Category: Research

The quality of images obtained with a synthetic aperture radar (SAR) strictly depends on the platform trajectory measurement precision. A crucial point is that even expensive navigation systems often do not fulfill the accuracy requirements. This is especially critical for a high-resolution SAR imaging. In the paper a novel approach which can successfully handle the residual deviations is proposed.

Synthetic aperture radar (SAR) is an effective instrument for the formation of high-resolution images of the earth surface. Trajectory instability is one of the factors that affect the quality of the obtained images. The problem is that even in the case of application of a common motion compensation procedure, which is necessary for synthetic aperture formation, instabilities of the antenna beam orientation are still left uncompensated. This leads to radiometric distortions in SAR images. This problem is especially critical in the case of SAR processing with frame-based algorithms based on FFT (for example, the range-Doppler algorithm), which assume the constant antenna beam orientation within the large data blocks. In this paper the method of radiometric correction of SAR images is considered in the case of processing of SAR data in the frequency domain. Its idea is based on the formation of extended number of looks which corresponds to the broadening of the processed Doppler bandwidth. It is shown that such broadening allows to compensate the influence of two effects: changes of the real antenna beam orientation angles in respect to the aircraft velocity vector and also the possible additional broadening of the Doppler bandwidth of the received signals due to the application of the common motion compensation procedure. The obtained extended number of looks is used for the restoration of the multi-look SAR image without radiometric errors. The introduced methods of radiometric correction are an efficient alternative to expensive antenna stabilization systems and actively used in operating SAR systems. The efficiency of the developed algorithms is illustrated by real SAR data examples.

The synthetic aperture radar (SAR) is a widely used instrument for high-resolution imaging from aircraft or satellite platforms. In the paper, the problem of the defocusing of multi-look SAR images by uncompensated phase errors presented in the received data is analyzed. It is shown that the phase errors on a multi-look processing interval can be effectively described in terms of local quadratic and local linear phase errors. Approximate analytical expressions are derived to describe the azimuth resolution degradation. Criteria for acceptable phase errors are given. The obtained results are verified by numerical simulations. The approach is illustrated by two typical motion errors: slow deflections of a SAR platform trajectory from a reference flight line and periodic trajectory deviations.

Insufficient accuracy of trajectory measurements is a pressing problem for modern high-resolution airborne SAR systems. In the paper, a novel method for the estimation of residual trajectory deviations from SAR data is proposed. The method is based on the map-drift autofocus principle used to estimate the cross-track components of the aircraft acceleration on short time intervals. The estimated acceleration is then integrated to retrieve the residual trajectory deviations on the whole data frame. The proposed approach has been successfully tested with an X-band airborne SAR system.

Uncompensated phase errors lead to quality degradation of images obtained by synthetic aperture radars (SAR). This effect is especially critical for systems with high spatial resolution. A novel approach to estimating residual phase errors in stripmap SAR signals is suggested. The idea of the method is to estimate the local quadratic phase errors on short time intervals with the subsequent reconstruction of the unknown phase error on the time intervals of arbitrary length. The map-drift autofocus (MDA) algorithm is used for the local estimation. The performance of the proposed method is demonstrated with real radar data examples.

Uncompensated phase errors lead to quality degradation of SAR images what is especially critical for high-resolution systems. In the paper, a novel approach to the stripmap autofocus is proposed. The idea of the method is to estimate the local quadratic phase errors by processing small data blocks. The conventional map-drift autofocus (MDA) algorithm is used for such estimation. Then, by a double integration of the estimated quadratic errors, an arbitrary residual phase error for large data blocks is evaluated. The performance of the proposed method is demonstrated with data obtained with an X-band airborne SAR system.

Uncompensated phase errors lead to a significant quality degradation of images obtained with a synthetic aperture radar (SAR). In the paper, an efficient approach to the SAR autofocusing is proposed. The main idea of the method is the usage of independent local phase error estimates on short time intervals for an arbitrary phase error reconstruction. The residual phase error is retrieved via a double integration of the estimated time-series of the second derivative of the phase error. A high efficiency of the approach is confirmed by experimental results.

Synthetic aperture radar (SAR) systems onboard small aircrafts suffer from trajectory deviations and instabilities of antenna orientation. These kinds of motion errors lead to significant geometric distortions in SAR images. In order to correct the distortions, we propose a time-domain multi-look stripmap SAR processing algorithm with built-in geometric correction. In the algorithm, the azimuth reference functions and range migration curves are designed to produce SAR images directly on a correct rectangular grid on the ground plane. The proposed technique has been successfully tested by using a Ku-band airborne SAR system installed onboard light-weight aircraft.