282 IEEE TRANSACTIONS ON AEROSPACE AND NAVIGATIONAL ELECTRONICS September

PROJECTED PLANS

The theoretical and experimental studies reportedhere have established a practical technique for trackingpartially illuminated bodies at intermediate ranges. Inpractical application to space vehicles, the preliminarypointing and alignment of the line of cusps will be ac-complished automatically using preliminary or approxi-mate informatioin available in the navigation computer.

Refinements of design are in progress to simplify thereticle subsystem, alignment requirements, and to inin-iaturize the computer.An extension of the same technique permits the track-

ing of partially illuminated bodies at very close range.

Special devices are required to extend the angular rangeand to minimize errors due to irregularities in the con-tour of the bright limb. These developmenits are inprogress and will be reported later.

ACKNOWLEDGMENT

The theoretical and experimental study was the jointeffort of a team of physicists, mathematicians, engineersand master craftsmiien drawn from Northrop SpaceLaboratories and Nortronics, a Division of NorthropCorporation. The contributions of the teami membersare gratefully acknowledged.

E1ectro-Optica1 Image Matcher for Space

Guidance Applications*

JOHN N. PACKARDt

Summary-Pattern recognition and the determination of relativeattitude are capabilities of an electro-optical map matching devicethat utilizes two-dimensional correlation data processing techniques.This device is the basic element of a Universal Space Tracker thatprovides pitch, yaw and roll data with respect to the recognized field(either planetary surface, star field, or planetary rim) for spacevehicle guidance applications. The correlation detector and dataquantizer is a digital image camera of special design. A feasibilityevaluation model of the map matcher has successfully correlated theconstellation Gemini under a Maryland atmosphere.

The implementation of optical correlation makes possible auniversal tracking system with excellent performance characteristics.An advanced version of the tracker is expected to have a 20-arcsec-ond accuracy, a 5-degree instantaneous acquisition cone, a 20-degreefield of view, a weight of 5 pounds, a 4-cubic foot volume and apower consumption of 3 watts.

I NTRODUCTION

PACE MISSIONS, whether manned or unmannedmay have similar objectives such as a soft landingon or circumspection of a celestial body; however,

the requirements for achieving the objectives differ. Formanned missions, the human is an integral part of thesystem and with proper man-machine relationships can

* Received June 11, 1963.t Electronic Systems & Products Division. Martin Company,

Baltimore, Md.

significantly lower the degree of subsystem automaticitvand sophisitication required. The human can performfunctions such as failure correction and certain types ofdecision making which the machine may not be capableof performing efficiently. On the other hand, in un-manned missions, a high degree of automaticity andsophistication is required. Equipment must operate re-liablv for extended periods and perform basic decisionfunctions. If the required degree of system performanceis unattainable, the human elenment must be inserted bymeans of communication and control links with theground.The problem of re-entry for manned missions is of

major importance since the ultimate earth landing loca-tion is largely determined during this period. For thisreason, the precise determination of vehicle position andvelocity just prior to re-entry is an absoltute necessity.With precise information, the on-board guidance equip-ment (e.g., an autopilot) can provide the proper steeringsignals during re-entry to achieve a predetermined land-ing position.The major navigation atnd guidance problem in un-

manned missionis occurs during the transplanet tra-jectory and is concerned with midcourse guidance.Accurate midcourse guidance permits the minimization

Packard: Elec tro- Optical Image Matcher

of trajectory errors and resultant steering correctionsand permnits the predetermined trajectory to be main-tained. As a result, the risk of mission failure is greatlyreduced and on-board stores are utilized most efficiently.

In either type of space mission, the precise determina-tion of the vehicle orientation and trajectory are essen-tial for successful completion of the mission objective.The Universal Space Tracker, an implementation ofoptical correlation image matching, is capable of per-forming these functions in either mission, as well asproviding the capability for additional useful functions.Because of its versatility, this concept eliminates theneed for many currently used space navigation devices,thereby permitting an economy of system design withno loss of performance.

