U.S. patent application number 10/603707 was filed with the patent office on 2005-03-31 for multi-spectral ladar.
Invention is credited to Evans, Bruno J., Mitra, Pradip.
Application Number | 20050068517 10/603707 |
Document ID | / |
Family ID | 34375631 |
Filed Date | 2005-03-31 |
United States Patent
Application |
20050068517 |
Kind Code |
A1 |
Evans, Bruno J. ; et
al. |
March 31, 2005 |
MULTI-SPECTRAL LADAR
Abstract
A multi-spectral detector for use in a passive/active system and
a method for use in identifying an object in a field of view are
disclosed. The multi-spectral detection system comprises an
optically dispersive element, a detector array, and an integrated
circuit. The optically dispersive element is capable of separating
received LADAR radiation and radiation received from a scene into a
plurality of spectral components and distributing the separated
spectral components; and a detector array. The detector array
includes a plurality of detectors capable of detecting the LADAR
radiation; and a plurality of detectors capable of detecting the
spectral components of the scene radiation. The integrated circuit
is capable of generating a plurality of electrical signals
representative of predetermined characteristics of the detected
LADAR radiation and the detected spectral components. The method
comprises passively detecting scene radiation employing a detector
array; and actively detecting LADAR radiation through the detector
array in parallel with passively detecting the scene radiation.
Inventors: |
Evans, Bruno J.; (Keller,
TX) ; Mitra, Pradip; (Colleyville, TX) |
Correspondence
Address: |
WILLIAMS, MORGAN & AMERSON, P.C.
10333 RICHMOND, SUITE 1100
HOUSTON
TX
77042
US
|
Family ID: |
34375631 |
Appl. No.: |
10/603707 |
Filed: |
June 25, 2003 |
Current U.S.
Class: |
356/5.01 ;
356/141.5 |
Current CPC
Class: |
G01S 7/4863 20130101;
G01S 17/894 20200101; G01S 7/4802 20130101; G01S 7/4816 20130101;
G01J 3/10 20130101; G01J 3/0264 20130101; G01J 3/2823 20130101;
G01J 3/36 20130101; G01J 3/0208 20130101; G01J 3/18 20130101; G01J
3/26 20130101; G01J 2003/2826 20130101; G01J 3/0291 20130101; G01J
3/027 20130101; G01J 3/0289 20130101; G01S 7/481 20130101; G01S
17/87 20130101 |
Class at
Publication: |
356/005.01 ;
356/141.5 |
International
Class: |
G01C 003/08; G01C
001/00; G01B 011/26 |
Claims
1. A multi-spectral detector for use in a passive/active system,
comprising: an optically dispersive element capable of separating
received LADAR radiation and radiation received from a scene into a
plurality of spectral components and distributing the separated
spectral components; and a detector array including: a plurality of
detectors capable of detecting the LADAR radiation; and a plurality
of detectors capable of detecting the spectral components of the
scene radiation; and an integrated circuit capable of generating a
plurality of electrical signals representative of predetermined
characteristics of the detected LADAR radiation and the detected
spectral components.
2. The detector of claim 1, wherein the optically dispersive
element comprises a diffraction grating or a linear variable
filter.
3. The detector of claim 2, wherein the optically dispersive
element is integrated with the detector array.
4. The detector of claim 1, wherein the optically dispersive
element is integrated with the detector array.
5. The detector of claim 1, wherein the detectors capable of
detecting the LADAR radiation or the detectors capable of detecting
the spectral components of the scene radiation comprise QWIPs or
EQWIPs.
6. The detector of claim 1, wherein the detectors capable of
detecting the LADAR radiation or the detectors capable of detecting
the spectral components of the scene radiation have varied widths
and are separated by varied pitches.
7. The detector of claim 6, wherein the detectors capable of
detecting the LADAR radiation or the detectors capable of detecting
the spectral components of the scene radiation comprise QWIPs or
EQWIPs.
8. The detector of claim 1, wherein the detector array is
integrated with the integrated circuit.
9. The detector of claim 8, wherein the optically dispersive
element is integrated with the detector array.
