OPTICAL FUSING ARRANGEMENT
United States Patent 3793958
An improved optical sensor, together with logic and control circuitry to improve reliability, is shown to provide a fusing arrangement for a missile. The sensor, which may be active or passive, includes a number of reflecting wedges affixed to the missile in such a manner that hollow, but complete, conical detecting fields are formed without detracting from the structural integrity of the missile. The logic and control circuitry includes means for differentiating between true target signals and noise signals to detonate ordance carried by the missile only in response to true target signals.
US Patent References:
Optical fuze
Arthaber et al. - May 1962 - 3034436
Distance responsive device
Rabinow - April 1964 - 3129424
Proximity fuse
Cosse et al. - July 1962 - 3046892
Optical fuze
Arthaber et al. - May 1962 - 3034436
Distance responsive device
Rabinow - April 1964 - 3129424
Proximity fuse
Cosse et al. - July 1962 - 3046892
Inventors:
Holt, Jon G. (Woburn, MA)
Hayward, Gary G. (Lexington, MA)
Goldstein, Irving (Lexington, MA)
Hayward, Gary G. (Lexington, MA)
Goldstein, Irving (Lexington, MA)
Application Number:
05/265130
Publication Date:
02/26/1974
Filing Date:
06/22/1972
View Patent Images:
Export Citation:
Assignee:
Raytheon Company (Lexington, MA)
Primary Class:
Other Classes:
244/3.160
International Classes:
F42C13/02; F42C13/00; F42C13/02; F42C11/00
Field of Search:
102/7.2P
Primary Examiner:
Borchelt, Benjamin A.
Assistant Examiner:
Webb, Thomas H.
Attorney, Agent or Firm:
Mcfarland, Philip Pannone Joseph J. D.
Claims:
What is claimed is
1. In fusing apparatus for ordnance carried by a missile having a cylindrical body, such apparatus being responsive to optical energy from an airborne target, the improvement comprising:
2. The improvement as in claim 1 having, additionally:
3. The improvement as in claim 2 having, additionally: an optical filter disposed across the tapered reflective element between the first conical mirror and the photodetector.
4. The improvement as in claim 2 having, additionally:
5. In fusing apparatus for ordnance carried by a missile, such apparatus including a photodetector responsive to optical energy from a target within a hollow conical detection field to produce electrical signals representative of such target and responsive to optical energy from sources other than a target to produce noise-like electrical signals, improved signal processing means comprising:
6. Improved signal processing means as in claim 5 having, additionally:
1. In fusing apparatus for ordnance carried by a missile having a cylindrical body, such apparatus being responsive to optical energy from an airborne target, the improvement comprising:
2. The improvement as in claim 1 having, additionally:
3. The improvement as in claim 2 having, additionally: an optical filter disposed across the tapered reflective element between the first conical mirror and the photodetector.
4. The improvement as in claim 2 having, additionally:
5. In fusing apparatus for ordnance carried by a missile, such apparatus including a photodetector responsive to optical energy from a target within a hollow conical detection field to produce electrical signals representative of such target and responsive to optical energy from sources other than a target to produce noise-like electrical signals, improved signal processing means comprising:
6. Improved signal processing means as in claim 5 having, additionally:
Description:
BACKGROUND OF THE INVENTION
This invention pertains generally to ordnance fusing apparatus and particularly to apparatus of such type using an optical radar to derive a required fusing signal.
It is known in the art that optical radar techniques may be used to advantage to derive a command signal for detonating ordnance carried by an intercepting missile during its flight toward an airborne target. With a properly designed optical radar system the probability that the command signal will be generated at the optimum moment is extremely high, making it necessary only that the intercepting missile be directed in any known manner to the near vicinity of an airborne target. Once this is done, the command signal may be derived, under widely varying atmospheric conditions and almost without regard for evasive maneuvers or countermeasures taken by the airborne target.
A satisfactory optical radar system for airborne fusing applications is now required to provide a hollow conical detection field around the longitudinal axis of the intercepting missile. With such a detection field extending a finite distance from the intercepting missile (to correspond with the extent of the explosive field of the ordnance), the presence of a target is all that is needed to allow the command signal to be generated. Unfortuantely, however, it is extremely difficult, if not impossible, to arrange an optical radar in a missile so that a complete conical detection field is formed and the structural integrity of the missile is maintained.
