Full aperture image synthesis using rotating strip aperture image measurements

• US 5,243,351 A
• Filed: 06/25/1992
• Issued: 09/07/1993
• Est. Priority Date: 06/25/1992
• Status: Expired due to Term

• Current Assignee: Raytheon Company (Rtx Corporation)
• Original Assignee: Hughes Aircraft Company (Rtx Corporation)
• Inventors: Rehfield, Mark J., Rafanelli, Gerard L.
• Primary Examiner(s): Sotomayor, John B.
• Application Number:
  US07/904,235
• Time in Patent Office: 439 Days
• Field of Search: 342/351, 342/25, 342/21, 342/52, 342/53, 342/54, 342/191, 342/179, 342/194, 342/195, 342/196, 342/197, 342/424, 342/378
• US Class Current: 342/351
• CPC Class Codes: G01K 11/006 using measurement of the effect of a material on microwaves or longer electromagnetic waves, e.g. measuring temperature via microwaves emitted by the object G01K17/003, G01J5/00 take precedence; measuring the effect of a material on X-, gamma- or particle radiation G01K11/30
• CPC Class Codes: G01S 17/90 using synthetic aperture techniques
• CPC Class Codes: G01S 3/7865 using correlation of the live video image with a stored image

Abstract

A spinning strip aperture imaging radiometer sensor system and data processing method that is capable of synthesizing full circular aperture images from a plurality of image frames acquired by the strip aperture imaging sensor. One embodiment of the imaging system comprises a rotating strip aperture wide field of view telescope, a two dimensional detector array used to detect images in the telescope'"'"'s focal plane, a rotation compensation device used to prevent rotational smear during the integration time of the detectors, a signal processor used to record a plurality of image frames of a target scene that is imaged by the telescope as it rotates around its optical axis, and a signal processor and method used to synthesize the full circular aperture image from the recorded images. The operation of the full aperture image synthesis method hinges upon the ability of the rotating strip aperture to measure all of the spatial frequencies contained in a full circular aperture image measurement. Having knowledge of the strip apertures'"'"' spatial frequency passband, and the temporal registrations of each of the recorded strip aperture images permits synthesis of the full aperture image by the sensor system and image synthesis method. Image synthesis may be accomplished in the spatial or spatial frequency domains. General and illustrative examples of the image synthesis procedure and first order noise performance predictions are described. A general form of the invention utilizes two dimensional spatial frequency information to greatly reduce the line of sight stability requirements of the telescope.

Claims

1. A spinning strip radiometer system comprising:

a telescope comprised of a rotating strip aperture that rotates around an optical axis and that produces temporally sequential images of a scene;

a two-dimensional detector array for detecting images located in the focal plane of the telescope;

a signal processor coupled to the detector array for recording a plurality of image frames of the image scene as the strip aperture rotates around the optical axis of the telescope, and for synthesizing a full circular aperture image from the recorded image frames.

2. The spinning strip radiometer system of claim 1 wherein one dimension of the rotating strip aperture is substantially identical to a diameter of a full aperture optical system to be synthesized.

3. The spinning strip radiometer system of claim 1 wherein the spinning strip aperture telescope comprises a multiple-strip aperture in which several radial strips are used as an image acquisition sensor.

4. The spinning strip radiometer system of claim 1 wherein the spinning strip aperture telescope comprises a phased array in which two or more telescopes are coherently joined to produce the images used during image synthesis.

5. The spinning strip radiometer system of claim 1 wherein the spinning strip aperture telescope comprises a sparse phased array comprising a plurality of telescopes, and wherein rotation of the telescopes in conjunction with predetermined variations of the separation distances between telescopes are used to fill gaps in the spatial frequency passband of the measurements.

6. The spinning strip radiometer system of claim 1 wherein the signal processor is adapted to:

record an image frame as the spinning strip aperture rotates about its optical axis;

interpolate the data in the image frame;

perform a coordinate transformation on the data in the image frame;

Fourier transform the data in the image frame;

estimate the frame to frame misregistration of the data in the image frame;

correct for misregistration of the data in the image frame;

add the corrected image frame data to a buffer;

repeat the above steps for each image frame, and wherein after all image frames have been processed and added to the buffer, the data in the buffer is representative of image information comprising a full aperture image;

spatially filter the image data representative of the full aperture image information with a synthesizer; and

inverse Fourier transform the spatially filtered image data to provide a synthesized full aperture image.