10. A multi-spectral detector for use in a passive/active system,
comprising: means for distributing a plurality of spectral
components of received LADAR radiation and radiation received from
a scene; means for detecting the distributed LADAR radiation; means
for detecting the spectral components of the infrared radiation;
and means for generating a plurality of electrical signals
representative of predetermined characteristics of the detected
LADAR radiation and the detected spectral components.
11. The detector of claim 10, wherein the distributing means
diffracts the received LADAR and radiation.
12. The detector of claim 11, wherein the distributing means
comprises a diffraction grating.
13. The detector of claim 10, wherein the distributing means
comprises a diffraction grating.
14. The detector of claim 10, wherein the distributing means is
integrated with the detecting means.
15. The detector of claim 10, wherein the detecting means comprises
QWIPs or EQWIPs.
16. The detector of claim 10, wherein detecting means comprises a
plurality of detectors have varied widths and are separated by
varied pitches.
17. The detector of claim 10, wherein the detecting means is
integrated with the generating means.
18. An imaging system, comprising: a laser capable of transmitting
LADAR radiation; a multi-spectral detector for use in a
passive/active system, comprising: an optically dispersive element
capable of separating received LADAR radiation and radiation
received from a scene into a plurality of spectral components and
distributing the separated spectral components; and a detector
array including: a plurality of detectors capable of detecting the
LADAR radiation; and a plurality of detectors capable of detecting
the spectral components of the scene radiation; and an integrated
circuit capable of generating a plurality of electrical signals
representative of predetermined characteristics of the detected
LADAR radiation and the detected spectral components; and a
processor for processing the electrical signals.
19. The imaging system of claim 18, wherein the optically
dispersive element comprises a diffraction grating or a linear
variable filter.
20. The imaging system of claim 18, wherein the optically
dispersive element is integrated with the detector array.
21. The imaging system of claim 18, wherein the detectors capable
of detecting the LADAR radiation or the detectors capable of
detecting the spectral components of the scene radiation comprise
QWIPs or EQWIPs.
22. The imaging system of claim 18, wherein the detectors capable
of detecting the LADAR radiation or the detectors capable of
detecting the spectral components of the scene radiation have
varied widths and are separated by varied pitches.
23. The imaging system of claim 18, wherein the detector array is
integrated with the integrated circuit.
24. A method for use in identifying an object in a field of view,
comprising: passively detecting radiation from a scene, the
detection employing a detector array; and actively detecting LADAR
radiation through the detector array in parallel with passively
detecting the scene radiation.
25. The method of claim 24, wherein passively detecting scene
radiation includes passively detecting infrared radiation.
26. The method of claim 25, wherein passively detecting infrared
radiation includes passively detecting hyperspectral infrared
radiation.
27. The method of claim 24, wherein passively detecting scene
radiation includes passively detecting hyperspectral scene
radiation.
28. The method of claim 24, further comprising receiving the scene
and LADAR radiation through the same optical train.
29. The method of claim 28, wherein detecting the scene and LADAR
radiation includes separating the received LADAR and scene
radiation into a plurality of spectral components and distributing
the separated spectral components across the detector array.
30. The method of claim 24, further comprising generating a
plurality of electrical signals representative of predetermined
characteristics of the detected LADAR radiation and the detected
spectral components.
31. An apparatus for use in identifying an object in a field of
view, comprising: means for passively detecting scene radiation
employing a detector array; and means for actively detecting LADAR
radiation through the detector array in parallel with passively
detecting the scene radiation.
32. The apparatus of claim 31, wherein the means for passively
detecting scene radiation includes means for passively detecting
infrared radiation.
33. The apparatus of claim 31, wherein the means for passively
detecting scene radiation includes means for passively detecting
hyperspectral scene radiation.
34. The apparatus of claim 31, further comprising means for
receiving the scene and LADAR radiation through the same optical
train.
35. The apparatus of claim 31, further comprising means for
generating a plurality of electrical signals representative of
predetermined characteristics of the detected LADAR radiation and
the detected spectral components.
