Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01C—MEASURING DISTANCES, LEVELS OR BEARINGS; SURVEYING; NAVIGATION; GYROSCOPIC INSTRUMENTS; PHOTOGRAMMETRY OR VIDEOGRAMMETRY
  • G01C11/00—Photogrammetry or videogrammetry, e.g. stereogrammetry; Photographic surveying
  • G01C11/02—Picture taking arrangements specially adapted for photogrammetry or photographic surveying, e.g. controlling overlapping of pictures
  • G01C11/025—Picture taking arrangements specially adapted for photogrammetry or photographic surveying, e.g. controlling overlapping of pictures by scanning the object
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B17/00—Systems with reflecting surfaces, with or without refracting elements
  • G02B17/08—Catadioptric systems
  • G02B17/0804—Catadioptric systems using two curved mirrors
  • G02B17/0808—Catadioptric systems using two curved mirrors on-axis systems with at least one of the mirrors having a central aperture
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B17/00—Systems with reflecting surfaces, with or without refracting elements
  • G02B17/08—Catadioptric systems
  • G02B17/0852—Catadioptric systems having a field corrector only
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B17/00—Systems with reflecting surfaces, with or without refracting elements
  • G02B17/08—Catadioptric systems
  • G02B17/0896—Catadioptric systems with variable magnification or multiple imaging planes, including multispectral systems
• • G—PHYSICS
  • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
  • G03B—APPARATUS OR ARRANGEMENTS FOR TAKING PHOTOGRAPHS OR FOR PROJECTING OR VIEWING THEM; APPARATUS OR ARRANGEMENTS EMPLOYING ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ACCESSORIES THEREFOR
  • G03B37/00—Panoramic or wide-screen photography; Photographing extended surfaces, e.g. for surveying; Photographing internal surfaces, e.g. of pipe
  • G03B37/02—Panoramic or wide-screen photography; Photographing extended surfaces, e.g. for surveying; Photographing internal surfaces, e.g. of pipe with scanning movement of lens or cameras
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N23/00—Cameras or camera modules comprising electronic image sensors; Control thereof
  • H04N23/50—Constructional details
  • H04N23/55—Optical parts specially adapted for electronic image sensors; Mounting thereof
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N23/00—Cameras or camera modules comprising electronic image sensors; Control thereof
  • H04N23/60—Control of cameras or camera modules
  • H04N23/68—Control of cameras or camera modules for stable pick-up of the scene, e.g. compensating for camera body vibrations
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N23/00—Cameras or camera modules comprising electronic image sensors; Control thereof
  • H04N23/60—Control of cameras or camera modules
  • H04N23/68—Control of cameras or camera modules for stable pick-up of the scene, e.g. compensating for camera body vibrations
  • H04N23/681—Motion detection
  • H04N23/6812—Motion detection based on additional sensors, e.g. acceleration sensors
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N23/00—Cameras or camera modules comprising electronic image sensors; Control thereof
  • H04N23/60—Control of cameras or camera modules
  • H04N23/68—Control of cameras or camera modules for stable pick-up of the scene, e.g. compensating for camera body vibrations
  • H04N23/682—Vibration or motion blur correction
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N23/00—Cameras or camera modules comprising electronic image sensors; Control thereof
  • H04N23/60—Control of cameras or camera modules
  • H04N23/68—Control of cameras or camera modules for stable pick-up of the scene, e.g. compensating for camera body vibrations
  • H04N23/682—Vibration or motion blur correction
  • H04N23/685—Vibration or motion blur correction performed by mechanical compensation
  • H04N23/687—Vibration or motion blur correction performed by mechanical compensation by shifting the lens or sensor position
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N23/00—Cameras or camera modules comprising electronic image sensors; Control thereof
  • H04N23/60—Control of cameras or camera modules
  • H04N23/695—Control of camera direction for changing a field of view, e.g. pan, tilt or based on tracking of objects
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N7/00—Television systems
  • H04N7/18—Closed-circuit television [CCTV] systems, i.e. systems in which the video signal is not broadcast
  • H04N7/183—Closed-circuit television [CCTV] systems, i.e. systems in which the video signal is not broadcast for receiving images from a single remote source
  • H04N7/185—Closed-circuit television [CCTV] systems, i.e. systems in which the video signal is not broadcast for receiving images from a single remote source from a mobile camera, e.g. for remote control

Definitions

• A. Field of the Invention This invention relates generally to the field of aerial reconnaissance, remote sensing, mapping and surveillance camera systems.
• aerial reconnaissance cameras are available in framing and pan scanning configurations, in both film and electro- optical implementations.
• the present invention relates to both types of camera configurations, in that a roll framing camera such as described herein generates individual frames of imagery, while the smooth rolling operation provides similar scene coverage and inertial load reductions found in pan scanning cameras.
• an exposure is taken over a large scene of fixed format.
• the field of view of the camera is stepped across a large area using mechanically driven stepping hardware while using image motion compensation techniques to compensate for forward motion of the aircraft.
• the field of view of the camera is a function of lens focal length and the geometrical format size of the image recording media.
• the exposure time is generally controlled by a shutter and is a function of 1) the sensitivity of the photosensitive media, 2) lens transmittance and relative aperture, and 3) available scene brightness.
• the photosensitive material can be film, an area array Charge Coupled Device (CCD), or any other media which records an image for later retrieval.
• CCD Charge Coupled Device
• Forward Motion Compensation is a technique used in framing cameras to correct for the image motion on the recording media caused by forward motion of the aircraft during the exposure interval. This correction is generally introduced by moving the film or the lens to keep the image stationary in the fore/aft direction while the exposure is taking place. In a framing camera, the correction is usually accomplished by moving the film to match the rate of image motion.
• U.S. Patent No. 5,668,593 to Lareau et al. assigned to the assignee of the present invention, the contents of which hare incorporated by reference herein, describes a electro-optical step frame camera system in which the forward motion compensation is achieved electronically in the focal plane of the electro-optical detector.
• the Lareau et al. '597 patent which is incorporated by reference herein, describes an electro-optical imaging array that accomplishes FMC electronically and without moving parts by dividing the columns of the array into multiple column groups, and by transferring pixel information in the column groups at a rate that substantially matches the rates of image motion in the column groups.
• Another operational function of a framing camera is the generation of an overlap between successive frames of imagery.
• the overlap is used to ensure complete coverage of all areas of the scene, and/or to provide a view of the scene from two different angular perspectives yielding stereo imagery.
• the amount of overlap is selectable and nearly always takes place in the direction of flight.
• the overlap L(OL) of the two frames of imagery is typically of 10% or 12%, or as much as 56%.
• An overlap of at least 50% allows all imagery in the adjacent frames to be exposed from two different angular perspectives.
• These images can be recombined by means of a stereo viewing system to achieve depth perception.
• stereo images are often used by a photointerpreter to gather additional information about the scene.
• Panoramic (pan) camera technology is another well-established means of imaging.
• a panoramic scanning camera the scene is exposed continuously by rotating a scanning mechanism (such as a double dove prism) so as to scan the image across the photosensitive medium .
• the photosensitive medium is moved in synchronism with the image.
• this may be accomplished by moving the film at a constant rate past an exposure slit which is located on the lens optical axis.
• a scan prism located in front of the lens is rotated in synchronism with the film rate such that the image of the scene remains stationary on the film during the exposure period.
• the slit opening is adjusted to a predetermined width to control exposure time.
• pan camera One major advantage of a pan camera is its ability to image a large area in the direction across the line of flight. Scan angles across the line of flight on the order of 120 to over 180 degrees are typical.
• the lens field of view in a pan camera is generally only required to be large enough to cover the width of the film. Overlapping of images and stereo imagery may also be obtained with pan cameras. Image overlap in a conventional fixed mounted pan camera is obtained as in the case of a framing camera, that is, in the common area between successive scans.
• FMC for both the film and electro-optical versions of the pan camera is usually accomplished by a conventional electro-mechanical means. Translating the lens during the scan is a popular means to achieve graded FMC as a function of instantaneous slant range to the scene. As noted above, the FMC can be done electronically as taught in the Lareau et al. US Patent 5,668,593. Prior art mechanically stepped framing panoramic cameras, such as described in the
• the image motion due to camera roll is compensated for electronically in the detector array.
