Patent Publication Number: US-6222479-B1

Title: Process and apparatus for finding stealthcraft

Description:
This is a division of application Ser. No. 07/714,328, filed Jun. 11, 1991, now U.S. Pat. No. 5,990,822 which is a continuation of U.S. Pat. application Ser. No. 07/338,975 filed Apr. 14, 1989 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to radar and sonar and more particularly to finding both conventional and stealth-modified targets by these means. 
     Radar and sonar equipment and techniques are well developed and widely used both militarily and otherwise, and the literature on these subjects abounds. Skolnik, for example, lists over one thousand references in his  Introduction to Radar Systems,  and his  Radar Handbook  covers the subject in even more detail. 
     Military and non-military applications differ, however, because enemy targets are attacked when detected and so best survive when they are hard to find, and are modified to make them so, while non-military targets survive best when they are deleted before there is any risk of collision, and are modified accordingly. While the most refined techniques for making military targets stealthy and the techniques for finding them despite stealth presumably have not been made public, it is clear that, at least with respect to detection of stealthcraft with prior art radars or sonars, this task becomes more difficult when these targets are absorbers rather than reflectors of the beams transmitted to find them, and this task has been made so, at least in part, by the conventional techniques used to suppress background returns. 
     SUMMARY OF THE INVENTION 
     According to the present invention, I have developed radar and sonar systems that are indifferent to whether the objects to be found are absorbers, reflectors or refractors of the beams transmitted to detect them. 
     According to one preferred embodiment of the invention, a dual-beam radar that can illuminate a reflective background such as earth, the sea, clouds, or the ionosphere is fitted with narrow transmit-beam antennas to radar-illuminate that background, receivers that monitor background illumination, processors responsive to local change in that illumination, and a display that highlights those changes. Thus, when a beam from this radar is absorbed, reflected, or refracted by a target, the resulting dark path in the background illumination pattern is displayed as a target. 
     In this dual-beam embodiment, targets are ranged by triangulation, and one important feature of this invention is a search process in which one beam follows conventional search patterns, and the second intersects with and scans the useful length of the first. 
     Another important feature of this invention is the way in which target-produced background illumination changes are distinguished from clutter. With conventional radars, the reflections from targets are the returns of interest and the returns from backgrounds are considered clutter, so that stealth-modified targets can hide in chaff or operate in such a way that their returns blend in with those from background, and so avoid detection. In this invention, however, the returns of interest are distinguished from clutter by the order in which they were transmitted, so that these target-masking techniques are ineffective here. 
     Yet another important feature of the dual-beam embodiment of this invention is a “false alarm” reduction process that does not compromise sensitivity. 
     This invention is also served by single beam embodiments, and in one such preferred embodiment, a transmitter with a broader beam antenna is dispatched to the far side of the target, the receiver tracking the silhouette of the target in that beam. 
     These and other features, modifications, and advantages of the radar systems of the present invention are applicable to sonar systems as well, and both radar and sonar embodiments of the present invention will be more fully described with reference to the annexed drawings of the presently preferred embodiments and some of the applications thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a block diagram of the preferred dual-beam radars and sonars in accordance with the present invention; 
     FIG. 2 is a pictorial representation of a searchcraft using the radar embodiment of FIG. 1 to detect and range stealthcraft; 
     FIG. 3 is a pictorial representation of some of the radar applications of the present invention that can be served by a single beam; 
     FIG. 4 is a block diagram of the preferred single-beam radar and sonar embodiments of the present invention for use aboard missiles; and 
     FIG. 5 is a pictorial representation of some of the sonar applications of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring initially to FIG. 1 in the drawings, the embodiment of the block diagram, generally designated  100 , is the preferred radar or sonar embodiment of this invention, having a transmit portion  110 , a conventional receive portion  120 , a display  130 , and a stealthfinder portion  140 . As is clear from the drawing, each of the portions  110 ,  120 , and  140  is dual, so that there are, using a radar embodiment as example, two conventional narrow beam transmit antennas  111 A and  111 B, each driven by its own conventional transmitter  115 A or  115 B, each transmitter being modulated by its own conventional modulator  116 A and  116 B respectively. The modulators  116 A and  116 B are driven by transmit signal encoder  117 , the purpose of which will be explained in greater detail later herein. 
