Abstract： During the construction of airborne Synthetic Aperture Radar (SAR) hardware systems, amplitude-phase errors are inevitably introduced in the transmit-receive channel links, which deteriorate the actual imaging performance of the SAR system. To address this issue, an amplitude-phase error compensation scheme for airborne SAR echo data based on internal calibration is designed, and imaging processing is completed in combination with an improved autofocus imaging algorithm. This scheme accurately estimates the amplitude-phase error values based on the reference, transmit, and receive calibration data in the internal calibration design, and compensates the echo data according to the amplitude-frequency and phase-frequency characteristics. Meanwhile, the traditional autofocus algorithm is improved by introducing quadratic phase terms, which enhances the estimation accuracy of high-order azimuth phase errors and optimizes the two-dimensional focusing effect of images. The processing results of measured data demonstrate that, after applying the proposed scheme, the range resolution improves by 6.6% and the azimuth resolution increases by 25.9%. Moreover, the ISLR meets the specified requirements of -10 dB and PSLR meets the specified requirements of -13 dB. Through the analysis of amplitude-phase error compensation and the comparative study of different imaging schemes, this research effectively enhances the image focusing accuracy, offering crucial guidance for the engineering implementation and practical applications of airborne SAR systems.
Key words : spaceborne synthetic aperture radar；airborne test platform；internal calibration；amplitude and phase errors；engineering verification
在傳統(tǒng) SAR 成像處理框架中，線性調(diào)頻信號(hào) (LFM) 通常被假定為理想信號(hào)模型，其回波數(shù)據(jù)僅考慮時(shí)延與幅度調(diào)制效應(yīng)。然而，隨著成像分辨率需求的持續(xù)提高，LFM 信號(hào)帶寬不斷拓展，致使信號(hào)幅頻特性與相頻特性難以維持理想狀態(tài)。此外，SAR 系統(tǒng)發(fā)射與接收通道不可避免地引入幅相畸變誤差，該類誤差對 SAR 載荷成像質(zhì)量形成顯著制約。此外，機(jī)載平臺(tái)運(yùn)動(dòng)的非平穩(wěn)性會(huì)給 SAR 載荷運(yùn)行帶來運(yùn)動(dòng)誤差。雖然高精度慣性測量單元（IMU）可實(shí)現(xiàn)部分誤差補(bǔ)償，但考慮到載荷的高采樣頻率特性，仍需依托自聚焦算法對高階運(yùn)動(dòng)誤差進(jìn)行精準(zhǔn)估計(jì)并補(bǔ)償至 SAR 數(shù)據(jù)中，才能獲得聚焦良好的 SAR 圖像[6-7]。
針對幅相誤差校正，文獻(xiàn)[8]提出一種基于回波信號(hào)時(shí)域特征的相位估計(jì)與補(bǔ)償算法，該算法通過分析圖像強(qiáng)點(diǎn)目標(biāo)特性實(shí)現(xiàn)回波數(shù)據(jù)的幅相誤差補(bǔ)償，但算法計(jì)算過程對地面目標(biāo)場景存在依賴性，需對圖像中的角反射器等強(qiáng)點(diǎn)目標(biāo)進(jìn)行信號(hào)提取與特征分析。在方位向高階運(yùn)動(dòng)誤差處理領(lǐng)域，相位梯度自聚焦（PGA）算法憑借其高效性與廣泛適用性脫穎而出，該算法通過分析場景中多個(gè)特顯點(diǎn)的多普勒頻率偏移差異，實(shí)現(xiàn)相位誤差的精確估計(jì)與補(bǔ)償。但隨著 SAR 影像分辨率的不斷提升，傳統(tǒng)一次或二次相位誤差補(bǔ)償已無法滿足需求，對更高階相位誤差進(jìn)行精確估計(jì)與補(bǔ)償成為研究關(guān)鍵。
本文基于團(tuán)隊(duì)工程實(shí)踐經(jīng)驗(yàn)，在載荷設(shè)計(jì)階段即前瞻性部署內(nèi)定標(biāo)模塊，通過多模塊定標(biāo)數(shù)據(jù)協(xié)同分析與誤差提取，實(shí)現(xiàn)了收發(fā)通道及信號(hào)源的幅相誤差的有效補(bǔ)償。同時(shí)，在信號(hào)處理環(huán)節(jié)對PGA算法進(jìn)行適應(yīng)性改進(jìn)，顯著提升了高階相位誤差的估計(jì)精度。研究成果為機(jī)載 SAR 系統(tǒng)的工程化驗(yàn)證提供了切實(shí)可行的技術(shù)方案與實(shí)踐范例。
