starts moving at time zero from location B with a

* Original manuscript received by the IRE, January 26, 1955.f Diamond Ordnance Fuze Labs., Washington 25, D. C.

velocity V which is supposed to be positive for increas-ing, and negative for decreasing, distance.

E1 is the transmitted signal voltage and E2 is the re-ceived voltage.

E1= E sin wt2D\

E2 = KE sinw t--\ c/

whereby K is an attenuation factor, c, velocity of light.

D = D' + vtE KE ( 2D' + 2vt)

E2 = KE sin co tt- -\ c /

/ D'o v\E2= KE sin t - +'T 2 -t)

\ c c/

2D'w= a represents a fixed phase angle.

c

2(v/c)w =Wd represents the angular doppler frequency.The signals E1 and E2 are fed into a mixer so that a thirdsignal Em is produced with the amplitude of the re-ceived signal E2 and with a phase-angle which is the dif-ference between the angles of E1 and E2.

Em- KE cos (a ±W dt).

A second mixer is arranged in such a way that an addi-tional phase-shift of 7r/2 is produced between E1 and E2.

/ 7rElm = KE cos (a+-+ Wdt).

Neglecting the constant phase-angle a, we have the fol-lowing conditions. For increasing distance:

Em = KE cos (COdt)

Elm = KE cos (COdt + )

For decreasing distance:Em = KE cos (COdt)

Elm = KE cos (Xdt- -).2

A rotating magnetic or electrostatic field can, therefore,be produced whose direction of rotation depends on thedirection of the radial relative motion of the target.

Fig. 2 shows a block diagram of the first experimentalarrangement. An x-band klystron K is employed as thecw transmitter. The energy is radiated by horn AT, re-flected by the moving target, and received by horn AR.A small part of the transmitted signal is branched off

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Kalmus: Direction Sensitive Doppler Device

and fed to the two detectors D and D'. The delay line Lproduces a phase-shift of 7r/2 between the two local sig-nals. The return signal is split symmetrically and fed tothe detectors. The mixers produce, according to thetheory described, the two doppler signals E. and E'm.

K

E,, -

Ast

Fig. 2-Block diagram of the first experimental arrangement.

Instead of using mathematics, the operation of thedevice can also be explained by the use of rotatingphasors as shown in Fig. 3. E1 and E'1 represent the twolocal signals, 7r/2 radians out of phase. E2 and E'2 arethe return signals. They rotate with the angular dopplerfrequency and the mixer output is represented by theirprojection on the local phasors E1.

E2

liii>__Em

>Time

,1

loss duplexing scheme with a single antenna can bebuilt, using a gyrator.

Fig. 4 shows the block diagram and the position of theelectric field vectors in space. K is the klystron, produc-

WI Wz H

D D

I tSf~~~~~~~~I1E,+

Fig. 4-Single antenna device with gyrator and position ofelectric fields in space.

ing a horizontally polarized wave E1 which passes therectangular waveguide WI with the standard TEo, mode.Next, the wave transverses the round guide W2 with theTE11 mode. The gyrator F, consisting of a ferrite-rod,surrounded by the coil C, turns the polarization planeclockwise by 45 degrees, so that the energy is radiatedby horn H, shifted by a 45-degree angle with respect toW1. The two detectors D and D' are arranged to receivea very small part of E1 which serves as the local signal.

jEm

Time

Fig. 3-Phasor representation of the two doppler signals.

Increasing distance corresponds to clockwise rotationand diminishing distance to counterclockwise rotation.Hence, the mixer output voltage Em is the same for bothdirections. E'm, however, is 7r/2 radians advanced or de-layed with respect to Em, depending on the sense ofrotation of E'2. A device as shown in Fig. 2 was builtand performs according to theory.

SINGLE ANTENNA DEVICE WITH GYRATORThe first experimental setup had the disadvantage

that two separate antennas had to be employed. A low-

Fig. 5-Photograph of the single antenna device with gyrator.

The signal is reflected by the target and enters thehorn with its plane of polarization shifted by -135 de-grees with respect to W1. It is represented by vector E2.After passing the gyrator, the wave is again turned by45 degrees so that the plane of polarization is now verti-cal. The two detectors receive freely the reflected signaland the two doppler signals are produced. The detectorsare spaced by the distance S= (2n+ 1)X/8, so that, if thephase-shift between local and return signal is ¢ in D, itis 0+((r/2) in D'. It was shown before that this is thecondition for the production of a rotating field by thetwo doppler signals. Fig. 5 shows the arrangement.

