Abstract:
A polarimetric radar with V and H polarized array antenna elements which transmit a V component modulated by a V code and the H component modulated by an H code orthogonal thereto. Unwanted polarization errors occur at off-boresight angles, which tend to obscure the polarization characteristics of the target. Radar returns are processed to form four independent channels by convolving the signals received by the V and H antennas with the mutually orthogonal V and H codes. This overcomes the polarization obscuration and allows determination of the polarimetric ratio of the target with high accuracy.

Description:
BACKGROUND OF THE INVENTION 
     Those skilled in the arts of antennas and arrays know that antennas are transducers which transduce electromagnetic energy between unguided- and guided-wave forms. More particularly, the unguided form of electromagnetic energy is that propagating in “free space,” while guided electromagnetic energy follows a defined path established by a “transmission line” of some sort. Transmission lines include coaxial cables, rectangular and circular conductive waveguides, dielectric paths, and the like. Antennas are totally reciprocal devices, which have the same beam characteristics in both transmission and reception modes. For historic reasons, the guided-wave port of an antenna is termed a “feed” port, regardless of whether the antenna operates in transmission or reception. The beam characteristics of an antenna are established, in part, by the size of the radiating portions of the antenna relative to the wavelength. Small antennas make for broad or nondirective beams, and large antennas make for small, narrow or directive beams. 
     When more directivity (narrower beamwidth) is desired than can be achieved from a single antenna, several antennas may be grouped together into an “array” and fed together in a phase-controlled manner, to generate the beam characteristics of an antenna larger than that of any single antenna element. The structures which control the apportionment of power to (or from) the antenna elements are termed “beamformers,” and a beamformer includes a beam port and a plurality of element ports. In a transmit mode, the signal to be transmitted is applied to the beam port and is distributed by the beamformer to the various element ports. In the receive mode, the unguided electromagnetic signals received by the antenna elements and coupled in guided form to the element ports are combined to produce a beam signal at the beam port of the beamformer. A salient advantage of sophisticated beamformers is that they may include a plurality of beam ports, each of which distributes the electromagnetic energy in such a fashion that different antenna beams may be generated simultaneously. The advantages of antenna arrays over single-antenna transducers has led to extensive use of array antennas. A notable disadvantage of antenna arrays is that, when the antenna beam is scanned away from broadside to the array, the polarization response of the elemental antennas (elements) of the array is degraded, and the larger the scan angle, the greater the degradation, because the element-level phase progression that steers the beam results in tilting of the beam relative to each element. Reflector-type antennas, however, are mechanically steered to point the beam, and provide high cross-polarization purity. The degraded off-axis cross-polarization performance of antenna arrays has inhibited the use of such arrays for polarimetric weather measurements and research. 
     Improved radar using array antennas for polarimetric measurements is desired. 
     SUMMARY OF THE INVENTION 
