Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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DUAL BAND FRAMING RECONNAISSANCE CAMERA
BACKGROLTID OF The INVE~ITiOI~T
A. Field of the Invention
This invention relates generally to the. field of aerial reconnaissance,
remoie. sensing,
i 0 mapping and surveillance camera systems. Generally speaking, aerial
reconnaissance
cameras are available in framing and pan scanning configurations, in both film
and electlo-
optical implementations. The present invention relates to both types of camera
configurations, in that a roll framing camera such as descnbed herein
generates individual
frames of imagery, while. the smooth rolling operation provides similar scent
coverage and
1 ~ inertial load reductions found in pan scanning cameras.
B. Description of Related Art
In prior art framing 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
20 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. ?) lens transmittance and relative aperture, and ~) available scene
brightness. The
2~ photosensitive material can be film, an area array Charge Coupled Device
(CCD), or any
other media which records an image for later retrieval.
Forward Motion Compensation (FbiC) 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 expossre interval. This correction is generally introduced by
moving the filin or
~0 the lens to keep the image stationary in the fore,~aft direction while the
e,~cposure 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. ~,668,~93 to Lareau et al.,
assigned to the
assignee of the present invention,
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describes a electro-optical step frame camera system in which the forward
motion
compensation is achieved electronicall~~ in the local plane of the eiectro-
optical detector.
One limitation of a conventional film or CCD framzag camera is that only a
single
F14ZC rate can be applied to any given frame regardless of the field of view.
Consequently, the
motion can exacthr be corrected for only a portion of the image. When eiposure
times are
short and the field angles small; this is acceptable. However, for I~er fields
of view and
where longer exposure times are required (as at dusk or under other low light
level
conditions), the dilrferential rate of motion between the film and the image
increases with the
field angle and can be lame enough result in image blur at the edges of the
field. A major
advance in forward motion compensation in alectro-optical framing cameras is.
disclosed in
the Lareau et aI. patent, U.S. Pat. hio. 5,1»,~9?, assigned to the assignee of
the present
invention. The Lareau et al. '597 patent, d~~bes
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
pi.Yel 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 as
overlap
bet-.veen successive i13n1e5 Gf iina'~V. 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. In a conventional framing camera, the
amount of
overlap is selectable and nearly always takes place in the direction of
flight.
In step frame cameras, the overlap L(OL) of the two frames of ima?ery is
typically of
10°,'0 or 12°.'0, or as much as 56°io. 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
~5 recombined by means of a stereo viewing systeiri to achieve depth
perception. Such stereo
images are often used by a photointerpreter to gather additional information
about the scene.
The operation of a film-type framing camera in a stepping mode is known in the
art.
For example, the article entitled "The KS-1~.6A LOROP Camera System", Thomas
C.
Augustvn, SPII: ProcoedinGs Vol.9, Aug. 27-23 1981, paper 309-11 p.76,
describes as
automatic stepping mode in which the camera cycle rate is proportional to
aircraft velocity,
altitude and selected depression angle, to achieve 56°.'0 overlap for
stereo viewing or 12°to
overlap for maa-imum flight Line coverage. V~ith the camera Iine of sight
normal to the flight
path, the scan head provides either 1, 2, 4, or 6 lateral-step cycles. A
similar stepping
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operation for a frame camera is described in the article entitled "KS-127A
Long Range
Oblique Reconnaissance Camera for RF-4 Aircraft", Richard C. Rucle and Oliver
J. Smith,
SPIE Proceedings Vol. 242, Jul. 29-30, 1980 San Diego Paper 242-02, p.22.
Panoramic (pan) camera technology is another well-established means of
imaging.
In 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 synchronsm with the image. In
the case of
a film camera, 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.
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
'593 patent and in the KS-146A camera are limited in size and the stepping
rate by the mass
and commensurate inertial loading created by trying to step that mass across
the area of
interest. Since the size and mass of the camera increases with operation in
multiple spectral
bands (i.e., with two or more detectors incorporated into the camera), the
capability of
mechanically stepped cameras is limited to smaller and more limited camera
configurations.
Thus, there exists a need in the art for an electro-optical camera which
obtains broad
area coverage in the manner of a panning or step framing camera without the
above
limitations. The present invention meets that need by providing a novel roll
framing
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technique for generating broad area coverage with an area array image
recording medium,
described in more detail herein. 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.
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SUNJNiARY OF THE INVENTION
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". ?~s the
~ camera rotates about the roll a~;is in a continuous fashion; the roll motion
is compensated for
electronically. This enables High resolution imaeer3~ 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 lame 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 spectnrm, multi-spectrum and
hyperspectral optical
systems. The invention is also applicable to cameras with mechanical shutters,
electronic
shutters, aceustical~optical switches, and other electronic ea~po~ure
controls.
Thus, in a first aspect of the invention, a method is provided for imaging a
scene vv~ittl
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 rotatin' the camera about the rotation axis v~riih 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.
?0 due to the rotation of the camera;
(c) readine out the pixel information from the array; and
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(d) repeating said steps (b), and (c) while the vehicle flies past a scene of
interest
and while the camera continuously rotates about the roll axis, thus generating
a series of
frames of imagery.
In a preferred embodiment, 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.
However, 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. In this less preferred embodiment, the camera could roll essentially
about the pitch
axis and generate a series of images in the forward oblique direction towards
nadir.
In a typical embodiment, 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. Alternatively, 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. As yet
another
alternative embodiment, 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.
In another aspect of the invention, 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
information onto
the optical system and array. Further, roll motion compensation circuitry is
provided for
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electronically transfen-ing 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.
In the illustrated embodiment, 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. In an electro-optical embodiment of the
image recording
media, the roll motion compensation means is preferably comprised of
electronic circuitry for
clocking or transfernng 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.
In the preferred embodiment, 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.
In a preferred embodiment, 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
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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.
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
S 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 (LWIR). In either of the
embodiment of
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. Thus, the roll motion
compensation can be
performed electronically on-chip.
