Abstract:
Pulse integration is utilized on board an optically guided missile or other ordinance device used as a part of a target designation system for extending the lock-on range of the missile by approximately 18%, when a double pulse laser is utilized to illuminate and designate the target. Pulse integration is accomplished with a recirculating delay unit which superimposes the first pulse on the second pulse, such that while the pulses add coherently, noise does not add in phase. The pulse integration technique therefore enhances the signal-to-noise ratio when the missile is at the outer limits of its operating range. When the missile is sufficiently close to the target, doublet decoding is actuated to offer countermeasure resistance. At this point, pulse integration may proceed in lieu of doublet decoding or may be dispensed with in view of the increased signal-to-noise ratio due to the close range of the missile to the target.

Description:
FIELD OF THE INVENTION 
   This invention relates to target designators, and more particularly, to a method and apparatus for extending the range of a target designator. 
   BACKGROUND OF THE INVENTION 
   Target designators, usually employing lasers, have in the past made use of a so-called “doublet pulse”, a pulse burst containing two pulses, in which the inter-pulse spacing is varied to provide a code which can be recognized by incoming missiles, guided shells or other ordinance devices. Doublet pulse laser target designators suffer from the disadvantage of requiring almost twice the laser energy per doublet pair,in order to provide range performance equal to a single pulse system. 
   In a recent development, a large portion of the extra energy requirement is compensated for by the superior efficiency provided by multi-cavity single-pump lasers or single-pump multi-laser systems in which sequentially activated Q-switches are provided in the laser-cavities. In these lasers a single pumping pulse provides the energy for the multiple pulse out-put. Typical efficiency increases measured in the laboratory and during field tests show that doublet pulses generated in this manner contain 50% more energy than those from a single cavity, multi-pump laser or lasers for a given amount of flash lamp energy. 
   While the multiple cavity, single-pump lasers provide a substantial increase in range, a still further substantial improvement can be obtained by utilizing doublet pulse integration. While the subject invention will be described in terms of two pulses, it will be appreciated that pulse integration of any number of regularly spaced pulses will result in substantial range increases in terms of the range at which a missile carrying a seeker or optical tracking device will lock onto a target illuminated by the multiple pulses. 
   The purpose of the pulse integration is to time superimpose a first pulse and a second pulse in a doublet pair, such that the signal components add in-phase, while the noise components add in random phase. In the two pulse case, this results in an increase in signal-to-noise ratio (SNR) of at  2  at least 3 db over direct doublet detection, which corresponds to an increase in the lock-on range of approximately 18%. 
   If pulse integration is utilized, the seeker of the missile will lock on to the target at ranges which exceed that which would ordinarily be achievable by detection of the doublet pulse. Thus, the seeker is able to lock on to relatively weak signals due to the pulse integration technique. In the subject system there are two modes of operation, namely, the short range or doublet decode mode and the long range or extended range mode. In the extended range mode doublet pulse decoding is not utilized, and therefore, there is a certain amount of countermeasure susceptibility at distances which are at the fringe of system performance. 
   However, in the extended range mode, countermeasure effectiveness is minimized, since even if countermeasuring is employed, the missile will, nonetheless, approach the target. As the missile or guided shell approaches the target, the intensity of the received radiation increases. When this intensity increases above a predetermined threshold, a doublet decoder within the missile&#39;s seeker is activated and signals resulting from detected radiation are only gated to the guidance system of the missile if doublet pulses having a known predetermined inter-pulse spacing have been received. Thus, in this embodiment, the seeker system is switched from its long range or extended range mode to its short range or doublet decode mode when the detected signals reach a predetermined threshold. At this point, the seeker is hardened against countermeasuring. 
   In summary, in the doublet decode mode, each incoming doublet pair is decoded and if the pair has the appropriate inter-pulse spacing the outputs from the pulse integrators are sampled and transmitted to the missile&#39;s guidance system. Any signals not having the requisite pulse coding are inhibited from reaching the guidance system of the missile. 
   The system described, while operating in a long range—short range mode, also has two additional modes of operation. The first mode of operation leaves the pulse integration system in operation all the time, whereas, the second mode of operation disconnects the integration system when the missile is operated in the short range or doublet decode mode. In either case, range extension is achieved by the pulse integration. 