     Each transmit antenna  111 A and  111 B has a drive mount that it shares with a conventional receive antenna, so that transmit antenna  111 A and receive antenna  121 A share drive mount  112 A as suggested by the dashed lines connecting these elements, and transmit antenna  111 B and receive antenna  121 B share drive mount  112 B as confirmed by the dashed lines connecting those elements. 
     Antenna drive mounts  112 A and  112 B orient their respective antennas in accordance with signals from antenna drive coordinator  114 , the actual orientation of the respective transmit antennas  111 A and  111 B being reported to antenna drive coordinator  114  by antenna orientation monitors  113 A and  113 B respectively, so that errors due to wind, drive train irregularities, etc., can be eliminated. 
     The signals transmitted by transmit antenna  111 A that are reflected by targets, backgrounds, clutter, chaff, etc., are collected by receive antenna  121 A, amplified by conventional low noise amplifier  125 A, processed in a conventional way to change from RF to IF frequency and to suppress noise and clutter by first detector and noise and clutter suppression circuitry  126 A which, if the radar were ground-based or mounted on a slow-moving platform such as a ship or a blimp, and the targets were faster-moving aircraft, could be Doppler circuitry. These signals are then processed by second detector  127 A, and made available for presentation by display  130 . 
     Doppler processing is, however, inappropriate for radar moving at approximately stealthcraft velocity for obvious reasons, and matched filters are better noise suppressors for such applications. Where Doppler processing is inappropriate, background clutter can be eliminated by circuitry (corresponding to the dotted paths in FIG. 1) that introduces a replica of the output from transmit signal encoder  117  into block  127 A, where it is stored temporarily for purposes of comparison with corresponding portions of the signals received. Because flying targets are closer to transmit antenna  111 A than the patch of background behind them, the signals returned from these targets will clearly be out of sequence (early) with respect to those from the background, and background clutter is eliminated by deleting in-sequence returns from the signals made available for further processing or display. (For purposes of this invention, “in-sequence” returns are those that, allowing for lost pulses, are returned in the sequence that would have been expected had the background been the only reflector, and sequence recognition is facilitated by transmit signal encoder  117 , the output of which sequence-codes transmission, say by changing the length, phase, spacing or grouping of pulses, or by modulating the carrier with respect to amplitude, frequency, etc.) 
     The returns from the signals transmitted by transmit antenna  111 B and collected by receive antenna  121 B are similarly processed by the corresponding blocks bearing the “B” suffix, so that the target-returned portions of these signals, which are the outputs of blocks  127 A and  127 B, can be made available for further processing and display. The signals from blocks  127 A and  127 B are also fed to conventional target confirmation processor  128 , as are those from transmit signal encoder  117 , antenna drive coordinator  114 , and, via block  114 , also those from searchcraft monitor  118 . 
     In conventional single-beam radars and in the “A” suffixed and “B” suffixed portions of the FIG. 1 embodiment of this invention as well, noise can be mistaken for target returns, and this “false alarm” problem, familiar from the literature and explained in Skolnik, has been previously addressed by raising threshold levels and/or by ignoring fewer than some arbitrary number of “hits”. While the “false alarm” rate is reduced by these techniques, so too is the ability to find small and/or distant targets. In the embodiment of FIG. 1, however, the “false alarm” problem is addressed without the compromises of the prior art. 
     In this invention, the preferred dual-beam search pattern is one in which both beams intersect, and in which one beam, say that from antenna  111 B, scans the length of the other, that from antenna  111 A (excepting, of course, an arbitrarily small portion of the beam from antenna  111 A nearest that antenna). For this search pattern, the point of intersection is known a priori from the orientation of the antennas, and in the preferred embodiment the transmitted beams are modulated in accordance with instructions from transmit signal encoder  117  not only to accommodate elimination of background returns as explained earlier herein, but also to identify returns that result when the beams impinge upon a target at a point of intersection. Conventional target confirmation processor, block  128 , confirms targets on the basis of corresponding returns from both beams, and because the probability of “false alarms” on this basis is low, sensitivity and its advantages with respect to target size and range can be preserved. 