This paper discusses the advantages to be gained byusing optical correlation imuage matching techniques, asincorporated in the Universal Space Tracker, for spaceguidance applications. A discussion of noncoherentoptical correlation data processing is included. A prede-cessor of the Universal Space Tracker concept, the feasi-bility model Star Field Tracker, is described.

OPTICAL CORRELATION CAPABILITIES

The optical correlation data processing technique pos-sesses characteristics of versatility and accuracy whichmake it especially applicable to the mechanization of anadvanced celestial tracker. Some of the salient featuresare

1) optical correlation tracking of star fields, celestialbodies and ground landmarks,

2) tracking of bodies over a wide range of illuminationlevel and phase since the position of the correlationpeak is not a function of illumination,

3) maximumii utilization of available light by integra-tion of all useful informatioin in the main correla-tion peak,

4) smoothing of random variations such as appear inthe contour of a viewed body disk by means of thecorrelation process.

The above characteristics result in a concept whichpermits high precision tracking of the center of the sun,moon, planets and artificial satellites during wide rangesof phase and levels of illumination. The versatility ofoptical correlation data processing enhances the systemaccuracy by taking advantage of the best physical char-acteristics of the observed body. Thus, at close ranges,the accuracy of determination of the local vertical canbe improved by tracking ground landmarks rather thanthe rinm of the viewed body disk. Ground landmarktracking also could be used to provide the data requiredfor a soft landing navigation system. In order for track-

ing of the center of near celestial bodies to be effective,it must be referred to a celestial or inertial reference.The correlation data processing technique can take careof both problems, the final configuration of the totaltracking concept being relatively simple.

For an optical correlation tracker in the conventionalsense, the tracking of all types of celestial bodies (exceptindividual stars with the same instrument) is feasiblewhen variable focal length optics are employed. Itshould also be noted that the physical elements of thetracker make possible the performance of functionsother than correlation tracking. For example, the systemcan operate as a conventional star tracker if the correla-tor elements are removed and the location of the opticsis adjusted to allow imaging of incident radiation on thephotodetector. This pernmits the performance of naviga-tion system functions requiring the observation of indi-vidual stars such as in occultation. Similarly, by employ-ing the above change in configuration, the system canserve as an image camera.

SYSTEM DESCRIPTIONThe basic elements of the optical correlation map

matcher are shown in Fig. 1. The system consists ofthree units: a telescope, map matching mechanism andcorrelator electronics.

PRIMARYLENS

Y I IMAGE REFERENCE INTEGRATING IMAGE SENSOR

SCREEN MAP LENS (VIDICONI\ 1

TELESCOPE MAP MATCHINGMECHANISM

CORRELATORELECTRONICS

Fig. 1-Optical correlation map matcher.

TelescopeThe telescope portion is comprised simply of the pri-

mary lens and a diffusion screen. The primary lens formsthe image that is to be correlated on the diffusionscreeni. The screen is specially designed to forward-scatter the incident light through small angles [1]. Thisaction tends to conserve available light for the correla-tion processing.

Map MlatcherThe map matching mechanism is comprised of the

reference map, hardware for storing and positioning themap, and the secondary (integrating) lens. The refer-ence map is a photographic transparency that representsthe information contained in the desired object field.When the desired field is viewed, the secondary lens

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284 IEEE TRANSACTIONS ON AEROSPACE AND NAVIGATIONAL ELECTRONICS September

images the autocorrelation of the stored reference onthe photosensor of the correlator electronics. The cor-relation processing depends upon diffused light from theimage pattern that is incident upon the secondary lensafter traversing the reference transparency. This light isintegrated in the focal plane of the secondary lens, form-ing a two-dimensional distribution of light intensity thatis the optical correlation function.

