Effects Of Atmospheric Turbulence On Inverse Synthetic Aperture LADAR Imaging

Inverse Synthetic Aperture Lidar(ISAL)is a new kind of active imaging radar with advantages of synthetic aperture technology and laser radar.This optical imaging technology could provide centimeter-level resolution at a long distance with a limited optical aperture.The advantages of long-range high-resolution imaging make ISAL widely used in airborne and spaceborne target detection and recognition.Because of laser’s short wave length,fine directionality and good coherence,ISAL can generate greater Doppler bandwidth by transmitting large time-bandwidth(linear frequency modulation)signals,thus improve the range resolution.Thanks to optical heterodyne detection technology,ISAL can detect extremely weak echoes and record the phase data from the receiver with a short synthetic aperture acquisition time,thus achieve high azimuth resolution.As the operating wavelength is short,ISAL system is susceptible to phase errors caused by target motion,vibration and atmospheric turbulence.Laser decoherence caused by atmospheric turbulence will cause intensity fluctuation and phase distortion of the echo,resulting in reduction of heterodyne detection efficiency and image resolution.Therefore,correcting the phase error caused by turbulence is one of the challenges to accurately form ISAL images.This paper focuses on effects of atmospheric turbulence on ISAL image quality.The primary research contents and creative research findings can be summarized as follows:(1)Effects of atmospheric turbulence on optical heterodyne detectionOptical heterodyne(coherent)detection enables pulse compression as the most suitable means of detection and it provides a powerful technique for ISAL imaging.The atmospheric turbulence causes phase shift,beam drift and beam expansion of the echo signal,making it decoherent,and the single-mode beam becomes multimode beam.The mismatch between the eigen light and the signal light mode results in the performance reduction of the heterodyne detection system.Therefore,based on requirements of optical heterodyne detection,the matching mode of the eigen signal and the echo signal is obtained by transverse mode decomposition,and the beat signal expression under mode matching is deduced.By theoretical derivation and numerical simulation,the influence mechanism of atmospheric turbulence on the detection performance of heterodyne is studied.The temporal decoherence of the echo signal caused by atmospheric turbulence will introduce additional phase random fluctuation,which will reduce the phase stability of the echo,cause the local oscillator and the echo signal to be incoherent,and cause the heterodyne detection fail to work normally.Therefore,the phase change of the echo signal with time is the main factor that destroys the heterodyne process,which makes the high-resolution ISAL imaging lose its unique advantages.(2)Effect of atmospheric turbulence on the temporal coherence of the received laser fieldAtmospheric turbulence will degrade the spatial and temporal coherence of laser in the transmission process,and then affect the heterodyne process in ISAL,making the image quality degraded.In practical engineering applications,random fluctuation of the received light field phase caused by turbulence is the core technical difficulty of ISAL.Based on ISAL principle,the atmospheric coherence time is defined to measure temporal coherence characteristics of the echo light field.The atmospheric coherence time is positively correlated with wavelength,receiving aperture and atmospheric coherence length,and inversely proportional to wind speed.By improving optical design,laser frequency selection and other methods,the temporal coherence of the received light field can be improved,and the adverse effect of turbulence on the coherent detection process and ISAL imaging quality can be effectively reduced.(3)The influence of atmospheric coherence time on ISAL imaging qualityBased on ISAL process and heterodyne detection principle,the influence of atmospheric turbulence on ISAL imaging quality is investigated.Atmospheric turbulence affects ISAL imaging quality by randomly modulating the amplitude and phase of the echo signal.The relative size of coherence time and pulse group time in ISAL is the key factor to determine the imaging quality of ISAL.Therefore,the relative size between atmospheric coherence time and ISAL imaging integration time determines the quality of ISAL imaging under turbulent conditions.There is a linear relationship between the atmospheric coherence time and the ISAL imaging quality.The atmospheric coherence time being greater than the ISAL imaging time is helpful to reduce the adverse impact of atmospheric turbulence on the ISAL imaging to a certain extent.The TMC(Translation Motion Compensation)algorithm can compensate phase errors caused by turbulence in the azimuth direction,but cannot do so in the range direction(within a pulse).Two important conclusions can be drawn.(1)For a certain range resolution,increasing the tuning frequency means reducing the pulse duration time,thus overcoming the adverse effect of atmospheric turbulence on ISAL imaging.(2)When TMC algorithm is used to compensate ISAL images affected by atmospheric turbulence,the impact of atmospheric turbulence is independent of ISAL imaging time.With the increase of ISAL imaging time,the target rotation angle becomes larger and the azimuth resolution is improved.Therefore,shortening ISAL imaging time can only reduce the cross-range resolution,but cannot overcome the adverse effects of turbulence.