• the invention is also particularly suitable for larger, more massive, and more complex cameras, including a camera which carries two or more imaging detectors in order to generate frames of imagery in two or more different bands of the electromagnetic spectrum simultaneously.
• the present invention provides the capability for collection of imagery using a framing camera in which a continuous scan motion about the roll axis of the aerial reconnaissance vehicle is performed, a technique referred to herein as "roll framing".
• roll framing As the camera rototes about the roll axis in a continuous fashion, the roll motion is compensated for electronically.
• This enables high resolution imagery to be generated without loss of resolution or blur, due to the fact that relative motion of the image with respect to the image recording media caused by the roll motion is compensated for using the techniques described herein.
• the continuous roll motion of the camera facilitates image collection without large inertial accelerations and decelerations or large power spikes, as are found in prior art step frame camera system when the camera mass is physically stepped across the terrain of interest in a series of start and stop movements.
• the present invention is believed superior to prior art step framing cameras since the problems inherent with mechanical stepping are eliminated.
• the camera and method are applicable to all sizes and arrangements of cameras, including cameras implementing single spectrum, multi-spectrum and hyperspectral optical systems.
• the invention is also applicable to cameras with mechanical shutters, electronic shutters, acoustical/optical switches, and other electronic exposure controls.
• a method for imaging a scene with a framing camera installed in an aerial reconnaissance vehicle.
• the camera comprises a two dimensional array of photosensitive cells, an optical system directing scene radiation onto said array, and a mechanism for rolling the camera about a rotation axis.
• the array of cells store pixel information and is arranged in a plurality of rows and columns.
• the method comprises the steps of : (a) continuously rotating the camera about the rotation axis with the roll mechanism to thereby direct scene information onto the two dimensional array; (b) exposing the array while the camera is rotating and transferring pixel information in the array at a rate substantially equal to an image motion rate due to the rotation of the camera;
• the camera is mounted to the aerial reconnaissance vehicle such that the step of continuously rotating comprises the step of rotating said camera about an axis substantially parallel to the direction of forward motion of the reconnaissance vehicle.
• the camera could also be mounted in an orthogonal configuration such that the step of continuously rotating comprises the step of rotating the camera about an axis in a direction substantially orthogonal to the direction of forward motion of said aerial reconnaissance vehicle.
• the camera could roll essentially about the pitch axis and generate a series of images in the forward oblique direction towards nadir.
• the steps (a), (b), (c), and (d) recited above are performed in a a series of cycles as the aircraft flies past a scene of interest.
• the frames overlap one another so as to avoid gaps in scene coverage. If the overlap is sufficient, it would be possible to obtain stereo imagery of the scene of interest.
• the camera can be configured with just a single detector and generate imagery in a single band of the electro-magnetic spectrum.
• the camera includes a second electro-optical detector and the camera generates imagery in two bands of the electromagnetic spectrum simultaneously from the first and second detectors.
• the preferred embodiment described in detail herein is an example of a dual band imaging system.
• the camera includes an electro-optical detector and optical system for generating imagery in a pan-chromatic spectral band, such as a hyperspectral electro- optical imaging array.
• an electro-optical roll framing camera with electronic roll motion compensation is provided.
• the camera is adapted for installation in an aerial reconnaissance vehicle.
• the camera comprises an electro-optical detector comprising a two-dimensional array of photosensitive cells that store pixel information.
• the array is arranged in a plurality of rows and columns and has at least one readout register for reading out pixel information from the array.
• the camera further includes an optical system directing scene radiation onto the array.
• a servo-mechanical system is provide which couples the camera to the aerial reconnaissance vehicle which is adapted or configured for continuously rolling the camera about a rotation axis to thereby direct scene info ⁇ nation onto the optical system and array.
• roll motion compensation circuitry is provided for electronically transferring pixel information in the array of photosensitive cells at a rate substantially matching the rate of image motion due to the rotation of the camera, whereby the resolution of images generated by said array may be preserved.
• the servo-mechanical subsystem includes a first motor system coupled to the camera housing that rotates the camera housing (including the optical system as recited above) about a first axis.
• the camera housing is preferably installed in the aerial reconnaissance vehicle such that this first axis of rotation is parallel to the roll axis of the aerial reconnaissance vehicle (referred to herein for simplicity as "the roll axis").
• the image recording media are exposed to the scene to generate frames of imagery as the first motor system rotates the camera housing in a continuous fashion about the roll axis.
• the first and second image recording media have a means for compensating for image motion due to the rotation of the camera housing.
• the roll motion compensation means is preferably comprised of electronic circuitry for clocking or transferring pixel information through the electro-optical detectors at a uniform rate substantially equal to the rate of image motion due to camera rotation.
• a method of calculating the image motion rate, and thus pixel information transfer rate, due to roll of the camera housing is disclosed herein. If a film camera is used for the image recording media, a mechanical system is used to move the film at a rate substantially equal to the image motion rate.
• the servo-mechanical subsystem also includes a second motor system coupled to a catoptric Cassegrain optical system.
• the second motor system rotates the Cassegrain optical system about a second axis in the direction of forward motion of the reconnaissance vehicle to compensate for forward motion of the aerial reconnaissance vehicle.
• the action of the first motor assembly to rotate the entire camera housing about the roll axis occurs at the same time (i.e., simultaneously with) the action of the second motor system to rotate the Cassegrain optical system in the line of flight to accomplish forward motion compensation.
• the net effect of the action of the second motor system and the roll motion compensation system is that the image of the scene of interest is essentially frozen in the focal plane while the image recording media obtain the frames of imagery, allowing high resolution images of the scene to be obtained.
• the camera is a dual band framing camera, and there are first and second image recording media each comprising two dimensional area array electro- optical detectors.
• One may be manufactured from materials sensitive to radiation in the visible portion of the electromagnetic spectrum, and in a preferred embodiment is a charge- coupled device (CCD) detector of say 5,000 X 5,000 or 9,000 X 9,000 pixels.
• CCD charge- coupled device
• the other of the electro-optical detectors is made from a material sensitive to radiation in the infrared portion of the electromagnetic spectrum, and may be a platinum suicide array of photo diode detectors or other suitable type of electro-optical detector suitable for IR detection.
• the detector sensitive to radiation in the infrared portion of the electromagnetic spectrum is preferably sensitive to radiation having a wavelength of between 3.0 and 5.0 microns (MWIR), or from about 8.0 to about 14.0 microns (LWTR).
• MWIR 3.0 and 5.0 microns
• LWTR 14.0 microns
• electro-optical detectors they will typically comprise an array of pixel elements arranged in a plurality of rows and columns.
• the means for compensation for roll motion of the camera housing comprises electronic circuitry for transferring pixel information in the electro-optical detectors from row to adjacent row at a pixel information transfer rate (uniform across the array) substantially equal to the rate of image motion in the plane of the electro-optical detectors due to roll of the camera housing.
• the roll motion compensation can be performed electronically on-chip.
• electro-optical detectors with the capability for transferring pixel information in both row and column directions independently, such as described in Lareau et al., U.S. Patent No. 5,798,786, could be used for the image recording media. Forward motion compensation and roll motion compensation could be performed on- chip in the detectors.
• the present invention required the solution to several difficult technical challenges, including optical, servo-mechanical and operational difficulties.
• the challenge is to accurately compensate for the roll motion electronically at a focal plane detector with (1) good image quality and satisfactory modulation transfer function, (2) while minimizing inertial loading, and (3) enabling the use of a relatively large two-dimensional area array as a focal plane detector to get an adequate field of view and resolution.
• these optical challenges were solved by an on-chip roll motion compensation described in more detail herein.
• the inventive multi-band LOROP/Tactical camera using electronic roll motion compensation does not lend itself to the use of servo-mechanical systems developed for prior art LOROP systems, particularly those used in prior art step frame cameras (such as described in the Lareau et al. '593 patent).
• the prior art step frame cameras use a stepping mirror to step across the line of flight and direct radiation onto the array, and require a de-rotation mechanism, such as a Pechan prism, to de-rotate the images.
• a de-rotation mechanism such as a Pechan prism
• the applications of the present invention are flexible enough to include both strategic and tactical aircraft, as well as the new breed of aircraft being used by the military for reconnaissance known as unmanned aerial vehicles (including low observables).
