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

ecoustluct the irnage, specially at falther ralge. This results may be pa'-tly
because of the nature of the naclil mocle configuration and the inhel'ent
problem of the far field apploxirnation, ancl partly because of the platforrr
velocity changes discussed ir section 4.5. Futthel focusing is necessary in
this case.
Thus an aclditional focusing process is carried out over. the bottorn half
of the test alea. Estiuation of the platfolm i'elocity is fir'st lequiled to
propelly ¡relform the focusing plocess of Eq.(a.36). Moleir.a [83] suggested
a method to estirnate platform velocity by estirnating two adjacent azirnuth
spectla. The changes in platforn velocity ale estirnatecl, using the algolithm
described by Moleira [83], at every 64 azirnuth pixels and avelaged over. 16
azirluth lines. After exauing the prelirninary processecl image (Figule 5.10),
512 azimuth pixels wele used as the operatol ivindow lelgth, and 32 pixels
at both ends were overlapped witli neighboring tvindorv output.
Aftel the fulther focusing, a flnal single-look image in Figule 5,11 is obtained.
Noiv point scattelels arid details of the ulban stlucture have l¡ecome
clearly visible.
The upper half (neal range) of the irnage is still too dalk to be discernible
even aftel a lineal histoglam stretching. The glay levels of the subinage of
the uppel half rvere lescalecl to make it visible as shown in Figur.e 5.12.
Both Figures 5.10 ancl 5.11 rvere plinted on a HP laser plinter. lvhich has
a 150 ppi (point per inch) r'esolution. The resolution of the printel is not
goocl enough to provide a high quality printout. Fol ilstalce, an azimuth
line consists of 820 pixels while HP lasel printel can only plint 360 poilts
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