Abstract:
A method and apparatus is provided, whereby a scanning, polarization and frequency diverse radar system measures the complete polarimetric characterization of weather targets without loss of scanning speed and without an additional ambiguity in the Doppler velocity beyond that given by Nyquist&#39;s sampling theorem. In one embodiment, a linear combination of a horizontally and a vertically polarized signal are transmitted at a predetermined first frequency. Cotemporaneously or nearly cotemporaneously with the transmitted signal of the first frequency, a horizontally polarized signal is transmitted at a predetermined second frequency. Horizontal and vertical receive channels receive echoes at the predetermined first frequency to determine, but not limited to determine, the co-polar elements of the scattering matrix. Horizontal and vertical receive channels receive echoes at the predetermined second frequency to determine, but not limited to determine, the cross-polar elements of the polarization matrix. The predetermined first and second frequencies are selected to maximize isolation yet allow practical implementation.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
   This patent application claims the benefit of U.S. Provisional Applications No. 60/236,638, filed on Sep. 29, 2000, and No. 60/259,681, filed on Jan. 4, 2001, each of which is incorporated herein by reference. 
   TECHNICAL FIELD 
   This invention relates to radar systems that transmit and/or receive multiply polarized beams of high frequency energy in a scanning mode to identify the presence, locus and characteristics of scatters in a region of space. 
   BACKGROUND OF THE INVENTION 
   The first radars for weather detection employed a single, fixed polarization transmission signal and receivers that were optimized to receive that particular polarization. 
   These first radars usually employed substantially linear polarization. Horizontal or Vertical polarization was most commonly used. It was less common to find single polarization scanning radars for weather detection that employ circular polarization or linear polarization at other than Horizontal or Vertical for a number reasons. One reason is that water droplets are asymmetric but aligned with vertical with respect to the surface of the Earth. 
   Since the first radars, it has been discovered that employing two or more fixed, orthogonal polarization signals in a radar is helpful in classifying and distinguishing targets such as distinguishing between essentially spherically symmetric ice particles and oblate water droplets. Polarization diversity also allows other advances and improvements over single polarization radars. 
   Polarization diversity can characterize a scatterer by what is known as the full polarimetric covariance matrix, which examines covariances between the co-polar and cross-polar received signals. The matrix consists of the 16 possible covariance combinations of the four possible time series from polarimetric scattering. They are S AA , the signal received on the A polarization channel due to an A polarized transmit signal, S AB , the signal received on the A polarization channel due to a B polarized transmit signal, S BA , the signal received on the B polarization channel due to an A polarized transmit signal, and S BB , the signal received on the B polarization channel due to a B polarized transmit signal. Here polarizations A and B refer to any two orthogonal basis polarizations that can be used. Due to the underlying physics and math, several of the 16 possible values are degenerate. Namely, by reciprocity, S BA  and S AB  are degenerate, and the covariance commutes (within a sign). This means that only a subset of all possible scattering scenarios and covariance computations are needed to generate the full polarimetric covariance matrix. The full polarimetric covariance matrix allows for complete polarimetric characterization of scatterers. 
   The covariances reveal the characteristics of the scatterer such as scattering coefficient, Doppler frequency, spectrum width, etc. Scattering coefficient relates to number and size of scatterers, Doppler frequency is directly proportional to the mean radial velocity of the scatterers. Spectrum width relates to turbulence within a sample volume. Some important parameters obtained from the full polarimetric covariance matrix are: Differential phase (Φ DP ), Differential Reflectivity (Z DR ), Horizontal Reflectivity (Z H ), Vertical Reflectivity (Z V ), Correlation (ρ HV ), Linear Depolarization (L DR ). 
   For example, one of the elements of the matrix is the scattering amplitude of the target received on the Horizontal channel when illuminated with a Vertically polarized transmit pulse. Matrix parameters involving both polarizations are known as cross-polar. These polarization products are well known to those skilled in the art and are fully described in the literature, including detailed performance aspects determined through decades of field experiments. Cross-polar measurements are especially useful for particle identification measurements, such as ice detection. Cross-polar measurements require an antenna optimized for cross-polarization isolation (ICPR). Co-polar measurements are generally useful for determining total liquid water content. 
