Abstract:
Method and apparatus for a frequency diverse array. Radio frequency signals are generated by a plurality of independent waveform generators and simultaneously applied to a transmit/receive module. A progressive frequency shift is applied to all radio frequency signals across all spatial channels. Amplitude weighting signals are applied for sidelobe control. Phase control is included for channel compensation and to provide nominal beam steering. The progressive frequency offsets generate a new term which cause the antenna beam to focus in different directions as a function of range.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a divisional application of and claims priority from related, co-pending, and commonly assigned U.S. patent application Ser. No. 11/312,805 filed on Dec. 20, 2005, entitled “Method and Apparatus for a Frequency Diverse Array” also by Michael C. Wicks and Paul Antonik. Accordingly, U.S. patent application Ser. No. 11/312,805 is herein incorporated by reference. 
     
    
     STATEMENT OF GOVERNMENT INTEREST 
       [0002]    The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon. 
       BACKGROUND OF THE INVENTION 
       [0003]    This invention relates generally to the field of electronically-scanned phased array antennas. More specifically, the present invention relates to electronic beamformers for such antennas. 
         [0004]    Phased array antennas have been developed to provide electronic beam steering of radiated or received electromagnetic signals. In traditional phased arrays, the signal applied to all radiating elements is identical. An amplifier is often placed near the radiating element to provide gain and to provide amplitude control for weighting to control sidelobe levels. A phase shifter is placed near the radiating element for beam steering. It is well known in the art that a linear phase shift applied across the radiating elements will cause the mainbeam of the antenna pattern to scan in varying degrees of angle from the boresight or axis of the array. 
         [0005]    Frequency scanned arrays achieve similar off-axis mainbeam steering by varying the frequency of the radiated signal as a function of time. 
         [0006]    Adaptive nulling was developed to control interference in the sidelobes of the antenna pattern. In this application, a constraint is placed on the amplitude and phase of each element such that the amplitude of the antenna pattern is small in the direction of an interfering signal, thereby attenuating the level of the interfering signal in the sidelobes relative to the amplitude of the desired signal in the mainbeam. 
         [0007]    Space-time adaptive processing was developed to provide additional control of signals upon reception, downstream of the antenna. 
         [0008]    Synthetic aperture radar was developed to produce long virtual apertures, thereby producing long dwell times and fine resolution of ground objects. In SAR, a small physical aperture is translated in space by the motion of the host platform. As the physical aperture is moved, the signals transmitted and received by the aperture are phase-shifted and added to produce a resultant sum that is similar to that of a larger physical aperture with many elements or subarrays. The virtual aperture is N times larger than the physical aperture, where N is the number of signals integrated, and results in a corresponding improvement in spatial resolution on the ground. 
         [0009]    A limitation of the prior art is that, for any instant of time, beam steering is fixed in angle for all ranges. In the current state of the relevant art, multiple antennas or a multiple-beam antenna is required to direct radiated energy to different directions at various ranges. 
         [0010]    In some applications, antenna patterns which focus in different directions with range would be very desirable. Such a mechanism would provide more flexible beam scan options, such as multiple transmit beams without spoiling the transmit pattern. Range dependent beamforming would also reduce interference arriving from fixed directions such as multipath. 
       OBJECTS AND SUMMARY OF THE INVENTION 
       [0011]    The present invention provides a range dependent beamformer. Different signals are applied to each radiating element. Input signals are controlled such that the combined signal focuses in different directions depending on range. The present invention provides beam focusing and beam pointing that vary with range by providing for the control of adaptive transmit signals resulting in multiple transmit beams without spoiling, and simultaneous use of radiated energy for multiple conflicting requirements. 
         [0012]    It is therefore an object of the present invention to provide an apparatus that overcomes the prior art&#39;s limitation of fixed beam scan for a given range. 
         [0013]    It is a further object of the present invention to provide reduction of interference from sources located at fixed angles, such as multipath. 
         [0014]    It is still a further object of the present invention to provide an apparatus wherein spotlight and strip map synthetic aperture radar can be performed simultaneously through common equipment. 
         [0015]    It is yet still a further object of the present invention to provide an apparatus wherein signals of multiple classes can be radiated and utilized at the same time, such as synthetic aperture radar signals simultaneously with ground moving target indication signals, or communications signals simultaneously with radar signals. 
         [0016]    An additional object of the present invention is to overcome a fundamental limitation of conventional synthetic aperture radar, wherein a small aperture is required for long dwell and fine cross-range resolution. 
         [0017]    An additional object of the present invention is to also simultaneously provide multiple transmit beams without spoiling. 
         [0018]    Briefly stated, the present invention achieves these and other objects through independent control of signals applied to radiating elements. Independently generated radio frequency signals are applied to each radiating element. Signal generation by means of multiple independent waveform sources is under the control of a waveform control subsystem. The waveform control subsystem adjusts the frequency, phase, polarization, and amplitude of all input signals. Input signals are selected to achieve range dependent beamforming. 
         [0019]    A progressive frequency shift is applied to all radio frequency signals across all spatial channels. Amplitude weighting signals are applied for sidelobe control. Phase control is included for channel compensation and to provide nominal beam steering. The progressive frequency offsets generate a new term which cause the antenna beam to focus in different directions as a function of range. 
         [0020]    A plurality of waveform generators produces a plurality of independent radio frequency signals, each being input to a respective spatial channel of a transmit/receive module. The input radio frequency signals each possess a relative frequency shift under the direction of a waveform control subsystem. The nominal frequency shift of each channel varies linearly with position in the array, and the frequency shifts of all elements or spatial channels are applied simultaneously. The frequency-shifted signals are then amplified for gain and to apply amplitude weighting for sidelobe control. The signals are also phase shifted for nominal steering of the radiation pattern. 