To meet both requirements just mentioned, it is known to combine several optical radars, each having a detection field which covers a different sector of the desired conical field. Each one of such radars, made up of a laser transmitter, a receiver and associated lens arrangements, is a complete system. It is evident, therfore, that the combination of a number of such systems entails critical alignment problems along with a requirement that overly complex and expensive apparatus be used.
Therefore, it is a primary object of this invention to provide improved optical fusing apparatus for ordnance in an airborne vehicle, as an intercepting missile.
Another object of this invention is to provide improved optical fusing apparatus using a single optical radar to form a complete conical detection field for deriving fusing command signals for ordnance carried by an intercepting missile.
Still another object of this invention is to provide improved optical fusing apparatus by which a complete conical detection field is formed without compromising the structural integrity of an intercepting missile.
SUMMARY OF THE INVENTION
The foregoing and other objects of this invention are attained generally by providing, in an optical radar installed in a convenient location in an intercepting missile, a detection field forming catoptric lens arrangement for the transmitter and the receiver sections of such radar. Such lens arrangement includes a pair of conical reflectors and a number of reflecting wedges surrounding the two, the bases of each one of such wedges being affixed to the shell of the intercepting missile so that a number of windows are formed between adjacent wedges. To isolate the transmitter section from the receiver section and to fix the relative positions in space of the detection field, the conical reflectors are mounted base-to-base on a septum which divides each one of the reflecting wedges into two parts. In addition, logic means are provided in the receiver to differentiate between true signals from a target and noise signals.
BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of this invention reference is now made to the accompanying description of a complete optical fusing apparatus according to the invention, such apparatus being illustrated in the drawings, in which:
FIG. 1 is a sketch showing generally how optical fusing apparatus according to the invention may be installed in an intercepting missile;
FIG. 1A is a cross-sectional view, taken across plane A--A of FIG. 1, showing the detection field forming wedges in particular;
FIG. 2 is a block diagram, somewaht simplified, of the logic and control section shown in FIG. 1; and
FIGS. 3A and 3B each is a sketch showing, respectively, the manner in which the amplitude of the detected signal from a target and from random backscatterers varies with time.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to FIG. 1, it may be seen that the transmitter section of the contemplated optical radar includes a laser 10t, conventional power and control circuitry 12 (to pulse such array at any desired rate), a collimating lens 14t, reflecting cone 16t, and a number, say eight, of reflecting wedges 18t and windows 20t mounted in the shell 22 of an intercepting missiel 24. The receiver section of the contemplated optical radar includes (in reverse order) windows 20r, reflecting wedges 18r, a reflecting cone 16r, a concentrator 14r and a photodetector 10t connected to a logic and control section 26. To isolate the transmitter and receiver sections, a septum 28 is disposed in any convenient manner as shown.
Referring now to FIG. 1A it may be seen that each one of the windows 20t here covers an arc of 22.5° along the shell 22 of the intercepting missile 24. The bases of each one of the reflecting wedges 18t are affixed to the shell 22 in any convenient manner so that each projects radially inwardly toward the reflecting cone 16t. The exposed sides of each one of the reflecting wedges 18t are provided with a mirror surface in any known manner. The apex angle of each one of the reflecting wedges 18 (which angle obviously in greater than 22.5°) varies with the height of the reflecting wedges. In turn, the optimum height of the reflecting wedges 18t preferably is such that any radially distributed optical energy from the relfecting cone 16t will be reflected no more than a single time before passing through a window 20t. That is, the optimum height of the reflecting wedges 18t is the height at which one-half of the radially distributed optical energy from the reflecting cone 16t (such energy apparently having originated at a virtual source at the center of the intercepting missile) passes directly through the windows 20t and substantially all of the remaining part of such energy is reflected once from a reflecting wedge before passing through a window. It follows, then, that the optimum height of the reflecting wedges 18t is such that: (a) the central angle subtended between the apices of successive weges is twice the central angle subtended by the interposed window; and, (b) the angle between a ray reflected from a point adjacent to the apex of a wedge and the outermost ray passing directly through the window is equal to one-half the central angle subtended by such window. With the foregoing in mind it may be shown that: X = R (sin A/2)/sin (3A/2) (1)
where X is the distance measured formthe center of the intercepting missile of the apex of any reflecting wedge along a radius, R, and
A is the central angle subtended by a window.
