1. Introduction

A Microwave HologramRadar SystemRICHARD W. LARSON, Member, ILEEEJERRY S. ZELENKA, Member, IEEEELMER L. JOHANSEN, Member, IEEEInstitute of Science and TechnologyThe University of MichiganAnn Arbor, Mich. 48107

Abstract

An airborne microwave hologram radar system has been developed

which is a two-dimensional analog to optical holography. The field

of view of the radar is directly below and to either side of the

aircraft. Resolution is realized in the along-track direct,ion by

utilizing the synthetic aperture technique, and in the cross-track

direction by means of a phased receiving array. The theory of

operation is summarized, the demonstration system is described,

and results for both the normal and contouring modes of operation

are presented.

Manuscript received July 26, 197 1; revised September 30, 197 1.

This work was supported by the Air Force Avionics Laboratory,Wright-Patterson AFB, Ohio, under Contract F33615-67-C-1814.

During the past twenty years considerable advances havebeen made in radar imaging systems. This work hasincluded the application of optical processing techniques tosynthetic aperture side-looking radar to obtain fine resolu-tion maps of the ground [1 ] -[3] and moving targets [4]. Ithas been shown that the azimuth channel of syntheticaperture radar systems is an analog to optical holography[2], [3], [5] -[7] . With synthetic aperture radars fine rangeresolution is obtained by the use of short pulsewidths orpulse compression techniques.

Recently an airborne microwave hologram radar systemhas been developed which is an analog to optical hologra-phy in two dimensions. The field of view of the radar isdirectly below and to either side of the aircraft (Fig. 1); thisfield of view is similar to that of airborne infrared andphotographiC systems. Resolution is realized in the along-track direction by utilizing the synthetic aperture tech-nique, and in the cross-track direction by means of a phasedarray. The hologram radar system operates in a CW modeand radiation from the transmitting antenna in the aircraftilluminates the ground area. The backscattered energy iscoherently detected and recorded on film. Signal processingis then accomplished using two-dimensional optical tech-niques similar to those developed for side-looking syntheticaperture systems. Synthetic aperture compression is per-formed in the along-track direction and antenna beamforming is accomplished simultaneously in the across-trackdirection. Photographic film serves to record the image atthe output plane of the optical processor.A unique feature of the microwave hologram radar is the

range contcuring capability. When the frequency of thetransmitter is switched rapidly between two values fi andf2, the received signals will interfere either constructively ordestructively at slant range increments separated by thevelocity of light divided by twice the frequency separationof f1 andf2.

This paper summarizes the theoretical and experimentalhologram radar program conducted at The University ofMichigan, which started with theoretical studies in 1964.An experimental hologram radar system was built, andground tests were performed in 1967 and 1968 [4].Fourteen imaging flights in a C-131 aircraft were made in1968 and 1969.

I. Basic Theory

Fig. 2 displays the geometry of the hologram radarsystem for a point target. The point target is located at x, yin the ground plane (x-y plane). Above this plane at aheight h there is a receiving array which is parallel to theyaxis. This array is of length 2a and it moves in the xdirection with velocity V. At the center of the array,(xo, O. h), there is a source of coherent radiation illuminat-ing the ground plane. To simplify this initial discussion weshall imagine that there are an infinite number of receivers

IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS VOL. AES-8, NO. 2 MARCH 1972

K +~~>____Velocity

Transmitting 60

Atitenna Xh

SyntheticAperture

Side View

Fig. 1. Hologram radar geometry.

Here Xr is the radar wavelength. It may be shown that j¾can be written as the sum of two terms. The first term (/2represents the phase history recorded by a perfect holo-gram, and the second AO can be considered as an error termthat causes aberrations in the reconstructed image; thisterm arises from the relative motion between the object andthe illuminating source. Let

1 /2

Front View Ro [(xo x(2+ ) ] (7)

then

4= r

Fig. 2. Hologram radar geometryfor point target.