• the diversity of these applications posed a power and stability problem that prevents application of prior art solutions.
• the task of stepping a 400 lb. camera mass two to four times a second creates tremendous inertial loads as well as power spikes that would be unacceptable.
• Even the inertia and associated settling times of a stepped scan head assembly pose problems in some applications.
• the solution was to (1) rotate the entire camera (including the entire optical system and the image recording media) smoothly in a continuous fashion about an axis parallel to the aircraft roll axis, similar to the pan-type movement, but without the starts and stops used in a traditional step-frame camera system, and (2) operating the camera as a framing camera while the camera undergoes the smooth rotation. Frames of imagery are thus taken while the camera smoothly rotates about the roll axis at a constant angular velocity.
• the present invention also electronically compensates for, i.e., stops, the image motion due to roll while the camera is scanning in a smooth motion.
• a novel forward motion compensation technique is performed by the Cassegrain optical assembly to cancel out image motion effects due to the forward motion of the aircraft. The result enables exposures of the image recording media to the scene while compensating for roll and forward motion, enabling high-resolution images to be obtained.
• the present invention thus solves the difficult optical, servo-mechanical and operational problems and provides a dual band framing electro-optical LOROP camera that delivers a performance and technical capability that has never before been achieved.
• it provides a system by which high-resolution frames of imagery in two different portions of the electromagnetic spectrum can be generated simultaneously.
• the inventive camera can be used in a quasi-stepping mode, in which overlapping frames of imagery are obtained across the line of flight. It can also be used in a spot mode, in which the camera is oriented in a particular direction to take an image of a target expected to be in the field of view.
• the teachings of the present invention are particularly applicable to a dual band electro-optical framing reconnaissance camera, and such a camera is the preferred embodiment.
• some of the techniques and methods of the subject camera system such as the roll-framing operation and unique roll and forward motion compensation techniques, are applicable to a camera system that images terrain in only one portion of the electromagnetic spectrum.
• the camera is basically as set forth as described above, except that only a single detector is used and a spectrum-dividing prism and second optical path are not needed.
• the inventive camera system can be adapted to use film or other types of detectors for the photosensitive recording medium.
• roll motion compensation could be performed by moving the film in a manner such that the film velocity substantially matches the image velocity due to camera roll.
• a dual-band framing aerial reconnaissance camera for installation in an aerial reconnaissance vehicle.
• the camera includes an optical system incorporated into a camera housing.
• the optical system comprises an objective optical subassembly that receives incident radiation from a scene external of the vehicle. Radiation from the scene is reflected from the objective optical subassembly to a spectrum-dividing prism.
• the prism directs radiation in a first band of the electromagnetic spectrum, such as visible and near IR, into a first optical path and directs radiation in a second band of the electromagnetic spectrum, such as midwavelength IR or long wavelength IR, into a second optical path different from the first optical path.
• the first optical path includes suitable image forming and focusing lenses and a first two-dimensional image-recording medium for generating frames of imagery in the first band of the electromagnetic spectrum.
• the second optical path also includes suitable image forming and focusing lenses and a second two- dimensional image-recording medium generating frames of imagery in the second band of the electromagnetic spectrum.
• the camera further includes a servo-mechanical subsystem including the first and second motor systems as described above.
• the first and second image recording media comprise two dimensional area array electro-optical detectors.
• One may be manufactured from materials sensitive to radiation in the visible and near-LR portion of the electromagnetic spectrum, and in a preferred embodiment is a charge-coupled device (CCD) detector of say 5,000 X 5,000 pixels.
• CCD charge-coupled device
• the other of the electro-optical detectors is made from a material sensitive to radiation in the infrared portion of the electromagnetic spectrum, and may be a platinum silicide array of photo diode detectors or other suitable type of electro-optical detector suitable for IR detection.
• the reader is directed to U.S. Patent 5,925,883 to Woolaway, III, the contents of which are incorporated by reference herein, for a description of an IR detector.
• the detector sensitive to radiation in the infrared portion of the electromagnetic spectrum is preferably sensitive to radiation having a wavelength of between 1.0 and 2.0 microns (SWIR), 3.0 and 5.0 microns (MWIR), or from about 8.0 to about 14.0 microns (LWTR).
• electro-optical detectors they will typically comprise an array of pixel elements arranged in a plurality of rows and columns.
• the means for compensation for roll motion of the camera housing comprises electronic circuitry for transferring pixel information in the electro-optical detectors from row to adjacent row at a pixel information transfer rate (uniform across the array) substantially equal to the rate of image motion in the plane of the electro-optical detectors due to roll of the camera housing.
• the transfer of pixel information occurs while the pixel elements are integrating charge representing scene information.
• the roll motion compensation can be performed electronically on-chip.
• electro-optical detectors with the capability for transferring pixel information in both row and column directions independently, such as described in Lareau et al., U.S. Patent No. 5,798,786, could be used for the image recording media. Forward motion compensation and roll motion compensation could be performed on-chip in the detectors.
• the present invention required the solution to several difficult technical challenges, including optical, servo-mechanical and operational difficulties.
• the optical challenge is to focus panchromatic energy (e.g. visible through IR) on a focal plane detector with (1) good image quality and satisfactory modulation transfer function, (2) while baffling stray energy, (3) meeting space constraints, and (4) enabling the use of a relatively large two- dimensional area array as a focal plane detector to get an adequate field of view and resolution.
• these optical challenges were solved by a unique catoptric Cassegrain objective optical subassembly incorporating an azimuth mirror and utilizing separate field optics for each band of the spectrum, described in more detail herein.
• a method for generating a frame of imagery of a scene of interest with a camera while compensating for forward motion of a reconnaissance aircraft.
• the camera includes a Cassegrain objective optical subassembly having a primary mirror, a secondary mirror fixedly mounted relative to the primary mirror, and a flat azimuth mirror positioned in the optical path between the primary mirror and the secondary mirror.
• the camera also includes at least one framing image recording medium, such as a two dimensional electro-optical detector.
• the Cassegrain optical subassembly and framing image recording medium are incorporated into a camera housing mounted to the vehicle, with the camera housing defining an axis.
• the method comprises the steps of:
• step (3) while rotating the primary mirror and secondary mirror as recited in step (2), rotating the azimuth mirror in the direction of flight at a rate one half the rate of rotation in step (2).
• the azimuth mirror is rotated about an axis coincident with the axis about which the primary and secondary mirrors are rotated.
• the azimuth mirror is located substantially at the center of the primary mirror.
• the method is performed in a camera implementing a roll-framing technique for generating sequential images of a terrain of interest.
• the camera housing including the objective lens subassembly and the framing image recording media, is continuously rotated about a roll axis.
• the invention can be practiced in conjunction with single band, dual band or hyperspectral imaging implementations.
• the illustrated embodiment is a dual band system in which the camera includes first and second electro-optical detectors and first and second optical paths for different portions of the electromagnetic spectrum.
• the detectors generating frames of imagery of the scene of interest in two different portions of the electro-magnetic spectrum simultaneously.
• a dual band optical system for a framing aerial reconnaissance camera wherein the camera includes at least two two- dimensional image recording media for generating frames of imagery of a scene external of an aerial reconnaissance vehicle carrying the camera.
• the optical system comprises a Cassegrain objective optical subsystem forming an objective lens for the optical system.
• the Cassegrain optical system receives incident radiation from the scene and comprises a primary mirror having a central aperture and a secondary mirror, the primary and secondary mirrors aligned along an objective optical axis.
• An azimuth mirror which receives radiation from the secondary mirror.
• the azimuth mirror is placed in front of the central aperture of the primary mirror and directs radiation in a direction away from the objective optical axis.
• a spectrum dividing element receives radiation from the azimuth mirror. This element directs radiation in a first band of the electromagnetic spectrum into a first optical path and directs radiation in a second band of the electromagnetic spectrum into a second optical path different from the first optical path.
• the first two-dimensional image recording medium is placed in the first optical path and a second two-dimensional image recording medium is placed in the second optical path.
• the first and second image recording media generate first and second frames of imagery in two different portions of the electromagnetic spectrum simultaneously.
• the aerial reconnaissance vehicle defines a roll axis.
• the first and second paths are oriented and extend for a spatial extent generally in the direction of the roll axis, and the objective optical axis is substantially orthogonal to the roll axis, whereby the dual band optical system provides a compact arrangement the camera.
