Abstract
A common assumption in SAR image formation and processing algorithms is that the chirp rates of the transmitted and received radar signals are exactly the same. Dechirp processing is also done based on this common assumption. In real scenarios, the chirp rate of the received signal is different from that of the transmitted signal due to several reasons. In case the difference between the chirp rates of the transmitted and received signals is obvious, the demodulation and compression of the received pulse are not carried out precisely and defocusing the targets and the output images of the SAR processor results. In the present paper, a new technique is proposed to improve the image formation quality of SAR by exploiting chirp rate estimation methods. Based on the proposed technique, the chirp rate of the received signal is estimated, and then, dechirp is carried out by using a time-reversed complex conjugate filter constructed based on the estimated chirp rate. In this stage, the existing chirp rate estimation algorithms can be used. The quality of the output image is assessed using PSLR as a quantitative criterion and the average number of point target extension pixels along the azimuth direction. Simulation results indicated that the smaller the average number of point target extension pixels along with azimuth and the higher the PSLR average is, the better the output image quality would be. Therefore, output images obtained from the proposed method by exploiting chirp rate estimation algorithms would have a better quality with a higher PSLR average (14.1 and 13.6) and also the lower average number of point target extension pixels along the azimuth directions (2.1 and 4.9) than the common method with PSLR average (8.3) and an average number of point target extension pixels along the azimuth direction (7.1).
Highlights
Considering the principle restrictions of the optical imaging systems with respect to the climate, the acceptable resolution of the image, the limitation of taking images during the daytime, and other challenges formed the idea of using the radar systems
While in the real scenarios, due to different reasons such as vibrations of the platform or lack of precise calibration of the systems, the chip rate of the received signal will be different from the transmitted signals
This paper proposed a novel technique for enhancing the image formation quality of Synthetic aperture radars (SARs) by exploiting chirp rate estimation method
Summary
Considering the principle restrictions of the optical imaging systems with respect to the climate, the acceptable resolution of the image, the limitation of taking images during the daytime, and other challenges formed the idea of using the radar systems. SAR radar uses an antenna with small real dimensions, which distributes waves along with the platform movement trajectory and receives the return echoes from the respective area. One of its special and significant applications regarding the estimation of the chirp signal parameters in the radar processing algorithm is the synthetic aperture radar (SAR). A common assumption in the proposed algorithms in the references for the SAR image formation and processing is that the chirp rates of the radar’s transmitted and received signals are precisely the same. In case the difference between the chirp rates of the transmitted and received signals is obvious, the demodulation and compression of the received pulse are not carried out precisely; the targets and output images of the SAR processor are defocused.
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