     In the embodiment of FIG. 1, block  18  is fed information from blocks  114  and  117 , and stores that information long enough to compare it with outputs of blocks  127 A and  127 B to confirm that these outputs are from returns from a point of intersection of the beams and thus a (conventional) target. Returns that do not correspond to points of intersection of the beams can either be assumed to be “false alarms”, or be used to refine the search pattern. 
     This confirmation process requires not only that returns from each of the beams is correctly identified by the corresponding portion of the receiver, but also that background patches simultaneously illuminated by both beams are not mistaken for targets. The first of these problems is preferably addressed by operating each transmitter at a distinguishably different frequency, and the second by ignoring in block  128  the in-sequence returns that were not deleted earlier (by Doppler processing, or by ignoring in-sequence returns in blocks  127 A and  127 B as explained. 
     The outputs of low noise amplifiers  125 A and  125 B are also fed to stealthcraft detection and ranging portion  140  where, with reference to the “A” suffixed blocks, the output of low noise amplifier  125 A is changed in frequency by RF-to-IF first detector  142 A, cleaned up with respect to noise by noise suppression circuitry  144 A which is, for example, a matched filter, and converted to a form suitable for further processing and display by second detector  146 A. Alternately, if block  126 A processes signals exactly as described for blocks  142 A, and  144 A, the output of block  126 A can be fed directly to block  146 A, and blocks  142 A and  144 A omitted. In either case, output from the second detector is fed to background image comparitor  148 A. The signals from low noise amplifier  125 B are, of course, similarly processed by the “B” suffixed blocks. 
     The frequencies of interest for the radars and sonars that detect and range stealthcraft in accordance with this invention are those for which the respective backgrounds, earth, water, water vapor (clouds), the ionosphere, sea bottoms, etc. are reflective. When the radar or sonar beams that illuminate these backgrounds also illuminate targets, these beams are at least partially absorbed, reflected, or refracted and, for beams narrow enough or targets large enough, dark patches corresponding to the positions of these targets will appear in the background reflection pattern. Because these dark patches will appear whether the transmitted beams are absorbed, reflected, or refracted by targets, it becomes a matter of indifference whether these targets are stealth-modified or conventional, provided, of course, that reflections from targets are not confused with those coming from the background. 
     These dark patches are highlighted for further processing or display by background image processors  148 A and  148 B that are, at least in part, digital image processing computers like those in conventional airborne mapping equipment such as digital synthetic aperture radars (SAR) that produce maps of the background that can be processed further, or made available for display “as is”. Further processing is preferred here, however, because these dark patches may be hard to find in the “raw” images. 
     The first step in this further processing is the elimination of the returns from targets, chaff, etc. that, because they arrive out of sequence (early) with respect to returns from backgrounds, can be identified by comparing suitably delayed replicas of the signals transmitted with those returned. In the embodiment of FIG. 1, transmit signal encoder  117  information is supplied to blocks  148 A and  148 B, where it is stored until the returns processed by blocks  146 A and  146 B can be compared with replicas of those transmitted, and the parts of the block  148 A and  148 B images due to out-of-sequence returns removed. These “sanitized” background images are then compared on a sequential basis so that the common features of consecutive images can be suppressed, and the target-produced dark patches highlighted for further processing or display. (While image processing to highlight changes in successive images is not normally a part of conventional mapping radar, such processing and the computer programs needed to do so are known from the art of deep space exploration, where transmissions from imaging satellites are, for practical reasons, limited to differences from previously transmitted images.) In practicing this invention, the basis for clutter removal, at least with respect to stealthcraft detection, is distinguishing between in-sequence and out-of-sequence returns, and is preferably facilitated by feeding information from searchcraft orientation monitor  118  to antenna drive coordinator  114  so that beams can follow prescribed search patterns despite pitch, roll, or yaw of the searchcraft. 
