Abstract:
According to typical practice of an inventive radar system, a switching device is capable of activating a receiver array one at a time so that when a receiver is activated the remaining receivers are inactivated. A switch control circuit is pre-programmed with control logic that is based on the counting of radio pulses that are emitted by a signal generator (for transmission by a transmitter). The control logic dictates, via the switching device, the rapid sequential cycling through of the arrayed receivers so that each receiver is activated for the same prescribed period of time, which corresponds to a pre-programmed number N of emitted radio pulses wherein N=[the number of frequencies in the wave table]×[the number of pulse integrations in the wave table]×[1 polarization or 2 polarizations]. Radio pulse input from the receivers is interleaved in a manner associable with individual receivers.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. provisional application No. 60/672,561, filing date 13 Apr. 2005, hereby incorporated herein by reference, entitled “Method And Apparatus Using Fast Electronic Switching For Multi-Channelizing a Single-Channel Radar System,” joint inventors Jerry Rosson Smith, Jr., Donald G. Morgan and Paul E. Ransom. 

   STATEMENT OF GOVERNMENT INTEREST 
   The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to radar, more particularly to apparatuses and methods that employ one or more radar receiver channels for effecting monostatic or bistatic radar. 
   Both single-channel and multi-channel radar systems have been used effectively for a variety of applications. The term “radar” is acronymous for “radio detection and ranging.” A single-channel radar system describes a single conveyance path of radio signals from a radar receiver element to a radar data storage device. Many applications require plural (e.g., multiple) radar receiver elements; typically, the radar receiver elements are selectively arranged as a “radar array.” Radar arrays have traditionally been designed from the start with multiple receiver channels in parallel, wherein each receiver channel describes a separate conveyance path of radio signals from a radar receiver element to a radar data storage device. These multi-channel radar systems are usually large and expensive due to the duplication of the receiver components (e.g., the signal conditioners and amplifiers that are connected to a receiver horn antenna). Until the present, the modification of a single channel radar system into a radar array system has been problematical and has not been favored practice. 
   SUMMARY OF THE INVENTION 
   In view of the foregoing, it is an object of the present invention to provide a practical and economical radar array system that utilizes a single radar receiver channel. 
   The present invention uniquely combines a switching circuit with a single-channel radar device (“core radar system”) so as to, in effect, convert a single-channel radar system to a plural-channel (e.g., multi-channel) radar system. The present invention can be practiced for either bistatic or monostatic radar systems. Inventive practice typically provides (for use in association with a single-channel radar device, single-polarized or dual-polarized) a fast electronic switching system that is combined with a radar receiver array. As often practiced, the inventive methodology constructs a radar array system from a single-channel core radar system by introducing quick switching between receiver array horns, the quick switch being effected by means of counting range gate signals (transmitter pulses). Thus, a single-channel radar system that is capable of implementing one receiver element at a time is inventively modified into a multi-channel radar system that is capable of implementing plural (e.g., an array of many) receiver elements one at a time but effectively at the same time. 
   Featured by the present invention is a switch control logic circuit, and the direction thereby of switches, to cycle through the receiver elements. The front-end receiver apparatus functions as a radar array but plugs directly into a single-channel radar device (“core radar system”). Furthermore, the inventive cycling through the receiver elements is typically characterized by great rapidity. The inventive switch control means electronically switches through the receiver array elements fast enough to permit clear imaging in post-processing. An analogy can be drawn to the sleight of hand of a dexterous playing card trickster, whose “hand is quicker than the eye”; in a sense, through its celerity and adroitness, the present invention “tricks” a single-input, single-output radar system into becoming a single-input, plural-output (e.g., multiple-output) radar system. Although the receiver array elements are actually sending signals sequentially rather than simultaneously, they are doing so at such a rapid rate that effective simultaneity of the respective signals is achieved. 
   According to typical embodiments of the present invention, an array of receiver elements feeds into a bank of fast electronic switches. The electronic switches control as to which receiver element is “open.” The electronic switches are alternatively opened and closed based on a switch control logic circuit, which counts each individual radar pulse from the transmitter; each such transmitted radar “pulse” is also referred to herein as the “range gate signal” or “transmitter output signal.” Once a predetermined number of pulses is encountered (wherein the number of pulses equals the number of integrations times the number of frequency steps; or, wherein the number of pulses equals the number of integrations times the number of frequency steps times the number of polarizations), the switch control logic circuit closes the current receiver path and opens the next consecutive receiver path. This switching, which occurs very quickly (typically within 800 nanoseconds), allows the individual elements of the radar array to be individually observed and fed into the single channel of the radar system and subsequently stored in the data file. The observations from the array elements are interleaved in the data file. In data translation, this interleaved data can be extracted into sequential data files for each element, thereby simplifying the post-processing data analysis. 
