Coherent two-dimensional image formation by passive synthetic aperture collection and processing of multi-frequency radio signals scattered by cultural features of terrestrial region

• US 6,700,527 B1
• Filed: 11/15/2000
• Issued: 03/02/2004
• Est. Priority Date: 11/15/2000
• Status: Expired due to Term

• Current Assignee: Harris Corporation (L3Harris Technologies, Inc.)
• Original Assignee: Harris Corporation (L3Harris Technologies, Inc.)
• Inventors: Shipley, John W., Martin, Gayle Patrick
• Primary Examiner(s): Trost, William
• Assistant Examiner(s): MILLER, BRANDON J
• Application Number:
  US09/713,378
• Time in Patent Office: 1,203 Days
• Field of Search: 455/423-425, 455/63, 455/62, 455/12.1, 455/13.1, 455/427, 455/226.1, 342/172-178, 342/25, 342/191, 342/192, 342/199
• US Class Current: 342/25.00R
• CPC Class Codes: G01S 13/904 SAR modes
• CPC Class Codes: G01S 13/9058 Bistatic or multistatic SAR
• CPC Class Codes: G01S 7/41 using analysis of echo signal for target characterisation; Target signature; Target cross-section

Abstract

An imaging system uses wideband ‘RF daylight’ created by plural narrowband RF illumination sources, to passively generate spectrally different sets of RF scattering coefficients for multiple points within a prescribed three-dimensional volume being illuminated by the narrowband RF transmitters. To correct for the lack of mutual coherence among different RF illumination sources, the respective sets of scattering coefficient data are applied to a cultural feature extraction operator, to locate one or more strong cultural features spatially common to multiple images. For spatial points along the extracted cultural feature theoretical scattering coefficients are calculated. Differences between phase values of these calculated scattering coefficients and those of the collected and processed scattering energy are used to modify the measured scattering coefficient values for all spatial points in the illuminated region. This allows the scattering coefficients of that narrowband frequency set to be coherently combined with those of another spectrally different narrowband set of scattering coefficients whose phase components have been similarly corrected, based upon the same extracted cultural feature.

Claims

1. A method of generating an image of a terrestrial region upon which electromagnetic energy from respectively different narrowband energy sources is incident comprising the steps of:

(a) passively collecting electromagnetic energy emitted by said respectively different narrowband energy sources by way of at least one first energy collector;

(b) passively collecting, by at least one second energy collector moving independently and arbitrarily with respect to said respectively different narrowband energy sources among a plurality of distributed energy collection locations, electromagnetic energy emitted by said electromagnetic energy sources and scattered by features of said terrestrial region of interest;

(c) processing a reference signal representative of electromagnetic energy collected in step (a), in accordance with information representative of the collection geometry of said at least one first energy collector and the geolocations of said electromagnetic energy sources, so as to derive a time- and location-corrected reference signal;

(d) correlating the time- and location-corrected reference signal derived in step (c) with image signals representative of electromagnetic energy collected by said at least one moving second energy collector in step (b), so as to derive amplitude and phase values of scattering components for each of said narrowband sources for plural locations of said terrestrial region as received by said at least one moving second energy collector as a function of spatial position; and

(e) processing scattering components derived in step (d) for each of said respectively different narrowband sources to produce said multidimensional image of said terrestrial region of interest.

2. A method according to claim 1, wherein said narrowband electromagnetic energy sources comprise television signal transmitters operating at respectively different frequencies, and step (b) comprises passively collecting electromagnetic energy emitted by said television signal transmitters, and scattered by features of said terrestrial region of interest, by means of an airborne or spaceborne electromagnetic energy collection platform moving among said plurality of distributed energy collection locations.

3. A method according to claim 1, wherein step (e) comprises performing phase coherence processing of said scattering components for each of said narrowband sources so as to produce a composite set of scattering component values for said terrestrial region of interest from which said multidimensional image may be generated.