Further, it may be shown that the half-apex angle, Q/2, of each reflecting wedge is:
Q/2 = tan -1 (sin A/2)/[cos A/2-(sin A/2)/(sin 3A/2)] (2)
with the radius of the intercepting missile known, it is apparent now that Eq. (1) and Eq. (2) may be solved to allow windows and reflecting wedges to be disposed in areas around the skin of such missile at locations which do not reduce the strength of the missile's body or interference with the desired location of other equipment. In this connection, it is noted that, if desired, the various windows and wedges need not be the same, but rather may differ, within wide limits, to avoid structural members within the bodh of the intercepting missile.
The apex angle of the reflecting cone 16t and the diameter of the beam from the collimating lens 14t determine, in the first instance, the apex angle of the "detection" field of the transmitter section and the thickness of such field. Obviously, then, the longitudinal dimension (meaning the length parallel to the longitudinal axis of the intercepting missile 24) of each one of the windows 20t needs only to be long enough to prevent obscuration of energy being transmitted.
The receiving windows 20r, reflecting wedges 18r and reflecting cone 16r operate together to form a conical detection field for the receiver section in the same way as the corresponding elements just described form a detection field for the transmitter section. Because of the displacement between the two detection fields (resulting from the required displacement between the reflecting cones 16t, 16r), it is desirable that the longitudinal dimension of the reflecting wedges for the receiver section be greater than the longitudinal dimension of the corresponding wedges in the transmitter section. The two fields then intersect at a shorter distance from the intercepting missile 24 (FIG. 1) than would otherwise be the case. Obviously, however, to reduce the effect of spurious signals to the receiver section, the longitudinal dimension of the windows 20r should not be any greater than required to attain intersection of the detection fields at a given distance to reduce spillover from the transmitter section to the receiver section to an acceptable level.
To complete the description of the receiver optical section, the reflecting cone 16r supports a concentrator 14r, a filter 26 and a photodetector 28. The concentrator 14r corresponds generally to the frustrum of a cone, with one base thereof shaped to fit snugly on the reflecting cone 16r. The concentrator 14r is preferably made from a material which has a high index of refraction for the optical energy of the wavelength of the laser 10t. Thus, energy falling on the concentrator 14r from the reflecting wedges 18r is first refracted to fall on the reflecting cone 16r. After reflection from the latter, the optical energy impinging on the wall of the concentrator 14r is internally reflected to pass through the filter 26 to the photodetector 28. To ensure the desired total internal reflection, the wall of the concentrator 14r may be mirrored adjacent to the filter 26. The filter 26 may be made from any conventional material so that is passes only a narrow band of optical energy having wavelengths centered at the wavelength of the optical energy from the laser 10t, with due allowance made for any expected Doppler shift.
In passing it is noted that the concentrator 14r and the filter 26 are not essential to the invention but are desirable to concentrate optical energy reflected from a target on the photodetector 28 and to attenuate optical energy of other wavelengths. The photodetector 28 is responsive to optical energy reflected from a target when such target is in th overlapping detection fields of the transmitter and the receiver of the optical radar. The result, then, is that an electrical signal is produced, such signal being indicative of the presence and angular position of a target. Thus, the electrical signals out of the photodetector 28 (which signals include laser echo signals from any targets within th detection field and noise signals, as energy backscattered by particles in the atmosphere or spillover) are amplified in a conventional amplifier 30 and passed to any known threshold detector 32 which is arranged to have its threshold change with the level of the electrical signals out of the amplifier 30. To accomplish such end, the electrical signals out of the threshold detector 32 are integrate in an integrator 34 and the resulting bias signal is used to change the detection level of such detector. As is known, then, a constant false alarm rate may be determined so that only true echo signals having an amplitude significantly greater than the mean value of the noise signals are likely to be passed through the threshold detector 32. The electrical signals corresponding to echo signals out of the threshold detector 32 are passed through an AND gate 36 to a shift register 38. The former is, in turn, enabled by a sampling gate generator 40 (as a monostable multivibrator). As is conventional, such generator is actuated during a portion of the time between successive pulses out of the laser transmitter 42 by a synchronizing pulse out of a synchronizer 44. It will be apparent that the temporal relationship between the trigger pulse to the laser transmitter 42 and the trigger pulse to the sampling gate generator 40 and the length of unstable equilibrium of the latter combine to determine the minimum and maximum range of any "processible" target. The signals loaded during each period of time in which AND gate 36 is enabled are shifted successively through the m-stages of the shift register 38 as either a logic "one" (meaning the signal most likely indicates a signal representative of a target) or as a logic "zero" (meaning the signal indicates no target). When the state of the shift register 38 is such that n-stages (less than m) contain logic "ones," a conventional decoder 46, as a diode matrix, is caused to produce a logic "one" to actuate a firing circuit 48. The latter may be conventional, as for example a solenoid and a switch, as a silicon controlled rectifier, in circuit with a power supply to trigger a detonator when the rectifier is actuated.
