Radar polarization calibration and correction

A polarimetric radar system transmits a signal which is nominally the desired polarization, but which may deviate therefrom. A calibration operation is performed using a symmetrical radar reflector, to determine the gains and phases of reception of two mutually orthogonal components of the received reflected signal which result in nulling of the two polarization components of the reflected signal. These gains and phases represent receive corrections which result in a simulation of perfect polarization purity on both transmission and reception. The corrections established during calibration are applied to the receive signals during normal (non-calibration) operation, to improve the effective polarization of the transmission and reception.

FIELD OF THE INVENTION

This invention relates to correction of the polarization characteristics of a radar system by calibration using a standard reflector to generate calibration functions. The calibration functions are applied during normal operation to determine the polarization characteristics of noncalibration targets.

BACKGROUND OF THE INVENTION

Polarimetric radar (radar which has the ability to transmit and receive in more than one sense of polarization) has uses in weather surveillance and in air traffic control applications. It may also have applications in other areas such as ballistic missile defense. The calibration of polarimetric radars is based, at least in part, on the electromagnetic reflection characteristics of planar and spherical targets to incident circular polarization. A reflective planar target returns the opposite sense of both linear and circular polarization, and a spherical target returns the opposite hand of circular polarization relative to the incident polarization. If a radar system transmits a particular hand of circular polarization, such as right-hand circular polarization (RHCP), raindrops, which are generally spherical, will reflect left-hand circular polarization (LHCP) signals. Assuming that the same antenna is used for radar reception of the returns as for transmission (monostatic radar), or at least assuming that the receive antenna has RHCP characteristics (in a bistatic radar context), the LHCP return signal will tend to be rejected by the receive antenna. Reflection of circular polarization signals, such as RHCP signals, from nonspherical targets are more complex, and do not necessarily simply reverse the polarization, but instead tend to return noncircular elliptical polarization. Thus, an aircraft target will generate reflections in response to incident RHCP which include elliptical RHP together with mutually orthogonal linear polarizations. A RHCP reflected energy receiving antenna will not reject these reflected signals. It should be noted that mutually orthogonal linear polarizations of electromagnetic energy are often referred to as vertical (V) and horizontal (H) regardless of the actual orientation of the electric field.

In the context of weather radar, it is possible to estimate the shape of precipitation by alternately transmitting two mutually orthogonal electromagnetic signals. The return signals from hailstones, which tend to be round, differ from those of raindrops, which tend to be flattened, and these differences can be used to distinguish between hailstones and raindrops.

Thus, there are important uses for radar systems which can transmit selected circular or linear polarizations and selectively respond to particular return signal polarizations. More specifically, weather surveillance and air traffic control radar systems require various forms of polarization diversity, including (a) transmission and reception of circular polarization, (b) transmission of circular polarization and reception of orthogonal linear polarizations and (c) transmission of ±45° slant polarization and simultaneous reception of orthogonal linear polarizations. Array antennas capable of transmitting diverse polarizations are known. In such array antennas, each antenna element includes a pair of crossed linear radiating/receiving elements such as crossed or mutually orthogonal dipoles. Those skilled in the art view such crossed radiating/receiving elements as being a single elemental antenna of the array. The individual crossed radiating-receiving elements are referred to herein as “radiators” regardless of whether they are operated in a radiating or receiving mode, or both.

Unfortunately, the imperfections of antennas and real systems tend to work against the use of polarimetric radar. It is difficult, if not impossible, to make an antenna which transduces only a particular polarization to the exclusion of other polarizations, and this difficulty is compounded by the high power which a transmit antenna must handle. An aspect of this difficulty lies in the precision with which the antenna itself can be fabricated, and another aspect lies in the associated electronics, beamformers, and cables which interconnect elemental antennas of an array antenna, and especially the transmit module which is associated with antenna elements or element subgroups in an active array antenna.

One possible way to adjust the transmitted polarization in the context of an array antenna is to adjust the phase and amplitude of the signal applied to each transmit/receive radiating element of the elemental antenna relative to those of other elemental antennas, so that the polarization of the resulting combined far-field radiation, in a particular direction, meets the desired standard. It is difficult to separate out the far-field contributions of any one elemental antenna, so the correction applied to a given antenna element may be such as to cause that particular antenna element to be far from the desired polarization even though the polarization of the sum radiation is correct. This has the effect of tending to degrade the sum polarization at other aspect angles. Additionally, the correction of phase and amplitude in a phased-array antenna is ordinarily accomplished by digital adjusters, which have fairly coarse adjustment steps. The coarse steps make achieving the desired polarization more difficult than if continuous adjustment were possible. Extremely fine adjustments of amplitude and phase may be possible, but may be unacceptably expensive.

Improved polarimetric radar systems are desired.