• the camera further includes a servo-mechanical subsystem.
• This subsystem includes a first motor system coupled to the camera housing that rotates the entire camera housing (including the optical system as recited above) about a first axis.
• the camera housing is installed in the aerial reconnaissance vehicle such that this first axis of rotation is parallel to the roll axis of the aerial reconnaissance vehicle (referred to herein for simplicity as "the roll axis").
• the image recording media are exposed to the scene to generate frames of imagery as the first motor system rotates the camera housing in a continuous fashion about the roll axis.
• the first and second image recording media have a means for compensating for image motion due to the rotation of the camera housing.
• the roll motion compensation means is preferably comprised of electronic circuitry for clocking or transferring pixel information through the electro-optical detectors at a uniform rate substantially equal to the rate of image motion due to camera rotation.
• a method of calculating the image motion rate, and thus pixel information transfer rate, due to roll of the camera housing is disclosed herein. If a film camera is used for the image recording media, a mechanical system is used to move the film at a rate substantially equal to the image motion rate.
• the servo-mechanical subsystem also includes a second motor system coupled to the objective optical subassembly.
• the objective optical subassembly comprises a catoptric Cassegrain optical system.
• the second motor system rotates the Cassegrain optical system about a second axis in the direction of forward motion of the reconnaissance vehicle to compensate for the forward motion of the aerial reconnaissance vehicle.
• the action of the first motor assembly to rotate the entire camera housing about the roll axis occurs at the same time (i.e., simultaneously with) the action of the second motor system to rotate the Cassegrain optical system in the line of flight to accomplish forward motion compensation.
• the net effect of the action of the Cassegrain motor system and the roll motion compensation system is that the image of the scene of interest is essentially frozen relative to the focal plane of the image recording media while the media obtain the frames of imagery, allowing high resolution images of the scene in two different bands of the spectrum to be obtained simultaneously. Furthermore, the rotation of the image scene caused by the roll motion of the objective subassembly is simultaneously detrotated by the roll motion of the rest of the camera, in view of the fact that the entire camera assembly is rolled as a unit, thereby eliminating the need for a separate derotation mechanism such as a pechan prism. Other types of optical arrangements for the objective optical subassembly are possible, but are less preferred.
• Figure 1 is a perspective view of an aircraft flying over a terrain of interest with a camera in accordance with the preferred embodiment operating to generate frames of imagery of the terrain in two bands of the electromagnetic spectrum simultaneously.
• Figure 2 is a schematic representation of the aircraft of Figure 1 taking a series of 5 frames of images in a series of cycles while flying past the terrain of interest;
• Figures 2A and 2B are perspective view of the camera system of Figure 1, shown isolated from the rest of the aircraft, and with protective covers removed in order to better illustrate the components of the camera;
• Figure 2C is a perspective view of the camera of Figures 2 A and 2B, with the protective covers installed, and showing the entrance aperture for the catoptric Cassegrain optical system;
• Figure 3 is a top plan view of a presently preferred embodiment of the dual band framing reconnaissance camera system of Figures 2A-2C, with the covers removed;
• Figure 4 is a cross-sectional view of the camera system of Figure 3, taken along the lines 4-4 of Figure 3;
• Figure 4A is a simplified ray diagram of the optical system of Figure 3 and 4;
• Figures 4B and C are more detailed cross-sectional views of the optical elements in the visible and MWLR paths of Figures 4 and 4A;
• Figure 5 is an end view of the camera system of Figure 3-4, shown from the right- hand end of the camera housing and with the roll motor and cover plate at that end removed in order to better illustrate the other structures in the camera;
• Figure 6 is a perspective view of the assembly of the Cassegrain subsystem, showing in better detail the structure that retains the Cassegrain primary mirror and showing the secondary mirror, azimuth mirror, Cassegrain motor assembly and azimuth 2-1 drive assembly in greater detail.
• the primary mirror itself is removed from Figure 6 in order to better illustrate the components of the Cassegrain optical system.
• Figure 7 is another perspective view of the Cassegrain primary mirror retaining assembly of Figure 6;
• Figure 8 is another perspective view of the Cassegrain primary mirror retaining assembly as seen generally from the rear, shown partially in section;
• Figure 9 is a top view of the Cassegrain optical system of Figure 6;
• Figure 10 is a cross-sectional view of the Cassegrain optical system of Figure 6, taken along the line 10- 10 of Figure 9 ;
• Figure 11 is a detailed sectional view of the azimuth mirror 2-1 drive assembly that rotates the azimuth mirror at one half the rate of rotation of the entire Cassegrain optical subsystem;
• Figure 12 is a detailed perspective view of one of the roll motor assemblies of Figure 2, showing the L shaped brackets that mount to the stator of the motor and rigidly couple the roll motor to the pod or aircraft;
• Figure 13 is an elevational view of the roll motor of Figure 12;
• Figure 14 is a cross-sectional view of the roll motor of Figure 14;
• Figure 15 is a detailed illustration of a portion of the roll motor of Figure 14;
• Figure 16A is a ray diagram of the visible path in the embodiment of Figure 4;
• Figure 16B is a ray diagram of the MWIR path in the embodiment of Figure 4.
• Figure 16C is a graph of the visible path diffraction modulation transfer function
• Figure 17 is a block diagram of the electronics for the camera system of Figures 2-5;
• Figure 18 is schematic representation on an image recording medium in the form of a two dimensional electro-optical array, showing the image motion in the array due to the roll of the camera;
• Figure 19 is another schematic representation of the array of Figure 18, showing the electronic circuitry that controls the transfer of pixel information in the array at the same velocity as the image in order to provide roll motion compensation.
• an aerial reconnaissance camera system 20 in accordance with a preferred embodiment of the invention is shown installed in a reconnaissance aircraft 22 flying over a terrain of interest 42 at an altitude H and with forward velocity V, moving in a direction of flight FL.
• the aerial reconnaissance camera system 20 includes a camera 36, shown in greater detail in Figures 2A-2C and 3-5, a camera control computer 34 and associated electronics described in further detail in Figures 17 and 19.
• the camera control computer receives certain navigational information from the aircraft avionics system 24, including current aircraft velocity and height data. Additional camera system inputs may come from a console 28 in the cockpit, such as start and stop commands or camera depression (roll angle) settings.
• the aircraft body defines a roll axis R, a pitch axis PI and a yaw axis Y passing through the center of gravity CG of the aircraft.
• the camera 36 is shown orientated at a camera depression angle ⁇ relative to a bilateral plane BP that is horizontal during level flight.
• the line of sight LOS of the camera 36 is nominally orthogonal to the roll axis in a side oblique or nadir orientation.
• the preferred embodiment of the subject camera system 20 operates like a step-frame electro-optic (E-O) sensor, capable of taking a sequence of overlapped frames in the cross- track, i.e., cross-line of flight, direction. This is shown in Figure 2.
• E-O electro-optic
• the camera is rotated about the roll axis in a continuous fashion (i.e., without starts and stops between frames), with frames of imagery taken at different depression (roll) angles, e.g., angles ⁇ l, ⁇ 2, ⁇ 3, ⁇ 4 and ⁇ 5, resulting in frames 1, 2, 3, 4 and 5.
• a nominal rate of rotation about the roll axis is used (based on focal length, array frame size and framing rate, such as 8-10 degrees per second, but the roll rate is adjustable by the camera control computer.
• the camera rotates back, i.e., retraces, to its initial roll position ( ⁇ l), and the second cycle of frames of imagery is obtained (IA, 2A, 3 A, 4A, 5 A).
• the process repeats for a third and subsequent cycles of operation.
• the cross-track framing sequence 1, 2, 3, 4, 5; IA, 2A, 3A, 4A, 5A; etc. (which is
• V/H dependent can be made in either spectrum individually or in both spectrums simultaneously, dependent on the time of day and the purpose of the reconnaissance mission.
• the roll action of the camera can encompass both sides of nadir, for example with frames 1-4 obtained at one side of nadir and frame 5 obtained at the other side of nadir.
• the camera can also be used in a spot mode, in which the camera is rotated to a particular depression angle and frames of imagery obtained of the scene of interest.
• the number of frames per cycle of roll, N can thus vary from 1 to say 5 or 10 or until horizon to horizon coverage is obtained.
