INVERSION OF SYNTHETIC APERTUR,E R,ADAR (SAR) - MSpace ...

ard tlrus the residual phase telm 02 approxitnates to a constant r f4. Since
p:2nLf lT and (ø-cu6) = 2nn\f 12, the argurne¡t of the Fres¡el i¡teg¡als
in Eq. (3.27) becor¡es
(3.32)
where n is a normalizecl flequeucy par.ameter. The spectra in Eq.(3.26) ale
thus functions of the tirne-bandwiclth product
"A/
(i.e. cornpressiorr ratio
Tlr).
Duling clesign of a matchecl filter', weighting is usually incluclecl to recluce
sicle lobes. The first, ancl lalgest, siclelobe of an ideal matched filter output
signal is only 13 clB below the peak of the complessed pulse and near siclelobes
fall off at approxirnately 4 dB per sidelobe intelval, with the siclelol¡e null
poilts being spacecl 1/A/ apatt. To reduce the uldesirable siclelobes, ther.e
are various apploaches such as weighting the spectrurn by frequency clornaiu
fllteling, or arnplitucle rveighting of the envelope of the transrnittecl FM signal
il the tirne domain. The frequency dornain weighting is gener.ally adopted
(Cook alcl Bemfeld [19]).
4:\Ft¡(#)
3.2.2 Doppler Phase History
The azirnuth compression of the SAR signal is basecl on the analysis of
Doppler shift along arì arÌay. Thus the irnage formation out of SAR data
requires accurate estimation of the expectetl phase histor.ies of scatterels
on the Ealth's surface. It is tlte sequence of relative clistances between the
tnoving sensol and a scatterer that cletermines its phase histor.y. Fol convenierce,
a flat-surface model is assumed first and the or.bital SAR case rvill be
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