As a further possible embodiment, 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. For an
electro-optical
framing LOROP camera to operate in a continous sweep with a framing array with
at least
two discrete bands of the electromagnetic spectrum at the same time, 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. In
accordance with one
aspect of the invention, 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
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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 Pechaii prism, to de-rotate the images. The standard
solution of
stepping the entire LOROP camera system or even a large scan mirror assembly
at the
operational frame rate are not acceptable alternatives for large LOROP
cameras, and in
particular large dual band systems. In particular, 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.
This servo-mechanical situation required a unique inventive solution,
described in
detail herein. The solution, as provided in one aspect of the present
invention, 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 wlule 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. In addition to this novel "roll-
framing" technique,
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. Meanwhile, 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. In
particular, 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
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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.
Many of the teachings of the present invention are particularly applicable to
a dual
band electro-optical fraaning reconnaissance camera, and such a camera is the
preferred
embodiment. However, as explained below, 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. Thus, in an alternative embodiment
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. Furthermore,
while a
preferred embodiment uses a two-dimensional electro-optical imaging array for
the detector
in each of the bands of the electromagnetic spectrum, the inventive camera
system can be
adapted to use film or other types of detectors for the photosensitive
recording medium. In
the film camera embodiment, 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.
In a second aspect, a dual-band framing aerial reconnaissance camera for
installation
in an aerial reconnaissance vehicle has been invented. 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.
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The car~e~~ further includes a servo-rzechanical subsystem including the fLrst
and
second meter s~-starns as d~scribec above. In a preferred embociment, the
first and second
image recoraing media comprise t<vo dimensional area array electro-optical
detectors. One
ma<< be manufactured from materials sensitive to radiation in the visible and
near-IR portion
of the electromagnetic specu,nn, and in a preferred embodiment is a charge-
coupled deuce
(CCD) detector of sav x,000 1 x,000 pixels. The other of the electro-optical
detectors. is
made from a material sensitive to radiation in the infrared portion of the
electroma~etic
spectrum, and may be a platinum silicide array of photo diode detector or
other suitable type
of electro-optical detector suitable for IR detection. The reader is directed
to U.S. Patent
=.,9~~;883 to VGoolaway, III; for a
description ef 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 ~.0 microns (MWIR), or from about 8.0 to
about 14.0
microns (LW>R). In either of the embodiment of electro-optical detectors, they
wt7.I typically
comprise an array of pixel elements arranged in a plurality of rows and
columas. 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 mw at'~
pi.Yel 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. Thus, the roll motion compensation can be
performed
electronically on-chip. As a further possible embodiment, electro-optical
detectors with the
capability for transferrinD pixel information in both row and column
directions independently,
such as described in Lareau et al., LT.S. Patent No. 5,798,786, could be used
for the image
?5 recording media. Forward motion compensation and roll lotion compensation
could be
performed on-chip in the detectors.
As noted above, the present invention required the solution to several di~cult
technical challenges; including optical, servo-mechanic:1 and operational
dif"nculties. For an
electro-optical framing LOROP (Long Range Oblique Photo~aphy) camera to
operate in at
least two discrete bands of the electromagnetic spectrum at the same time, the
optical
challenge is to focus panchromatic ener~~ (e.g. visible tluough 1R) on a focal
plane detector
with (1) Qood image qualii<r and satisfactory modulation transfer ftmction,
(2) while baling
stray energy, (_) me~~g space constraints, and (4) enabling the use of a
relatively large trvo-
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dimensional area array as a focal plane detector to get an adequate field of
view and
resolution. In accordance with one aspect of the invention, 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.
In still another aspect of the invention, a method is provided 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;
(1) orienting, e.g., installing, the camera housing such that the camera
housing is
substantially parallel to the roll axis of the aircraft;
(2) rotating the primary mirror and secondary mirror about an axis orthogonal
to the
roll axis in the direction of flight of the aircraft, while maintaining the
image
recording medium in a fixed condition relative to the camera housing; and
(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. In the illustrated embodiment, the
azimuth
mirror is located substantially at the center of the primary mirror.
In the illustrated embodiment, the method is performed in a camera
implementing a
roll-framing technique for generating sequential images of a terrain of
interest. In this
technique, 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
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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.
In yet another aspect of the invention, a dual band optical system for a
framing aerial
reconnaissance camera is described, 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 is provided 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 obj ective 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. In the illustrated
embodiment
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
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the first mOtOr S:~'St2~1 rotates the camera housing in a continuous fashion
about the roll axis.
The nrst and seccnd in:ase recording media have a means for compensating for
image motion
due to the rotation of the camera housing. Ir: an electro-optical embodiment
of the image
recording media_ the roll motion compensation means is preferably comprised of
electronic
circuitry for clocl~n~ or transfezring pilel 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 pi.Yel information
transfer rate, due to
roll of the camera liousing 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
substaatially equal to
the image motion rate.
The servo-mechanical subsystem also includes a second motor system coupled to
the
objective optical subassembly. In the illustrated embodiment, the objective
optical
subassembly comprises a catoptric Cassegrain optical system. The second motor
system
rotates the Cassegrain optical s~~stem 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 CasseS gin optical system in t_he line
of flight to
accomplish forward motion compensation. The net effect of the action of the
CasseQrain
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 viev~~ of the fact
that the entire
camera assembly is rolled as a unit, thereb;~ 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. The operation of the
camera with
the different type of objective subassembly is the same.
14
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According to another aspect of the present
invention, there is provided a method for imaging a scene with
a dual band framing camera installed in an aerial
reconnaissance vehicle and generating frames of imagery of the
scene of interest in different bands of the electromagnetic
spectrum; the camera comprising a camera housing containing an
optical system for directing scene radiation, a spectrum
dividing element receiving radiation from said optical system,
said element directing radiation in a first band of the
electromagnetic spectrum into a first optical path and
directing radiation in a second band of the electromagnetic
spectrum into a second optical path different from said first
optical path; and a first motor system coupled to said camera
housing for rotating said camera about a first axis; the
camera further comprising a first framing two-dimensional
electro-optical image detector array of photosensitive cells
arranged in rows and columns, said first array in said first
optical path, said first array for generating frames of
imagery in said first band of the electromagnetic spectrum,
and a second framing two-dimensional electro-optical image
detector array of photosensitive cells arranged in rows and
columns, said second array in said second optical path, said
second array for generating frames of imagery in said second
band of the electromagnetic spectrum; the method comprising
the steps of: a) orientating said camera housing in an
initial position; b) directing radiation from said scene into
said optical system; c) exposing said first and second arrays
to said scene radiation; d) reading out scene information from
said first and second arrays to thereby generate two-
dimensional frames of imagery in each of said first and second
bands; e) moving said camera housing about said first axis
whereby a new portion of said scene is in the field of view of
said optical system; f) repeating said steps b) - e) while
said vehicle flies past said scene of interest to thereby
14a
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generate a series of frames of imagery in said first and
second bands.