Electronics

The correlator electronics processes the video infor-mation contained in the correlation function and derivesuseful data fromii its position and structure. The elec-tronics comprises a vidicon camera for scanning the cor-

relation function, a peak detector that identifies themiaximum intensity of the function, digital circuits that

locate the peak in terms of a system of rectangular co-

ordinates applied to the vidicon target, a convergence

integrator that determuines the roll condition and a

memory for storage of this information.

TWTO-DIMIFNSIONAL NONCOHERENT OPTICALCORRELATION

Signal Mlultiplication

Consider the horizontal and vertical edges of a photo-graphic transparency to be called x and y, respectively.The varying transparency on the plate constitutes a

two-dinmensional space signal, and can be described bya function Ti(x, y, zo), where T1 represents the trans-

mittance and zo indicates the location of the plate on the

optical axis. If a light ray of unit intensity passes throughthe point (xn, yin, zo), it will emerge with intensityT1(xn, Ym, ZO). If the ray then passes through a point(x,, y, z1) in a second plate with picture T2(x, y, Z1),the resultant intensity on emergence is Tl(x,, yin, z0)T9(x,, yq, z1) (Fig. 2).

Correlation Functfion Generation

Assume that the map matcher stored reference con-

tains a precise prediction of the image pattern, althoughin actual practice this condition generally does not exist.

Furthermore, assume for the monment a one-dimen-

sional situation (Fig. 3).The image pattern is given by T(x, zo) and the stored

reference pattern by T(x+-a, z1). The s-ymibol a repre-

sents the x displacemiient of the imlage pattern with re-

spect to the corresponding portion of the stored refer-

ence caused by a position- error of the systemii axis.

If a ray of unit intensity originates at (Xm, z0) anid

passes through the reference picture at (xo+a-+K, z1),it will emerge with intensity T(xm, zo)T'(xm+a+K, Z1).Since the reference picture and image pattern are

assumed to be identical,

T(xm, zO)T(x,+xa+KK, zi)=T(xm, Z)T(xm+k, z0). (1)

Fig. 2 TMvo-dimensional signial operatioin.

Fig. 3 Correlation projection.

Anv other unit ray originating from any point (xn, zO)on the image pattern and parallel to the previous ray

will pass through (x7-+-a+K, z1) and emerge with inten-sity

T(x,,r z0) T(x, + a + K, z1) = T(x,, z0) 7F(X,1 + K, z0), (2)

and the total intensitv caused by all rays in this parallelray bundle will be on emergence fromi zI,

x

+(K) = T(x, zo)T(x + K, zo)dx. (3)

Considering an extensioni to two dimlensions, the integralbecomes

(K, L) J J T(x, y, zo)T(x + K, y + L, zo)dxdy (4)

which is the two-diimiensional autocorrelatioin of theimnage pattern.The integral exists physically as the sum of the in-

tensity of the parallel ray bundle on emerging from thereference map at z1 and, as such, the rays in this set

occupy an area on emnergence equal to that of the image

zo

Packard: Electro-Optical Image Matcher

pattern. However, by placing a converging lens at Z2, allof these rays are imaged to a common point on thelens focal plane at z3, so that the intensity at this pointis the value of the autocorrelation 4(K, L).The discussion has involved only a fixed K and L

corresponding to one set of parallel rays. But K and Lare variables, and from the same points on the imagepattern and at the same time a multitude of sets ofmutually parallel rays originate, each set emerging in adifferent direction and each intercepting the syntheticin a distinct value of K and L. Each such set is focusedat a different point on the screen with an intensity pro-portional to q5(K, L), so that the two-dimensional auto-correlation function of the image pattern is painted onthe lens focal surface as a function of light intensity vsx and y.

Pattern RecognitionThe functional problem involved in the optical cor-

relator map matcher is to determine the identity of in-formation contained in two spatial signals, a distribu-tion of light intensity and a distribution of opticaltransparancy. It is known from the theory of correla-tion data-processing [2] that. if two signals match, thecross correlation between them will be a maximum. Thismaximum value is the autocorrelation peak which cor-responds to the two signals in perfect overlap or whenK = L =0 in (4). No other term in the correlation func-tion has a value that exceeds 0(0, 0). Therefore, if theautocorrelation peak exists, the two signals are matched.This is the pattern recognition capability of the opticalcorrelation map matcher.