SUMMARY OF THE INVENTION

A method according to an aspect of the invention is for compensating errors in transmitted circular polarization such that reflections from spherical objects tend to be cancelled. The method comprises the step of transmitting from an antenna an electromagnetic signal at a carrier frequency. The transmitted signal includes mutually orthogonal linear components nominally phased to generate circular polarization, but which may instead generate noncircular or elliptical polarization transmissions. The method also includes the step of receiving, by means of first and second linear radiators, first and second mutually orthogonal linear components of reflections of the transmitted signals, to thereby produce first and second received signals. A set of correction factors to the first and second received signals is procured. The correction factors are a function of (a) beam direction in the case of monostatic radar and (b) beam directions in the case of bistatic radar. These correction factors may also be a function of carrier frequency if the system operates at a plurality of frequencies. The first and second received signals are processed with the correction factors in such a fashion as to cause the received signal to appear as if substantially perfect circular polarization had been transmitted and received. In a particular mode of this method, the step of procuring a set of correction factors comprises the steps of placing a sphere to receive the transmitted signal and to reflect the reflected signal. The correction factors may be generated for a plurality of beam directions. The amplitudes and phases of the first and second received signals are adjusted to cancel the combined echoes at each of the beam directions. The amplitudes and phases at which the cancellation occurs for each beam direction are tabulated to generate the set of correction factors.

A method for determining the ellipticity of a radar target according to another aspect of the invention comprises the step of transmitting from an antenna an electromagnetic signal at a carrier frequency, where the transmitted signal includes mutually orthogonal linear components nominally phased to generate circular polarization, but which may instead generate noncircular or elliptical polarization, to thereby produce transmitted signals. The method also includes the step of receiving, by means of first and second linear radiators, first and second mutually orthogonal linear components of reflections of the transmitted signals, to thereby produce first and second received signals. A set of correction factors to the first and second received signals is procured, which correction factors are a function of (a) beam direction in the case of monostatic radar and (b) beam directions in the case of bistatic radar. If the transmitted signals are at different frequencies, the correction factors may take frequency into account. The first and second received signals are processed with the correction factors in such a fashion as to cause the received signal to appear as if substantially perfect circular polarization had been transmitted and received. The method also includes the step of receiving, from a nonspherical target, third and fourth mutually orthogonal linear components of reflections of the transmitted signals. The third and fourth received signals are processed with the correction factors to thereby generate corrected mutually orthogonal received components. The characteristics of the corrected mutually orthogonal received components are compared to determine the nonsphericity or ellipticity of the target.

Another method according to an aspect of the invention is for compensating errors in transmitted and received circular polarization. The method comprises the step of transmitting an electromagnetic signal at a carrier frequency from an antenna, to thereby produce transmitted electromagnetic signals. The transmitted electromagnetic signal includes mutually orthogonal linear components nominally phased to generate circular polarization, but which may instead generate noncircular or elliptical polarization. This method also includes the step of receiving, by means of first and second linear radiators, first and second mutually orthogonal linear components of reflections of the transmitted signals, to thereby produce first and second received signals. A set of correction factors to the first and second received signals is procured. The correction factors are a function of (a) beam direction in the case of monostatic radar and (b) beam directions in the case of bistatic radar. The correction factors may be a function of carrier frequency if the transmitted signals range over a plurality of frequencies. The first and second received signals are processed with the correction factors in such a fashion as to cause the received signal to appear as if substantially perfect circular polarization had been transmitted and received.

A method for compensating errors in transmitted andor received linear polarization according to yet another aspect of the invention comprises the steps of transmitting, from an antenna, an electromagnetic signal at a carrier frequency, which signal includes, or can be decomposed into, two mutually orthogonal linear components nominally controlled in phase and amplitude to produce linear polarization to thereby produce first and second transmitted signals. The actual phase and amplitude may be such that (a) the polarization of the transmitted nominally linearly polarized signal is at an undesired angle andor (b) the transmitted signal includes a circular component. By means of first and second linear radiators, first and second mutually orthogonal linear components of reflections of the transmitted signals are received, to thereby produce first and second received signals. A set of correction factors to the first and second received signals is procured, where the correction factors are a function of (a) beam direction in the case of monostatic radar and (b) beam directions in the case of bistatic radar. The first and second received signals are processed with the correction factors in such a fashion as to cause the received signal to appear as if (a) substantially perfect linear polarization had been transmitted and received in the desired direction. In a particular mode of this method, the step of procuring a set of correction factors comprises the steps of placing a sphere to receive the transmitted signal and reflect the reflected signal at a plurality of beam directions, adjusting the amplitudes and phases of the first and second received signals to cancel the combined echoes at each of the beam directions, and tabulating the amplitudes and phases at which the cancellation occurs for each beam direction to generate the set of correction factors. The correction factors may be a function of carrier frequency if the transmitted signals are at various frequencies.