36. A method, comprising: receiving LADAR and scene radiation from
a field of view; separating the received LADAR and scene radiation
into a plurality of spectral components; directing the spectral
components to respective detectors; detecting the spectral
components; and generating an electrical signal representative of
predetermined characteristics of the detected spectral
components.
37. The method of claim 36, wherein receiving the scene radiation
includes receiving infrared radiation.
38. The method of claim 36, wherein receiving the scene radiation
includes receiving hyperspectral scene radiation.
39. The method of claim 36, wherein receiving the scene and LADAR
radiation includes receiving the scene and LADAR radiation through
the same optical train.
40. An apparatus, comprising: means for receiving LADAR and scene
radiation from a field of view; means for separating the received
LADAR and scene radiation into a plurality of spectral components;
means for directing the spectral components to respective
detectors; means for detecting the spectral components; and means
for generating an electrical signal representative of predetermined
characteristics of the detected spectral components.
41. The apparatus of claim 40, wherein the means for receiving the
scene radiation includes means for receiving infrared
radiation.
42. The apparatus of claim 40, wherein the means for receiving the
scene radiation includes means for receiving hyperspectral scene
radiation.
43. The apparatus of claim 40, wherein the means for receiving the
scene and LADAR radiation includes means for receiving the scene
and LADAR radiation through the same optical train.
44. The apparatus of claim 31, further comprising: means for
separating received LADAR radiation and radiation received from a
scene into a plurality of spectral components; and means for
distributing the separated spectral components across the detector
array.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention pertains to remote sensing and, more
particularly, to a combined passive/active system, e.g., a passive
infrared/active LADAR system.
[0003] 2. Description of the Related Art
[0004] A need of great importance in military and some civilian
operations is the ability to quickly detect and identify objects,
frequently referred to as "targets," in a "field of view." A common
problem in military operations, for example, is to detect and
identify targets, such as tanks, vehicles, guns, and similar items,
which have been camouflaged or which are operating at night or in
foggy weather. It is important in many instances to be able to
distinguish reliably between enemy and friendly forces. As the pace
of battlefield operations increases, so does the need for quick and
accurate identification of potential targets as friend or foe, and
as a target or not.
[0005] Techniques for identifying targets have existed for many
years. For instance, in World War II, the British developed and
utilized radio detection and ranging ("RADAR") systems for
identifying the incoming planes of the German Luftwaffe. RADAR uses
radio waves to locate objects at great distances even in bad
weather or in total darkness. Sound navigation and ranging
("SONAR") has found similar utility and application in environments
where signals propagate through water, as opposed to the
atmosphere. While RADAR and SONAR have proven quite effective in
many areas, they are inherently limited by a number of factors. For
instance, RADAR is limited because of its use of radio frequency
signals and the size of the resultant antennas used to transmit and
receive such signals. Sonar suffers similar types of limitations.
Thus, alternative technologies have been developed and
deployed.
[0006] One such alternative technology is laser detection and
ranging ("LADAR"). Similar to RADAR systems, which transmit radio
waves and receive radio waves reflected from objects, LADAR systems
transmit laser beams and receive reflections from targets. Systems
that both transmit signals and receive reflections, such as RADAR
and LADAR, are known as "active systems." Because of the short
wavelengths associated with laser beam transmissions, LADAR data
exhibits much greater resolution than RADAR data.
[0007] Lasers are also used in "semi-active" laser ("SAL") systems.
With the SAL system, a narrow laser beam is produced and
transmitted toward a target. The laser radiation is typically
generated and transmitted from a laser designator aircraft manned
by a forward operator or by a ground-based operator. The operator
directs the laser radiation to a selected target, thereby
designating the target. The laser radiation reflected from the
target can then be detected by the laser seeker head of a missile,
aircraft, or other platform located remote from both the target and
the laser energy transmitter. Because the transmitter is not
located on the same platform as the receiver, such systems are not
considered "active" systems. Although SAL systems have proven
effective, the next generation of receiver platforms are expected
to fly to ranges well beyond the range of imaging sensors on board
the designator platform.