7. A spinning strip radiometer system that comprises:

a rotating strip aperture telescope that comprises an optical axis and that produces temporally sequential images of a scene;

a two-dimensional detector array for detecting images located in the focal plane of the telescope;

a rotation compensation means for producing a stationary image by compensating for the image rotation during the integration time of the detector; and

a signal processor for recording a plurality of image frames of a scene imaged by the telescope as it rotates around the optical axis of the telescope, and for synthesizing a full circular aperture image from the recorded image frames;

whereby the spinning strip radiometer system synthesizes a circular full aperture radiometric image from a plurality of rotating strip aperture image measurements, while compensating for random line of sight errors between individual strip aperture images to provide a high resolution image.

8. The system of claim 7 wherein the rotation compensation means counter-rotates the image during the integration time of the detector comprising the detector array to provide a stationary image.

9. The spinning strip radiometer system of claim 7 wherein a longest dimension of the rotating strip aperture telescope is substantially identical to a diameter of a full aperture optical system to be synthesized.

10. The spinning strip radiometer system of claim 7 wherein the spinning strip aperture telescope comprises a multiple-strip aperture in which several radial strips are used as an image acquisition sensor.

11. The spinning strip radiometer system of claim 7 wherein the spinning strip aperture telescope comprises a phased array in which two or more telescopes are coherently joined to produce the images used during image synthesis.

12. The spinning strip radiometer system of claim 7 wherein the spinning strip aperture telescope comprises a sparse phased array comprising a plurality of telescopes, and wherein rotation of the telescopes in conjunction with predetermined variations of the separation distances between telescopes are used to fill gaps in the spatial frequency passband of the measurements.

13. The spinning strip radiometer system of claim 7 wherein the rotation compensation means prevents rotational smear during integration time of the detectors of the array.

14. An image synthesizing method for use with a spinning strip aperture radiometer that records image data as a spinning strip aperture rotates about its optical axis, said method comprising the steps of:

recording an image frame as the spinning strip aperture rotates about an optical axis;

interpolating the data in the image frame;

performing a coordinate transformation on the data in the image frame;

Fourier transforming the data in the image frame;

estimating the frame to frame misregistration of the data in the image frame;

correcting for misregistration of the data in the image frame;

adding the corrected image frame data to a buffer;

repeating the above steps for each image frame, and wherein after all image frames have been processed and added to the buffer, the data in the buffer is representative of the full aperture image information;

spatially filtering the image data representative of the full aperture image information with a synthesizer; and

inverse Fourier transforming the spatially filtered image data to provide a synthesized image.

15. An image synthesizing method for use in an imaging system having a spinning strip aperture telescope that rotates around its optical axis, a two-dimensional detector array for detecting images located in the focal plane of the telescope, rotation compensation means for providing a stationary image during the integration time of detectors of the detector array, and a signal processor for recording a plurality of image frames of a scene imaged by the telescope as it rotates around the optical axis of the telescope, and for synthesizing a full circular aperture image from the recorded image frames, wherein the synthesizing method comprises the following steps:

recording and storing an image frame;

interpolating and coordinate transforming the stored frame such that it corresponds to a reference coordinate grid of the synthesized image;

spatially Fourier transforming and storing the data in the image frame;

recording and storing the remainder of the image frames;

estimating the frame-to-frame misregistration of the data due to random line of sight errors;

correcting a selected one of the strip aperture images, or its Fourier transform, for the line of sight errors and storing the corrected image or Fourier transform;

sequentially repeating the preceding steps for each strip aperture image frame; and

after all image frames have been processed and saved;

synthesizing a full aperture image spectrum by summing redundant individual spatial frequency components from the strip aperture measurements that correspond to the reference coordinate grid of the full aperture spatial frequencies to provide a summed result;

dividing the summed result by the number of frames contributing to the spatial frequency redundancy;

spatial frequency filtering the resultant data using an appropriate spatial frequency synthesizer to produce a synthesized image spectrum; and

inverse Fourier transforming the synthesized image spectrum to provide a full aperture image.

