1. Field of the Invention
The present invention relates to an image processor and an image processing method for a synthetic aperture radar capable of displaying information originally held by a small target without damaging it.
2. Description of Related Art
Conventionally, a method disclosed in Japanese Patent Application Laid-Open No. 2000-249527 (Patent Document 1) has been used for performing image processing. In the method, a low resolution processing unit performs thinning out to a digital conversion image so as to express the image feature amount with low resolution, and a high resolution processing unit does not perform thinning out to the digital conversion image so as to express the image feature amount with high resolution.
Further, Japanese Patent Application Laid-Open No. 09-081756 (Patent Document 2) discloses art including a maximum value filter, a minimum value filter and a subtractor, as level extraction filters. In the maximum value filter, the maximum value of the pixel level of each pixel of a prescribed pixel length, which is a filter length of before and after the target pixel along the image scanning axis, is set as a value of the target pixel. In the minimum value filter, the minimum value of the pixel level of each pixel of a filter length in right and left of the target pixel is set as a value of the targeted value, from the output image of the maximum value filter. In the subtractor, an input image is subtracted from the output image of the minimum value filter, whereby only levels not more than the prescribed pixel length are extracted from the input image. By combining these techniques, it can be considered to process a sea surface image.
However, as shown in FIG. 8, a conventional synthetic aperture radar image display has the following problem. That is, in order to monitor a wide area, if processing is performed with low resolution by a low resolution processing unit 20 and the image signal processed with the low resolution is displayed by a display unit 21, the signal energy of a target smaller than one pixel size when imaged is averaged in one pixel. Therefore, it is difficult to distinguish it from the energy of sea clutter, causing a problem in detecting capability.
In other words, in the case of performing low resolution processing, signal to noise ratio (S/N) of a target is lowered due to a decrease in T*B relating to the Doppler band B and the observation time T. Besides, since the observation time T is short, sea clutter suppressing capability is also lowered.
Further, as shown in FIG. 9, in the case of processing with high resolution by a high resolution processing unit 22 and displaying the image signal processed with the high resolution by the display unit 23, the signal energy of the target can be easily distinguished from the energy of sea clutter. However, in order to monitor a wide area, a large high-definition display, displaying all pixels, is needed. Therefore, it is required to realize target detecting capability same as that of the case of high resolution, even in displaying a wide area with low resolution.