According to a further aspect of the invention, a method for determining the nonsphericity of a radar target comprises the step of transmitting, from an antenna, an electromagnetic signal at a carrier frequency, which signal includes two mutually orthogonal linear components nominally controlled in phase and amplitude to produce linear polarization to thereby produce first and second transmitted signals. The two mutually orthogonal linear components may be such that (a) the polarization of the transmitted signal is at an undesired angle and (b) the transmitted signal includes a circular component. By means of first and second linear radiators, first and second mutually orthogonal linear components of reflections of the transmitted signals are received, to thereby produce first and second received signals. A set of correction factors to the first and second received signals is procured. The correction factors are a function of (a) beam direction in the case of monostatic radar and (b) beam directions in the case of bistatic radar. The first and second received signals are processed with the correction factors in such a fashion as to cause the received signal to appear as if substantially perfect linear polarization had been transmitted and received in the desired direction. A radar target is placed for reflection of the first and second transmitted signals, to thereby produce third and fourth received signals. The correction factors are applied to the third and fourth received signals, to thereby produce corrected third and fourth received signals. The corrected third and fourth received signals are compared to determine the nonsphericity of the radar target. The correction factors may be a function of carrier frequency if the transmitted signals are at plural frequencies.

DESCRIPTION OF THE INVENTION

FIG. 1ais a simplified diagram in block and schematic form of a portion10of a radar system including a transmit/receive (TR or T/R) module12and elemental crossed (mutually orthogonal) antenna radiators1and2of an array element3of an array antenna designated generally as4. Antenna array4also includes other array elements, some of which are illustrated as3a,3b,3c, and3d. Thus, antenna radiators1and2together make up an antenna element or elemental antenna3of antenna array4. The elemental radiators may be dipoles, and for convenience in explanation they are termed dipoles, although they may be other types of linear antenna radiator elements. The physical orientation of the radiator elements1and2may be expressed as ±45°, and, looking outward, radiator dipole1is canted from upper left to lower right, while radiator dipole2is perpendicular or orthogonal to radiator dipole1. A beamformer14receives signals to be transmitted at a port14t, and distributes the signals to be transmitted to at least n ports, one of which is designated14n. InFIG. 1a, a portion of the signal to be transmitted by antenna radiators1and2is applied from a port14nof beamformer14to switches16and18. More particularly, the signal to be transmitted is applied to switch terminals or electrodes161and181, respectively. Switches16and18are represented as ganged mechanical switches, each with fixed electrodes and a movable element connected to a common electrode, as is conventional for purposes of explanation. Those skilled in the art know that electronic switches are ordinarily preferred for this purpose, and that the illustrated mechanical equivalents are only for purposes of explanation. In the arrangement ofFIG. 1a, the movable element of switch16is designated16m, which is shown in the “transmit” mode as connecting a switch terminal or electrode162to common electrode16c. Also inFIG. 1athe movable element18mof switch18connects switch electrode181to common electrode18c. The signal applied from beamformer14to switch element161goes no further for lack of a connection. The signal applied from beamformer14to switch element181is coupled by switch18through a steering phase block20. Steering phase block20adjusts the phase of the signal to be transmitted by crossed dipole radiators1and2relative to other dipole elements (not illustrated) of the array4to steer the transmit beam in a selected direction. The steering-phase-adjusted signal to be transmitted is coupled from steering phase block20and by way of movable switch element16mto a port221of a circulator22. Circulator22circulates the steering-phase-controlled signal to be transmitted in the direction of circulation, indicated by the arrow, to a further port222of the circulator22.

The signal to be transmitted which leaves port222of circulator22ofFIG. 1ais coupled to a 3 dB power divider/combiner41and by individual paths within a phase control block24to crossed linear radiators or dipole elements1and2of antenna element3. Block24is often referred to as a “polarization switch.” A first path extends from circulator port222to dipole radiator1and passes through a phase shifter designated φ1, and a second path extends from circulator port222to dipole radiator2and passes through a phase shifter designated φ2. Phase shifters φ1and φ2adjust the electrical phase of the signals to be transmitted by dipole radiators1and2relative to each other. The polarization of the transmitted signal can be selected by adjustment of the phase shifters φ1and φ2. For right circular polarization, φ1−φ2=π/2 radians; for left circular polarization, φ2−φ1=π/2 radians; for linear vertical polarization φ1=φ2; and for horizontal linear polarization φ1=φ2. Thus, control of phase shifters φ1and φ2allows control or selection of the polarization of the transmitted signal26produced by the radiation of the two dipole radiators1and2. For equal amplitude radiation from radiators1and2, the gain of the paths through phase shifters φ1and φ2of phase control24must be equal.