On the one hand, the shift register 38 is actuated, i.e., shift pulses are impressed on it, when the amplitude of the signals out of the amplifier 30 increases in a prescribed manner (as shown in FIG. 3A) within a fixed period of time, say four pulse intervals of the laser transmitter 42, to indicate a closing target within the detection field. On the other hand, the shift register 38 is cleared whenever the amplitude of the signals out of the amplifier 30 during an interval of the same length does not change in the prescribed manner, meaning that such signals are not from a closing target within the detection field. Thus, a counter 50 is caused to count up "one" each time the synchronizer 44 produces a signal to pulse the laser transmitter 42. The counter 50 in turn actuates a selector switch 52 (here four AND gates, not numbered) to direct the signals out of the amplifier 30 to successive ones of four peak detectors 54A, 54B, 54C, 54D. These elements then produce DC signals indicative of the amplitude of the maximum return on four successive pulses out of the laser transmitter 42. A counter 61 actuates a selector switch 56 (again four AND gates, not numbered) successively to impress, during a single interpulse period, the DC signal out of each one of the peak detectors 54A, 54B, 54C, 54D on one input of a differential amplifier 58. The second input of such amplifier is connected to a sweep generator 60 controlled by the counter 61 as shown. It will be recognized that the sweep generator 60 is synchronized with the selector switch 56 to cause the output of the differential amplifier 58 to be a pulse width coded signal, the width of each pulse being indicative of the DC level out of the peak detectors 54A, 54B, 54C, 54D. Each time the counter 61 is filled, a bistable multivibrator f/f62) is "set" or "reset." During each "set" interval, f/f62 enables an AND gate 64, thereby allowing clock pulses from the synchronizer 44 to pass to an AND gate 66. The latter, being enabled by each pulse out of the differential amplifier 58, allows some of the clock pulses to be passed to a counter 68. A moment's thought will make it clear now that, with a given clock frequency, the counter 68 may be caused to count a known number of clock pulses if the pulse width coded signal out of the differential amplifier 58 is as shown in FIG. 3A and that any other count indicates that the pulse width coded signal out of the differential amplifier 58 is of the nature shown in FIG. 3A. In other words, when the count in the counter 68 is such that a decoder 69t indicates a proper count for a target, an AND gate 70t is enabled to allow shift pulses to be passed to the shift register 38. On the other hand, when the count in the counter 68 is such that a decoder 69n indicates an improper count for a target, an AND gate 70n is enabled to cause the shift register 70n to be cleared. It follows then that, in the absence of return signals from a target within a predetermined range, it is impossible for the firing circuit to be actuated.
Having described an embodiment of this invention, it will now be apparent to one of skill in the art that many changes may be made without departing from our inventive concepts. It is felt, therefore, that the invention should not be restricted to its disclosed embodiment but rather should be limited only by the spirit and scope of the appended claims.
This invention pertains generally to ordnance fusing apparatus and particularly to apparatus of such type using an optical radar to derive a required fusing signal.
It is known in the art that optical radar techniques may be used to advantage to derive a command signal for detonating ordnance carried by an intercepting missile during its flight toward an airborne target. With a properly designed optical radar system the probability that the command signal will be generated at the optimum moment is extremely high, making it necessary only that the intercepting missile be directed in any known manner to the near vicinity of an airborne target. Once this is done, the command signal may be derived, under widely varying atmospheric conditions and almost without regard for evasive maneuvers or countermeasures taken by the airborne target.
