U.S. patent application number 09/773733 was filed with the patent office on 2001-08-23 for electro-optical reconnaissance system with forward motion compensation.
Invention is credited to Coon, Bryan H., Mathews, Bruce A..
Application Number | 20010015755 09/773733 |
Document ID | / |
Family ID | 21852437 |
Filed Date | 2001-08-23 |
United States Patent
Application |
20010015755 |
Kind Code |
A1 |
Mathews, Bruce A. ; et
al. |
August 23, 2001 |
Electro-optical reconnaissance system with forward motion
compensation
Abstract
An electro-optical framing camera forward motion compensation
(FMC) system comprising a moving shutter and a full frame focal
plane array detector is disclosed. The reconnaissance system is
designed to minimize the variation of image motion from a target
scene across the focal plane array. The full frame focal plane
array, such as a Charge Coupled Device (CCD), is designed to
transfer and add the image from pixel to pixel at a predetermined
rate of image motion corresponding to the region exposed by the
focal plane shutter. The focal plane shutter aperture and velocity
are set to predetermined values coordinated with the available
illumination. The CCD image transfer rate is set to minimize the
smear effects due to image motion in the region of the scene
exposed by the focal plane shutter. This rate is variable with line
of sight depression angle, aircraft altitude, and aircraft
velocity/altitude ratio. Further, a method of FMC utilizes a
comparison of a measured light level to a standard value in order
to determine the appropriate exposure time and shutter motion rate.
An optimal FMC clocking signal is calculated based on image motion
equations incorporated in the processing unit of the reconnaissance
system.
Inventors: |
Mathews, Bruce A.; (Kings
Park, NY) ; Coon, Bryan H.; (Hicksville, NY) |
Correspondence
Address: |
STERNE, KESSLER, GOLDSTEIN & FOX PLLC
1100 NEW YORK AVENUE, N.W., SUITE 600
WASHINGTON
DC
20005-3934
US
|
Family ID: |
21852437 |
Appl. No.: |
09/773733 |
Filed: |
February 2, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09773733 |
Feb 2, 2001 |
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09080452 |
May 19, 1998 |
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6256057 |
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09080452 |
May 19, 1998 |
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PCT/US97/19897 |
Nov 5, 1997 |
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60030089 |
Nov 5, 1996 |
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Current U.S.
Class: |
348/144 ;
348/169; 348/E3.018; 348/E3.019; 348/E3.023 |
Current CPC
Class: |
H04N 5/372 20130101;
H04N 5/353 20130101; G01C 11/02 20130101; H04N 5/37206
20130101 |
Class at
Publication: |
348/144 ;
348/169 |
International
Class: |
H04N 007/18 |
Claims
What is claimed is:
1. An electro-optical reconnaissance system, comprising: a focal
plane array including a main format area having-a plurality of
photo-sensitive cells arranged in rows and columns, wherein said
focal plane array is configured to detect a projected image of a
scene and to convert said image into an electronic charge
representation of said image; and a shutter having a controllable
exposure slit proximate to said focal plane array, wherein said
exposure slit is moved across said focal plane array to define
areas of exposure having approximately equal image motions and
wherein said charges representing said image are transferred at a
charge transfer rate corresponding to said image motion in said
area of said scene exposed by said shutter exposure slit.
2. The electro-optical reconnaissance system of claim 1, further
comprising: a camera control electronics unit driving said
plurality of photo-sensitive cells with a clocking signal for an
exposed portion of said focal plane array corresponding to said
charge transfer rate, wherein said clocking signal corresponds to a
position of said exposure slit and said image motion.
3. The electro-optical reconnaissance system of claim 2, wherein a
width and a speed of said exposure slit are adjustable, and wherein
said camera control electronics unit controls said exposure slit
width.
4. The electro-optical reconnaissance system of claim 3, wherein
said focal plane array is a charge coupled device (CCD).
5. The electro-optical reconnaissance system of claim 4, wherein
said focal plane array further comprises: a horizontal output
register having a predetermined number of segments, wherein each of
said segments includes an output detector/amplifier structure.
6. The electro-optical reconnaissance system of claim 2, wherein
the reconnaissance system is installed in a vehicle capable of
moving in a forward direction, and wherein said camera control
electronics unit comprises: an imaging electronics section
comprising an analog processor to process said transferred
electronic charges representing said image, focal plane array (FPA)
drive electronics to generate said clocking signal to drive said
FPA, a shutter exposure control unit to control shutter parameters,
wherein said shutter parameters include said exposure slit width
and said speed of said exposure slit; a signal processing
electronics unit comprising a digital preprocessor coupled to said
FPA drive electronics and to said analog processor, to receive and
further process said electronic charge representation of said
image, and to provide a digital processed image signal; and a
camera central processing unit (CPU), to process mission parameter
inputs and provide processed mission parameter information to said
imaging section to perform forward motion compensation (FMC) of
said image.
7. The electro-optical reconnaissance system of claim 6 further
comprising: a lens to focus said scene onto said focal plane array;
signal recording means coupled to an output of said signal
compression means to record a forward motion corrected image of
said scene; and a power supply to provide power for said camera
control electronics unit.
8. The electro-optical reconnaissance system of claim 7, further
comprising a light sensor in communication with said shutter
control unit; and a thermoelectric cooler to control an operating
temperature of said focal plane array.
9. The electro-optical reconnaissance system of claim 8, wherein
said focal plane array is mounted on an adjustable mount coupled to
said vehicle, wherein the electro-optical reconnaissance system
performs forward motion compensation in a forward oblique mode of
operation, a side oblique mode of operation, and a vertical mode of
operation.
10. The electro-optical reconnaissance system of claim 6, wherein
said shutter control unit comprises: a buffer to receive a signal
generated by said light sensor indicating lighting conditions of
the scene; an analog to digital converter coupled to said buffer to
convert said light sensor signal into a digital signal; and a
look-up table to convert said digitized signal into a look up table
value to drive said shutter, wherein said look-up table provides
drive signals corresponding to said exposure slit speed and said
exposure slit width.
11. The electro-optical reconnaissance system of claim 6, wherein
said FPA control electronics comprise: a timing generator to
generate a master timing signal and to provide for focal plane
array readout and FMC, wherein said master timing signal is divided
by a predetermined value to provide a local timing signal; a
horizontal counter to provide a time base for pixel counting
operations; a vertical counter to provide a time base in the
vertical direction of said focal plane array; a horizontal clock
generator coupled to said horizontal and vertical counters, to
provide a horizontal clocking signal to said focal plane array; a
vertical clock generator coupled to said horizontal and vertical
counters, to provide a vertical clocking signal to said focal plane
array; and a frame synchronization unit, coupled to said horizontal
and vertical counters, to generate frame sync signals and line sync
signals.
12. The electro-optical reconnaissance system of claim 11, wherein
said FPA control electronics further comprise: a plurality of
multi-tap delay lines to define a phase relationship of said
horizontal and vertical clocking signals.
13. The electro-optical reconnaissance system of claim 11, wherein
said digital preprocessor comprises: a circuit card assembly (CCA)
to process inputs from said imaging electronics section.
14. The electro-optical reconnaissance system of claim 13, wherein
said CCA comprises: a Static Random Access Memory (SRAM) configured
as First In/First Out (FIFO) memory to store pixel data from said
focal plane array, wherein said FIFO memory facilitates replacing
defective pixels with nearest neighbor processing; a timing
generator coupled to said frame sync and line sync signals; a Field
Programmable Gate Array (FPGA) address generator coupled to said
timing generator to generate memory addressing; a Programmable Read
Only Memory (PROM) coupled to said FPGA address generator to store
locations of said defective pixels; an Automatic Gain Control (AGC)
ASIC to reduce said pixel data without degradation of the original
image, wherein said pixel data is reduced from twelve-bit form to
eight-bit form; an illumination chip to correct for vignetting
effects of said image; and an image bus coupled to said AGC ASIC to
receive said eight-bit data format.
15. The electro-optical reconnaissance system of claim 14, wherein
said AGC ASIC includes means to subtract out specular reflections
contained on said image, subtract out haze contributions contained
on said image, and maintain a running average of said image
data.
16. An electro-optical reconnaissance system for performing forward
motion compensation, wherein said reconnaissance system is
installed in a vehicle capable of moving in a forward direction,
comprising: a focal plane array including a main format area having
a plurality of photo-sensitive cells arranged in rows and columns,
wherein said focal plane array is configured to detect a projected
image of a scene and to convert said image into an electronic
charge representation of said image, and wherein said focal plane
array is oriented to view said scene in a forward oblique mode of
operation; and a focal plane shutter, having a controllable
exposure slit proximate to said focal plane array, wherein said
exposure slit is moved across said focal plane array to define
areas of exposure having approximately equal image motions, wherein
said exposure slit is oriented parallel to a direction of said
rows, and wherein said charges representing said image are
transferred at a charge transfer rate corresponding to said image
motion in said area of said scene exposed by said shutter exposure
slit.
17. The electro-optical reconnaissance system of claim 16, further
comprising: a lens to focus said scene onto said focal plane array;
and a camera control electronics unit driving said plurality of
photo-sensitive cells with a clocking signal for an exposed portion
of said focal plane array corresponding to said charge transfer
rate, wherein a width and a speed of said exposure slit are
adjustable, wherein said camera control electronics unit controls
said exposure slit width, wherein said clocking signal corresponds
to a position of said exposure slit and said speed of said exposure
slit, and wherein said clocking signal corresponds to a rate of
motion of objects contained in a portion of said scene viewed by
said focal plane array.