     The output of blocks  148 A and  148 B intended for further processing is that in which dark patches are highlighted, and this output, as well as the “raw” or “sanitized” images from these blocks, is fed to display  130  by switching devices that are familiar from the prior art and are assumed to be part of the embodiment, but have been omitted from the drawing. 
     Stealthcraft are ranged and their detection confirmed by blocks  147  and  149  of FIG. 1, but the process is best explained with reference first to FIG. 2, in which a searchcraft  260  is shown finding and ranging a first stealthcraft  270  below, and also a second stealthcraft  275  above. 
     With respect to stealthcraft  270  below, transmit antenna  211 A, receive antenna  221 A, and their common drive mount  212 A (corresponding to  111 A,  121 A, and  112 A of FIG. 1) are mounted behind beam-transparent shield  261 A which, for radar, is a radome. Similarly, transmit antenna  211 B, receive antenna  221 B, and their common drive mount  212 B are mounted behind radome  261 B. Baseline  262  is the line connecting the effective pivot point of transmit antenna  211 A with that of transmit antenna  211 B. (These pivot points are the actual pivot points for antennas that are mechanically steered, and their effective equivalents for antennas that are purely electronically or both electronically and mechanically steered.) Angle  263  is the angle that baseline  262  makes with respect to a parallel to the horizon in the plane of the triangles that, as will be explained, are used to calculate range. 
     Recalling that in the preferred search mode one transmitted beam, say beam  264 B from transmit antenna  211 B, sweeps the length of and intersects with beam  264 A from transmit antenna  211 A, these beams are shown intersecting at and being absorbed by stealthcraft  270 , so that when these beams are so oriented, beam  264 B leaves one dark patch  271 B in the radar illumination pattern of sea surface  273 , beam  264 A another,  271 A, each dark patch being highlighted by the corresponding background image comparitor  148 A or  148 B of FIG.  1 . 
     The transmitted beams intersect at the stealthcraft as shown, and the portion of these beams above stealthcraft  270  plus baseline  262  form one triangle, while the extension of these beams from their point of intersection to the sea surface plus background baseline  272 , the distance between dark patches  271 A and  271 B, form another. These triangles and the known altitude of the searchcraft are used to calculate the range, bearing, and altitude of the stealth-modified target  270 . 
     For the first triangle, the angles  265 A and  265 B that the beams  264 A and  264 B make with baseline  262  are known because they are set by antenna drive coordinator  114 , so that the distance from the target to each pivot point as well as the height of the searchcraft with respect to the target can be found by trigonometry. The target altitude is found by subtracting the height of the searchcraft with respect to the target from the altitude of the searchcraft itself, and the target bearing directly from angles  265 A and  265 B. 
     Target range and altitude can also be calculated from the second triangle, recognizing that angle  274 B is the difference between angles  265 B and  263 , that angle  274 A is the sum of angles  265 A and  263 , and that background baseline  272  can be calculated from angles  265 A and  265 B and the altitude and orientation of the searchcraft. In the preferred embodiment, antenna baseline rangefinder  147  of FIG. 1 calculates target range from the triangle that includes baseline  262  of FIG.  2 . 
     Returning now to FIG. 1, antenna drive coordinator  114  feeds not only antenna orientation data corresponding to the FIG. 2 angles  265 A,  265 B to antenna baseline rangefinder  147 , but also the FIG. 2 angle  263  and the searchcraft altitude, the last-mentioned angle as well as the searchcraft altitude having been supplied to block  114  by searchcraft monitor  118 . 
     Antenna baseline rangefinder  147 , a computer (or portion thereof), also receives and temporarily stores replicas of signals from transmit signal encoder  117 , compares them with those from detectors  146 A and  146 B, and, for reasons that have been explained earlier herein, suppresses those that are out of sequence. It also looks for a loss of signal, say because transmit beam  264 A of FIG. 2 has been absorbed by stealthcraft  270 , “memorizes” the value of angle  265 A supplied by antenna drive coordinator  114  corresponding to that loss of signal, looks for the loss of signal from transmit beam  264 B that corresponds to a point of intersection of the beams and that confirms target detection, “memorizes” the angle  265 B, and calculates the range, bearing, and altitude of the target as explained, making this information available to display  130 , and then resets to repeat the process. Recalling that in the preferred scan pattern the beams intersect, and that the second beam scans the length of the first, the second beam must be interrupted within a time corresponding to one full scan of the first to confirm a target. If there is no confirmation during that time, the computer is reset, and the “memorized” data for the first dark patch is either used as a basis for refining the search pattern or ignored. 