, SOURCE OF RADIATION

2a

z=hTARGET

(8)

To record the signal history on film we must introduce abias term S so that signal values can be recorded. It is alsonecessary to scale down the dimensions of the diffractionpattern in space so that the pattern will fit on the film. Inthe x dimension the scaling factor Px is the ratio of filmspeed v to array speed V:

P.x= V/V . (9)In the y direction the scaling factor Py is the ratio of filmwidth w to array width 2a:

p _ wY -2aclosely spaced along the array so that the return from the

point target can be continuously recorded along the lengthof the array.

Suppose the transmitted signal St is in a sine wave

(10)

By substituting x/Px for x and y/Py for y in Ro, the phaseterm 02 recorded on film becomes

St = exp jwt; (1) 02 =- 47 [(Xthen the received signal SR at a point on the array "Xrx L(xo, yo, h) has the form

SR = K exp jw(t - r) (2)

at arbitrary time t where t is equal to x/V. Here K is aconstant which accounts for transmitter power, antennagain, range to the target, etc., and r is the time required forthe transmitted signal to reach the target and return to apoint on the array. In our analysis we shall assume that thearray velocity V is much less than the velocity of light sothat the array moves only a small fraction of a wavelengthduring Xr

Rt +Rr(3)

with

Rt = [(xo- x)2 + y2 + h2 1'/2

Rr = [(Xo X)2 + (yo _y)2 + h2 112 (5

After synchronous detection, the phase of the returnbecomes

1 =---?T7(Rt + Rr) -)r

±+( 2P~ -PXYO ) Px2h2] 1/2

(11)

It has been shown that if a hologram is constructed at alonger wavelength than the reconstruction wavelength, thehologram should be scaled down approximately by thewavelength ratio before the image is viewed [8] -[101.Without such scaling the lateral and longitudinal magnifica-tions of the image are unequal and aberrations areintroduced. Reference [11] discusses these problems ingreater detail. The motion of the hologram radar arrayintroduces an additional factor of 2 which complicates thereconstruction. A more detailed analysis shows that AOcontains a quadratic phase error in the y dimension. Thisphase error is a function of target location; however, it canbe eliminated by setting

Px _ p _Nl\X W=

(12)

(6) where X1 is the wavelength of light used in the reconstruc-tion. If this relationship is satisfied, the along-track or x

LARSON ETAL.: MICROWAVE HOLOGRAM RADAR

(5)

209

Timen

tiologram Output Image

I--------aY yI

H

FIR, - NRIR R

Fig. 3. Range-contouring geometry.

dimension of the image will be larger than the cross-track ory dimension by the factor v/ If we choose

PX - 2PY =- Xr (13)

the output image will have unity aspect ratio; however, thelength of the array (2a) cannot be too large or the quadraticphase error in AOb will be excessive [11]. The quadraticphase error can be compensated whenever

h >4a 1-cos3 3 /2)

; Rl - -%R

,- \\Z S V

H'OR

where fx is the antenna beamwidth in this direction. In thecross-track direction the array length determines the resolu-tionpy,

AY=Xr,] h sec3 oy °Py ~-: hsc3a (16)

Here A is a constant near unity, which depends on theaperture weighting, Oy is the angle measured from thevertical, and h is the height.

(14)

where I30 denotes the cross-track beamwidth.When (13) and the distance criteria are satisfied, t2 is a

good approximation to the signal available for recording on

film. A linear phase term is introduced before recording so

that the desired output image can be separated from theconjugate image and the undiffracted light in the processor.When collimated, monochromatic light is incident on thehologram, a demagnified image similar to the scene illumi-nated by the hologram radar will appear.

With the hologram radar it is impractical to utilize theideal demagnification ratio from radar to reconstructionspace (2Xi/Ir) because this ratio is too small, ranging inpractice from 10-5 to 10-3. Current recorders cannot packinformation on film densely enough to permit the idealscaling, and even if the ideal scaling could be realized,practical limitations would restrict the length of the arrayso that the size of the scaled hologram would be too smallfor convenient viewing. A lens system is therefore neededto reconstruct the image and magnify it to a reasonablesize.