In the context of a monostatic radar system, the signal transmitted by the arrangement ofFIG. 1, and by the overall array4of which antenna element3(radiators1and2) are a part, propagates away from the array antenna4, as known in the art, and may impinge on or be intercepted by a reflective target, such as an airplane (not illustrated). The reflective target tends to reflect the radar signal, and a portion of the reflected signal returns toward the radar system from which it originated. Those skilled in the art know that the polarization of the reflected signal depends, in part, on the polarization of the transmitted signal, and in part on the electrical characteristics of the target.FIG. 1brepresents the same portion of a radar system10as that ofFIG. 1a, but with switches16and18in the position for reception of the reflected signals.

InFIG. 1b, the movable elements16mand18mof switches16and18connect the switch common terminals16cand18cto terminals161and182, respectively. With switches16and18in the positions illustrated inFIG. 1b, the portion10of the radar is arranged for reception of reflected signals28. Radiator or dipole element1ofFIG. 1breceives or transduces a slant +45° component of that portion of the reflected signal arriving at the radar system, and radiator or dipole element2receives or transduces the −45° component. These +45° and −45° components will in general be of different amplitudes and phases. The +45° component from radiator or dipole element1passes through phase shifter φ1of phase control block24, and the −45° component from radiator or dipole element2passes through phase shifter φ2. The phase shifters may be in the same phase condition as for transmission, in which case the array element3, representing the combination of radiator or dipole elements1and2, will respond to the same received polarization as that which was transmitted. If the phase conditions of phase shifters φ1and φ2are changed after transmission of the signal to be transmitted, then the antenna element3(combination of radiators1and2) will respond to the polarization defined by the phase settings of phase shifters φ1and φ2. In general, the phase settings of the phase shifters φ1and φ2, together with the physical positions of radiators or dipoles1and2, determine those polarizations which the arrangement ofFIG. 1bwill receive or accept, and those which it will reject. Power splitter/combiner41combines the two phase shifted received signals and applies them to port222of circulator22.

In the receive mode ofFIG. 1b, portion10of the radar receives the selected polarization(s) as established by the physical positions of the radiator elements1and2and by the phase settings of phase shifters φ1and φ2, and applies the signals to port222of circulator22. Circulator22circulates the signals in the direction of the arrow to port223. The signals exit port223of circulator22and flow by way of switch terminal182, movable element18m, and common terminal18cto steering phase block20. In block20, the phase of array element3relative to other array elements of the array4is set, to thereby establish the receive beam pointing direction. The receive beam direction is often selected to be the same as the transmit beam direction. The received signals flow from steering phase adjustment block20to port14nof beamformer by way of common terminal16c, movable element16m, and terminal161of switch16. The received signal is processed by the beamformer to combine the signals from all the antenna elements3of the entire array, as known in the art, and make the combined signals available at a port14o.

FIG. 2ais a simplified diagram in block and schematic form of a portion200of a radar system according to an aspect of the invention, arranged for transmission of signals, andFIG. 2bis similar toFIG. 2a, but arranged for reception of reflected signal and for correction of polarization errors. InFIG. 2a, elements corresponding to those ofFIG. 1aare designated by like reference alphanumerics. In the transmit arrangement ofFIG. 2a, signals from output port14nof beamformer14are coupled through switch18, steering phase block20, and switch16as described in conjunction withFIG. 1a. The signals to be transmitted leave terminal162of switch16, are split in splitter/combiner41into first and second transmit portions, which proceed to the input ports of multipliers A1and A2. Multipliers A1and A2adjust the amplitudes of the two transmit signal portions. The first amplitude-adjusted transmit signal portion is applied from multiplier A1to port2211of circulator221, and the second amplitude-adjusted transmit signal portion is applied from multiplier A2to port2221of circulator222. Circulators221and222circulate their respective first and second transmit signal portions to their ports2212and2222, respectively. The first and second transmit signal portions leaving circulator ports2212and2222are applied to polarization phase control blocks φ1and φ2, respectively. Phase control blocks φ1and φ2determine the polarization of the transmit signals26transmitted by dipole antenna elements1and2of antenna element3.

Circulators221and222of portion200of the radar system ofFIG. 2ain the transmit mode of operation nominally circulate no signal to their ports2213and2223, respectively, but any signal received during the transmit mode of operation, and appearing at ports2213and2223, are coupled to a block224, and ultimately coupled to open switch terminal182of switch18, which prevents further propagation of the signal toward the beamformer14.