     Searchcraft  260  of FIG. 2 is also shown finding and ranging a stealthcraft  275  above, and now familiar are beam-transparent shields  266 A and  266 B, beams  268 A and  268 B making angles  269 A and  269 B with baseline  267  respectively, the interruptions of these beams resulting in dark patches  276 A and  276 B on reflective background  278  that, depending upon operating frequencies and conditions, is the ionosphere, clouds, a man-made reflector such as chaff, etc. Also familiar are the angles  279 A and  279 B that the extensions of beams  268 A and  268 B make with background baseline  277 . Here, however, the beams are shown having a measurable width, so that for targets small enough, or at ranges great enough, beams will be wide enough to illuminate backgrounds and compromise detection despite absorption of part of the beam by the stealthcraft. 
     Skolnik mentions a lowest practical beam angle of 0.4 degrees for conventional radar frequencies and antennas, and with such equipment stealth-modified flightcraft can be reliably detected if distances no greater than a few miles. While this range limit can be addressed by increasing the number of searchcraft and decreasing the spacing between them, the preferred approach is that of changing frequencies from those of conventional radar to those at which narrower beam antennas can be routinely fabricated, or to even higher frequencies where transmitters and antennas are replaced by lasers, and receive circuitry is modified accordingly. This high frequency circuitry is mentioned in Skolnik and described in detail in the references cited herein. 
     Beam spreading, of course, imposes no practical range limit upon laser radar, and such devices can be effective even from space. Laser radars can, however, fail to detect targets that monitor scan patterns and adjust flight paths accordingly because, as noted in Skolnik, beam coverage and/or scan repetition rates are compromised by extremely narrow beams. The coverage problem can, however, be addressed without compromising the range advantage by artificially broadening the beam, say by bouncing it off a vibrating mirror, or preferably by clustering lasers of different frequencies, the beams of which are preset to deviate from parallel just enough so that the distance between beams is somewhat less than the smallest principal dimension of the targets when the targets are at their maximum expected ranges, or adjusted to this deviation from parallel as a variable of operation. 
     Returning now to FIG. 2, the relative sizes and proximities of searchcraft, stealthcraft, and antenna baselines have been exaggerated for purposes of clarity of illustration, the actual beams, say those corresponding to  264 A and  264 B, becoming almost parallel at ranges of just a few miles, so that small errors in measuring angles results in the large errors in range unless the antennas of this invention, shown separated by the fuselage length of the searchcraft in the drawing, are actually separated by distances that are realizable only on large ships, or on land. When the errors due to this near-parallelism become intolerable, range is preferably determined by applying the laws of similar triangles. 
     In FIG. 2, for example, two such triangles are those that meet at stealthcraft  270 , the upper triangle being made similar to the lower by substituting for line  262  line  259 , the line that originates at the pivot point for beam  264 B, terminates at beam  264 A, and is parallel to the horizon and to line  272 . The length of line  269  can be calculated by trigonometry because side  262  and the angles  263  and  265 B are known, and because this calculation is less sensitive to small errors in measuring angles, the range calculation is less sensitive as well, so that for a first approximation when searchcraft  260  is in level flight over level terrain, line  269  is reasonably approximated by baseline  262 . 
     Returning again to FIG. 1, and recalling that transmitter  115 A is preferably operated at a frequency distinguishably different from that of transmitter  115 B, it becomes clear that the details of dark patch  271 A that are available from background image comparitor  148 A are not available from background image comparitor  148 B, nor vice versa with respect to dark patch  271 B, and that these separate images must be combined to learn the details of background baseline  272  needed to calculate the range of stealthcraft  270 . 