Location of the Peak

A light ray originating at an element xm of the imagepicture and passing through the element x, on the storedreference (and hence through the center of the lens)will emerge undeviated and proceed to the place of con-vergence on the focal plane at z3 (Fig. 4). All other raysof the same set will converge at the same point. Similartriangles provide

xm - X /ILmn(5)

Zi - Zo Z3- Z2

Because x,, =xn +a+K,

Z2 - Ztmn = (a + K) - (+ K)P

ZI - Z°

where

Z2 - Z3P =

Z1 - Zo

IIMAGE MAPSCREENf`mn x - x

3 2 O

2LENS

"imnz

Z3DETECTOR

BUT Xn= Xm+ a+KSO THAT (z3K) 2 -z +z

Hmn=(o+K) 3z =( +K)p,K=p 3 2

SIMILARLY, z +Z

Vmn = (l L) zi -zo ( L)p

[ +K)P,(SL)P, z3] K=L=O (aP,tP,p z3).Fig. 4 Location of correlation peak.

Similarly,

z2 - Z3vmn = (3+ L) = (O + L)P

Z1 - ZO(7)

where ,u anid v are the x, y coordinates in the focal plane.The coordinates of the point corresponding to thevalue 4(K, L) of the correlation function are

(a + K)P, (3 + L)P, Z3.The peak value is obtained when K= L =0, so that thepeak of the correlation function k(0, 0) is located at(aP, 3P, z3). The location of the peak, consequently, isa measure of a and /, the displacement of the imiagepattern relative to the stored reference.

Effect of Roll

As the reference map is rotated relative to perfectalignment with the image pattern, the correlation peakdecreases in amplitude and spreads into the intensitydistribution. Maximum spreading occurs at a roll angleof Xr radians. As the perfect alignment position is ap-proached, a pronounced convergence of intensity is ob-served and the amplitude of the correlation peak buildsup rapidly. It is possible to detect the degree of con-vergence of the correlation peak and, therefore, deter-mine the roll angle of the system.

In Fig. 5, the line OPQ represents a light ray from apoint (xn, yi, zo) on the image pattern through the point(xv, yq, z1) on the reference store. When the referencestore and image pattern are in perfect alignment, thisray is imaged by the lens to point R on the lens focalsurface. This ray represents the parallel set that formsthe center of the correlation peak. There are a multitudeof parallel sets that add to form the finite intensity thatis the peak. Under a rotation of the reference store 0 thepoint P is now situated at P', and the ray OP'Q' repre-

-i

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286 IEEE TRANSACTIONS ON AEROSPACE AND NAVIGATIONAL ELECTRONICS September

' -2 , -=8c0,= I/2~

I,1414 B (I _ 0)

Fig. 5-Effect of roll.

sents the shifted ray OPQ. This ray will be imaged topoint R' on the focal plane which is displaced a distanceAr from point R. It can be shown that

Ar = (Z3 - Z2/Zl - Zo)S; (8)or, written in terms of the distance from the center ofrotation to the point (x,, Yq, z1),

Ar = 1.414r(1 - cos O)i/2(Z3- Z2/Zl - zO). (9)

This is a cardioid-like pattern with its origin at thepoint R and gives the locus of the points R' in the focalplane of the lens for each set of mutually parallelrays. It is apparent that the correlation function divergesas a function of roll angle to a maximum value at 7rradians.By way of summarizing, it should now be evident

that a peak of maximum intensity will be formed whenthe autocorrelation of the image pattern is generated,and that the coordinates and structure of this peak ofintensity determine the pitch, yaw and roll of the opticalaxis relative to the image pattern.