   Two general methods are used currently to implement polarization diversity. The first method relies on transmitting one of two polarizations in succession (usually alternately). The switching is normally accomplished using a high power A-B switch and an antenna with a separate feed for each polarization. The switch alternatively routes the transmit signal to one or the other of the antenna feeds depending on the polarization desired. Some systems use two high power amplifiers preceded by a similar switching arrangement. In other words the A-B switching is done at low powers prior to being amplified. 
   Two receivers are used to simultaneously receive co-polar and cross-polar returns for the scatterers. The full polarization matrix can be deduced (within certain limitations and using certain assumptions), but requires twice the number of transmit pulses (since each polarization is alternated) and hence the scan speed must be reduced by a factor of two to regain signal sensitivity and statistics. An additional ambiguity (beyond that given by the Nyquist sampling theorem) exists in the measurement of Doppler velocity using this technique. It is resolved by an assumption of typical scatterer behavior. However, in some cases this assumption is incorrect causing an erroneous Doppler velocity measurement. 
   A second method (referred to as ‘simultaneous transmit’ or ‘45 degree transmit’) transmits a linear combination of Horizontally and Vertically polarized energy. This is usually accomplished using a high power splitter to simultaneously route the transmitter energy to the two feeds of a dual polarization antenna. A system with two high power amplifiers and appropriate drive circuitry can also be used. The result is in general elliptically polarized, but the signal processing techniques used can easily account for any amplitude and phase offsets encountered. With this technique, many but not all of the parameters of the scattering matrix can be deduced. This technique does not suffer the loss of scanning speed or include the additional ambiguity in Doppler velocity, as does the alternating scheme above. However, certain of the parameters in the scattering matrix that cannot be obtained with this technique are meteorologically significant. 
   What is needed is a polarization diverse radar system capable of measuring the full scattering matrix of weather targets (unlike the simultaneous transmit technique) without loss of scanning speed and without an additional ambiguity in the Doppler velocity (associated with the alternating polarization technique). It is therefore an object of the invention to provide a polarization diverse radar system to determine the full scattering matrix of scatterers. It is a further object of this invention to provide this measurement without the need to reduce the scanning speed. It is a further object of this invention to provide these measurements without an additional ambiguity in Doppler velocity beyond that required by the Nyquist sampling theorem. 
   SUMMARY OF THE INVENTION 
   In accordance with the invention, a radar system is provided a means of simultaneously transmitting a horizontally and vertically polarized pulse at a predetermined first frequency. 
   In accordance with the invention, a radar system is provided a means of transmitting a horizontally polarized pulse at a predetermined second frequency. This pulse is coincident or nearly coincident with the pulse transmitted at the predetermined first frequency. 
   In accordance with the invention a radar system is provided a means of receiving echoes returned from scatterers at said predetermined first frequency and polarized horizontally. 
   In accordance with the invention a radar system is provided a means of receiving echoes returned from scatterers at said predetermined first frequency and polarized vertically. 
   In accordance with the invention a radar system is provided a means of receiving echoes returned from scatterers at said predetermined second frequency and polarized horizontally. 
   In accordance with the invention a radar system is provided a means of receiving echoes returned from scatterers at said predetermined second frequency and polarized vertically. 