In the along-track direction standard synthetic aperturetheory applies, and for nearly vertical viewing the theoreti-cally achievable resolution Px is

Px =

4 sin (ox/2) (

MII. Range Contouring

In optical holography, two-frequency light can be usedto generate holograms which contain contour lines [121.The same principle applies to the hologram radar. When thefrequency of the transmitted radiation is switched rapidlybetween two values f1 and f2, it is clear that the receivedsignals will interfere either constructively or destructivelywhen received from range increments given by

C

- 2(f2 -fl)(17)

where c is the velocity of light. Fig. 3 shows the contouringgeometry. Assume that signals received along the path R,at the frequencies f1 and f2 interfere; similarly, signalsreceived along path R1 + AR, at transverse angle 02, alsowill interfere. Lines of minimum intensity will appear in theoutput image corresponding to the transverse angle associ-ated with range R 1 and R 1 + AR.

It is clear that the indications obtained showing terrainrelief are not "map contour lines," but slant range contourlines referenced to the aircraft. Since the aircraft geometryis known, knowledge of the aircraft altitude and groundelevation references will permit the surface contour to beobtained from the range contour lines on the microwavehologram imagery.

IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS MARCH 1972

.5

2

21()

Fig. 4. View looking aft toward the forward radome showing the108-element antenna and one row of receivers.

IV. A Description of the Demonstration System

The basic requirements for the construction of themicrowave hologram are that 1) the terrain be illuminatedby a microwave signal, 2) the backscattered wavefront mustbe adequately sampled in two dimensions, 3) the phaseinformation of the return must be recovered by coherentdetection, and 4) the phase information of the return mustbe recorded on a film or some other recording medium. TheUniversity of Michigan has constructed a demonstrationsystem meeting these requirements. This radar operates atlow altitudes with a relatively short receiving array.

In the theoretical discussion, the transmitting antennawas located at the center of the receiving array; in thedemonstration system, however, the transmitting horn wasmoved 10 meters behind the receiving array to isolate thetransmitter from the receiving elements. A receiving array of100 elements served to sample the received field across theaircraft. The elements were equally spaced; the spacing ofslightly over half a wavelength was sufficient to preventgrating lobes from appearing on the output image. Fig. 4shows the receiving array; it consists of 108 H-plane hornsmounted side by side. The outer four elements on each endwere not utilized because their patterns differed fromthose of the other elements.

Fig. 1 shows the antenna beam coverage. In thealong-track direction the beam is narrow, but across track abroad 900 beam is used for wide angular coverage. Thetransmitting antenna was also an H-plane horn with thesame beamwidth.An operating frequency of 16.8 GHz was selected

because suitable components could not be obtained at areasonable cost at a higher frequency. A high operatingfrequency is, of course, desirable for good cross-trackresolution. The space available in the radome of the C-131test aircraft determined the length of the array.

Fig. 5 is a simplified block diagram of the completesystem. Table I lists the operating parameters. A cavitystabilized klystron generates the basic operating frequencyof 16.8 GHz. Power output from the klystron is divided intwo with each output line driving the input of a single-

sideband generator. Offset frequencies used to drive thesingle-sideband (SSB) generators are obtained by multiply-ing the output of a 5-MHz crystal oscillator by factors of 12and 13. The difference between the output frequencies ofthe two SSB generators is equal to the intermediatefrequency of the receiver amplifier. Power output from oneSSB generator is amplified in a TWT and radiated as thetransmitted signal; power output from the other SSBgenerator is also amplified in a TWT which drives the localoscillator distribution system.