     A radar system according to an aspect of the invention is for illuminating a target. The radar system comprises a first array of antenna elements. The first array of antenna elements includes at least one guided-field input-output port. The first array of antenna elements is for transducing electromagnetic fields of a first polarization. The radar system also comprises a second array of antenna elements. The second array of antenna elements includes at least one guided-wave input-output port. The second array of antenna elements is for transducing electromagnetic fields of a second polarization which is orthogonal to the first polarization. The radar includes a source of radio-frequency energy at a frequency related to a radar transmit frequency. A source is provided of first and second mutually orthogonal coding waveforms. A convolver is coupled to the source of radio-frequency energy and to the source of mutually orthogonal coding waveforms, for convolving the radio-frequency energy with the first and second mutually orthogonal coding waveforms, to thereby generate first and second modulated radio-frequency energy with first and second mutually orthogonal modulations, respectively. A first transmit-receive arrangement is coupled to the input-output port of the first array of antenna elements and to the convolver, for, in a transmit operating mode, applying the first modulated radio-frequency energy to the input-output port of the first array of antenna elements for transduction into unguided waves of the first polarization, and for, in a receive operating mode, receiving guided waves transduced by the first array of antenna elements. A second transmit-receive arrangement is coupled to the input-output port of the second array of antenna elements and to the convolver, for, in a transmit operating mode, applying the second modulated radio-frequency energy to the input-output port of the second array of antenna elements for transduction into unguided waves of the second polarization, and for, in a receive operating mode, receiving guided waves transduced by the second array of antenna elements. A deconvolver comprises first and second delay elements coupled to the source of first and second mutually orthogonal coding waveforms for delaying the first and second mutually orthogonal coding waveforms, respectively, by a selected amount, to thereby generate first and second delayed mutually orthogonal signals, the deconvolver also including first, second, third, and fourth multipliers, each of the first and second multipliers being coupled to receive the first delayed mutually orthogonal signals, and each of the third and fourth multipliers being coupled to receive the second delayed mutually orthogonal signals, the first and third multipliers also being coupled to the first transmit-receive arrangement for, in the receive mode of operation, receiving guided electromagnetic signals originating from the first array of antenna elements, for thereby generating first and third components of received signals, the second and fourth multipliers being coupled to the second transmit-receive arrangement, for, in the receive mode of operation, receiving guided electromagnetic signals originating from the second array of antenna elements, for thereby generating second and fourth components of received signals. A processor is coupled to receive the first, second, third, and fourth components of received signals, for determining the polarimetric ratio or differential reflectivity of the illuminated target. 
     A particular embodiment of this aspect of the invention further comprises a display for displaying a plan-position representation of the return signals representing each of the first and second polarizations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1A  is a simplified diagram in block and schematic form of a radar system according to an aspect of the invention, and  FIG. 1B  illustrates some details of antenna arrays of  FIG. 1A . 
     
