| In today’s fiercely competitive world,territorial disputes and resource competition among countries may lead to serious military conflicts.Airfields,military ports,military depots and other bases suitable for large-scale militarization operations are the targets that the enemy focuses on and strikes first,so as to quickly weaken the defense forces of the other side.Missiles and medium and large conventional bombs are the main weapons to complete the strike.Some of these bombs are not triggered and buried underground when the attack is completed.Another type of buried unexploded ordnance(UXO)are preset weapon that wait for the targets to appear and then targets to be destroyed.These unexploded bombs,if triggered,pose a serious threat to aircraft at airfields,ships in shipping lanes,and other military equipment and personnel.Therefore,it is of great military significance to develop detection and identification technology for buried unexploded bomb targets.The shell of the buried unexploded bomb target is usually made of iron material,which produces magnetic moment after being magnetized by the geomagnetic field,and makes the geomagnetic field distribution around the target abnormal,which provides the basis for detecting and identifying the buried unexploded bomb targets.UAV has the advantages of high detection efficiency and low cost,and can be used as a platform for magnetic detection system.In the process of detecting the unexploded bomb targets during the rendezvous period,the high maneuverability of UAV and the complexity of the magnetic field environment put forward higher requirements on the interference magnetic field compensation,the identification and extraction of the magnetic anomaly signals of the unexploded bomb targets,and the parameter estimation of the unexploded bomb targets.In order to solve the problems of compensating the interference magnetic field,identifying the magnetic anomaly signals of the unexploded bomb targets and estimating the parameters of the unexploded bomb targets in the complex noise background of the UAV magnetic detection system,this paper has analyzed the generation mechanism of the magnetic anomaly signals of the unexploded bomb targets and the time-frequency characteristics of the magnetic anomaly signals of the unexploded bomb targets under the action of the earth magnetic field during the rendezvous period of the aircraft.On this basis,the method of interference magnetic field compensation based on angle increment,the method of rapid detection of magnetic anomaly signals of unexploded bomb targets with fourth-order aperitional stochastic resonance algorithm at low SNR,the method of accurate identification of magnetic anomaly signals of unexploded bomb targets with wavelet packet and energy domain fusion,and the method of parameter estimation of unexploded bomb target during the rendezvous period have been systematically studied.It has provided technical support for the detection and identification of buried unexploded bomb targets by the UAV magnetic detection system.Specific work is as follows:The induced magnetic field excitation model of the unexploded bomb target has been constructed,and the spatial magnetic field distribution characteristics of the unexploded bomb target have been analyzed.On this basis,the equivalent magnetic dipole model and the rotating ellipsoid model of the unexploded bomb target have been established,and the spatial magnetic field distribution characteristics of the unexploded bomb target have been focused.The time-frequency characteristics of the magnetic anomaly signals of the unexploded bomb targets under the influence of the earth magnetic field have been investigated in the UAV magnetic detection system at different initial points,different detection directions and different flight speeds during the rendezvous period.Aiming at the problem of interference magnetic field compensation for UAV platform maneuvering,an Angle increment interference magnetic field compensation method has been proposed by using multiple magnetometers to form magnetic field gradient,and the original data containing unexploded bomb target signals at heights of 2 m,3 m,4 m and 5 m above the ground in the aeromagnetic detection system have been compensated in real time.The new aeromagnetic compensation method has an average improvement ratio of 39.69,and can compensate the maneuvering noise from a peak value of 10 n T to a peak value of 0.18 n T.Compared with the compensation results of traditional interference magnetic field compensation model,the average improvement ratio of interference magnetic field compensation of UAV magnetic detection system based on angle increment interference magnetic field compensation method proposed in this paper is increased by 15.39,which has better realized maneuvering interference magnetic field compensation.When the UAV magnetic detection system rapidly detects the magnetic anomaly signals of unexploded bomb targets in a large range,the magnetic anomaly signals of unexploded bomb targets are submerged in strong noise,and it is difficult to detect the magnetic anomaly signals of unexploded bomb targets through compensation,filtering,time-frequency,energy domain and other signal processing technologies.A four-order aperiodic stochastic resonance(ASR-4)detection algorithm for unexploded bomb target magnetic anomaly signals is proposed,and the detection of single and multi-target signals with signal-to-noise ratio of-22.1d B and-27.9d B under aeromagnetic noise background has been realized respectively.The real magnetic noise sample data of UAV magnetic detection system was coupled with the simulated magnetic field signals of unexploded bomb targets,and the signal detection performance of ASR-4 algorithm,empirical mode decomposition algorithm and wavelet decomposition and reconstruction algorithm has been compared under the background of strong noise.Experimental results show that when the SNR is(-30～-15)d B,the signal detection probabilities of ASR-4 algorithm,empirical mode decomposition algorithm and wavelet decomposition and reconstruction algorithm are 99.3%,25.9% and 63.9%.The signal detection capability of ASR-4 algorithm is superior to the other two comparison algorithms,which provides a new method for detecting the magnetic anomaly signals of unexploded bomb targets under strong noise background.In the process of fine detection of the local area of the unexploded bomb targets distribution by the UAV magnetic detection system,after compensation by the interference magnetic field compensation model,the signal-to-noise ratio of the magnetic anomaly signals and noise of the unexploded bomb targets are distributed in the lower range of(-15～0)d B,and the magnetic anomaly characteristics of the unexploded bomb targets cannot be directly and clearly identified and extracted from the time domain and frequency domain.The recognition algorithm of the magnetic anomaly signals of the unexploded bomb targets by the fusion of the single and multi-target magnetic field simulation signals and the measured noise have been proposed.The effectiveness of the algorithm has been verified.In the measured noise background,the recognition rate of the magnetic anomaly signals of the unexploded bomb targets are better than that of the wavelet decomposition and reconstruction algorithm and Kalman filter algorithm.Aiming at the multivariate nonlinear problem of estimating the spatial coordinates and magnetic moment of buried unexploded projectile,an interior point method has been proposed to optimize the target inversion model,and the estimation of magnetic moment and coordinate position of unexploded projectile have been completed respectively under the condition of no noise and adding Gaussian white noise.At the same time,1000 statistical experiments have been conducted on six kinds of unexploded bomb targets.The experimental results have showed that the proposed target parameter estimation algorithm can accurately estimate the magnetic moments and coordinates of various unexploded projectiles.A magnetic detection system for unmanned aerial vehicle has been developed,and the real flight test of unexploded bomb target signals recognition during the rendezvous period has been completed by using this system.The actual flight tests show that the system can accurately detect the original magnetic field signal of the target at different flight altitudes.The validity and feasibility of the proposed algorithms have been verified by the original experimental data. |
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