The radar return is received by 100 individual receivingelements simultaneously. Each return is converted down toan IF of 15 kHz, amplified, demodulated, and then sent toa multiplexer. The required synchronous demodulationfrequency is obtained from the basic 5-MHz crystaloscillator through a countdown circuit dividing by 300. Themultiplexer output is recorded on film. The 100 channelsare recorded across the film as the multiplexer switchesfrom one channel to the next and the Doppler signalhistories are recorded along the film as it passes through therecorder. Fig. 6 is an enlarged section of hologram signalfilm.

Alignment of the system consisted of adjusting allreceivers to indicate signals of equal phase when theequivalent of a plane wave was incident on the array.Amplitude weighting of the receiving array was accom-plished as a part of the same operation, by adjusting thegains of the receivers. Practical difficulties were encounter-ed when using a far-field source to provide an incidentplane wave with long-term stability across the array. Toavoid this difficulty, a small source that could be movedacross the face of the array was used instead of a planewave. Results of a number of phase measurements indicatethat the phases can be aligned within 10 degrees peak-to-peak error for the 100 receivers. Once aligned, phasetolerances of 20 degrees peak-to-peak over a period ofseveral months were realized. Considerable design effortand temperature cycling testing were necessary in order toarrive at the final phase-stable circuitry.

When the radar was being designed, provision was madefor pulsed operation in the event of inadequate isolation

LARSON ETAL.: MICROWAVF HOLOGRAM RADAR 211

TABLE I

Hologram Radar Parameters

TransmitterRadiated frequencyPolarizationTransmitter power (maximum)Frequency difference in contouring mode

Receiving ArrayTotal array elementsActive array elementsAcross-track spacing of array elementsAcross-track array lengthAlong-track width (both array and elements)

ReceiversIntermediate frequencyBandwidth (receivers)Receiver system noise figure (average)Receiver sampling rate (multiplexer)Average array sampling ratePeak-to-peak phase variation (maximum)Amplitude taper across array

Recording FilmWidth of recorded stripAverage film speed (at 150 knots)Scaling factors:

P (across-track)PZ(along-track)

ImageGround swath width (across-track)

Across-track resolutionAlong-track resolutionRange-contour intervals

16.8 GHz (A = 1.78 cm)horizontal400 mW5 MHz

1081000.56A1.01 meters0.15 meters

15 kHz1800 Hz17 dB500 kHz3800 Hz± 200Taylor, for 20-dB

sidelobes

5 mm40 mm/s

6.8 X 10-35.2 X 10-4

Twice aircraft altitudeabove terrain

0.018 radians0.076 meters30 meters (100 feet)

Fig. 5. Simplified block diagram of the experimental system. Fig. 6. Section of microwave hologramsignal film. Receiver channel numbers arein the vertical direction, and time is in thehorizontal direction. Actual film width is 5millimeters.

100Receivers

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between the transmitter and receiver. Fortunately, the10-meter separation of the transmitter horn gave sufficientisolation and many of the maps were made in the CWmode.

So that range contours can be generated, provisions havebeen made to permit operation in the two-frequency mode.This is accomplished by switching the SSB generators, asshown in Fig. 5. Alternate pulses consist of two transmittedfrequencies differing by 5 MHz. The contours that resultdenote slant range intervals of 30 meters.

V. Processing

As mentioned in Section II, recorder limitations do notpermit the microwave wavefront to be scaled down by theideal wavelength ratio, resulting in phase errors across theaperture. An additional complication is usually introducedby choosing along-track and cross-track scale factors whichdiffer greatly in order to record data on film efficiently. Asa result the holographic image does not come to a commonfocus in the along-track and cross-track directions. Relative-ly simple optical systems will serve to compensate forpractical scaling factors and to adjust the reconstructedimage to a suitable size. Standard spatial filtering tech-niques in the processor permit the removal of both thedc bias and the unwanted sideband. There are severaloptical configurations capable of generating an image froma microwave hologram, but we shall only discuss theprocessor in Fig. 7, which was the processor used mostfrequently in the program.