   Superposition of the first and second pulses is accomplished, in one embodiment, by a recirculating delay line and summation network combination. In another embodiment, the delay line is replaced by a lighter and more versatile device, called a serial analog delay or SAD. 
   Assuming that the seeker utilizes the conventional quad cell detector, the output from each of these detectors is amplified and mixed in three channels such that if the quad cells are designated A, B, C, and D, then the outputs of the amplifiers are applied to processors which perform the following functions: (A+B)−(C+D); (A+B)−(B+C); and A+B+C+D. This provides three channels of information from the quad cell detector. The first two of these channels constitute the up-down channel and the right-left channels respectively which provide the directional signals for the missile&#39;s guidance system. 
   In one embodiment, the outputs of these processors are applied to respective recirculating delay units each of which have a feedback circuit to a summing network which adds the output of the recirculating delay unit to the incoming signal. The recirculating delay is exactly equal to the expected inter-pulse spacing such that the two;pulses coherently add in the summing network. The output of each recirculating delay unit is applied through a gate to either the up-down or right-left signal channels of the guidance system of the missile. 
   The third channel is utilized to detect when the missile is within a predetermined short range of the target. This is accomplished by applying the output of this channel to the same type of recirculating delay unit described for the first two channels. The output of this recirculating delay unit is applied to a high level trigger, which, in essence, activates a doublet decoder when the level of the radiation at the seeker reaches a predetermined level indicating that the missile is within a predetermined short range of the target. 
   In order to provide a signal to the doublet decoder, the third channel is coupled to the doublet decoder. The output of the doublet decoder is applied to a switching or gating system such that the switching or gating system generates a pulse which activates the gates in the first two channels in the presence of an appropriate pulse doublet to sample the quad cell output. When the system is in its short range or doublet decode mode, assuming that the appropriate signals are available at the quad cell, the doublet decoder decodes the fact that the pulses have the requisite interpulse spacing and applies a gating pulse to the gates of the first two channels. The delay throughout the doublet decoder/switching system corresponds to the expected inter-pulse spacing. Thus, if pulses of the appropriate inter-pulse spacing impinge on the quad cell their integrated values will be sampled and transmitted to the guidance system of the missile or ordinance device. Countermeasure signals are rejected by this system and do not affect the guidance system of the missile or ordinance device. 
   Alternatively, the pulse integration system may be completely taken out of the loop once the doublet mode threshold has been reached. This may have some advantages in close range situations where the signal-to-noise ratio is sufficiently high. However, it will be appreciated that by utilization of the recirculating delay units in the first two channels, an even higher signal-to-noise ratio can be obtained in these channels, with a consequent reduction in the false alarm rate. 
   It will also be appreciated that the system described includes “preprocessing” the signals from the quad cell detector prior to recirculation. If a recirculating delay line channel is provided for each quad cell detector, the processing for guidance purposes may be accomplished after the pulse integration in a “postprocessing” step. 
   It is therefore an object of this invention to provide an improved target designating system utilizing pulse integration for extending the range of the system; 
   It is another object of this invention to provide a target desiganting system which operates in a long range and a short range mode, in which at least the long range mode includes utilization of pulse integration for range extension; 
   It is another object of this invention to provide a method and apparatus for increasing the signal-to-noise ratio in seekers utilized with multiple pulse target designating systems. 
   These and other objects of the invention will be better understood in connection with the appended drawings in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagrammatic representation of a doublet pulse laser target designating system indicating the extended range achievable by the utilization of doublet pulse integration; 
       FIG. 2  is a schematic and block diagram of one type system for utilization in the seeker of the missile depicted in  FIG. 1 , in which a long range/short range mode is defined and in which multiple-pulse integration is utilized in the long range and short range modes. 
       FIG. 3  is a graph showing the increased signal-to-noise ratio achieved within the pulse integration system as a function of normalized range, also indicating the extent of the range extension obtainable by doublet pulse integration; 
       FIG. 4  is a schematic diagram of one type of pulse integrator usable in the system described in connection with  FIG. 2 ; 
       FIG. 5  is a schematic diagram illustrating a simplified doublet decoding system utilizable in the  FIG. 2  embodiment; and, 
       FIG. 6  is a schematic diagram illustrating a simplified system for one channel in which pulse integration is used only in the long range mode, with normal detection taking place in the short range doublet decode mode. 