18. The electro-optical reconnaissance system of claim 17, wherein
said clocking signal is generated in accordance with an in-track
image motion, wherein said in-track image motion is determined by
14 FV ALT sin 2 ( ) cos 2 and VF ALT sin 2 where F=Focal length,
ALT=Altitude of the vehicle, V=velocity, .theta.=in track angle,
and .gamma.=depression angle.
19. The electro-optical reconnaissance system of claim 16, wherein
said focal plane array is a column-segmented charge coupled device
(CCD).
20. An electro-optical reconnaissance system for performing forward
motion compensation, wherein said reconnaissance system is
installed in a vehicle capable of moving in a forward direction,
comprising: a focal plane array including a main format area having
a plurality of photo-sensitive cells arranged in rows and columns,
wherein said focal plane array is configured to detect a projected
image of a scene and to convert said image into an electronic
charge representation of said image, and wherein said focal plane
array is oriented to view said scene in a side oblique mode of
operation; and a focal plane shutter, having a controllable
exposure slit proximate to said focal plane array, wherein said
exposure slit is moved across said focal plane array to define
areas of exposure having approximately equal image motions, wherein
said exposure slit is oriented parallel to a direction of said
columns, and wherein said charges representing said image are
transferred at a charge transfer rate corresponding to said image
motion in said area of said scene exposed by said shutter exposure
slit.
21. The electro-optical reconnaissance system of claim 20, further
comprising: a lens to focus said scene onto said focal plane array;
and a camera control electronics unit driving said plurality of
photo-sensitive cells with a clocking signal for an exposed portion
of said focal plane array corresponding to said charge transfer
rate, wherein a width and a speed of said exposure slit are
adjustable, wherein said camera control electronics unit controls
said exposure slit width, wherein said clocking signal corresponds
to a position of said exposure slit and to said image motion in
said area of said scene exposed by said shutter exposure slit.
22. The electro-optical reconnaissance system of claim 21, wherein
said clocking signal is generated in accordance with an in-track
image motion, wherein said in-track image motion is determined by
15 VF ALT sin and FV ALT sin ( ) cos where F=Focal length,
ALT=Altitude, V=velocity, .phi.=in track angle, .theta.=cross track
angle, and .gamma.=depression angle.
23. The electro-optical reconnaissance system of claim 20, wherein
the focal plane array is a column-segmented charge coupled device
(CCD).
24. A method for providing forward motion compensation (FMC) for an
electro-optical reconnaissance system in a vehicle capable of
forward motion, comprising the steps of: (1) measuring a light
level of a scene to be imaged by the reconnaissance system; (2)
comparing the measured light level to a predetermined light level
value; (3) determining an exposure time by comparing the measured
light level to an exposure time look-up table, (4) determining a
forward motion compensation profile corresponding to the exposure
time and mission parameter inputs; and (5) sending a signal
corresponding to said forward motion compensation profile to an
electronics unit of the electro-optical reconnaissance system to
perform FMC.
25. The method of claim 24, wherein step (3) comprises the steps
of: (a) determining an exposure time by comparing the measured
light level to a primary exposure time look-up table, if the
measured light level is greater than the predetermined light level
value; and (b) determining an exposure time by comparing the
measured light level to a low light exposure time look-up table, if
the measured light level is less than the predetermined light level
value.
26. The method according to claim 24, wherein step 3(a) further
comprises the step of: sending a shutter speed signal corresponding
to the determined exposure time to a shutter exposure control unit,
wherein a faster shutter speed corresponds to shorter exposure
times, and wherein a slower shutter speed corresponds to longer
exposure times.
27. The method according to claim 24, wherein step 3(b) further
comprises the step of: utilizing a set of instantaneous mission
parameters to determine the exposure time, wherein the set of
instantaneous mission parameters includes at least one of aircraft
velocity, altitude, and camera look angle.
28. The method according to claim 24, further comprising the step
of: (6) determining a exposure slit width for the exposure slit
corresponding to product of the exposure time and the exposure slit
speed.
29. The method according to claim 24, wherein the forward motion
compensation profile determined in step (4) corresponds to a
look-up table value, wherein said look up table value is calculated
based on in-track image motion rate equations, and wherein the
in-track image motion rate equations utilize a set of mission
parameter inputs that include: aircraft velocity, V; aircraft
altitude, H; depression angle of camera (fixed for flight); camera
installation location (fixed for flight); shutter trigger pulse;
and focal length.
30. The method according to claim 29, wherein the electro-optical
reconnaissance system is operating in a side oblique mode of
operation, wherein the in-track image motion is determined by 16 VF
ALT sin and FV ALT sin ( ) cos where F=Focal length, ALT=Altitude,
V=Aircraft velocity, .phi.=in track angle, .theta.=cross track
angle, and .gamma.=depression angle.
31. The method according to claim 29, wherein the electro-optical
reconnaissance system is operating in a forward oblique mode of
operation, wherein the in-track image motion is determined by 17 FV
ALT sin 2 ( ) cos 2 and VF ALT sin 2 where F=Focal length,
ALT=Altitude, V=Aircraft velocity, .theta.=in track angle, and
.gamma.=depression angle.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of the commonly owned,
co-pending PCT Appl. No. PCT/US97/19897, filed Nov. 5, 1997
(incorporated by reference herein), which claims the benefit of
U.S. Appl. No. 60/030,089, filed Nov. 5, 1996 (incorporated by
reference herein).
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates generally to electro-optical
reconnaissance systems whose angular resolution is greater than the
product of the exposure time and the angular rate of image motion.
The invention is a forward motion compensation (FMC) system that
permits full resolution performance when the target
line-of-sight-angular-rate-exposure-time product is greater than
the angular resolution of the system. The system includes optics, a
mechanical shutter and full frame CCD.
[0004] 2. Related Art
[0005] Aerial reconnaissance systems have undergone a dramatic
transition in the past two decades with the replacement of
photographic film by electro-optic image sensors. With the advent
of wafer-scale focal planes that provide sufficient coverage and
resolution, reconnaissance systems are being designed to utilize
electro-optic sensors configured as large format area arrays. These
electro-optic ("EO") reconnaissance imaging systems most often
employ charge-coupled devices ("CCDs") operating in the visible and
near-infrared regions of the electromagnetic spectrum to capture
the image of the target or scene. The ability to operate in a
real-time environment and in low ambient light conditions are just
a few of the reasons why electro-optical-based reconnaissance
imaging systems are increasingly replacing film-based
reconnaissance systems.
[0006] One of the more frequently encountered problems in designing
aerial reconnaissance imaging systems is determining the most
effective method of compensating for image smear or blurring.
Typically, smearing occurs when low ambient light conditions
prevent an imaging system from using sufficiently short exposure
times, resulting in a blurred image due to the forward motion of
the aircraft. In other words, smearing occurs as a result of the
relative motion between a scene or target to be imaged and the
imaging system. Therefore, in order to prevent the degradation of
the information contained in a recorded image, an ideal
reconnaissance imaging system must utilize some means of image
motion compensation ("IMC") for image smear.
[0007] Different reconnaissance mission operating scenarios can
present different image motions that should be compensated for. The
goal of any image motion compensation system, of which a forward
motion compensation ("FMC") system is a specific category, is to
reduce the image smear that occurs when the target
line-of-sight-angular velocity is significantly different from the
camera angular velocity.
[0008] Early reconnaissance systems comprised linear arrays that
operated at high altitudes, thereby minimizing the angular motion
effects proportional to the aircraft velocity/altitude ratio.
However, when low flying mission scenarios are required to avoid
detection of the reconnaissance aircraft, forward motion
compensation is necessary to maintain image resolution. Several
conventional methods of IMC have been developed to meet these image
resolution requirements.
[0009] For example, U.S. Pat. No. 4,505,559, issued Mar. 19, 1985
to Prinz, discloses an approach wherein an instantaneous
line-of-sight controls the motion of the film used to record the
image. U.S. Pat. No. 4,157,218, issued Jun. 5, 1979 to Gordon et
al., also uses a film drive to compensate for the forward motion of
the image. Mechanical means are used in U.S. Pat. No. 4,908,705,
issued Mar. 13, 1990 to Wight, where the imaging array physically
moves to reduce the smear.
[0010] U.S. Pat. No. 5,155,597 to Lareau et al., issued Oct. 13,
1992, discloses an equation that described the correction for the
image motion in the side oblique scenario by transferring the
charge in a column segmented CCD array at different transfer rates
corresponding to the depression angle.
[0011] However, these aforementioned image motion compensation
techniques are inadequate to provide for image motion compensation
in each of the various mission scenarios described above. What is
needed is an electro-optical reconnaissance system that provides
adequate image motion compensation in forward oblique, side
oblique, and vertical orientations. In addition, it is desirable
that this reconnaissance system be low cost.
SUMMARY OF THE INVENTION
[0012] The present invention provides a system and method for the
compensation of image motion during reconnaissance missions.
According to a first embodiment of the present invention, the
electro-optical reconnaissance system includes an imaging focal
plane array (FPA), such as a charge-coupled device (CCD), to record
a target scene. The focal plane array includes a main format area
having a plurality of photo-sensitive cells arranged in rows and
columns. The reconnaissance system also includes a shutter having a
window (or exposure slit) that moves across the imaging device. In
order to compensate for the forward motion of the vehicle, such as
an aircraft, the charge in the imaging device is transferred across
the device. The rate of charge transfer is uniform across the focal
plane array, but varies in time in accordance with the portion of
the target being imaged, where the portion of the target scene
being imaged is defined by the position and width of the shutter
slit. The charge transfer rate is varied based on the position of
the shutter slit over the imaging device. A camera control
electronics unit controls the position of the shutter slit and
processes target scene information, light levels, and
reconnaissance mission requirements in order to determine to the
rate of motion of objects contained in the portion of the target
scene viewed by the focal plane array. As a result, the camera
control electronics unit can generate an appropriate clocking
signal to perform forward motion compensation (FMC) in a variety of
target viewing modes, including forward oblique, side oblique, and
vertical modes of operation.
