李銀偉 鄧 袁 向茂生
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波束中心近似對(duì)機(jī)載干涉SAR運(yùn)動(dòng)補(bǔ)償?shù)挠绊懛治?/p>
李銀偉*①②鄧 袁①②向茂生①
①(中國(guó)科學(xué)院電子學(xué)研究所微波成像技術(shù)國(guó)家級(jí)重點(diǎn)實(shí)驗(yàn)室 北京 100190)②(中國(guó)科學(xué)院大學(xué) 北京 100039)
為了定量分析波束中心近似對(duì)機(jī)載干涉SAR運(yùn)動(dòng)補(bǔ)償?shù)挠绊?,該文首先建立斜視條件下波束中心近似時(shí)運(yùn)動(dòng)補(bǔ)償殘余誤差的數(shù)學(xué)模型,其形式類似于斜距誤差。隨后推導(dǎo)斜視條件下二次斜距誤差對(duì)干涉SAR的影響,通過(guò)仿真驗(yàn)證了理論推導(dǎo)的正確性。最后詳細(xì)討論不同波段、斜視角、軌跡偏移、地形變化和斜距情況下波束中心近似對(duì)干涉SAR運(yùn)動(dòng)補(bǔ)償后圖像質(zhì)量和相干系數(shù)的影響。該文的分析結(jié)果為機(jī)載重軌干涉SAR數(shù)據(jù)處理中運(yùn)動(dòng)補(bǔ)償精度的估計(jì)提供了技術(shù)支持。
機(jī)載干涉SAR;波束中心近似;運(yùn)動(dòng)補(bǔ)償;殘余誤差;相干系數(shù)
波束中心近似是指在同一方位時(shí)刻,對(duì)波束照射范圍內(nèi)的所有目標(biāo)均按照波束中心目標(biāo)的運(yùn)動(dòng)補(bǔ)償量進(jìn)行補(bǔ)償。而運(yùn)動(dòng)誤差的方位空變性將導(dǎo)致運(yùn)動(dòng)補(bǔ)償殘余誤差。DLR的Potsis等人[8]和JPL的Madsen[9]首次指出波束中心近似假設(shè)不成立,研究者相繼提出了子孔徑運(yùn)動(dòng)補(bǔ)償算法,如SATA[10], PTA[11]和FD[12],但這些算法沒(méi)有定量分析波束中心近似對(duì)干涉SAR的影響,無(wú)法確保在殘余運(yùn)動(dòng)估計(jì)時(shí)可完全忽略這些殘余誤差。文獻(xiàn)[13]定量地分析了正側(cè)視模型下波束中心近似對(duì)SAR圖像質(zhì)量的影響,但載機(jī)平臺(tái)的抖動(dòng)總會(huì)導(dǎo)致斜視角的出現(xiàn),同時(shí)也沒(méi)有考慮對(duì)相干性的影響。另外,與只重視SAR圖像的系統(tǒng)相比,需要精確相位信息的干涉SAR對(duì)運(yùn)動(dòng)補(bǔ)償精度的要求更高。
針對(duì)上述問(wèn)題,本文針對(duì)斜視條件下波束中心近似時(shí)的運(yùn)動(dòng)補(bǔ)償殘余誤差建立數(shù)學(xué)模型,得到其形式類似于斜距誤差。隨后詳細(xì)推導(dǎo)了斜視條件下二次斜距誤差對(duì)干涉SAR的影響,通過(guò)仿真驗(yàn)證了理論推導(dǎo)的正確性。最后在此基礎(chǔ)上詳細(xì)討論了不同波段、斜視角、軌跡偏移、地形變化和斜距情況下殘余誤差對(duì)干涉SAR圖像質(zhì)量和相干性的影響,為機(jī)載重軌干涉SAR數(shù)據(jù)處理中運(yùn)動(dòng)補(bǔ)償精度的估計(jì)提供了技術(shù)支持。
圖1 斜視條件下機(jī)載SAR系統(tǒng)的運(yùn)動(dòng)補(bǔ)償幾何關(guān)系圖
則波束中心近似導(dǎo)致的運(yùn)動(dòng)補(bǔ)償殘余誤差為
其中
則對(duì)式(7)進(jìn)行距離壓縮,并且對(duì)其進(jìn)行方位向FFT,得到距離壓縮后的2維頻域信號(hào)
假設(shè)方位向處理帶寬足夠大,對(duì)2維頻域信號(hào)進(jìn)行距離徙動(dòng)校正(Range Cell Migration Correction, RCMC)和方位向壓縮可得其2維頻域表達(dá)式為
其中
接下來(lái),忽略基線去相干、噪聲去相干等因素,分析雙通道間相對(duì)的斜距誤差對(duì)相干系數(shù)的影響。假設(shè)主通道沒(méi)有受到運(yùn)動(dòng)誤差的影響,副通道受到二次斜距誤差的影響,則相干系數(shù)的幅度為
由式(15)可知,相干系數(shù)主要由兩部分構(gòu)成:一部分是由線性項(xiàng)引起圖像偏移而導(dǎo)致的去相干,另一部分是由二次項(xiàng)引起圖像方位向散焦而導(dǎo)致的去相干。
圖2給出了方位向理想脈沖響應(yīng)函數(shù)(Impulse Response Function, IRF)和存在運(yùn)動(dòng)誤差情況下的散焦IRF。由圖2理想IRF和散焦IRF的3 dB分辨率得到其脈沖響應(yīng)寬度(Impulse Response Width, IRW)展寬大約為9.8%,而根據(jù)理論計(jì)算得到其IRW展寬為8%,兩者結(jié)果比較吻合。同時(shí)二次誤差對(duì)PSLR和ISLR也都有影響。
表1仿真系統(tǒng)參數(shù)
中心頻率中心斜距中心視角波束寬度斜視角帶寬速度采樣率脈寬PRF 9.6 GHz5000 m40°3°2°100 MHz169 m/s150 MHz51186 Hz
表2線性誤差影響定位的仿真驗(yàn)證
