function?[dataoutphase_correctionentropy_his]?=?PGA（datainvarlimiteration_num）
%PGA?phase?gradient?autofocus?of?ISAR
%?function?[dataout]=PGA（datain）?performs?phase?gradient?
%?autofocus?of?ISAR.
%
%?datain?is?range-time?data?of?ISAR.
%?dataout?is?range-time?data?that?have?been?removed?translational
%?phase?error.
%
%?Reference:
%???D.E.?Wahl?P.H.?Eichel?D.C.?Ghiglia?C.V.?Jakowatz?“Phase
%???gradient?autofocus-a?robust?tool?for?high?resolution?SAR?phase
%???correction“?IEEE?Trans.?Aero.?&?Elec.?Sys.?vol.?30?no.?3?1994.

if?nargin?????varlim?=?0.2;?
????iteration_num=10;
end
[MN]?=?size（datain）;
dataout?=?datain;
%?varlim?=?0.2;
[dataslvarslen]?=?sltbin（datainvarlim）;
clear?datain
datain?=?datasl;
phase_correction?=?ones（1N）;

%?實驗
%?datain=dataout;
%?temp=0;
%?Iterative?Phase?Correction
%?iteration_num=10
for?iii?=?1:iteration_num
????orig_img?=?fftshift（fft（datain[]2）2）;???????????%做行FFT
????
????%?Circular?Shifting
????center_az_idx?=?ceil（N/2）;
????[tmp?maximum_az_idx]?=?max（abs（orig_img）?[]?2）;
????new_img?=?zeros（lenN）;
????for?i?=?1:size（orig_img1）
????????new_img（i:）?=?circshift（transpose（orig_img（i:））?center_az_idx?-?maximum_az_idx（i））;
????end
???%?
%????figurecontour（abs（new_img）20）xlabel（‘方位‘）ylabel（‘距離‘）
???
????%?Windowing
????if?1
????????window?=?20*log10（sum（abs（new_img）.^2?1））;
????????scatter_power?=?window;
????????db_down?=?30;
????????window_cutoff?=?max（window）-db_down;
????????leftidx?=?find（window（1:center_az_idx）?-?window_cutoff<0?1?‘last‘?）;
????????rightidx?=?find（window（center_az_idx+1:end）?-?window_cutoff<0?1?‘first‘）;
????????leftidx?=?leftidx+1;
???????