   A typical inventive radar array system comprises a switch control logic circuit, plural electronic switches, and plural receiver elements. This inventive radar array system “plugs” directly into the existing single channel of the core radar system. Inventive practice is thus versatile, as most single-channel radar systems can be inventively converted into a multi-channel radar array without direct modification of the core radar system. Moreover, an inventive radar array system is inherently less complicated and less expensive than a “from-the-ground-up” multi-channel radar array system, because the only duplication of components in inventive practice is in the receiver apertures. The present invention therefore permits radar arrays to be fielded more quickly and more economically than can the conventional multi-channel radar arrays. 
   The principles of the present invention are applicable to any of diverse types of energetic communication, wherein the term “energetic communication” is broadly defined as any system, process or activity involving the conveying of a signal or signals, and wherein the term “signal” is broadly defined as any form of energy, either electromagnetic (e.g., radio or light) or acoustic, that carries information or is otherwise informative. In accordance with typical practice of the present invention, a method for performing energetic communication comprises emitting plural energy pulses, and activating in succession a plurality of energy reception devices so that each energy reception device is activated in turn for a duration commensurate with a selected number of emitted energy pulses. Typically, the inventive method further comprises storing data so as to be associable with individual energy reception devices, the data being based on emitted energy pulses that are received by the energy reception devices. The energy reception devices are activated in succession for one or more (e.g., many) cycles of activations, each duration being greater than or equal to 800 nanoseconds, each energy reception device being activated for the same duration. 
   Examples of inventive applications involving electromagnetic energy include radar (radio energy), lidar (light energy), and telecommunications (radio energy or light energy). For instance, for typical radar applications of the inventive method, the energy pulses are radio pulses, the energy reception devices are receivers, and one or more transmitters transmit emitted radio energy. Some inventive applications, such as sonar, involve acoustic (sound) energy. For typical sonar applications of the inventive method, the energy pulses are sound pulses, the energy reception devices are hydrophones, and one or more projectors emit sound energy. 
   Other objects, advantages and features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described, by way of example, with reference to the accompanying drawings, wherein like numbers indicate the same or similar components, and wherein: 
       FIG. 1  is a schematic of a typical radar array system in accordance with the present invention. 
       FIG. 2  is a table setting forth the sequential ranges, in terms of numbers of pulses, of N number of receiver antennae (e.g., receiver horns), as typically practiced in accordance with the present invention. 
       FIG. 3  is a schematic of an experimental embodiment that was tested on or about 4 Sep. 2004 by the U.S. Navy in accordance with the present invention. 
       FIG. 4  is a schematic illustrating, in three dimensions, an embodiment of a bistatic radar array proposed for future U.S. Navy testing in accordance with the present invention. 
       FIG. 5  is a schematic, in elevation view (not to scale or proportion), of an embodiment of a forward scattering test (bistatic radar) that can be practiced in accordance with the present invention. 
       FIG. 6  is a schematic, in elevation view (not to scale or proportion), of an embodiment of a target scattering test (monostatic radar) that can be practiced in accordance with the present invention. 
       FIG. 7  is a schematic, in elevation view, of an embodiment of bistatic radar involving a movable receiver array, such inventive embodiment being practicable in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Reference is now made to  FIG. 1 , which is a flow diagram of a typical radar array system in accordance with the present invention. Among its novel functions and aspects as typically practiced, the present invention uniquely provides for: (i) the counting of the transmitted pulses; and, (ii) the adjustment of the receiver horn switching system in accordance with the counting of the transmitted pulses. 
   Core radar system  10  emits and receives pulses of electromagnetic radiation, e.g., microwaves or other radio waves. Core radar system  10  includes a signal generator  12  (for emitting pulses), a transmitter clock  14  (for timing the emitted pulses), a receiver clock  16  (for synchronizing the received pulses with the emitted pulses), and a data recorder  18  (for recording data associated with the received pulses). Receiver dock  16  and transmitter dock  14  are synchronized to ensure the validity of the data received by the data recorder  18 . 
   Core radar system  10  has information storage capability such as provided by data recorder  18 , which not only includes a memory for storing digital data but also includes means for digitizing the analog signals received from the antennae  41 . This stored digital data can be processed at a later time by a separate machine having a processor and a memory, e.g., computer  90  shown in  FIG. 1 . Instead of subsequently processing the data, some inventive embodiments provide for immediate receipt of data and processing thereof by a computer  90 , such as one that is included in the core radar system  10 . Data recorder  18  is a data storage device that is capable of receiving a single data stream of electromagnetic signals. In effect, the inventive methodology imparts plural-channel capability to the single-channel data storage device. 