[0013] According to a second embodiment of the present invention, a
method for providing forward motion compensation for the
electro-optical reconnaissance system is utilized in the camera
control electronics unit. First, a light sensor measures the light
level of the scene to be imaged by the reconnaissance system. Next,
the measured light level is compared to a predetermined light level
value. For example, the predetermined light level value can
correspond to a given solar angle above the horizon. If the
measured light value is greater than the standard value, an
exposure time is determined by comparing the measured light level
to a primary exposure time look-up table. If the measured light
value is less than the standard value, the exposure time is
determined by comparing the measured light level to a low exposure
time look-up table. By determining the proper exposure time, the
proper shutter slit width and shutter slit speed are determined.
Next, a forward motion compensation profile is determined
corresponding to the exposure time and mission parameter inputs.
For example, the mission parameters can include aircraft velocity,
altitude, and camera look angle. This FMC profile corresponds to
the clocking signal that is used to drive the focal plane array of
the reconnaissance system in order to perform FMC.
[0014] Further features and advantages of the present invention, as
well as the structure and operation of various embodiments of the
present invention, are described in detail below with reference to
the accompanying drawings.
BRIEF DESCRIPTION OF THE FIGURES
[0015] The accompanying drawings, which are incorporated herein and
form part of the specification, illustrate the present invention
and, together with the description, further serve to explain the
principles of the invention and to enable a person skilled in the
pertinent art to make and use the invention. In the drawings, like
reference numbers indicate identical or functionally similar
elements. Additionally, the left-most digit(s) of a reference
number identifies the drawing in which the reference number first
appears.
[0016] FIG. 1 illustrates various image motions due to the forward
motion of an aircraft;
[0017] FIGS. 2A-D illustrate alternative conventional
electro-optical imaging modes;
[0018] FIG. 3 illustrates the shifting of charges in a simplified
Charge Coupled Device (CCD);
[0019] FIG. 4 illustrates a conventional detector array;
[0020] FIG. 5 illustrates a detector array employing graded forward
motion compensation (FMC) according to the present invention (shown
in the forward oblique mode);
[0021] FIG. 6 illustrates a conventional detector array employing
column-segmented forward motion compensation;
[0022] FIG. 7 illustrates a projection of a focal plane array onto
the ground for side oblique image collection;
[0023] FIG. 8 illustrates the operation of a graded FMC detector
array operating in a side oblique viewing mode;
[0024] FIG. 9A illustrates the image motion rate and FIG. 9B
illustrates the CCD line rate for a graded FMC detector array
operating in a side oblique viewing mode;
[0025] FIG. 10 is an illustration of an example environment for the
electro-optical reconnaissance system operating in a side oblique
viewing mode;
[0026] FIG. 11 is an illustration of an example environment for the
electro-optical reconnaissance system operating in a forward
oblique viewing mode;
[0027] FIGS. 12 and 13 illustrate the operation of a graded FMC
detector array in a forward oblique mode according to the present
invention;
[0028] FIG. 14 is an illustration of an example environment for the
electro-optical reconnaissance system operating in a vertical
viewing mode;
[0029] FIG. 15 illustrates the operation of a graded FMC detector
array in a vertical mode according to the present invention;
[0030] FIG. 16 illustrates a layout of a focal plane array
according to the present invention;
[0031] FIG. 17 illustrates a preferred embodiment of the focal
plane array with side bus connections;
[0032] FIGS. 18 and 19 illustrate the clocking sections of a
detector array with a column-segmented imaging area according to an
alternative embodiment of the present invention;
[0033] FIG. 20 illustrates a pixel model for determining a time
constant for V-phase gates according to the present invention;
[0034] FIG. 21 is a block diagram of the electro-optical
reconnaissance system's camera electronics according to a preferred
embodiment of the present invention;
[0035] FIG. 22 is a flow chart of the camera motion compensation
control process according to the present invention;
[0036] FIG. 23 is a block diagram of the timing generator and CCD
drive electronics implemented in the reconnaissance system
according to the present invention;
[0037] FIG. 24 illustrates example frame timing and line timing
signals according to the present invention;
[0038] FIG. 25 is a block diagram of the shutter exposure control
according to the present invention;
[0039] FIG. 26 plots exposure time versus slit width for two
example shutter speeds according to the present invention;
[0040] FIG. 27 plots exposure time as a function of solar altitude
for various lenses according to the present invention; and
[0041] FIG. 28 is a block diagram of the digital preprocessor
according to a preferred embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0042] 1. Overview and Discussion of the Invention
[0043] The present invention is directed to a method and system for
the compensation of image motion during aircraft reconnaissance
missions. In particular an imaging focal plane array (FPA), such as
a charge-coupled device (CCD) is utilized to record a target scene.
In order to compensate for the forward motion of the aircraft, the
charge in the imaging device is transferred across the device. The
rate of transfer is uniform across the CCD, but varies in time in
accordance with the portion of the target being imaged. This is
accomplished by using a moving window (or slit) shutter which scans
the projected image of the target across the imaging device. The
charge transfer rate is varied based on the position of the
projected image on the imaging device. The present invention
controls the design and the specification of the optics, mechanical
shutter, and the CCD to construct a low cost and producible image
motion compensation (IMC) system for a very high performance
reconnaissance system. The manner in which this is accomplished is
described in detail below.
[0044] To put the invention in context, a brief discussion of some
of the problems associated with current aerial reconnaissance
systems will be described. For example, image smear is normally
present in several different aerial reconnaissance mission
scenarios. FIG. 1 illustrates the types of image motions
encountered when the camera is looking directly downward
(vertical), looking toward one side (side oblique), and looking
forward at a selected angle of depression from the horizon (forward
oblique). In FIG. 1, the rectangular focal plane array is shown as
projected onto the ground. The relative magnitude and direction of
the image motion within the frame is indicated by the length and
direction of the motion-indicating arrows.
[0045] In the vertical example 102, the aircraft 104 flies directly
over top of the target or scene of interest. Thus, all of the image
motion is of a singular direction (opposite to the flight
direction) and magnitude. That is, all the parts of the image move
together, parallel to the line of flight. This motion is uniform in
magnitude throughout the frame 106.
[0046] In the side oblique case 112, the motion remains parallel to
the line of flight 111, but is not of the same magnitude throughout
the frame. Objects nearest to the flight path, represented by arrow
114, appear to move fastest. Those objects further away from the
line of flight, represented by arrow 116, appear to move more
slowly, in proportion to their distance from the flight path
111.
[0047] The forward oblique case 122 is more complex. In this case,
the image motion is composed of two vectors. The first is parallel
to the line of flight as in the examples discussed above. Here
again, the magnitude of this motion vector varies with range from
the aircraft: the motion vector is larger the nearer a given point
in the image is to the aircraft. Away from the line of flight, a
second motion (of much lower absolute magnitude) occurs. As points
approach the aircraft, they appear to "fly off" to the side of the
format. Points to the left of the flight path "fly off" to the
left, and points to the right of the flight path "fly off" to the
right. The magnitude of this vector increases the closer a given
point is to the aircraft.
[0048] 2. Example Environment
[0049] Before describing the invention in detail, it is useful to
discuss example reconnaissance techniques in which the invention
can be utilized. Preferably, the present invention can be
implemented in a variety of electro optical reconnaissance systems.
For example, four basic types of imagery sensors are: strip mode,
pushbroom, panoramic sector scan and framing. The choice of a
particular technique depends on the specific operational need.
These four example reconnaissance systems are illustrated in FIG.
2.
[0050] FIG. 2A illustrates a stip mode sensor. Strip mode sensors
create an image by pointing an EO (electro-optical) focal plane at
the ground, through a lens, orientated perpendicular to the line of
flight. The EO focal plane device can be a body-fixed sensor or a
moveable sensor. Ground geometry is maintained and/or corrected by
adjusting the line rate of the focal plane to compensate for image
motion. For example, as shown in FIG. 2A, a linear detector array
(not shown), located in aircraft 202 is oriented perpendicular to
the flight direction. The array comprises a line of pixels to
create a first dimension of scene coverage. The forward motion of
the aircraft creates a second dimension of coverage along the
flight path of the aircraft.
[0051] A second reconnaissance technique is called a "pushbroom"
technique and is illustrated in FIG. 2B. Pushbroom sensors are a
variation of strip mode sensors, in which the linear instantaneous
field of view is moved fore and aft in the in-track or along-track
(along the flight path) direction in order to achieve stereo
imaging, compensate for image motion, and/or create an image
"frame." Once again, as shown in FIG. 2B, a linear array is
oriented perpendicular to the flight direction. The combination of
the forward motion of the aircraft and the fore/aft scan of the
array produces an overlapping second dimension of coverage.
[0052] A panoramic sector scan sensor technique, illustrated in
FIG. 2C, creates a "frame" of imagery by moving the instantaneous
field of view of the focal plane perpendicular (or across-track) to
the line of flight. The width of the frame (in degrees) is fixed by
the focal length of the sensor and the length of the focal plane,
and the across-track coverage is determined by the scan speed and
the duration of the scan of the sensor. In this example, a linear
detector array is oriented parallel to the flight direction. The
scan of the array, e.g., from the horizon down, produces the second
dimension of coverage.