A satisfactory optical radar system for airborne fusing applications is now required to provide a hollow conical detection field around the longitudinal axis of the intercepting missile. With such a detection field extending a finite distance from the intercepting missile (to correspond with the extent of the explosive field of the ordnance), the presence of a target is all that is needed to allow the command signal to be generated. Unfortuantely, however, it is extremely difficult, if not impossible, to arrange an optical radar in a missile so that a complete conical detection field is formed and the structural integrity of the missile is maintained.
To meet both requirements just mentioned, it is known to combine several optical radars, each having a detection field which covers a different sector of the desired conical field. Each one of such radars, made up of a laser transmitter, a receiver and associated lens arrangements, is a complete system. It is evident, therfore, that the combination of a number of such systems entails critical alignment problems along with a requirement that overly complex and expensive apparatus be used.
Therefore, it is a primary object of this invention to provide improved optical fusing apparatus for ordnance in an airborne vehicle, as an intercepting missile.
Another object of this invention is to provide improved optical fusing apparatus using a single optical radar to form a complete conical detection field for deriving fusing command signals for ordnance carried by an intercepting missile.
Still another object of this invention is to provide improved optical fusing apparatus by which a complete conical detection field is formed without compromising the structural integrity of an intercepting missile.
SUMMARY OF THE INVENTION
The foregoing and other objects of this invention are attained generally by providing, in an optical radar installed in a convenient location in an intercepting missile, a detection field forming catoptric lens arrangement for the transmitter and the receiver sections of such radar. Such lens arrangement includes a pair of conical reflectors and a number of reflecting wedges surrounding the two, the bases of each one of such wedges being affixed to the shell of the intercepting missile so that a number of windows are formed between adjacent wedges. To isolate the transmitter section from the receiver section and to fix the relative positions in space of the detection field, the conical reflectors are mounted base-to-base on a septum which divides each one of the reflecting wedges into two parts. In addition, logic means are provided in the receiver to differentiate between true signals from a target and noise signals.
BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of this invention reference is now made to the accompanying description of a complete optical fusing apparatus according to the invention, such apparatus being illustrated in the drawings, in which:
FIG. 1 is a sketch showing generally how optical fusing apparatus according to the invention may be installed in an intercepting missile;
FIG. 1A is a cross-sectional view, taken across plane A--A of FIG. 1, showing the detection field forming wedges in particular;
FIG. 2 is a block diagram, somewaht simplified, of the logic and control section shown in FIG. 1; and
FIGS. 3A and 3B each is a sketch showing, respectively, the manner in which the amplitude of the detected signal from a target and from random backscatterers varies with time.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to FIG. 1, it may be seen that the transmitter section of the contemplated optical radar includes a laser 10t, conventional power and control circuitry 12 (to pulse such array at any desired rate), a collimating lens 14t, reflecting cone 16t, and a number, say eight, of reflecting wedges 18t and windows 20t mounted in the shell 22 of an intercepting missiel 24. The receiver section of the contemplated optical radar includes (in reverse order) windows 20r, reflecting wedges 18r, a reflecting cone 16r, a concentrator 14r and a photodetector 10t connected to a logic and control section 26. To isolate the transmitter and receiver sections, a septum 28 is disposed in any convenient manner as shown.
Referring now to FIG. 1A it may be seen that each one of the windows 20t here covers an arc of 22.5° along the shell 22 of the intercepting missile 24. The bases of each one of the reflecting wedges 18t are affixed to the shell 22 in any convenient manner so that each projects radially inwardly toward the reflecting cone 16t. The exposed sides of each one of the reflecting wedges 18t are provided with a mirror surface in any known manner. The apex angle of each one of the reflecting wedges 18 (which angle obviously in greater than 22.5°) varies with the height of the reflecting wedges. In turn, the optimum height of the reflecting wedges 18t preferably is such that any radially distributed optical energy from the relfecting cone 16t will be reflected no more than a single time before passing through a window 20t. That is, the optimum height of the reflecting wedges 18t is the height at which one-half of the radially distributed optical energy from the reflecting cone 16t (such energy apparently having originated at a virtual source at the center of the intercepting missile) passes directly through the windows 20t and substantially all of the remaining part of such energy is reflected once from a reflecting wedge before passing through a window. It follows, then, that the optimum height of the reflecting wedges 18t is such that: (a) the central angle subtended between the apices of successive weges is twice the central angle subtended by the interposed window; and, (b) the angle between a ray reflected from a point adjacent to the apex of a wedge and the outermost ray passing directly through the window is equal to one-half the central angle subtended by such window. With the foregoing in mind it may be shown that: X = R (sin A/2)/sin (3A/2) (1)
where X is the distance measured formthe center of the intercepting missile of the apex of any reflecting wedge along a radius, R, and
A is the central angle subtended by a window.
