Extended SAR imaging capability for ship classification

• US 4,723,124 A
• Filed: 03/21/1986
• Issued: 02/02/1988
• Est. Priority Date: 03/21/1986
• Status: Expired due to Term

• Current Assignee: Grumman Aerospace Corporation (Northrop Grumman Corporation)
• Original Assignee: Grumman Aerospace Corporation (Northrop Grumman Corporation)
• Inventors: Boles, Sol
• Primary Examiner(s): Tubbesing, T. H.
• Assistant Examiner(s): GREGORY, BERNARR E
• Application Number:
  US06/842,459
• Time in Patent Office: 683 Days
• Field of Search: 342/25, 342/27, 342/28, 342/55, 342/61-63, 342/66, 342/67, 342/69, 342/104, 342/106-109, 342/111-116, 342/133, 342/146, 342/156, 342/179, 342/184, 342/192, 342/193 , 342/195-197, 364/516, 364/517
• US Class Current: 342/25.00C
• CPC Class Codes: G01S 13/9023 combined with interferometric techniques
• CPC Class Codes: G01S 13/9092 combined with monopulse techniques

Abstract

Capability is provided using coherent synthetic aperture radar (SAR) techniques for substantially extending the useful range for producing 3 scaled high resolution orthogonal image projections on a CRT of a translating ship under the influence of rotational motions arising from sea state conditions, for the purpose of ship classification and weapon delivery from an airborne platform at long stand-off ranges. This advantage is brought about by determining image coordinates on the basis of range, doppler, and doppler rate measurements of individual scatterers and from ship angular rotational velocities derived from a weighted multivariate regression solution to doppler processed interferometric azimuth and elevation angle measurements of all significant ship target scatterers. In this manner, the image degradation suffered by plotting angular measurements directly, whose location accuracies are known to deteriorate rapidly with increased range due to high signal-to-noise requirements, is circumvented.

Claims

1. A method for forming high resolution synthetic aperture radar imagery of a ship target under the influence of sea state conditions comprising the steps of:

(a) processing signals received from ship scatterers comprising the ship target to obtain estimates of (1) range, velocity, and acceleration of ship scatterers, and (2) ship rotational velocities and rotational velocity rates;

(b) determining from these estimated ship scatterer motional parameters the coordinate values of the ship scatterer centers to form a high resolution image of the ship target; and

(c) displaying the formed imagery of the ship target including a range/azimuth projection, an elevation/azimuth profile projection, and a range/elevation profile projection.

2. A method as recited in claim 1 wherein the step of processing the received signals includes:

(a) Determining scatterer velocities from polynomial curve fitting to interpolated and sorted doppler processed frequency measurements over a prescribed tracking time interval; and

(b) determining scatterer accelerations from the time derivatives of polynomial curve fits to frequency data.

3. A method as recited in claim 1 wherein the step of processing the received signal includes:

(a) performing a weighted multivariate regression fit to processed doppler, azimuth, and elevation angle measurements to obtain cross-line-of-sight ship rotational velocities using an iterative process of minimizing error residuals; and

(b) determining line-of-sight ship rotational velocity using sorted azimuth and elevation angle measurements, in conjunction with elevation angle rates determined for several bright scatterers and estimated cross-line-of-sight ship rotational velocity.

4. A method as recited in claim 3 where the step of processing the received signals includes:

(a) obtaining cross-line-of-sight ship rotational velocity rates from time derivatives of polynomial curve fits to stored estimates of the cross-line-of-sight rotational velocities derived from regression solutions.

5. A method of claim 3 wherein said doppler, azimuth and elevation angle measurements are determined from Fourier transform signal processing using an antenna angle sensing system.

6. A method of claim 5 wherein the antenna system is an interferometric antenna system.

7. A method of claim 5 wherein the antenna system is a phase monopulse antenna system.

8. An airborne synthetic aperture radar system for forming high resolution synthetic aperture radar imaging of a ship target under the influence of sea state conditions, said system comprising:

(a) a multiple section interferometer antenna connected to a two channel receiver;

(b) means coupled to said receiver for processing signals received by said antenna from ship scatterers comprising the ship target to obtain estimates of (1) range, velocity, and acceleration of ship scatterers, and (2) ship rotational velocities and rotational velocity rates;

(c) means for determining from these estimated ship scatterer motional parameters the coordinate values of the ship scatterer centers to form a high resolution image of the ship target; and

(d) means for displaying the formed imagery of the ship target including a range/azimuth projection, an elevation/azimuth profile projection, and a range/elevation profile projection.

9. The system of claim 8 wherein said processing means includes:

(a) means for determining scatterer velocities from polynomial curve fitting to interpolated and sorted doppler processed frequency measurements over a prescribed tracking time interval; and

(b) means for determining scatterer accelerations from the time derivatives of polynomial curve fits to frequency data.

10. The system of claim 8 wherein said processing means includes:

(a) means for performing a weighted multivariate regression fit to processed doppler, azimuth, and elevation angle measurements to obtain cross-line-of-sight ship rotational velocities using an iterative process of minimizing error residuals; and

(b) means for determining line-of-sight ship rotational velocity using sorted azimuth and elevation angle measurements, in conjunction with elevation angle rates determined for several bright scatterers and estimated cross-line-of-sight ship rotational velocity.

11. The system of claim 10 wherein said processing means includes means for obtaining cross-line-of-sight ship rotational velocity rates from time derivatives of polynomial curve fits to stored estimates of the cross-line-of-sight rotational velocities derived from regression solution.

12. An airborne synthetic aperture radar system for forming high resolution synthetic aperture radar imaging of a ship target under the influence of sea state conditions, said system comprising:

(a) means for processing the received signals from ship scatterers (1) using polynomial curve fitting to interpolated and sorted doppler processed frequency measurements over a prescribed tracking time interval to yield scatterer velocities;

(2) determining scatterer accelerations from the derivatives of polynomial curve fits to frequency data;

(3) performing a weighted multivariate regression fit to processed doppler, azimuth, and elevation angle measurements to obtain the two cross-line-of-sight ship rotational velocities using an iterative process of minimizing error residuals;

(4) determining the line-of-sight ship rotational velocity using sorted azimuth, elevation, and elevation angle rates determined for several bright scatterers, in conjunction with estimated horizontal cross-line-of-sight ship rotational velocity;

(5) obtaining the two cross-line-of-sight ship rotational velocity rates from the time derivatives of polynomial curve fits to stored estimates of the two cross-line-of-sight rotational velocities derived from regression solutions; and

(6) determining from the estimated scatterer velocity and acceleration, and the estimated ship rotational parameters, the coordinate values of the ship scattering centers; and

(b) means for displaying the formed imagery of the ship target including a range/azimuth projection, an azimuth/elevation profile projection, and a range/elevation profile projection.