[0053] A fourth technique, known as a framing sensor technique, is
illustrated in FIG. 2D. Framing sensors "instantaneously" collect
area images, much as a snapshot camera does. For example, an area
detector array is pointed to a target, then frames of imagery are
collected. Up until recently, frame size and resolution were
limited, due to the size and fewer number of pixels in staring
arrays.
[0054] Framing sensors are advantageous for specific applications.
For example, a framing sensor is uniquely able to capture forward
oblique imagery containing the horizon. This capability can give
the reconnaissance pilot flexibility when maneuvering the aircraft
near the target. In addition, a framing camera can provide
improvements in low light level performance, continuous stereo
coverage and reduced image artifacts resulting from low frequency
motions.
[0055] Until recently, EO framing cameras were not operationally
viable due to their small image area and technology limitations
associated with data processing and storage. Improvements in wafer
fabrication and image processing technologies now make this type of
camera feasible. As described below, such high resolution framing
cameras can compliment other tactical reconnaissance sensors,
especially in the forward oblique mode and for stereo imaging.
[0056] The present invention is described in terms of this example
environment. Description in these terms is provided for convenience
only. It is not intended that the invention be limited to
application in this example environment. In fact, after reading the
following description, it will become apparent to a person skilled
in the relevant art how to implement the invention in alternative
environments.
[0057] 3. Electronic Forward Motion Compensation (FMC)
[0058] The use of Charge Coupled Devices (CCDs) in place of film
allows for a method of "electronic" image motion compensation. In
this method, the electronic signal being formed in the detector
array by the image can be shifted to move along with the motion of
the image falling on the array. This charge transfer concept is
illustrated schematically in FIG. 3. This method can be used to
reduce smear caused by the image motion while allowing for longer
exposure times.
[0059] FIG. 3 shows a simplified CCD detector represented as a
single column of pixels 302. The incident light on each pixel (or
potential well), for example pixel 304, generates free electrons
which are collected at the pixel site. By the application of
clocking waveforms A, B, and C at input 306, the charges
(electrons) collected in a pixel well are shifted down the column.
If all pixels are clocked together, a "bucket brigade" like
transfer of the signals is achieved. According to the present
invention, the pixels are clocked during the exposure period at a
rate equal to the rate of image motion. Thus, the signal generated
by a specific image point will move to stay with that image point.
This method of charge transfer eliminates smear due to the image
motion while increasing the effective exposure time.
[0060] There are at least three methods of compensating for image
motion which are electronic in nature; average FMC, graded FMC and
segmented FMC. All of these methods of FMC can be utilized in the
present invention. These methods are each described below.
Additionally, these methods are contrasted with an uncompensated
imager.
[0061] a. No FMC
[0062] FIG. 4 illustrates an uncompensated imager. The
uncompensated imager is a simple, but very large imager comprising
rows and columns of pixels, as well as attendant readout structure
(e.g., amplifiers, etc.). In the uncompensated imager no attempt is
made to eliminate or minimize the image-smearing effects of image
motion. The film camera used with an uncompensated imager is
typically one in which the film is flat and fixed, and the shutter
(between-the-lens or focal plane) is simply opened to produce the
desired length of exposure. Whatever image motions occur during
exposure, together with the associated smear-induced loss of image
quality, are simply tolerated.
[0063] For example, a detector array 402 based on an uncompensated
imager includes X columns, each of which has Y pixels. The array is
exposed to a moving image and the signal is then shifted out as in
a CCD or read by the application of an X and Y address clock. Image
smear causes a loss of image quality as a function of its
magnitude.
[0064] b. Average FMC
[0065] An improvement over the uncompensated imager configuration
is an average FMC imager. In average FMC imagers, as charge is
collected during exposure, the charge is also moved in
synchronization with the motion of the image which produced it. For
example, the IMMIC (Integrating Mode Moving Image Chip) is a known
average FMC imager.
[0066] An average FMC imager can also be described with respect to
FIG. 4. The detector array 402 comprises X columns, all of which
transfer charge at the same rate. Each column comprises Y pixels.
During an exposure, all charges can be moved at a rate equal to the
average motion rate. A detector array based on average FMC has one
set of clock lines (.phi..sub.1, .phi..sub.2, .phi..sub.3) which
transfer charge for the entire array at a rate proportional to
clock speed.
[0067] Also referred to as Time Delay and Integration (TDI), the
average FMC method is used to increase the signal-to-noise ratio.
Such charge motion can be implemented using a variant of the simple
large chip discussed above by shifting data along the CCD columns
while the shutter is open. The image motion rate for all columns is
the same. The image motion rate is selected by a camera system
housing the average FMC imager to match the average image velocity
over the chip during exposure. While average FMC imagers provide
average EMC correction, they do not compensate for the different
magnitudes of the image vectors at different distances from the
flight path. Therefore, unless the image motions are uniform, even
with a perfect match to the desired average charge/image velocity,
certain columns would exhibit lead or lag smear errors. Even though
these errors are much less than for the uncompensated imager, these
errors result in less than optimum performance. In average FMC,
either a between-the-lens or focal plane shutter can be used.
[0068] c. Graded FMC
[0069] As discussed above, in either side oblique or forward
oblique imaging applications, forward image motions are not the
same at all positions in the field of view (FOV). In order to solve
the problem of non-uniform image motion as a function of position
of the object being imaged, it is necessary to alter the charge
motion rate for each column (or group of columns). The present
invention is based on graded FMC. Graded FMC imagers use a
combination of a time varying charge motion rate and an exposure
controlled by a focal plane shutter slit. With graded FMC imagers,
the charge motion rate is uniform across the columns of the array,
but it varies in time as a function of the portion of the imager
being exposed by the slit at a given instant.
[0070] This graded FMC approach utilizes a focal plane shutter so
that only a portion of the array is exposed at any one time. Thus,
the invention matches the charge motion rate with the position of
the shutter slit as it traverses the chip. As a result, the optimum
charge motion rate can be selected corresponding to the exposed
portion of the array. Because only one area is exposed during the
time associated with a given charge transfer rate, a nominally
ideal image motion compensation can be obtained on-chip. Concerning
focal plane array chip design, graded FMC imagers are similar to
average FMC imagers. However, a reconnaissance system based on
graded FMC imagers, utilizes the added refinements of a focal plane
shutter and column clocks synchronized to the changes in image
motion. For example, FIG. 5 illustrates the graded FMC concept used
in the forward oblique imaging mode.
[0071] A detector array utilizing graded FMC has one set of clock
lines which transfer charge for the entire array at a rate
proportional to clock speed. The array is made up of X columns, all
of which transfer charge at the same rate. Each column is made up
of a number of pixels. Pixel signal is shifted along all columns
keeping pace with the rate of image motion seen through the slit.
Thus, according to the present invention, the moving exposure-slit
is used to determine the exposure time for any portion of the
array, and to allow tracking of variable image motion across the
array.
[0072] d. Segmented FMC
[0073] A fourth category of detector chip, referred to as segmented
FMC, can be designed to work with either a between-the-lens shutter
or a focal plane shutter camera. The segmented FMC concept is shown
in FIG. 6. Here the area of the imager is broken up into some
number of segments, where each segment is a group of columns. The
size of the segments will be dictated by the magnitude of the
differential motion from "side" to "side" of the array and the
practicality of adding ever more segments. The average FMC and
graded FMC systems represent an example of a single segment. For
example, a segmented FMC imager having 16-segments requires
16-times the drives/clocks, etc. of a graded FMC device.
[0074] The column segmented detector array is segmented into
sections. Each section is clocked to move charge at a different
rate in order to keep pace with differing image rates. Each segment
is made up of a number of columns, all of which transfer charge at
the same rate. Each column is made up of a number of pixels.
[0075] For example, an array which can be implemented in
conjunction with the segmented FMC is disclosed in the Lareau '597
patent, incorporated herein by reference.
[0076] According to the present invention, a graded FMC detector
array is incorporated into an electro-optical reconnaissance
system. Further, the reconnaissance system utilizes a moving
shutter or slit to perform image motion compensation in the
vertical, forward oblique, and side oblique modes of operation.
Using a graded FMC imager approach reduces the potential complexity
to the design and cost of the focal plane array. Alternatively, the
present invention can be implemented using a column-segmented focal
plane array. The manner in which these FMC imagers operate is
described in detail below.
[0077] 4. Side Oblique Operation in Detail
[0078] Image motions and operations of the graded FMC imager
according to the present invention are described in detail for the
side oblique mode of operation. In the side oblique image
collection mode, compensation of the image motion is complex.
Although image motion remains monotonic in direction, the magnitude
of the image motion is a function of the position of a given column
of pixels in the focal plane array (FPA) relative to its position
within the format.
[0079] The projection of the FPA onto the ground in a side oblique
mode of operation is illustrated in FIG. 7. The further away the
projection of a particular column of pixels is from the flight
track, the more slowly the scene image traverses it. To correct for
this unidirectional, but unequal, image motion throughout the
format, alternative means to transfer the charges along each column
can be implemented.
[0080] For example, one method of achieving FMC is to divide the
columns into sub-groups of a few columns each and to transfer
charges along each sub-group at some average rate deemed to be
sufficiently approximate for that sub-group. This column-segmented
FMC method is described above. Each sub-group requires a separate
set of adjustable clocks. Such a set of clocks is calculated to
work at the best compromise for a given V/H and depression angle
combination. In addition to adding greatly to the complexity of the
chip drive electronics, the added complexity of the detector chip
makes it difficult, expensive, and risky to produce. These same
factors tend to limit the ultimate size (i.e., pixel count) of a
column-segmented imaging device.