In Fig. 7, the microwave hologram is placed in plane Hwhere it is illuminated with a collimated monochromaticsource. Following the hologram plane are two telescopes intandem. The first is a spherical telescope composed oflenses L1 and L2, which have respective focal lengths f1and f2. The second is a cylindrical telescope with lenses L3and L4.

The spherical telescope transports desired wavefrontsemanating from the hologram to the plane H'. Theunwanted wavefronts (i.e., the undiffracted wave resultingfrom the dc bias together with one of the first-order wavesresulting from having the signal on a spatial carrier) areeasily removed by placing the appropriate slit in thefrequency plane F. Of course, any spacial filtering of thesignal can be readily accomplished by placing the properoptical element, usually a transparency or lens, in plane F.

The cylindrical telescope is used to compensate for theusual mismatch in scaling factors. This telescope does notaffect any wavefront appreciably in a direction parallel tothe axes of its lenses, say the along-track dimension. In theother dimension, that is, cross-track, it operates on thevirtual wavefront formed by the spherical telescope at planeH' to form a scaled-down wavefront at the same location.For the cylindrical telescope to form a rescaled wavefrontat H', it is required that

X2 f4 3 +f4/f3 (18)1 -(f4/f ) (8

L2 L3 L4eJV f3+ -4 H

A--,F- - -~ -

si p os 1- f fl t- f2 f2 -4 X2

F ig. 7. Decoupled telescopic processor.

I

The complex wavefront at H' is identical to one of thefirst-order waves emanating from H, except that it has beenrescaled or demagnified by an amount

(19)Ma =f2/fi

in the along-track direction and

MC = f2/fl)f4/f3) (20)in the cross-track direction.We can view the reconstruction of the target field

directly from the H' plane. As described below, this imagecan be three-dimensional in nature providing two condi-tions are met. First, the aperture associated with thedemagnified wavefront at H' must be large enough toaccept both the viewer's eyes. Second, the reconstructionhas to form a reasonable distance behind the hologram, sayone-half to ten feet. When these processors are operated inthe mapping mode, it is usually convenient to have anadditional spherical lens L5 in the vicinity of H'. This lenscan be easily used for adjusting the final image to somespecified size.

The processors are tracking processors when operating inthe mapping mode. That is, all targets formed in the outputplane move in synchronism with the input data. Trackingprocessors have two distinct advantages over other types,both advantages arising from the fact that the output filmcan be exposed via an adjacent slit several resolutionelements wide. First, such a wide slit reduces the exposuretime required to make the map, since for nontrackingprocessors, this slit can be no wider than a resolution cell.Second, the peak output noise resulting from dust particles,lens imperfections, etc., is greatly reduced by averaging.

The processor depicted in Fig. 7 is suitable for modify-ing scaling factors from 4 to 25 times. Another processormust be used for scale factors higher than 25 [11 ] .

Compensating for scaling factors less than 4 can beaccomplished with the processor simply by placing acylindrical shift lens in the frequency plane F. When theaxis of this lens is at right angles to those of the cylindricaltelescope, the along-track focus position HI is shiftedfurther down the system axis. Thus, more space becomesavailable for the use of a cylindrical lens L4 with asufficiently long focal length.

With the experimental hologram radar the intensity ofthe imagery varied considerably with slant range andantenna gain, and at the edges of the swath the attenuationwas 10 dB higher than at the center. Since this attenuationis a function of the cross-track position of the targets, wecan easily compensate for it in the frequency plane of the

LARSON ETAL.: MICROWAVE HOLOGRAM RADAR

t l l z

v

H L, IIA 4

213

(A)

(B)

Fig. 8. (A) Photograph of Willow Run Airport, Ypsilanti, Mich., and factory complex(300-meter altitude). (B) Microwave hologram image of factory complex (CW operation; 4-mWaverage power; 300-meter altitude).

optical processor. The reflectivity of the terrain is depend-ent upon the depression angle from the radar, andvariations in image intensity may also result from thisfactor.

