WO1987007389A2 - Adaptive radar for reducing background clutter - Google Patents
Adaptive radar for reducing background clutter Download PDFInfo
- Publication number
- WO1987007389A2 WO1987007389A2 PCT/US1987/001156 US8701156W WO8707389A2 WO 1987007389 A2 WO1987007389 A2 WO 1987007389A2 US 8701156 W US8701156 W US 8701156W WO 8707389 A2 WO8707389 A2 WO 8707389A2
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- WO
- WIPO (PCT)
- Prior art keywords
- radar
- signal
- null
- return
- signals
- Prior art date
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Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
- G01S7/02—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00
- G01S7/024—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00 using polarisation effects
- G01S7/026—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00 using polarisation effects involving the transmission of elliptically or circularly polarised waves
Definitions
- the present invention relates generally to £he field of radar and / more particularly, to radar apparatus having provision for substantially reducing background clutter, especially sea clutter, at low radar grazing angles.
- Doppler filtering Both range gating and Doppler filtering or Doppler frequency discrimination are known techniques for reducing the relative effects of background clutter in radar systems.
- Doppler filtering it is well known that frequency shifts, known as Doppler frequency shifts, are obtained from radar returns from moving objects. Since radar returns from objects that are stationary, or are stationary with respect to the radar transmitter and receiver, do not exhibit Doppler shifts, the possibility exists, through Doppler frequency filtering, to discriminate moving objects from stationary objects and, hence, moving targets from stationary background. Ground and sea clutter are, however, very widely dispersed and range and Doppler ambiguities greatly com ⁇ pound the problem of discriminating targets from their background.
- Range ambiguities in effect, break the range profile of radar target and background echos into zones which are superimposed upon one another.
- the radar echo from a target may be received simultaneously with clutter not only from the target's own particular range but also from the corresponding range in every other range zone.
- An unambiguous range zone in nautical miles, is approximately equal to 80 divided by the radar pulse repetition frequency (PRF) 'expressed in kHz; therefore, increasing the radar PRF narrows the zone ranges and increases the number of superimposed zones, making it increasingly difficult to isolate the target echo from the background clutter.
- PRF radar pulse repetition frequency
- Doppler ambiguities cause successive repetitions of the target and background Doppler profile to overlap, resulting in a target echo having to compete with background clutter whose true Doppler frequency is quite different from that of the target.
- increasing the radar PFF has the effect of moving the successive repetitions of the main lobe clutter further apart on the frequency axis, thereby making isolation of the target echo easier.
- radar PRF Doppler ambiguities dominate, whereas at high radar PRF's, range ambiguities dominate.
- the selection of radar PRF so as to reduce the effects of background clutter can, therefore, be seen not to be entirely effective.
- the polarization state of transmitted and reflected radar signals is also known to have an effect on the ability to discriminate targets from background clutter.
- radar signal polarization can be understood by considering the radar signals, both transmitted and reflected, to comprise electromagnetic waves having an transverse oscillatory motion defined by the orientation of the associated electric field vector.
- Elliptically polarized transmitted signals are provided by shifting the phase between the horizontal and vertical components of the signal.
- the receiving antenna is geometrically similar to the transmitting antenna and, in fact, the same antenna is usually used, on a time sharing basis, as both a trans ⁇ mitting and a receiving antenna.
- Radar return signals are commonly processed so that it appears that the transmitted and reflected signals have the same polari ⁇ zation state. In such regard, it is, however, generally known that the maximum amount of energy can be extracted from a reflected signal when the polarization state of the return signal is the same as the "polarization" of the receiving antenna.
- the polarization state of the reflected radar signal is opposite to the "polarization" of the receiving antenna, a minimum amount of energy is extracted from the return signal.
- Advantage of such effect is taken, for example, by using circularly polarized radar signals or receiver antenna processes to reduce the radar return clutter from rain. It should further be observed that depending upon characteristics of the target, the polarization state of the reflected signal may not be the same as that of the transmitted signal; that is, a target may "depolarize" the signal.