   Signal generator (e.g., frequency synthesizer)  12  of core radar system  10  emits an A 0  initialization pulse to transmitter  51 . Switch control circuit  20  receives the A 0  initialization pulse, sets the pulse counter to zero (Pulse_Ctr=0), and re-initializes the state of the switching system  30 . Radar begins to work through the wave table. A “wave table” is an organized sequence of radar pulses that are emitted from core radar system  10 &#39;s signal generator  12  to the transmitter  51 . During continuous operations, the core radar system  10  repeatedly “passes through” the wave table. Typically, before each pass, signal generator  12  emits an initialization pulse, called the “A-naught” (“A 0 ”) pulse. Each transmission pulse in the wave table is processed in several “phases,” including the phases referred to hereinbelow as “Phase  1 ,” “Phase  2 ,” and “Phase  3 .” 
   Phase  1   
   Signal generator  12  of core radar system  10  emits a transmitter output pulse for transmission to front end apparatus  500  and to switch control circuit  20 . In emitting the transmission pulse, signal generator  12  opens the range gate of a receiver horn  41 . A “range gate” is a window in time when the radar system  10  “listens” to a receiver horn  41 ; the time corresponds with a fixed distance or “range” from the receiver  41 . The longer is the delay in the range gate, the farther is the range. 
   Phase  2   
   Front end apparatus  500  includes a conditioner  451  and an amplifier  452 . The transmission pulse is conditioned and amplified by the front end  500  and is transmitted via input line  74   T  to the transmitter horn  51 . 
   The transmission pulse is received by the switch control logic circuit  20  via an input line  74   S . The switch control circuit  20  counts the electromagnetic pulses emitted by the signal generator  12 . Each emitted pulse that is received by the switch control circuit  20  causes the switch control circuit  20  to increment its pulse counter by one (i.e., Pulse_Ctr=Pulse_Ctr+1). The switch control circuit  20  then determines the appropriate receiver horn  41  state, based on the newly incremented pulse counter, using the formula set forth in  FIG. 2 . “NumFreq” is the number of frequencies in the wave table. “NumInte” is the number of pulse integrations in the wave table. “NumPole” is the number of polarizations. 
   Each receiver horn  41  is open for the same designated number of pulses, which equals the product (NumFreq)×(NumInte)×(NumPole). According to typical inventive practice, in order to reduce system noise, the signal is integrated over multiple, consecutive pulses. These are referred to as “pulse integrations.” “Polarization” is determined by the direction of the electric field. “NumPole” is selected to be either one or two; that is, “NumPole” is one (horizontal direction), or one (vertical direction), or two (both horizontal and vertical directions). 
   The switch control circuit  20  adjusts the state of the switching system  30  so that the output of the appropriate receiver horn  41  is directed to the back end apparatus  400 . According to typical inventive practice, the switch control circuit  20  causes the switching system  30  to at least once “cycle through” antennae  41 , in turn for a predetermined duration (e.g., greater than or equal to 800 nanoseconds), so that each receiver antenna  41  is activated (receptive to incoming radio signals) while the remaining receiver antennae  41  are inactivated (non-receptive to incoming radio signals). The term “cycle” in this context refers to one complete performance of sequential activations through all of the receiver antennae  41  of the receiver array  40 ; upon the conclusion of a cycle, switch control circuit  20  returns switching system  30  to the starting point (viz., the sequentially first receiver  41 ) of the next cycle. In each cycle, receiver horns  41  are activated by switching system  30  in succession, one receiver horn  41  at a time, each receiver horn  41  being activated for a duration that accords with the counting of the emitted electromagnetic signals by switch control circuit  20 . 
   The present invention is frequently practiced so that the antennae  41  are cycled through repeatedly (i.e., at least twice—more typically, numerous times); the complete succession of events (i.e., all of the activations, in turn, of receivers  41 ) is repeated again and again, continually and uniformly in the same order. The duration of activation for each receiver horn  41  is the same and is equal to either (i) the product of the number of frequencies in said wave table times the number of pulse integrations in said wave table or (ii) the product of the number of frequencies in said wave table times the number of pulse integrations in said wave table times the number of polarizations (the number of polarizations being either “1” or “2”). 