    
    
     DESCRIPTION OF THE INVENTION 
       FIG. 1A  is a simplified representation of a radar system  10  of the polarimetric type, in which the target  8  is typically a weather phenomenon such as a cloud or source of precipitation. Radar system  10  of  FIG. 1A  includes an exciter illustrated as a block  24 , which is controlled to produce pulses of guided electromagnetic energy (as opposed to unguided energy as would occur in free space) on a path  26 . Guided electromagnetic energy follows a defined path established by a “transmission line” of some sort. A salient difference between a “transmission line” and an ordinary electrical conductor arrangement is that the transmission line has constant surge or characteristic impedance along its length, or if the impedance varies along its length the variation is controlled rather than uncontrolled. Transmission lines include coaxial cables, rectangular and circular conductive waveguides, dielectric paths, and the like. Each of these structures has the property that transverse dimensions and cross-sectional topology remain constant (or change in a controlled manner) as a function of length. Thus, the diameter of the center conductor and bore of the outer conductor of a coaxial transmission line remain constant along the pertinent length. Topological transformations of the cross-section of a transmission line can result in a different type of transmission line, as for example a topological transformation of a coaxial transmission line can result in a microstrip transmission line configuration. 
     The frequency and pulse characteristics of the exciter  24  of  FIG. 1A  are controlled by a radar control computer (RCC), illustrated as a block  90 , so that inter alia the “radio frequency” of the electromagnetic signals is related to the radar transmission frequency. In the past, the term “radio frequencies” was interpreted to mean a limited range of frequencies, such as, for example, the range extending from about 20 KHz to 2 MHz. Those skilled in the art know that “radio” frequencies as now understood extends over the entire frequency spectrum, including those frequencies in the “microwave” and “millimeter-wave” regions, and up to light-wave frequencies. Many of these frequencies are very important for commercial purposes, as they include the frequencies at which radar systems, global positioning systems, satellite cellular communications and ordinary terrestrial cellphone systems operate. The exciter signal is applied to a convolver illustrated as a block  32 . 
     RCC  90  of  FIG. 1A  also controls other parameters and aspects of radar  10 , as known in the art. Radar  10  also includes a source  28  of sequences of mutually orthogonal (⊥) or mutually complementary binary signals. The salient properties of the mutually orthogonal signals or codes are that when combined, they do not add, and when the combined signal is deconvolved with one of the codes, only the data contained in that channel is extracted. Thus, each of the mutually orthogonal codes produces an independent data channel on a common radio frequency (RF) carrier which has good isolation from the other data channels based on the degree of orthogonality of codes themselves. Mutually orthogonal or complementary digital or binary signals are well known in the art, as for example in U.S. Pat. No. 5,440,311 issued Aug. 8, 1995 in the name of Gallagher et al., and require no further description. The two sequences of mutually orthogonal binary signals produced by block  28  of  FIG. 1A  are separately applied by way of paths  38  and  40  to convolver  32  and to a delay function illustrated as a pair of blocks  42   a  and  42   b . Convolver block  32  separately convolves the exciter signal from block  24  with each of the two mutually orthogonal signals from block  28 . Thus, the radio-frequency signal from exciter  24  is convolved within convolver  32  with the binary sequence appearing on path  38 , to produce what amounts to modulated signal, which is applied over a path  34  to a first transmit/receive (TR) device  20 . Similarly, the radio-frequency (RF) signal from exciter  24  is convolved within convolver  32  with the binary sequence appearing on path  40 , to produce what amounts to modulated signal which is applied over a path  36  to a second transmit/receive (TR) device  22 . Transmit-receive blocks  20  and  22  route the modulated RF signals by way of paths  21  and  37 , respectively, to input/output (IO) ports  12   io  and  16   io , respectively, of first and second antenna arrays  12  and  16 , respectively. First antenna array  12  is polarized or oriented to transduce “vertical” (V) polarized unguided radiation illustrated by a “lightning bolt” symbol  14 , and second array  16  is polarized or oriented to transducer “horizontal” (H) unguided radiation illustrated by a “lightning bolt” symbol  18 . 