[0081] An alternative approach, according to a preferred embodiment
of the present invention, is to combine the charge motion handling
technique described above with the incorporation of a focal plane
shutter within the camera. This combination achieves graded
FMC.
[0082] For example, recall the FPA projection illustrated in FIG.
7. According to the present invention, a focal plane shutter having
an exposure slit which runs parallel to the columns of CCD pixels
is added to the arrangement. This shutter traverses the array from
left to right, sequentially exposing different columns of pixels to
the target scene.
[0083] The operation of a FPA with graded FMC is illustrated in
FIG. 8. For example, at the far point of the frame 802, the
magnitude of the image motion is at its smallest, as illustrated by
arrow 803. At the mid frame point 804, the magnitude of the image
motion has increased. At the near point of the frame 806, the
magnitude of the image motion is at its greatest, as this
corresponds to the objects closest to the aircraft. The rate at
which charge is swept down the columns is varied uniformly in
magnitude across the entire FPA. Without a focal plane shutter in
place, this charge motion rate might be correct for one column or
small group of columns, but would be incorrect for all other column
groups. However, by making the charge transfer rate (for the entire
FPA) consistent at any one instant but variable with the position
of the exposure slit in the focal plane shutter, the charge
transfer rate is varied such that the charge transfer rate is
matched to the image motion rate in that particular column in the
center of the exposure slit. This principle is illustrated in FIG.
9A, which shows how the image motion rates vary as a function of
distance. Thus, only the image rate corresponding to the position
of the slit is "seen" by the array at any given time.
[0084] The CCD line rate is made to vary as a function of slit
position to match the image motion rate, as shown in FIG. 9B. At
the conclusion of slit travel, the complete array is read out at a
maximum rate. The charge motion in the unexposed areas is not
matched to the correct image motion rate, but this is of no
consequence since no imaging light is being collected outside of
the exposure slit area. As illustrated in FIG. 8, for side oblique
operation, the focal plane shutter (or slit) is oriented parallel
to the charge transfer columns and is moved perpendicular to the
image motions.
[0085] In the side oblique mode of operation, the graded FMC imager
approach provides FMC without the added design and charge transfer
complexities of the segmented FMC approach. In particular, these
risks are significantly reduced for large scale FPAs, such as a
9216 pixel by 9216 pixel CCD.
[0086] To understand the rate at which charge is transferred in
order to perform FMC, it is beneficial to examine the geometry of
the focal plane array and the target. FIG. 10 illustrates the side
oblique geometry. In this geometry:
[0087] F=Focal length
[0088] H=Altitude (ALT)
[0089] V=Aircraft velocity
[0090] .phi.=in track angle
[0091] .theta.=cross track angle
[0092] .gamma.=depression angle
[0093] The position at which a point of the target is focused on
the focal plane (i.e., the CCD) as a result of the lens is, given
by the Rectilinear Lens Image Transfer Relation. In the side
oblique mode, the derivative of the image transfer relation
determines the motion in the x-direction (i.e., V.sub.CCD in the
x-direction). For an ideal lens, the image transfer relation
is:
x=F(tan .phi.)
[0094] Note that for non-ideal lenses, this transfer relation
changes, depending on the imperfections in the lens (i.e., x=F(tan
.phi.+k.sub.1.phi..sup.3+k.sub.2.phi..sup.5)).
[0095] The point where .phi.=0 indicates the center of the area of
the target "seen" by the lens. Values of .phi. other than zero
indicate a point on the target separated from the center point in
the in-track direction by that angle.
[0096] The rate of change of distance x across the focal plane with
respect to the in-track motion is given by: 1 x = F cos 2
[0097] The rate of change in the in-track direction as a function
of time is dictated by the target geometry and shown by the
relationship: 2 t = V cos H sin cos = V H sin cos 2
[0098] The rate of motion of a point of the target across the CCD
(dx/dt) is the product of the motion across the focal plane and the
change in the in-track direction. The charge transfer velocity is
always in track (along the direction of flight). Thus the charge
transfer velocity, V.sub.CCD, (in-track, i.e., perpendicular to the
principle plane or along line B from FIG. 10) is determined by: 3 V
CCD = x t = x t = VF H sin cos 2 cos 2 = VF H sin
[0099] Thus, V.sub.CCD is independent of .phi. when .theta.=0.
[0100] In order to determine the charge transfer velocity variation
along the principle plane, the tangential effects must be examined
instead of image transfer effects. The effective focal length is
given by: 4 F EFF = F cos
[0101] The change in position on the image sensor as a function of
the change in the cross-track direction is given by: 5 x = F
cos
[0102] Similar to the case with the in-track direction (i.e., along
line B), the velocity V.sub.CCD of the image along line P (of FIG.
10) is given by: 6 V CCD = x t = x t = F cos V H sin ( ) = FV H sin
( ) cos Where : t = V H sin ( )
[0103] These equations describe the image motions that are
compensated for imaging in the side oblique mode of operation.
These equations can be used to form a look-up table that is
utilized in the camera control processing system described below in
Section 7(b)(i).
[0104] 5. Forward Oblique Operation in Detail
[0105] Image motions and operations of the example FMC methods are
now described in detail for forward oblique look angles. According
to the present invention, forward oblique motions are compensated
by using a focal plane shutter for a graded FMC imager. The
advantages in using the graded FMC approach are described below by
way of a comparison to the previously discussed FMC approaches.
[0106] The forward oblique geometry is illustrated schematically in
FIG. 11. In the forward oblique mode of image collection, the
center line of the FPA is aligned to the direction of flight just
as in the vertical case. Now, however, the optical axis of the
camera is pointed upward from Nadir (i.e., the point directly below
the aircraft).
[0107] For a FPA operating in the forward oblique mode, as shown in
FIG. 12, the columns of pixels 1202 run from the "top" to the
"bottom" of the FPA 1203. The magnitude of charge motion in the
forward oblique orientation varies with position along the Y-Y axis
of FIG. 12. All columns share a common value for image and charge
velocities for any given point along the Y-Y axis, whereas all
column velocities are common along the X-X axis.
[0108] The variation in apparent in-track (along a column) image
motion is similar, but not equal, for all the rows. Image motion
varies from being slower at the "top" and being faster at the
"bottom". This image motion is compensated by using a graded FMC
approach.
[0109] By way of comparison, in forward oblique operation, average
FMC area imagers (oriented with their columns parallel to the Y-Y
axis of FIG. 12) move charge along all columns at the same rate.
Preferably, the rate is selected to be an average value of image
motion and correctly compensates only at one point along the Y-Y
axis.
[0110] According to the present invention, graded FMC imagers (with
their columns aligned parallel to the Y-Y axis) operate in the
forward oblique mode in the same manner as in the side oblique case
with one exception: the array (chip) is rotated 90.degree. with
respect to the direction of exposure slit travel. This method of
application is illustrated in detail in FIG. 13. A focal plane
shutter traverses the image area from top to bottom, and the charge
motion rate is varied as a function of the position of that shutter
along the Y-Y axis. For example, in FIG. 13A, position 1302
corresponds to the position of the exposure slit at the far point
of the frame, position 1303 corresponds to the position of the
exposure slit at the mid point of the frame, and position 1304
corresponds to the position of the exposure slit at the near point
of the frame. Here, the slit is oriented perpendicular to the
charge transfer columns and is moved parallel to the vector of
image motion. In addition, as shown in FIG. 13B, the forward image
motion rates vary as a function of V/H. As in the side oblique case
described above, only the image rate at the position of the slit is
"seen" by the array at any given time. The CCD line rate is made to
vary as a function of slit position to match the image motion rate,
as shown in FIG. 13C. At the conclusion of slit travel, the
complete array is read out at a maximum rate. Thus, according to
the present invention, ideal matching of image motion and charge
motion can be maintained.
[0111] Column-segmented imagers can also perform image motion
compensation in the forward oblique mode of operation by utilizing
a moving focal plane shutter as discussed above. In particular,
each column segment is clocked at the same rate because there is
essentially no differential motion across the row. However, the
complexities of multiple vertical clocks and potentially low yield
CCD architecture are still present.
[0112] The equations describing the forward oblique in-track image
motion, V.sub.CCD, are derived in a similar manner as described
above in Section 4. Referring back to FIG. 11, in this
geometry:
[0113] F=Focal length
[0114] H=Altitude (ALT)
[0115] V=Aircraft velocity
[0116] .phi.=cross track angle
[0117] .theta.=in track angle
[0118] .gamma.=depression angle
[0119] The image transfer relationship of a lens determines the
y-velocity of the image on the focal plane and is determined
by:
y=F(tan .theta.)
[0120] The image motion on the focal plane as a function of the
offset angle is given by: 7 y = F cos 2
[0121] The target geometry provides the rate of change of the image
Line of Sight (LOS) to target as follows: 8 t = V sin ( ) H sin ( )
= V H sin 2 ( )
[0122] Therefore, the velocity across the focal plane (in-track)of
the image is given by: 9 V CCD = y t = y t = FV H sin 2 ( ) cos
2
[0123] For values perpendicular to the principle plane (where
.theta.=0), the effective focal length F.sub.EFF is given by: 10 F
EFF = F cos
[0124] Therefore, the change in the position of a point y on the
focal plane is given by: 11 y = F cos
[0125] The target geometry provides: 12 t = V sin H sin cos = V H
sin 2 cos
[0126] Therefore, the velocity of the imaged point across the focal
plane is 13 V CCD = y t = VF H sin 2
[0127] Note that, as the above equation shows, for the forward
oblique case there is no dependence on cross-track angle .phi. when
.theta.=0. As in the side oblique case, these image motion
equations can be used to form a look-up table that is utilized in
the camera control processing system described below in Section
7(b)(i).