FEASIBILITY MODEL

A feasibility model of the optical correlation map

matcher has been developed for ap;plication to starfields (Fig. 6). The system is comprised of optics, mapmechanism, vidicon camera, computer, CRT monitor,digital display and power supply.

Vidicon Camera and ComputerThe correlation detector is a vidicon image tube. It

is operated in synchronism to a binary computer whichis the data quantizer. Fig. 7 is a block diagram of thecorrelation and quantizing system. Since the positionand structure of the autocorrelation peak are desired,it is detected and located. A video signal whose ampli-tude is proportional to the light intensity is generatedby scanning the vidicon. A peak detector determines thepresence of the video signal corresponding to the auto-correlation peak and develops a bias proportional to thepeak amplitude. The data quantizer is designed to

Fig. 6-Feasibility model optical correlation map matcher.

VIDICON SEPVERTICAL SYNCHRONIZATIONGENERATR HOIOTL ZATHROIZONANDCAMERA CONTRO I

1 ~~~~1111111 v 1111111|CONVERCENtEr^WO IHORIZONTAL MEMRYI IVERTICAL MEMORYIFig. 7-Correlation detector and data quantizer.

operate on a single scan of the vidicon to minimize thetime required for the generation of error signals. Itconsists of a master clock oscillator, a synchronizer,two 8-stage binary counters and a 16-stage binaryregister and shift matrix. Operation is as follows: thefirst clock pulse starts both the horizontal and verticalsweeps of the vidicon. The first video peak above somenoise threshold appears at the peak detector outputand triggers a shift of the number contained in thecounters to the binary register. The counters continueto count with no interruption. A bias is stored in thepeak detector corresponding to the first amplitude peak.For additional video to trigger the system, it must ex-ceed the stored amplitude. In this way, successivelylarger video peaks are stored until the maximum isreached. Since a shift is generated for each pulse thataffects the peak detector, the count in the register isthe localizing number for the most recent video peak.Therefore, the number remaining in the register at thecompletion of the scan locates the correlation peak. Thefirst eight digits of the binary number designate thehorizontal coordinate, the last eight digits designate thevertical coordinate.

Convergence IntegratorUnder a roll misalignment, the correlation peak de-

creases in amplitude and spreads into the intensity dis-tribution of the correlation function. Maximum spread-ing occurs at a roll angle of pi radians. The roll state ofthe system is indicated by the convergence integrator.

R A ,

Packard: Elec tro- Optical Image Matcher

It is a charging circuit with a decay tinme that is shortwith respect to the time required for each scan line. Asroll alignment is optimized, convergence of the correla-tion peak to a single scan line occurs, thereby causiingthle convergence integrator to charge to a maximnumoutput. Roll alignment is achieved by adjusting formaximum convergence integrator output.

Deflection CircutitsHorizontal and vertical deflection are obtained from

a bootstrap sawtooth generator that is time triggered bythe camera control unit. A parabolic function is intro-duced in the sawtooth generator horizontal amplifier tocorrect for nonlinearity due to transistor beta and theoutput transformer saturation curve. Deflection yokedamping and recovery characteristics are accomplishedby conventional means. The vertical deflection output isa push-pull complementary amplifier. Since the verticaldeflection is all dc coupled, a means is provided to blankthe camera by stopping the vertical counter for anydesired length of time, consequently stopping thevertical sweep. Simultaneously, the vidicon beam is cutoff and the video output shorted. This technique perinitsintegration of the signal on the vidicon target.