16. The image synthesizing method of claim 15 wherein information comprising two dimensional spatial frequency content of each frame is used to estimate frame to frame line of sight errors using predetermined cross-coherence techniques, and the line of sight error estimates are used to register predetermined aperture images.

17. The image synthesizing method of claim 16 wherein the predetermined aperture images comprise individual strip aperture images.

18. The image synthesizing method of claim 16 wherein the predetermined aperture images comprise all of the strip aperture images, and the line of sight error estimates are used to noise optimally register all of the strip aperture images.

19. The image synthesizing method of claim 15 wherein information comprising two dimensional spatial frequency content of each frame is used to estimate frame to frame line of sight errors using predetermined cross-correlation or cross-coherence techniques, and the line of sight error estimates are used to register individual strip aperture images.

20. The image synthesizing method of claim 19 wherein the predetermined aperture images comprise all of the strip aperture images, and the line of sight error estimates are used to noise optimally register all of the strip aperture images.

21. The image synthesizing method of claim 15 wherein the two dimensional spatial frequency information provides redundant frame to frame image spectrum information, that increases the effective signal to noise ratio of the spatial frequencies of the synthesized full aperture image.

22. The method of claim 15 wherein the step of correcting for misregistration of the data in the image frame comprises the steps of:

processing the individual strip aperture images with a spatially weighted window, wherein the weighting in the window is adapted to eliminate non-common image data at outer edges of the image caused by line of sight jitter between image frames;

performing a Fourier transform on each strip aperture image frame;

calculating the cross-coherence between strip aperture image frames for the common spatial frequency regions;

determining the line of sight shift between image frames using the phase of the cross-coherence between strip aperture frames;

compensating the strip aperture image spectra for line of sight shifts by using a predetermined Fourier transform shift theorem; and

synthesizing the full aperture image.

23. The method of claim 15 wherein the step of synthesizing the image frame comprises the step of:

processing the image data using an estimation procedure that accounts for signal, noise, and measurement statistics and processes to optimally use redundant measurement information.

24. The method of claim 15 wherein the step of synthesizing the image frame comprises the steps of:

estimating a particular spatial frequency image components, or spatial frequency image component in the full aperture optical system'"'"'s passband by;

establishing a set of n spatial frequency measurements in the neighborhood of the k spatial frequency image components, or spatial frequency image component, to be estimated, where the term "in the neighborhood" refers to a temporal and spatial frequency region in the vicinity of the frequency image component, or frequency image components, to be estimated, and wherein the neighborhood includes an entire spatial frequency passband of the full aperture, all the time domain measurements, or any fraction thereof;

incorporating apriori and aposteriori knowledge concerning the strip aperture sensor system, the full aperture sensor system, the meausurement process, and the noise and signal statistics into the Gauss-Markov estimation matrix;

applying the Gauss-Markov estimation matrix to the measured neighborhood of points to obtain the estimate of the selected spatial frequency image component, or spatial frequency image components, of the full aperture image;

repeating the process until all spatial frequency image components in the passband of the full aperture have been estimated; and

performing an inverse Fourier transform on the spatial frequency image components in the passband to obtain a synthesized image.

25. The method of claim 15 wherein the step of synthesizing the image frame comprises spatial domain image synthesizing of the image frame comprising the steps of:

generating an estimation matrix for transforming idealized strip aperture spatial and temporal measurements associated with the strip aperture'"'"'s point spread function to measurements obtained using a full aperture point spread function by;

establishing a set of n spatial measurements in the neighborhood of the spatial image component, or spatial image components, to be estimated, wherein the term "in the neighborhood" refers to a temporal, as well as, a spatial region in the vicinity of the point, or points, to be estimated, wherein the neighborhood can include the entire spatial region of the focal plane, and all the time domain measurements, or any fraction thereof;

incorporating apriori and aposteriori knowledge concerning the strip aperture sensor system, the full aperture sensor system, the measurement process, and the noise and signal statistics into the Gauss-Markov estimation matrix;

applying the Gauss-Markov estimation matrix to the measured neighborhood of points to obtain the estimate of the selected spatial image component, or spatial image components; and

repeating the process until all spatial components in the full aperture image have been determined, and wherein the Gauss-Markov estimation matrix is calculated for each of the discrete spatial image components or spatial image components to be estimated in the full aperture image.

