Abstract:
A method for calibrating an ESA radar to mitigate degradation of its radiation system. The radiation system includes the distribution manifold, the ESA radiating elements and the radome the ESA may be operating through. Sensing of degradation takes place both locally with a switchable matched load to identify individual ESA element characteristics and globally with target based radar return data. The data generated by this sensing of both local and global characteristics is then used to modify available ESA element complex weighting to produce a desired far-field radiation pattern.

Description:
FIELD OF THE INVENTION 
     The present invention is directed generally toward aircraft mounted radar, and more particularly toward optimizing and calibrating electronically steerable antennas (ESAs). 
     BACKGROUND OF THE INVENTION 
     Aircraft radars are commonly housed inside a radome. A radome is a housing specially designed to be transparent at the radar&#39;s frequencies. Over time, the radome can accumulate defects that interfere with radar signals. Radome degradation traditionally requires the radome to be replaced. 
     Also, some radars have ESAs. ESAs are antennas composed of a number of radiating elements. By manipulating the signal sent to each radiating element, a computer can alter the direction of a signal transmitted by an ESA. ESAs can degrade over time as radiating elements of the ESA fail. 
     Also, some radars have distribution manifolds that split or distribute the transmitted signal to the individual or groups of ESA elements. The manifold may also combine radar return signals from individual or groups of ESA radiating elements. The amplitude and phase of signals passing through the distribution manifold may vary over time from aging effects such as temperature cycling, component variation, or vibrations. These variations can cause the radar beam to be sufficiently degraded as to require the radar to be removed from the aircraft for repair or replacement. 
     Radome degradations, ESA failures, and Distribution Manifold variations adversely impact the usable life of a radar system and impose maintenance requirements. The useable life of radomes and ESA based radars could be extended if they could be re-calibrated and optimized to account for certain degradations. 
     Consequently, it would be advantageous if an apparatus existed that is suitable for optimizing and calibrating an ESA radar in a radome and/or radiating elements in an ESA radar in a radome. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a novel method and apparatus for optimizing and calibrating an ESA radar in a radome and/or radiating elements in an ESA. 
     In one embodiment, a known matching load may be used to accurately measure coupling between radiating elements in an ESA radar. By measuring coupling, a computer may determine if any radiating elements in the ESA radar have a soft failure with operating characteristics outside the tolerance of their desired commanded characteristics or have a hard failure. If any radiating elements have failed or are operating outside their commanded characteristics tolerance, the computer may alter signals sent to either the element in question or surrounding radiating elements for functional mitigation. 
     In another embodiment of the present invention, an ESA radar may send a signal toward a dominant target. The ESA radar may receive a return signal and analyze the return signal to identify any distortion due to defects in the radome. A computer may then alter future signals sent by the ESA to functionally mitigate the measured radome distortion. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention claimed. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention and together with the general description, serve to explain the principles. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The numerous objects and advantages of the present invention may be better understood by those skilled in the art by reference to the accompanying figures in which: 
         FIG. 1  shows a block diagram of a computer system useful for implementing embodiments of the present application; 
         FIG. 2  shows a block diagram of radiating elements in an ESA antenna; 
         FIG. 3  shows a cross-sectional representation of signal interaction between multiple radiating elements in an ESA antenna; 
         FIG. 4  shows a diagram of near and far field radiation patterns from an ESA radar in a radome; 
         FIG. 5  shows a block diagram of a circuit for testing radiating elements in an ESA antenna in an ESA radar while in transmit mode; 
         FIG. 6  shows a block diagram of a circuit for testing radiating elements in an ESA antenna in an ESA radar while in receive mode; 
         FIG. 7  shows a block diagram of a circuit for testing radiating elements in an ESA antenna in an ESA radar while connected to a matching load; 
         FIG. 8  shows waveforms of a transmitted and received signal in an ESA antenna; 
         FIG. 9  shows a representation of an ESA radar in a radome sending signals to a known target; 
         FIG. 10  shows a representation of an ESA radar in a radome sending modified signals to a known target; 
         FIG. 11  shows a representation of signal interaction between multiple radiating elements in an ESA antenna in a radome; 
         FIG. 12  shows a flowchart of a method for calibrating an ESA radar based on a dominant target; and 
         FIG. 13  shows a flowchart of a method for identifying sub-optimal ESA radiating elements. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to the subject matter disclosed, which is illustrated in the accompanying drawings. The scope of the invention is limited only by the claims; numerous alternatives, modifications and equivalents are encompassed. For the purpose of clarity, technical material that is known in the technical fields related to the embodiments has not been described in detail to avoid unnecessarily obscuring the description. 
