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Principles of Modern Radar - Volume 2 1891121537

Principles of Modern Radar - Volume 2 1891121537

Principles of Modern Radar - Volume 2 1891121537

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252 CHAPTER 6 Spotlight Synthetic Aperture <strong>Radar</strong>FIGURE 6-22Steps for the phasedifference aut<strong>of</strong>ocusalgorithm. (Adaptedfrom Carraraet al. [4].)Range-compressedimage domainDivide into two equalsubaperturesConjugateMultiplyFourier transformMagnitudeSum over rangeFind cross-range peak<strong>of</strong> cross-correlationFind QPE coefficient6.8.1 Phase Difference Aut<strong>of</strong>ocusPhase difference aut<strong>of</strong>ocus (PDA) is a noniterative parametric technique for estimatingthe quadratic phase error present in a SAR image [4]. The procedure is diagrammedin Figure 6-22. If we consider a complex-valued SAR image to be our source <strong>of</strong> data,we first inverse Fourier transform in the cross-range dimension. This new domain correspondsroughly to the slow-time (or azimuth or pulse) domain <strong>of</strong> the data and is the inputfor PDA. The data are then divided into a pair <strong>of</strong> equal-duration slow-time blocks. Theblocks are known as subapertures because their Fourier transforms should yield the sameimagery [22], having coarser cross-range resolution and independent speckle realizations.Each subaperture now contains half <strong>of</strong> the original quadratic phase. These new phaseterms can be approximated by a linear phase plus a residual quadratic phase. The linearcomponent is proportional to the quadratic phase error coefficient β and causes thesubaperture-based images to shift in opposite directions. The residual quadratic phasecauses each subaperture image to be slightly defocused. PDA operates by cross-correlatingthe image pair and measuring the <strong>of</strong>fset <strong>of</strong> the resulting peak to estimate β.The next step is to multiply the subapertures after having first taken the complex conjugate<strong>of</strong> one <strong>of</strong> them. Recall from Table 6-1 that the operation <strong>of</strong> conjugate multiplicationin one Fourier domain is the same as correlation in the other. The data are then Fouriertransformed giving the cross-range cross-correlation function evaluated at each range bin.We then take the magnitude <strong>of</strong> the cross-correlation and then average over all ranges toobtain a single estimate <strong>of</strong> the subaperture image cross-correlation function. The <strong>of</strong>fset <strong>of</strong>

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