[0008] "Passive" systems are also employed. In passive systems, a
detector is used to sense energy produced or reflected from the
objects in the scene of interest. One example of a passive system
is an infrared sensor that detects heat produced by objects.
Alternatively, a light sensor, such as an array of photodiodes, may
be used to sense the scene light reflected by the objects in the
field of view. Passive, multi-spectral detection in narrow spectral
bands is recognized to a highly effective approach for target
detection in a thermally cluttered or camouflaged environment.
Correlated hyper-spectral radiometric measurements in the
atmospheric windows in the short wavelength infrared ("SWIR") and
mid-wave infrared ("MWIR") bands have been shown to be effective in
detecting low contrast, partially obscured and camouflaged targets.
However, when considered in other applications, passive systems
have been found to have a number of limitations. For instance, data
provided by passive systems is sometimes difficult to interpret,
since a given level of light intensity may indicate an object with
low reflectivity, or the same intensity level may indicate a
distant object with high reflectivity.
[0009] Various combinations of active, semi-active, and passive
systems have been attempted in the past. However, these
combinations usually have many problems limiting their
practicability. For instance, SAL systems do not carry the
advantages of passive infrared detection while suffering the
limitations of active LADAR systems. Combined active LADAR and
passive IR systems overcome this problem, but frequently suffer
from other problems. For instance, space and weight limitations are
usually severe because of the platform requirements on which the
systems are deployed. Attempts to combine active LADAR and passive
IR systems have problems with these constraints because, e.g., they
employ separate optical trains or separate detectors for the LADAR
and IR radiation.
[0010] The present invention is directed to resolving, or at least
reducing, one or all of the problems mentioned above.
SUMMARY OF THE INVENTION
[0011] The invention includes, in various aspects, a multi-spectral
detector for use in a passive/active system and a method for use in
identifying an object in a field of view. The multi-spectral
detection system comprises an optically dispersive element, a
detector array, and an integrated circuit. The optically dispersive
element is capable of separating received LADAR radiation and
radiation received from a scene into a plurality of spectral
components and distributing the separated spectral components; and
a detector array. The detector array includes a plurality of
detectors capable of detecting the LADAR radiation; and a plurality
of detectors capable of detecting the spectral components of the
scene radiation. The integrated circuit is capable of generating a
plurality of electrical signals representative of predetermined
characteristics of the detected LADAR radiation and the detected
spectral components. The method comprises passively detecting scene
radiation employing a detector array; and actively detecting LADAR
radiation through the detector array in parallel with passively
detecting the scene radiation.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The invention may be understood by reference to the
following description taken in conjunction with the accompanying
drawings, in which like reference numerals identify like elements,
and in which:
[0013] FIG. 1 is a system diagram incorporating the present
invention showing the major operational parts of the system as used
in a field environment;
[0014] FIG. 2 depicts a passive infrared/active LADAR system
constructed and operated in accordance with the present invention
to acquire data about a field of view through an optics package
aboard a platform shown therein;
[0015] FIG. 3 is a block diagram of a sensor system capable of
active and passive acquisition of LADAR and infrared data,
respectively, in accordance with the present invention;
[0016] FIG. 4 conceptually illustrates selected elements of the
system in FIG. 2;
[0017] FIG. 5A and FIG. 5B conceptually depict the detector array
and read-only integrated circuit, respectively, of the focal plane
array of the detector of FIG. 4 in top, plan views;
[0018] FIG. 6 is a partial, sectional view of a first
implementation of a detector in the detector array in FIG. 5A;
[0019] FIG. 7 is a partial, sectional view of a second
implementation of a detector in the detector array in FIG. 5A;
[0020] FIG. 8A-FIG. 8B illustrate, in block diagrams, the
electronics for the read-only integrated circuit in FIG. 5B in
alternative embodiments;
[0021] FIG. 9 is a conceptualized top, plan view of a detector
array for the focal plane array alternative to the detector array
of FIG. 5A; and
[0022] FIG. 10 is a flow diagram illustrating data collection,
processing and transfer in accordance with one embodiment of the
present invention.