斜視角方位向(m)距離向(m) 仿真值理論值仿真值理論值近似值 斜視角-4.9997-5.00000-0.0025無(wú) 斜視角-5.0008-5.0028-0.1863-0.1773-0.1748
圖2 方位向理想IRF和散焦IRF對(duì)比圖
圖3 地形變化中二階誤差的散焦影響
圖4 地形變化中線性誤差導(dǎo)致的相位誤差
圖5 地形變化中殘余誤差導(dǎo)致的去相干
本文針對(duì)波束中心近似對(duì)機(jī)載干涉SAR圖像質(zhì)系數(shù)的影響進(jìn)行了深入研究。詳細(xì)推導(dǎo)了斜視條件下二次斜距誤差對(duì)干涉SAR的影響,并通過(guò)仿真驗(yàn)證了理論推導(dǎo)的正確性。在此基礎(chǔ)上,從理論上推導(dǎo)了斜視量和相干條件下波束中心近似造成的運(yùn)動(dòng)補(bǔ)償殘余誤差對(duì)干涉SAR的影響,從而得到了如下結(jié)論:波束中心近似殘余誤差會(huì)導(dǎo)致圖像散焦、定位偏移、相位誤差和去相干,且對(duì)寬波束高頻系統(tǒng)的影響更大;地形變化殘余誤差中斜視角的存在會(huì)使得相位誤差急劇增加;LOS近似殘余誤差的影響與斜距無(wú)關(guān),且斜視角和軌跡偏移的增大會(huì)加劇其不利影響。該文的分析結(jié)果為機(jī)載重軌干涉SAR數(shù)據(jù)處理中運(yùn)動(dòng)補(bǔ)償精度提供了理論依據(jù)。同時(shí)在殘余運(yùn)動(dòng)誤差估計(jì)和補(bǔ)償中都假設(shè)相位誤差全是由IMU/GPS測(cè)量數(shù)據(jù)導(dǎo)致的,因此在殘余運(yùn)動(dòng)誤差估計(jì)之前必須要消除波束中心近似誤差的影響。
圖6 隨的變化
圖7 IRW<2%時(shí)的相位誤差
圖8 合成孔徑長(zhǎng)度內(nèi)軌跡偏移隨的變化曲線
圖9 LOS近似中線性誤差導(dǎo)致的相位誤差
圖10 LOS近似中二次誤差導(dǎo)致的去相干
圖11 LOS近似中線性誤差導(dǎo)致的去相干
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李銀偉: 男,1985年生,博士生,研究方向?yàn)镾AR成像、運(yùn)動(dòng)補(bǔ)償、干涉SAR信號(hào)處理.
鄧 袁: 男,1989年生,碩士生,研究方向?yàn)槎嗷€干涉SAR信號(hào)處理.
向茂生: 男,1964年生,研究員,博士生導(dǎo)師,長(zhǎng)期從事干涉SAR技術(shù)及方法的研究工作.
Effects of Center-beam Approximation on Motion Compensation for Airborne Interferometric SAR
Li Yin-wei①②Deng Yuan①②Xiang Mao-sheng①
①(,,,100190,)②(,100039,)
In order to analyze quantitatively the effects of Center-Beam Approximation (CBA) on MOtion COmpensation (MOCO) for airborne Interferometric SAR (InSAR), a mathematical model of MOCO residual error under the condition of squint is firstly established. The form of residual error is similar to the slant range error. Then, the effects of quadratic slant range error on InSAR are deduced on condition that the squint angle is not zero, and the accuracy of the theoretical derivation is verified with simulation data. Finally, the effects of CBA on image quality and coherence coefficient for airborne InSAR are discussed in detail for different bands, squint angles, trajectory deviations, topography variation and slant ranges. The research provides technical support for the estimation of MOCO precision in signal processing of airborne repeat-pass interferometric SAR.
Airborne Interferometric SAR (InSAR); Center-Beam Approximation (CBA); MOtion COmpensation (MOCO); Residual error; Correlation coefficient
TN959.3
A
1009-5896(2014)02-0415-07
10.3724/SP.J.1146.2013.00479
李銀偉 liyinwei19@163.com
2013-04-11收到,2013-10-18改回
國(guó)家973計(jì)劃項(xiàng)目(2009CB724003)資助課題