A Range Ambiguity Suppression Processing Method for Spaceborne SAR with Up and Down Chirp Modulation  
[PERSON]  
1 Laboratory of spatial information intelligent processing system, Institute of Electronics, Chinese Academy of Sciences, Suzhou 215000, China; 3 [EMAIL_ADDRESS]  
[PERSON]  
2 Institute of Electronics, Chinese Academy of Sciences, Beijing 100190, China; 3 [EMAIL_ADDRESS]  
[PERSON]  
2 Institute of Electronics, Chinese Academy of Sciences, Beijing 100190, China; 3 [EMAIL_ADDRESS] (B.H.); 3 [EMAIL_ADDRESS] (C.D.); 3 [EMAIL_ADDRESS] (B.L.)  
[PERSON]  
2 Institute of Electronics, Chinese Academy of Sciences, Beijing 100190, China; 3 [EMAIL_ADDRESS] (B.H.); 3 [EMAIL_ADDRESS] (C.D.); 3 [EMAIL_ADDRESS] (B.L.)  
[PERSON]  
2 Institute of Electronics, Chinese Academy of Sciences, Beijing 100190, China; 3 [EMAIL_ADDRESS] (B.H.); 3 [EMAIL_ADDRESS] (C.D.); 3 [EMAIL_ADDRESS] (C.D.); 3 [EMAIL_ADDRESS] (B.L.)  
[PERSON]  
3 China Center for Resources Satellite Data and Application, Beijing 100094, China; 3 [EMAIL_ADDRESS]  
Correspondence: [EMAIL_ADDRESS]; Tel.: +86-138-1171-17823233  
decline in the ambiguity ratio appears in cross polarization mode. With the development of new SAR systems, some new techniques have been put forward in recent years, for example, coding the transmitted pulses using an orthogonal frequency-division multiplexing (OFDM) pulse [14], frequency division segregation scheme of uniformly dividing the entire bandwidth into several smaller bands according to the ground-range resolution [15]. In order to solve the conflict between range and azimuth ambiguities in spaceborne high-resolution and wide-swath (HRWS) SAR, some new techniques based on frequency diverse array (FDA) are proposed [16; 17]. Furthermore, the performances of range ambiguity suppression in multichannel ScanSAR with up and down chirp modulation have been studied in [13].  
It can be seen that in order to adapt to the new mode SAR, many improvements in radar system design for range ambiguity suppression have been put forward. However, there is a lack of analysis and solutions to the problems that may exist in these system designs. Take the technique of up and down chirp modulation for example, it will show severe destructions to the image quality under certain conditions where targets appearing in the ambiguous area have a stronger backscattering intensity than the imaging targets of interest, such as ships or towns in the ambiguous area for the imaging targets like rivers or mountains. The energy of ambiguous area after SAR imaging process are still too high to reduce the ambiguity ratio which leads to line defocusing ambiguity in the image, having severe impact on visual effects and subsequent applications.  
Gaofen-3 satellite is the first full-polarization SAR sensor of China, which implemented alternatively up and down chirp modulation for range ambiguity suppression. The line defocusing ambiguity problem appeared in certain images acquired in full-polarization mode. Therefore, this paper proposed a novel hybrid range ambiguity suppression method for up and down chirp modulation. The method can obtain the ambiguity area image and reduce the ambiguity signal power appropriately, by applying pulse compression using contrary modulation rate and Constant False Alarm Rate (CFAR) detecting method.  
In Section 2 the effect of range ambiguity under the condition of transmitting up and down chirp modulation signals is analyzed. In Section 3 the principles of the proposed range ambiguity suppression technique are introduced. Section 4 gives the processing results of Gaofen-3 full-polarization strip mode data and conclusion is summarized in the Section 5.