KEYWORDS: Synthetic aperture radar, Signal processing, Signal attenuation, Signal to noise ratio, Filtering (signal processing), Interference (communication), Doppler effect, Detection theory, Data processing, Image quality
Generally the pulse repetition frequency in airborne SAR system is much higher than signal bandwidth to guarantee the maximum slant range requested. In order to reduce the large processing amount caused by high PRF, we always preprocess the echo signal in azimuth before the whole processing procedure. The conventional method is pre-filtering, which has some approximation and relatively large computation amount. This paper mainly studies pre-accumulation as another preprocessing method with relatively small computation amount. On the basis of SAR signal processing theory, we analyze and simulate the result of direct accumulation of incoherent signal in azimuth, as well as the phase loss caused by incoherence and the factors affecting the phase loss. Furthermore, present a resolution to eliminate the phase loss, which adjusting signal phase by multiplying phase difference before accumulating, and compare the computation amount with that of pre-filtering. After that, point out the condition suitable to use this method. Finally, we apply this method to raw data processing of airborne SAR and the result shows that the proposed method can achieve the image result with the similar quality to that of original processing, while reduce the computation amount considerably.
KEYWORDS: Synthetic aperture radar, Imaging systems, Motion analysis, Image processing, Real time imaging, Digital signal processing, Multispectral imaging, Hyperspectral imaging, Image acquisition, Inertial navigation systems
Motion compensation of airborne synthetic aperture radar (SAR) consists of motion compensation based on inertial navigation system (INS) and autofocus algorithms. This paper deeply analyzes both the two aspects on the principle and gives the implementation solution. By the motion compensation based on INS, it gives not only the influence of the precision but also the effect of the data rate and points out that the data rate of the INS is the same important as the precision of INS for better result of motion compensation. Because of its excellent compensation ability and wide compensation scope, phase gradient autofocus (PGA) is used as an effective algorithm to do autofocus work. Based on the principle of PGA, the detail approach and simulation result is presented. Additionally, this paper also points out that the bottle-neck of the real time imaging processing of airborne SAR is not the computing capacity of the processing chip but the amount of memorize of the system. Then it gives an implementation solution for airborne SAR system by using high speed DSPs and huge amount memorize, with which the real time signal processing of the imaging algorithm combined with the motion compensation can be realized.
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