US 3923273 A
A system is presented for locking an electro-optical seeker to a desired target without recourse to a television type reproduction of the target scene. A special gunsight is provided in which the axis of an electro-optical scanning system is automatically aligned with the cross-hairs that the operator uses to sight the target. The seeker axis is then varied until signals from the seeker correlate with signals from the scanning system within the gunsight, at which time the seeker axis is in proper alignment with the target sighted with the crosshairs.
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Description (OCR text may contain errors)
United States atem 1 91 1111 3,923,273 Alpers 1 Dec. 2, 1975  AIM POINT CORRELATOR 3,337,l6l 8/1967 Halton 244/3.l4
 Inventor: Frederick C. Alpers, Riverside,
Calif Primary ExammerR1chard A. Farley Assistant ExaminerS. C. Buczinski 1 Asslgnee! The Umted States of Amenca as Attorney, Agent, or FirmRichard S. Sciascia; Joseph represented by the Secretary of the St Amand; M. Phillips Navy, Washington, DC.
 Filed: Jan. 30, 1967 57 ABSTRACT ] Appl. No.: 614,527 A system is presented for locking an electro-optical seeker to a desired target without recourse to a televi-  U S Cl 244/3 178/6 8, 178/1310 21, sion type reproduction of the target scene. A special 250/203; 1 gunsight is provided in which the axis of an electro- [511 Int C12 7/00 optical scanning system is automatically aligned with  Fieid R 203 CT the cross-hairs that the operator uses to sight the targ'L'6' 8 7 6 i 350/11: get. The seeker axis is then varied until signals from 4 3 the seeker correlate with signals from the scanning system within the gunsight, at which time the seeker  References Cited axis is in proper alignment with the target sighted with the cro'sh UNITED STATES PATENTS 5 alts 3,290,506 12/1966 Bertram 1. 250/203 2 Claims 6 Drawing Figures /IO REFLECTED LIGHT W FROM TARGET m ---1 1 l L i i l r 13 i i MULTl-APERTURE I AIRCRAFT I 1 IMAGE DISSECTOR 2:222:51 STICK 1 A rl7 sEEKER AH LOCK H5 BUTTON fi rag-WV? WW? r24 r 11 fie ge SCAN QUADRATURE MASTER CIRCUIT PULSE I SCAN CUIT GENERATOR j OSCILLATOR i V r 38 a0 4- l MATCH SEARCH SEQUENCE DETECTORS PATTERN CONTROL w GENERATOR clRculTs TO SEEKER To sEEKER 22%;? A66 CIRCUIT SCAN CONTROL To SEEKER CIRCUIT MEMORY CIRCUITS US. Patent Dec. 2, 1975 Sheet 2 of2 3,923,273
ALIGNMENT OF REFERENCE SYSTEM MATCH DETECTION SEEKER SCAN 4I'\ 4O CENTER APERTURE MAID SCAN MAID SCANS TOP-RIGHT MAID VIDEO PHOTOMULTIPLIERS MULTI-APERTURE IMAGE DISSECTOR (MAID) 3x3 ARRAY OF APERTURES PHOTOCATHODE FIG.6
CORRELATION SEARCH BY MISSILE SEEKER AUTOMATIC TRACKING FREDERICK C. ALPERS Q Q 3 RESOLUTION ELEMENTS N R. BETWEEN BARS I l BY ,4 FIG. 3
ATTORNEYS AIM POINT CORRELATOR The invention herein described may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.
BACKGROUND The present invention relates to electro-optical missile guidance systems and more particularly to electrooptical missile guidance systems wherein the electrooptical seeker is locked to a desired target independent of a television type reproduction of the target scene, imperfections in the mounting alignment of'the missile and in-flight flexing of the aircraft structures.
