Discrete autofocus for ultrahigh resolution synthetic aperture radar imaging

Claim 1

A system for automatically focusing a synthetic aperture radar (SAR) image having multiple, isolated and grouped targets (42, 43, 44, 52), the image expressed as range com­pressed SAR data, a log-detected SAR image (40) and a corre­sponding complex SAR image (54), characterized by: a) means (10, 12, 14) for storing the range compressed SAR data, the log-detected SAR image and the corresponding complex SAR image; b) means (18, 70, 71) for selecting an isolated target (42, 43, 52) from the log-detected SAR image (40) and the corresponding complex SAR image (54); c) a band pass filter (22) responsive to the selecting means (18, 70, 71) to operate on the corresponding complex SAR image (54) thereby filtering all signals other than the isolated target (42, 43, 52) to produce band pass filtered range bin data; d) calculation means (24) receiving the band pass filtered range bin data for determining a non-­linear residual phase of the isolated target (42, 43, 52); and e) means (26) for incorporating the non-linear residual phase corrections into the range compressed SAR data whereby azimuth compression will then produce a well focused SAR image.

Claim 2

A system for automatically focusing a synthetic aperture radar (SAR) image having multiple, isolated and grouped targets (42, 43, 44, 52), the image expressed as range com­pressed SAR data, a log-detected SAR image (40) and a corre­sponding complex SAR image (54), characterized by: a) means (10, 12, 14) for storing the range compressed SAR data, the log-detected SAR image and the corresponding complex SAR image; b) means (18, 70, 71) for selecting an isolated target (42, 43, 52) from the log-detected SAR image (40) and the corresponding complex SAR image (54); c) a band pass filter (22) responsive to the selecting means (18, 70, 71) to operate on the corresponding complex SAR image (54) thereby filtering all signals other than the isolated target (42, 43, 52) to produce band pass filtered range bin data; d) calculation means (24) receiving the band pass filtered range bin data for determining a non-­linear residual phase of the isolated target (42, 43, 52); and e) means (26) for incorporating the non-linear residual phase corrections into the range compressed SAR data whereby azimuth compression will then produce a well focused SAR image.

Claim 5

The system of any of claims 1 through 3, charac­terized by the calculation means (24) comprising: a) evaluation means (74) for determining the peak doppler filter and the associated linear phase; b) first means (76) for performing an inverse Fast Fourier Transform; c) second means (78) for subtracting the linear phase progression; and d) third means (80) for calculating the arc tangent of the resultant to provide residual phase.

Claim 6

The system of any of claims 1 through 4, charac­terized by means (36) for initial focusing of range compressed SAR data.

Claim 7

A system for automatically focusing a SAR image having multiple, isolated and grouped targets (42, 43, 44, 52), the image expressed as range compressed SAR data, a log-­detected SAR image (40) and a corresponding complex SAR image (54), characterized by: a) first storage means (10) for the log-detected SAR image (40) having a plurality of separable first range bins (46) and azimuth filters (48), the intersection of a range bin (46) and an azimuth filter (48) defining a resolution cell (50); b) second storage means (12) for the azimuth compressed complex SAR image (54) having second range bins (56) in one to one correspondence to the first range bins (46) and doppler filter outputs (62) in one to one correspondence with the resolution cells (50); c) third storage means (14) for the range compressed SAR data corresponding to the SAR image having third range bins (100) in one to one correspondence with the second range bins (56), each of the third range bins (100) having complex number time samples; d) first selection means (18) to select the resolution cells (50) of an isolated target (42, 43, 52) in the SAR image (40); e) second selection means (70, 71) responsive to the first selection means (18) to select the range bin (46) and proximate first doppler filter outputs (48) of the isolated target (42, 43, 52); f) a band pass filter operation (22) of second doppler filters responsive to the second selection means (70, 71); g) an evaluation means (74) to determine the peak second doppler filter output and corresponding linear phase progression; h) first calculation means (76) for performing an inverse Fast Fourier Transform (FFT) on the second doppler filter outputs; i) second calculation means (78) for subtracting the linear phase progression; j) third calculation means (80) for determining residual phase by calculating the arc tangent; k) fourth calculation means for subtracting the residual phase from the corresponding complex number time sample in each of the third range bins (100); and l) means for reforming and storing the SAR image on the first storage means (10).

