合成孔徑雷達之移動目標物速度估測研究; The estimator for Estimating Target Velocity with SAR

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題名:	合成孔徑雷達之移動目標物速度估測研究;The estimator for Estimating Target Velocity with SAR
作者:	郭進民;Jin-Min Kuo
貢獻者:	太空科學研究所
關鍵詞:	小波轉換;船跡;相位展開法;有限微分;都卜勒系數;速度估測骯;合成孔徑雷達;空間相交法;finite difference;doppler coefficients;Synthetic aperture radar;SAR;phas
日期:	2002-06-17
上傳時間:	2009-09-22 09:44:36 (UTC+8)
出版者:	國立中央大學圖書館
摘要:	移動目標物會改變合成孔徑雷達(Synthetic aperture radar, SAR)回波係數，造成移動目標物影像模糊及成像位置偏移。所以如能估測回波係數或移動目標物速度，就得以正確補償回波訊號相位變化，提昇影像品質。本文將分成兩個主題，一是直接由雷達回波訊號估測其地物速度，另一是由船跡影像估測船速。合成孔徑雷達訊號是一種都卜勒訊號(doppler signals), 其系數為速度的函數，雖可藉由直接微分法所得之相位差，推得目標物速度，但此法也將使得雜訊變異量(variance)快速增加。主題一討論利用有限微分(finite difference)法壓抑雜訊變異量，使得在較高雜訊比且取樣數量不大於延遲函數(delay parameters)時仍可高於Cramer-Rao Lower bounds (CRLB)的標準，但都卜勒系數的可測範圍也將減小，文中另提出相位展開法(phase unwrapping)克服此缺點。本法只有一維的加減與平均等運算也不需初始值，相較於其它方法，本法的運算量較少，且在資料量相對較少時有較好的抗雜訊效能。移動船隻在SAR影像中會產生多尺度的V形尾跡，其隱含船速、船向及船隻尺寸等資訊。但SAR影像往往伴隨有高雜訊，使其資訊不易取出。主題二提出以小波轉換(Wavelet transform)結合Radon轉換的混合方法，增強船跡邊緣以利粹取開角資訊。本法首先藉由小波轉換產生一組多尺度影像，每一尺度影像均包括垂直、水平及對角等三張小波轉換之正交基底的高通系數影像，再藉由空間相交法(spatial correlator)累乘不同尺度影像，減抑雜訊、增強V形尾跡邊緣，此有利於V形尾跡邊緣的偵獲。第二步驟則藉由Radon轉換法估測V形尾跡開角。相較於僅直接使用Radon轉換法估測V形尾跡開角，本法可更有效減抑雜訊的影響。 A moving target will change the coefficients of the synthetic aperture radar (SAR) chirp signals. Vice versa, the velocity could be inferred from the SAR images or signals. This paper proposes two algorithms for estimating a moving target’s velocity in the SAR image or signal. Topic-1 describes the velocity estimation of a moving target in the SAR signals; Topic-2 describes that in the SAR images. Topic-1 describes a new approach for estimating the Doppler coefficients from a finite number of noisy discrete-time observations, which are functions of the speed variation of target/radar. The approach adopts the finite difference method to estimate the chirp signal coefficients. It is motivated with the concept of the HAF. But the finite difference method directly calculates the phase of the signal. The finite difference method, with respect to the HAF, replaces the correlation operations with addition and average operations. That reduces the computation load. A closed-form expression is derived that describes the relationship between the phase differences and the chirp signal parameters. The difference method could derive the phase differences, but cause the deterioration of the signal variance. The finite difference method is a good way to reduce the noise deviations, but the measurable spans will become smaller. The measurable span could be improved by adopting a phase unwrapping method, as proposed in Topic-1. Unwrapping the phase differences could recover the chirp signal coefficients from bias estimates. The maximum measurable span of the coefficients will be significantly larger. The statistical analysis for the finite difference estimation proves that the variance could attain the Cramer-Rao lower bounds in higher SNR. In conclusion, our algorithm can reduce the computational complexity and remove the effect of the signal amplitude variation. A moving ship on SAR image produces multiscale wake with a characteristic linear V-shaped pattern. Detection of the wake can provide substantial information about the ship, such as its size, direction and speed of movement. In general, ship-generated wakes in SAR images are associated with high sea clutter, which causes the deterioration of detection performance. Topic-2 presents a hybrid method that combines the wavelet technique and the Radon transforms technique to detect the ship wake. The wavelet technique is first applied to generate a set of multiscale images. An orthogonal basis function is adopted so that three high-pass images in horizontal, vertical and diagonal direction are generated for each resolution scale. Then a spatial correlator is applied among the moduli of different scale, where modulus images are formed from three high-pass images. The output of the correlation process is shown to be highly representative at ship wake edges. Comparisons with other methods indicate the superior performance of the present approach in that not only are the wakes detected but also the V-shaped pattern is well pre-served. The second stage of the method involves the application of the Radon transform technique to estimate the V-opening angle from the detected ship wakes. Compared with a direct Radon transform, the greater effectiveness of the proposed scheme is demonstrated in a terms of efficiency as well as reliability for ship wake detection in noisy backgrounds.
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