Further, it may be shown that the half-apex angle, Q/2, of each reflecting wedge is:
Q/2 = tan -1 (sin A/2)/[cos A/2-(sin A/2)/(sin 3A/2)] (2)
with the radius of the intercepting missile known, it is apparent now that Eq. (1) and Eq. (2) may be solved to allow windows and reflecting wedges to be disposed in areas around the skin of such missile at locations which do not reduce the strength of the missile's body or interference with the desired location of other equipment. In this connection, it is noted that, if desired, the various windows and wedges need not be the same, but rather may differ, within wide limits, to avoid structural members within the bodh of the intercepting missile.
The apex angle of the reflecting cone 16t and the diameter of the beam from the collimating lens 14t determine, in the first instance, the apex angle of the "detection" field of the transmitter section and the thickness of such field. Obviously, then, the longitudinal dimension (meaning the length parallel to the longitudinal axis of the intercepting missile 24) of each one of the windows 20t needs only to be long enough to prevent obscuration of energy being transmitted.
The receiving windows 20r, reflecting wedges 18r and reflecting cone 16r operate together to form a conical detection field for the receiver section in the same way as the corresponding elements just described form a detection field for the transmitter section. Because of the displacement between the two detection fields (resulting from the required displacement between the reflecting cones 16t, 16r), it is desirable that the longitudinal dimension of the reflecting wedges for the receiver section be greater than the longitudinal dimension of the corresponding wedges in the transmitter section. The two fields then intersect at a shorter distance from the intercepting missile 24 (FIG. 1) than would otherwise be the case. Obviously, however, to reduce the effect of spurious signals to the receiver section, the longitudinal dimension of the windows 20r should not be any greater than required to attain intersection of the detection fields at a given distance to reduce spillover from the transmitter section to the receiver section to an acceptable level.
To complete the description of the receiver optical section, the reflecting cone 16r supports a concentrator 14r, a filter 26 and a photodetector 28. The concentrator 14r corresponds generally to the frustrum of a cone, with one base thereof shaped to fit snugly on the reflecting cone 16r. The concentrator 14r is preferably made from a material which has a high index of refraction for the optical energy of the wavelength of the laser 10t. Thus, energy falling on the concentrator 14r from the reflecting wedges 18r is first refracted to fall on the reflecting cone 16r. After reflection from the latter, the optical energy impinging on the wall of the concentrator 14r is internally reflected to pass through the filter 26 to the photodetector 28. To ensure the desired total internal reflection, the wall of the concentrator 14r may be mirrored adjacent to the filter 26. The filter 26 may be made from any conventional material so that is passes only a narrow band of optical energy having wavelengths centered at the wavelength of the optical energy from the laser 10t, with due allowance made for any expected Doppler shift.