[0023] While the invention is susceptible to various modifications
and alternative forms, specific embodiments thereof have been shown
by way of example in the drawings and are herein described in
detail. It should be understood, however, that the description
herein of specific embodiments is not intended to limit the
invention to the particular forms disclosed, but on the contrary,
the intention is to cover all modifications, equivalents, and
alternatives falling within the scope of the invention as defined
by the appended claims.
DETAILED DESCRIPTION OF THE INVENTION
[0024] Illustrative embodiments of the invention are described
below. In the interest of clarity, not all features of an actual
implementation are described in this specification. It will of
course be appreciated that in the development of any such actual
embodiment, numerous implementation-specific decisions must be made
to achieve the developers' specific goals, such as compliance with
system-related and business-related constraints, which will vary
from one implementation to another. Moreover, it will be
appreciated that such a development effort, even if complex and
time-consuming, would be a routine undertaking for those of
ordinary skill in the art having the benefit of this
disclosure.
[0025] FIG. 1 illustrates an imaging system 100 constructed and
operated in accordance with the present invention. The imaging
system 100 includes a laser 110 that produces a laser beam 115, a
detector subsystem 120, a processor 125 connected through a line
130 for operating the laser 110, and a display 135 connected via a
line 140 to the processor 125. The processor 125 is also connected
through a bi-directional data and control line 145 to the detector
subsystem 120 and processes data collected by the imaging system
100 from an exemplary scene 150 of an outdoor area. The illustrated
scene includes trees 155 and 160, a military tank 165, a building
170, and a truck 175. The tree 155, tank 165, and building 170 are
all located at the same distance from the system 100. Note,
however, that the scene 150 may have any composition. One
application of the imaging system 100, as shown in FIG. 1, is to
detect the presence of the tank 165 within the scene 150 under
adverse conditions such as darkness, smoke, haze, and other
barriers (such as the trees and building) which would prevent a
direct visible optical viewing of the tank 165.
[0026] As described in further detail below, the imaging system 100
images the scene 150 using both two-dimensional, passively detected
data and three-dimensional, actively detected data. In the
two-dimensional passive image data set, each pixel (picture
element) includes passive intensity information corresponding to
the magnitude of the passive energy collected for that pixel. The
second mode initializes the laser 110 to produce an active LADAR
image by detecting the reflected laser energy. This produces a
three-dimensional active image data set in which each pixel of the
image has both z (range) and active intensity data. In both the
passive and active image data sets, discussed more fully below,
each pixel has x (horizontal) and y (vertical) coordinates.
[0027] The active LADAR can be implemented using any suitable LADAR
transceiver currently known in the art, modified as described below
to implement the invention. The operation of one such transceiver
200 is conceptually illustrated in FIG. 2. The LADAR transceiver
200 includes the laser 110 of FIG. 1 as well as some portions of
the detector subassembly 120. In the embodiment illustrated in FIG.
2, the imaging system 100 is packaged on a platform 210 and
collects data from a field of view 225, shown in FIG. 2, within the
scene 150, shown in FIG. 1.
[0028] The laser signal 115 is transmitted by an optics package
(not shown) of the LADAR transceiver 200 on the platform 210 to
scan a geographical area called a scan pattern 220. Each scan
pattern 220 is generated by scanning elevationally, or vertically,
several times while scanning azimuthally, or horizontally, once
within the field of view 225 for the platform 210 within the scene
150, shown in FIG. 1. The scan patterns are sometimes, and will be
hereafter herein, referred to as "footprints." FIG. 2 illustrates a
single elevational scan 230 during the azimuthal scan 240 for one
of the footprints 220. Thus, each footprint 220 is defined by a
plurality of elevational scans 250 such as the elevational scan 230
and the azimuthal scan 240. The principal difference between the
successive footprints 220 is the location of the platform 210 at
the start of the scanning process. An overlap 260 between the
footprints 220 is determined by the velocity of the platform 210 in
the direction of an arrow 265. The velocity, depression angle of
the sensor with respect to the horizon, and total azimuth scan
angle of the LADAR platform 210 determine the footprint 220 on the
ground.