In prior known systems of high-accuracy air-to-surface homing missiles it has been necessary to provide a television display in the launching aircraft cockpit to show precisely the scene being viewed by the missile seeker in order that the seeker tracking circuits may be locked to the desired target. Without this provision to show precisely the scene being viewed by the missile seeker, slight misalignments in mounting the missile to the aircraft, drifts in the electronic circuitry of the missile seeker, flexure of the aircraft structures due to variations in flight conditions, and other sources of error might cause the seeker to be locked to an object other than the desired target. With the television display arrangement, the pilot first maneuvers the launch aircraft to point toward the desired target, and then shifts his attention to his cockpit display and remotely controls the seeker tracking gates (or corrects the aircraft heading) to bring the gates into spatial coincidence with the image of the selected target. For multiple launching of missiles, the pilot would repeat the gate controlling sequence for each seeker. The television display must be sufficiently bright that the pilot does not have to become dark adapted after viewing bright clouds and landscape.
SUMMARY The present invention provides a missile tracking system which forces the missile seekers to have the same aim point as the aircraft gunsight by correlating what the sensor or each seeker sees with what is seen by a similar sensor that looks through the gunsight. By the use of an array of overlapping circular scans, a simultaneous comparison basis is provided for retaining the alignment or tracking once the alignment has been achieved. A simple coded output from each of the parallel match detectors provides the error signal to correct the seeker scan position to counter any drift.
The present invention makes it possible to eliminate a cockpit television display, permits a free interchange of weapons between guided and unguidedtypes, provides quicker target acquisition, and avoids the pilot diversion associated with viewing a television display.
Many of the attendant advantages of this invention will become readily appreciated as the same becomes betterunderstood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:
FIG. 1 is a schematic diagram of a preferred embodiment of the invention.
FIG. 2 is a graph showing the alignment of reference system of the embodiment of FIG. 1.
FIG. 3 is a graph showing a correlation search of a missile seeker.
FIG. 4 is a graph showing the match detection mode.
FIG. 5 is a graph showing the automatic tracking mode.
FIG. 6 shows one form of a multi-aperture image dissector used in the invention.
Referring now to the drawings there is shown in FIG. 1 a gunsight 10 which receives reflected light energy from a target. A partially silvered mirror 12 reflects a portion of the light received to a multi-aperture image dissector (MAID) l4 and also passes light to the pilot 16 where the image of the target can be viewed. By use of the aircraft control stick 13 the aircraft airframe 15 can be made to maneuver to bring gunsight 10 into line with the target. A master scan oscillator 18 provides the sine and cosine scanning voltages that will produce a circular scan in dissector 14. An automatic gain control circuit 20 has as its input the video from the center aperture of dissector 14 which provides an output voltage to control the gain of dissector 14 in accordance with the average scene brightness encountered in the course of the center circular scan. The sine and cosine outputs of master scan oscillator 18 are also coupled to quadrature pulse generator 22 which for each of scan quadrant generates a pulse that is fed to scan centering circuit 24. The output signals from multi-aperture image dissector 14 are fed to match detectors 26 where they are compared with the video signal received at terminal 28 from the seeker (not shown). The output signals from match detectors 26 are fed to sequence control circuits 30 which produce seeker control signals that are fed to the seeker AGC circuit, scan control circuit and memory circuits at output terminals 32, 34,
and 36, respectively. Pulses for triggering sequence control circuits 30 are provided by a search pattern generator 38. Pilot 16 also provides an input to sequence control circuits 30 via seeker lock button 17.