According to still another aspect of the present
invention, there is provided a dual band framing aerial
reconnaissance camera for installation in an airborne vehicle,
comprising; (a) a camera housing; (b) an optical system
incorporated into said camera housing, comprising: (1) an
objective optical subassembly for receiving incident radiation
from a scene external of said vehicle; (2) a spectrum
dividing element receiving radiation from said objective
optical subassembly, said element directing radiation in a
first band of the electromagnetic spectrum into a first
optical path and directing radiation in a second band of the
electromagnetic spectrum into a second optical path different
from said first optical path; (3) a first framing two-
:4~
dimensional electro-optical detector array in said first
optical path for generating a first series of frames of
imagery of said scene from radiation received in said first
band of the electromagnetic spectrum; and (4) a second framing
two-dimensional electro-optical detector array in said second
optical path for generating a second series of frames of
imagery of said scene from radiation received in said second
band of the electromagnetic spectrum; (5) a first motor
system coupled to said camera housing capable of rotating said
camera about a first axis, wherein both said first and second
arrays are exposed to said scene to generate said first and
second series of frames of imagery in said first and second
bands of said electromagnetic spectrum as said first motor
system moves said camera about said first axis.
According to yet another aspect of the present
invention, there is provided a dual band framing aerial
reconnaissance camera for installation in an airborne vehicle,
comprising; (a) a camera housing; (b) an optical system
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incorporated into said camera housing, comprising: (1) an
objective optical subassembly for receiving incident radiation
from a scene external of said vehicle; (2) a spectrum dividing
element receiving radiation from said objective optical
subassembly, said element directing radiation in a first band
of the electromagnetic spectrum into a first optical path and
directing radiation in a second band of the electromagnetic
spectrum into a second optical path different from said first
optical path; (3) a first two-dimensional image recording
medium in said first optical path for generating frames of
imagery in said first band of the electromagnetic spectrum;
and (4) a second two-dimensional image recording medium in
said second optical path for generating frames of imagery in
said second band of the electromagnetic spectrum; (c) a first
motor system coupled to said camera housing capable of
rotating said camera about a first axis, wherein said first
and second image recording media are exposed to said scene to
generate frames of imagery in said first and second bands of
said electromagnetic spectrum as said first motor system steps
said camera in a series of steps about said first axis.
According to a further aspect of the present
invention, there is provided a dual band hyperspectral framing
aerial reconnaissance camera for installation in an airborne
vehicle, comprising; (a) a camera housing; (b) an optical
system incorporated into said camera housing, comprising: (1)
an objective optical subassembly for receiving incident
radiation from a scene external of said vehicle; (2) a
dividing element receiving radiation from said objective
optical subassembly, said element directing radiation into a
first optical path and directing radiation into a second
optical path different from said first optical path; (3) a
first two-dimensional electro-optical detector in said first
optical path for generating a first series of images of said
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scene; and (4) a second two-dimensional electro-optical
detector in said second optical path for generating a second
series of images of said scene; wherein at least one of said
first and second electro-optical detectors comprises a
hyperspectral imager, and wherein said camera operates to
obtain said first and second series of images, at least one of
said series of images comprising hyperspectral imagery.
According to yet a further aspect of the present
invention, there is provided a dual band hyperspectral framing
aerial reconnaissance camera for installation in an airborne
vehicle, comprising; (a) a camera housing; (b) an optical
system incorporated into said camera housing, comprising: (1)
an objective optical subassembly for receiving incident
radiation from a scene external of said vehicle; (2) a
spectrum dividing element receiving radiation from said
objective optical subassembly, said element directing
radiation in a first band of the electromagnetic spectrum into
a first optical path and directing radiation in a second band
of the electromagnetic spectrum into a second optical path
different from said first optical path; (3) a first two-
dimensional electro-optical detector in said first optical
path for generating a first series of images in said first
band of the electromagnetic spectrum; and (4) a second two-
dimensional electro-optical detector in said second optical
path for generating a second series of images in said second
band of the electromagnetic spectrum; wherein said first band
comprises the visible spectrum and said first two-dimensional
electro-optical detector is sensitive to radiation in the
visible band, and wherein said second two-dimensional electro-
optical detector comprises a hyperspectral imager; whereby
said camera operates to obtain visible spectrum and
hyperspectral imagery of said scene.
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According to still a further aspect of the present
invention, there is provided a dual band hyperspectral framing
aerial reconnaissance camera for installation in an airborne
vehicle, comprising; (a) a camera housing; (b) an optical
system incorporated into said camera housing, comprising: (1)
an objective optical subassembly for receiving incident
radiation from a scene external of said vehicle; (2) a
spectrum dividing element receiving radiation from said
objective optical subassembly, said element directing
radiation in a first band of the electromagnetic spectrum into
a first optical path and directing radiation in a second band
of the electromagnetic spectrum into a second optical path
different from said first optical path; (3) a first two-
dimensional electro-optical detector in said first optical
path for generating a first series of images in said first
band of the electromagnetic spectrum; and (4) a second two-
dimensional electro-optical detector in said second optical
path for generating a second series of images in said second
band of the electromagnetic spectrum; wherein said first
electro-optical detector is sensitive to radiation in the
infrared portion of the electromagnetic spectrum and said
second electro-optical detector comprises a hyperspectral
imager, and wherein said camera operates to obtain infrared
and hyperspectral imagery of said scene.