     In the preferred embodiment of FIG. 1, these images, available separately from background image comparitors  148 A and  148 B, are combined in proper registration by background image combiner  149 , preferably a computer or portion thereof that effects registration on the basis of common image portions that are the result of beams illuminating common background patches. This patch commonality is confirmed on the basis of information supplied to background image combiner  149  by antenna orientation coordinator  114 , transmit signal encoder  117 , and, of course, background image comparitors  148 A and  148 B. 
     This combined image provides the additional information, the length of background baseline  272 , needed to calculate the range of stealthcraft  270  by trigonometry from the triangle including that baseline, by the laws of similar triangles, or both. This range data is calculated by background image comparitor  149 , and is made available for presentation by display  130 , as are the combined background images. Here too, returns processed by the “A” suffixed stealthfinder blocks of FIG. 1 but not confirmed by the “B” blocks are either used to refine the search pattern or ignored. 
     As FIG. 1 makes clear, the preferred radar and sonar embodiments of this invention are dual with respect to transmit and receive portions, and are so because, using FIG. 2 as example, stealthcraft  270  anywhere along beam  264 A would produce the same dark patch  271 A, so that the only universally viable option for targets that absorb beams and cannot be ranged by timing reflections is triangulation, and triangulation, of course, requires tow beams. There are, however, special situations that can be served by a single beam because stealthcraft altitude or course and speed are known, because targets move so slowly that successive scans by a moving searchcraft accommodate triangulation, or because, in the particular application, actual range is a matter of indifference. Some of these special situations are shown in FIG. 3, the radars for which might, for example, be embodiments of FIG. 1 in which the beams are steered independently, newly designed FIG. 4 embodiments, or conventional radar reworked to include at least the additional blocks  142 A,  144 A,  146 A, and  148 A of FIG. 1, or their functional equivalents. 
     With reference now to FIG. 3, one such stealth-modified target is ship  370 , appearing as a dark patch in the background image on the screen of the stealthfinder radar aboard searchcraft  360 . Because ship  370  is on the surface of the sea, its altitude is obviously known, and its range can be calculated from the right triangle that includes the altitude of the searchcraft, the target range, and the angle between. 
     Once found, ship  370  becomes the target for two weapons of destruction, one a sea-skimming stealth-modified cruise missile  373 , and the other a stealth-modified missile  375  launched from searchcraft  360 . For missile  375 , the range to its target, ship  370 , is a matter of indifference, and a single-beam radar in accordance with this invention can be used to steer missile  375  to the dark patch in the sea-glint pattern that is, of course, ship  370 . 
     Missile  375 , however, is a target the destruction of which is of prime importance to ship  370 , and even if that ship did not carry stealthfinder radar in accordance with this invention, it could certainly detect radar transmissions from the missile and attack it on that basis, so that the homing radar of missile  375  is preferably operated as if it were the receiver portion of a bistatic pair, the transmit portion being the transmitter(s) of searchcraft  360 . 
     Stealth maintenance is somewhat more complicated for missile  373 , however, because at sea-skimming altitudes the dark patch in the sea glint produced by the transmitter(s) of searchcraft  360  that is the stealth-modified ship  370  might not be discernable as such by the receive equipment aboard missile  373 , and at altitudes at which these dark patches are so discernable, the stealth advantage inherent in a sea-skimming approach is lost. In the embodiment of FIG. 3, the benefits of both bistatic radar and the sea-skimming approach are preserved with respect to stealth missile  373  by radar flare  381  that is a broad-beam transmitter suspended from parachute  382  as shown, and is buoyant or is fitted with a device to make it so. 
     In the battle scene of FIG. 3, ship  370  is shown protected by stealth-modified blimp  385  that can effectively detect and range missile  373  with a single-beam stealthfinder radar because that missile cruises at approximately sea level. Blimp  385  itself, however, can be both detected and ranged by a single beam from the stealthfinder radar aboard searchcraft  360  despite the fact that the blimp is not at sea level, because the blimp moves so slowly with respect to searchcraft  360  that it appears to remain stationary while a baseline  362  that can be used for rangefinding by triangulation is generated by the motion of the searchcraft. 
