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The processing of high resolution spotlight synthetic aperture radar (SAR) data suffers from Doppler spectrum aliasing because of the limited pulse repetition frequency (PRF). A modified two-step a...

The processing of synthetic aperture radar (SAR) echoes collected during radar antenna beam steering, such as in the staring spotlight, sliding spotlight, and TOPS operating modes, requires additional efforts due to the limited pulse repetition frequency (PRF). Subaperture processing is mostly used in these cases, but the complexity arising from subaperture dividing and recombination is preferably avoided. The two-step approach (TSA) is an elegant solution to full-aperture processing. Nevertheless, for high-resolution airborne and spaceborne SAR processing, the TSA is still faced with a number of difficulties, which does not occur, however, in subaperture processing. For instance, the high range bandwidth induces Doppler spectrum aliasing during the spectral analysis (SPECAN) stage of the original TSA. Moreover, the Doppler spectrum obtained in TSA, when inverse Fourier transformed back to the slow time domain, might also suffer from aliasing, which precludes the estimation and correction of the unknown residual motion error or tropospheric disturbance. In this article, we extend the TSA to address these problems and present a full-aperture processing framework to precisely focus the high-resolution airborne and spaceborne SAR data. Simulation of X-band spaceborne SAR echoes with transmission signal bandwidth of 3.6 GHz and coherent integration angle of 12.5° has validated the extended TSA (ETSA). Meanwhile, experimental X-band airborne SAR data with the same range and azimuth resolution are also processed using the ETSA, where the conventional autofocusing techniques have been smoothly incorporated.

With the circular-scanning mode, we can obtain real-time synthetic aperture radar (SAR) images of wide areas including both sides of the nadir line, which considerably benefits the terrain matching procedure. However, due to the continuous rotation of the antenna beam, the azimuth Doppler bandwidth of the circular-scanning SAR data will increase noticeably, which in turn will lead to the azimuth spectrum aliasing phenomenon. Existing approaches for the azimuth processing of the circular-scanning SAR data are based on the subaperture technique. However, when the pulse repetition frequency (PRF) is limited, these approaches result in the loss of azimuth resolution. In this letter, a new circular-scanning approach based on the deramping operation is proposed. The deramping operation could solve the problem of spectrum aliasing and the loss of azimuth resolution can be avoided. The performance of the proposed imaging approach is demonstrated by point targets simulation and real circular-scanning SAR data processing results.

It is possible to obtain real-time images of broad regions, including the both sides under flight path, for image-matching by using the circular-scanning synthetic aperture radar (SAR). However, due to the continuous rotation of the antenna beam, the azimuth Doppler bandwidth of circular-scanning SAR data will increase noticeably, which will lead to the azimuth spectrum aliasing phenomenon. To solve this problem, in this paper, we propose an efficient imaging approach based on the SPECAN technique for the focusing of data acquired in circular-scanning mode. The reference function, adopted by the deramping operation for obtaining the unaliasing spectrum in azimuth direction, should be defined as two formats since the data acquisition of the circular-scanning SAR can be divided into two parts: the first one is the data acquisition of terrain observation by progressive scan (TOPS) SAR and the second one is the data acquisition of inverse TOPS SAR. The performance of the proposed imaging approach is demonstrated by point targets simulation.

With the development of spaceborne synthetic aperture radar (SAR), the high resolution has become a hot topic again in recent years. However, the higher the resolution is, the worse the accuracy of the traditional hyperbolic range model (HRM) gets. In the case of ultra-high resolution spaceborne SAR, the curved orbit should be taken into account. To overcome this problem, we present a modified data-focusing scheme integrated with state-of-the-art airborne SAR motion compensation methods for spaceborne spotlight SAR. Firstly, one-step motion compensation is selected to correct the orbit curvature effectively, which has been applied in ultra-high resolution airborne SAR systems. Meanwhile, the two-step approach (TSA) is introduced to solve the azimuth spectral folding phenomenon. The simulation results validate the effectiveness of the scheme.

With the increasing complexity of the surrounding maritime situation, the high resolution imaging of ship targets has become a hot topic again in recent years. The optimal time selection based imaging is an effective approach for ship ISAR imaging. In this paper, two optimal time selection based imaging methods for ship targets, which use an improved Doppler Spread (DS) estimation and the Doppler Centroid Frequency (DCF) estimation, are exploited to process extensive airborne ISAR data. The performance of the DS- and DCF-estimation based methods in airborne ISAR imaging is evaluated and compared explicitly. The results demonstrate the Doppler estimation based imaging methods.

For synthetic aperture radar(SAR) imaging of ship targets, it's difficult to generate high quality images because their motion characteristics are blind in most cases. Aiming at this problem, this paper presents a processing scheme for estimating the radial velocity based on the digital spotlight processing. This method is implemented after the coarse-resolution wide area image is obtained. Then the digital spotlight technique is carried out to extract the interested ship target from the original image and reform the motion compensation processing according to the real center of the extracted target. This processing greatly improves the SAR image quality of the moving target, while its accurate radial velocity is determined by finding the minimum value of the image entropy curve obtained from a searching procedure. The ship-target simulations fully prove the effectiveness of the technology.