   In accordance with the invention there is provided a method and apparatus, whereby a scanning, polarization and frequency diverse radar system measures the complete polarimetric characterization of weather targets without loss of scanning speed and without an additional ambiguity in the Doppler velocity beyond that given by Nyquist&#39;s sampling theorem. In one embodiment, a linear combination of a horizontally and a vertically polarized signal are transmitted at a predetermined first frequency. Cotemporaneously or nearly cotemporaneously with the transmitted signal of the first frequency, a horizontally polarized signal is transmitted at a predetermined second frequency. Horizontal and vertical receive channels receive echoes at the predetermined first frequency to determine, but not limited to determine, the co-polar elements of the scattering matrix. Horizontal and vertical receive channels receive echoes at the predetermined second frequency to determine, but not limited to determine, the cross-polar elements of the polarization matrix. The predetermined first and second frequencies are selected to maximize isolation yet allow practical implementation. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS  
     The accompanying drawings, which are incorporated in and form a part of the specification, illustrate the preferred embodiments of the present invention, and together with the written description and claims, serve to explain the principles of the invention. In the drawings: 
       FIG. 1  illustrates the coordinate system of an azimuth and elevation gimbal; 
       FIG. 2  illustrates a block diagram of the overall invention; 
       FIG. 3  illustrates the details of the processing system; and, 
       FIG. 4  illustrates an improved receiver configuration. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
   For purposes of discussion, ground based radars will be assumed in which a gimbaled mount is used to scan a pencil beam antenna  32  in azimuth and elevation as per the general coordinate system  200  shown in  FIG. 1 . The reference plane  210  is assumed substantially parallel to the local surface of the Earth. The elevation angle  206  defines the pointing direction  202  of antenna  32 . Zenith vector  214  is perpendicular to reference plane  210 . The projection  216  of pointing direction  202  lying on plane  210  defines the azimuth angle  208  to a northern reference vector  212 . An elevation angle of zero degrees is defined to point at the horizon (points lying in plane  210 ), and an elevation angle of 90 degrees is defined to point to the zenith  214 . Azimuth angles  208  are defined about the zenith  214 . The zero azimuth reference point is generally aligned to north  212 . However, this is only by convention and does not affect the geometry. Radars employing ground-based scanners are introduced here only to simplify the following discussion. The principles herein can be applied to scanners on other or moving reference frames (such as aircraft), or scanners using various other coordinate systems (such as a polar mount). 
   By fixed polarization it is meant the single unique polarization of the traveling electromagnetic wave generated by the antenna  32 , and with respect to the antenna  32  when excited through a particular input port such as inputs  28 ,  30  or in a particular way. An antenna  32  capable of transmitting one or more fixed polarizations and gimbaled to perform scanning will generate a traveling wave who&#39;s polarization with respect to a fixed coordinate system such as the Earth will not remain fixed as the antenna  32  scans. For that reason care has to be used to specify the elevation angle  206  and/or the azimuth angle  208  where needed to define a polarization. 
   For the purpose of this discussion, horizontal polarization is defined as substantially linear polarization with the E-field of the traveling electromagnetic wave parallel to the horizon (plane  210 ) when the elevation angle  206  of antenna  32  is zero. Vertical polarization is defined as substantially linear polarization with the E-field of the traveling electromagnetic wave perpendicular to the horizon (plane  210 ) when the elevation angle  206  of antenna  32  is zero. These are the generally accepted definitions known to those skilled in the art. 
     FIG. 2  shows a block diagram of the general system  100 . Transmitter  10  generates a pulse of high frequency energy at a predefined first frequency. Splitter  14  divides the energy from transmitter  10  equally to two paths. One path travels through circulator  16 , then through directional coupler  22  and into the V port  28  of antenna  32 . Antenna  32  radiates this energy into a pencil beam with a specific polarization such as vertical. The other path of energy from splitter  14  is passed to frequency duplexer  20 . Frequency duplexer  20  combines either of its inputs  21   a,    21   b  to its output  21   c  with substantially no loss. Energy from transmitter  10  through splitter  14  and into frequency duplexer  20  input  21   a  therefore emerges at frequency duplexer  20  output  21   c  and flows to circulator  24 . From circulator  24  the energy flows through directional coupler  26  and into H port  30  of antenna  32 . Antenna  32  radiates this energy into a pencil beam with a specific polarization such as horizontal, and usually substantially orthogonal to that radiated as resulting from energy input into V port  28  of antenna  32 . 
   A fraction of the energy passing from circulator  16  through directional coupler  22  to V port  28  of antenna  32  is coupled to receiver  44 . Processing system  52  uses the detected signal from receiver  44  for calibration and coherent Doppler processing. 
   Energy incident on antenna  32  at a predefined first frequency as transmitted by transmitter  10  and having reflected from scatterers in the atmosphere emerges from V port  28  and H port  30  of antenna  32 . The energy from V port  28  passes through directional coupler  22  and through circulator  16  to Low Noise Amplifier (LNA)  18 . LNA  18  greatly amplifies the signal for subsequent reception and detection. Splitter  36  routes substantially equal amounts of the signal from LNA  18  to receiver  40  and receiver  42 . Receiver  42  contains frequency selective electronics such that little output of receiver  42  arises from signals at the predetermined first frequency. Receiver  40  also contains frequency selective electronics that allow efficient detection of signals at the predetermined first frequency. The detected signal from receiver  40  is used by the processing system  52  to generate the complete polarimetric characterization  54  of scatterers. 