     While the detection and ranging of stealth-modified blimp  385  using a single beam from the radar aboard searchcraft  360  has been explained, the preferred radar aboard that searchcraft is, of course, the dual-beam embodiment of FIG. 1 that also accommodates detection and ranging as explained for the embodiment of FIG.  2 . When the searchcraft is a satellite and the targets are on or near the surface of the earth, however, the longest baseline aboard the searchcraft may be too short for effective range calculation even with dual beam laser radar, and the motionally generated baseline corresponding to  362  of FIG. 3 is an effective alternative. 
     Single beams, say the beam  264 A of FIG. 2, can also be used to track targets previously detected and ranged by dual beams. 
     Stealthcraft maneuverability is limited not only by the design of these craft, but also because certain maneuvers reveal target positions in other ways, so that pilots believing themselves to be invisible to radar will maintain a course and speed that can be confirmed by adjusting searchcraft antenna orientation and transmission in such a way that a predetermined portion of the transmission, say a group of pulses, will either be absorbed or returned out of sequence if the target is where expected. 
     The radar embodiment of FIG. 4 is the preferred radar for the missiles  373  and  375  of FIG. 3, and its sonar embodiment is the preferred sonar for the torpedo-like missiles  573  and  575  of FIG.  5 . Most of the blocks of FIG. 4 are now familiar from FIG.  1 . 
     Thus, assuming the embodiment of FIG. 4 to be a radar,  411  is the antenna,  415  the transmitter,  416  the modulator,  425  the low noise amplifier,  426  the first detector and noise filter and  427  the second detector, these blocks comprising a conventional radar much like the corresponding “A” suffixed blocks of FIG.  1 . The transmit and receive portions of FIG. 4 are shown sharing a common antenna, however, and duplexer  419  has been introduced to protect the receive elements from the damaging effects of the high output levels of the transmitter. 
     The blocks  442 ,  444 ,  446 , and  448  are also FIG. 4 counterparts of FIG. 1, corresponding to FIG. 1 blocks  142 A,  144 A,  146 A, and  148 A respectively, so that the embodiment of FIG. 4 is substantially half of an embodiment of FIG.  1 . Because the embodiments of FIG. 4 are single-beam radars or sonars that guide missiles to their targets, however, blocks corresponding to  128 ,  147  and  149  of FIG. 1 have been omitted as has the display, and the outputs of blocks  427  and  448  are fed directly to missile flightpath controller  435 . Controller  435  also steers antenna drive mount  412  so that the antenna can scan the target and its background, and the radar can supply course correction information to missile flightpath controller  435 . 
     The radar and sonar embodiments of FIG. 4 preferably have both conventional and stealth capability as do those of FIG. 1 as shown, not only because the exact nature of a target cannot be known a priori, or because, at the present stage of development, stealth-modified targets are still measurably reflective, but also because some applications will be best served on the basis of target reflections. These FIG. 4 embodiments also include transmitters, the returns from which are one way to home in on stealth-modified targets, and, in this mode of operation, background returns are distinguished from those from clutter by comparing returns with suitably delayed signals from modulator  416 , the block corresponding to transmit signal encoder  117  of FIG. 1 having been omitted from FIG.  4 . While the signals from transmitter  415  can be used to home in on targets as explained, they can also be used by targets to find, track, and kill missiles, so that the preferred mode of operation of the FIG. 4 embodiments is bistatic, the transmitters being rendered inoperative, say by a switch (not shown), the missiles homing in on conventional target reflections from transmitters mounted elsewhere, or on target-produced dark patches or silhouettes from such transmitters, as explained in the descriptions of FIGS. 3 and 5. 
     The radar techniques used to find and track targets on or above the sea and illustrated by FIGS. 2 and 3 are, of course, directly applicable to targets such as tanks, helicopters, terrain-hugging stealthcraft, etc. on or above land, but are not as directly applicable to underwater targets such as stealth-modified submarine  570  of FIG. 5, because the electromagnetic waves used to detect and range targets in air do not propagate well in water, and acoustic waves and sonar techniques are used instead. 