[0029] The laser signal 115 is typically a pulsed signal and may be
either a single beam or a split beam. Because of many inherent
performance advantages, split beam laser signals are typically
employed by most LADAR systems. A single beam may be split into
several beamlets spaced apart from one another by an amount
determined by the optics package (not shown) aboard the platform
210 transmitting the laser signal 115. Each pulse of the single
beam is split, and so the laser signal 115 transmitted during the
elevational scan 250 in FIG. 2 is actually, in the illustrated
embodiment, a series of grouped beamlets. The optics package aboard
the platform 210 transmits the laser signal 115 while scanning
elevationally 250 and azimuthally 240. The laser signal 115 is
continuously reflected back to the platform 210, which receives the
reflected laser signal. Suitable mechanisms for use in generation
and acquiring active LADAR signals are disclosed in:
[0030] U.S. Pat. No. 5,200,606, entitled "Laser Radar Scanning
System," issued Apr. 6, 1993, to LTV Missiles and Electronics Group
as assignee of the inventors Nicholas J. Krasutsky, et al.;
[0031] U.S. Pat. No. 5,224,109, entitled "Laser Radar Transceiver,"
issued Jun. 29, 1993, to LTV Missiles and Electronics Group as
assignee of the inventors Nicholas J. Krasutsky, et al.; and
[0032] U.S. Pat. No. 5,285,461, entitled "Method for Increasing
Single-Pulse Range Resolution," issued to Feb. 8, 1994, to Loral
Vought Systems Corporation as assignee of the inventors Nicholas J.
Krasutsky, et al.
[0033] However, any suitable mechanism known to the art may be
employed.
[0034] One particular embodiment of the invention includes a LADAR
seeker head ("LASH") aboard a flying submunition such as the
platform 210 in FIG. 2. The manner in which this LASH generates,
transmits, and receives a LADAR scan pattern is fully disclosed and
claimed in U.S. Pat. No. 5,200,606, first mentioned above. This
particular LASH splits a single 0.2 mRad l/e.sup.2 laser pulse into
septets with a laser beam divergence for each spot of 0.2 mRad with
beam separations of 0.4 mRad. The optics package includes fiber
optical array (not shown) having a row of seven fibers spaced apart
to collect the return light. The fibers have an acceptance angle of
0.3 mRad and a spacing between fibers that matches the 0.4 mRad far
field beam separation. An elevation scanner (not shown) spreads the
septets vertically by 0.4 mRad as it produces the vertical scan
angle. The optical transceiver including the scanner is then
scanned azimuthally to create a full scan raster.
[0035] FIG. 2 also conceptually illustrates the passive acquisition
of infrared data from the same field of view 225 in which the
footprints 220 are defined. The infrared data is extracted from the
passive detection of infrared electromagnetic radiation,
represented by the arrow 270. The radiation 270 may be, e.g., light
waves reflected from or thermal emissions of objects (e.g., the
tank 165, building 170, truck 175) within the field of view 225.
The radiation 270 will be multi-spectral, i.e., it will include
electromagnetic waves of many frequencies. The radiation 270
comprises a portion of the scene conditions within the field of
view 225, and do not result from any transmission from the platform
210. Hence, the waves 270 are "passively" detected. As used herein,
the phrase "scene radiation" shall mean any radiation existing
within the field of view but not having been actively introduced by
the imaging system 100.
[0036] Both the passive image data set and the near simultaneously
collected active LADAR image data set are transmitted via the line
145 to the processor 125. Within the processor 125, data from both
the passive image data set and the active image data set are
combined to produce a multi-dimensional image data set. Because the
passive and active image data sets are collected by the same sensor
assembly 105 at approximately the same time (i.e., are temporally
aligned), the pixels within the two image data sets are also
spatially aligned, that is, objects in the scene 150 are in the
same positions x and y for both the passive image and the active
LADAR image.
[0037] However, the illustrated embodiment provides a hyperspectral
resolution within this broadband detection scheme that can yield
greater discrimination in identifying objects within the field of
view. The illustrated embodiment does this by (1) passively
detecting multiple bands of infrared radiation, and (2) doing so at
the same time it actively detects the returned LADAR radiation.