Claim 8

A system for automatically focusing a SAR image having multiple, isolated and grouped targets (42, 43, 44, 52), the image expressed as range compressed SAR data, a log-­detected SAR image (40) and a corresponding complex SAR image (54), characterized by: a) first storage means (10) for the log-detected SAR image (40) having a plurality of separable first range bins (46) and azimuth filters (48), the intersection of a range bin (46) and an azimuth filter (48) defining a resolution cell (50); b) second storage means (12) for the azimuth compressed complex SAR image (54) having second range bins (56) in one to one correspondence to the first range bins (46) and doppler filter outputs (62) in one to one correspondence with the resolution cells (50); c) third storage means (14) for the range compressed SAR data corresponding to the SAR image having third range bins (100) in one to one correspondence with the second range bins (56), each of the third range bins (100) having complex number time samples; d) first selection means (18) to select the resolution cells (50) of an isolated target (42, 43, 52) in the SAR image (40); e) second selection means (70, 71) responsive to the first selection means (18) to select the range bin (46) and proximate first doppler filter outputs (48) of the isolated target (42, 43, 52); f) a band pass filter operation (22) of second doppler filters responsive to the second selection means (70, 71); g) an evaluation means (74) to determine the peak second doppler filter output and corresponding linear phase progression; h) first calculation means (76) for performing an inverse Fast Fourier Transform (FFT) on the second doppler filter outputs; i) second calculation means (78) for subtracting the linear phase progression; j) third calculation means (80) for determining residual phase by calculating the arc tangent; k) fourth calculation means for subtracting the residual phase from the corresponding complex number time sample in each of the third range bins (100); and l) means for reforming and storing the SAR image on the first storage means (10).

Claim 9

A method for focusing a SAR image having multiple isolated and grouped targets (42, 43, 44, 52), characterized by: a) storing the log-detected SAR image (40) and the corresponding complex SAR image (54); b) selecting an isolated target (42, 43, 52) from the log-detected SAR image (40) and the correspon­ding complex SAR image (54); c) filtering the corresponding range bin (46) from the SAR image (40) with a band pass filter (48) to remove all signal other than that isolated target (52); d) calculating the peak doppler filter linear phase and residual phase angle of the isolated target (52); and e) incorporating the non-linear residual phase correc­tions into the range compressed SAR data prior to azimuth compression to produce a well focused SAR image.

Claim 10

A method for focusing a SAR image having multiple isolated and grouped targets (42, 43, 44, 52), characterized by: a) storing the log-detected SAR image (40) and the corresponding complex SAR image (54); b) selecting an isolated target (42, 43, 52) from the log-detected SAR image (40) and the correspon­ding complex SAR image (54); c) filtering the corresponding range bin (46) from the SAR image (40) with a band pass filter (48) to remove all signal other than that isolated target (52); d) calculating the peak doppler filter linear phase and residual phase angle of the isolated target (52); and e) incorporating the non-linear residual phase correc­tions into the range compressed SAR data prior to azimuth compression to produce a well focused SAR image.

Claim 11

A method for focusing a SAR image having multiple, isolated, and grouped targets (42, 43, 44, 52), characterized by: a) selecting an isolated target (52); b) determining the target range bin (46) and resolution cells (50) and compassing the target (52); c) determing the target doppler filters (48) corre­sponding to the resolution cells (50); d) performing a band pass filter operation (114) to obtain time samples of radar return from only the isolated target (52) where a band pass filtering operation (114) is realized by first performing a forward Fast Fourier Transform (FFT) (112), zeroing (110) all FFT filters outside of the extent of the target (52), and performing an inverse FFT (116); e) operating the band pass filtered range bin to form doppler filters with a finer filter spacing by performing a FFT (124) with a larger size; f) determining the peak filter (126) in the doppler filter bank having a finer filter spacing; g) generating the linear phase progression (127) corresponding to the peak filter; h) subtracting the linear phase from the band pass filtered range bin; i) calculating the arc tangent (130) of the linearized band pass filtered range bin to determine the non-linear residual phase (132); j) subtracting the non-linear residual phase from the range compressed SAR data; and k) reforming the SAR image.

Claim 12

A method for focusing a SAR image having multiple, isolated, and grouped targets (42, 43, 44, 52), characterized by: a) selecting an isolated target (52); b) determining the target range bin (46) and resolution cells (50) and compassing the target (52); c) determing the target doppler filters (48) corre­sponding to the resolution cells (50); d) performing a band pass filter operation (114) to obtain time samples of radar return from only the isolated target (52) where a band pass filtering operation (114) is realized by first performing a forward Fast Fourier Transform (FFT) (112), zeroing (110) all FFT filters outside of the extent of the target (52), and performing an inverse FFT (116); e) operating the band pass filtered range bin to form doppler filters with a finer filter spacing by performing a FFT (124) with a larger size; f) determining the peak filter (126) in the doppler filter bank having a finer filter spacing; g) generating the linear phase progression (127) corresponding to the peak filter; h) subtracting the linear phase from the band pass filtered range bin; i) calculating the arc tangent (130) of the linearized band pass filtered range bin to determine the non-linear residual phase (132); j) subtracting the non-linear residual phase from the range compressed SAR data; and k) reforming the SAR image.