       FIG. 1B  illustrates some details of the array antennas  12  and  16  of  FIG. 1A . In  FIG. 1B , elements corresponding to those of  FIG. 1A  are designated by the same reference alphanumerics. As illustrated in  FIG. 1B , array antenna arrangement  12  includes a beamformer illustrated as a block  84 V, connected to common or beam port  12   io  and to the array  86  of elemental antennas. Array  86  as illustrated includes antenna elements  86 Va,  86 Vb, . . . ,  86 Vn. Similarly, array antenna arrangement  16  includes a beamformer illustrated as a block  84 H, connected to common or beam port  16   io  and to the array  88  of elemental antennas. Array  88  as illustrated includes antenna elements  88 Ha,  88 Hb, . . . ,  88 Hm. When excitation energy is applied to port  12   io , the energy is distributed by beamformer  84 V among the elemental antennas of set  86 , to thereby form the desired vertically polarized beam or beams. Similarly, when excitation energy is applied to port  16   io , the energy is distributed by beamformer  84 H among the elemental antennas of set  88 , to thereby form the desired horizontally polarized beam or beams. The direction in which the beam or beams are formed depends upon relative phase and possibly amplitude control of the excitation applied to the various antenna elements of the vertically and horizontally polarized portions, as is known in the art. The phase and amplitude control is established by the radar control computer (RCC)  90  of  FIG. 1A . 
     As mentioned, when the antenna beams are steered away from broadside to the plane of the antenna array, the polarization purity is compromised. More particularly, at large angles away from a broadside condition, changes in the apparent aspect ratio of the antenna element(s) to the ground as seen from the direction of the impinging wave and increases in feed-point effects on the radiation pattern result in changes in the cross-polarization of all antennas. These changes in cross-polarization may be unacceptable in the context of radar polarimetric measurements. Thus, the unguided radiation arriving at a target such as cloud  90  of  FIG. 1B  does not exhibit pure vertical or pure horizontal polarization. Instead, the target receives an intermixture of vertically and horizontally polarized radiation components depending upon the aspect angle of the array as seen from the target. Reflected energy from the cloud  90  thus contains an intermixture of vertically polarized and horizontally polarized components. Some of the reflected RF components are reflected back toward the antenna arrays  12  and  16  (the radar “return”), and are received and transduced into guided waves by the antenna arrays  12  and  16 . The guided “return” signals or waves received by first antenna array  12  in a reception mode are coupled by way of path  21  to TR  20 , which passes the received signals on to an input port  50   i   1  of a deconvolver  50 . The guided waves received by second antenna array  16  in the reception mode are coupled by way of path  37  to TR  22 , which passes the received signals on to an input port  50   i   2  of deconvolver  50 . 
     It will be understood that, because of the off-axis or off-boresight polarization purity problems of an array antenna, the transmitted V and H signals will arrive at the target as a mixture of V and H components. Consequently, the reflection from the target (the return signal) starts out as a mixture of V and H components, and this mixture is further mixed by the off-boresight response of the array antennas. Even if the radar signals reflected by the target were pure vertical andor horizontal, the polarization properties of the array antennas  12  and  16  would result in cross-contamination of the polarization components. According to an aspect of the invention, the mutually orthogonal modulation of the V and H beams allows identification of the desired and undesired components of the signal received from the target, and allows them to be separated. 
     Deconvolver  50  of  FIG. 1A  also receives from block  28  the sequences of mutually orthogonal or coding signals, delayed by an amount equal to the time required for electromagnetic radiation to make the trip from the radar to the target and for the reflected energy to return to the radar. This round-trip time is well known to correspond to 12.4 microseconds per nautical mile of range. The delay is imposed on the mutually orthogonal signal sequences by delay blocks  42   a  and  42   b , under the control of the radar control computer  90 . Thus, deconvolver  50  of  FIG. 1A  receives at paths  52  and  54  the mutually orthogonal binary sequences with delays appropriate to the round-trip time between the radar  10  and the target  8 . The delayed sequence on path  52  is designated d 1 , and the delayed sequence on path  54  is designated d 2 . 
     Deconvolver  50  of  FIG. 1A  also includes a first multiplier (X)  61 , a second multiplier  62 , a third multiplier  63 , and a fourth multiplier  64 . Multipliers  61  and  63  are connected by way of port  50   i   1  and first TR  20  to receive the return or reflected signal transduced by the first antenna array  12 , and multipliers  62  and  64  are connected by way of a port  50   i   2  and second TR  22  to receive the return or reflected signal transduced by the second antenna array  16 . 