Although hologram radars have the capability of generat-ing three-dimensional imagery, the experimental system wasnot well suited for this purpose. Since it had only 100cross-track receiving elements and an along-track beam of60, an ideal microwave hologram generated from an altitudeof 300 meters would have dimensions of only 0.036 by 2.2mm. Even if such a hologram were enlarged ten times, itpresents a very small aperture for direct viewing with botheyes. Some experiments on the generation of three-dimensional imagery were seen with enlarged holograms;however, only those people having considerable experiencewith stereo viewers were able to see the three-dimensionalimage associated with tall objects. Although the generationof three-dimensional images has been experimentally veri-fied, the resulting images are not of any practical use.Hologram apertures an order of magnitude larger in bothdimensions would give more satisfactory results.

VI. Imagery

The flight test program consisted of fourteen mappingflights, each one hour or less in duration, and imagery wasobtained on all flights. All flights were made at a lowaltitude, 300 meters above ground level or less, in order to

obtain the best possible resolution. No motion compensa-tion was used during any flight; consequently, the genera-tion of a good synthetic array was dependent upon a stableflight path.

Fig. 8(A) is an aerial photograph of a factory complexnear the Willow Run Airport, and 8(B) is a hologram radarimage of the same area when the flight path was directlyover the factory. A large number of features in the imagecan be identified in the photograph. For instance, there is alake on the right side next to a group of railroad tracks.

Fig. 9(B) is a hologram radar image of a ramp area at theWillow Run Airport taken from a 150-meter altitude, and9(A) is a photograph of the boxed area in the image. Alarge hanger facing the ramp is at the bottom of the figure.Five C-46 aircraft can be seen in the center of the ramparea, and a twin-engine beechcraft is at the left end of theline of parked aircraft. A single aircraft, a four-engineElectra turbo prop, is at the top of the ramp. There are alarge number of stored parts, boxes, and crates at the rightof the ramp.

The radar system was also operated in the two-frequencymode to produce range contours in the output image. Witha 5-MHz frequency difference, the slant range differencebetween contours is 30 meters. Fig. 10(A) isa photograph ofan industrial plant and gravel pit on opposite sides of ariver. The constant range lines in the image [Fig. l0(B)]show the steep slopes in the gravel pit (A), the downwardslope from the building toward the river on its right side

IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS MARCH 1972214

(A)

(B)Fig. 9. (A) Photograph of air-freight parking ramp, Willow Run Airport. (B) Microwavehologram of air-freight parking ramp and hanger, Willow Run Airport, at 150-meter altitude.

Fig. 10. (A) Photograph of an industrial plant and a gravel pit northwest of Ann Arbor.(B) Microwave hologram image of industrial plant and gravel pit at 300-meter altitude, rangecontour mode.

(A)

(B)

LARSON ETAL: MICROWAVE HOLOGRAM RADAR 2 15

Fig. 11. Microwave hologram of hill west of Chattanooga, Tenn.Minimum altitude is 300 meters; maximum is 510 meters.

(B), and the sudden shift in range contour due to the heightof the building (C). Within the gravel pit, depressions (D)and mounds (E) can be seen though the two cannot bedifferentiated as such without additional information, andsignal returns from many objects in the pit are also evident.It can reasonably be deduced from the image that the slopefrom the building to the river is downhill. It may beobserved that the lines converge toward the center of theimages, becoming closer to the aircraft's ground track as itis moved from the left side of the image toward the river.Since the slant range is constant for a given line, the terrainshould slope downward in that direction. The closed line inthe center of the image is ambiguous and could be causedby either a hill or a depression beneath the aircraft. Theshift in the range contour line which goes over the buildingcan be measured, and using the measured value, the verticalheight of the building was found by simple geometry to be5 meters. The structure is, in fact, a one-story factorybuilding about 5 to 6 meters high.