• Figure 2A shows the camera 36 in a perspective view as seen from below, with a set of protective cover plates removed in order to better illustrate the structure of the camera.
• Figure 2B is another perspective view, shown from above, and
• Figure 2C is a perspective view of the camera 36 with the cover plates 33 installed, showing the entrance pupil 35 for the camera.
• the camera 36 per se is shown in top, sectional and end views, respectively.
• a rear support plate 41 and a roll motor 70A are removed in order to better illustrate the rest of the camera 36.
• the camera 36 mounts to the reconnaissance pod or airframe of the aerial reconnaissance vehicle via four mounting brackets 39, each connected to the pod or airframe via passive shock isolation mounts in conventional fashion.
• the mounting brackets 39 are bolted to the sides of the stator of the roll motor assemblies 70A and 70B as shown in Figure 12 and described below.
• the entire camera cylinder comprising all the components between the two support plates 41 and 41A can rotate relative to the roll axis 37 while the stator of the roll motors 70A and 70B and mounting brackets 39 remain in a fixed position relative to the aerial reconnaissance vehicle.
• the basic configuration of the camera 36 is a cylinder, as perhaps best illustrated in Figure 2C, which in the illustrated embodiment is approximately 20 inches in diameter and 48 inches in length.
• the camera 36 is installed in an aircraft reconnaissance pod via the mounting brackets 39 such that the cylinder axis 37 is oriented nominally parallel to the flight direction of the aircraft, i.e., the roll axis of the aircraft.
• the fore/aft orientation of the camera can be either way.
• the camera 36 can be installed such that it is oriented perpendicular to the line of flight.
• a typical use of the camera is to take overlapping frames of images in the cross-track direction as the aircraft flies over the scene of interest as shown in Figure 2, similar in concept to the step frame operation described in the prior art patent of Lareau, et al. U.S. Patent No. 5,668,593 and earlier step frame film cameras.
• the manner in which the camera achieves this result is very different from that taught in the prior art.
• a stepping mirror is rotated in discrete steps to image the terrain, and forward motion compensation is performed in the array itself electronically
• the entire camera 36 is rotated at a constant angular velocity, and in a continuous fashion, about the roll axis 37.
• the roll rate is determined by the optical system focal length, frame size, frame rate and the desired cross-track overlap (typically 5%) between consecutive frames. Moreover, forward motion compensation is achieved by means of rotation of the Cassegrain optical system about an axis 75, as described below, not in the array.
• the camera includes an optical system 50 which is incorporated into (i.e., mounted to) a camera housing or superstructure 52.
• the optical system 50 in the preferred embodiment comprises a novel catoptric Cassegrain objective optical subassembly 54 which receives incident radiation from a scene external of the vehicle.
• Figure 4A shows a simplified ray diagram for the optical system 50.
• the Cassegrain objective optical subassembly includes a primary mirror 80, a secondary mirror 82 and a flat azimuth mirror 84.
• the secondary mirror 82 is centrally located in the entrance aperture of the Cassegrain optical system. Radiation from the scene is reflected from the Cassegrain objective subassembly 54 to a spectrum-dividing prism 56.
• the prism 56 directs radiation in a first band of the electromagnetic spectrum, such as visible and near LR, into a first optical path 58 and directs radiation in a second band of the electromagnetic spectrum, such as mid- wavelength IR or long wavelength IR, into a second optical path 60 different from the first optical path.
• the first optical path 58 includes suitable image forming and focusing lenses 62 and a first two-dimensional image recording medium 64 for generating frames of imagery in the first band of the electromagnetic spectrum.
• the second optical path 60 includes a fold prism 61, suitable image forming and focusing lenses 66 and a second two-dimensional image recording medium 68 which generates frames of imagery in the second band of the electromagnetic spectrum.
• the camera further includes a novel servo-mechanical subsystem.
• This subsystem includes a first motor system 70A and 70B coupled to the camera housing 52 that rotates the camera housing 52 (including the optical system 50 as recited above) about the roll axis 37.
• the image recording media 64 and 68 are exposed to the scene to generate frames of imagery as the first motor system 70A and 70B rotates the camera housing 52 in a continuous fashion about the roll axis 37.
• the first and second image recording media have a means for compensating for image motion due to the rotation of the camera housing.
• the roll motion compensation means is preferably comprised of electronic circuitry for clocking or transferring pixel information through the electro-optical detectors at a uniform rate substantially equal to the rate of image motion due to camera rotation.
• a method of calculating the image motion rate, and thus pixel information transfer rate, due to roll of the camera housing is described below. If a film camera is used for the image recording media, a mechanical system is used to move the film at a rate substantially equal to the image motion rate.
• Film drive mechanisms for moving film for purposes of motion compensation are known in the art and can be adapted for a film framing camera for purposes of roll motion compensation by persons skilled in the art.
• the servo-mechanical subsystem also includes a second motor system 74, shown best in Figure 3, 5 and 6, coupled to the front end of the Cassegrain optical system 54.
• the second motor system 74 rotates the Cassegrain objective subassembly 54, including the primary, secondary and azimuth mirrors, about a second axis 75 in the direction of forward motion of the reconnaissance vehicle in a manner to compensate for forward motion of the aerial reconnaissance vehicle.
• the azimuth mirror 84 is rotated about the axis 75 at one half the rate of rotation of the Cassegrain primary and secondary mirrors 80 and 82 in the direction of forward motion.
• the action of the first motor assembly 70A and 70B to rotate the entire camera housing about the roll axis occurs at the same time (i.e., simultaneously with) the action of the second motor system 74 to rotate the Cassegrain optical system 80, 82 and 84 in the line of flight to accomplish forward motion compensation.
• the net effect of the action of the Cassegrain motor system 74 and the roll motion compensation technique is that the image of the scene of interest is essentially frozen relative to the focal plane of the image recording media while the image recording media obtain the frames of imagery, allowing high resolution images of the scene in two different bands of the spectrum to be obtained simultaneously.
• the entire camera 36 rotates by action of the roll motors 70A and
• exposure of the detectors 64 and 68 at the two focal planes is made.
• the exposure in the visible spectrum path 58 the exposure is executed by means of a mechanical focal plane shutter 88 which opens to allow incident photons to impinge on a two-dimensional charge-coupled device E-O detector array 64.
• exposure is executed by electronic switching (on/off) of IR-sensitive photocells arranged in a two-dimensional array 68, basically by dumping charge accumulating prior to the initiation of exposure and then accumulating and storing charge when the exposure period commences.
• any method of exposure control will work with this roll-framing camera.
• the data is read out from the two focal plane detectors 64 and 68 and they are placed in condition for a second exposure.
• the rotation of the entire camera assembly about the roll axis 37 continues smoothly (that is, without starting and stopping as for example found in a prior art step frame camera system).
• the shutter is opened in the visible/near IR path; similarly, in the MWTR path the charge dumping ceases and charge is accumulated.
• the data is then read out of the two focal plane sensors after the exposure period is over.
• the camera system roll rate (the cylinder angular velocity), ⁇ , is established as follows. First, determine the cross-track field of view per frame, ⁇ , according to equation (1):
• W detector array size in the cross-track direction
• lens focal length i.e, the focal length of the overall aggregate of optical components in the particular band of interest, e.g., the visible band).
• FR system frame rate (frames per second)
• OL c overlap between consecutive cross-track frames (expressed as a decimal). Note that the cylinder angular velocity ⁇ is independent of the aircraft's velocity and height above the earth. Typical angular rotations between the successive exposures of the array will be less than 10 degrees.
• the image motion due to camera roll is constant and uniform across the array.
• this image motion is synchronized with the velocity at which charge representing scene information is transferred within the detector arrays, thus eliminating relative motion between the image and the pixels imaging the scene and thus eliminating the image smear that would otherwise take place at the detector.
• pixel information in the entire array is transferred in the direction of image motion from row to adjacent row at a rate that substantially matches the image velocity v.
• the cylinder continues rotating to the next scene position while the collected scene signals are read-out of the detector array(s). Note that there is no rotational start and stop between exposures, as found in prior art step frame camera systems, thereby avoiding the servo loop settling times, load current surges, and power spikes produced by mechanical stepping systems as noted earlier.