     The return signal transduced by antenna array  12  and applied by way of port  50   i   1  to multiplier  61  is modulated or multiplied by delayed sequence d 1 , and the return signal transduced by antenna array  12  and applied by way of port  50   i   1  to multiplier  63  is modulated or multiplied by delayed sequence d 2 . Similarly, the return signal transduced by antenna array  16  and applied by way of port  50   i   2  to multiplier  62  is modulated or multiplied by delayed sequence d 1 , and the return signal transduced by antenna array  16  and applied by way of port  50   i   2  to multiplier  64  is modulated or multiplied by delayed sequence d 2 . The delay which is imposed depends upon the range of the target, and is readily determined by those skilled in the art. The multiplication of the return signals received by nominally V antenna  12  by the first orthogonal sequence d 1  in multiplier  61  will enhance any return which originated from a vertically polarized transmitted signal, and will attenuate or disregard any return originating from a horizontally-polarized transmission. Thus, the deconvolved product produced by multiplier  61  on signal path  71  will tend to represent pure vertical return signal, without contributions by the horizontal signal components. Similarly, The multiplication of the return signals received by nominally V antenna  12  by the second orthogonal sequence d 2  in multiplier  63  will enhance any return which originated from a horizontally polarized transmitted signal, and will attenuate or disregard any return originating from a vertically-polarized transmission. Thus, the deconvolved product produced by multiplier  63  on signal path  73  will tend to represent pure horizontal return signal, without contributions by the vertical signal components. 
     The multiplication of the return signals received by nominally H antenna  16  by the first orthogonal sequence d 1  in multiplier  62  of  FIG. 1A  will enhance any return which originated from a vertically polarized transmitted signal, and will attenuate or disregard any return originating from a horizontally-polarized transmission. Thus, the deconvolved product produced by multiplier  62  on signal path  72  will tend to represent pure vertical return signal, without contributions by the horizontal signal components. Similarly, the multiplication of the return signals received by nominally H antenna  16  by the second orthogonal sequence d 2  in multiplier  64  will enhance any return which originated from a horizontally polarized transmitted signal, and will attenuate or disregard any return originating from a vertically-polarized transmission. Thus, the deconvolved product produced by multiplier  62  on signal path  72  will tend to represent pure horizontal return signal, without contributions by the vertical signal components. 
     Thus, the deconvolved or multiplied signals on path  71  of  FIG. 1A  represent the response of the vertically polarized antenna  12  to the “vertical return”, the signals on path  72  represent the response of the horizontally polarized antenna  16  to the “vertical return, the signals on path  73  represent the response of the vertically polarized antenna  16  to the “horizontal return,” and the signals on path  74  represent the response of the horizontal antenna  16  to the “horizontal return.” Taken together, the signal components produced by deconvolver  50  on paths  71  and  72  represent the entirety of the vertically polarized response, and, taken together, the signal components produced by deconvolver  50  on paths  73  and  74  represent the entirety of the horizontally polarized response. 
     Since all of the vertically and horizontally polarized components are individually identified and separated onto paths  71 ,  72 ,  73 , and  74  of  FIG. 1A , a determination of the V and H responses can be made by simply processing the returns. The processing of the return signals on paths  71 ,  72 ,  73 , and  74  is performed in a processor designated  80 . 
     The processing in block  80  can be understood by the following. 
     Let IV be the transmit input voltage of the vertically polarized portion or element of the antenna, namely portion  12  of  FIG. 1A . The magnitude of IV is unknown. 
     Let IH be the transmit input voltage of the Horizontally polarized antenna portion  16 . The relationship between the vertical and horizontal element input or transmit voltages is established by calibration Such calibration is well known in the art and can be interleaved in time with the radar pulses or by periodic measurement of the output unguided wave to an external antenna/receiver. Therefore, IH=cal×IV, where cal is the calibration value. 
     Let X be the ratio V pol /H pol =Zdr of the target, which is to be measured by the radar. 
     Let Y represent the unknown cross-polarization component transmitted or received by the vertically polarized portion  12  of the antenna of  FIG. 1A . 
     Let Z represent the unknown cross-polarization component transmitted or received by the horizontally polarized portion  16  of the antenna of  FIG. 1A . 
     The convolution of the signal received by the vertically polarized element with the V orthogonal code is
 
