Research On Synthetic Aperture Ladar Imaging System

Synthetic aperture ladar(SAL) apply the concept of synthetic aperture to optical wavelengths. SAL can acquire higher resolution than traditional synthetic aperture radar, using its advantage of short wavelength and large bandwidth. It is an important direction for future development in the field of radar imaging and has broad application prospects. In this paper, the key problems faced by synthetic aperture ladar imaging process has been studied. new ideas and approaches has been provided for solving the problems in the aspects of transmitter, reception, transmission and process of laser signal. The main thesis work and innovations are as follows:The chirp signal model considering the linewidth of laser is established and the effects in synthetic aperture imaging caused by the linewidth of laser is analyzed in Chapter 2. It is indicated that the linewidth of laser is an important parameter that determines the performance of synthetic aperture ladar Imaging. It is indicated that the linewidth of laser will cause degradation of range resolution in synthetic aperture ladar imaging through computer simulation. The effects caused by nonlinear frequency modulation of laser signal is analyzed, and it will reduce the range resolution. A pointwise compensation method is porposed to compensate for the phase error in order to make up for the impact of nonlinear frequency modulation, using phase shift formula and setting the reference channel. And the effectiveness of the compensation method is verified by computer simulation.The synthetic aperture laser signal reception technologies has been studied in Chapter 3. Because of the high frequency and large bandwidth of laser signal, it is difficult to receive laser signal through direct detection. According to the characteristics of synthetic aperture laser signal, conditions of laser signal by optical heterodyne reception has been analyzed in this chapter. It is illustrated that we must strictly ensure the signal light and LO light parallel to each other when using optical heterodyne reception by computer simulation. The impact caused by that the incident light is not perpendicular to the surface of the photoelectric detector can be ignored when the signal light and LO light parallel to each other.The atmospheric effects on laser propagation and synthetic aperture ladar imaging caused by atmospheric turbulence is studied in Chapter 4. As the influence on laser propagation caused by atmospheric turbulence is complex, it can be analyzed by simplified as phase screen. Phase screen model is generated by the structure function method. The affects on laser signal propagation in the atmosphere is compensated through phase gradient autofocus(PGA) algorithm. And the effectiveness of PGA algorithm to compensate atmospheric propagation effects is verified by computer simulation.Algorithms that are suitable for banding pattern of synthetic aperture ladar is studied in Chapter 5, according to the characteristics of linear wavelength modulation of the laser signal. And improved two-dimensional backprojection(BP) algorithm is proposed. It is able to achieve precise synthetic aperture laser image by the use of phase shift formula to construct echo signal phase. This method apply echo signal to imaging process directly, reducing the non-linear processing steps before imaging. A two-dimensional synthetic aperture ladar imaging simulation is conducted to verify that this algorithm can acquire high resolution image with laser echo signal directly.Research on the effects in synthetic aperture imaging caused by the characteristics of laser signal can solve the major problems encountered by synthetic aperture imaging with optical band signal. It plays a promoting role in the design and application of synthetic aperture ladar system.