Research On Beamforming Algorithm Of Airborne Conformal Array

Both airborne warning aircraft and battlefield surveillance aircraft require antennas with large apertures.The installation of a large-aperture parabolic or planar antenna on an aircraft will often affect the performance of the aircraft,which will also increase the radar cross section of the aircraft and make it easier to be detected by the enemy.Compared with parabolic and planar antennas,the elements of conformal arrays are mounted on the surface of the aircraft to align the array with the airplane,which does not increase the RCS of the aircraft and does not affect the aerodynamic performance of the aircraft.At the same time,it can produce a relatively large antenna effective aperture and the smaller carrier load,which can significantly improve the performance of the aircraft.The conformal array beamforming technology can suppress clutter and spatial interference on the premise of giving full play to the advantage of conformal antennas,which can improve the detection performance of targets.However,there also exists many difficulties in the conformal array beamforming technology,for example,insufficient sample size,contamination of the sample by the desired signal,difficulty in correcting mutual coupling among array elements,etc.These issues seriously affect the performance of conformal array beamforming technology in practical applications.The thesis mainly focuses on the beamforming techniques of conformal arrays under nonideal conditions such as the sample being contaminated by the desired signal and the difficulty of the mutual coupling correction among array elements.Two kinds of interference-plus-noise covariance matrix reconstruction algorithms are proposed for the problem that the samples are contaminated by the desired signal.Aiming at the mismatch of steering vectors caused by the mutual coupling among array elements,an adaptive beamforming algorithm based on mutual coupling correction is proposed.The work done by thesis mainly includes the following aspects:1.The signal reception models of the three-dimensional arbitrary array and the uniform circular arrays are established.The performance of the optimal beamformer and the sample covariance matrix inversion beamformer applied to the conformal array is studied.The influence of the desired signal in samples and the number of samples were analyzed theoretically and simulated.2.The robust beamforming algorithms based on the conformal array are studied in the case of insufficient samples and the samples are contaminated by the desired signal.Firstly,the advantages and disadvantages of several traditional robust beamforming methods are analyzed.It is found that the principal component method requires a high accuracy of the subspace dimension estimation,the performance of the worst-case optimization algorithm will degrade when the steering vector mismatches and the choice of steering vector constraint range affects the performance of the beamformer too much.Then,against the above shortcomings,two covariance matrix reconstruction methods are proposed.The first method iteratively estimates the amplitude and phase of the signal and removes the target signal directly from the sample to reconstruct the covariance matrix.The second method uses the correlation to removal signal in the eigenspace and selects the appropriate diagonal loading level to reconstruct the covariance matrix.Finally,the uniform circular array is used as an example of conformal arrays to verify the performance of the two methods under nonideal conditions and compared with the aforementioned traditional robust beamforming methods.The simulation results show that both methods can effectively eliminate the desired signal in the samples,improve the output performance of the beamformer,and have good performance in the case of steering vector mismatch.3.A uniform circular array adaptive beamforming algorithm under the influence of mutual coupling was studied.Firstly,a uniform circular array signal receiving model with mutual coupling among array elements is established.Secondly,according to the special physical structure of the uniform circular array,the mathematical characteristics of the mutual coupling matrix are studied.Then,an adaptive beamforming algorithm based on mutual coupling correction is proposed by using the mathematical properties of the mutual coupling matrix for the problem of steering vectors mismatch caused by mutual coupling Finally,the simulation results show that the method can accurately estimate the mutual coupling matrix of the uniform circular array and improve the output signal-to-interference and noise ratio of the beamformer under the influence of mutual coupling.