According to another aspect of the present
invention, there is provided a dual band multispectral framing
aerial reconnaissance camera for installation in an airborne
vehicle, comprising; (a) a camera housing; (b) an optical
system incorporated into said camera housing, comprising: (1)
an objective optical subassembly for receiving incident
radiation from a scene external of said vehicle; (2) a
dividing element receiving radiation from said objective
optical subassembly, said element directing radiation into a
14e
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first optical path and directing radiation into a second
optical path different from said first optical path; (3) a
first two-dimensional electro-optical detector in said first
optical path for generating a first series of images of said
scene; and (4) a second two-dimensional electro-optical
detector in said second optical path for generating a second
series of images of said scene; wherein at least one of said
first and second electro-optical detectors comprises a
multispectral imager, and wherein said camera operates to
obtain said first and second series of images, at least one of
said series of images comprising multispectral imagery.
According to yet another aspect of the present
invention, there is provided a dual band multispectral framing
aerial reconnaissance camera for installation in an airborne
vehicle, comprising; (a) a camera housing; (b) an optical
system incorporated into said camera housing, comprising: (1)
an objective optical subassembly for receiving incident
radiation from a scene external of said vehicle; (2) a
spectrum dividing element receiving radiation from said
objective optical subassembly, said element directing
radiation in a first band of the electromagnetic spectrum into
a first optical path and directing radiation in a second band
of the electromagnetic spectrum into a second optical path
different from said first optical path; (3) a first two-
dimensional electro-optical detector in said first optical
path for generating a first series of images in said first
band of the electromagnetic spectrum; and (4) a second two-
dimensional electro-optical detector in said second optical
path for generating a second series of images in said second
band of the electromagnetic spectrum; wherein said first band
comprises the visible spectrum and said first two-dimensional
electro-optical detector is sensitive to radiation in the
visible band, and wherein said second two-dimensional electro-
14f
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optical detector comprises a multispectral imager; whereby
said camera operates to obtain visible spectrum and
multispectral imagery of said scene.
According to an even further aspect of the present
invention, there is provided a dual band multispectral framing
aerial reconnaissance camera for installation in an airborne
vehicle, comprising; (a) a camera housing; (b) an optical
system incorporated into said camera housing, comprising: (1)
an objective optical subassembly for receiving incident
radiation from a scene external of said vehicle; (2) a
spectrum dividing element receiving radiation from said
objective optical subassembly, said element directing
radiation in a first band of the electromagnetic spectrum into
a first optical path and directing radiation in a second band
of the electromagnetic spectrum into a second optical path
different from said first optical path; (3) a first two-
dimensional electro-optical detector in said first optical
path for generating a first series of images in said first
band of the electromagnetic spectrum; and (4) a second two-
dimensional electro-optical detector in said second optical
path for generating a second series of images in said second
band of the electromagnetic spectrum; wherein said first
electro-optical detector is sensitive to radiation in the
infrared portion of the electromagnetic spectrum and said
second electro-optical detector comprises a multispectral
imager, and wherein said camera operates to obtain infrared
and multispectral imagery of said scene.
While the foregoing summary has described some of
the highlights of the inventive camera system, further details
on these and other features will be described in the following
detailed description of a presently preferred embodiment of
the invention.
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BRIEF DESCRIPTION OF THE DRAWINGS
Presently preferred embodiments of the invention will be discussed below in
conjunction with the appended drawing figures, wherein like reference numerals
refer to like
elements in the various views, and wherein:
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 2A 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 MW112 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;
CA 02418917 2003-02-07
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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 I3 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; and
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.
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DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Overview and Method of Oueration:
Referring now to Figure 1, 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 8 relative to a bilateral plane BP that is horizontal
during level flight.
In the illustrated embodiment, 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. As
the aircraft flies by
the terrain of interest, 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 S1, 82, 83, 84 and 85, 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. When the fifth frame of imagery is obtained and the camera
rolled to its
roll limit position (either pre-set or commanded by the operator), the camera
rotates back, i.e.,
retraces, to its initial roll position (81), and the second cycle of frames of
imagery is obtained
(lA, 2A, 3A, 4A, SA). The process repeats for a third and subsequent cycles of
operation.
The cross-track framing sequence 1, 2, 3, 4, 5; lA, 2A, 3A, 4A, SA; 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.
As noted in Figure 2, the roll action of the camera can encompass both sides
of nadir, for
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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. Referring now to Figures 3-5, the camera 36 per se is shown in
top, sectional and
end views, respectively. In the end view of Figure 5, a rear support plate 41
and a roll motor
70A are removed in order to better illustrate the rest of the camera 36.
As shown best in Figure 3, 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 forelaft
orientation of the
camera can be either way. Additionally, 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. However, the manner
in which the
camera achieves this result is very different from that taught in the prior
art. Whereas in the
Lareau '593 patent, a stepping mirror is rotated in discrete steps to image
the terrain, and
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forward motion compensation is performed in the array itself electronically,
in the preferred
embodiment of the present invention 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 S%) 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.
Referring to Figures 3 and 4, 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 IR,
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. In an
electro-
optical embodiment of the image recording media, the roll motion compensation
means is
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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. Filin 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. In the illustrated embodiment, 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.
During operation, as the entire camera 36 rotates by action of the roll motors
70A and
?OB, exposure of the detectors 64 and 68 at the two focal planes is made. In
the illustrated
embodiment, 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. In the MWIR path
60,
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.
However, any method of exposure control will work with this roll-framing
camera.
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When the initial exposure is complete, 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).
When the next
exposure is ready to be taken, i.e., when depression angle b2 of Figure 2 has
been reached, the
shutter is opened in the visible/near IR path; similarly, in the MWIR 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. Meanwhile, the rotation of the entire
camera system about
the roll axis continues without interruption and a third and subsequent
exposure of both
cameras is taken if time permits The process continues until the angular
limits of the framing
cycle has been reached, at which time the roll motors 70A and 70B retrace
their angular
rotation and return to their original angular position. The process then
repeats for a new cycle
of framing, as indicated in Figure 2.
The camera system roll rate (the cylinder angular velocity), w, is established
as
follows. First, determine the cross-track field of view per frame, ~,
according to equation
(1):
(1) ~ = 2 ARCTAN (W/2f), where
W = detector array size in the cross-track direction; and
f = 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).