Fig. 11 is an interesting example of range contourimagery obtained from a flight over a wooded section ofmountains in Tennessee along a westward line fromChattanooga to Monteagle. An elevation change of 390meters in the vicinity of the Sequatchie Valley is shown onthe profile. The line intervals in the image represent30-meter range increments, and 14 of these lines can becounted.

VIl. Conclusions

The objective of the hologram radar program was tostudy the applicability of holographic techniques to air-borne terrain-imaging systems operating in the microwaveregion and to demonstrate feasibility. The theory underly-ing such a hologram system was examined, and anexperimental system designed, constructed, and operatedsuccessfully. Visible hologram images of good quality havebeen generated from airborne microwave radar signals.Synthetic-aperture beam compression and phased-arraybeam forming were accomplished simultaneously in anoptical processor. Measured system resolutions were ingood agreement with theoretically predicted values. Thecapability of a hologram radar to generate range contourson terrain images by using a two-frequency interferencetechnique has also been successfully demonstrated.

These hologram images show the terrain beneath theaircraft and out to 450 on either side of the aircraft'sground track, and they reveal the microwave appearance of

terrain and targets from a perspective unlike that oftraditional pulse-ranging radars. Because this type of radarsystem produces microwave images with a perspectivewhich nearly matches that of infrared and photographicsensors, it may be particularly well sui.ed to applications inthe new field of multispectral sensing.

The present experimental hologram radar system is thefirst to be constructed and it was intended only todemonstrate the principles of microwave holography, notto compete with state-of-the-art side-looking radars. Cross-track resolution could be improved in future hologramradar systems by using longer arrays or transmitter switch-ing techniques in various combinations. For arrays with agreater number of elements, thinning techniques (either therandom removal of elements or incorporation of pointtransmitters along the array to remove sections of redun-dant elements) should be utilized. Improvement in across-track resolution for a given array aperture size can berealized by increasing the operating frequency. With thepresent aperture size of 1 meter, a system designed tooperate at a wavelength of 3 mm would have an arraybeamwidth of 0.003 radian.

There should be considerable interest in the range-contouring capability of such radar systems. The groundswath can be increased by flying at higher altitudes, and therange-contour intervals can be varied by changing thedifference between the two frequencies used. Designers offuture systems should also consider the possible employ-ment of fringe sharpening to reduce the amount of imageblanking or loss resulting from destructive interference.This can be accomplished, for example, by transmittingmore than two equally spaced frequencies. Simultaneousimagery with and without range contouring, on two outputfilms, could be obtained with minor modifications to TheUniversity of Michigan system.

Acknowledgment

The authors wish to acknowledge the assistance in thisprogram of many other members of the Radar and OpticsDivision of the Institute of Science and Technology. Themeticulous efforts of F. Smith in constructing and operat-ing the equipment merit special attention. Dr. K. A. Haineswas responsible for the first theoretical analysis of the radarand R. A. Rendleman directed the flight test operations.

References

[11 L.J. Cutrona et al., "A high resolution radar combat-surveillance system," IRE Trans. Military Electronics, vol.MIL-5, pp. 127-131, April 1961.

[21 E.N. Leith, "Quasi-holographic techniques in the microwaveregion," Proc. IEEE, vol. 59, pp. 1305-1318, September1971.

(3j W.M. Brown and L.J. Porcello, "An introduction to syntheticapertute radar," IEEE Spectrum, vol. 6, pp. 52-62, September1969.

[41 R.W. Larson et al., "Microwave holography," Proc. IEEE, vol.57, pp. 2162-2164, December 1969.

[51 E. Leith, "Synthetic aperture radar viewed as a holographicprocess," Radar Lab., The University of Michigan, Ann Arbor,Unpublished Internal Memo., 1956.

IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS MARCH 1972216

[61 -, "Pulse compression in optics," in Introduction toOptical Data Processing, University of Michigan SummerCourse Notes, Ann Arbor, sec. 2, pt. 2, July 25-August 5,1966.