   

   DETAILED DESCRIPTION 
   Referring now to  FIG. 1 , a typical laser target designator system is depicted in which a source of multiple pulses, such as a laser  10 , directs a pulse doublet  12  with an interpulse delay ΔT towards a target  14 . The target scatters the light from the laser in all directions and scattered radiation in the form of a pulse doublet  16  propagates in the direction of a missile or guided ordinance device  18 . Under ordinary circumstances, without pulse integration, the seeker aboard missile  18  would acquire the target at a normalized range 1.0 as illustrated. With the system to be described, target acquisition would take place at an extended range such as that illustrated, which in the doublet pulse case, is on the order of a 18% increase. In one embodiment, the normalized range defines a range perimeter with the target at its center. It is the function of the subject circuit to provide pulse integration circuitry at points outside the range perimeter, while activating doublet decoding when the missile is within the range perimeter. As will be seen, the range perimeter is set in terms of the intensity of the scattered pulsed radiation from the target. In the subject system the range perimeter is set by thresholding circuits within the seeker which will be described hereinafter. In general, the range perimeter defines the crossover point between extended range operation and doublet decode operation. This range is variable and depends on the signal-to-noise ratio of the signals as detected by the seeker at various ranges from a target. In one embodiment, the SNR at the crossover point is ˜12. Obviously, the gain of the seeker utilized and the strength of the pulse source, along with the range of the pulse source to the target contribute to define the crossover threshold and thus the range perimeter. One system which provides the aforementioned extended range is now described in connection with FIG.  2 . 
   Referring to  FIG. 2 , an ordinance device typically includes a “seeker” which is the name given an optical tracker. In general, optical trackers are provided with a quad cell type detection unit such as that illustrated at  20 . The cell is divided into four segments A, B, C, and D, each carrying a detector, and these individual detectors are coupled to amplifying circuits  22 ,  24 ,  26 , and  28  respectively. The output signals from the amplifying circuits are combined as illustrated in processors  30 ,  32 , and  34 , such that the output of processor  30  defines the up-down directional signal which is applied to the guidance control circuit (not shown) of the missile, and such that the output of processor  32  defines the right-left directional signal. The output of processor  34  defines the so-called “sum” channel which is used in some instances for normalizing the system, for controlling the sampling of the directional channels, and more specifically in the present case for providing a signal, the amplitude of which is used in changing the mode of operation of the seeker. 
   As illustrated, the returns from the target are imaged on the quad cell as illustrated at  36 . As shown, the image at least partially overlaps all of the detectors in the quad cell. The resultant output signals from processors  30 ,  32 , and  34  as a result of this image are illustrated at  38 ,  40 , and  42  respectively. 
   Under normal circumstances, the outputs of processors would be applied directly to the guidance and control system for the missile or ordinance device. However, the effective range of such a system can be increased by as much as 18% in a doublet pulse case by pulse integration. 
   Pulse integration, as mentioned hereinbefore, involves the superposition of adjacent pulses which are then added in a coherent manner. Although this may be accomplished in a number of ways, one such system may include a summing network  44  and a recirculating delay unit  46  interposed in the first channel. The length of the de lay for the recirculating delay unit, Γ, is set equal to the expected interpulse spacing ΔT, such that the first pulse which is applied through the summing network to the recirculating delay unit, is delayed by Γ and then returned via line  48  to the summing network where it is coherently added with the second pulse. It will be appreciated that this is a coherent process for the signal only. Thus, noise, adding incoherently, is discriminated against. The result of the summation is then passed through the recirculating delay unit and applied to a gate  50  whose operation will be described hereinafter. 
   The second channel is likewise provided with a summing network  52  and a recirculating delay unit  54  with a feedback line  56  and is identical in operation to the first channel system. The output of recirculating delay unit  54  is applied to a second gate  58  whose function is the same as that of gate  50  and will likewise be described hereinafter. 
   The output of sum channel processor  34  is applied to a summing network  60  and thence to a recirculating delay unit  62  which has a feedback path  64  back to the summing network. The output of recirculating delay unit  62  is applied to a level trigger  66  and a high level trigger and latch circuit  68  which has Q and {overscore (Q)} outputs. The output of level trigger  66  and the {overscore (Q)} output of level trigger  68  are applied to a two input terminal AND gate  70  which has its output coupled to one input of a two input terminal OR gate  72 . The Q output of level trigger  68  is applied to one input terminal of a two input terminal AND gate  74 , which has its output coupled to the other input terminal of OR gate  72 . 