- polarization sphere of Poincare An elliptically polarized wave can be constructed of two orthogonal electric field vectors which represent minor and major axes of the ellipse and which have a ratio, r, of minor to major axes. Moreover, the major axis will be spatially oriented at an angle, ⁇ , relative to the local horizontal.
- a particular radar signal polarization state which produces a maximum signal return.
- This particular maximum polarization state can, of course, be represented by a point somewhere on the Poincare sphere. Over a period of time, if the object, for example, changes orientation relative to the radar, the maximum polari ⁇ zation state will typically change, giving rise to a series of maximum points for that particular object on the sphere. Depending on the nature of the object and its movement relative to the radar, these maximum points on the sphere may be quite widely scattered.
- any object at any point in time, will also have a null polarization state; that is, a polarization of transmitter and receiver producing zero signal return.
- any fixed reflecting target has two such null polarization states which are represented by a pair of points on the Poincare sphere. These two points representing null polarizations are understood to be on the same great circle of the sphere on which the two cross polarization nulls lie (wherein the transmitter and receiver are cross polarized) .
- null polarization techniques may possibly provide an approach to enhancing target detection in the presence of background clutter.
- Poelman has not, however, to the present inventor's knowledge, publicized radar data that illustrates the clutter rejection methods by considering average positions on the polarization (Poincare) sphere.
- Adaptive radar for substantially reducing relatively time-invarient background clutter, such as sea clutter, especially at low radar grazing angles, comprises radar signal trans ⁇ mitting means having orthogonal first and second radiating antenna elements and radar signal receiving means having orthogonal first and second receiving antenna elements. Included are controllable radar signal generating means for providing radar signals to the signal transmitting means at selected polarization states and controllable radar return signal processing means for processing return signals received by the signal receiving means in accordance with the selected polarization states.
- Null polarization computing and control means are included in the radar apparatus for determining, from signal returns received by the radar, a null polarization associated with background clutter and for causing the radar signal generating means and the radar return signal processing means to operate at a null polarization state corresponding to the background null polarization.
- the first and second radiating antenna elements may be generally rectangular apertures, which are associated with corresponding first and second rectangular radiating waveguides, and are respectively horizontally and vertically oriented.
- the first and second receiving antenna elements may be generally rectangular apertures which are associated with corresponding first and second rectangular receiving waveguides.
- the null polari ⁇ zation computing and control means are operative for causing a predetermined test signal to be provided by the radar signal generating means to the signal trans ⁇ mitting means and for determining from the return of the test signal the background null polarization.
- null polarization computing and control means also be operative for determining the background null polarizations for a predetermined number of ranges and azimuth cells and for determining therefrom a mean null polarization associated with the background clutter.
- the null polarization computing and control means are operative for computing a pair of first and second polarization nulls polarization of the background clutter and, in such case, for causing the radar signal generating means and the return signal processing means to operate at either first or second null polarization states corresponding respectively to the first and second polarization nulls.
- a corresponding method for substantially reducing, in radar and especially in low grazing angle radar, signal return clutter from relatively time- invarient background.
- the method comprises the steps of radiating, from a transmitting antenna, towards a relatively time-invarient backaground, a radar test signal; receiving from a receiving antenna return test signals from such background and determining from the return signals a null polarization of the background clutter.
- the method further comprises transmitting radar signals from the transmitting antenna at a polarization state equal to the background null polarization and processing radar return signals received from the receiving antenna at the null polarization of the ' ⁇ background clutter.
- the method includes transmitting the test and radar signals from separate, horizontally and vertically oriented transmitting elements and receiving the test and radar return signals through separate horizontally and vertically oriented receiving elements.
- the trans ⁇ mitting and receiving elements may be waveguides or dipoles or arrays thereof. Also, the transmitting and receiving elements may be common.
- the method includes determining a number of clutter null polarizations associated with a like number of radar range and azimuth cells, and then determining therefrom a clutter mean polarization null for all the cells involved.
- a pair of mean polarization nulls of the clutter may be determined, the method then including operating the radar alternately between two polarization states corresponding to the two clutter me % an polarization nulls.
- the radar return clutter associated with the background is eliminated, or at least substantially reduced, and target detection is thereby made easier.