   The energy from H port  28  passes through directional coupler  26  and through circulator  24  to Low Noise Amplifier (LNA)  34 . LNA  34  greatly amplifies the signal for subsequent reception and detection. Splitter  38  routes substantially equal amounts of the signal from LNA  34  to receiver  46  and receiver  48 . Receiver  48  contains frequency selective electronics such that little output of receiver  48  arises from signals at the predetermined first frequency. Receiver  46  also contains frequency selective electronics that allow efficient detection of signals at the predetermined first frequency. The detected signal from receiver  46  is used by the processing system  52  to generate the complete polarimetric characterization  54  of scatterers. 
   Transmitter  12  generates a pulse of high frequency energy at a predefined second frequency. This energy is incident on frequency duplexer input  21   b  and is passed with substantially low loss to frequency duplexer output  21   c  and through circulator  24 . From circulator  24  the energy passes through directional coupler  26  and into H port  30  of antenna  32 . 
   A fraction of the energy passing from circulator  24  through directional coupler  26  to H port  30  of antenna  32  is coupled to receiver  50 . Processing system  52  uses the detected signal from receiver  50  for calibration and coherent Doppler processing. 
   Energy incident on antenna  32  at a predefined second frequency as transmitted by transmitter  12  and having reflected from scatterers in the atmosphere emerges from V port  28  and H port  30  of antenna  32 . The energy from V port  28  passes through directional coupler  22  and through circulator  16  to Low Noise Amplifier (LNA)  18 . LNA  18  greatly amplifies the signal for subsequent reception and detection. Splitter  36  routes substantially equal amounts of the signal from LNA  18  to receiver  40  and receiver  42 . Receiver  40  contains frequency selective electronics such that little output of receiver  40  arises from signals at the predetermined first frequency. Receiver  40  also contains frequency selective electronics that allow efficient detection of signals at the predetermined first frequency. The detected signal from receiver  42  is used by the processing system  52  to generate the complete polarimetric characterization  54  of scatterers. 
   The energy from H port  28  passes through directional coupler  26  and through circulator  24  to Low Noise Amplifier (LNA)  34 . LNA  34  greatly amplifies the signal for subsequent reception and detection. Splitter  38  routes substantially equal amounts of the signal from LNA  34  to receiver  46  and receiver  48 . Receiver  48  contains frequency selective electronics such that little output of receiver  48  arises from signals at the predetermined first frequency. Receiver  46  also contains frequency selective electronics that allow efficient detection of signals at the predetermined first frequency. The detected signal from receiver  46  is used by the processing system  52  to generate the complete polarimetric characterization  54  of scatterers. 
   Receivers  40 ,  42 ,  44 ,  46 ,  48 , and  50  take as input a presumably weak signal modulated on a high frequency carrier, amplify and downconvert it to baseband and generate a stream of sampled, complex data (I&#39;s and Q&#39;s) for processing. In practice this is accomplished by a dual or triple hetrodyne process. A first frequency conversion mixes the high frequency input with a Stable Local Oscillator (STALO) resulting in a first intermediate frequency (1 st  IF). In a triple conversion system, another conversion occurs with a second local oscillator to generate a 2 nd  IF. Finally, the IF is downconverted to baseband to generate inphase and quadrature (I and Q) signals. The I and Q signals are digitally sampled for use by a digital signal processor. Often modern receivers digitize the 2 nd  IF and perform the final baseband conversion digitally. 
   Those skilled in the art understand that the H and V polarizations referred to in this discussion represent a typical set of polarizations, and that there are other possible sets of orthogonal or substantially orthogonal polarizations that can be used without changing the spirit or intent of the invention. It shall be understood that the H and V polarizations are used in this discussion as a particular set of polarizations, but in no way limit the scope of this disclosure to that particular set of polarizations. Other polarizations can be substituted for the stated H and V polarizations without changing the spirit and intent of the invention. 