   The output of sum channel processor  34  is also applied to an additional level trigger  76 , the output of which is applied to a doublet decoder  78  which produces an output pulse upon the detecting of pulse doublets having the expected inter-pulse spacing. The output of the doublet decoder is applied to the other input terminal of AND gate  74 . 
   In operation, a weak doublet signal arriving on boresight is enhanced in SNR by +3 db in the sum channel recirculating delay unit  62 . The output from recirculating delay unit  62  triggers level trigger  66  but is not large enough to trigger the high level trigger  68  which is set to a level corresponding to the desired crossover point. The output of level trigger  66  passes through AND gate  70 , OR gate  72  and gates the directional signals from recirculating delay units  46  and  54  to the guidance circuitry for the missile by virtue of actuating gates  50  and  58 . 
   On boresight, while the directional signals are zero, the sum channel in essence tells the system that the zero outputs from units  46  and  54  are valid directional commands. Off boresight the directional signals will have the appropriate amplitudes and polarities to indicate the angular error away from boresight. In the weak signal, extended range mode, the outputs of high level trigger  68  are Q=0 and {overscore (Q)}=1. This disables level trigger  76  by inhibiting AND gate  74  while enabling level trigger  66  by enabling AND gate  70 . 
   As the missile moves towards the target, the signal level increases until the high level trigger  68  is enabled. This sets the latch so that for the remainder of the mission Q =1 and {overscore (Q)}0. This disables level trigger  66  by inhibiting AND gate  70  and. enables level trigger  76  by enabling AND gate  74 , allowing the system to go into the doublet decode mode. 
   In this mode, the system has a much higher countermeasure resistance because the doublet decoder will accept as valid signals only those pulse pairs which have the precise spacing required to satisfy the code. The gating signals now proceed from decoder  78  through AND gate  74  and OR gate  72  to activate gates  50  and  58 . The delays in the recirculating delay lines and the doublet decoder are identical such that a doublet decoder pulse output coincides with a maximum SNR superimposed pulse pair from the recirculating delay units. 
   This means that upon decoding of the appropriate pair, gates  50  and  58  are opened at the right moment to allow the signals from delay units  46  and  54  to be applied to the follow-on guidance circuitry. 
   If high level jamming occurs when the system is in the low signal, extended range mode, the high level jamming signal would trigger the high level trigger  68 , immediately placing the system into the doublet decode or short range mode. In this mode random jamming would be rejected and the seeker would continue on a neutral guidance error until the signal level increased to a level sufficient to trigger level trigger  76 . Note, all of the level triggers are used to convert analog signals into digital signals for timing and switching purposes. 
   It is important to note that the overall delay through the summing networks and recirculating delay units in the first two channels, Γ, equal the overall delay through level trigger  76 , doublet decoder  78 , and the switching circuitry coupled thereto. 
   The advantages of the pulse integration described are as follows: The noise fluctuations in the vicinity of the two pulses in a doublet are usually independent because of the time separation. Noise, therefore, undergoes random phase addition in the summing network, causing the amplitude of the noise to increase by a factor of the square root of two. Therefore, the net change in signal-to-noise ratio is 2/√{square root over (2)}, or an improvement factor of the square root of two. Such an improvement in SNR translates to a 2 1/4  improvement in doublet laser target designation range performance, yielding an increase of 18% in lock-on range for the same laser target designator power, the same probability of detection and the same false alarm rate. 
   A typical system operating senario has been platted in FIG.  3 . Operation begins with initial system lock-on to the right of curve B. This is the extended range mode of operation. 
   The doublet decoder is not triggered because the signal does not have the benefit of the √{square root over (2 )}SNR enhancement and is, therefore, below threshold at this range. In the extended range mode the doublet decoder output is always zero. Hence the {overscore (Q)} output of trigger  68  is always at a logical one. The {overscore (Q)} output enables AND gate  70  which then accepts pulses from level trigger  66 , transfers them through OR gate  72  and enables gates  50  and  58 . This permits the outputs from the recirculating delay units to be sampled only as they appear. Gates  50  and  58  are closed when signals are not present, thereby rejecting noise between valid signals. 