- the radar apparatus is caused to operate alternately between two mean null poplarizations associated with the two background clutter null polarizations, the possibility of missing a target having a null polarization at one of the back ⁇ ground null polarizations is substantially reduced.
- FIG. 1 is a functional block diagram of a null polarization operating radar in accordance with the present invention, showing principal portions of the radar; and FIG. 2 is a functional block diagram showing a more detailed breakdown of the radar depicted in FIG. 1.
- a null polarization, adaptive radar 10, according to the present invention is shown in block diagram form in FIG. 1.
- radar 10 As more particularly described below, are transmitting antenna means 12, receiving antenna means 14, radar signal generating means 16, return signal processing means 18 and null polarization, background clutter computing and control means 20.
- Transmitting means 12 comprise first and second radar antenna portions 26 and 28, for example, con ⁇ ventional rectangular radar waveguide or dipole antenna, either individually or in an array, for respectively emitting horizontally and vertically polarized radar waves in a conventional manner.
- receiving means 14 comprise first and second, respective horizontal and vertical, radar return signal receiving antenna portions 30 and 32, which may also be of conventional configuration and which may comprise rectangular wave- guides, dipoles or arrays thereof.
- Radar transmitting means 12 are connected, by an electrical conduit or microwave guide conduit means 34, to radar signal generating means 16 for receiving therefrom "H" and "V" signals to be transmitted, respectively, by horizontal and vertical transmitter portions 26 and 28, as horizontally and vertically polarized signals or waves.
- radar receiving means 14 are connected, by an electrical
- Radar apparatus 10 is more particularly depicted, in block diagram form, in FIG. 2. Shown further comprising radar signal generating means 16 are respective "H” and “V” transmitters 56 and 58 as well as transmitter exciter and controller means 60. Internally, exciter and controller means provide control signals, over conduits 62 and 64, to respective "H” and “V” transmitters 56 and 58. Transmitters 56 and 58, in turn, provide » H" and "V” signals to respective "H” and “V” antenna portions 26 and 28 via conduit 34, such conduit comprising, as shown, separate conduits 66 and 68.
- "H” and “V” transmitters 56 and 58 and transmitter exciter and controller 60 are of known, conventional design, the exciter and controller adjusting the phase between "H” and “V” signals and their respective amplitudes to provide any desired linear, circular or elliptical polarization state of electromagnetic signals transmitted by antenna portions 26 and 28.
- Comprising return signal processing means are an "H” preamp 76, a “V” preamp 78, an "H” signal mixer 80, a “V” signal mixer 82, a coherent oscillator (reference oscillator) 84 (having a reference frequency f r ) , an IF amplifier 86, and a receiver polarization controller 88.
- Return signals from "H” and “V” receiving antenna portions 30 and 32 are input over respective conduits 90 and 92 (which comprise conduit 36) to "H” and “V” preamplifiers 76 and 78.
- Amplified "H” and “V” signals are fed from respective “H” and “V” preamplifiers 76 and 78, over conduits 94 and 96, to respective "H” and “V” mixers 80 and 82, which are fed, over conduits 98 and 100, a reference frequency (f r ) signal from coherent oscillator 84.
- IF intermediate frequency
- "H”, "V” and “ ⁇ ” data is, in turn, fed from IF amplifier 86, over respective conduits 112, 114 and 116, to receiver polarization controller 88.
- the "H” , "V” and “ ⁇ ” data from IF amplifier 86 are alternatively provided to null polarization computer and controller 20, over conduits 118, 120 and 122 (which comprise conduit 36) , as more particularly described below.
- Conduits 118, 120 and 122 tap into respective conduits 112, 114 and 116 through a switch 130.
- Receiver polarization controller 88 controls the processing therein of "H” and “V” data by establishing the polarization state therof, in actuality , by establishing the phase difference between the "H” and “V” return data and their relative amplitudes, as is known in the art.
- the return "H” and “V” data is processed at the same polarization state as the i radar signal is transmitted.
- Processed data is trans ⁇ mitted from receiver polarization controller 88, via conduit 44, to target report subsystem 46.