4. A method according to claim 3, wherein step (e) comprises

(e1) locating a cultural feature within multiple ones of respective sets of scattering coefficient data obtained for said narrowband illumination sources;

(e2) generating, for spatial points on the cultural feature located in step (e1), theoretical scattering coefficients; and

(e3) modifying scattering coefficient values of said scattering coefficient data obtained for said narrowband illumination sources in accordance with differences between phase values generated in step (e2) and those derived in step (d), so as to enable scattering coefficients of one narrowband frequency set to be coherently combined with those of other spectrally different narrowband sets of scattering coefficients.

5. A system for deriving image information representative of cultural features of a terrestrial region illuminated by spectrally different RF transmitters comprising:

a reference signal collection subsystem which is operative to collect non-scattered RF energy emitted by said spectrally different RF transmitters illuminating said terrestrial region;

a dynamic scattered image energy subsystem which is operative to travel past said terrestrial region, moving arbitrarily and independently with respect to said spectrally different RF transmitters and collect RF energy scattered from points of cultural features within said terrestrial region; and

a collected signal processing subsystem, which is operative to process information representative of said non-scattered RF energy as collected by said reference signal collection subsystem, to derive coherent reference signals corresponding to those transmitted by said RF transmitter illuminating said terrestrial region, time- and location-corrected as necessary to points within said terrestrial region, and to correlate said coherent reference signals with a scattered RF energy signals representative of electromagnetic energy collected by said dynamic scattered image energy subsystem, time- and location-corrected as necessary to said points within said terrestrial region, so as to derive respective sets of amplitude and phase values of scattering components for said points of said terrestrial region, and is operative to process respective sets of scattering components derived for each of said respectively different narrowband sources to coherently combinable sets of scattering coefficients.

6. A system according to claim 5, further including an image generator subsystem, which is operative to process said sets of scattering components to produce a multidimensional image of cultural features of said terrestrial region.

7. A system according to claim 5, wherein said reference signal collection subsystem and said dynamic scattered image energy subsystem are configured to employ a common RF energy collector that simultaneously collects said non-scattered RF energy emitted by said spectrally different RF transmitters illuminating said terrestrial region, and RF energy scattered from points of cultural features within said terrestrial region.

8. A system according to claim 5, wherein said reference signal collection subsystem and said dynamic scattered image energy subsystem employ respectively separate RF energy collectors that simultaneously collect said non-scattered RF energy emitted by said spectrally different RF transmitters illuminating said terrestrial region, and RF energy scattered from points of cultural features within said terrestrial region.

9. A system according to claim 7, wherein said collected signal processing subsystem includes:

a coherent reference signal processing section having a first Lorentz transform operator that accounts for signal propagation delay from a respective narrowband transmitter to said reference signal collection subsystem, and performs a Lorentz transform of an RF energy signal received thereby to a static frame of reference of a respective point in said illuminated terrestrial region, and a delay associated with said reference signal'"'"'s propagation time from said respective narrowband transmitter to said respective point, so as to effectively transform a reference signal component of RF energy received at the collection aperture of said reference signal collection subsystem to said respective point, and

a dynamic scattered signal processing section having a second Lorentz transform operator which accounts for signal propagation delay and performs a second Lorentz transform of RF energy received by said dynamic scattered image energy subsystem from its moving frame of reference to the static frame of reference of said respective point.

10. A system according to claim 9, wherein said dynamic scattered signal processing section includes a reference signal suppression operator coupled to remove a reference signal component from the scattered image component of RF energy received by said dynamic scattered image energy subsystem.

11. A system according to claim 9, wherein said collected signal processing subsystem includes a correlator, which is operative to correlate the output of said dynamic scattered signal processing section with the output of said coherent reference signal processing section, to derive a respective set of scattering coefficients.

