利用合成陣列技術進行雷達影像方位方向之聚焦處理

Radar Image Focusing Process in Azimuth Direction Using Synthetic Array Technique

曾裕強(Yu-Chang Tzeng) ； 朱得銘(De-Ming Chu)

《航測及遙測學刊》 12卷2期 (2007/06) Pp. 133-144

https://doi.org/10.6574/JPRS.2007.12(2).3

摘要

合成口徑雷達可利用聚焦處理的技術大幅改善真實孔徑雷達在方位方向的解析度，進而提高雷達系統在遙測應用上的實用性。在傳統上，合成口徑雷達利用載具以定速度移動而對目標物產生杜卜勒頻移，再利用時間域的傅立葉轉換而達到提高方位方向解析度之目的。國立聯合大學電子工程學系遙測影像處理研究室與國立中央大學太空及遙測研究中心微波遙測實驗室已共同設計完成一合成口徑雷達地面遙測系統。此系統在前一階段的研究中，已由中央大學的研究團隊利用頻率調變波－步階頻率連續波的技術發射電磁波，並對接收訊號進行反離散傅立葉的轉換，而達到距離方向聚焦的目的。然而，因此系統無移動裝置故無法對目標物產生杜卜勒頻移，以致在方位方向的聚焦處理上發生困難。因此，本論文之主要工作即在於提出一有效的方法以進行本系統在方位方向之聚焦處理。由於本系統與合成陣列之架構類似，故本論文提出利用合成陣列技術藉由一虛擬之位移速度將時間域的傅立葉轉換等效為空間域的傅立葉轉換，以達到方位方向聚焦的目的。首先，本論文將所提出之方法以實際之系統參數利用電腦模擬所產生之回波訊號來驗證其可行性。模擬結果顯示，此一方法在方位方向無法達到聚焦的目的。接著，本論文再對此系統實際測試所接收的資料，依本論文所提出之方法進行方位方向的聚焦處理。實驗結果與電腦模擬所獲得之結果相同，顯示此一方法在方位方向無法達到聚焦的目的。於是，本論文再對實際之系統參數作一驗證，發現此一系統所設計之參數，在距離方向取樣點過多而在方位方向卻是取樣點過少，以致在方位方向無法產生聚焦的效果。最後，本論文再以修正後之系統參數利用電腦模擬所產生之回波訊號來驗證本論文所提出之方法。模擬結果顯示，此一方法確實可達到方位方向聚焦的目的。總而言之，本論文已成功解決無移動裝置之合成口徑雷達地面遙測系統在方位方向無法聚焦的困難。同時，利用合成陣列技術藉由一虛擬之位移速度將時間域的傅立葉轉換等效為空間域的傅立葉轉換，使得傳統上用以進行方位方向聚焦的作法仍然可以使用，而無須付出其他多餘的代價。

關鍵字

合成孔徑雷達 ； 步階頻率連續波 ； 合成陣列

並列摘要

Synthetic aperture radar (SAR) which adopts a focusing process to improve the spatial resolution in azimuth direction of the real aperture radar (RAR) brings the use of radar system in remote sensing applications into practical. Traditionally, SAR platform moving with a constant speed along azimuth direction results in a Doppler frequency shift relative to a target on the ground. A time domain Fourier Transform is then applied so that the spatial resolution in azimuth direction is improved. The Remote Sensing Image Processing Lab at the Department of Electronic Engineering in the National United University cooperated with the Microwave Remote Sensing Lab at the Center for Space and Remote Sensing Research in the National Central University to design a ground based SAR remote sensing system. In the previous studies, which are performed by the research group in the National Central University, the step frequency continuous wave technique was adopted to modulate the transmitted electromagnetic waves so that Inverse Discrete Fourier Transform could be applied to the received radar echoes and image focusing in the range direction were achieved. However, without Doppler frequency, image focusing in the azimuth direction can not be realized by making use of the conventional approach because that this system has no moving mechanism. Therefore, this paper aims to propose an effective approach to conduct SAR image focusing process in azimuth direction for the ground based SAR remote sensing system. Because the architecture of this system is similar to that of synthetic array, this paper propose to apply synthetic array technique to transfer time domain Fourier Transform into spatial domain Fourier Transform with a virtual speed of the system so as to attain SAR image focusing in azimuth direction. At first, in this paper, a computer simulation according to the system parameters designed in the precious studies is done to verify the feasibility of this approach. Simulation result shows that this approach fails to perform SAR image focusing in azimuth direction. Then, this approach is applied to the real radar image acquired by the ground based SAR system. Experimental result indicates that this approach is still unable to achieve SAR image focusing in azimuth direction. Therefore, this paper attempts to re-design the system parameters. It is concluded that the failure of the proposed approach results from the sampling rates of the system which are over-sampled in the range direction and under-sampled in the azimuth direction. Finally, in this paper, a computer simulation according to the modified system parameters is done again to verify the feasibility of this approach. Simulation result shows that this approach does perform SAR image focusing in azimuth direction effectively. In conclusion, this paper has successfully overcome the difficulty of the SAR image focusing process in azimuth direction for a ground based SAR remote sensing system with no moving mechanism. The synthetic array technique is applied to transfer time domain Fourier Transform into spatial domain Fourier Transform with a virtual speed of the system so that the conventional SAR image focusing process in azimuth direction is still applicable without any extra efforts to be paid.

並列關鍵字

Synthetic Aperture Radar ； Step-Frequency Continuous-Wave ； Synthetic Array