[0128] 6. Downward Looking (Vertical) Operation in Detail
[0129] A third method of operation for the graded FMC imager
according to the present invention is in the straight downward
looking (vertical) orientation. This is illustrated schematically
in FIG. 14. In this mode of operation, the columns of the FPA 1402
are oriented to flow from "top to bottom" of the perceived frame of
imagery. The rate of motion is the same at all points of the FOV
for an undistorted lens looking perfectly vertically.
[0130] According to the present invention, graded FMC imagers,
oriented with the columns parallel to the Y-Y axis, move charge
along all rows at the same rate. Image motion is fully compensated
since it is uniform. Neither a between-the-lens nor focal plane
shutter is required to achieve FMC, although either type of shutter
can be used.
[0131] The operation of a graded FMC detector array in the vertical
orientation is shown schematically in FIG. 15. Since the graded FMC
imager of the present invention is already equipped with a focal
plane shutter for the side and front modes, that shutter can be
used for the vertical mode of operation. Since image motion is
uniform, the charge transfer rate remains fixed throughout the
shutter scan time. The orientation of the shutter with respect to
the transfer columns is optional because it is not necessary to
limit exposure to a specific column as a function of image motion
rate. This flexibility makes it convenient to move the camera from
side oblique to downward looking without the need to rotate the
chip with respect to the slit. Similarly, a camera initially
oriented to operate in the forward oblique mode can be rotated down
for downward looking operation without the need to re-orient the
chip with respect to the slit.
[0132] In the preferred embodiment of the present invention, the
focal plane shutter can be oriented such that the transparent slit
traverses either side-to-side, or top-to-bottom (the orientation of
which is illustrated in FIG. 15). For the vertical orientation, the
magnitude of charge motion is constant with the position of the
exposure slit along the Y-Y axis and is fixed for a given row along
the X-X axis.
[0133] 7. Preferred Embodiment of the Present Invention
[0134] The present invention can be incorporated in numerous
different reconnaissance systems using current and
yet-to-be-developed cameras, focal planes, and electronics systems
adapted to provide a charge transfer rate that is uniform across
the CCD and is time-varying in coordination with the focal plane
shutter motion. The present invention is designed to utilize a
variety of possible focal plane arrays, CCD imaging electronics,
and system electronics to meet a specific set of desired
performance specifications and parameters of the operating
environment (e.g., ambient light conditions, aircraft velocity,
altitude, distance to target, etc.). It will be apparent to one
skilled in the art that alternative embodiments and structures may
be utilized to meet these specifications and parameters.
Additionally, these or alternative embodiments and/or structures
may be utilized to meet alternative specifications and/or
parameters.
[0135] a. Focal Plane Array
[0136] Although the invention can be utilized with numerous
different focal plane array configurations, a preferred focal plane
array configuration in this example operating environment is
provided below. After reading this description, it will become
apparent to those skilled in the art how to implement the invention
using alternative focal plane arrays.
[0137] i. Focal Plane Array Size
[0138] Focal plane array size is driven by performance requirements
and application parameters. Preferably, a detector array is large
enough to meet the application's field of view (FOV) requirement
and to achieve the desired performance (such as that defined by the
National Imagery Interpretation Rating Scale (NIIRS)) from a
specified altitude. For example, a high quality reconnaissance
system can produce a NIIRS index of approximately 8. In this
example operating environment, a General Image Quality Equation
(GIQE) is used with an estimate of the Ground Sampled Distance
(GSD) to produce this high NIIRS index value. For this first order
analysis, it is assumed that no image enhancement is used, a system
modulation transfer function (MTF) of 15% is achieved at Nyquist
and the typical GIQE signal-to-noise ratio, for an f/4 optical
system using a typical detector array at 20.degree. solar altitude,
is about 23:1. Applying desired light level and contrasts, this
results in a GSD of about 2.4 inches to produce NIIRS 8
performance. These above mentioned standards are known to those of
skill in the reconnaissance art.
[0139] In the example operational embodiment, the required
cross-track field of view from a 500 foot altitude is 115.degree.,
which produces cross track coverage of 1570 feet. If this coverage
is resolved uniformly, approximately 7850 pixels to sample at 2.4
inches per pixel are required to achieve NIIRS 8 throughout the
field of view (FOV). To achieve a desired coverage of 1400 (2747
feet), approximately 13,737 pixels are needed to sample the FOV
uniformly at NIIRS 8. In the along-track direction, the required
field of view is 75.degree. (767 feet). This coverage is sampled to
NIIRS 8 with approximately 3836 pixels. Therefore, a preferred
imager has between approximately 7850 and 13,737 pixels in the
cross-track direction and at least 3836 pixels in the along-track
direction. For example, an imager with the performance equivalent
to a 100 megapixel framing camera .+-.20% requires between
approximately 9000 by 9000 pixels and 11,000 by 11,000 pixels, or
another appropriate multiple. Thus a large scale, monolithic CCD is
the preferred focal plane array according to the present
invention.
[0140] Alternatively, the present invention can also utilize a
step-stare imager, which is known in the relevant art. The line of
sight of the imager can be repositioned by either moving the lens
assembly or by moving a mirror/prism in front of the lens. However
step-stare approaches introduce an added level of mechanical
complexity to a reconnaissance system. In addition, increased
coverage can be achieved by mechanically butting two arrays
together, eliminating the problems associated with step-staring.
However, the added cost of matching four chips, processing
complexity, and the loss at the critical central region due to
butting creates an undesirable tradeoff for a framing camera.
[0141] The focal plane arrays described above are provided for
example only. The above example illustrates the manner in which the
array size is chosen for a particular set of performance
specifications and application criteria. For other applications or
performance specifications, alternative focal plane array sizes can
be implemented as would be apparent to one of ordinary skill in the
art.
[0142] ii. Array Architecture
[0143] Eliminating complexity in the device design and processing
is essential to obtaining a sufficient yield to make an imager
economically viable. The preferred array architecture for the rows
in the main format area is the conventional three-phase structure,
which is known to be straightforward to process with high yields.
The column structure depends on the type of on-chip forward motion
compensation (FMC). The preferred embodiment of the present
invention utilizes a graded (i.e., non-segmented) FPA for use in a
system based on the graded FMC approach described above.
[0144] The functional layout for a 9216 pixel by 9216 pixel device
according to one embodiment is shown in FIG. 16. According to a
preferred embodiment of the present invention, the full-frame CCD
imager has an 8.1 centimeter (cm) by 8.1 cm main format area 1602
containing an array of 9216.times.9216 pixels. Each pixel size is
approximately 8.75 micrometers (.mu.m).times.8.75 .mu.m. The serial
register 1604 at the bottom of main format 1602 has four
detector/amplifier outputs 1610-1613. The sampling rate for each
output is approximately 25 megapixels/second. A greater or lesser
number of amplifier outputs can be utilized depending on the
readout requirements.
[0145] As described above in connection with FIG. 3 and the
description of conventional CCD operation in section 3, during the
integration or exposure period, an electronic representation of an
image is formed when incident photons create free electrons that
are collected within the individual photosites. These
photoelectrons are collected locally by the bias action of the
three "V" electrodes 1606 and the column boundaries formed by the
P+ channel-stop implants. These column boundaries are illustrated
as channel stops 1705 in FIG. 17. FIG. 17 also illustrates that in
a preferred embodiment, poly V-phase gates 1706 with side bus
connections are utilized.
[0146] After an integration time, a shutter (such as a focal plane
shutter or a between the lens shutter described above) closes to
block illumination on the focal plane and the readout cycle begins.
During readout, the complete image is shifted out by changing the
potentials on electrodes V.sub.1, V.sub.2, and V.sub.3 in a
sequence which causes packets of signal charge to move line by line
into the horizontal output register.
[0147] Referring back to FIG. 16, during each line readout time
period, the voltage on the electrodes comprising the horizontal
shift registers are changed or "clocked" to shift pixel charges
into the output detector and amplifier structure (1610-1613).
One-by-one the charge packets are dumped on a small conductive area
called the floating diffusion (FD). There the charge packets change
the FD potential by an amount equal to nq/C, where n is the number
of electrons/packet, q is the electron charge in coulombs and C is
the FD capacitance. The FD voltage is sensed and buffered to the
signal output by an on-chip FET source follower structure located
within a detector/amplifier, such as amplifier 1610.
[0148] When FMC is required, the normally static bias condition for
the "V" electrode voltages are modified to cause charge packets to
transfer at a rate corresponding to the rate of image motion normal
to the line direction of the array matrix. The FMC charge shift is
always in the same direction. The rate of charge shift can vary as
the slit opening in the focal plane shutter moves from top to
bottom of the CCD format. The FMC line shifts that occur during the
exposure period are small in number compared to the total lines of
the CCD format.
[0149] In one embodiment, an approach to multiport operation is to
separate only the horizontal output register 1604 into segments,
where each segment contains an output detector/amplifier structure.
A unique feature of this output register design is a taper region
between the last active format line and throughput register. This
eliminates any gap between columns of the active format.
[0150] Alternatively, a column-segmented CCD, having as many as 16
segments can be utilized in order to achieve sufficient FMC to
produce good quality images. For example, FIG. 18 depicts the
architecture of a column-segmented CCD array 1802 having N column
segments. If the number N of column segments is 16, these 16 column
segments thus require an increase in the number of separate
variable V clocks from 3 to 48, with an associated increase of the
off-chip drive electronics. Additionally, as shown in insert 1804,
a column-segmented design requires metalization in the imaging area
which significantly reduces CCD yield. Moreover, a column-segmented
CCD requires an increased number of contact holes (the locations
where the metal makes contact to the underlying structures), as
shown in insert 1806. As discussed below, this added complexity is
required in order to vertically clock the column-segmented focal
plane array.