   Similarly, those skilled in the art understand that the H and V polarizations referred to in this discussion can be constructed as a linear combination of other orthogonal or substantially orthogonal sets of polarizations. The scope of this invention includes the technique of reconstructing H and V equivalent polarizations using other polarization sets. 
   The particular receivers  40 ,  42 ,  44 ,  46 ,  48 , and  50  shown in black-box form serve well to clearly illustrate the functionality, spirit and scope of the invention. However, an improved receiver system is given further below illustrating a more practical implementation while still providing the same functionality described above. 
     FIG. 3  shows processing system  52  in greater detail. Transmitters  10  and  12  may be coherent amplifiers such as a traveling wave tubes (TWT) or klystrons. They may also be magnetrons, which generate signals of unpredictable phase and relatively constant but somewhat uncertain frequency. Compensator  60  uses TX SAMPLE 1  as received by receiver  44  and standard algorithms known to those skilled in the art to compensate the S VV1  data from receiver  40  and the S HH1  data from receiver  46  for the transmitter  10 &#39;s random phase and/or fluctuating amplitude to generate coherent data S VV1 ′ and S HH1 ′ respectively. Coherent data S VV1 ′ is used as input to covariance algorithm  67 . Based on this input, covariance algorithm  67  generates power data P VV1  and velocity data V VV1 . In the preferred embodiment, the covariance algorithm  67  used is the pulse pair algorithm, known by that name to those skilled in the art. However, other algorithms such as Fourier processing algorithms or other algorithms known to those skilled in the art could be used without departing from the spirit or scope of this invention. Outputs P VV1  and V VV1  are interpreted as the vertical co-polar power and velocity respectively for frequency f 1 . 
   Coherent data S HH1 ′ and S VV1 ′ is used as input to covariance algorithm  66 . Based on this input, covariance algorithm  66  generates power data differential phase data Φ DP1 . In the preferred embodiment, the covariance algorithm  66  used is the pulse pair algorithm, known by that name to those skilled in the art. However, other algorithms such as Fourier processing algorithms or other algorithms known to those skilled in the art could be used without departing from the spirit or scope of this invention. Output Φ DP1  is interpreted as the differential phase respectively for frequency f 1 . 
   Coherent data S HH1 ′ is used as input to covariance algorithm  65 . Based on this input, covariance algorithm  65  generates power data P HH1  and velocity data V HH1 . In the preferred embodiment, the covariance algorithm  65  used is the pulse pair algorithm, known by that name to those skilled in the art. However, other algorithms such as Fourier processing algorithms or other algorithms known to those skilled in the art could be used without departing from the spirit or scope of this invention. Outputs P HH1  and V HH1  are interpreted as the horizontal co-polar power and velocity respectively for frequency f 1 . 
   Coherent data S HV2 ′ is used as input to covariance algorithm  64 . Based on this input, algorithm  64  generates power data P HV2 . In the preferred embodiment, the covariance algorithm  64  is the pulse pair algorithm, known by that name to those skilled in the art. However, other algorithms such as Fourier processing algorithms or other algorithms known to those skilled in the art could be used without departing from the spirit or scope of this invention. Output P HV2  is related to the linear depolarization ratio L DR  for frequency f 2 . 
   Coherent data S HH2 ′ is used as input to algorithm  63 . Based on this input, algorithm  63  generates power data P HH2  and velocity data V HH2 . In the preferred embodiment, the covariance algorithm  63  is the pulse pair algorithm, known by that name to those skilled in the art. However, other algorithms such as Fourier processing algorithms or other algorithms known to those skilled in the art could be used without departing from the spirit or scope of this invention. Outputs P HH2  and V HH2  are interpreted as the horizontal co-polar power and velocity respectively for frequency f 2 . 
   Product generator  68  combines the results from algorithms  63 ,  64 ,  65 ,  66 , and  67  to form a complete polarimetric characterization  54  of scatterers. The final velocity estimate is a weighted combination of the three velocity estimates V VV1 , V HH1 , and V HH2 . Although the data S VV1 ′ and S HH1 ′ that result in V VV1  and V HH1  are highly correlated, they together are independent of the data S HH2 ′ resulting in V HH2 . The independence arises from the use of two different predefined transmit frequencies f 1  and f 2 . The weighted combination of velocity estimates from statistically independent data constitutes a 41% improvement in the variance of the resultant velocity estimate. 