     Submarines, once quite detectable with active or passive sonars because they were noisy, acoustically reflective, and limited with respect to drive depth and range, have been so improved that they can now cruise under arctic ice and under thermoclines, hide in sea canyons, etc., and are not only quiet at rest or in motion, but have also been modified to absorb rather than reflect the acoustic beams intended to find them. Again, however, like the radar targets previously described, the bettery thay are at absorbing the beams intended to find them, the more vulnerable they are to the stealthcraft detection and ranging processes of this invention. 
     In FIG. 5, a stealth-modified submarine  570  is shown being detected and ranged by one dual-beam sonar embodiment of FIG. 1 on the sea floor, and another on a towed paravane  560 , the towline and towcraft for which have been omitted from the drawing. The sonar analogs of radar antennas are transducers, and the transducers for the sea-floor-based sonar of FIG. 5 are mounted on sea-floor transducer mounts  512 A and  512 B that are the sonar analogs of antenna mounts  112 A and  112 B of FIG.  1 . 
     In FIG. 5, the transducers on transducer mounts  512 A and  512 B produce the beams  568 A and  568 B that acoustically illuminate reflective background  578  that here is a density anomaly resulting from say a change in temperature or salinity, but might alternately be arctic ice, etc. These beams impinge upon and are absorbed by target  570 , the result being the dark patches  576 A and  576 B shown. Again, target bearing and range is calculated by triangulation, here from the triangle that includes baseline  567 , the distance between the effective pivot points of the beams, and the angles  569 A and  569 B that beams  568 A and  568 B make with this baseline, from the triangle that includes baseline  577  that is the distance between dark patches  576 A and  576 B, or by applying the laws of similarity to this pair of triangles. Bearing and range are similarly calculated from the paravane beams  564 A and  564 B that acoustically illuminate the sea floor. Here too, these beams, like their radar analogs, must be narrow enough so that targets can be detected by changes in background illumination, and beam frequencies must be those for which these backgrounds are reflective, so that some sonar applications will, like their radar analogs, be best served in these respects by mechanically, electronically, or both mechanically and electronically steered distributed transducer arrays. 
     The sonars of FIG. 5 that have been described thus far are the analogs of the radars of FIG. 2, and, like these radars, betray their presence by the beams that they transmit. In FIG. 3, however, the missiles  373  and  375  maintain stealth because they are parts of bistatic radar systems the transmitters for which are located elsewhere, and their FIG. 5 analogs, the torpedo-like anti-ship/anti-submarine missiles  573  and  575  maintain stealth in the same way. Thus missile  575  is shown homing in on submarine  570  silhouetted by illumination of background  578  by the sea-floor-based sonar, and missile  573  homes in on the same target that is the silhouette in the illumination pattern produced by sonar “flare”  581  that is preferably an acoustic radiator operating at the same frequency as the sonar receiver aboard missile  573 , but might alternately be a collection of charges preferably detonated underwater for this or other purposes. Missiles  573  and  575  can, of course, also be targeted against surface vessels, and workers of ordinary skill in the art will recognize that the appropriate reflective background for such targets is the air-water interface, and that the sonar flare or its equivalent must be at or near that interface. These workers will also recognize that differences between radars and sonars familiar from the prior art have been omitted from the descriptions herein, and will address these differences using known techniques when practicing this invention. 
     The new and novel features of this invention have now been disclosed, and two presently preferred embodiments, one a dual-beam configuration for detecting and ranging both stealth-modified and targets, and the other a single-beam configuration for special applications have been described, along with some of their applications. These descriptions are intended to be illustrative of the present invention, and not to limit its scope. 
     Thus the term “radar” as used here is intended to cover all those portions of the electromagnetic spectrum to which the new and novel features of this invention can be applied, including not only those in or near the visible portions of that spectrum and for which transmit and receive “antennas” and their associated circuitry are quite different from the parabolic reflectors and their associated circuitry implied by the antenna symbols in FIGS. 1 and 4, but also radar alternatives to sonar (perhaps using blue-green lasers, the beams from which are known to penetrate sea water and are presently being considered for use as a communication link between aircraft and submarines). By analogy, this disclosure also applies to sonar-like applications in which air is the wave propagation medium. Workers of ordinary skill in the art will recognize the changes in circuitry needed to serve these applications, and will effect these changes when practicing this invention.