InFIG. 2bthe switches16and18are positioned or conditioned for reception. In the receive mode of operation, antenna radiators1and2of antenna element3receive mutually orthogonal components of the arriving reflected signal28from the target, and couple the received signal components to polarization phase shifters φ1and φ2, which phase shift the received signal components so that the nominal polarization to which antenna elements1and2of antenna element3respond is the desired polarization. The two received signal components from phase shifters φ1and φ2are applied to ports2212and2222of circulators221and222, respectively. Circulators221and222circulate the received signal components in the direction of the arrow to ports2213and2223, respectively. As mentioned, the polarization of the transmitted signal may deviate from the desired polarization, and consequently the received signal may not properly represent or carry the characteristics of the radar target. According to an aspect of the invention, correction, in the form of external control signals A3, A4, φ3and φ4, is applied to the received signal to compensate for errors in transmission. This correction is performed on the individual signal components received by antenna radiators1and2of antenna element3.

In the receiving mode of the arrangement200ofFIG. 2a, the first individual received signal component received by radiator1is phase shifted by phase shifter φ1to select the nominal receive polarization, and is circulated to an input port2241of a correction block224. Similarly, the second individual received signal component received by radiator2is phase shifted by phase shifter φ2to select the nominal receive polarization, and is circulated to an input port2242of correction block224. The first individual received signal components applied from circulator221to correction block224are applied to the cascade of a phase shifter φ3and a multiplier or amplitude adjuster A3. The second individual received signal components applied from circulator222to correction block224are applied to the cascade of a phase shifter φ4 and a multiplier or amplitude adjuster A4. Thus, the received signal components can be individually adjusted in amplitude and phase independent of the amplitude and phase of the transmitted signals under the control of control signals A3, A4, φ3and φ4. The corrected first and second individual received signal components are coupled to terminal182of switch18, and thence by way of steering phase block20and switch16to port14nof beamformer14. Those skilled in the art know that the beamformer14combines the received signals applied to its port14nwith the received signals from other antenna elements of the array, such as3a,3b,3c, and3d, to produce a combined signal for further processing.

In the receive operating mode of the portion200of the radar system illustrated inFIG. 2b, the first received signal component (originating from radiator1) applied to phase shifter φ3and multiplier or amplitude adjuster A3is corrected by phase shifts and amplitude adjustments from an external control source. The second received signal component (originating from radiator2) applied to phase shifter φ4and multiplier or amplitude adjuster A4is corrected by phase shifts and amplitude adjustments from an external control source. The phase and amplitude correction for the two received signal components are selected to compensate for or correct for polarization errors occurring during transmission, and also incidentally compensate for polarization errors occurring on reception. In other words, the corrections applied to correction block224are such as to make it seem that both the transmit and receive polarizations were or are perfect. The correction values are determined by a calibration process.

FIG. 3illustrates the calibration apparatus300and suggests the process by which the polarization correction factors are determined for the arrangement ofFIG. 2b. In short, the calibration nulls the reflected components from a symmetrical reflector6, such as a conductive or dielectric sphere, to determine the polarization error which must be corrected. It is assumed that the radar system has transmitted a signal of a particular, nominally known polarization (as by setting φ1and φ2ofFIG. 2ato nominal values for the desired polarization).FIG. 3represents the operation of the radar in a receive mode to receive the symmetrically-reflected transmitted signal, with application of nominal or zero polarization correction signals A3, A4, φ3, φ4. The apparatus ofFIG. 3includes the array antenna A, of which antenna elements3are a part. The signals received from the symmetrical reflector6are processed with the polarization corrections set to a nominal or zero value, so that the imperfections in the received polarization may be evaluated. The beamformer couples signals to produce a combined array output330. A processing step332performs a search for the values of A3, A4, φ3, and φ4for each antenna element or TR module which result in mutual cancellation or nulling of the first and second received signal components. The gains and phases are varied under control of minimum search block332by means of any one of a number of multidimensional search procedures or methods, such as (a) gradient search, (b) multidimensional least squares, or (c) quasi-Newton minimization, described, for example, in M. A. Branch and A. Grace,Optimization Toolbox for use with MATLAB, user's guide, Mathworks, Inc. of Natlick, MA 1990, 1996. The search procedure produces a vector of gains and phases that produces a minimum of the array output. When nulling occurs, the first and second received signal components are known to be identical in amplitude and of mutually opposite phase. The values of A3, A4, φ3, and φ4for each antenna element of the array antenna which provide the nulling for each array antenna as a whole are stored in a memory334for later use during normal (non-calibration) operation. The values of A3, A4, φ3, and φ4are determined for each possible or potential beam direction or pointing angle. Thus, each pair of mutually orthogonal elemental antennas of the array antenna has its own particular values of A3, A4, φ3, and φ4at each beam pointing direction. If the values of A3, A4, φ3, and φ4are a function of operating frequency, then the values of A3, A4, φ3, and φ4must additionally be tabulated for the various operating frequencies.