Because they are detected at the same time, the image data sets
derived from the detected radiation can be readily combined. The
resulting combined image data, which comprises a multi-dimensional
image data set, is provided as a signal through the line 140 for
producing an image on the display 135. This image, preferably, can
be rotated on a display screen in a manner that produces a
three-dimensional representation of objects within the scene 150,
in particular a threat object such as the tank 165. The
three-dimensional representation of an object further provides an
indication of the thermal intensity of the object, e.g., a
three-dimensional image. Additionally, the processor 125 can
process the multi-dimensional image data set to identify or aid in
identifying threat objects such as the tank 165. The processing
identifies tank 165 as not only a tank, but whether it is a friend
or foe tank. This additional processing substantially decreases the
probability of fratricide or being killed by friendly fire.
[0038] FIG. 3 conceptually illustrates one particular embodiment of
a system 300 with which the detector subassembly 120 of FIG. 1 may
be implemented in accordance with the present invention. The system
300 comprises a collection of operative elements that work in
conjunction to collect, detect and process received infrared and
LADAR radiation 310. The received radiation 310 is received by an
optical train 320 comprising a steerable group of lenses (not
individually shown) that collect and focus the received radiation
310. The radiation 325 collected by the optical train 320 is passed
to an optically dispersive element 330. The optically dispersive
element 330 separates the received radiation 310 into its spectral
components 335, as is discussed more fully below, which are then
provided to a focal plane array 340. The focal plane array 340
senses each of the spectral components 335 produced by the
optically dispersive element 330, also as is discussed more fully
below, and generates a corresponding signal output 345. The signal
output 345 contains both actively acquired, three-dimensional LADAR
data and passively acquired, two-dimensional data.
[0039] The signal output 345 of the focal plane array 340 is
provided to a processor 350. The processor 350 may be, for example,
a heterogenous digital signal processor ("DSP") system which can be
expanded to have hundreds of individual DSPs. Any suitable
processor known to the art may used to implement the processor 350.
The processor 125 also generates elecromechanical command and
control signals for the optically dispersive elements 330 and/or
the optical train 320 to control their behavior in a manner more
fully discussed below. In some embodiments, the processor 125 may
delegate this task to a controller (not shown).
[0040] FIG. 4 conceptually illustrates one particular
implementation of the optical train 320, optically dispersive
element 330, and the focal plane array 340 of the system 300 in
FIG. 3. In the illustrated implementation, the optically dispersive
element 410 is a diffraction grating, but alternative embodiments
(not shown) may employ, e.g., a linear variable filter instead. The
received radiation 430 includes the laser radiation actively
generated and received by the active LADAR subsystem (not shown) of
the platform 110 and the electromagnetic radiation passively
received from the scene conditions as described above. The received
radiation 430 is directed through the lens 400, which focuses the
received radiation 430 onto the optically dispersive element 410.
The received radiation 430 is reflected by the optically dispersive
element 410 onto the focal plan array 420. In so doing, the
optically dispersive element 410 separates the received radiation
430 into its constituent spectral components 440, each having a
different, respective wavelength .lambda..sub.0-.lambda..sub.n.
[0041] Thus, by way of example and illustration, the diffraction
grating with which the optically dispersive element 410 is
implemented in FIG. 4 comprises but one means for separating the
received radiation 430 into the spectral components 440. The
optically dispersive element 410 separates by diffraction, but
other separation mechanisms may be employed. A linear variable
filter, for instance, as was mentioned above, separates by
filtering the received radiation 430 into spectral components 440.
Thus, a linear variable filter comprises a second means separating
the received radiation 430 into the spectral components 440. The
present invention therefore admits variation in the mechanism and
manner in which the spectral components 440 are obtained from the
received radiation 430.