In operation, the correlator accomplishes its function in four steps, which are illustrated in FIGS. 2 through 6. First, the multi-aperture dissector (FIG. 6) associated with the gunsight is caused to perform a circular scan 40 that is electronically aligned by means of scan centering circuit 24 to center on the point defined by the gunsight crosshairs 41; this scan produces a reference video signal 42 whose waveform depends on the scene being viewed at a given instant, except that signals 43 representing the crosshairs are superimposed at the points in the scan where the crosshairs come infront of the scene. Second, a synchronized circular scan of equal diameter 1 .0) is established in each seeker, and these scans are made by electronic deflection to search over their respective images through an angle of 12 in both pitch and yaw (FIG. 3) about the nominal seeker axis position (12 is the maximum alignment deviation anticipated between each seeker axis and the 'point identified by the gunsight crosshairs). Third, the video signal resulting from the scan in each seeker is continuously checked against the reference video from the gunsight dissector by match detectors 26, and when a match throughout a scan cycle is detected (F IG. 4), the searching action for that particular'seeker is stopped. Fourth, as the search actions are stopped, each seeker is switched into a condition in which automatic alignment tracking of the seeker axis with that of the center aperture of the gunsight dissector (FIG. 5) is maintained by the correlator. Each seeker is thus accurately and dynamically aligned so that the axis of its circular scan coincides with that of the scan of the gunsight dissector, which in turn is aligned with the axis of the gunof the scene viewed will be quickly discarded and asearch for true alignment reinstituted as the scene changes due to forward motion and maneuvering of the aircraft.
The type of automatic tracking action selected for the aim point correlator depends upon a simultaneous comparison technique that can be most easily implemented by the use of a multiple-aperture type of image dissector in the gunsight. The multi-aperture arrangement of the dissector in the gunsight is illustrated in FIG. 6 and the overlapping circular scans that are obtained with this arrangement are illustrated in FIGS. 4 and 5. The arrangement consists of nine apertures 44 in a 3 X 3 matrix. Channeltron type multiplierchannels 45 of the Bendix Research Laboratories may be utilized at the output end of the dissector which will make it possible to space these apertures only one resolution element apart. Thus nine video outputs will be derived: one from a circular scan centered on the crosshair position as discussed above, one from a circular scan displaced one resolution element to the right, one from a scan displaced both one element upward and one element to the right, one from a scan displaced one element upward, etc. In the correlator tracking action, coincidence of the video from a given seeker with that from the center scan of the gunsight dissector will produce no correction of the seeker scan axis, while coincidence of the seeker video with that of the right circle of the gunsight dissector will result in a small leftward correction of the seeker scan axis, coincidence with the up circle will result in a downward correction, etc. The alignment will thus be retained without reverting to the search mode despite possible electronic drifts or flexure of structures between the cockpit and the missile seeker. The multiple aperture arrangement not only makes possible this simultaneous comparison type of tracking but also speeds up the search time since it makes it possible to check the seeker video signal simultaneously against nine different reference video signals.
The firstof the four steps of the correlation process described above is implemented by quadrature pulse generator 22 and the scan centering circuit 24. The quadrature pulse generator 22 may be a set of four blocking oscillators arranged to trigger respectively at the four points in the scan cycle when either the sine or cosine voltage becomes zero, and generator 22 therefore generates a pulse 23 for each quadrant of advance of the circular scan. The scan centering circuit 24 may be a set of bistable multivibrator circuits (commonly called "flip flops"), each of which is turned on by a quadrature pulse 23 and turned off by a subsequent crosshair signal 43, followed by differential detector circuits that compare the on cycles of the opposing multivibrators to derive dc outputs that control the position of the dissector scanning and thereby center the scan. Thus the scan positioning is based on the relative timing of the quadrature pulses 23 and the black-level video signals 43 that result as the scan crosses each crosshair, and when the quadrature pulses and the corresponding black-level video signals are respectively in coincidence, the central scan circle is properly centered about the crosshair point. The scan centering circuit 24 should have a very long time constant so that integration will serve to eliminate any false centering effects caused by other black-level signals that appear 4 momentarily due to dark line-like objects (e.g., asphalt roads) in the scene viewed by the gunsight dissector.
The second step in the correlation process (that of causing seeker search) is performed by the search pattern generator 38. This unit includes a back-to-back sawtooth generator for horizontal search and a synchronized staircase waveform generator for vertical search that combine to produce the raster indicated in FIG. 3. The faster circuit scan is treated as an ac signal that is superimposed on the slower search pattern, which is treated as dc, and the two together are fed to the seekers in the missiles.
