The Imaging Research Of Ultra Wideband Airborne SAR System Using Stepped Frequency Waveforms

Synthetic Aperture Radar (SAR) achieves high azimuth resolution in virtue of the synthetic aperture principle, while the high range resolution lies on the large bandwidth of the transmitted signal. One of the trends for SAR's development is the ultra high resolution. For regular signal radars large bandwidth requests very high A/D (Analog to Digital) rate which increases the cost of hardware. Stepped-frequency can synthesize large bandwidth signals by transmitting and receiving stepped-frequency signals in separating time. Therefore, it becomes one of the ideal signal for ultra high resolution SAR system for its ability of reducing the instantaneous A/D rate effectively.It cannot be ignored that the range between radar and targets never remain a constant during the period that the airborne stepped-frequency SAR transmits a whole strand of stepped-frequency signals. As a result, the conventional"stop and go"hypothesis cannot come into existence any longer. Hence the signal processing becomes more difficult than ever before. In this paper, three aspects including signal parameter designing, imaging algorithm and motion compensation will be discussed.As for the design of system parameters, the high range profile imaging algorithm and ambiguous function of stepped-frequency are studied in this paper. By analyzing the characteristics of ambiguous function some parameters such as equivalent bandwidth, range resolution and Doppler resolution can be accordingly educed. Then on the basis of what is discussed ahead the elementary principium of airborne SAR using stepped-frequency is introduced.As to the imaging algorithm, the echo model of stationary point target is firstly established, the reasons for the fast-time Doppler Effect are discussed in details and then the fast-time Doppler compensation algorithm in echo domain is given. Considering the fact that the compensation factor of fast-time Doppler changes spatially and actualizes inefficiently, a new method for fast-time Doppler compensation, which is carried out in the image wavenumber domain, is introduced. The new method can decrease the calculating load from O(N 3M) to O(N 2 log 2N). Therefore it can greatly increase the efficiency of compensation. Meanwhile this new method can achieve the same compensation precision as time domain algorithm.The last part of this paper is about motion compensation. Stationary target echo model with motion errors including range, height and azimuth is established. The way of these errors affecting stepped-frequency SAR imaging is analyzed. And then the compensation factors of phase errors in the echo domain are given according to the approximate models. Owing to the fact that the compensation factors vary spatially, a new motion compensation method by dividing the imaging area into several sub-grids is brought forward. This new method can not only enhance the compensation efficiency, but also hold the same precision. Finally the PGA algorithm on images validated, which can eliminate residual phase errors, is studied. And it is available by simulation data that PGA can process stepped-frequency SAR images efficiently.