26. An image synthesizing method for use in an imaging system having a spinning strip aperture telescope that rotates around its optical axis, a two-dimensional detector array for detecting images located in the focal plane of the telescope, and a signal processor for recording a plurality of strip image frames of a scene imaged by the telescope as it rotates around an optical axis, and for synthesizing a full circular aperture image from the recorded image frames, wherein the synthesizing method comprises the following steps:

recording and storing a strip image frame;

interpolating and coordinate transforming the stored frame such that it corresponds to a reference coordinate grid of the synthesized image;

storing the data in the strip image frame;

recording and storing another image frame;

estimating the frame-to-frame misregistration of the data due to random line of sight errors;

correcting a selected one of the strip aperture images, or their Fourier transforms, for their line of sight errors and storing the corrected images or Fourier transforms, respectively;

sequentially repeating the preceding steps for each strip aperture image frame; and

after all image frames have been corrected for frame to frame registration errors and saved;

synthesizing a full aperture image or image spectrum using statistically optimal synthesizers.

27. The image synthesizing method of claim 26 wherein information comprising two dimensional spatial frequency content of each frame is used to estimate frame to frame line of sight errors using cross-coherence (or cross-correlation) techniques, and the line of sight errors estimates are used to register individual strip aperture images or noise optimally register all of the strip aperture images.

28. The image synthesizing method of claim 26 wherein the two dimensional spatial frequency information provides redundant frame to frame image spectrum information, that increases the effective signal to noise ratio of the spatial frequencies of the synthesized full aperture image.

29. The method of claim 26 wherein the step of synthesizing the image frame comprises the step of:

processing the image data using an estimation procedure that accounts for signal, noise, and measurement statistics to optimally use redundant measurement information.

30. The method of claim 26 wherein the step of synthesizing the image frame comprises the steps of:

estimating a particular spatial frequency image components, or spatial frequency image component in the full aperture optical system'"'"'s passband by;

establishing a set of n spatial frequency measurements in the neighborhood of the k spatial frequency image components, or spatial frequency image component, to be estimated, where the phrase "in the neighborhood" refers to a temporal and spatial frequency region in the vicinity of the image frequency component, or image frequency components, to be estimated, and wherein the neighborhood includes an entire spatial frequency passband of the full aperture, all the time domain measurements, or any fraction thereof;

incorporating apriori and aposteriori knowledge concerning the strip aperture sensor system, the full aperture sensor system, the measurement process, and the noise and signal statistics into the Gauss-Markov estimation matrix;

applying the Gauss-Markov estimation matrix to the measured neighborhood of points to obtain the estimate of the selected spatial frequency image component, or spatial frequency image components, of the full aperture image;

repeating the process until all spatial frequency image components in the passband of the full aperture have been estimated; and

performing an inverse Fourier transform on the spatial frequency image components in the passband to obtain a synthesized image.

31. The method of claim 27 wherein the step of synthesizing the image frame comprises spatial domain image synthesizing the image frame comprising the steps of:

generating an estimation matrix for transforming idealized strip aperture spatial and temporal measurements associated with the strip aperture'"'"'s point spread function to measurements obtained using a full aperture point spread function by;

establishing a set of n spatial measurements in the neighborhood of the spatial image component, or spatial image components, to be estimated, wherein the term "in the neighborhood" refers to a temporal, as well as, a spatial region in the vicinity of the point, or points, to be estimated, wherein the neighborhood can include the entire spatial region of the focal plane, and all the time domain measurements, or any fraction thereof;

incorporating apriori and aposteriori knowledge concerning the strip aperture sensor system, the full aperture sensor system, the measurement process, and the noise and signal statistics into the Gauss-Markov estimation matrix;

applying the Gauss-Markov estimation matrix to the measured neighborhood of points to obtain the estimate of the selected spatial image component, or spatial image components; and

repeating the process until all spatial components in the full aperture image have been determined, and whereby the Gauss-Markov estimation matrix must be calculated for each of the discrete spatial image components or spatial image components to be estimated in the full aperture passband.