   Differential reflectivity (Z DR ) is computed from frequency f 1  co-polar powers P VV1  and P HH1 . Since differential reflectivity (Z DR ) is computed from data S VV1 ′ and S HH1 ′ of the same frequency f 1 , and from the same instant in time, differential reflectivity measurement stability will be very high. 
   Differential phase (Φ DP ) is also computed from frequency f 1  data S VV1 ′ and S HH1 ′. Since differential phase (Φ DP ) is computed from data S VV1 ′ and S HH1 ′ of the same frequency f 1 , and from the same instant in time, differential phase measurement stability will be very high. 
   Linear depolarization (L DR ) is computed from frequency f 2  co-polar power P HH2  and cross-polar power P HV2 . 
   Reflectivity is derived from a weighted average of frequency f 1  co-polar power P HH1  and frequency f 2  co-polar power P HH2 . The two co-polar powers P HH1  and P HH2  are independent estimates since they arise from transmitted signals of two different frequencies f 1 , and f 2 . The weighted combination of reflectivity estimates from statistically independent data constitutes a 41% improvement in the variance of the resultant reflectivity estimate. 
     FIG. 4  shows an improved receiver chain designed around a set of IF receivers  80 ,  82 ,  84 ,  86 ,  88 , and  90 . This improvement does not depart from the spirit and scope of the invention but offers a simplified means of implementing receivers for the various signals in the present system. 
   The system of  FIG. 4  replaces splitters  36 ,  38  and receivers  40 ,  42 ,  44 ,  46 ,  48 , and  50  of  FIG. 3 . The output previously supplied by receiver  40  will then be supplied by IF receiver  80 . The output previously supplied by receiver  42  will then be supplied by IF receiver  82 . The output previously supplied by receiver  44  will then be supplied by IF receiver  84 . The output previously supplied by receiver  46  will then be supplied by IF receiver  86 . The output previously supplied by receiver  48  will then be supplied by IF receiver  88 . The output previously supplied by receiver  50  will then be supplied by IF receiver  90 . 
   The output of STALO  70  is split four ways by splitter  71 . The four splitter  71  signals drive frequency mixers  72 ,  73 ,  74 , and  75 . The signal from directional coupler  22  is mixed down by mixer  75  to a 1 st  IF frequency and received by IF receiver  84 . The output of LNA  18  is mixed down by mixer  72  to a 1 st  IF frequency. The 1 st  IF frequency output of mixer  72  is then split by splitter  78  and received by IF receiver  80  tuned for IF frequency IF 1  and IF receiver  82  tuned for IF frequency IF 2 . 
   The signal from directional coupler  26  is mixed down by mixer  74  to a 1 st  IF frequency and received by IF receiver  90 . The output of LNA  34  is mixed down by mixer  7 . 3  to a 1 st  IF frequency. The 1 st  IF frequency output of mixer  73  is then split by splitter  76  and received by IF receiver  86  tuned for IF frequency IF 1  and IF receiver  88  tuned for IF frequency IF 2 . 
   A practical implementation of the diagram of  FIG. 4  applies constraints to predetermined frequencies f 1 , and f 2 . If the frequencies f 1 , and f 2  differ too greatly, IF receivers  80 ,  82 ,  84 ,  86 ,  88 , and  90  will not be able to practically receive one or both signals. However, if the frequencies differ too little, frequency discrimination and hence isolation between channels will suffer. Within these constraints there is a range of frequency spacings giving adequate isolation, yet within a reasonable tenability range for practical IF receivers. 
   The foregoing description is considered as illustrative of the principles of the invention. Furthermore, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and process shown and described above. Accordingly, resort may be made to all suitable modifications and equivalents that fall within the scope of the invention; The words “comprise,” “comprises,” “comprising,” “include,” “including,” and “includes” when used in this specification are intended to specify the presence of stated features, integers, components, or steps, but they do not preclude the presence or addition of one or more other features, integers, components, steps, or groups thereof.