Numerical Simulation Of Synthetic Aperture Ladar Imaging Process Through Turbulence

As an active imaging sensor, the working frequency of synthetic aperture ladar (SAL) is much higher than conventional synthetic aperture radar (SAR) that works in radio domain. It results in the much improvement of both resolution and imaging time due to the achievement of higher Doppler frequency shift for targets having the same relative motions, and hence can be used to get high resolution images for certain areas. But because SAL works in a much higher frequency, it becomes much more sensitive to the working environment such as the atmospheric turbulence and the irregular targets motions. In this dissertation, three correlative topics in the process of SAL imaging are focused and discussed: the atmospheric turbulence effects on SAL backscatters, SAL imaging algorithms and phase error compensation.1. A new phase screen generating method called structure function method (SFM) is developed. Based on this SFM, a modified atmospheric turbulence model is induced to simulate the effects on SAL laser beams.Refractive index fluctuation induced by the atmospheric turbulence is one of the most important factors that limit the quality of SAL imaging. Based on the statistic characteristics of turbulence, we present here a new method of phase screen generation which is called structure function method. This method retrieves the distribution of phase fluctuation on the screen using the value of phase variation between different sampling points calculated by the wave structure function. Hence, numerical model of atmospheric turbulence can be developed to help us understand the distortion mechanism of atmospheric turbulence distortion effects on the amplitude and phase of transmitted and return laser beams. Simulation results show that a more accurate phase screen can be obtained by structure function method compared with those using traditional spectrum method and polynomial method in reasonable computation time. Low and high frequency component deficiency exist respectively in traditional spectrum and polynomial method can both be improved to certain extent. Furthermore, the computation time can be cut down due to the interval computation in SFM instead of Fourier transform in spectrum method and Zernike polynomial iteration in polynomial method.2. A suited SAL imaging algorithm modified from the traditional Chirp Scaling algorithm is presented and reported in the simulation of a SAL operated from a low altitude airborne platform.The chirp scaling (CS) algorithm is a universal method of spotlight mode SAR image processing in which the range cell migration correction operation is efficiently and accurately implemented by a range time, azimuth frequency domain multiply, thereby eliminating the traditional interpolation operation. The multiply uses the linear frequency modulation (LFM) property of the range chirp to scale the radar data in the range direction, achieving the desired range time and azimuth frequency dependent correction. Compared with pulse signal mode of traditional SAR in radio domain, continuous wave (CW) signal is a potential and practical transmitting source for SAL concentrated in this dissertation, which means original CS algorithm is not able to be completely suitable. To make the CS algorithm still available for SAL imaging, some modifications needed to be made. Here two solutions are given: one is converting the CW into pulses by discrete sampling, where the Nyquist sampling in the range direction and“neighbor return theory”in the azimuth direction are applied; the other is frequency error fore filtering ahead of azimuth fast Fourier transform (FFT), where the frequency shift needed to be eliminated is induced by fast time component in target sensor range computing. From the simulation results we can see, improvement of image resolution shows the effectiveness of such modifications.3. Phase error compensation algorithms that show the improvement of both accuracy and efficiency are presented to help realize a better solution of SAL imaging.The distortion of atmospheric turbulence and unforeseen vibration of sensor platform would induce phase error in laser beam, and hence seriously degrade the quality of SAL imaging. Based on the parametric model of phase error, or the point spread function (PSF) of turbulence, map return (MR) algorithm is reported here to estimate the phase error existed in each return by the correlation information contained in phase history data (PHD). On the other hand, as two previously devised methods, rank one phase error estimation (ROPE) algorithm and iterative blind deconvolution (IBD) are reexamined here, to which some modifications are given to be more effective. For ROPE algorithm, windowing and MR pre estimation can both improve the image signal to noise ratio and initialization accuracy. For IBD algorithm, Doppler information of backscatter signals can be used to find the suited iteration times. Simulation results show that modified SAL image processing algorithm can give us a satisfied solution in both computing accuracy and efficiency.