[7] D.E. Duffy, "Optical reconstruction from microwave holo-grams," J. Opt. Soc. Am., vol. 56, p. 832, 1966.

[81 E.B. Champagne, "Nonparaxial imaging, magnification, andaberration properties in holography," J. Opt. Soc. Am., vol.57, 1968.

[9] E.N. Leith, 1. Upatnieks, and K.A. Haines, "Microscopy bywavefront reconstruction," J. Opt. Soc. Am., vol. 55, pp.981-986, 1965.

[101 R.W. Meier, "Magnification and third-order aberrations inholography,"J. Opt. Soc. Am, vol. 55, pp. 987-992, 1965.

(111 J.S. Zelenka, "Microwave holography," in Principles ofImaging Radars, The University of Michigan, Ann Arbor, ClassNotes from Intensive Short Course, July 20-31, 1970.

[12] B.P. Hildebrand and K.A. Haines, "Multiple-wavelength andmultiple-source holography applied to contour generation," J.Opt. Soc. Am., vol. 57, p. 155, 1967.

[131 R.W. Larson et al., "Results obtained from The University ofMichigan hologram radar," presented at the Seventh Inter-natl. Symp. on Remote Sensing of the Environment, AnnArbor, Mich., May 17-21,1971.

Richard W. Larson (S'50-A'51-M'58) was born in Chicago, 111. He received the B.S.degree in electrical engineering from The University of Illinois, in 1952, and the M.S.degree in electrical engineering from The University of Michigan, Ann Arbor, in 1958.

He has been active in projects relating to microwave radar systems or relatedcomponents for 19 years. After three years in the U.S. Army Signal Corps, he was withthe Electron Physics Laboratory of The University of Michigan from 1955 to 1958. Hethen worked for a year in the Microwave Branch of Raytheon, Santa Barbara, Calif. Forthe past ten years he has been a Research Engineer with the Radar and OpticsLaboratory, Willow Run Laboratories, Institute of Science and Technology, at TheUniversity of Michigan. Work during this period has included various aspects of fineresolution radar systems design such as propagation characteristics, oscillation stability,and microwave hologram techniques. He has also been a Lecturer in the ElectricalEngineering Department since 1965.

Jerry S. Zelenka (S'57-M'60) was born in Cleveland, Ohio, on January 27, 1936. Hereceived the B.S., M.S., and Ph.D. degrees in electrical engineering from The University ofMichigan, Ann Arbor, in 1958, 1959, and 1966, respectively.

He joined the Bendix Research Division, Southfield, Mich., in 1959, and worked for twoyears in the Radar Group as a systems engineer. Since 1961 he has been with the Radarand Optics Laboratory, Willow Run Laboratories, Institute of Science and Technology, ofThe University of Michigan, where he has worked on systems analysis of side-lookingradar, coherent optical processing techniques, and applications of holography. He iscurrently Head of the Electro-Optics Section.

Dr. Zelenka is a member of the Optical Society of America, Sigma Xi, Tau Beta Pi, andEta Kappa Nu.

Elmer L. Johansen (S'54-A'55-M'60) was born in Lake Forest, Ill., on June 28, 1930.He received the B.A. degree from Harvard University, Cambridge, Mass., in 1952, and theM.S.E.E. and Ph.D. degrees from The University of Michigan, Ann Arbor, in 1954 and1964, respectively.

For two years following discharge from the U.S. Army in 1956 he was engaged inradar countermeasures for the Cook Research Laboratories, Morton Grove, Ill. Since1958 he has been at The University of Michigan, where most of his time has been spenton the staff of the Radar and Optics Laboratory, Willow Run Laboratories, Institute ofScience and Technology, working on synthetic aperture radar and target reflectivity.

Dr. Johansen is a member of Sigma Xi and Phi Kappa Phi.

LARSON ETAL: MICROWAVE HOLOGRAM RADAR 217