- "H” and “V” preamplifiers 76 and 78, mixers 80 and 82, coherent oscillator 84, IF amplifier 88 and receiver polarization controller 88 are of known configuration.
- Null polarization computing means 20 comprise clutter signal measurement unit 132, polarization null computer (all cells) 134 and mean null computer 136. °
- the functions of null polarization computing means 20 are to cause the generation of a test signal by trans ⁇ mitting antenna means 12 to determine, by the solving of the
- the mean null polarization determined by computing means 20 will be the mean null polarization of the background clutter (for example, of the sea clutter) . If necessaary, the mean of each azimuth cell 5 can be retained for processing returns if the variation among azimuth cells is considered to be too large for adequate clutter rejection.
- the mean null polarization determination of the present invention averages the null positions of 0 the clutter on the Poincare polarization sphere by finding the mean central angle to the center of the sphere.
- computing and control means 20 control radar signal generating means 16 so that antenna means 12 transmits radar signals at the mean null polarization state and also control return signal processing means 18 to operate at the same mean null polarization state.
- the return signals from the background that is, the background clutter
- null polarization computer and control means 20 make the polarization state selection based upon the determination of the computed mean null polarization of the measured background clutter.
- null polarization computing and control means can cause radar signal generating means 16 and return signal processing means 18 to alternately operate between both mean null polarization states so that any target which might have a null polarization state near one of the background null polarization state has a better chance of being detected.
- H the ratio of H and V
- ⁇ the relative phase
- Equation (1) the term ⁇ j refers to the sea surface radar cross section for cell combinations, (i, j), of vertical and horizontal transmission and to correponding vertical and horizontal reception.
- an can be written either as o-yv, a-gjj, ⁇ H t / and ⁇ vH or ⁇ ll' ⁇ 22' ⁇ 12' and &21 «
- H and V are unknown quantites having phase and amplitude.
- Radar apparatus 10 is first required to compute the radar cross section of each cell of interest, ⁇ ij, using the well known radar range equation:
- Equation (2) P r is the power density at a distance R (in meters) from a radar that radiates a power of P (in watts) from an antenna having a gain G t .
- ⁇ is the target cross section in meters 2 and A.r, is the effective aperture area, also in meters 2 .
- Equation (1) when the elements ⁇ ij are known is :
- ⁇ 0 indicates a phase angle relationship between H 0 and V 0 and wherein ⁇ 2 is equal to 021 (which accounts for only ⁇ .2 appearing in the right-hand side of the equation) .
- the system gain in the H and V channels are set at any suitable values such that their ratio is H 0 /V 0 and have a relative phase angle ⁇ .
- the values __£ and ⁇ for all cells are transmitted 0 from polarization null computer 134, via a conduit 146, to mean null computer 136, wherein a mean horizontal o control signal H 0 and a mean vertical control signal V 0 are computed.
- the mean values of H 0 , V 0 and ⁇ 0 are then transmitted from computer and control 136 to transmitter exciter controller 60 over respective conduits 146, 148 and 150 (which comprise conduit 40) .
- mean H 0 , V 0 and ⁇ Q values are transmitted from mean null computer and controller 136, via respective conduits 152, 154 and- 156 (which comprise conduit 42), to receiver polarization controller 88.
- mean null computer 136 may be connected, by conduit 158, to switch 130 to control the operation thereof, the switch blocking the transfer of V, H and ⁇ data to clutter signal measuring unit 132 except during test periods in which the background null polarization determinations are made. Further, mean null computer 136 may have provision for displaying the values of H 0 , V 0 and ⁇ 0 associated with the mean null polarization to enable monitoring by the radar operators.
- Such method includes the transmitting of a test signal which may, for example, comprise a hori ⁇ zontally polarized signal followed by a vertically polarized signal (or vice versa); processing the back ⁇ ground return signals from the test signals to obtain a null polarization state; determining a mean null polarization over a selected number of cells or zones; controlling the transmission of radar signals so that the polarization state of the transmitted signals is the same as the mean null polarization state of the background and controlling the processing of return signals so that the polarization state of the receiver is also the same as the mean null polarization state of the background.
