Research On DOA Estimation Method For Low-altitude Turbofan Aircraft Based On A Tri-axis Geophone

Detecting and estimating the direction of arrival(DOA)of enemy aircraft in border environments has important reconnaissance and strike significance.Aircraft with turbofan engines are more common at the border,but the border environment is complex and harsh.For the DOA estimation of turbofan aircraft flying at low altitude in this environment,radar technology has a low-altitude blind spot,and mainstream technology based on time difference acoustic array also has 3 main problems.Firstly,sound sensors are severely affected by ground reflected sound waves and are susceptible to damage by wild animals;Secondly,the array method renders the entire array ineffective when the acoustic sensor is damaged;In addition,the mountains and forests at the border can block the propagation of sound signals,which reflects the limitations of sound sensing technology.Based on the principle of acoustic-seismic coupling,geophones have more advantages in detecting airborne sound sources.For example,it is not affected by ground reflected sound waves and is not easily destroyed by concealed deployment.Moreover,the precursor acoustic-seismic coupling waves triggered by the aircraft mainly propagate underground and can bypass some obstacles above ground,making them suitable for mountainous and forest environments on the border.Due to the poor adaptability of array technology in harsh environments,it is necessary to estimate DOA with as few geophones as possible.In fact,the polarization characteristics of seismic waves can reflect the DOA of the sound source,which enables a tri-axis geophone to estimate DOA of the sound source.Therefore,this article proposes a DOA estimation method for low-altitude turbofan aircraft based on a tri-axis geophone.However,estimating the DOA of low altitude turbofan aircraft using a tri-axis geophone poses significant challenges.Firstly,there is currently no theoretical research on using a tri-axis geophone to estimate the DOA of turbofan aircraft.Secondly,coherent and incoherent noise can affect the effectiveness of detecting useful waves and the accuracy of estimating DOA.In addition,the low signal-to-noise ratio characteristics of precursor acoustic-seismic coupling waves increase the difficulty of estimating the DOA of turbofan aircraft.Therefore,this article analyzes the current research status and difficulties at home and abroad.By conducting research on the current status of relevant research both domestically and internationally,and focusing on method error analysis,the above issues are analyzed and studied in the following four aspects.(1)Study the feasibility of estimating DOA of low-altitude turbofan aircraft based on a tri-axis geophone and investigate the systematic error of this method in the absence of noise.Firstly,the basic theory of DOA estimation for turbofan aircraft based on a tri-axis geophone was analyzed,and then the error in estimating DOA for turbofan aircraft under different flight parameters was quantitatively analyzed through numerical simulation.We also analyzed the polarization characteristics of acoustic-seismic coupling signals under typical different flight trajectories as the theoretical basis for the DOA algorithm in the following text.(2)We investigated the impact of coherent/incoherent noise on the accuracy of DOA results in border environments,and studied corresponding DOA estimation methods.A DOA estimation method based on polarization and spectral-peak limitation is proposed for coherent noise.A DOA estimation algorithm based on optimal polarization Rayleigh waves is proposed for incoherent noise.A DOA estimation method based on Rayleigh wave frequency domain reconstruction is proposed for complex noise when two types of noise are mixed.Simulation results demonstrate the effectiveness of the method.(3)A method for estimating the DOA of low-altitude turbofan aircraft using precursor acoustic-seismic coupling waves is proposed in response to the presence of obstructions at the border.The frequency characteristics of precursor acoustic-seismic coupling waves are analyzed through theoretical derivation,and precursor acoustic-seismic coupling waves are detected.In the simulation example,the polarization characteristics of the precursor acoustic-seismic coupling wave were used to estimate the aircraft DOA within 3km with an error of less than 30 °.(4)Complete DOA estimation experiments for low-altitude turbofan aircraft.Considering bad repeatability of aircraft experiments,a DOA estimation algorithm performance verification experiment was first designed.Then,the DOA estimation experiment of turbofan aircraft near the airport was analyzed,and the results obtained using the method proposed in this paper were more accurate and had less fluctuation.We also analyzed the reasons why it is difficult to detect precursor acoustic-seismic coupling waves from actual data.The innovation of this article mainly lies in 3 aspects:(1)A DOA estimation method based on optimal polarization Rayleigh waves(OPR)is proposed to address the impact of incoherent noise on signal polarization.By analyzing actual data,the average error and standard deviation of existing methods are 13.00 ° and 6.54 °,while the OPR method is reduced to 4.95 ° and1.82 °,effectively improving the accuracy of DOA estimation.(2)A DOA estimation method(PSL)based on polarization characteristics and spectral-peak limitation is proposed to address the interference of coherent noise on signal detection in the frequency domain.By analyzing actual data,this method can reduce errors by 44% compared to existing methods and has significant noise resistance in environments with severe coherent noise.(3)A low-altitude turbofan aircraft detection and DOA estimation method based on precursor acoustic-seismic coupling waves is proposed to address the issue of aircraft acoustic signals not being directly transmitted to geophones in mountainous and forested environments.The Doppler frequency shift formula for precursor acoustic-seismic coupled waves has been derived for the first time.Numerical simulation results show that under experimental conditions,for aircraft within 3km of the geophone,when the time window length is reasonably selected,the DOA error of this method can be below 30 °.In summary,this article is of great significance for ensuring the security of border airspace.This article has important reference significance for DOA estimation of turboprop aircraft and turbojet aircraft.In addition,the method proposed in this article is expected to be extended to monitoring intrusion flying targets on the border using seismic networks,thereby achieving a larger range of monitoring.