Normal operation of the radar can begin once the values of A3, A4, φ3, and φ4which null the reflected signal components are known and stored in memory334ofFIG. 3. The following equations must be satisfied. For right circular polarization,
(φ1+φ3)−(φ2+φ4)=±π;A1A3=A2A4(1)
where φ represents phase shift and A represents gain

For left circular polarization,
(φ1+φ3)+(φ2+φ4)=±π;A1A3=A2A4(2)

It is possible to perform the polarization correction at the antenna element level, at the subarray level, or at the array level. Polarization correction at the antenna element level (where each antenna element consists of a pair of mutually orthogonal dipole radiators) is described in conjunction with the description ofFIG. 2b. Polarization correction at the array or subarray level is described in conjunction withFIG. 4. In this type of correction, the array or subarray signals are collected, reduced to coherent digital baseband to produce digitized complex envelope, and the correction is applied to the resulting digital signals. This type of correction is advantageous in that the correction may be made very fine, limited only by the correction word length. A variant of the array or subarray level correction allows parallel and simultaneous correction of any desired polarization properties such that the effective transmissions and corresponding receptions are as though they were perfectly polarized.

FIG. 4is a simplified diagram in block and schematic form illustrating a portion of a radar system400according to an aspect of the invention, in which separate beam steering phase is used for transmission and reception, and the received signals from each pair of mutually orthogonal dipoles are combined before generating digital signals and processing the digital signals to produce the desired overall receive beam polarizationFIG. 4illustrates the application of polarization correction signals to signals collected from a plurality of elemental antennas (where an elemental antenna consists of means for separately receiving two mutually orthogonal linear polarizations). InFIG. 4, beamformer14is a transmit-only beamformer, unlike the corresponding beamformer ofFIGS. 2aand2b. The signal to be transmitted is applied through transmit beam steering phase block20and power divider410to a block424, which includes multipliers or amplifiers A1and A2and phase shifters φ1and φ2, for setting the transmit polarization to be transmitted from the antenna element3, including radiators or dipoles1and2, to produce transmitted signal26.

In the receive mode of operation, the antenna element3including radiators or dipoles1and2receives signal28, which is or are independently circulated (or switched, if switches are used instead of circulators) by way of a pair of paths to phase shifters410and412. The values or settings of phase shifters410and412are selected to be the same as the steering phase imposed by block20, so as to steer the receive beam in the same direction as the transmit beam. The steering phase must be applied independently to each path to maintain equal receive signal phase characteristics. The steering-phase-corrected received signals from phase shifters410and412are applied to signal collectors414and416, respectively, which correspond to receive beamformers (or possibly sub-beamformers), collecting the steering-phase-corrected signals from a antenna elements, such as elements3a,3b,3c, and3ddistributed over the radar antenna array. The beamforming characteristics may be the same as those of the transmit beamformer14, to thereby provide receive beam characteristics generally similar to those of the transmit beam. The outputs of signal collectors414and416are collected received signals from antenna element3and other antenna elements of the array antenna4.

The corrected received signals from signal collectors414and416ofFIG. 4are expected to have sufficiently high signal-to-noise ratio for proper processing. The corrected received signals from collectors414and416are applied to receivers418and420, respectively, which downconvert the signals to intermediate frequency or baseband, and which may also amplify the signal. The downconverted or baseband signals from receivers418and420are converted to coherent digital form (two baseband components) for processing in analog-to-digital converters (A/D or ADC)422and424, respectively. The digitized baseband received signals appearing at the outputs of A/Ds422and424are applied to polarization correction blocks426and428, respectively. Within blocks426and428, amplitude correction multipliers A3and A4are set to the correction values for the receive beam pointing direction, and phase shifters φ3and φ4are set to the phase correction values for the receive beam pointing direction and the desired polarization. Finally, digital combiner438combines the phase-corrected values to produce the desired polarization characteristics in the receive mode. It should be noted that the calibration values for the arrangement ofFIG. 4, being based on summed received signals rather than on individual received signals, may be different from those used in the arrangement ofFIG. 2b.

FIG. 5represents a portion of the arrangement ofFIG. 4, modified to have additional correctors to simultaneously provide diverse desired receive polarizations. InFIG. 5, elements corresponding to those ofFIG. 4are designated by the same alphanumerics. InFIG. 5, only one pair of amplitude correctors A3and A4is necessary. The outputs of amplifiers or multipliers A3and A4are applied by way of buses to a plurality of phase-corrector/summer blocks, two of which are illustrated as510aand510n. Each of the phase-corrector/summer blocks includes a phase shifter connected to the output of amplifier A3, and another phase shifter connected to the output of amplifier A4. The outputs of the phase shifters of each phase-corrector/summer block are connected to a summing (Σ) circuit which sums the two applied phase-shifted signals to produce a digital output signal representing the reception of signal at the desired polarization. Thus, phase-corrector/summer block510aincludes a phase shifter φ3(1)having its input port coupled to the output of amplifier A3and its output connected to an input port of summing circuit538a, and another phase shifter φ4(1)having its input port coupled to the output of amplifier A4and its output connected to a second input port of summing circuit538a, for generating a signal on path540arepresenting one of RHCP reception, LHCP reception, +45° linear, or −45°, or any other polarization. Similarly, phase-corrector/summer block510nincludes a phase shifter φ3(n)having its input port coupled to the output of amplifier A3and its output connected to an input port of summing circuit538n, and another phase shifter φ4(n)having its input port coupled to the output of amplifier A4and its output connected to a second input port of summing circuit538n, for generating a signal on path540nrepresenting another polarization. These signals representing various receive polarizations occur simultaneously.