[0151] iii. Vertical Clocking
[0152] FIG. 19 further illustrates a portion of a column-segmented
CCD shown above in insert 1806 of FIG. 18. Note that array 1902
includes metal straps 1904 over the corresponding channel stops
with thru-hole metal-poly contacts, such as contact hole 1906. For
very large area arrays, side bussed polysilicon gate lines as
illustrated in FIG. 17 are much easier to process with high yields
than the more complex metal strapped structure shown in FIG. 19.
Metal strapping, which is usually done to achieve very fast V
clocking, is a required feature for column-segmented arrays.
[0153] The yield limitations of metal strapping arise from the need
to make small diameter openings in the insulating dielectric
coatings such that the metal straps make contact with each of the
polysilicon gate lines (see e.g., contact hole 1906). As shown in
FIG. 19, for a three phase CCD, every row of N pixels contains N/3
contact regions (one every third pixel). In small pixel devices
(<12 .mu.m.sup.2) with 1/2 to 1 .mu.m overlap of the phase
gates, there is very little room in each of the three gates to etch
down and contact the first poly layer. This is further aggravated
by alignment inaccuracies between .phi..sub.1, .phi..sub.2,
.phi..sub.3 and the contact layer.
[0154] The contact problems are exacerbated when the array image
section is segmented, such as in array 1802 from FIG. 18. To
minimize loss of information, the gap between segments must be kept
small: yet the metal over the channel stop must be kept from
causing an electrical short between adjacent segments of the same
phase.
[0155] As mentioned above, a graded FMC imager having a side bussed
polysilicon gate structure without metal strapping or format
segmentation, such as the array structure illustrated in FIG. 17,
is preferred for full-frame imager production. According to a
preferred embodiment of the present invention, for the 9216 by 9216
array (totaling approximately 85 megapixels) operating at an output
of 100 megapixels per second, the full format can be read out in
85/100 or 0.85 seconds. The corresponding line shift time is
0.85/9216 or 92.2 microseconds (.mu.s), which is the maximum time
allowed for clocking each line to the output serial register.
[0156] Burst clocking, as is the case for TV cameras where the line
is only shifted during horizontal blanking, can require even
shorter shift time. Although some feedthrough of clock into the
video does occur, this signal contamination is line coherent and
readily removed with a digital stored compensating signal. FIG. 20
shows a model used to determine the vertical poly line time
constant for the preferred 9216 pixel.times.9216 pixel CCD. A time
constant (T) value of 11.3 microseconds (.mu.s) is calculated based
on the pixel resistance (Rpix) and pixel capacitance (Cpix) values
listed on the right hand side of model 2002. This T value fully
supports clocking with a preferred 92.2 .mu.s line time
interval.
[0157] iv. CCD Imager
[0158] Characteristics for the CCD imager in a preferred embodiment
of the present invention are listed below in Table 1. Other CCD's
with other characteristics can be used as would be apparent to
those of skill in the art.
1TABLE 1 FULL-FRAME IMAGE SENSOR SPECIFICATIONS Active Pixels per
line 9216 Active lines (progressive readout) 9216 Pixel size, .mu.m
8.75 .times. 8.75 Image format, mm 80.64 .times. 80.64 Number of
output registers 4 (on one side) Number of outputs 4 Data rate 100
MHz Resolution: MTF at Nyquist 50% Q Saturation (100% pixels) 70 k
electrons RMS noise electrons 18 Dynamic range 72 dB Pixel random
nonuniformity 3% Dark current (20.degree. C., 1 second) <480
electrons Fixed pattern noise (20.degree. C., 1 second) <75
electrons QE, 550 nm 29% 650 nm 44% 750 nm 35% 850 nm 20% Number of
clocks (vertical) 3 Number of overscan columns 1/segment Number of
black reference columns 2 .times. 20 (20 L .times. 20 R) Number of
black reference lines 20 (bottom) Clock amplitude (vertical) 10 V
Total number of lines 9236 Number of clocks (horizontal) 2 Clock
amplitude (horizontal) 10 V Conversion factor, .mu.V/electron 3
Linearity 99% Pixel rate per output 25 MHz
[0159] b. System Electronics
[0160] A block diagram of an example system electronics
architecture is illustrated in FIG. 21. In this example
architecture, the camera back electronics 2100 comprise an imaging
section 2104 and an electronics unit 2106. According to this
example architecture, the imaging section 2104 includes imaging
electronics 2108 comprising an analog processor 2110,
thermo-electric (TE) cooler controller 2116, shutter exposure
control 2114 and the FPA (or CCD) drive electronics 2112. These
electronics are used to command and communicate with the focal
plane array 2123, in conjunction with the FMC methods discussed
above. As described above, a lens 2120 collects the target image
onto FPA 2123. A focal plane shutter 2121 traverses across FPA 2123
at a rate corresponding to image motion of the objects viewed in
the scene. The rate at which shutter 2121 traverses FPA 2123, as
well as the slit width of shutter 2121 are determined based on the
commands of imaging electronics 2108. The TE cooler controller 2116
controls a TE cooler 2114, which maintains the operating
temperature of FPA 2123. The camera control electronics also
include a power supply module 2145.
[0161] The camera back electronics 2100 also include an electronics
unit 2106 to ultimately process the image of the target scene as
viewed by FPA 2123. The electronics unit 2106 includes the camera
host processor (or CPU) 2140, two digital preprocessors 2130 and
2131, the data compression electronics 2134, the tape recorder
interface 2138, and a DCRSI 240 recorder 2139. The digital
preprocessors 2130 and 2131 utilize ASIC technology. In addition,
the camera CPU 2140 controls the CCD clock speed and its variation
to implement FMC. The functionality of these individual components
is discussed below in detail. Except where noted below, these
electronics can be conventional electronics that are known in the
art. Alternative architectures can be implemented to perform these
functions, as would be apparent to one of skill in the art.
[0162] i. Camera Control Process
[0163] The electronics illustrated in FIG. 21 are used to perform
FMC according to the present invention. An exemplary FMC method
utilizing these electronics, and based on the image motion
equations described above in sections 4 and 5 is shown in FIG.
22.
[0164] FIG. 22 is a flow diagram that describes the camera control
process according to one embodiment. The control of light level
involves decision points and simple look-up tables in steps 2206
and 2208. Referring to both FIGS. 21 and 22, the process starts at
the start of a frame at step 2202, where the input light level to
the camera is measured by a light sensor, such as light sensor
2122. This light level is compared to a standard light level at
step 2204. For example, the chosen standard light level is 277 foot
candles, which corresponds to a solar altitude of approximately
3.degree. above the horizon. At high solar altitudes (>277 foot
candles), the decision is made to use the Primary Exposure Time
Look-up Table to determine exposure time (step 2208). For longer
exposure times (>2 milliseconds (ms)) required for low ambient
lighting conditions, the shutter speed is slowed down to
approximately 50 inches/second in order to keep the slit width
narrow enough for graded FMC. For short exposure times (.ltoreq.2
ms), a shutter speed of approximately 300 inches/second is
selected.
[0165] For the lowest ambient light conditions, an exposure time is
determined from the Low Light Exposure Time Look-up Table (step
2206). This table utilizes the instantaneous values of aircraft
velocity, altitude, and camera look angle at the start of each
frame in addition to the light level. One example of when the low
light exposure time look-up table is used is when the measured
light level is <277 foot candles, such as when the solar
altitude is less than 3.degree.. Other thresholds can be defined
for the lowest ambient light condition based on mission
requirements.
[0166] The output of the exposure time look-up steps 2206 or 2208
is the optimal exposure time and selection of a shutter speed. The
corresponding slit width is determined in step 2210, where the slit
width chosen is a product of the exposure time and the shutter
speed. Correspondingly, the exposure time can be determined by
dividing the slit width by the shutter speed.
[0167] Once exposure time is determined, for each frame, the CCD
clocking profile is calculated in step 2212 to accomplish FMC. In
one embodiment, this profile is determined by the host processor
2140 (in step 2212). In step 2212 a look-up table based on the
in-track image motion rate equations described above in sections 4
and 5 (depending on the oblique mode of operation) is used with the
following inputs: exposure time; aircraft velocity, V; aircraft
altitude, H; depression angle of camera (fixed for flight); camera
installation location (fixed for flight); shutter trigger pulse;
and focal length. In a preferred embodiment, the process is
re-initiated at the start of each camera frame. The resulting FMC
clocking signal is sent to imaging electronics 2108 to perform FMC
(step 2214).
[0168] ii. CCD Drive Electronics
[0169] The CCD drive electronics (such as CCD drive electronics
2112 from FIG. 21) comprise two essential parts, a timing
generator, and a clock drive stage. These elements are shown in
detail in FIG. 23, which represents an exemplary design to perform
optimum CCD and system clocking. The timing generator is
responsible for two functions, CCD readout and Forward Motion
Compensation (FMC).
[0170] As shown in FIG. 23, a 150 MHz master clock signal is
divided by six (at location 2302) to provide the local 25 MHz pixel
clock, from which all CCD clocks and digital controls are derived.
The horizontal counter 2304 provides a time base for pixel counting
operations, which include defining the vertical shift interval at
FPA vertical clock generator 2310 and clocking of the horizontal
output CCD registers at FPA horizontal clock generator 2311. The
vertical counter 2306 likewise provides a time base in the vertical
direction of the CCD. Alternatively, higher frequency clocks may
also be utilized to provide for greater smoothness of steps to the
vertical clocks.