Video AmplifierThe video anmplifier achieves a high input impedance

(ca 50 kQ ohms) by use of a transistor, selected for itslow noise properties, operated as an emitter follower.The four succeeding stages are grounded emitter stagesproviding a 3-db frequency response at 200 cps and 700kc. The upper limit is determined by a 680-kc clock rateand permits a maximum definition of 200 TV lines. Tomake it unnecessary that the low-frequency response ofthe amplifier be compatible with the vertical sweep rate,a dc restorer operating at the horizontal sweep frequencyis incorporated in the amplifier final stage. The advan-tage of limiting low-frequency response is that of reduc-ing the low-frequency noise inherent in transistors. Apinch-type video clamp (controlled by count 0 and 200or both the horizontal and vertical counters) shorts thevideo output to remove unwanted spikes, which oftenoccur at the start and the finish of the scan. The 1000-ohm output of the video amplifier is provided by anemitter follower.The vertical counter is driven by the output of the

horizontal counter at a rate of 2656/sec. The numberof stages and the configuration of the vertical counterare the same as that of the horizontal. The outputs ofthe vertical counter gates are routed to the camera con-trol system where they are used for vertical synchroniza-tion pulses for the camera. The vertical counter, andthus the vertical sweep, is gated on and off to control thecamera scans. Output of the vertical counter is alsodirected to the vertical memory.

The purpose of the camera control circuit is to providesuitable synchronization signals. Consider the pulses re-ceived from the vertical counter that are derived fromthe counter gates. These six pulses are used as triggersfor three bistable multivibrators. The pulses are pro-duced by counts, 0, 200, 206, 211, 244, and 249. Fig. 8is a sketch of the gate pulses as they occur in time dur-ing one cycle of the vertical counter. Coincidence isshown to indicate the particular count number thattriggers the bistable.

MfemoryThe horizontal storage system is conmposed of eight

memory stages, plus error correcting circuitry. For eachstage in the horizontal counter, there exists a similarstage in the horizontal memory. The steering diodes ofthe memory stage are connected to the collectors of theassociated counter stages; thus, these diodes are alter-nately reversed biased at the rate of operation of thecounter stages. When a peak detector pulse occurs, it ispassed by a diode that is not reverse biased and switchesthe memory stage into the same state as the counter,consequently "remembering" the state of the counter atthe occurrence of a peak detector pulse.The error elimination portion of the horizontal mem-

ory is best explained by reference to Fig. 9. This deviceis comprised of a two input gate and a 1-nmicrosecond"one shot" multivibrator. The period of the "one shot"is longer than the propagation delay of the counter.Essentially, the "one shot" locks out the memorytransfer function in the event of a peak detector outputuntil the position count is registered, at which time theposition count is permitted to be transferred to thememory. Systems considerations require that the un-stable period of the "one shot" be longer than one-halfthe period of the frequency divider and less than the fullperiod of the divider multivibrator. The figure is asketch of the wave forms in two cases, 1) the peakdetector pulse occurs at a time when the frequencydivider output is -20 volts and 2) the peak detectorpulse occurs when the frequency divider is -12 volts.The vertical memory operates in the same manner as

the horizontal memory. The error correcting signal,however, instead of being supplied by the frequencydivider is supplied by the last stage of the horizontalcounter.

OpticsThe system optics are f/2.5, 12-inch focal length

primary lens and f/1.2, 50-millimeter focal lengthsecondary lens. In general, for optimum system per-formance, the primary lens should have a large aperturefor light gathering power (star fields only) and a shortfocal length for maximum field of view. This featureimplies a low "f" number. Lens aperture must be con-

1963 287

288 IEEE TRANSACTIONS ON AEROSPACE AND NAVIGATIONAL ELECTRONICS September

244 249 0 200 20f 2 I

CountU

1~~~~~~~~~~~

F.ideob'ian~king B

1" dicon1) a iik n1 c

SwveepSI'rIc 1)

Fig. 8-Synchroinizationi signals.

CASE 1 CASE 2

n f/~~~~~ete c torI } ~~~~~~~liornost,aLleI l o~~~~~~~~~~utput

(;%NID atet l ~~~~~~~~~~~input.N,,.

