Discrete autofocus for ultra-high resolution synthetic aperture radar

• US 5,043,734 A
• Filed: 12/22/1988
• Issued: 08/27/1991
• Est. Priority Date: 12/22/1988
• Status: Expired due to Term

• Current Assignee: Raytheon Company (Rtx Corporation)
• Original Assignee: Hughes Aircraft Company (Rtx Corporation)
• Inventors: Niho, Yoji G.
• Primary Examiner(s): Tarcza, Thomas H.
• Assistant Examiner(s): Sotomayor, John B.
• Application Number:
  US07/288,741
• Time in Patent Office: 978 Days
• Field of Search: 342/25, 342/179, 342/195, 342/197, 342/161, 342/162
• US Class Current: 342/25.00A
• CPC Class Codes: G01S 13/9011 with frequency domain processing of the SAR signals in azimuth G01S13/9005 takes precedence
• CPC Class Codes: G01S 13/9019 Auto-focussing of the SAR signals

Abstract

The invention provides improved focus by phase corrections for Synthetic Aperture Radar images by operation on the range bin containing a selected isolated target. A phase correction signal is generated by first obtaining a non-interfering radar return from the selected target through band pass filtering operation and then extracting a non-linear residual phase from the band pass filtered data with an arc-tangent generator. The residual phase derived by the arc-tangent generator is then applied to the range compressed SAR data as a phase correction signal.

Claims

1. A system for automatically focusing a synthetic aperture radar image having multiple, isolated and grouped targets, the image expressed as range compressed SAR data, a log-detected SAR image and a corresponding complex SAR image, the system comprising:

(a) means for storing the range compressed SAR data, the log-detected SAR image and the corresponding complex SAR image;

(b) means for selecting an isolated target from the log-detected SAR image and the corresponding complex SAR image;

(c) a band pass filter responsive to the selecting means to operate on the corresponding complex SAR image thereby filtering all signals other than the isolated target to produce band pas filtered range bin data;

(d) calculation means receiving the band pass filtered range bin data for determining a non-linear residual phase of the isolated target; and

(e) means 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.

2. A system as defined in claim 1 in which the storage means comprises:

(a) first storage means for the log-detected SAR image;

(b) second storage means for the azimuth compressed complex SAR image; and

(c) third storage means for the range compressed SAR data prior to azimuth compression.

3. A system as defined in claim 2 in which the identification means comprises:

(a) first selection means for manually selecting an isolated target in the log-detected SAR image and identifying that target in the first storage means; and

(b) second selection means responsive to the first selection means to identify the isolated target in either the second or the third storage means.

4. A system as defined in claim 3 in which the calculation means comprises:

(a) evaluation means for determining the peak doppler filter and the associated linear phase;

(b) first means for performing an inverse Fast Fourier Transform;

(c) second means for subtracting the linear phase progression; and

(d) third means for calculating the arc tangent of the resultant to provide residual phase.

5. A system as defined in claim 1 further comprising means for initial focusing of range compressed SAR data.

6. A system for automatically focusing a SAR image having multiple, isolated and grouped targets the image expressed as range compressed SAR data, a log-detected SAR image and a corresponding complex SAR image, the system comprising:

(a) first storage means for the log-detected SAR image having a plurality of separable first range bins and azimuth filters, the intersection of a range bin and an azimuth filter defining a resolution cell;

(b) second storage means for the azimuth compressed complex SAR image having second range bins in one to one correspondence to the first range bins and doppler filter outputs in one to one correspondence with the resolution cells;

(c) third storage means for the range compressed SAR data corresponding to the SAR image having third range bins in one to one correspondence with the second range bins, each of the third range bins having complex number time samples.

(d) first selection means to select the resolution cells of an isolated target in the SAR image;

(e) second selection means responsive to the first selection means to select the range bin and proximate first doppler filter outputs of the isolated target;

(f) a band pass filter operation of second doppler filters responsive to the second selection means;

(g) an evaluation means to determine the peak second doppler filter output and corresponding linear phase progression;

(h) first calculation means for performing an inverse FFT on the second doppler filter outputs;

(i) second calculation means for subtracting the linear phase progression;

(j) third calculation means 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; and

(l) means for reforming and storing the SAR image on the first storage means.

7. A method for focusing a SAR image having multiple isolated and grouping targets comprising:

(a) storing the log-detected SAR image and the corresponding complex SAR image;

(b) selecting an isolated target from the log-detected SAR image and the corresponding complex SAR image;

(c) filtering the corresponding range bin from the SAR image with a band pass filter to remove all signal other than that isolated target;

(d) calculating the peak doppler filter linear phase and residual phase angle of the isolated target; 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.

8. A method for focusing a SAR image having multiple, isolated, and grouped targets comprising:

(a) selecting an isolated target;

(b) determining the target range bin and resolution cells encompassing the target;

(c) determining the target doppler filters corresponding to the resolution cells;

(d) performing a band pass filter operation to obtain time samples of radar return from only the isolated target where a band pass filtering operation is realized by first performing a forward FFT, zeroing all FFT filters outside of the extent of the target, and performing an inverse FFT;

(e) operating the band pass filtered range bin to form doppler filters with a finer filter spacing by performing an FFT with a large size;

(f) determining the peak filter in the doppler filter bank having a finer filter spacing;

(g) generating the linear phase progression corresponding to the peak filter;

(h) subtracting the linear phase from the band pass filtered range bin;

(i) calculating the arc tangent of the linearized band pass filtered range bin to determine the non-linear residual phase;

(j) subtracting the non-linear residual phase from the range compressed SAR data; and

(k) reforming the SAR image.