人们普遍认为,SAR点目标响应是sinc函数形状,但是,在通常条件下,SAR点目标看上去不象sinc函数形状。文章从驻留相位原理的近似性,数字信号处理和连续信号处理的差别,人类视觉,检波造成的频谱扩展,SAR的斜视等方面,指出其原因,并阐述了使点目标形状象sinc函数形状需要满足的条件。并提出SAR图像从复数图到模值图的转化过程需要满足的条件和处理。

It was said that the shape of a point target response is just a sinc function after formating in synthetic aperture radar(SAR) systems. But this idea is a mistake actually. The shape of a point target response can be closen as a sinc function shape under some limited conditions. For example, if the time-band width product of a chirp signal is large enough and the spectrum of the chirp signal is close to rectangular, the sequence is long enough to enable more energy of signal being kept down, and the sampling frequency is high enough to ensure less aliasing of the spectrum.Since the last shape is the modulus of a point target, the spectrum of the last shape is extended far away and the spectrum may have more aliasing, and any low-pass filters can not restore the signal satisfactorily. Usually, a liner-interpolation, which is a first-order hold low-pass filter(LPF), is used to restore signal. It may have more errors than ideal LPF under lower sampling frequency. A higher sampling frequency may make the last shape of point target response more close to the shape of sinc function.Because the spectrum will be extended during the process of demodulation, it may result in aliasing. To change the complex image to modular image, the signal needs upsampling and a low-pass filter before demodulation, and downsampling after demodulation.Specially, more difference may be derived from a two-dimension sinc function shape for the shape of a point target response of a Squint Mode compared with a side-look SAR since its spectrum has more difference from rectangular than a side-look SAR.The reasons of the shape of a point target response differing from a sinc frunction are analyzed, and simulation results have been presented to support the analysis in this paper.