Then, the cylinder angular velocity cu is computed according to equation (2):
(2) cc = c~ (FR) (1-OL~), where
FR = system frame rate (frames per second)
OL~ = overlap between consecutive cross-track frames (expressed as a decimal).
Note that the cylinder angular velocity cu 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.
Since the focal plane detectors are rotating about the roll axis during the
exposure
period, the scene image is translating across each of the detector arrays in
the cross-track
direction at a fixed velocity v = f~. The image motion due to camera roll is
constant and
uniform across the array. To compensate for this image motion, and thereby
preserve
resolution, this image motion is synchronized with the velocity at which
charge representing
scene information is transferred within the detector arrays, thus eliminating
relative motion
21
CA 02418917 2003-02-07
WO 02/019030 PCT/USO1/23680
between the image and the pixels imaging the scene and thus eliminating the
image smear
that would otherwise take place at the detector. In other words, 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.
At the end of the exposure period (typically 0.0005 to 0.020 seconds), 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.
In this "roll-framing" type of operation, 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. 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. In maximum coverage mode, 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. At the end of
the cross-
track cycle, 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. The
camera can generate overlapping frames of imagery similar to that shown in
Figure l, where
N in the illustrated example = 5.
In spot mode, a one or two frames/cycle is executed, with the camera aimed at
a
specific predetermined depression (roll) angle and fore/aft azimuth angle
where a target or
specific interest is expected to be. In this example, N will typically equal 1
or 2. The cycle
may repeat for as many times as needed.
As another mode of operation, 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.
22
CA 02418917 2005-04-21
76909-232
The prefered forward motion compensation method will now be des~~'bed with a
little more specif~ciry. As he exposures are made at Either of the two
detectors 64 and 58,
the aircraft is moving at some lmown velocity. The fonvard motion of the
aircraft is
neutralizes in a novel way in the prefe:Ted embodiment. ~~ereas in the prior
art Lareau et
al. '~ 86 patent forward :notion compensation is pe~'omied on-chip in the
arr2y, the forward
motion compensation of the preferred embodiment is performed by rotation of
the Casse_srain
objective subassembly. i.e, the Casse~ain primary and secondary minor
assembly, in the
flight direction at a rate = V/R (in units of radians per second) where V is
the aircraft velocity
and R is the range to the scene of interest. The value of R can be derived
from simple
geometry from the Imown aircraft height and camera depression angle (b ~ and
assuming the
earth is flat in the scene of interest, fi~am a Global Positioning System on
board the aircraft,
using an active range finder, or by computing range from successive frames of
imagery as
descnbed in the patent of Lareau et al., US Patent No. ~,b92,062.
As the Cassegrain primary and secondary mirrors 80 and 82, restively,
1 ~ are rotated at the V/R rate in the direction of flight, the flat azimuth
minor $4, located in the
optical path between the secondary reflector and the Cassegrain image plane
86, is rotated at
a rate equal to lip (V/R) in the same direction, thus "slopping" image motion
due to aircraft
forward motion at the image plane. Thus, the rotating Cassegrain objective
lens and the half
speed azimuth mirror provide the needed forward motion compensation function.
As an alternative embodiment, the Cassegrain optical system could remain
fi:~ted wad
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
direction's in
accordance with the principles of the Lareau et al. patent, U.S. Patent No.
x,798,786.
From the Figures I-5 and 18 and the above discussion, it will be appreciated
that we
have invented a method of generating frames of imagery of a scene of interest
with an aerial
reconnaissance camera in tvo different ~ bands of the electromagnetic spectrum
simultaneously. The method includes the steps of
(a) providing tvvo photosensitive electro-optical detectors 64, 68 in the
camera 36,
each of the detectors comprising an array of pixel elements arranged in a
plurality of cows
3 0 and columns;
(b) rotating the camera 3ti in a continuous fashion about a roll ax sxi 37
either
coincident with or parallel to a roll axis R of an aerial reconnaissance
vehicle carrying the
camer~~;
7J
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(c) while rotating the camera 36, simultaneously exposing the electro-optical
detectors
64, 68 to a scene of interest in a series of exposures;
(d) while rotating the camera 36 and while exposing the electro-optical
detectors 64
and 68 to the scene, rotating an optical system 54 providing an objective lens
for the camera
in the direction of forward motion of the vehicle at a predetermined rate to
cancel out image
motion due to forward motion of the vehicle; and
(e) while the electro-optical detectors 64 and 68 are being exposed to the
scene,
moving pixel information in the arrays at a rate and in a direction
substantially equal to the
rate of image motion due to rotation of the camera about the roll axis, to
thereby preserve
resolution of images generated by the detectors.
Performance specifications for a presently preferred dual band step frame
camera
system in accordance with the invention are listed below.
Focal Length & f / #
Visible Channel 50.0 inches - f /4.0
(Options) 72.0 inches -f /5.8
84.0 inches - f /6.7
MWIR Channel 50.0 inches -f /4.0
Optical System
Type: Cassegrain objective lens with spectrum
beam divider and individual visible
channel and MWIR channel relay lenses.
Operating Spectrums =
Visible Channel - 0.50 to 0.90 microns
MWIR Channel - 3.0 to 5.0 microns
Entrance Pupil Diameter:
12.5 inches, both channels, all focal lengths.
Detectors:
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.
24
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WO 02/019030 PCT/USO1/23680
MWIR 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.
MWIR Channel: 2520 x 2520 pixels
(future) .020 mm x .020 mm pixel pitch
50.4 min x 50.4 nnm array size
4.0 frames/sec max.
FOV (per frame):
VIS Channel: 2.27° x 2.27° (50 inch F.L.)
1.58° x 1.58° (72 inch F.L.)
1.35° x 1.35° (84 inch F.L.)
MWIR Channel: 2.27° x 2.27° (SO inch F.L.)
Frame Rates: Variable, up to 4.0 fr/sec
Both channels, all focal lengths.
PixeIIFOV:
VIS Channel: 7.9 x 10-6 RAD (50 inch F.L.)