     Referring to  FIG. 1 , a block diagram of a computer system useful for implementing embodiments of the present application is shown. A computer system may include a processor  102  configured to implement computer executable program code, memory  104  connected to the processor  102 , a distribution manifold  106  connected to the processor  102  for distributing signals to individual radiating elements in an ESA and combining signals received from individual radiating elements in an ESA, and an antenna  108  to send and receive signals. The antenna  108  may be an ESA having a plurality of radiating elements that interact to produce a signal. The processor  102  may send a distinct signal to two or more radiating elements in the antenna  108 , through the distribution manifold  106 , such that each radiating element produces a signal that interacts with signals produced by other radiating elements. 
     Referring to  FIG. 2 , a block diagram of radiating elements in an ESA radar is shown. An ESA radar may comprise an ESA antenna having an array of radiating elements  202 ,  204 ,  206 ,  208 . By manipulating the signal sent to each radiating element  202 ,  204 ,  206 ,  208 , a computer processor may alter the properties of a combined signal. For example, a first radiating element  202  may produce a signal that constructively or destructively interferes with a signal produced by a second radiating element  204 . Likewise, a third radiating element  206  may produce a signal that constructively or destructively interferes with a signal produced by a fourth radiating element  208 . Furthermore, the first radiating element  202  may produce a signal that constructively or destructively interferes with a signal produced by the third radiating element  206 . All of the interactions of radiating elements  202 ,  204 ,  206 ,  208  in an ESA may work to steer the direction of a combined signal or alter the waveform or manipulate other properties. A person skilled in the art may appreciate that even though four radiating elements  202 ,  204 ,  206 ,  208  are shown, in actual implementation an ESA antenna may comprise more than four radiating elements  202 ,  204 ,  206 ,  208 . 
     Referring to  FIG. 3 , a cross-sectional representation of signal interaction between multiple radiating elements of an ESA antenna in an ESA radar is shown. Radiating elements  302 ,  304 ,  306 ,  308 ,  310  in an ESA antenna may be arranged such that signals from two or more radiating elements  302 ,  304 ,  306 ,  308 ,  310  may interact to produce a combined signal having certain characteristics. For example, a first radiating element  302  and second radiating element  304  may interact through constructive and destructive interference to produce a first signal  312  having certain specific characteristics such as direction, phase and amplitude. Likewise, the first radiating element  302 , second radiating element  304  and a third radiating element  306  may interact through constructive and destructive interference to produce a second signal  314  having certain, different characteristics such as direction, phase and amplitude. The second signal  314  may be a variation of the first signal  312  due to the added interaction of the third radiating element  306 . Furthermore, the first radiating element  302 , second radiating element  304 , third radiating element  306  and a fourth radiating element  308  may interact through constructive and destructive interference to produce a third signal  316  having certain, different characteristics such as direction, phase and amplitude. The third signal  316  may be a variation of the second signal  314  due to the added interaction of the fourth radiating element  308 . 
     Referring to  FIG. 4 , a diagram of near and far field radiation patterns from an ESA radar in a radome is shown. Through interactions of radiating elements, an ESA radar  400  may produce a signal having properties expressed as an aperture phase  402  and an aperture amplitude  404 . The aperture phase  402  and aperture amplitude  404  may be specifically calibrated to work with a radome  408  such that the radome  408  is effectively transparent to the signal. Outside the radome  408 , the signal may be perceived as a well-defined far-field radiation pattern  406  having certain desirable properties. A hard or soft failure of any one radiating element in the ESA radar  400  may adversely impact the properties of the far-field radiation pattern  406 . 