-20 volts W L a X ( A utNo. 2)

Tv_ p'.uLls-s <)Cc 1 AN9D ga.ne outputSeconpra(Iodlu e s rn(w;mor (tr igge. s 11ltomory stag,es)'

Fig. 9 \\Wave forms to explain propagation error eliminationportion of horizontal memory circuit.

sistent with system weight and size requirements. Tominimize reference mnap errors and for optimumi field ofview, the secondary lens should also have a low "f"number. The diffusion screen is designed to have hightransnmittance and a narrow diffusion angle.

System Parameters

The feasibility model has been used successfully to

demonstrate correlation of the constellation Gemini.Parameters of this systerm follow:

field of view 70 48'acquisition angle 36'pointing accuracy 10.8"nominal star field illumination--0. 5 X 10-13 watts /cm2vidicon WNL 7290, 1-inch slow scanresolution 200 lines.

An advanced version of the tracker is expected to havethe following specifications:

field of view 200acquisition angle 50pointing accuracy 20"nominal star field illumination-10-11 watts/cm .

THE UNIVERSAL SPACE TRACKER CONCEPT

The general concept of a Universal Space Tracker de-

veloped while proving feasibility of the optical cor-

relation map matcher for star fields. The existing feasi-

bility model may be considered an advanced techniquefor star field tracking. From this device, a broader con-

cept has been projected which has the inherent ca-

pability of providing solutions to currently recognized

problems in space navigation and guidance. As en-visioned, the Universal Space Tracker will be an ad-vanced electro-optical device capable of operating in anyof three rmodes or combinations thereof, 1) star fieldtracking, 2) planet rim tracking and 3) map matching.

It is quite reasonable to consider the design of anavigation and stabilization system made up solely ofUniversal Space Trackers without requiring an inertialreference system. For exalmple, two such trackersaboard a stabilized satellite could be oriented by havingone of them acquire the sun, after which the other couldbe progranmnmed to acquire a star field. Once referenceto a star field has been established, the vehicle couildbe rotated about this axis of view simply by program-ming a rotation of the stored reference. Quite accuratenavigation could be achieved near the earth or otherplanets by using one correlation tracker for celestialstabilization and the other for correlating on landmarks.In some other mission an occultation technique might beindicated. The actual configuration of such a systemnwould, of course, depend upon the mission and manyother factors.

For short range to a tracked body, the correlator isan excellent stadiametric instrunment and, through cor-relating with stored maps appropriately simulated, itwould be able to determine range accurately. At thelonger ranges, position can be computed by trackingtwo celestial bodies anad applying triangulation tech-niques. Velocity can be derived from an analysis of aseries of position determinations or, if near a planet ormoon, by 'IH techniques. The optical correlationtracker is a suitable inform-iation sensor for any of thesetechniques.The tracker could even provide some characteristics

which would be useful within the atnmosphere. At night,of course, correlation with star fields is quite reliable aslong as there is no cloud cover problem. During the day,the skv background would require heavy filtering whichwould linmit tracking to star fields whose energy levelsare high enough to permlit correlation after the attenua-tion of the filters.By combining the star field tracking and celestial

body and ground landmnark tracking features in asingle sy7stem, a self-contained navigation system, with-out recourse to an inertial guidance system, becomesfeasible for space applications. The optical correlationtracker concept will thus permit a new and versatileapproach to space vehicle navigation and guidance.

CONCLUSIONS

1) An electro-optical map matcher using correlationdata processing can recognize an image field and pro-vide attitude data relative to this field.

2) A feasibility model of the optical correlation mapmatcher has successfully correlated a star field.

3) The optical correlation map matcher concept may

Moskowitz and Weinschel: Instrumentation for Space Navigation

be extended to a broader concept of a Universal SpaceTracker that is capable of solving many space naviga-tion and guidance problems in a highly versatile andaccurate manner.

AcKNOWLEDGMENTS

The author wishes to acknowledge the contributionsof the following individuals who participated in the de-velopment of the optical correlation map matcher: M. A.Robbins, J. A. Bourget, C. H. Vaughan and R. C. Tagler.