S.5 x 10-6 RAD (72 inch F.L.)
4.7 x 10-GRAD (84 inch F.L.)
MWIR Channel: 19.7 x 10-6 RAD (50 inch F.L.)
15.8 x 10-6 RAD (50 inch F.L.) (future)
Ground Resolvable Distance (GRD) (at range, perpendicular to the LOS).
VIS Channel: - 3 ft @ 31 N mi. (IVIIRS-5) (50")
- 3 ft @ 45 N mi. (hTIIRS-5) (72")
- 3 ft @ 52 N mi. (NlIRS-5) (84")
MWIR Channel: - 3 ft @ 12.5 N mi. (TTIIRS-5) (50")
(Future) - 3 ft @ 15.6 N ini. (NIIRS-5) (50")
Field of Regard: Horizon to Horizon, or as limited by vehicle windows
(5° to 30° depression below horizon (&) is typical).
Scene coverage rate: Variable cross-track.
Roll rate: 8.6°/sec - (50 inch focal length, 4 Fr/Sec.)
CA 02418917 2003-02-07
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Preferred Dual Band Camera Detailed Mechanical and Servo-Mechanical
Description
With the above overall description in mind, attention is directed primarily to
Figures
2A, 2B, and 3-S. The more important mechanical aspects of the camera will now
be
described. The optical system 50, including the Cassegrain optical system,
spectnun
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 41A.
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 70A 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
41A, 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.
In the
illustrated embodiment, 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.
As is shown best in Figure 3 and 6, 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
26
CA 02418917 2003-02-07
WO 02/019030 PCT/USO1/23680
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 126A 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. Thus, 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.
Refernng to Figure 12, 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.
27
CA 02418917 2003-02-07
WO 02/019030 PCT/USO1/23680
The module includes a power amplifier and associated DC electronic components,
which are
conventional.
As shown in the cross-sectional view of Figure 14, and the detail of Figure
15, the
motor assembly 70A also includes an annular shim 220, an annular bearing 222,
a lock
washer 224 and locking nut 226, a trumiion 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.
O tip cal System Detailed Description
The 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. Thus, while the particular optical design described herein is
optimized for a
given set of spatial constraints, variation from the illustrated embodiment is
considered to be
within the scope of the invention.
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.
Referring now again primarily Figures 4, 4A, 4B and 4C, 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 ixnnediately in front of the prism 56.
The spectrum-
dividing prism 56 is constructed such that radiation in the visible and near
IR band (about 0.5
to about 0.9 microns) passes through the prism 56 into the visible/near IR
optical path 58
while radiation in the MWIR 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
MWIR optical path 60.
In the visible path, 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
28
CA 02418917 2003-02-07
WO 02/019030 PCT/USO1/23680
shutter 88 opens and closes to control exposure of the visible spectrum image
recording
medium 64. In the illustrated embodiment, 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
faros 146.
In the MWIR path, 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 IR-detecting two dimensional array 68. The MWIR sensor comprises the
array 68, a
cold stop 69, and an internal filter, all enclosed in a cryogenic dewar 63.
The 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 obj ective
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.
Furthermore, 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/casnera 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 8Ø
Figures 16A and 16B are ray diagrams for the visible and MWIR 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/IR spectral range of
500 to 900 nm
with system spectral weights. The Cassegrain objective subsystem introduces a
central
29
CA 02418917 2003-02-07
WO 02/019030 PCT/USO1/23680
obscuration into the light forming beams, and therefore reduces the
diffraction-limited
performance limits that can be achieved.
In the interest of completeness of the disclosure of the best mode
contemplated for
practicing the invention, optical prescription, fabrication and aperture data
are set forth below
in the following tables for the embodiment of Figure 4. Of course, 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. Furthermore, 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.
In the tables for the visible and MWIR paths, 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.
CA 02418917 2003-02-07
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TABLE 1
VISIBLE PATH PRESCRIPTION
F_4BRTCATION DATA
30-Aug-00
Modified 50", F/4 VISIHI~E~PATH
ELEMENTRADIUS APERTURE
OF 37IAMETER
CURVATURE
NUMBERFRONT BACK THICKNESS FRONT HACK GLASS
OBJECT. INF INFINITY*1
C-I
11.3468
APERTURE C-2
STOP
1 A(1) -11.3468 C-2 REFL
Z A(2) 10.3x68 4.8650 REEL
DECENTER(,1)
3 INF 0.0000 C-3 REFL
3.3525
-8.2000
4 -14.5000 CX INF -0.3000 2.9916 3.0328CAF2
-0.0200
INF INF -3.1000 3_0382 3.6125CAF2
-0.4000 '
6 INF 5.6254 CX-0.6360 3.7191 3.7662'F9474/30'
-0.2000
7 INE' INF -0.2500 3_6228 3.5785'OG515'
-0.3000
S -5.4347CX 33.2980 CX-0.5926 3.4210 3.3057'A2334/2'
-0.020'0
9 -4_0545CX. 3.6785 C_X-0.8920 3.0291 2.7699'B2601/lA'
3_6785CC -1.4940 CC-0.2500 2.7699 2.0369'12549414'
-4.4633
11 -2.1854CX 1.4016 CX-0.8744 1.9875 1.8522'135662/A'
12 1.4016CC -0_8475 CC-0.2500 1.8522 1.4085'B2651/2'
13 -0.8475CY -1.5553 CC-0.4845 1.4085 1.3037'A2334/2'
-0.3747
31
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WO 02/019030 PCT/USO1/23680
I4 1,2750 CC -1.5275 -0.3634 1.3029 1.6318'B2650/I'
CC
15 -1.5275 CX 2.1577 -0.6838 1_6318 1.7656'11646656'
CX
-0.0270
I6 -5.0620 C<Y 3.3440 -0.4730 1.8086 1.8207'A2334/2'
CX
-0.4251*2
17 -7.5223 CX 19.2220 -0.1985 1.6606 1.6522'A9420/8'
CX
-0.5725*3
18 -?-5775 CX -1.1718 -0.2500 1..6800 1.5948'D1741/4'
CC
19 -1.1718 CX 2.7961 -0.7869 1.5948 1.5304'E2601/lA'
CX
20 2.7961 CC -2.2230 -0.25D0 1.5301 1.4526'D1741/4'
CC
-1.7614
CC 3x97 CX 3802 1.7611 2.1914'Fi9418/35'
21 .