   In the extended range mode, the system provides only a modest amount of interference rejection. When the seeker reaches the normalized range, 1.0, the signal at the doublet decoder has increased inversely as the square of range by a factor of the √{square root over (2)}, At this point, the doublet decoder begins to emit output pulses, assuming of course, that the correct code is received. The directional signals are now sampled only when valid signals are present. All incorrectly timed jamming is rejected automatically, and transfer to the doublet decoder is done automatically upon achieving sufficient signal strength. 
   As can be seen from the graphs of  FIG. 3 , there is significant advantage in the extended range guidance mode, since curve B defines a higher signal-to-noise ratio than curve A, which refers to operation without pulse integration. 
   Referring now to  FIG. 4 , doublet pulse integration can be alternatively accomplished by a unit diagrammatically shown as enclosed in dotted box  80 . The input to this unit is the signal from the preprocessing circuits described above. In this embodiment, the signal from the processor is applied to serial analog delay  82  which is clocked at a rate which determines the delay. These devices are available as SAD Model No. SAD 100 from Reticon Corporation. The output of the serial analog delay is applied to an amplifying circuit  84  and thence to a summation network  86 . The input signal to this unit is also applie over line  88  to the summing network such that the output of the serial analog delay is summed with the original signal. It will be appreciated that the first pulse of the doublet is delayed by the serial analog delay unit and reaches the summing network at the same time that the second pulse is directly delivered over line  88 . The amplitudes of the summing network input signals are equalized to provide maximum signal-to-noise enhancement. Of course, the first pulse is also applied directly to the summing unit  86 , but the gate to which this unit is attached is inactivated at this time and the first pulse is therefore inhibited. 
   Referring now to  FIG. 5 , a simplified doublet decoder is illustrated within dotted box  90  to include a delay line  92  and a two terminal AND gate  94 . The delay line is set such that its delay corresponds to the expected interpulse spacing. In this case, the first pulse is delayed and is applied to one input terminal of AND gate  94 , whereas the second pulse is applied over line  96  directly to the other input terminal of AND gate  94 . It will be appreciated that with the first pulse delayed, the first and second pulses will arrive at the input terminals to AND gate  94  simultaneously if the incoming doublet has the expected interpulse spacing. The output of AND gate  94  is a pulse which is generated simultaneously with the arrival of the output from the delay unit  92  and a signal on line  96 . It will be appreciated that the input and output of the doublet decoder are digital signals. 
   As mentioned hereinbefore, it is sometimes desirable to take the pulse integration circuits completely out of the loop when the seeker is operating in the doublet decode mode. One simplified circuit for doing this is illustrated in  FIG. 6  in which only one channel of the system is illustrated. Typically, this system might use bang-bang directional logic as illustrated. In this system the B detector of a quad cell  100  is amplified at  102  and applied to a terminal  106  of a single-pole, double throw switch or relay generally indicated at  106 . The switch has an output terminal  108  coupled to a summation network  110  which is, in turn, coupled to a recirculating delay unit  112 . The output of unit  112  is coupled through level trigger  114  to one terminal of a two terminal OR gate  116 , the output of which is coupled to one channel of the guidance system of a missile or ordinance device. The output of recirculating delay unit  112  is also coupled to a high level detector  118  which may be any kind of adaptive threshold device. This detector is, in turn, coupled to a relay or switch control unit  120  which controls the position of switch  106 . 
   Output terminal  128  is coupled through a level trigger  132  to a doublet decode unit  134 , with the level trigger and doublet decode units similar to that described hereinbefore. The output of the doublet decode unit is applied to one input terminal of a two input terminal OR gate  116 , the output of which feeds the systems bang-bang guidance logic. 
   In operation, in the extended mode range, the switch  106  is in the position illustrated. In this position the output signals from the channel B detector are applied through the recirculating delay unit  112  to level trigger  114  which feeds the bang-bang guidance logic via OR gate  116 . 
   When the level of the signal at the output of recirculating delay unit  112  reaches a predetermined level, high level trigger  118  is enabled and relay control latch unit  120  repositions switch  106  downwardly. Simultaneously, output terminal  128  is connected to the output of the channel B output and doublet decoding begins. The output of the doublet decoder feeds the bang-bang guidance logic via OR gate  116 . 
   As will be seen in the doublet decode mode, the output of the channel B detector is fed directly through to the guidance system. Thus, in the doublet decode mode there is notpulse integration.