Range/azimuth ship imaging for ordnance control

• US 4,546,354 A
• Filed: 06/17/1982
• Issued: 10/08/1985
• Est. Priority Date: 06/17/1982
• Status: Expired due to Fees

• Current Assignee: Grumman Aerospace Corporation Bethpage NY 11714
• Original Assignee: Grumman Aerospace Corporation (Northrop Grumman Corporation)
• Inventors: Boles, Sol
• Primary Examiner(s): Tubbesing, T. H.
• Assistant Examiner(s): Barron Jr., Gilberto
• Application Number:
  US06/389,367
• Time in Patent Office: 1,209 Days
• Field of Search: 343/5 CM, 343/17, 343/7 ED, 343/5 SA, 343/8, 343/7.7, 343/16 M, 364/516, 364/517
• US Class Current: 342/179
• CPC Class Codes: G01S 13/66 Radar-tracking systems; Analogous systems
• CPC Class Codes: G01S 13/90 using synthetic aperture techniques , e.g. synthetic aperture radar [SAR] techniques
• CPC Class Codes: G01S 13/9092 combined with monopulse techniques

Abstract

Distortions inherent in the formation of a range/doppler image by an airborne Synthetic Aperture Radar (SAR) of a ship under the influence of roll, pitch, and yaw motions characteristic of a sea state conditions, are removed by the formation of a scaled range/azimuth image generated with the use of an interferometer antenna in conjunction wth the SAR. A least squares linear regression solution to doppler processed interferometric azimuth angle data derived from ship radar reflections permits the determination of aircraft to ship relative rotational motion essential to the development of such an improved high resolution radar image, so that continuous automatic tracking of a cursor imbedded in a single designate resolution cell of the ship'"'"'s displayed image essential to carrying out precision standoff command weapon guidance to that selected ship target cell, can be accomplished.

Claims

1. In conjunction with an airborne synthetic aperture radar system having an interferometer antenna and a range versus azimuth angle display, a method for forming with improved azimuth resolution a scaled range/azimuth image of a ship target under the influence of sea state conditions comprising the steps of:

(a) steering the pointing of the interferometer antenna boresight to provide radar illumination of the ship target;

(b) controlling the range sampling timing so that corresponding samples from pulse-to-pulse over the coherent integration interval correspond to the same range increment of the ship target;

(c) compensating for phase variations in the received signals due to the respective motions of the radar bearing aircraft and the ship;

(d) measuring interferometrically the compensated received signals for each doppler filter in each range bin;

(e) plotting as interferometer azimuth angle versus filter doppler frequency the measured data for each doppler filter in each range bin;

(f) performing a weighted least squares linear regression straight line fit to the plotted data points of the azimuth angle/doppler distribution for each range bin;

(g) determining the slope of the straight line fit to the azimuth angle/doppler distribution for each range bin;

(h) averaging the slope values determined over all range bins; and

(i) determining from the averaged slope value the values of predetermined system parameters including synthetic aperture radar integration time and doppler filter bandwidths and spacings to be used predictively in the succeeding coherent integration interval in forming the range/azimuth image.

2. A method as recited in claim 1 including the further steps of:

(a) displaying the formed range/azimuth image of the ship target on the range versus azimuth angle display;

(b) cursoring a designated range/azimuth resolution cell of the displayed image of the ship target; and

(c) tracking from aperture to aperture the designated resolution cell of the ship target.

3. A method as recited in claim 2 including the further step of applying to the cursor location on an aperture to aperture basis a tracking correction to compensate for the rotation of the ship about an axis orthogonal to both the aircraft horizontal cross line-of-sight velocity and the radar line-of-sight to the center of rotation tracking point of the ship.

4. A method as recited in claim 3 including the further steps of excluding any data points located outside of a prescribed threshold band about the least squares fit formed from all available data points of the azimuth angle/doppler distribution for each range bin;

and refitting to the data after such exclusion.

5. In conjunction with an airborne synthetic aperture radar system having an interferometer antenna and a range versus azimuth display, a method for forming with improved azimuth resolution a range/azimuth image of a ship target under the influence of sea state conditions comprising the steps of:

(a) processing the received signals from the scatterers comprising the ship target to obtain an estimate of the net doppler producing cross line-of-sight velocity of the radar bearing aircraft relative to the ship;

(b) determining from the estimated cross line-of-sight relative velocity the values of predetermined system parameters to be used predictively in the succeeding coherent integration interval in forming the range/azimuth image;

(c) displaying the formed range/azimuth image of the ship target on the range versus azimuth display;

(d) cursoring a designated range/azimuth resolution cell of the displayed image of the ship target;

(e) tracking from aperture to aperture the range and interferometrically determined azimuth of the designated resolution target cell;

(f) compensating for phase variations in the received signals due to the respective motions of the radar bearing aircraft and the ship;

(g) steering the pointing of the interferometer antenna boresight to provide radar illumination of the ship target;

(h) controlling the range sampling timing so that corresponding range samples from pulse-to-pulse over the coherent integration interval correspond to the same range increment of the ship target;