-0
21 1.4142 . .
-0.0245
22 -3.4440 CX 12.7030 -0.8500 2.4681 2.5450'D1?41/6'
CX
-0.0500
INF 0800 5554 2.5603BK7 Schott
-0 2
23 INF . .
IMAGE DISTANCE -0.9162
=
IMAGEINF 2.6440
NOTES- Positive ius indicatesthe centerof curvatureto right
rad is the
Negative ius indicatesthe centerof curvatureto lef t
rad is the
- Dimensionsre given
a in inches
- Thickness a.,tial surface
is distance
to next
- Image diameter shown
above
is a paraxial
value,
it is not ray traced
a value
-.Other glasssuppliers be used their materialsare
can if
functionally o the t needed design;
equivalent exten by the
t
contact the designer approvalsubstitutions.
for of
32
CA 02418917 2003-02-07
WO 02/019030 PCT/USO1/23680
APx_'.~.T~~ DAi~-
'
D DEC~'ER
T=biF."TER
pn~~~, Wing X Y K Y 30T AT=ON
C- I CT~2CLE 13.268
CIRCLE (OBSC) z:80nx.800
c- z c=RCLE (aBSC) a.ean~.aoa
CgM,F I2.60n12.600
C- 3 RECTANGLe. 3.500. a.000 0.000 0.100 0.0
ASPHER=C CONSTAN:~S
2
(~V) Y 4 6 8 ZO
Z = ______-__________________ (A)Y(H)Y ~r (C)Y f (D)Y
f +
2 2 1/2
1 + (1-(1+FC) Y )
(CQRV)
ASPHERIC CORY K_ A S C D
A( 1) -n.n2881267-1.000000
A( 2) -n.05583a'13~3.928273
DECENTERING CONSTANfiS
DECENTER X Y Z ALPHA BETA GAMIL~
________ __-
_____- '_-
___
-
--_____
ni 1)--____0-QOOO____o-oooo-_ a a,oonn
____ oooo (HENn)
o
onan
3s.anao
33
CA 02418917 2003-02-07
WO 02/019030 PCT/USO1/23680
TABLE 2
MWIR PRESCRIPTION
FAHFtICATION DATA
ModifiedI.14~IR LEPIS 2 '~ FiC Testplates
50, F/4 to POD .
EL RADIUS QE CURVATURE APEtfCURE
CxlEN'P DIAMETER
LVIl:LSt;~FRDNT ' HACK TEiICIiNESSFROPIT HACK GLASS
~B.TECTINF INFINITY*1
C-1
11_3468
1 A(ll (Paraboloid)-11.3468C-2 REEL
2 A(?y (Ellipsoid)10.39b8 4.4000 FiEFL
DCNTER( 11
3 INg -8.2000 3.9482 REFL (A2imuth
Mirrory
4 -14_5000 -0.3000 3.2080 3.2480CAF2 (Field Lens)
CX
INF
-0.0200
_
SNF INF -1.5500 C-3 , C-4 CAF2
DECENTER( 2) . CaF2 Prism
~
INF C-4 REFL
INF INF 1.5500 C-A C-5 CAF2
0.525 0
6 INF INF 2.90D0 C-6 C-7 SILICON
DECENTER( 3) Silicon Prism
INF C-7 REFL
INF INF -2.5000 C-7 C-8- SILICON
-0 _1000
? -5.2363 CX CC-0.6000 _4.34D0 4.170SILICON
-11.8133 0'
-1.2602
g 7.0285 CC AI3) -0.3500 3.1100 3.1800GEFMMW
-0.9399
9~ 3.5768 .CC CX-0.3500 3.2000 3.?4002NS
10.0904 ,
-0.0638
27.0245 CC CX-D.6000 4.1200 4.2400SILICON
5.1234
-1.9286
11 A(4) 10.148? C?t-0,4000.3.8400 3.82D02NS
-D.9223 .
12 -2.2473 CX CC-0.3500 2.6700 2.420D22dSE
-2.5307 '
-0.2?022
13 -2.067'1 CX CC-0.5000 2.1000 1.56002NS
-1.4306
34
CA 02418917 2003-02-07
WO 02/019030 PCT/USO1/23680
-0.6180'3
19 INF ICdF -C.118U 1.1400 I.14CUStLICQN
-0.2290
APECtTCTRE STEP C-9 iCOld Stopl
-1.2070
15 INF INF -0,0900 C-10 C-11 SILICON
IL7ACE UTST~.NCE = -2.1300
IMAGE INF 2.8293
NOTES Positive radius indieateS Ehe centerCo right
- of curvature is the
Necfative radius indicates the centerto Ieft
of curvature is tYce
- Dimensions are given in inches
- Thickness is axial distance to
next surface
- Image diameter sho~,rn above is
a paraxial value,
it is not a ray traced'value
- Other glass suppliers can be used are
if their materials
unctionally equivalent to the extentdesign;
needed by the
contact the designer for approval
oE.substitutions.