IV*(1−Y)2*X+IV*(1−Y)2*(1−X)
 
     where * represents multiplication. 
     The convolution of the signal received by the horizontally polarized element with the H orthogonal code is
 
cal*IV*(1−Z)2*(1−X)+cal*IV*(Z)2*X
 
     The convolution of the signal received by the vertically polarized element with the H orthogonal code is
 
cal*IV*(1−Z)*(1−X)*Y+cal*IV**(Z)*(1−Y)*X
 
     The convolution of the signal received by the horizontally polarized element with the V orthogonal code is
 
IV*(1−Y)*X*Z+IV*Y*(1−Z)*(1−X).
 
     The four equations relating the four received and convolved inputs to the processor  80  of  FIG. 1  are solved simultaneously for the polarization ratio (Zdr) of the target. The polarization ratio is represented by X in each equation. A system of four equations with four unknowns can be uniquely solved as known in the art. 
     When the vertical and horizontal components of the reflection from the target  8  are known, the information can be displayed in the form of image density or image color (or both) on a plan-position-indicator (PPI) display, as is well known in the radar and weather arts, to give a “map-like” display of precipitation characteristics in the radar coverage area. 
     A radar system ( 10 ) according to an aspect of the invention is for illuminating a target ( 90 ). The radar system ( 10 ) comprises a first array ( 12 ) of antenna elements ( 86 ). The first array of antenna elements ( 12 ) includes at least one guided-field input-output port ( 12   io ). The first array ( 12 ) of antenna elements is for transducing electromagnetic fields ( 14 ) of a first polarization (V). The radar system ( 10 ) also comprises a second array ( 16 ) of antenna elements ( 88 ). The second array of antenna elements ( 16 ) includes at least one guided-wave input-output port ( 16   io ). The second array ( 16 ) of antenna elements ( 88 ) is for transducing electromagnetic fields ( 18 ) of a second polarization (H) which is orthogonal to the first polarization (V). The radar ( 10 ) includes a source ( 24 ) of radio-frequency energy at a frequency related to a radar ( 10 ) transmit frequency. A source ( 28 ) is provided of first and second mutually orthogonal coding waveforms. A convolver ( 32 ) is coupled to the source of radio-frequency energy ( 24 ) and to the source ( 28 ) of mutually orthogonal coding waveforms, for convolving the radio-frequency energy with the first and second mutually orthogonal coding waveforms, to thereby generate first (on path  34 ) and second (on path  36 ) modulated radio-frequency energy with first and second mutually orthogonal modulations, respectively. A first transmit-receive arrangement ( 20 ) is coupled ( 21 ) to the input-output port ( 12   io ) of the first array ( 12 ) of antenna elements ( 86 ) and (by way of  34 ) to the convolver ( 32 ), for, in a transmit operating mode, applying the first modulated radio-frequency energy to the input-output port ( 12   io ) of the first array ( 12 ) of antenna elements ( 86 ) for transduction into unguided waves ( 14 ) of the first polarization (V), and for, in a receive operating mode, receiving guided waves transduced (onto path  21 ) by the first array ( 12 ) of antenna elements ( 86 ). A second transmit-receive arrangement ( 22 ) is coupled ( 37 ) to the input-output port ( 16   io ) of the second array ( 16 ) of antenna elements ( 88 ) and to the convolver ( 32 ), for, in a transmit operating mode, applying the second modulated radio-frequency energy to the input-output port ( 16   io ) of the second array ( 16 ) of antenna elements ( 88 ) for transduction into unguided waves ( 18 ) of the second polarization (H), and for, in a receive operating mode, receiving guided waves transduced (onto path  37 ) by the second array ( 16 ) of antenna elements ( 88 ). A deconvolver ( 50 ) comprises first ( 42   a ) and second ( 42   b ) delay elements coupled to the source of first and second mutually orthogonal coding waveforms ( 28 ) for delaying the first and second mutually orthogonal coding waveforms, respectively, by a selected amount, to thereby generate first (d 1 ) and second (d 2 ) delayed mutually orthogonal signals, the deconvolver also including first ( 61 ), second ( 62 ), third ( 63 ), and fourth ( 64 ) multipliers, each of the first ( 61 ) and second ( 62 ) multipliers being coupled to receive the first delayed mutually orthogonal signals (d 1 ), and each of the third ( 63 ) and fourth ( 64 ) multipliers being coupled to receive the second delayed mutually orthogonal signals (d 2 ), the first ( 61 ) and third ( 63 ) multipliers also being coupled to the first transmit-receive arrangement ( 20 ) for, in the receive mode of operation, receiving guided electromagnetic signals originating from the first array ( 16 ) of antenna elements ( 86 ), for thereby generating first (on path  71 ) and third (on path  73 ) components of received signals, the second ( 62 ) and fourth ( 64 ) multipliers being coupled to the second transmit-receive arrangement ( 22 ), for, in the receive mode of operation, receiving guided electromagnetic signals originating from the second array ( 16 ) of antenna elements ( 88 ), for thereby generating second (on path  72 ) and fourth (on path  74 ) components of received signals. A processor ( 80 ) is coupled to receive the first, second, third, and fourth components of received signals, for determining the polarimetric ratio or differential reflectivity of the illuminated target. 
     A particular embodiment of this aspect of the invention further comprises a display for displaying a plan-position representation of the return signals representing each of the first (V) and second (H) polarizations.