In passing it is noted that the concentrator 14r and the filter 26 are not essential to the invention but are desirable to concentrate optical energy reflected from a target on the photodetector 28 and to attenuate optical energy of other wavelengths. The photodetector 28 is responsive to optical energy reflected from a target when such target is in th overlapping detection fields of the transmitter and the receiver of the optical radar. The result, then, is that an electrical signal is produced, such signal being indicative of the presence and angular position of a target. Thus, the electrical signals out of the photodetector 28 (which signals include laser echo signals from any targets within th detection field and noise signals, as energy backscattered by particles in the atmosphere or spillover) are amplified in a conventional amplifier 30 and passed to any known threshold detector 32 which is arranged to have its threshold change with the level of the electrical signals out of the amplifier 30. To accomplish such end, the electrical signals out of the threshold detector 32 are integrate in an integrator 34 and the resulting bias signal is used to change the detection level of such detector. As is known, then, a constant false alarm rate may be determined so that only true echo signals having an amplitude significantly greater than the mean value of the noise signals are likely to be passed through the threshold detector 32. The electrical signals corresponding to echo signals out of the threshold detector 32 are passed through an AND gate 36 to a shift register 38. The former is, in turn, enabled by a sampling gate generator 40 (as a monostable multivibrator). As is conventional, such generator is actuated during a portion of the time between successive pulses out of the laser transmitter 42 by a synchronizing pulse out of a synchronizer 44. It will be apparent that the temporal relationship between the trigger pulse to the laser transmitter 42 and the trigger pulse to the sampling gate generator 40 and the length of unstable equilibrium of the latter combine to determine the minimum and maximum range of any "processible" target. The signals loaded during each period of time in which AND gate 36 is enabled are shifted successively through the m-stages of the shift register 38 as either a logic "one" (meaning the signal most likely indicates a signal representative of a target) or as a logic "zero" (meaning the signal indicates no target). When the state of the shift register 38 is such that n-stages (less than m) contain logic "ones," a conventional decoder 46, as a diode matrix, is caused to produce a logic "one" to actuate a firing circuit 48. The latter may be conventional, as for example a solenoid and a switch, as a silicon controlled rectifier, in circuit with a power supply to trigger a detonator when the rectifier is actuated.
On the one hand, the shift register 38 is actuated, i.e., shift pulses are impressed on it, when the amplitude of the signals out of the amplifier 30 increases in a prescribed manner (as shown in FIG. 3A) within a fixed period of time, say four pulse intervals of the laser transmitter 42, to indicate a closing target within the detection field. On the other hand, the shift register 38 is cleared whenever the amplitude of the signals out of the amplifier 30 during an interval of the same length does not change in the prescribed manner, meaning that such signals are not from a closing target within the detection field. Thus, a counter 50 is caused to count up "one" each time the synchronizer 44 produces a signal to pulse the laser transmitter 42. The counter 50 in turn actuates a selector switch 52 (here four AND gates, not numbered) to direct the signals out of the amplifier 30 to successive ones of four peak detectors 54A, 54B, 54C, 54D. These elements then produce DC signals indicative of the amplitude of the maximum return on four successive pulses out of the laser transmitter 42. A counter 61 actuates a selector switch 56 (again four AND gates, not numbered) successively to impress, during a single interpulse period, the DC signal out of each one of the peak detectors 54A, 54B, 54C, 54D on one input of a differential amplifier 58. The second input of such amplifier is connected to a sweep generator 60 controlled by the counter 61 as shown. It will be recognized that the sweep generator 60 is synchronized with the selector switch 56 to cause the output of the differential amplifier 58 to be a pulse width coded signal, the width of each pulse being indicative of the DC level out of the peak detectors 54A, 54B, 54C, 54D. Each time the counter 61 is filled, a bistable multivibrator f/f62) is "set" or "reset." During each "set" interval, f/f62 enables an AND gate 64, thereby allowing clock pulses from the synchronizer 44 to pass to an AND gate 66. The latter, being enabled by each pulse out of the differential amplifier 58, allows some of the clock pulses to be passed to a counter 68. A moment's thought will make it clear now that, with a given clock frequency, the counter 68 may be caused to count a known number of clock pulses if the pulse width coded signal out of the differential amplifier 58 is as shown in FIG. 3A and that any other count indicates that the pulse width coded signal out of the differential amplifier 58 is of the nature shown in FIG. 3A. In other words, when the count in the counter 68 is such that a decoder 69t indicates a proper count for a target, an AND gate 70t is enabled to allow shift pulses to be passed to the shift register 38. On the other hand, when the count in the counter 68 is such that a decoder 69n indicates an improper count for a target, an AND gate 70n is enabled to cause the shift register 70n to be cleared. It follows then that, in the absence of return signals from a target within a predetermined range, it is impossible for the firing circuit to be actuated.
Having described an embodiment of this invention, it will now be apparent to one of skill in the art that many changes may be made without departing from our inventive concepts. It is felt, therefore, that the invention should not be restricted to its disclosed embodiment but rather should be limited only by the spirit and scope of the appended claims.