Both the third and fourth steps of the correlation process are performed basically by the match detectors 26, while the sequence control circuits 30, which can readily be implemented by a suitable connection of relays, regulate the switching of individual seekers from step to step as the correlation progresses. Each match detector may consist of a difference detector, an integrating circuit (to determine that the match is obtained over a complete cycle or more), and a bistable output; each detector will accept two analog video inputs and give a single digital-type yes-no output that indicates whether or not the two inputs match each other over a complete scan cycle. An entire bank of match detectors will be requireda separate one to compare the video from each seeker with each of the nine videos from the gunsight dissector. For each seeker being correlated, the outputs of the match detectors as a group will give (1) an indication of whether or not a match is present at that instant, and (2) an indication of the position of any existing match with respect to the desired match involving the center aperture of thematrix. The
first output from match detectors 26 can be supplied as a voltage to initiate appropriate relay action in the sequence control circuits 30; the second can be supplied in the form of small dc yaw and/or pitch correction signals that can be delivered to the seeker via terminals 36 to center the scan as required. During the correlation search and tracking processes, the sequence control circuits 30 cause the AGC circuit within each seeker to switch to a functioning mode that directly parallels that of the correlator AGC circuit in order that the amplification of the respective dissectors will be equal and the resultant videos will be of comparable voltage levels; and these sequence circuits also cause the seekers own scanning circuits to be temporarily deactivated and the scan/search signals from the correlator to be substituted.
When the pilot has a target sighted in his crosshairs and pushes the Seeker Lock" button, 17, the sequence control circuits 30 cause the seeker in the selected missile to commence independent scanning through activation of the seekers own scan circuit, and also cause the seeker to convert to an AGC mode that is optimized for seeker gray-level tracking, as described in copending application Ser. No. 434,740 filed Feb. 18, 1965 entitled Gray-Level Angle-Gated Electro- Optical Seeker. In the lock-on process, the seeker memory circuits retain the aim point positional information supplied by the correlator at the instant of switchover; the seeker commences its circular scan about this point at essentially zero scan diameter, and gradually increases this diameter until changes in gray level occur which signify that the edges of the target have been reached whereupon normal seeker tracking and scan size control functions take over. The graylevel memory in the seeker first memorizes" the target gray level encountered when the scanning circle is at essentially zero diameter, and thereafter tracks the gray level in the manner described in the above-referenced copending application.
Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.
What is claimed is:
1. For use in an electro-optical guidance system an aim point correlator comprising:
a. a gunsight having cross-hairs for aligning a missile carrying aircraft on a target of interest,
b. a multi-aperture image dissector,
c. a partially silvered mirror positioned in said gunsight for reflecting the image viewed by said gunsight on to said multi-aperture image dissector,
d. circular scan circuit means coupled to said multiaperture image dissector for producing a circular scan of the target image to produce output signals proportional to the light reflected from said target,
e. comparison circuit means having afirst input coupled to the output of said multi-aperture image dissector and a second input adapted to receive video signals from the seeker of a missile carried by said aircraft and producing output signals proportional to a mis-match of the target viewed by the gunsight and target viewed by the missile seeker,
f. control circuit means coupled to said comparison circuit means for producing error signals which can be fed to the seeker circuits for aligning the seeker with the gunsight. I
2. For use in an electro-optical guidance system an aim point correlator for correlating the image of a target as viewed by the gun-sight of an aircraft and the image of the target as viewed by the seeker of a missile to be launched against the target from the aircraft, the combination comprising:
a. a multi-aperture dissector associated with said gunsight,
b. circular scan circuit means coupled to said multiaperture dissector for causing said dissector to scan the image of the target, and produce a reference video signal whose waveform depends on the scene being viewed at a given instant,
c. scan centering circuit means for centering the circular scan on a point defined by the gunsight crosshairs,
d. an input terminal for receiving video signals from seekers of missiles to be launched by the aircraft,
e. comparator circuit means coupled to said multiaperture dissector and to said input terminal for comparing the two video signals for producing a stop search signal when there is a match of the two video signals throughout a scan cycle and then producing aligning signals for keeping the seeker axis aligned with that of the center aperture of the gunsight dissector.