Simultaneous achievement of several polarizations or polarization components from the arrangement ofFIG. 5requires that the values of A3, A4, φ3(1)and φ4(1)satisfy equations (1), (2), (3), and (4), and that the values of A3, A4, φ3(n), and φ4(n)also satisfy equations (1), (2), (3), and (4).

The beam steering direction selection during normal operation of the polarization-calibrated radar system also aids or contributes to selection of the correction factors for the selected beam direction.

A major advantage of the polarization correction method according to an aspect of the invention is that the phase and amplitude controls are executed in the receive portion of the radar system, which operates at low power relative to the transmit portion of the radar. This also tends to reduce the need for tight specifications on the transmit polarization because the correction can be made upon reception.

It should be noted that while horn, flat-plate or spherical reflectors can be used to calibrate monostatic radar, only an isotropic reflector, such as a sphere, can be used for calibration of bistatic or multistatic radars.

Those skilled in the art know that many variants of the illustrated arrangements are possible. For example, radio-frequency (RF) switches may be used instead of circulators in the arrangements ofFIGS. 2aand2b. While the antenna elements of the array antenna have been described as dipoles, other types of antennas can be used, so long as on reception they transduce linear components of the received signal. While the minimization technique may not be capable of complete nulling of the radiation from the calibration reflector, reasonable nulling should still be sufficient to provide improvement in the polarization operation.

A method according to an aspect of the invention is for compensating errors in transmitted circular polarization such that reflections from spherical objects (6) tend to be cancelled. The method comprises the step of transmitting from an antenna (1,2;3;4) an electromagnetic signal (26) at a carrier frequency. The transmitted signal (26) includes mutually orthogonal linear components nominally phased to generate circular polarization, but which may instead generate noncircular or elliptical polarization transmissions. The method also includes the step of receiving, by means of first (1) and second (2) linear radiators, first and second mutually orthogonal linear components of reflections of the transmitted signals (26), to thereby produce first and second received signals. A set of correction factors (A3, A4, φ3and φ4) to the first and second received signals is procured. The correction factors (A3, A4, φ3and φ4) are a function of (a) beam direction in the case of monostatic radar and (b) beam directions in the case of bistatic radar. These correction factors (A3, A4, φ3and φ4) may also be a function of carrier frequency if the system operates at a plurality of frequencies. The first and second received signals are processed with the correction factors (A3, A4, φ3and φ4) in such a fashion as to cause the received signal to appear as if substantially perfect circular polarization had been transmitted and received. In a particular mode of this method, the step of procuring a set of correction factors (A3, A4, φ3and φ4) comprises the steps of placing a sphere (6) to receive the transmitted signal (26) and to reflect the reflected signal (28). The correction factors (A3, A4, φ3and φ4) may be generated for a plurality of beam directions. The amplitudes and phases of the first and second received signals are adjusted to cancel the combined echoes at each of the beam directions. The amplitudes and phases at which the cancellation occurs for each beam direction are tabulated to generate the set of correction factors (A3, A4, φ3and φ4).

A method for determining the ellipticity of a radar target according to another aspect of the invention comprises the step of transmitting from an antenna (1,2;3;4) an electromagnetic signal at a carrier frequency, where the transmitted signal includes mutually orthogonal linear components nominally phased to generate circular polarization, but which may instead generate noncircular or elliptical polarization, to thereby produce transmitted signals (26). The method also includes the step of receiving, by means of first (1) and second (2) linear radiators, first and second mutually orthogonal linear components of reflections (28) of the transmitted signals (26), to thereby produce first and second received signals. A set of correction factors (A3, A4, φ3and φ4) to the first and second received signals is procured, which correction factors (A3, A4, φ3and φ4) are a function of (a) beam direction in the case of monostatic radar and (b) beam directions in the case of bistatic radar. If the transmitted signals are at different frequencies, the correction factors (A3, A4, φ3and φ4) may take frequency into account. The first and second received signals are processed with the correction factors (A3, A4, φ3and φ4) in such a fashion as to cause the received signal (28) to appear as if substantially perfect circular polarization had been transmitted and received. The method also includes the step of receiving, from a nonspherical target, third and fourth mutually orthogonal linear components of reflections (28) of the transmitted signals (26). The third and fourth received signals are processed with the correction factors (A3, A4, φ3and φ4) to thereby generate corrected mutually orthogonal received components. The characteristics of the corrected mutually orthogonal received components are compared to determine the nonsphericity or ellipticity of the target.

