Method and apparatus to minimize antenna backside signal response and ambiguity

• US 8,013,792 B2
• Filed: 08/06/2009
• Issued: 09/06/2011
• Est. Priority Date: 09/17/2008
• Status: Active Grant

• Current Assignee: United States Air Force (Government of the United States of America)
• Original Assignee: United States Of America As Represented By The Secretary Of The Air Force
• Inventors: Paschen, Dean, Pratt, Frank, McCarthy, Daniel, Eichel, Lary
• Primary Examiner(s): Tarcza; Thomas
• Assistant Examiner(s): GALT, CASSI J
• Application Number:
  US12/462,632
• Publication Number:
  US 20100066608A1
• Time in Patent Office: 761 Days
• Field of Search: 342/359, 342/375, 342/383
• US Class Current: 342/375
• CPC Class Codes: G01S 13/90 using synthetic aperture techniques , e.g. synthetic aperture radar [SAR] techniques
• CPC Class Codes: G01S 7/2813 Means providing a modification of the radiation pattern for cancelling noise, clutter or interfering signals, e.g. side lobe suppression, side lobe blanking, null-steering arrays specially adapted to secondary radar systems G01S13/762; aerials or aerials systems H01Q21/29, H01Q25/00

Abstract

Method and apparatus for minimizing antenna backside signal response and ambiguity in low frequency applications, particularly low frequency synthetic aperture radar (SAR). Various time delay elements are selectably switched into the signal path so as to cause a null to be placed in the antenna response pattern in the direction of undesired radar returns. The means for selectably switching may be dithered so as to introduce modulation onto the undesired radar return to aid in the discrimination and removal of the undesired radar return from the SAR image during post processing.

Claims

1. In an antenna system having at least two elements, an apparatus for minimizing antenna backside signal response and ambiguity, comprising:

a means for causing a phase difference having a plurality of outputs;

a signal path connecting each of said at least two elements to a corresponding output of said means for causing a phase difference;

a means for causing a variable time delay disposed in at least one said signal path;

a processor; and

a software program;

said software program comprising computer implementable instructions capable of being executed in said processor wherein, when executed, causes said processor to control said means for causing a variable time delay so as to iteratively

cause at least one null to be positioned in the antenna pattern of said antenna in the direction of an undesired received signal;

cause at least one null to be positioned in the antenna pattern of said antenna in the direction of a desired received signal;

dither the position of said nulls according to a predetermined pattern;

detect, at fixed weighting, amplitude modulations on signals received in the spatial direction of said undesired received signal and said desired received signal;

tag strongly amplitude modulated signals as ambiguous artifacts;

tag non amplitude modulated signals and weakly amplitude modulated signals as desirous;

subtract said ambiguous artifacts from said desirous signals; and

integrate the results of said subtraction for each iteration.

2. The apparatus of claim 1, wherein said means for causing a phase difference comprises at least one pair of outputs having a 180 degree relative phase difference.

3. The apparatus of claim 1, wherein said means for causing a variable time delay further comprises an individually selectable plurality of radio frequency transmission media of various electrical length, each of said plurality having a first end and a second end.

4. The apparatus of claim 3, wherein said means for causing a variable time delay further comprises a first switch placed at said first end of said plurality of radio frequency transmission media and a second switch placed at said second end of said plurality of radio frequency transmission media so as to cause said signal path to route through a selected said radio frequency transmission media.

5. The apparatus of claim 4, wherein said processor cooperates with a means for detecting signal amplitude in said at least one signal path.

11. The antenna system of claim 1, wherein

said weakly amplitude modulated signals further comprise amplitude modulations of substantially less than 1 decibel;

and

wherein said strongly amplitude modulated signals further comprise amplitude modulations of substantially greater than 1 decibel.

6. A method for minimizing antenna backside signal response and ambiguity, comprising the steps of:

introducing a relative phase difference between signal paths each supplying signals to at least two antenna elements of an antenna;

causing a variable time delay in at least one said signal path; and

iteratively

positioning at least one null in the antenna pattern of said antenna in the direction of an undesired received signal;

positioning at least one null in the antenna pattern of said antenna in the direction of a desired received signal;

dithering the position of said nulls according to a predetermined pattern;

detecting, at fixed weighting, amplitude modulations on signals received in the spatial direction of said undesired received signal and said desired received signal;

tagging strongly amplitude modulated signals as ambiguous artifacts;

tagging non-amplitude modulated signals and weakly amplitude modulated signals as desirous;

subtracting said ambiguous artifacts from said desirous signals; and

integrating the results of said subtraction for each iteration.

7. The method of claim 6, wherein said step of introducing a relative phase difference further comprises introducing a 180 degree phase difference.

8. The method of claim 7, wherein said step of causing a variable time delay further comprises the step of selecting a time delay from a plurality of discrete selectable time delay elements.

9. The method of claim 8, wherein said discrete selectable time delay elements are radio frequency transmission media of various electrical length, each having a first end and a second end.

10. The method of claim 9, wherein said step of selecting further comprises electrically switching said first end and said second end of any of said frequency transmission media into said at least one signal path.

12. The method of claim 6, wherein

said weakly amplitude modulated signals further comprise amplitude modulations of substantially less than 1 decibel;

and

wherein said strongly amplitude modulated signals further comprise amplitude modulations of substantially greater than 1 decibel.