• the two focal plane detectors 64 and 68 operate in the above manner, taking N consecutive cross-track frames, N being dependent on the time available or by the intended mode (maximum coverage, limited coverage or spot mode) of operation.
• N is dependent on the time available or by the intended mode (maximum coverage, limited coverage or spot mode) of operation.
• the result is a series of frames of images similar to that produced with a step frame camera system, as indicated in Figure 2, each frame taken in two different bands of the electromagnetic spectrum.
• N is determined by the V/H ratio of the mission, the camera system depression angle range and the framing rate, and N can be as many as 10 frames/cycle (or more) in normal operation.
• the camera system or cylinder is rotated back (reset) to the first frame angular position and the cycle repeats until the intended in-flight direction coverage is achieved.
• a one or two frames/cycle is executed, with the camera aimed at a specific predete ⁇ nined depression (roll) angle and fore/aft azimuth angle where a target or specific interest is expected to be.
• N will typically equal 1 or 2.
• the cycle may repeat for as many times as needed.
• the camera could be used in a traditional step frame operation. In this mode, the camera would rotate between successive angular positions, and the photosensitive media would generate two-dimensional images of the terrain. If the camera body rotation is stopped during scene exposure, forward motion compensation could be performed in the photosensitive media, such as described in the earlier Lareau et al. patents.
• the preferred forward motion compensation method will now be described with a little more specificity. As the exposures are made at either of the two detectors 64 and 68, the aircraft is moving at some known velocity. The forward motion of the aircraft is neutralized in a novel way in the preferred embodiment. Whereas in the prior art Lareau et al.
• the value of R can be derived from simple geometry from the known aircraft height and camera depression angle ( ⁇ ; ) and assuming the earth is flat in the scene of interest, from a Global Positioning System on board the aircraft, using an active range finder, or by computing range from successive frames of imagery as described in the patent of Lareau et al., US Patent No.
• the Cassegrain optical system could remain fixed and both forward motion compensation and roll motion compensation could be performed in the focal plane detector by transferring pixel information in both row and column directions in accordance with the principles of the Lareau et al. patent, U.S. Patent No. 5,798,786.
• Visible Channel 50.0 inches -//4.0 (Options) 72.0 inches -f/5.8 84.0 inches -f/6.1
• Type Cassegrain objective lens with spectrum beam divider and individual visible channel and MWTR channel relay lenses. Operating Spectrums :
• Visible Channel 5040 x 5040 pixels .010 mm x .010 mm pixel pitch 50.4 mm x 50.4 mm array size 4.0 frames/sec max.
• MWTR Channel 2016 x 2016 pixels .025 mm x .025 pixel pitch 50.4 mm x 50.4 mm array size 4.0 frames/sec max.
• MWTR Channel 2520 x 2520 pixels (future) .020 mm x .020 mm pixel pitch 50.4 mm x 50.4 mm array size 4.0 frames/sec max.
• GRD Ground Resolvable Distance
• VIS Channel 3 ft @ 31 N mi. (NILRS-5) (50”) 3 ft @ 45 N mi. (NIIRS-5) (72”) 3 ft @ 52 N mi. (NIIRS-5) (84”)
• the optical system 50 including the Cassegrain optical system, spectrum dividing prism 56, and optical components in the optical paths 58 and 60, are rigidly mounted to a camera housing or superstructure 52.
• This camera housing 52 takes the form of a pair of opposed, elongate C-shaped frames extending transversely on opposite sides of the roll axis substantially the entire length of the camera.
• the C-shaped frame members 52 provide a structure in which to mount the various optical and mechanical components of the camera, including the end plates 41 and 41 A.
• the end plate 41 is bolted to the right hand end of the C-shaped frames 52, as shown in Figure 3.
• the rotor portion of the roll motor 70 A is in turn bolted to the end plate 41, thereby coupling the rotational portion of the roll motor 70A to the camera frame 52.
• the stator portion of the roll motor 70A is fixedly coupled to the aircraft frame or pod via two L- shaped brackets 39 and the associated passive isolation mounts (conventional, not shown).
• the left-hand end of the C-shaped frame 52 is similarly bolted to an end plate 41 A, and the rotor portion of the roll motor 70B is bolted to the end plate 41A, with the stator portion bolted to two L-shaped brackets 39.
• Two roll motors 70A and 70B are conventional frameless DC torque motors, adapted to mount to the camera 36. Two are used in the illustrated embodiment in order supply enough torque to rotate the camera housing 52 and all the attached components, but one motor may suffice if it is powerful enough.
• the roll motors 70A and 70B are frameless DC torque motors, adapted to fit to the camera housing, a task within the ability of persons skilled in the art. The roll motors are described below in further detail in conjunction with Figures 12-15.
• Figure 3 is a top view of the camera 36, looking towards to the back side of the primary mirror 80.
• the Cassegrain objective lens optical subassembly 54 includes a primary mirror cell 100 which includes four mounting flanges 102 with bolt holes 104 for mounting via bolts to the top flange 106 of the C-shaped frames 52.
• the Cassegrain optical system is shown isolated in Figures 6-10. In Figures 6 and 7, the primary mirror is removed in order to better illustrate the rest of the structure in the Cassegrain optical system.
• a spider 120 consisting of eight arms 122 extends between an inner primary mirror holding ring 110 and an azimuth mirror mounting plate 124 located at the center of the primary mirror 80.
• the mounting plate 124 incorporates three adjustment screws 126 for adjusting the tilt of the azimuth mirror 84.
• a fiber optic gyroscope 128 is also mounted to the plate 124 and is provided for purposes of inertial stiffness and stabilization of the Cassegrain optical system 50.
• the secondary mirror assembly 113 includes a set of three adjustment screws 126 A for adjusting and aligning the orientation of the secondary mirror relative to the primary mirror.
• the stator portion of the Cassegrain motor 74 is fixed with respect to the primary mirror cell 100.
• the rotor portion of the motor 74 is mounted to an annular ring 111 shown in Figure 10, which is attached to the inner primary mirror holding ring 110.
• the secondary mirror 82 is fixed with respect to the primary mirror by means of three arms 112.
• the motor 74 rotates both the primary, secondary and azimuth mirrors about axis 75 in the direction of the line of flight in unison.
• the Cassegrain motor 74 is based on a DC direct drive motor adapted as required to the Cassegrain primary mirror holding structure, again a task within the ability of persons skilled in the art.
• the rotation of the inner mirror holding ring 110 by the Cassegrain motor 74 is reduced by a two-to-one reduction tape drive assembly 114, shown best in figures 5, 6 7, 9 and 11.
• the tape drive assembly 114 rotates an azimuth mirror drive shaft 116 that extends from the tape drive assembly 114 to the azimuth mirror 84.
• the azimuth tape drive assembly 114 rotates the azimuth mirror drive shaft 116 and thus the azimuth mirror 84 at one half of the rate of rotation of the primary and secondary mirrors by the Cassegrain motor 74.
• the tape drive assembly 114 includes a two-to-one drive housing 150, two-to-one drive couplings 152 and 154, a shaft locking coupling 156, and a pair of stainless steel tapes 158 and 160, the thickness of which is shown exaggerated in Figure 11.
• the roll motor 70A is shown isolated from the rest of the camera in a perspective view.
• Figure 13 is an elevational view of the motor as seen from the other side.
• Figure 14 is a cross-sectional view of the motor 70A.
• the roll motor 70B is identical to the motor of Figures 12-14. Additional details concerning the tape drive assembly 114 are conventional and therefore omitted for the sake of brevity.
• the motor 70A includes a trunnion 200, a journal 202 and a DC frameless motor 204.
• the journal 202 bolts to the plate 41 ( Figure 3) via six bolt holes 208.
• a set of apertures 210 is provided in the face of the journal 202 to reduce weight.
• the sides of the trunnion 200 have opposed, parallel flat surfaces 212 with a series of mounting holes for enabling the L- shaped mounting brackets 39 to mount to the trunnion 200 in a plurality of different positions.
• the motor 70A also includes an electronics module contained in a housing 214.
• the module includes a power amplifier and associated DC electronic components, which are conventional.
• the motor assembly 70A also includes an annular shim 220, an annular bearing 222, a lock washer 224 and locking nut 226, a trunnion sleeve 228, a bearing spacer 230, a bearing insert 232 and a bearing adjustment plate 234. Additional mechanical features shown in Figures 14 and 15 are not particularly important and therefore are omitted from the present discussion.