Another method according to an aspect of the invention is for compensating errors in transmitted and received circular polarization. The method comprises the step of transmitting an electromagnetic signal (26) at a carrier frequency from an antenna (1,2;3;4), to thereby produce transmitted electromagnetic signals. The transmitted electromagnetic signal (26) includes mutually orthogonal linear components nominally phased to generate circular polarization, but which may instead generate noncircular or elliptical polarization. This method also includes the step of receiving, by means of first and second linear radiators, first and second mutually orthogonal linear components of reflections (28) of the transmitted signals (26), to thereby produce first and second received signals. A set of correction factors (A3, A4, φ3and φ4) to the first and second received signals is procured. The correction factors (A3, A4, φ3and φ4) are a function of (a) beam direction in the case of monostatic radar and (b) beam directions in the case of bistatic radar. The correction factors (A3, A4, φ3and φ4) may be a function of carrier frequency if the transmitted signals range over a plurality of frequencies. The first and second received signals are processed with the correction factors (A3, A4, φ3and φ4) in such a fashion as to cause the received signal to appear as if substantially perfect circular polarization had been transmitted and received.

A method for compensating errors in transmitted andor received linear polarization according to yet another aspect of the invention comprises the steps of transmitting, from an antenna (1,2;3;4), an electromagnetic signal at a carrier frequency, which signal includes, or can be decomposed into, two mutually orthogonal linear components nominally controlled in phase and amplitude to produce linear polarization to thereby produce first and second transmitted signals. The actual phase and amplitude may be such that (a) the polarization of the transmitted nominally linearly polarized signal is at an undesired angle andor (b) the transmitted signal includes a circular component. By means of first (1) and second (2) linear radiators, first and second mutually orthogonal linear components of reflections (28) of the transmitted signals (26) are received, to thereby produce first and second received signals. A set of correction factors (A3, A4, φ3and φ4) to the first and second received signals is procured, where the correction factors (A3, A4, φ3and φ4) are a function of (a) beam direction in the case of monostatic radar and (b) beam directions in the case of bistatic radar. The first and second received signals are processed with the correction factors (A3, A4, φ3and φ4) in such a fashion as to cause the received signal to appear as if (a) substantially perfect linear polarization had been transmitted and received in the desired direction. In a particular mode of this method, the step of procuring a set of correction factors (A3, A4, φ3and φ4) comprises the steps of placing a sphere to receive the transmitted signal and reflect the reflected signal at a plurality of beam directions, adjusting the amplitudes and phases of the first and second received signals to cancel the combined echoes at each of the beam directions, and tabulating the amplitudes and phases at which the cancellation occurs for each beam direction to generate the set of correction factors (A3, A4, φ3and φ4). The correction factors (A3, A4, φ3and φ4) may be a function of carrier frequency if the transmitted signals are at various frequencies.

According to a further aspect of the invention, a method for determining the nonsphericity of a radar target comprises the step of transmitting, from an antenna (1,2;3;4), an electromagnetic signal at a carrier frequency, which signal includes two mutually orthogonal linear components nominally controlled in phase and amplitude to produce linear polarization to thereby produce first and second transmitted signals. The two mutually orthogonal linear components may be such that (a) the polarization of the transmitted signal is at an undesired angle and (b) the transmitted signal includes a circular component. By means of first (1) and second (2) linear radiators, first and second mutually orthogonal linear components of reflections of the transmitted signals are received, to thereby produce first and second received signals. A set of correction factors (A3, A4, φ3and φ4) to the first and second received signals is procured. The correction factors (A3, A4, φ3and φ4) are a function of (a) beam direction in the case of monostatic radar and (b) beam directions in the case of bistatic radar. The first and second received signals are processed with the correction factors (A3, A4, φ3and φ4) in such a fashion as to cause the received signal to appear as if substantially perfect linear polarization had been transmitted and received in the desired direction. A radar target, which may be nonspherical, is placed for reflection of the first and second transmitted signals, to thereby produce third and fourth received signals. The correction factors (A3, A4, φ3and φ4) are applied to the third and fourth received signals, to thereby produce corrected third and fourth received signals. The corrected third and fourth received signals are compared to determine the nonsphericity of the radar target. The correction factors (A3, A4, φ3and φ4) may be a function of carrier frequency if the transmitted signals are at plural frequencies.