(i) applying to the cursor location on an aperture to aperture basis a tracking correction to compensate for the rotation of the ship about the axis orthogonal to both the aircraft horizontal cross line-of-sight velocity and the radar line-of-sight to the center of rotation tracking point of the ship; and

(j) processing the compensated received signals by;

(1) measuring interferometrically the compensated received signals for each doppler filter in each range bin;

(2) plotting an interferometer azimuth angle versus filter doppler frequency the measured data for each doppler frequency in each range bin;

(3) performing a straight line fit to the plotted data points of the azimuth angle/doppler distribution for each range bin;

(4) determining the slope of the straight line fit to the azimuth angle/doppler distribution for each range bin;

(5) averaging the slope values determined over all range bins; and

(6) obtaining from the averaged slope value an estimate of the net doppler producing cross line-of-sight velocity of the radar bearing aircraft relative to the ship.

6. In conjunction with an airborne synthetic aperture radar system having an interferometer antenna and a range versus azimuth display, a method for forming with improved azimuth resolution a range/azimuth image of a ship target under the influence of sea state conditions comprising the steps of:

(a) processing the received signals from the scatterers comprising the ship target to obtain an estimate of the net doppler producing cross line-of-sight velocity of the radar bearing aircraft relative to the ship;

(b) determining from the estimated cross line-of-sight relative velocity the values of predetermined system parameters to be used predictively in the succeeding coherent integration interval in forming the range/azimuth image;

(c) displaying the formed range/azimuth image of the ship target on the range versus azimuth display;

(d) cursoring a designated range/azimuth resolution cell of the displayed image of the ship target;

(e) tracking from aperture to aperture the range and interferometrically determined azimuth of the designated resolution target cell;

(f) compensating for phase variations in the received signals due to the respective motions of the radar bearing aircraft and the ship;

(g) steering the pointing of the interferometer antenna boresight to provide radar illumination of the ship target;

(h) controlling the range sampling timing so that corresponding range samples from pulse-to-pulse over the coherent integration interval correspond to the same range increment of the ship target;

(i) applying to the cursor location on an aperture to aperture basis a tracking correction to compensate for the rotation of the ship about the axis orthogonal to both the aircraft horizontal cross line-of-sight velocity and the radar line-of-sight to the center of rotation tracking point of the ship; and

(j) processing the compensated received signals by;

(1) measuring interferometrically the compensated received signals for each doppler filter in each range bin;

(2) plotting as interferometer azimuth angle versus filter doppler frequency the measured data for each doppler frequency in each range bin;

(3) performing a straight line fit to the plotted data points of the azimuth angle/doppler distribution for each range bin;

(4) determining the slope of the straight line fit to the azimuth angle/doppler distribution for each range bin;

(5) averaging the slope values determined over all range bins;

(6) obtaining from the averaged slope value and estimate of the net doppler producing cross line-of-sight velocity the radar bearing aircraft relative to the ship; and

(7) excluding any data points located outside a prescribed threshold band about the straight line fit formed from all available data points of the azimuth angle/doppler distribution for each range bin and refitting to the data after such exclusion.

7. In combination with an airborne synthetic aperture radar system including a two section interferometer antenna operatively connected to the input of a two channel receiver and doppler processing system, and a display of range versus azimuth operatively connected to the output of said two channel receiver and doppler processing system, image signal processing means for forming with improved azimuth resolution a scaled range/azimuth image of a ship target under the influence of sea state conditions comprising:

(a) means for steering the pointing of the interferometer antenna boresight to provide radar illumination of the ship target;

(b) means for controlling the range sampling timing so that corresponding samples from pulse-to-pulse over the coherent integration interval correspond to the same range increment of the ship target;

(c) means for compensating for phase variations in the received signals due to the respective motions of the radar bearing aircraft and the ship;

(d) means for measuring interferometrically the compensated received signals for each doppler filter in each range bin;

(e) means for

(1) plotting as an interferometer azimuth angle versus doppler filter frequency the measured data for each doppler filter in each range bin;

(2) performing a weighted least squares linear regression straight line fit to the plotted data points of the azimuth angle/doppler distribution for each range bin;

(3) excluding any data points located outside of a prescribed threshold band about the straight line fit formed from all available data points of the azimuth angle/doppler distribution for each range bin and refitting to the data after such exclusion;

(4) determining the slope of the straight line fit to the azimuth angle/doppler distribution for each range bin; and

(5) averaging the slope values determined over all range bins;

(f) means for determining from the averaged slope value the values of predetermined system parameters including synthetic aperture radar integration time and doppler filter bandwidths;

(g) means for displaying the formed range/azimuth image of the ship target on the range versus azimuth display;

(h) means for cursoring a designated range/azimuth resolution cell of the displayed image of the ship target;

(i) means for tracking from aperture to aperture the designated resolution cell of the ship target and for applying to the cursor location on an aperture to aperture basis a tracking correction to compensate for the rotation of the ship about an axis orthogonal to both the aircraft horizontal cross line-of-sight velocity and the radar line-of-sight to the center of rotation tracking point of the ship.