     A computing device sending signals to radiating elements in an ESA antenna in an ESA radar  400  could alter signals to certain radiating elements to mitigate the effects of one or more radiating elements having sub-optimal performance if the computing device could receive an accurate measurement of mutual coupling terms between discrete radiating elements. For example, referring to  FIG. 2 , if a computing device could receive an accurate measurement of the mutual coupling between the first radiating element  202  and the second radiating element  204  without contribution of a third radiating element  206  and a fourth radiating element  208 , the computing device could determine if the first radiating element  202  is performing sub-optimally and thereby determine a correction to apply to surrounding radiating elements  204 ,  206 ,  208 . The computing device may calibrate each radiating element  202 ,  204 ,  206 ,  208  in turn to produce amplitude and phase adjustments for each radiating element  202 ,  204 ,  206 ,  208 . Amplitude and phase adjustments for each radiating element  202 ,  204 ,  206 ,  208  may be stored in a matrix or some other appropriate data structure. 
     Referring to  FIG. 5 , block diagram of a circuit for testing radiating elements of an ESA antenna in an ESA radar while in transmit mode is shown. While calibrating radiating elements in an ESA antenna, a first radiating element  502  may be connected to a high-power transmitter receiver switch  504 . The high-power transmitter receiver switch  504  may have switch states; a first state, corresponding to a transmit mode, may connect to a power amplifier  506 ; a second state, corresponding to a receive mode, may connect to a low-noise amplifier  508 ; and a third state may connect to a matched load  510 . The matched load  510  may provide a known load to any radiating elements in the third state. 
     The circuit may also include a variable attenuator  512  connected to a phase shifter  514 . The variable attenuator  512  and phase shifter  514  are elements for sending and receiving signals to radiating elements in an ESA. The variable attenuator  512  may be connected to the power amplifier  506  or the low-noise amplifier  508  depending on the state of the high-power transmitter receiver switch  504 . The phase shifter  514  for each radiating element  502  may be connected to a distribution manifold (not shown); or, alternatively, one or more elements of the circuit may comprise portions of a distribution manifold. 
     When calibrating the first radiating element  502 , a high-power transmitter receiver switch  504  connected to the first radiating element  502  may be set to a transmit mode so that the first radiating element  502  is connected to the power amplifier  506  and the variable attenuator  512  is connected to the power amplifier  506 . A computing device may then induce a signal in the first radiating element  502  through the phase shifter  514  and variable attenuator  512 . 
     Referring to  FIG. 6 , a block diagram of a circuit for testing radiating elements of an ESA antenna in an ESA radar while in receive mode is shown. When calibrating a first radiating element, an adjacent or associated second radiating element  602  may be connected to a high-power transmitter receiver switch  604  substantially similar to the high-power transmitter receiver switch of  FIG. 5 . The high-power transmitter receiver switch  604  connected to the second radiating element  602  may be set to a receive mode so that the second radiating element  602  is connected to a low-noise amplifier  608  and a variable attenuator  612  is connected to the low-noise amplifier  608 . A computing device may then receive a signal through the second radiating element  602  through the phase shifter  614  and variable attenuator  612 . The signal received through the second radiating element  602  may be sent though the first radiating element; for example, a radiating element such as shown in  FIG. 5 . Coupling between a first radiating element and a second radiating element  602  may thereby be measured. 
     Referring to  FIG. 7 , a block diagram of a circuit for testing radiating elements of an ESA antenna in an ESA radar while connected to a matching load is shown. When calibrating a first radiating element, remaining radiating elements  702  not associated with the first radiating element, or for which coupling is not being measured, may be connected to a high-power transmitter receiver switch  704  substantially similar to the high-power transmitter receiver switch of  FIG. 5  and  FIG. 6 . High-power transmitter receiver switches  704  connected to the remaining radiating elements  702  may be set to connect the remaining radiating elements  702  to identical matching loads  710 . Variable attenuators  712  associated with high-power transmitter receiver switches  704  may be connected to low-noise amplifiers  708 . The remaining radiating elements  702  may thereby be segregated from a first radiating element and a second radiating element so that coupling between the first radiating element and the second radiating element can be measured accurately. 