12. A system according to claim 11, further including an image generator subsystem, which is operative to process said amplitude and phase values of scattering components for said points to produce a multidimensional image of cultural features of said terrestrial region.

13. A system according to claim 9, wherein said collected signal processing subsystem is operative to perform phase coherence processing said scattering components for each of said narrowband sources so as to produce a composite set of scattering component values for said terrestrial region from which a multidimensional image may be generated.

14. A system according to claim 13, wherein said collected signal processing subsystem is operative to locate a cultural feature within multiple ones of respective sets of scattering coefficient data obtained for said narrowband illumination sources, generate, for spatial points on the located cultural feature, theoretical scattering coefficients, and to modify scattering coefficient values of said scattering coefficient data obtained for said narrowband illumination sources in accordance with differences between derived and generated phase values, so as to enable scattering coefficients of one narrowband frequency set to be coherently combined with those of other spectrally different narrowband sets of scattering coefficients.

15. A method for passively deriving image information representative of cultural features of a region illuminated by a plurality of spectrally different narrowband RF transmitters comprising the steps of:

(a) providing coherent reference signals representative of RF energy emitted by said spectrally different RF transmitters illuminating said terrestrial region;

(b) collecting, from at least one energy collector moving arbitrarily and independently of said plurality of spatially different narrowband RF transmitters along a prescribed portion of a travel path offset from said terrestrial region, RF energy scattered, as a result of illumination by RF energy emitted by said spectrally different RF transmitters, from points that are capable of defining cultural features within said terrestrial region;

(c) correlating said coherent reference signal, time- and location-corrected as necessary to said points of said terrestrial region, with scattered RF energy signals associated with said spectrally different transmitters and representative of electromagnetic energy collected by said dynamic scattered image energy subsystem, time- and location-corrected as necessary to said points of said terrestrial region, so as to derive respective sets of scattering components for said spectrally different narrowband sources for said points of said terrestrial region; and

(d) performing phase coherence processing of said scattering, components for each of said spectrally different narrowband RF sources so as to produce a composite set of scattering components for said terrestrial region from which a multidimensional image may be generated.

16. A method according to claim 15, wherein step (d) comprises

(d1) locating a cultural feature within multiple ones of respective sets of scattering coefficient data obtained for said narrowband illumination sources;

(d2) generating, for spatial points on the cultural feature located in step (d1), theoretical scattering coefficients; and

(d3) modifying scattering coefficient values of said scattering coefficient data obtained for said narrowband illumination sources in accordance with differences between phase values generated in step (d2) and those derived in step (c), so as to enable scattering coefficients of one narrowband frequency set to be coherently combined with those of other spectrally different narrowband sets of scattering coefficients.

17. A method according to claim 15, further including the step (e) of processing said composite set of scattering components to produce a multidimensional image of cultural features of said terrestrial region.

18. A method according to claim 15, wherein steps (a) and (b) include employing a common RF energy collector to simultaneously collect non-scattered RF energy emitted by said narrowband RF transmitters illuminating said terrestrial region, and RF energy scattered from cultural features of said terrestrial region.

19. A method according to claim 15, wherein steps (a) and (b) include employing respectively separate RF energy collectors to collect RF energy emitted by said narrowband RF transmitters illuminating said terrestrial region, and RF energy scattered from said points of cultural features of said terrestrial region.

20. A method according to claim 15, wherein step (c) comprises:

(c1) processing a respective coherent reference signal in accordance with a first Lorentz transform that accounts for signal propagation delay from a respective narrowband RF transmitter to a collector for said respective coherent signal and performs a Lorentz transform of an RF energy signal received thereby to a static frame of reference of a respective point in a three-dimensional space of said terrestrial region, and providing a delay associated with said reference signal'"'"'s propagation time from said respective narrowband RF transmitter to said respective point, so as to effectively transform a reference signal component of RF energy received by said collector to said respective point, and

(c2) processing said scattered RF energy signal in accordance with a second Lorentz transform operator which accounts for signal propagation delay and performs a second Lorentz transform of RF energy received by a collector for said scattered RF energy signal from its moving frame of reference to the static frame of reference of said respective point in said three-dimensional space.