   Phase  3   
   The incident radar energy impinges on the receiver array  40 . Similarly as the front end apparatus  500 , the back end apparatus  400  includes a conditioner  451  and an amplifier  452 . The radar energy from the appropriate receiver horn  41  is sent to the back end  400  for signal conditioning and amplifying. The amplified and conditioned signal (receiver input signal) is then transmitted via line  74   R  so as to be received by the core radar system  10  and recorded by its data recorder  18 . Included in the recorded data are the pulse number and the burst number, which can be used in post-processing to determine precisely to which receiver horn  41 , frequency, polarization, and integration number the recorded data corresponds. 
   The pulse signals received from the antenna array  40  are interleaved by data recorder  18 ; that is, the input data from the receiver horns  41  are arranged by data recorder  18  in alternating levels, sectors or blocks so that each level/block/sector carries a piece of a different data stream. In essence, plural data recorder  18  “channels,” corresponding one-to-one to the plural receiver antenna horns  41 , are created by data recorder  18  through time-division multiplexing (TDM) of the radio signals that are carried thereto by receiver input line  74 R. The input signals are interleaved so that each data recorder  18  “channel” corresponds to a different receiver horn  41 . 
   After the radar has passed through the wave table, the radar begins again by emitting an A 0  initialization pulse to the transmitter horn  51 , and the sequence repeats until interrupted by the operator. 
   With reference to  FIG. 3 , the U.S. Navy&#39;s experimental 32-element, dual-polarized array of September 2004 cost approximately $100 k to implement, not including the non-recurring design and engineering costs. The inventive experimental system was capable of toggling through the receiver (Rx) horns  41  of the linear array  40  to provide SAR-like data. The electronic switching control circuit  20  controlled the states (e.g., polarization and horn number) of the receiver horns  41 . Cross-range mapping was performed in post-processing, wherein the phase relationship(s) between/among receiver horns  41  was/were similar to that for conventional SAR (Synthetic Aperture Radar) systems. 
   Each of the receivers  41  that were used (in the two linear arrays of sixteen receivers  41  each) in the September 2004 inventive experimentation was an 8-18 GHz, dual polarized receiver (dual polarized quad-ridged horn antenna, type number 201187-4, manufactured by TECOM Industries, Inc., located at 375 Conejo Ridge Avenue, Thousand Oaks, Calif., website www.tecom-ind.com). A 2-18 GHz, dual polarized transmitter  51  (dual polarized quad-ridged horn antenna, series DF240, model DP240-AB, manufactured by Flann Microwave, located at Dunmere Road, Bodmin, Cornwall, PL31 2Ql, UK, and Baldwin Park 1, 12 Alfred St., Ste. 300, Woburn, Mass., website www.flann.com) was used in the September 2004 inventive experimentation. The core radar system  10  was a commercially available single-channel radar unit manufactured by Lintek, now owned by Aeroflex Incorporated, 35 South Service Road, Plainview, N.Y., website www.aeroflex.com. 
   A commercial programmable circuit board was used for the switch control circuit  20 . The programmable board was patched into the transmitter line (isolated with one-way opto-isolators so that the programmable board would not corrupt the transmission signal). This circuit board could be reprogrammed at will whenever the number of receiver horns  41 , number of frequencies, etc., was/were adjusted. Any programmable electronic means can be used as the switch control circuitry  20  in accordance with the present invention, such as that which includes a programmable logic device or a processor (e.g., microprocessor). 
   As shown in  FIG. 3 , thirty-two receiver horns were cycled through, using cascading switches  31 . The Robinson switch  31   R  alternated between the “A” group of receiver horns  41  (by choosing switch  31   A ) and the “B” group of receiver horns  41  (by choosing switch  31   B ). Group “A” switch  31   A  and group “B” switch  31   B  each alternated between two banks of eight horns  41  each. The group “A” switch, switch  31   A , chose either switch  31   A-1  or switch  31   A-2 ; the group “B” switch, switch  31   B , chose either switch  31   B-1  or switch  31   B-2 . The inventors desired sixteen H-pole (horizontal-pole) receiver horns  41  and sixteen V-pole (vertical-pole) receiver horns  41  for this application; accordingly, group A (consisting of banks A- 1  and A- 2 ) was H-pole, and group B (consisting of banks B- 1  and B- 2 ) was V-pole. 
   Front end apparatus  500  and back end apparatus  400  were powered by front end power supply  80   R  and back end power supply  80   B , respectively. Switching system  30  was also provided with its own power supply, viz., switch power supply  80 S. Front end apparatus  500  is shown in  FIG. 3  to include a signal conditioner  451 , an amplifier  452 , a directional coupler  33  (which is connected to the core radar system  10 ), a detector  35 , a Robinson switch  31   R-P , and a transmit module  37  (which is connected between Robinson switch  31   R-P  and transmitter horn  51 ). Robinson switch  31   R-P  chose between horizontal polarity and vertical polarity (or both horizontal and vertical polarities) of the radar energy as was transmitted by transmitter horn  51 . 