APERTURE DRTA
DIAC3ETER
DGCEN'1'EF
AE'ET.~TURE____SHAE-____________________~___________~___________ ,
________Y________kW'ATION-,___
C- 1 CIRCLH 12.&33
'
CIRCLE (OSSC) 4.800 4.800
C- 2 CIRCLE 12.500 12.500
CIF1CLE (UASC) 4.800 4.800
C- 3 RECTANGT~E 2.900 2,900 ~CsF2 Prism - Entrance Eace)
C- 4 RL:CTANGL~ 2.900 4.101 (CaF2 Prism - Splitter Facet
CA 02418917 2003-02-07
WO 02/019030 PCT/USO1/23680
C- ~ RF_CT.'-.PiCi.c Z.~:OC 2.00 (Ce_'2 Pris:~ - _.._t Face;
.C- R=CaALd6LE 3.OSD 3.0=C !Silicon ?ris- - S'ncra~tcc
6 Facc-)
C- 7 RECTAP16L'c 3.300 S.fi40 0.000 -v.14 D_D
C- 8 RECTAtdGLE 3 _ 3. 4.0 (Silicon .?r i=m - E;ci
410 a Fac _)
C- CIRCLE D.B~24 0.844 (Cold SCOD)
CIRCLE (U3SC 0-.320 j (Occulting Disk)
0.32D
C- 10 RECT.=,P!GLB 1.280 2.280
C- 11 RECTRI4GL6 1.280 1.280
__ _______
_____________________z_________ _____
________________________________..____________...
AS9ftERIC ___
CUtJSTAL.7TS
2
(CURV)Y 4 6 B 1D
_ _________________________ ~ (A)Y + (3)Y + (C)Y + (D1Y
2 2 1l2
1 ,. (1-(3.i-r,) tcuev) Y 1
ASPHERIC CURV fC A H C D
_______________________________________________________________________________
_________________.
A( 1) -0.028812fi7-1.000000
At 2) -D.05583473-3.928273 .
A( 3) -0.02571678D.00000D -3.39014E-03 3.984fi0E-04 -2.25842E-05
O.OD000~'rC0
A( 4) -0.020189300.000000 6.40826E-09 8.943B7T'-OS -1.85905E-OS
2.939910-06
36
CA 02418917 2003-02-07
WO 02/019030 PCT/USO1/23680
Electronics S, s
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 I28
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. 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 I9 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 IPU 401. In an
alternative embodiment,
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 TPU 401.
Visible and IR 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.
Typically, images
generated by the arrays 64 and 68 are either stored for later retrieval or
downlinked to a
ground station. In the illustrated embodiment, the imagery is compressed by a
data
37
CA 02418917 2003-02-07
WO 02/019030 PCT/USO1/23680
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 IPU 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 IPU 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.
Except as noted herein and elsewhere in this document, the individual modules
and
components in the electronics are considered to be conventional and therefore
can be readily
derived be persons skilled in the art. Accordingly, a detailed discussion of
the modules per
se is omitted from the present discussion.
Roll Motion Compensation
Referring now to Figure 18, a presently preferred implementation of roll
motion
compensation in an electro-optical area array detector will now be described.
The
38
CA 02418917 2003-02-07
WO 02/019030 PCT/USO1/23680
visiblelnear 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 xmn x .010 rmn pixel pitch and a 50.4 mm x 50.4 mm array size. The reader
is directed
to the Lareau et al. U.S. patent 5,155,597 patent for a suitable detector,
however the array
need not be organized into column groups as described in the '597 patent and
could be
configured as a single column group, all columns of pixels clocked at the same
rate.
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 f
multiplied by the
rate of rotation w. Since f 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 adj acent 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.
To accomplish this, and with reference to Figure 19, the camera electronics
includes a
counter and clock driver circuit 304 (one for each detector 64, 68) wluch
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
39
CA 02418917 2003-02-07
WO 02/019030 PCT/USO1/23680
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 mrn).
The master
computer 34 supplies the counter value to the counter 310, along with a start
and stop
commands.
At the moment the array 64 is exposed to the scene, the counter 310 starts
counting at
the clock rate up to the counter value. When the counter value is reached, 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. When the counter value is reached, 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. At the end of the exposure period, 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. In alternative embodiments, the
image motion
compensation could be performed in other readout structures depending on the
architecture
for the array. The IR detector could be sensitive to radiation in the Short
Wavelength Infra-
Red (SWIR) band (1.0 to 2.5 microns), Mid-Wavelength IR (MWIR) band (3.0 to
5.0
microns) or Long Wavelength IR (LWIR) band (8.0 to 14.0 microns). In such an
array, 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.
The process of roll motion compensation can be more finely tuned by deriving
the rate
of rotation (w) used in the algorithm from the actual inertial rate sensed by
a fiber optic
gyroscope mounted to the camera housing or frame. Such 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. By doing this, any rate
inaccuracy or
CA 02418917 2003-02-07
WO 02/019030 PCT/USO1/23680
stabilization shortcomings associate with the roll motion could be overcome.
The roll motion
compensation becomes, in effect, a fme 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 above-described roll motion compensation will produce some minor edge
effects
at the bottom of the array, which are typically ignored since they are a very
small fraction of
the image generated by the array.
Other Embodiments
As noted above, 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. In such an embodiment, 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.
As another alternative embodiment, three or more detectors could image the
three or
more bands of the electromagnetic spectrum simultaneously. In such an
embodiment, an
additional spectrum separating prism would be placed in either the visible or
IR 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. As an
example, the visible/near IR 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. Meanwhile, the IR
portion of
the spectrum could be similarly divided into two separate bands, such as SWIR,
MWIR,
and/or LWIR bands, and each band associated with a distinct optical path and
associated
image forming and focusing lenses and an image recording medium. Obviously, in
such an
embodiment 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. However,
persons skilled in
41
CA 02418917 2003-02-07
WO 02/019030 PCT/USO1/23680
the art will be able to make such a modification from the illustrated
embodiment using routine
skill.
As yet an another possible embodiment, the camera may be designed for
hyperspectral
imaging. In such an embodiment, 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.
As yet another alternative, 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.
As yet another alternative embodiment, 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. In
this embodiment,
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, I~ 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.
As a variation on the above embodiment, 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 transfernng pixel
information
in the array at substantially the same rate as the rate of image motion due to
scan head rotation.
Less preferred embodiments of the invention include other types of optical
arrangements. While 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. In
such embodiments,
the optical subassembly comprising the objective lens would be rotated in the
direction of
42
CA 02418917 2003-02-07
WO 02/19030 PCT/USO1/23680
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.
Presently preferred and alternative embodiments of the invention have been
described
with particularity. Considerable variation from the disclosed embodiments is
possible
without departure from the spirit and scope of the invention. For example, the
type and
structure of the image recording medium is not critical. The details of the
optical design, the
mechanical system and the electronics may vary from the illustrated, presently
preferred
embodiments. This true scope and spirit is to be determined by the appended
claims,
interpreted in light of the foregoing.
43