• optical system design of the subject camera is driven by the need to illuminate a large focal plane image recording medium and by space constraints, namely the total axial length and the total diameter, which have to be accounted for in potential aircraft installation applications.
• space constraints namely the total axial length and the total diameter, which have to be accounted for in potential aircraft installation applications.
• the optical system 50 of Figure 3 and 4 represents a 50-inch, F/4 optical system designed to operate over an extended spectral region.
• the objective lens module consists of the Cassegrain optical subsystem 54, comprising the primary and secondary mirrors 80 and 82.
• the azimuth mirror 84 is utilized to redirect the image forming light bundles into the remainder of the optical system, namely the spectrum dividing prism and the relay lenses and other optical components in the optical paths 58 and 60.
• radiation is reflected off the flat azimuth mirror 84 towards a calcium fluoride spectrum-dividing prism 56.
• An image is formed at a Cassegrain image plane 130 immediately in front of the prism 56.
• the spectrum- dividing prism 56 is constructed such that radiation in the visible and near LR band (about 0.5 to about 0.9 microns) passes through the prism 56 into the visible/near TR optical path 58 while radiation in the MWTR portion of the spectrum (about 3 to about 5 microns) is reflected upwards through a fold prism 132, made from an infrared transmitting material, into the MWTR optical path 60.
• the radiation passes through a relay lens assembly 62 enclosed in a suitable enclosure 134, a focus element 136 adjusting a set of focus lenses 138, and finally to a shutter 88.
• An image is formed on the focal plane of the image recording medium 64.
• the shutter 88 opens and closes to control exposure of the visible spectrum image recording medium 64.
• the medium 64 is a charge-coupled device E-O detector, comprising an array of pixels arranged in rows and columns.
• the array 64 is cooled by a thermo-electric cooler 140.
• the array and thermo-electric cooler are enclosed in their own housing 142, which includes electronics boards 144 and a set of heat dissipating cooling fans 146.
• the light passes through a relay lens assembly 66 contained in a suitable housing, through a focus lens assembly 67 and an image is formed at the focal plane of an TR-detecting two dimensional array 68.
• the MWTR sensor comprises the array 68, a cold stop 69, and an internal filter, all enclosed in a cryogenic dewar 63.
• optical axis of the objective Cassegrain optical subassembly is shown vertical in Figure 4. This arrangement provides a very compact assembly; if the objective were arranged along a horizontal axis the total length required for the system would have been intolerably large.
• the use of only reflecting components (catoptric) in the objective allows the collection of light from a very wide spectral region. Such imaging would be impractical or impossible with a refracting objective design.
• the point of intersection of the visible relay optical axis and the objective optical axis is an important datum feature of this system.
• the azimuth mirror 84 reflecting surface is designed to rotate about an axis that contains this intersection point.
• the entire Cassegrain objective subassembly 54 is arranged to also rotate about this same axis for forward motion compensation. Rotation of the objective permits locking onto ground image detail while the camera and aircraft are moving forward. If the azimuth mirror 84 rotates at half the angular rate of the objective module with respect to the aircraft/camera frame of reference, the selected ground image is effectively locked or frozen onto the detectors. Consequently, the image can be recorded without blur of relative forward motion between the camera and the scene.
• Presently preferred embodiments of the subject optical system have focal lengths of between 50 and 100 inches, and an f/number of between 4.0 and 8.0.
• Figures 16A and 16B are ray diagrams for the visible and MWTR paths of the embodiment of Figure 4.
• Figure 16C is a graph of the diffraction MTF for the visible path.
• the MTF curves are wavelength-averaged over the visible/TR spectral range of 500 to 900 nm with system spectral weights.
• the Cassegrain objective subsystem introduces a central obscuration into the light forming beams, and therefore reduces the diffraction-limited performance limits that can be achieved.
• optical prescription, fabrication and aperture data are set forth below in the following tables for the embodiment of Figure 4.
• the data set forth in the tables is by no means limiting of the scope of the invention, and departure thereof is expected in other embodiments of the invention.
• selection and design of the optical components for any given implementation of the invention is considered to be a matter within the ability of persons skilled in the art of optical design of aerial reconnaissance cameras, with such additional designs being considered obvious modifications of the illustrated embodiment.
• the numbering of the elements in the left-hand column corresponds to the optical elements shown in Figures 16A and 16B in progression from the entrance aperture to the detectors.
• Thickness is axial distance to next surface
• Thickness is axial distance to next surface
• the electronics for the camera 36 of Figures 1 and 3 is shown in block diagram form in Figure 17.
• the electronics includes an image processing unit (IPU) 401 which contains the master control computer 34 of Figure 1.
• the master control computer 34 supplies control signals along a conductor 400 to a camera body and stabilization electronics module, represented by block 402.
• the camera body and stabilization electronics 402 basically includes digital signal processing cards that provide commands to the roll motors 70A and 70B and the Cassegrain or azimuth motor 74 of Figures 3, 5 and 6, and receive signals from the stabilization system consisting of the azimuth fiber optic gyroscope 128 mounted on the azimuth mirror and a roll fiber optic gyroscope (not shown) mounted on the camera housing 52.
• the camera body electronics 402 also receives current roll angle and roll rate data from resolvers in the roll motors 70A and 70B, and from the roll gyroscope, and supplies the roll information to the camera control computer.
• the camera control computer 34 also generates control signals, such as start, stop, and counter values, and supplies them via conductor 406 to an IR sensor module (IRSM) 408 and a Visible Sensor Module 410.
• IRSM IR sensor module
• the IRSM 408 includes a cryogenic dewar or cooler 63, the IR detector 68 ( Figure 4) and associated readout circuitry, and electronic circuitry shown in Figure 19 and described subsequently for transferring charge through the IR array to achieve roll motion compensation. Pixel information representing IR imagery is read out of the array 68, digitized, and sent along a conductor 412 to the EPU 401.
• the electronic circuitry shown in Figure 19 could be incorporated into the camera body electronics 402 or in the Image Processing Unit 401.
• the visible sensor module 410 includes a mechanical shutter 88, a visible spectrum electro-optical detector 64 ( Figure 4) and associate readout registers, and electronic circuitry described in Figure 19 and described subsequently for transferring charge through the visible spectrum detector 64 to achieve roll motion compensation. Pixel information representing visible spectrum imagery is read out of the detector 64, digitized, and sent along a conductor 414 to the IPU 401.
• Visible and LR imagery supplied by the Visible Sensor Module and the IR Sensor Module is received by a dual band input module 420 and supplied to an image processor 422 for purposes of contrast adjustment, filtering, radiometric correction, etc.
• images generated by the arrays 64 and 68 are either stored for later retrieval or downlinked to a ground station.
• the imagery is compressed by a data compression module 424, supplied to an output formatter 426 and sent along a conductor 428 to a digital recording module 430 for recording of the imagery on board the aircraft.
• Aircraft inertial navigation system data such as aircraft velocity, height, aircraft attitude angles, and possibly other information, is obtained from an aircraft 1553 bus, represented by conductor 432.
• Operator inputs such as start, stop and roll angle commands from a manual cockpit or camera console or control panel, can also be supplied along the conductor 432 or by an optional control conductor 434.
• the INS and operator commands are processed in an INS interface circuit 436 and supplied to the camera control computer 34 and used in the algorithms described above.
• the camera control computer also has a non-volatile memory (not shown) storing fixed parameters or constants that are used in generating the roll motion compensation commands, such as the pixel pitch, array size, master clock rate, and optical system focal length.
• the image processor 422 and a graphics module 438 are used to generate thumbnail imagery and supply the imagery to an RS- 170 output 440 for viewing in near real time by the operator or pilot on board the reconnaissance vehicle, or for downloading to the ground station.
• Other format options for the thumbnail imagery are also possible.
• Aircraft power is supplied to a power conversion unit 442, which filters, converts and distributes it to two power modules 444.
• the power modules 444 supply the required AC or DC voltages to the various electronic components in the camera 36.
• An RS-232 diagnostic port 446 is provided in the LPU 401 for remote provisioning, diagnostics, and software downloads or upgrades or debugging by a technician.