- a test signal which may, for example, comprise a hori ⁇ zontally polarized signal followed by a vertically polarized signal (or vice versa)
- processing the back ⁇ ground return signals from the test signals to obtain a null polarization state
- determining a mean null polarization over a selected number of cells or zones controlling the transmission of radar signals so that the polar
- the method may further include the determination of both mean null polarization states of the background and the alternating of the polarization state of the radar transmitter and receiver between the two mean null polarization states.
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- Computer Networks & Wireless Communication (AREA)
- Physics & Mathematics (AREA)
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Abstract
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Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
US06/867,866 US4766435A (en) | 1986-05-27 | 1986-05-27 | Adaptive radar for reducing background clutter |
US867,866 | 1986-05-27 |
Publications (2)
Publication Number | Publication Date |
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WO1987007389A2 true WO1987007389A2 (en) | 1987-12-03 |
WO1987007389A3 WO1987007389A3 (en) | 1988-01-28 |
Family
ID=25350624
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
PCT/US1987/001156 WO1987007389A2 (en) | 1986-05-27 | 1987-05-13 | Adaptive radar for reducing background clutter |
Country Status (4)
Country | Link |
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US (1) | US4766435A (en) |
EP (1) | EP0344147A1 (en) |
JP (1) | JPS63503405A (en) |
WO (1) | WO1987007389A2 (en) |
Cited By (4)
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GB2265515A (en) * | 1991-04-26 | 1993-09-29 | Thorn Emi Electronics Ltd | Method of enhancing the target : clutter ratio in a radar return |
US8111191B2 (en) | 2008-02-07 | 2012-02-07 | Saab Ab | Wideband antenna pattern |
US8115679B2 (en) | 2008-02-07 | 2012-02-14 | Saab Ab | Side lobe suppression |
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FR2609224B1 (en) * | 1986-12-30 | 1989-04-07 | Thomson Csf | DEVICE AND METHOD FOR TRANSMITTING AND / OR ACQUIRING DATA USING TWO CROSS POLARIZATIONS OF AN ELECTROMAGNETIC WAVE AND MAGNETIC RECORDING DEVICE |
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US4937582A (en) * | 1989-07-19 | 1990-06-26 | Itt Corporation | Polarization adaptive active aperture system |
US5036331A (en) * | 1989-09-18 | 1991-07-30 | The Boeing Company | Adaptive polarization combiner |
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US9213088B2 (en) * | 2011-05-17 | 2015-12-15 | Navico Holding As | Radar clutter suppression system |
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US9348022B1 (en) * | 2013-12-16 | 2016-05-24 | Lockheed Martin Corporation | Identifying obstacles in a landing zone |
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US11002848B2 (en) * | 2019-05-15 | 2021-05-11 | The United States Of America As Represented By The Secretary Of The Army | Interferometric synthetic aperture radar imaging of subsurface structures |
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Cited By (6)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
GB2265515A (en) * | 1991-04-26 | 1993-09-29 | Thorn Emi Electronics Ltd | Method of enhancing the target : clutter ratio in a radar return |
GB2265515B (en) * | 1991-04-26 | 1995-02-01 | Thorn Emi Electronics Ltd | Method of enhancing the target:clutter ratio in a radar return |
US8111191B2 (en) | 2008-02-07 | 2012-02-07 | Saab Ab | Wideband antenna pattern |
US8115679B2 (en) | 2008-02-07 | 2012-02-14 | Saab Ab | Side lobe suppression |
WO2012067557A1 (en) * | 2010-11-19 | 2012-05-24 | Saab Ab | A method and radar system for repetition jammer and clutter suppression |
US9170321B2 (en) | 2010-11-19 | 2015-10-27 | Saab Ab | Method and radar system for repetition jammer and clutter supression |
Also Published As
Publication number | Publication date |
---|---|
US4766435A (en) | 1988-08-23 |
EP0344147A1 (en) | 1989-12-06 |
JPS63503405A (en) | 1988-12-08 |
WO1987007389A3 (en) | 1988-01-28 |
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