Phase Gradient Autofocus-A Robust Tool for High Resolution SAR Phase Correction

Phase Gradient Autofocus-A Robust Tool for High Resolution SAR Phase Correction No'^]r  
" iKX-VIl  
I.  INTRODUCTION -t5DcEAb$  
The ability of a synthetic aperture radar (SAR) M.[A%_|
P  
to form a high resolution image is dependent on +RuPfw{z  
maintaining subwavelength knowledge of platform #Q6.r.3@x  
position from pulse to pulse during the aperture d{gj8  
generation time. As imaging standoff distances and A18&9gY  
aperture generation times increase, the need to ^EG@tB $<  
measure and accurately track the antenna phase t!u{sr{j=  
center puts an extreme burden on any motion *1>zE>
nlP  
compensation system. In addition, phase perturbations =VctG>ct|  
due to propagation effects can destroy the utility of HgY#O r(  
high resolution imagery, even with perfect motion ,^>WCG  
compensation. :kaHvf  
In a typical SAR application, an inertial :J'ibb1  
measurement system is used to sense platform motions knPo"GQW  
over some range of frequencies. These may then be ]t23qA@^2  
removed from the data prior to image formation Hy_}e"  
[l]. For motion errors beyond the measurement o|FY-+  
capabilities of inertial measurement units or for c^$+=-G{fd  
propagation induced errors, it is necessary to use 6pKb!JJ  
data-driven autofocus techniques [2, 31. 4)IRm2
G  
It seems unlikely then, that any high resolution V&j |St[  
SAR system will fully meet its ultimate utility without S>'S4MJE`  
some means for automatically focusing the imagery, .QaHE`e{  
independent of the source or the form of the phase qM|-2Zl!+  
error. Simple automatic focusing methods have On^jHqLaE  
been proposed and built since the early days of xIrRFK9[Q  
SAR development. However, these techniques and (ohza<X;6  
extensions thereof, that basically track low-order og! d  
polynomial-like motion errors, have not kept pace with mHnHB.OL  
the requirements. High-order phase-error correction cl4Vi%  
has usually been relegated to postprocessing techniques L'*P;z7<  
generally requiring human intervention. In addition, I|{A&G}|q  
it has been difficult to focus scenes containing these :Hn*|+'  
errors without having strong isolated scatterers present. Wb|xEwqd`  
Since the initial publication of phase gradient B
!#F!Wk"  
autofocus (PGA) [4], the algorithm has seen M< T[%)v  
widespread utilization throughout the SAR community. 9qqzCMrI0e  
PGA has been demonstrated, by ourselves and N?7vcN+-t)  
others in the SAR community, to provide near neHozmm|  
diffraction-limited performance on a wide variety of ZgXn8O[a  
real-world images. SAR designers now have a practical /{j")  
tool by which phase-degraded imagery can be restored HLPnbI-+  
to designed impulse response (IPR) specifications .\W6XRw  
in nearly all cases. It is important to note that this Yy_o*Ozq  
performance is achieved with PGA independent RyxEZ7dC<y  
of SAR scene content. Specifically, bright isolated n1Y3b~E?E  
point-like reflections are not needed. Therefore scenes _;M46o%h  
completely devoid of cultural features can be restored. CXzN4!  
Some concern has been levied toward PGA in |.s#m^"  
terms of its purported computational burden. A natural P/'9k0zs)  
technical questio .. 'A|OVyH  
+&T;jad2  

未注册仅能浏览部分内容，查看全部内容及附件请先 登录 或 注册