1955 699

PROCEEDINGS OF THE IRE

SINGLE-ANTENNA, SINGLE-DETECTOR DEVICE

Duplexing schemes can be replaced by an arrange-ment in which the two doppler signals are prcoduced al-ternately in quick succession. This method is especiallyapplicable if vhf and uhf carriers are employed.

Use of a triode oscillator, the rf output of which is fedto the antenna through transmission line L, is shown inFig. 6. The reflected signal is mixed with the local signalby the diode action of the grid of the triode. The dopplersignal is fed into amplifier A. There is a delay networkN inserted in the transmission line which is switched inand out periodically at a rate much faster than the dop-pler frequency. A synchronously-driven switch feeds theamplifier output alternately to the integrators I, and I2so that, from their terminals, the two doppler signalscan be derived. The mechanical switch shown can, ofcourse, be replaced by an electronic device with an in-herently higher rate. In this case, it is advantageous toemploy saturable ferrite reactors in the delay line, sothat the delay time can be changed alternately by ap-plying a square wave to the reactors.

Fig. 6-Single antenna, single detector device.

APPLICATIONS

Distance Determination

The device can be used to measure the actual dis-tance from the antenna to the target if the original dis-tance is known. Hence, it can be employed as an altim-eter for an aircraft. 'The angular velocity of the syn-chronous motor is proportional to velocity, whereby, indistinction to conventional doppler devices, positive or

negative velocity is measured by clockwise or counter-clockwise rotation. The number of armature revolutionsis determined by a counter so that, by this integratingprocess, distance is measured. If an x-band generator isemployed, one revolution is obtained for traversing a

distance of 1.5 cm so that a high accuracy can beachieved.

If the doppler frequency is so high that the motor re-sponse is limited by mechanical inertia, electronic phasecomparison and counting devices can be applied.The new method is especially applicable for the alti-

tude control of a missile in level flight. The motor shaftoutput can be used directly to correct the altitude (tocontrol the hydraulic system).

Moving- Target, Indicator

Continuous revolution of the armature is producedonly by continuously moving targets. Hence, it becomespossible to discriminate against ground clatter. Thearmature inertia is actually put to good use in this ap-plication because it serves as a memory device for thedoppler frequency. The same end can be achieved elec-tronically only by many narrow-band circuits (vibrat-ing reeds) or equivalent complicated arrangements.

Temperature Measurement

When a sonic wave is transmitted between two fixedtransducers in a gas atmosphere, a change in the fre-quency of the received wave can be observed when thetemperature is changed. This "doppler" effect is due tothe change of the propagation velocity of sound.' If now,instead of conventional doppler methods, the describeddouble-channel system in combination with a synchro-nous motor is used, the temperature of the gas can bedetermined at any time if the initial temperature isknown. The counter on the motor, or the cycle counterin an electronic system, acts as an integrator for tem-perature changes in such a way that the correct temper-ature is indicated independent of whether precedingchanges were positive or negative. In this way, a fast,direct-acting thermometer can be designed, whichmakes it possible to measure temperature in a veryshort time. Work is now in progress to determine thetemperature of the gas in the explosion chamber of arecoilless rifle or in the interior of cylinders in internalcombustion motors.

Frequency Measurement

If the frequency of a wave has to be determined, it isnormal to compare the unknown frequency with aknown one by beat methods. This way, the differencefrequency can be measured, but it is not easy to findout whether this difference frequency is positive ornegative. By the use of two detectors, arranged in sucha way that the two beat-notes are 90 degrees out-of-phase, it becomes readily possible to determine the signof the difference frequency. Again, a two-phase motorcan be employed or an electronic equivalent thereof.

ACKNOWLEDGMENTThe help of H. Dropkin, who built the first model of

the doppler device and also designed the required wave-guide arrangement, is acknowledged.

1 H. P. Kalmus, A. L. Hedrich, and D. R. Pardue, Diamond Ord-nance Fuze Labs., Washington, D. C., Tech. Rep. No. TR-72.

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