REFERENCES[1] J. Dyson, "Optical diffusion screens of high efficiency," J. Opt.

Soc. Am., vol. 50, pp. 519-520; June, 1960.[2] Y. WV. Lee, "Statistical Theory of Communications," John WViley

and Sons, Inc., New York, N. Y.; 1960.[3] Norman S. Potter, "Orientation Sensing in Inertial Space by

Celestial Pattern Recognition Fechniques, " presented at the 15thAnnual Meeting of Americani Rocket Society, XXashington, D. C.,1960.

[4] "Advanced concepts studied for pattern recognition," Staff Rept.,Electronlic Design, vol. IX, pp. 28-46; March 1, 1961.

[5] F. A. Jenkins and H. E. XX hite, "Fuindamentals of Optics,"McGraw-Hill Book Co., Inic., New York, N. Y., chs. 7, 9, 10, 22,23; 1957.

Instrumentation for Space Navigation*

S. MOSKOWITZt, MEMBER, IEEE AND P. WEINSCHELt

Summary-If a practical system is to be realized within the pre-dictable future, the design of instrumentation for space navigation iscontingent upon the meaningful definition of the mission, properchoice of navigation or guidance concept, optimum integration of thehuman operator and independence from technological breakthroughs.The mission must be considered and categorized in terms of thecharacteristics of the observables rather than by any other methodof classification. The mission also dictates the choice betweenimplicit or explicit navigation concepts. The capabilities and limita-tions of the astronaut must be considered in the design of all equip-ment requiring viewing and manual manipulation, especially forcomputing purposes. The resulting equipment configuration may beconsidered in terms of the "spectrum of navigation instrumentation",ranging from completely manual to fully automatic operation. Such adesign philosophy leads to a maximum probability of mission successdespite partial equipment malfunctions.

This paper is based in part upon the unclassified aspects of astudy on space position fixing under sponsorship of the Air ForceAeronautical Systems Division and in part upon other space naviga-tion programs at the Kollsman Instrument Corporation. The materialpresented here serves as an introduction to the general topic ofinstrumentation for space navigation and does not include the de-tailed exploration of any facet thereof.

I. INTRODUCTION

WX BEwEARE TODAY at the threshold of an historicaltransition in manned space flight. 'Man's essen-tially passive function in space travel is about

to be replaced by his active participation in this opera-tion. Spacecraft "guidance" is about to be supersededby space navigation.

This article presents an analysis of the instrumenta-

* Received June 11, 1963.t Navigation and Guidance Systems, Space Division, Kollsman

Instrument Corporation, Elmhurst, N. Y.

tion requiremients for space navigation. Space naviga-tion implies the transformiiation of primary observeddata into position and trajectory informiiation. The re-quirements exist, therefore, for performance of therequisite primary imieasurements and the calculationsrequired to provide meaningful navigation infornmation.The exact form of these operations is largely dependentupon the mlission. (The problenm of circunmlunar flight,for example, encomiipasses certain unique aspects whichare not at all encountered during earth-orbital flight.)In ali cases the limlitinig constraints imposed on anyspace navigation system are the characteristics of theavailable observables. A property of a terrestrial,celestial, solar or orbiting body imust be nmeasurable be-fore it can provide useful information. These observablesmust be defined before specific instrumentation or datareduction techniques can be set forth.The fundamental data (observables) available for

space navigation are not dependent upon the particularnavigation schemes employed. The selection and utiliza-tion of the data, however, does vary to a large extentwith the specific concept. It is possible to employ anyone of a nunmber of imiiplicit or explicit navigationschemes for a given mission whether the mission beearth-orbital, rendezvous, circumlunar or deep spaceflight. The choice between imiplicit or explicit techniquesis dependent upon on-board computational capacities,the available observables and the operational require-ments of the mission.The capabilities and limitations of the human opera-

tor in the performance of the space navigation function

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