   The multi-level switching system illustrated in  FIG. 3  was designed in this manner mainly for cost-saving reasons and in fact performed adequately, but it is not suggested herein as representative of a preferred switching system  30  configuration in accordance with the present invention. Preferred inventive practice is for the present invention&#39;s switching system  30  to be as efficient as possible. Such efficiency would tend to be promoted by employment of as few switches and/or switching levels as practicable, perhaps by employing even a single electronic switching device. 
   According to generally preferred inventive practice, it is important that the practitioner make sure that the switch control circuit  20  is fast enough to: (a) count the transmitter pulses, (b) determine the necessary state of the switches, and (c) transmit any adjustments to the switching system  30 . In addition, it is important for the practitioner to make sure that the switching system  30  reacts sufficiently fast. If there is too much lag, then the radar energy intended for the “N+1” receiver horn  41  will actually be circuited through the “N” receiver horn  41 , and hence the intendedly organized data stream will become disorganized. This guidance is particularly significant for radar sets that run extremely fast. 
   With reference to  FIG. 4  through  FIG. 7 , the present invention can be practiced for either bistatic radar systems (for example, as shown in  FIG. 4 ,  FIG. 5  and  FIG. 7 ) or monostatic radar systems (for example, as shown in  FIG. 6 ).  FIG. 5  (bistatic radar) and  FIG. 6  (monostatic radar) are more generally illustrative. The term “bistatic” as used herein is synonymous with the term “multistatic.” 
   As shown in  FIG. 5 , according to a typical bistatic application of the present invention, the transmitter horn  51  and the receiver array  40  (of receiver horns  41 ) are situated at different locations. As shown in  FIG. 6 , according to a typical monostatic application of the present invention, the transmitter horn  51  and the receiver array  40  (of receiver horns  41 ) are situated at the same or approximately the same location. The separation distance between transmitter horn  51  and a receiver horn  41  is the “range” or “baseline,” which is significant for bistatic radar and insignificant (or nonexistent) for monostatic radar. Angle “LDA” indicated in  FIG. 5  is the geometric elevation angle to transmitter  51  at the base of a receiver horn  41 . Grazing angle “{acute over (Ø)} EM ” indicated in  FIG. 5  is the elevation angle, at the forward scatter location of the mean water level “MWL,” of the forward scatter of the electromagnetic radiation (e.g., radio waves) from a receiver horn  41  to transmitter horn  51 . Angle “LDA” indicated in  FIG. 6  is the geometric elevation angle to transmitter  51  at the base of a target  61 . Grazing angle “{acute over (Ø)} EM ” indicated in  FIG. 6  is the elevation angle, at the forward scatter location of the mean water level “MWL,” of the forward scatter of the electromagnetic radiation (e.g., radio waves) from target  61  to a receiver horn  41 . 
   According to some inventive embodiments, such as illustrated in  FIG. 4 , measurements are made to map scattering from a sea surface. A proposed MASK Test 2 by the U.S. Navy will include: more sea states (0, 2, 3, 3½, 4, 4½, 5, 6); multiple wave directions; 3-D laser profiling of waves; effects with and without wind (a wind generator, such as wind generator  47  shown in  FIG. 7 , will be used); a wider range of radar threat bands (X and Ku). The proposed radar measurement will be for the X and Ku Bands (8-18 GHz). Wide bandwidth will allow down-range imaging of sea surface, providing time-of-flight information. An array  40  of receivers  41  will allow cross-range imaging of the sea surface, similar to an acoustic line array. This uniquely designed radar imaging system in accordance with the present invention can thus perform 2-D mapping of sea reflection. 
     FIG. 7  shows a preliminary test setup, contemplated by the U.S. Navy, featuring a linear array  40  of receiver horns  41  that is movable via rails  49 . Among the notable aspects of such an inventive system are: testing of multiple wave directions (e.g., 360° FS/MP map); testing of more sea states (e.g., to map transition from coherent to diffuse, and/or to consider limited swell effects); testing of wind effects using wind generator  47  (e.g., to determine the effects of small-scale roughness); mapping of the scattering sea surface using linear array  40 . 
   The present invention, which is disclosed herein, is not to be limited by the embodiments described or illustrated herein, which are given by way of example and not of limitation. Other embodiments of the present invention will be apparent to those skilled in the art from a consideration of the instant disclosure or from practice of the present invention. Various omissions, modifications and changes to the principles disclosed herein may be made by one skilled in the art without departing from the true scope and spirit of the present invention, which is indicated by the following claims.