         [0021]    According to the present invention, method and apparatus for a frequency diverse array to provide range dependent beamforming comprises a plurality of independent radio frequency signal sources, a bank of amplifiers, a bank of phase shifters, an array of radiating elements, and a waveform control subsystem. 
         [0022]    Application of a linear frequency shift across the aperture results in an antenna radiation pattern that varies with range. A greater or lesser degree of variation can be achieved by increasing or decreasing the amount of frequency shift between spatial channels. By varying the applied frequency shift with time, the antenna beam pattern can be made to scan a volume as directed by the waveform control subsystem. 
         [0023]    In contrast to prior art devices, the present invention produces an antenna radiation pattern that varies with range. Nothing in the prior art teaches or suggests this feature of the present invention. 
         [0024]    Therefore, it is accurate to say that the present invention (1.) can produce an antenna radiation pattern that varies with range; and (2.) can therefore mitigate the effects of interference from fixed angular positions such as multipath. As such, the present invention represents a significant improvement over prior art methods and apparatus. 
         [0025]    The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  is a schematic diagram representation of the present invention which provides independent control over synthesis of transmitted signals. 
           [0027]      FIG. 2  is a graphical representation of beam scan (steering angle) versus range in meters for an antenna array operating at 10 Giga Hertz (GHz) for frequency shifts (offsets) of 0 Hz, 200 Hz, and 400 Hz. 
           [0028]      FIG. 3  is a graphical representation of the present invention configured to achieve spotlight and strip map synthetic aperture radar simultaneously. 
           [0029]      FIG. 4  is a graphical representation of the present invention configured to achieve synthetic aperture radar and ground moving target indication simultaneously. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0030]    Referring to  FIG. 1 , depicts how the present invention provides enhanced control over the synthesis of transmitted signals. A plurality of waveform generators  101 ,  102  through  103  output radio frequency signals which are provided to a transmit/receive module  125 . The outputs of the transmit/receive module  125  are provided to a like plurality of antenna radiating/receiving elements  141 ,  142  through  143 . A waveform control subsystem  180  provides frequency modulation control signals  181 ,  182 ,  183  and phase modulation control signals  184 ,  185 ,  186  to the waveform generators  101 ,  102  through  103 . The frequency and phase modulation control signals provide pulse-to-pulse and element-to-element frequency and phase diversity to the waveform generators as a function of time. The waveform control subsystem  180  also provides amplitude control signals  134 ,  135 ,  136  for power control and antenna weighting, and first phase control signals  137 ,  138 ,  139  for nominal beam steering. The frequency modulation control signals  181 ,  182 ,  183  and the second phase (modulation) control signals  184 ,  185 ,  186  permit the radiation of multiple signal modes at the same time. 
         [0031]    The first through the nth waveform generators  101 ,  102  and  103  independently synthesize signals to be transmitted. These signals are ultimately distributed to each of the first and second through the nth radiating/receiving elements  141 ,  142 ,  143 . The signals are applied to each input of a transmitter/receiver module  125  consisting of a set of first and second through an nth radio frequency amplifier  161 ,  162 ,  163  and a first and second through an nth phase shifter  171 ,  172 ,  173 . The transmitter/receiver module  125  is controlled by a waveform control subsystem  180 , which sends a plurality of control signals for each of amplitude  134 ,  135 ,  136 , and phase  137 ,  138 ,  139 . The outputs of the transmitter/receiver module  125  are provided to an antenna array  140  consisting of radiating/receiving elements  141 ,  142 ,  143 , which may, in turn, be subarrays of radiating/receiving elements. 
         [0032]    Still referring to  FIG. 1 , a plurality of spatial channels is depicted. The actual number of transmitter/receiver module  125  signal outputs W 1 (t) . . . W N (t) depends upon the number of antenna elements  141 ,  142 , and  143 . It follows that the number of amplifiers  161 ,  162  and  163 , and phase shifters  171 ,  172  and  173  will be identical to the number of waveform generators  101 ,  102  and  103 . 
         [0033]    Still referring to  FIG. 1 , the waveform control subsystem  180  provides a plurality of amplitude modulation control signals  134 ,  135 ,  136  and phase modulation control signals  137 ,  138 ,  139  to each respective amplitude and phase modulation section of the transmit/receive module  125 . The amplitude modulation control signal  134 ,  135 ,  136  permits power control as well as a mechanism to apply amplitude weighting for antenna sidelobe control. The phase modulation control signal  137 ,  138 ,  139  introduces a radiating/receive element-to-radiating/receive element phase shift for conventional or nominal beam steering, which is independent of the range-dependent beam steering afforded by the frequency modulation control provided by each frequency modulation control signal  181 ,  182 ,  183 . Frequency modulation control signals provides a frequency shift which increases linearly across radiating/receive elements at any point in time. 
         [0034]    If all of the signal output waveforms W 1 (t) . . . W N (t) being radiated or received from the radiating/receiving elements  141 ,  142  and  143 , are identical with identical phase, the antenna beam will point at broadside, or orthogonal to the face of the antenna aperture. Now consider a far field target at an angle θ with respect to broadside direction. If all of the waveforms are identical continuous wave signals, then the only difference between the returns from adjacent radiating elements  141  and  142  is due to path length difference: 
         [0000]        R   1   −R   2   =d  sin(θ), 
         [0035]    where d is the spacing between any two adjacent elements  141  and  142 . 
         [0000]    The path length difference results in a phase shift from element  141  to element  142 : 
         [0000]      ψ=2π d /λ sin(θ) 
         [0000]    An incremental phase shift ψ from element-to-element (linear phase progression across the aperture) will steer the antenna mainbeam to angle θ. 
         [0036]    Next, allowing the frequency of the waveform radiated/received from each element to increase by a small amount, Δf, from element-to-element, then for element  141 , the one-way electrical path length in wavelengths is: 
         [0000]        l   1   =R   1 /λ 1   =R   1   f   1   /c.    
         [0000]    For element  142 , the electrical path length becomes: 
         [0000]    
       