• the port 446 provides an interface to the master control computer 34, and the other modules in the LPU 401 and allows the technician to access these units with a general purpose computer. Changes to fixed parameters stored in non-volatile memory, such as a change in the focal length of the camera, are also made via the port 446.
• the visible/near IR E-O detector 64 is shown in a plan view.
• the detector consists of an array of pixel elements 300 arranged in a plurality of rows and columns, with the column direction chosen to be across the line of flight and the row direction in the direction of flight.
• the array 64 can be any suitable imaging detector including a charge-coupled device, and preferably will comprise at least 5,000 pixels in the row direction and at least 5,000 pixels in the column direction.
• the illustrated embodiment consists of 5040 x 5040 pixels, with a .010 mm x .010 mm pixel pitch and a 50.4 mm x 50.4 mm array size.
• the architecture for the array is not critical, but a full frame imager, as opposed to an interline transfer architecture, is presently preferred.
• the imager can use either a mechanical shutter or an electronic shutter to expose the array.
• the roll motion caused by camera roll motors 70A and 70B produces an image motion indicated by the arrows 302 in the plane of the array 64.
• the roll motion is in the cross-line of flight direction and the image velocity v is nearly constant throughout the array.
• the velocity v is equal to the product of the optical system focal length /multiplied by the rate of rotation ⁇ . Since/ is fixed (and the value stored in memory for the camera control computer), and the rate of rotation is known by virtue of outputs of the fiber-optic gyroscope 128 or from resolvers in the roll motors, the velocity of the image due to roll can be precisely determined for every exposure.
• the velocity can be expressed in terms of mm/sec, in terms of rows of pixels per second, or in terms of the fraction of a second it takes for a point in the image to move from one row of pixels to the adjacent row, given the known pixel pitch.
• the pixel information (i.e., stored charge) in the individual pixels 300 is transferred row by row throughout the entire array 64 at the same rate and in the same direction of image motion during the exposure time, thereby avoiding image smear due to the roll motion.
• the camera electronics includes a counter and clock driver circuit 304 (one for each detector 64, 68) which generates voltage pulses and supplies them to a set of three phase conductors 308 which are coupled to each row of the array.
• One cycle of three-phase clocking effectuates a transfer of charge from one row to the adjacent row.
• a master clock 306 generates clock signals at a master clock frequency and supplies them to a counter 310.
• the camera control computer calculates a counter value which determines the number the counter 310 is supposed to count to at the known master clock rate to time the transfer of charge from one row to another in synchronism with the movement of the image by one row of pixels (0.010 mm).
• the master computer 34 supplies the counter value to the counter 310, along with a start and stop commands.
• the counter 310 starts counting at the clock rate up to the counter value.
• a trigger signal is sent to a clock driver 312.
• the clock driver 312 initiates one cycle of three phase clocking on conductors 308, causing the pixel information from row 1 to be transferred to row 2, from row 2 to row 3, etc.
• the counter 310 resets itself and immediately begins counting again to the counter value, another cycle of clocking is triggered, and the process repeats continuously while the array is exposed and charge is integrated in the detectors.
• a stop signal is sent to the counter 310.
• the pixel information in the array 64 is read out of the array into read-out registers at the bottom of the array (not shown), converted into digital form, and either stored locally on a digital recording medium for later use or transmitted to a remote location such as a base station.
• the process described for array 64 is essentially how the IR detector operates as well for accomplishing roll motion compensation, hi alternative embodiments, the image motion compensation could be performed in other readout structures depending on the architecture for the array.
• the LR detector could be sensitive to radiation in the Short Wavelength Infra- Red (SWIR) band (1.0 to 2.5 microns), Mid-Wavelength LR (MWLR) band (3.0 to 5.0 microns) or Long Wavelength LR (LWLR) band (8.0 to 14.0 microns).
• SWIR Short Wavelength Infra- Red
• MWLR Mid-Wavelength LR
• LWLR Long Wavelength LR
• the output of the each photosensitive photodiode detector is coupled to a charge storage device, such as a capacitor or CCD structure, and the charge is shifted from one charge storage device to the adjacent charge storage device in synchronism with the image velocity while charge is being integrated in the charge storage devices.
• a charge storage device such as a capacitor or CCD structure
• the process of roll motion compensation can be more finely tuned by deriving the rate of rotation (co) used in the algorithm from the actual inertial rate sensed by a fiber optic gyroscope mounted to the camera housing or frame.
• a gyroscope can count with a resolution of 1 microradian or better.
• the gyroscope generates a signal that is supplied to a DSP card in the camera control electronics module 402 ( Figure 17).
• a signal could also be constructed for imaging array clocking purposes in the form of a pulse train which the imaging array clock generator could phase lock to.
• the roll motion compensation becomes, in effect, a fine stabilization system which removes the residual error from the more coarse electro-mechanical stabilization system. Having a fine system, based on a closed loop feedback from the roll fiber optic gyroscope, would allow for a larger range of roll motion without image degradation.
• the principles of roll framing and forward motion compensation described above are applicable to a camera that images in a single band of the electromagnetic spectrum.
• the spectrum dividing prism would not been needed and the objective optical subassembly (Cassegrain or otherwise) would direct the radiation in the band of interest to a single optical path having a photo-sensitive image recording medium placed herein.
• the spectrum dividing prism and second optical channel are not needed. Otherwise, the operation of the camera in roll framing and spot modes of operation would be the same as described above.
• three or more detectors could image the three or more bands of the electromagnetic spectrum simultaneously.
• an additional spectrum separating prism would be placed in either the visible or LR paths to further subdivide the incident radiation into the desired bands and direct such radiation into additional optical paths, each with its own photo-sensitive image recording medium.
• the visible/near LR band could be divided into a sub 700 nanometer band and a 700 to 1000 nanometer band, each associated with a distinct optical path and associated image forming and focusing lenses and an image recording medium.
• the IR portion of the spectrum could be similarly divided into two separate bands, such as SWLR, MWLR, and/or LWLR bands, and each band associated with a distinct optical path and associated image forming and focusing lenses and an image recording medium.
• the arrangement of optical components in the camera housing will be different from the illustrated embodiment due to the additional spectrum dividing prisms, additional optical paths and optical components, and additional detectors.
• persons skilled in the art will be able to make such a modification from the illustrated embodiment using routine skill.
• the camera may be designed for hyperspectral imaging.
• one of the optical paths may be devoted to visible spectrum imaging, while the other path is fitted with a spectroradiometer, an imaging spectrometer, or spectrograph to divide the incident radiation into a large number of sub-bands in the spectrum, such as 50 of such sub-bands.
• Each sub-band of radiation in the scene is imaged by the hyperspectral imaging array.
• the camera could be mounted transverse to the roll axis of the aircraft.
• Such a camera could be used for dual spectrum, full framing imaging in a forward oblique mode, either in a spot mode of operation or in a mode in which overlapping frames of images are generated in a forward oblique orientation.
• the smooth roll motion and roll motion compensation feature could be adapted to a step framing camera, such as the KS-127A camera or the step frame camera of the Lareau et al. patent, U.S. No. 5,688,593.
• the roll motors are coupled to the step frame scan head assembly, and continuously rotate the scan head about the roll axis in a smooth, continuous fashion.
• the detector array and associated relay and focusing optical elements remain stationary with respect to the aircraft.
• the image acquired by the scan head assembly would need to be derotated with a pechan prism, K mirror or other suitable element, as described in the '593 patent.
• Roll motion compensation would be performed electronically in the array, as described at length above.
• the roll motors are coupled to the step frame scan head and continuously rotate the step frame scan head assembly, while the image derotation is achieved by rotation of the imaging array in synchronism with the rotation of the scan head assembly.
• Roll motion compensation is achieved by transferring pixel information in the array at substantially the same rate as the rate of image motion due to scan head rotation.
• the catoptric Cassegrain optical system is the preferred embodiment, refractive optical systems, catadioptric optical systems, and still other types of optical arrangements may be used, for example where only single spectrum imaging is performed, where space requirements are not so important, or when other considerations dictate that a different type of optical arrangement for the objective lens is suitable.
• the optical subassembly comprising the objective lens would be rotated in the direction of flight to accomplish forward motion compensation as described above, while the entire camera housing including the objective lens is rotated about an axis to thereby generate sweeping coverage of the field of interest, either about the roll axis or about an axis perpendicular to the roll axis.

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