Short-exposure imaging-seeker for spin-stabilized projectiles
A method is provided of generating a course-correction signal for a spin-stabilized projectile. The method includes capturing a time-sequence of images of a scene at a frame rate, comparing respective current images of the time-sequence of images to a corresponding previous image of the time-sequence of images, determining a rotation angle between the current and previous images, rotating the images using the rotation angle, identifying a target in the rotated images, generating target bearing angles to cause the projectile to correct its course toward the target using the target bearing angles, and adjusting the target bearing angles to compensate for the rotation of the images.
1. An imaging-seeker for a spin-stabilized projectile that spins about a longitudinal axis of the projectile, the imaging-seeker comprising:
a forward-looking imager configured to capture a time-sequence of images of a scene, the imager capturing the time-sequence of images at a frame rate;
an image correlation component configured to:
compare respective current images of the time-sequence of images to a corresponding previous image of the time-sequence of images; and
determine a rotation angle between the current and previous images based on the comparison;
shutter control logic configured to:
control the frame rate of the imager based on the rotation angle; and
estimate spin rate of the projectile based on rotation angles determined for iterations of a control loop;
an image rotator configured to rotate the time-sequence of images using the rotation angle;
an image processor configured to acquire a target in images of the rotated time-sequence of images;
a de-blur component configured to minimize blur in at least a portion of the time-sequence images, wherein minimizing the blur uses the estimated spin rate, minimizing the blur is performed before providing the at least a portion of the time-sequence images to the image processor for acquiring the target and only until the target is acquired and when target acquisition is repeated, and minimizing the blur is performed one of before and after rotating the image by the image processor; and
a signal generator configured to generate target bearing angles to cause the projectile to correct its course toward the target using the target bearing angles.
2. The imaging-seeker as recited in claim 1 , wherein the signal generator is configured to adjust the target bearing angles to compensate for the rotation of the time-sequence of images.
3. The imaging-seeker as recited in claim 1 , wherein the imaging processor further processes images of the time-sequence of images for tracking the target, wherein the de-blur component is configured to minimize blur in the images after the images are rotated by the image processor, and tracking the target is performed using a second portion of the rotated images that is different than the at least a portion of the rotated images, the second portion of the rotated images having not been processed by the de-blur component for minimizing blur.
4. The imaging-seeker as recited in claim 1 , wherein the shutter control logic is configured to adjust the exposure time of the imager for capturing the time-sequence of images to render the rotation angle between images of the time-sequence of images captured by the imager to be equal to about zero.
5. The imaging-seeker as recited in claim 4 , wherein the frame rate is controlled to be an integer division of an actual spin rate cop of the projectile.
6. The imaging-seeker as recited in claim 1 , wherein the shutter control logic is configured to provide a time stamp that coincides with a time each image is captured.
7. The imaging-seeker as recited in claim 1 , wherein the image correlation component uses an affine scale-invariant feature transform (ASIFT) algorithm.
8. A method of generating a course-correction signal for a spin-stabilized projectile, the method comprising:
capturing a time-sequence of images of a scene at a frame rate;
comparing respective current images of the time-sequence of images to a corresponding previous image of the time-sequence of images;
determining a rotation angle between the current and previous images based on the comparison; controlling the frame rate of the imager based on the rotation angle;
estimating spin rate of the projectile based on rotation angles determined for iterations of a control loop;
rotating the time-sequence of images using the rotation angle;
acquiring a target in the rotated time-sequence of images;
minimizing blur in at least a portion of the time-sequence images, wherein minimizing the blur uses the estimated spin rate, minimizing the blur is performed before acquiring the target and only until the target is acquired and when target acquisition is repeated, and minimizing the blur is performed one of before and after rotating the image;
generating target bearing angles to cause the projectile to correct its course toward the target using the target bearing angles; and
adjusting the target bearing angles to compensate for the rotation of the time-sequence of images.
9. The method as recited in claim 8 , further comprising controlling the frame rate based on the rotation angle.
10. The method as recited in claim 8 , further comprising tracking the target, wherein minimizing blur in the at least a portion of the rotated images is performed after rotating the image, and tracking the target is performed using a second portion of the rotated images that is different than the at least a portion of the rotated images, the second portion of the rotated images having not been operated on by the de-blur component for minimizing blur.
11. A spin-stabilized projectile that spins about a longitudinal axis of the projectile, the spin-stabilized projectile comprising:
an imaging-seeker comprising:
a forward-looking imager configured to capture a time-sequence of images of a scene, the imager capturing the time-sequence of images at a frame rate;
an image correlation component configured to:
receive the time-sequence of images and compare respective current images of the time-sequence of images to a corresponding previous image of the time-sequence of images; and
determine a rotation angle between the current and previous images based on the comparison; and
shutter control logic configured to:
control the frame rate of the imager based on the rotation angle; and
estimate spin rate of the projectile based on rotation angles determined for iterations of a control loop;
an image rotator configured to rotate the time-sequence of images using the rotation angle;
an image processor configured to acquire a target in images of the rotated time-sequence of images;
a de-blur component configured to minimize blur in at least a portion of the time-sequence images, wherein minimizing the blur uses the estimated spin rate, minimizing the blur is performed before providing the at least a portion of the time-sequence images to the image processor for acquiring the target and only until the target is acquired and when target acquisition is repeated, and minimizing the blur is performed one of before and after rotating the image by the image processor; and
a signal generator configured to generate target bearing angles to cause the projectile to correct its course toward the target using the target bearing angles.