         
           
             
               
                 
                   
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         [0000]    The electrical path length difference between element  141  and element  142 , in radians, is then: 
         [0000]      ψ=−2π d  sin(θ) f   1   /c+ 2π R   1   Δf/− 2π d  sin(θ)Δ f/c,    
         [0000]    provided that Δf is negligible in computing the path length difference. 
         [0037]    The new terms due to frequency diversity are 2πR 1 Δf/c and −2πd sin(θ)Δf/c. The first term is range and frequency offset dependent, while the second term is dependent on the scan angle and frequency offset. The first new term shows that for a frequency diverse array in the present invention the apparent scan angle of the antenna now depends on range. 
         [0038]    In a frequency diverse array a frequency shift is applied across elements rather than solely as a function of time. 
         [0039]    Referring now to  FIG. 2 , the effect of range-dependent beamforming for a frequency diverse array is depicted. Scan angle is plotted as a function of range for various frequency offsets at a nominal steering direction of 20 degrees. The most significant beam bending is achieved for larger frequency offsets. The frequency offset, Δf, must be less than the reciprocal of a receiver&#39;s coherent observation interval in order to make the individual waveforms inseparable. 
         [0040]    Referring to  FIG. 3  a space-time illumination wherein the waveform generators  101 ,  102 ,  103  (see  FIG. 1 ) output a plurality of linear frequency modulation signals to the transmit/receive module is depicted. A channel-to-channel frequency offset is also applied, as in the preferred embodiment. Different linear frequency modulation signals are applied to each antenna element  141 ,  142 ,  143  (see  FIG. 1 ), to permit spotlight synthetic aperture radar and stripmap synthetic aperture radar modes at the same time. By processing all received signals in combination as well as separately, the described illumination permits a large aperture on transmit for high gain while enabling a plurality of spotlight synthetic aperture radars to operate simultaneously. The invention therefore defeats a fundamental limitation of conventional synthetic aperture radar, wherein a small aperture is required for long dwell and fine cross-range resolution. 
         [0041]    Referring to  FIG. 4  a space-time illumination to achieve synthetic aperture radar and ground moving target indication at the same time is depicted. In the prior art, synthetic aperture radar and ground moving target indication are fundamentally different processes. Synthetic aperture radar is an integration process which requires on the order of hundreds of megahertz of bandwidth to achieve sufficient range resolution for imaging. Ground moving target indication is a differencing process that requires only several megahertz of bandwidth for detection. The present invention permits modes to be constructed to support synthetic aperture radar and ground moving target indication at the same time by providing chirp diversity and phase modulation across the transmit/receive elements  141 ,  142  through  143 , and processing all elements in combination and individually. 
         [0042]    Having described preferred embodiments of the invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention as defined in the appended claims.