| The synthetic aperture ladar(SAL)system is an advanced optical imaging radar system,which is the extension of synthetic aperture radar(SAR)in the optical band.It uses the Doppler shift produced by relative motion between the ladar and the target for imaging.Since its working wavelength is much smaller than that of SAR,a larger Doppler shift can be obtained and a higher azimuthal resolution can be reached.It is one of the development directions of a new generation of ultra-high resolution radar and has broad application prospects.However,the SAL system is easily affected by atmospheric turbulence.The atmospheric turbulence will deteriorate the azimuthal resolution of the SAL system and cause a large turbulence coupling loss.Therefore,it is of great significance to study the change rule of the SAL system characteristics under atmospheric turbulence and explore the corresponding turbulence compensation method.This paper focuses on the characteristics of the SAL system under atmospheric turbulence,studies its turbulence resolution characteristics and turbulence coupling characteristics.The main content of this paper is as follows:1.A numerical simulation model of the SAL system under atmospheric turbulence is studied.According to the working principle of the SAL system and Kolmogorov’s atmospheric turbulence theory,a SAL simulation system under atmospheric turbulence is constructed by using the multiple phase screens model.The selection rule of each system parameter is discussed.Based on this simulation system,the SAL system’s echo signal and imaging characteristics under different turbulence strengths are studied.And it is verified that this system can accurately simulate the phase and amplitude perturbations caused by turbulence.The results show that when the turbulence strength increases,the azimuthal resolution of the SAL system will deteriorate,and the signal phase tilt and image shift caused by atmospheric turbulence will increase.2.The turbulence resolution characteristics of the SAL system are studied.Based on Karr’s SAL resolution theory under turbulence,the change rule of turbulence resolution of the SAL system is analyzed,and the deterioration coefficient equation of atmospheric turbulence on the SAL system resolution is deduced.Then,the Karr’s resolution theory in the near-field case is modified by numerical simulation and a more accurate turbulence resolution equation is obtained.The maximum normalized turbulence resolution of the modified theory is about 37%higher than that of the unmodified Karr’s resolution.Then the influence of different turbulence strengths on the SAL system resolution is studied.When the relative synthetic aperture length is less than 4,the deterioration coefficient of atmospheric turbulence on the resolution is small.And when the relative synthetic aperture length is greater than 4,the deterioration coefficient of atmospheric turbulence on the resolution will increase rapidly.Finally,the turbulence resolution characteristics of the SAL system at different altitudes are analyzed.The results show that the resolution of the SAL system at high altitudes is less affected by turbulence than that of the SAL system at low altitudes.3.The turbulence coupling characteristics of the SAL system based on mode diversity are studied.Apply the mode diversity compensation technology to the SAL system and introduce the composition and simulation method of the SAL system using mode diversity.The difference of turbulence coupling characteristics between the few mode fiber and the single mode fiber in the SAL system is discussed.Compared with single mode fiber,the overall coupling loss of few-mode fiber can be reduced by about 6dB under strong turbulence.Then,a combining algorithm of the mode diversity suitable to the SAL system is studied.This algorithm has almost no signal combining loss under weak turbulence and only has a low combining loss of-0.47dB under strong turbulence.Finally,the turbulence coupling characteristics of the combined signal are analyzed.Compared with single mode fiber,it can bring 5.57dB turbulence coupling gain to the SAL system under strong turbulence,which can effectively restrain the increase of coupling loss caused by atmospheric turbulence. |
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