     Referring to  FIG. 8 , waveforms of a transmitted and received signal in an ESA antenna is shown. While calibrating a first radiating element by measuring coupling between the first radiating element and a second radiating element, the first radiating element may produce a transmission signal  802 . A computing device may receive a corresponding signal  804  through a second radiating element. The corresponding signal  804  may include bleed through discharge comprising a distortion to the trailing edge of the corresponding signal  804 . Depending on the condition of the first radiating element and the second radiating element, the bleed through discharge may be a healthy waveform  808  or some other unhealthy waveform  806 ,  810 . Based on the properties of the unhealthy waveform  806 ,  810 , a computing device may determine a correction to the amplitude and phase of the first radiating element to adjust the bleed through discharge back to a healthy waveform  808 . All future transmission through the first radiating element may be adjusted according to the correction. The computing device may also determine that one or more radiating elements are performing such that no correction is possible to mitigate the effects of the unhealthy waveform  808 , and that more extensive servicing is required. 
     Furthermore, a computing device may analyze data from one or more radiating elements to determine the health of a distribution manifold connected to each of the one or more radiating elements in an ESA radar. The computing may determine a modification to the distribution manifold; or alternatively, the computing device may determine that no modification to the distribution manifold is possible. 
     Referring to  FIG. 9 , a representation of an ESA radar in a radome sending signals to a dominant target is shown. An aircraft may comprise an ESA radar  904  inside a radome  902  configured to be effectively transparent to signals from the ESA radar  904 . Over time, the radome  902  may accumulate wear and defects that cause distortion to signals from the ESA radar  904 . The ESA radar  904  may produce a signal having properties expressed as an aperture phase  906  and an aperture amplitude  908 . Outside the radome  902 , where the radome  902  includes accumulated defects and wear, the signal may be perceived as a distorted far-field radiation pattern  910  having certain undesirable properties. 
     Where there is a dominant target  914 , a computing device may generate a signal and transmit the signal through the ESA radar  904  and the radome  902  to produce a distorted far-field radiation pattern  910 . The distorted far-field radiation pattern  910  may reflect off of the dominant target  914  and return to the ESA radar  904  as a reflected distorted far-field radiation pattern  912 . The dominant target  914  may comprise a point target. 
     A computing device receiving the reflected distorted far-field radiation pattern  912  may analyze the reflected distorted far-field radiation pattern  912  to determine what distortions are being caused by the defects in the radome  902 . The computing device may then superimpose an amplitude and phase pre-distortion onto the aperture excitation of the ESA radar  904  to produce a pre-distorted signal, such that the pre-distorted signal may be “distorted” into a desired far-field radiation pattern, and thereby mitigate the effect of wear and defects from the radome  902 . 
     The computing device may calibrate each radiating element in an ESA antenna of the ESA radar  904  to produce an adjustment to the aperture phase  906  and aperture amplitude  908 . Amplitude and phase adjustments for each radiating element may be stored in a matrix or some other appropriate data structure. 
     Referring to  FIG. 10 , a representation of an ESA radar in a radome sending modified signals to a dominant target is shown. A computing device may produce an adjusted aperture phase  1006  and an adjusted aperture amplitude  1008  through an amplitude and phase pre-distortion onto the aperture excitation to create a modified far-field radiation pattern  1010 . The adjusted aperture phase  1006  and adjusted aperture amplitude  1008  may interact with a radome having certain defects and wear that modify the adjusted aperture phase  1006  and adjusted aperture amplitude  1008  to create the modified far-field radiation pattern  1010  such that the modified far-field radiation pattern  1010  has certain desirable properties. 
     Where there is a dominant target  914 , a computing device may generate an adjusted aperture phase  1006  and an adjusted aperture amplitude  1008  and transmit the signal through the radome  902  to produce a modified far-field radiation pattern  1010 . The modified far-field radiation pattern  1010  may reflect off of the dominant target  914  and return to the ESA radar  904  as a reflected modified far-field radiation pattern  1012 . 
     A computing device receiving the reflected modified far-field radiation pattern  1012  may analyze the reflected modified far-field radiation pattern  1012  to determine that the modified far-field radiation pattern  1010  is properly modified. 
     Referring to  FIG. 11 , a representation of signal interaction between multiple radiating elements in an ESA antenna in a radome is shown. In another embodiment of the present invention, a computing device may analyze the condition of radiating elements  302 ,  304 ,  306 ,  308 ,  310  in an ESA antenna and determine adjustments to a far-field signal by receiving a signal reflected of the inside of a radome  1102 . 