[0171] Multi-tap delay lines 2314a-b are employed on the horizontal
and vertical clocks to permit minute refinements in phase
relationships, allowing optimization of vertical and horizontal
charge transfer efficiency.
[0172] Additionally, the 25 MHz clock is buffered and
skew-compensated to provide synchronous timing to both the video
sampling analog-to-digital converters (ADC) and the subsequent
digital preprocessing.
[0173] Synchronization signals are generated at frame
synchronization unit 2319, in the form of frame and line syncs
2320a-b, respectively. These sync signals synchronize the digital
preprocessors 2130 and 2131 (in FIG. 21) to the quantized video
stream.
[0174] FIG. 24 represents example line and frame timing output
pulses 2404 and 2402, respectively. For the example operating
conditions described above, the line and frame timing are derived
as follows:
[0175] Line Timing
[0176] 9216 pixels/line.div.4 segments=2304 pixels per segment
[0177] 2304 active pixels+10 pre/post scan=2314 pixels/line
[0178] 2314 pixels.div.25 MHz+10 .mu.s (vertical clock
interval)=102.6 .mu.s/line=9747 lines/second
[0179] Frame Timing
[0180] 9216 active lines+20 pre/post scan=9236 lines/frame
[0181] Readout time=9236.times.102.6 .mu.s=0.9472 second
[0182] Maximum exposure time (at 50 ips)=0.0638 second
[0183] Frame time=0.9472+0.0638=1.011 second/frame=0.989
frames/second.
[0184] iii. Forward Motion Compensation (FMC)
[0185] As explained above in section 3, during the CCD exposure
interval, the charge pattern formed in the CCD corresponding to the
optical image is shifted at the rate the image is moving in order
to compensate for the effect of the aircraft's forward motion. This
charge pattern movement is accomplished by applying variable clock
rate vertical transfer signals to the CCD during the exposure time.
Referring back to FIG. 21, these signals are generated in the clock
waveform generators of CCD drive electronics 2112, and are
controlled in both frequency and duration by CPU 2140. The V/H
signal is interpreted by the host processor 2140 to provide
rate-based CCD vertical shift commands (seen in FIG. 22, CCD clock
control 2214) to a timing generator (explained in detail above), in
turn commanding the CCD to shift for motion compensation. FMC
occurs during the integration time of the CCD. At this time, all
CCD clocks are in an inactive state until commanded by the
processor to perform a vertical shift for FMC.
[0186] The vertical clock drivers move the charge through the
integration sites, and into the horizontal output register (see
FIG. 16). According to a preferred embodiment, the vertical clock
drivers 2310 supply a 10 volt peak-to-peak drive waveform into 4
nano-fared (nf) gate capacity. A typical CCD readout rate according
to the present invention is approximately 9747 Hz. However, during
FMC, the vertical transfer can go as high as 12 KHz. For example,
in a preferred embodiment, a known MIC4451 driver (manufactured by
MICREL Semiconductor, Inc., of San Jose, Calif.) can be used as the
vertical driver 2310. Other known drivers can also be utilized
based upon cost and performance considerations.
[0187] The horizontal clock drivers 2311 move the charge through
the horizontal register to the floating diffusion (FD) section (of
amplifiers 1610-1613 in FIG. 16) where the output voltage signal is
formed. These horizontal drivers supply up to a 10 volt
peak-to-peak waveform into 125 pico-fared (pf) at the 25 MHz pixel
rate. A discrete component circuit known in the art can be utilized
as no satisfactory monolithic circuit drivers are currently
available. The horizontal and vertical drivers each have adjustable
offset and gain capabilities, to permit tuning to optimal
performance for each individual array.
[0188] iv. Shutter Exposure Control
[0189] In a preferred embodiment, CCD exposure is controlled by two
focal plane functions: the width of the focal plane shutter
exposure slit and the speed of the exposure slit (e.g., shutter
2121 illustrated in FIG. 21). In alternative embodiments, exposure
can be controlled by either function alone. As mentioned above, in
a preferred embodiment, the width of the slit is approximately 0.1
inch to 0.5 inches, and the speed of the exposure slit is
approximately 300 inches per second for shorter exposure times and
50 inches per second for longer exposure times. Thus, the speed of
the exposure slit can be constant across the FPA or varied,
depending upon the necessary forward motion compensation
required.
[0190] In the illustrated embodiment, the existing light sensor
output (such as from light sensor 2122 in FIG. 21) is digitized by
the shutter exposure control circuitry 2114. An example exposure
control block diagram is shown in FIG. 25. The light sensor signal
2502 is converted to look-up table values located in look-up table
2508 after buffering (at buffer 2504) and digitization (at ADC
2506) to separately drive the speed 2510 and slit width 2512 of the
shutter. An example exposure control profile is shown in FIG. 26,
where exposure time is plotted as a function of slit width. An
example plot of optimized exposure times for the CCD, in standard
daylight conditions, is a function of the sun angle as represented
in FIG. 27. It should be noted that illumination levels change
dramatically at dawn and dusk conditions. Also, the step indicated
by an asterisk (*) in FIG. 27 is due to a filter inserted in front
of the imager.
[0191] As described above in connection with FIG. 22, the CCD's
exposure time (t), which is the time it takes the slit to pass any
single pixel, is given by:
t=w/s
[0192] where w=slit width in inches, and s=shutter speed in
inches/second. In a preferred implementation, the exposure time is
set by the incident light. Because the final signal amplitude is
controlled by an automatic gain control (AGC) function in the
digital preprocessing sections 2130 and 2131, this open loop
control function, which mimics that used on the film camera,
represents the preferred approach.
[0193] For example, for light levels down to 3.degree. solar
altitude (i.e., the 277 foot candles level), the exposure time
follow the curves shown in FIG. 27. Below these light levels
(essentially at dusk), the length of exposure is limited by the
calculated image motion variations across the chip. Camera focal
length, V/H and depression angle are used to select proper look-up
table exposures at the very low light levels as previously shown in
FIG. 22, step 2206.
[0194] v. Digital Preprocessor
[0195] In a preferred embodiment, as shown in detail in FIG. 28,
digital preprocessing is performed on two identical Circuit Card
Assemblies (CCAs) 2802 and 2804. CCA 2802 processes inputs from
channels 1 and 2, and CCA 2804 processes inputs from channels 3 and
4. These CCAs respectively correspond to digital preprocessors 2130
and 2311, illustrated in block diagram form in FIG. 21.
[0196] Because CCA 2802 and CCA 2804 are similar, only the elements
comprising CCA 2802 are described. In one embodiment, CCD pixel
data is stored in high speed Static Random Access Memory (SRAM)
configured as First In/First Out (FIFO) memory (see location 2810).
This FIFO memory operates as line buffers to facilitate replacing
defective pixels with nearest neighbor processing. Defective pixels
are identified during laboratory testing and characterization of
the CCD. Locations of these defective pixels are stored as (X,Y)
coordinates in Programmable Read Only Memories (PROM) 2815 on the
digital preprocessor board. These locations are compared to the
(X,Y) coordinates of the FPA as it is read out. When a match
occurs, the defective pixel is replaced by a known nearest neighbor
processing routine. This implementation reduces the hardware
complexity required for defective pixel correction for the 9216
pixel.times.9216 pixel FPA.
[0197] In a preferred embodiment, memory addressing is generated by
a Field Programmable Gate Array (FPGA) based timing and address
generator 2814, which runs synchronously and in tandem with the CCD
timing generator 2812 on the EO module CCA 2802. CCD timing
generator 2812, which is synchronized by the frame and line sync
2809, uses the 25 MHz video sampling clock. This synchronous
operation eliminates any possibility of injecting uncorrelated
noise into the video.
[0198] The memories 2815 are read out into the Application Specific
Integrated Circuit (ASIC) 2820. The 12-bit video data at full scale
is equivalent to saturation of the CCD. For most operational
scenarios, the video sensor signal occupies only a fraction of an
ADC's 12-bit dynamic range. Specular reflections, manifest as high
intensity transients, are subtracted out with a digital low pass
filter within ASIC 2820. Haze, which manifests itself as a DC level
(i.e., no counts in the lower bins of the gray scale histogram), is
also subtracted out at ASIC 2820. The Automatic Gain Control (AGC)
functionality in ASIC 2820 detect the maximum and minimum amplitude
of the signals, maintaining a running average over multiple lines.
The AGC gain is then adjusted to take full advantage of the 8-bit
dynamic range. This 12 to 8-bit conversion eliminates the
non-essential video information while preserving the actual imagery
data. Further, the 8-bit data is in the proper format for the image
bus control ASIC 2824 and the data compressor 2825. The AGC action
optimizes sensor performance and reduces the raw data rate by 30%
without degradation of the original image.
[0199] Illumination (vignetting) correction is performed at chip
2822 by applying correction coefficients (e.g., for the 1", 3" and
12" lenses) to the gain input of AGC ASIC 2820. During factory
calibration, curves of the illumination roll-off across the FPA are
established. The inverse of these curves is programmed into
Programmable Read Only Memory (PROM) 2815, which provides these
gain corrections to the video.
[0200] As noted, the electronic components described above can be
conventional electronics that are known in the art. Alternative
architectures can be implemented to perform the aforementioned
functions, as would be apparent to one of skill in the art.
[0201] 4. Conclusion
[0202] While various embodiments of the present invention have been
described above, it should be understood that they have been
presented by way of example only, and not limitation. Thus, the
breadth and scope of the present invention should not be limited by
any of the above-described exemplary embodiments, but should be
defined only in accordance with the following claims and their
equivalents.
* * * * *