21. A method of generating an image of a terrestrial region comprising the steps of:

(a) passively collecting non-scattered electromagnetic energy, emitted by respectively different narrowband electromagnetic energy transmitters illuminating said terrestrial region, at a first, passive, electromagnetic energy collector that is exclusive of any electromagnetic energy transmission element employed by said respectively different narrowband electromagnetic energy transmitters to transmit narrowband electromagnetic energy;

(b) passively collecting, by a second passive electromagnetic energy collector, electromagnetic energy emitted by said narrowband electromagnetic energy transmitters and scattered by features of said terrestrial region of interest, said second passive electromagnetic energy collector moving independently and arbitrarily with respect to said respectively different narrowband electromagnetic energy transmitters illuminating said terrestrial region among a plurality of distributed energy collection locations and being exclusive of any electromagnetic energy transmission element employed by any of said respectively different narrowband electromagnetic energy transmitters;

(c) processing a reference signal representative of electromagnetic energy collected in step (a), in accordance with information representative of the collection geometry of said first energy collection element and geolocations of said electromagnetic energy transmitters, so as to derive a time- and location-corrected reference signal;

(d) correlating said time- and location-corrected reference signal derived in step (c) with image signals representative of electromagnetic energy collected by said moving second energy collector in step (b), so as to derive amplitude and phase values of scattering components for each of said narrowband electromagnetic energy transmitters for plural locations of said terrestrial region as received by said moving second energy collector as a function of spatial position; and

(e) processing scattering components derived in step (d) for each of said respectively different narrowband electromagnetic energy transmitters to produce said multidimensional image of said terrestrial region of interest.

22. The method according to claim 21, wherein step (d) comprises correlating said coherent reference signal, time-, and location-corrected as necessary to points of said terrestrial region, with scattered electromagnetic energy signals associated with spectrally different ones of said respectively different narrowband electromagnetic energy transmitters and representative of electromagnetic energy collected by said second electromagnetic energy collector, so as to derive respective sets of scattering components for said spectrally different narrowband electromagnetic energy transmitters for said points of said terrestrial region;

and wherein

step (e) comprises performing phase coherence processing of said scattering components for each of said spectrally different narrowband electromagnetic energy sources so as to produce a composite set of scattering components for said terrestrial region from which said multidimensional image is produced.

23. The method according to claim 21, wherein step (c) comprises:

(c1) processing a respective coherent reference signal in accordance with a first Lorentz transform that accounts for signal propagation delay from a respective narrowband electromagnetic energy transmitter to a passive collector for said respective coherent signal and performs a Lorentz transform of an electromagnetic energy signal received thereby to a static frame of reference of a respective point in a three-dimensional space of said terrestrial region, and providing a delay associated with said reference signal'"'"'s propagation time from said respective narrowband electromagnetic energy transmitter to said respective point, so as to effectively transform a reference signal component of narrowband electromagnetic energy received by said passive collector to said respective point, and

(c2) processing said scattered electromagnetic energy signal in accordance with a second Lorentz transform operator which accounts for signal propagation delay and performs a second Lorentz transform of electromagnetic energy received by a passive collector for said scattered electromagnetic energy signal from its moving frame of reference to the static frame of reference of said respective point in said three-dimensional space.

24. The method according to claim 21, wherein said narrowband electromagnetic energy transmitters comprise television signal transmission towers transmitting television signals at respectively different frequencies, and step (b) comprises passively collecting electromagnetic energy emitted by said television signal transmission towers, and scattered by features of said terrestrial region of interest, by means of said second passive electromagnetic energy collector that is mounted on an airborne or spaceborne electromagnetic energy collection platform moving among said plurality of distributed energy collection locations.