     Radiating elements  302 ,  304 ,  306 ,  308 ,  310  in an ESA antenna may be arranged such that signals from two or more radiating elements  302 ,  304 ,  306 ,  308 ,  310  may interact to produce a combined signal having certain characteristics. For example, a first radiating element  302  and second radiating element  304  may interact through constructive and destructive interference to produce a first signal  312  having certain specific characteristics such as direction, phase and amplitude. Likewise, the first radiating element  302 , second radiating element  304  and a third radiating element  306  may interact through constructive and destructive interference to produce a second signal  314  having certain, different characteristics such as direction, phase and amplitude. The second signal  314  may be a variation of the first signal  312  due to the added interaction of the third radiating element  306 . Furthermore, the first radiating element  302 , second radiating element  304 , third radiating element  306  and a fourth radiating element  308  may interact through constructive and destructive interference to produce a third signal  316  having certain, different characteristics such as direction, phase and amplitude. The third signal  316  may be a variation of the second signal  314  due to the added interaction of the fourth radiating element  308 . 
     Signals such as the third signal  316  may reflect off of the inside of the radome  1102 . In one embodiment of the present invention, a computing device may receive the reflected signal and compare the reflected signal to a reference signal corresponding to a healthy ESA radar and radome. 
     Referring to  FIG. 12 , a flowchart of a method for calibrating an ESA radar based on a dominant target is shown. A computing device may generate  1202  a signal through an ESA radar. The signal may pass through a radome surrounding the ESA radar and reflect off a dominant target. The ESA radar may then receive  1204  a high level return signal for analysis. If the radome has accumulated certain defects and wear, the signal and return signal may be distorted. In that case, the computing device may analyze  1206  the return signal to determine how the return signal differs from the expected far-field signal. The computing device may then determine a phase adjustment and an amplitude adjustment that will produce the expected far-field signal when distorted by the radome. The phase adjustment and amplitude adjustment may be embodied in one or more modifications to the aperture excitation of the ESA radar. When generating future signals, the computing device may modify  1208  the signal according to the phase adjustment and amplitude adjustment by superimposing the one or more modifications to the aperture excitation in the antenna. 
     The phase adjustment and amplitude adjustment may be embodied in one or more modifications to one or more radiating elements in an ESA antenna of the ESA radar; such modifications stored in an appropriate data structure. When generating future signals, the computing device may modify  1208  the ESA radar according to the phase adjustment and amplitude adjustment by applying the one or more modifications to the one or more radiating elements in the ESA antenna. 
     Referring to  FIG. 13 , a flowchart of a method for identifying sub-optimal ESA radiating elements in shown. In one embodiment of the present invention, a computing device attempting to analyze mutual coupling between a first radiating element and a second radiating element in an ESA antenna of an ESA radar may attach  1302  all radiating elements to a matching load except the first radiating element and the second radiating element. The computing device may then transmit a signal through the first radiating element and receive a signal through the second radiating element. The signal received by the second radiating element may include features due to mutual coupling. The computing device may then measure  1304  the mutual coupling to determine if the first radiating element is operating outside normal operating parameters or having operating characteristics outside the permissible tolerance window of operation. By performing such analysis on all of the radiating elements in an ESA, the computing device may identify  1306  all of the sub-optimal radiating elements and modify  1308  signals sent to the radiating elements in the ESA antenna accordingly. Alternatively, the computing device may determine that no modification is possible to mitigate the effects of one or more sub-optimal radiating elements, and that additional servicing is required. 
     Furthermore, the computing device may determine that a distribution manifold connected to the radiating elements is performing sub-optimally. In that case, the computing device may determine a modification to mitigate the effects of a sub-optimal distribution manifold; or the computing device may determine that no modification is possible and that the distribution manifold should be replaced. 
     It is believed that the present invention and many of its attendant advantages will be understood by the foregoing description, and it will be apparent that various changes may be made in the form, construction, and arrangement of the components thereof without departing from the scope and spirit of the invention or without sacrificing all of its material advantages. The form herein before described being merely an explanatory embodiment thereof, it is the intention of the following claims to encompass and include such changes.