[0042] In one particular implementation of the illustrated
embodiment, n=4, and the optically dispersive element 410, shown in
FIG. 4, diffracts the received radiation 430 into five spectral
components 440. The laser signal 100, shown in FIG. 3, has a
wavelength of either 1.5 .mu.m or 2.1 .mu.m. Thus,
.lambda..sub.0=1.5 .mu.m or .lambda..sub.0=2.1 .mu.m. The focal
plane array 420 therefore passively detects, e.g., infrared,
radiation in four narrow spectral bands. These four spectral bands
have peak wavelengths of 2.3 .mu.m, 3.5 .mu.m, 4.2 .mu.m, and 4.6
.mu.m, respectively. Thus, in this particular implementation,
.lambda..sub.1=1.5 .mu.m, .lambda..sub.2=2.1 .mu.m,
.lambda..sub.3=1.5 .mu.m, and .lambda..sub.4=2.1 .mu.m,
respectively. Note, however, that the invention is not limited to
these values for .lambda..sub.0-.lambda..sub.n. For instance, the
laser signal 100 may generally have any "eye-safe" wavelength.
Similarly, the wavelengths for the passively detected infrared
radiation 170 may be varied from the number provided for this
particular implementation.
[0043] One particular embodiment of the focal plane array 420,
first shown in FIG. 4, is more particularly illustrated in FIG. 5A
and FIG. 5B. The focal plane array 420 includes, in the illustrated
embodiment, a read out integrated circuit ("ROIC") 450 and a
detector array 460. FIG. 5A shows the "top" of the focal plane
array 420, i.e., the detector array 460, and FIG. 5B shows the
"bottom" of the focal plane array, i.e., the ROIC 450, in plan
views. The detector array 460 comprises a plurality of detectors
520 arrayed in columns 530.sub.0-530.sub.n. Each detector 520
represents one pixel of data. One column of detectors (e.g.,
530.sub.0) is dedicated to active imaging with LADAR data and the
remaining columns (e.g., 530.sub.1-530.sub.n) are dedicated to
passive infrared, multi-spectral imaging in the illustrated
embodiment.
[0044] In illustrated embodiment, however, the detectors 420 are
quantum well infrared photodetectors ("QWIPs"). The QWIPs 420 are
fabricated from GaAs/AlGaAs or InGaAs/AlGaAs or InGaAs/GaAs
multiple quantum well materials ("MQW"), possess attributes
suitable for both active and passive detection. QWIPs based on
GaAs/AlxGa1-xAs and other III-V MQW materials present the benefits
of high material uniformity, low noise and mature processing
techniques. This detector technology is readily amenable to
producing large format FPAs with high uniformity and operability
and exhibits excellent long-term stability. One difficulty has been
low optical coupling due to a quantum mechanical selection rule,
which only allows absorption of radiation with an electric field
component normal to the MQW layers. To facilitate effective
coupling of the radiation various approaches have been developed
that include adding diffraction gratings, random reflectors and
etched diffractive resonant optical cavity ("DROC") structures. The
DROC approach, also called the Enhanced QWIP ("EQWIP") has resulted
in quantum efficiency ("QE") as high as 50% in long wavelength
infrared ("LWIR") detectors the highest achieved in QWIPs.
[0045] Because EQWIPs are true resonant devices, the full width at
half maximum ("FWHM") of the detector spectral response is also the
narrowest of all QWIPs. In the present invention, the DROC
parameters are tuned to span the 1.5-14 .mu.m wavelength range
while retaining high detector performance. In this approach, as
discussed elsewhere, the adjacent rows of pixels detect at
specifically tuned peak wavelengths in narrow spectral bands but
cover a relatively broad spectral band across the FPA. Spectral
response FWHM as narrow as 0.4 .mu.m at 8.5 .mu.m has been achieved
despite the use of relatively low-resolution contact
photolithography based processing. EQWIPs offer several benefits
for high-speed LWIR detectors, including: lower detector
capacitance due to the reduced detector volume; tunability to match
laser wavelength, thereby permitting preset multi-wavelength
detection on the same chip so that laser wavelength can be varied;
efficient optical coupling; and higher operating temperature by 15
K over conventional QWIPs.
[0046] QWIPs are intrinsically high-speed detectors due to their
short carrier lifetime (.tau..sub.life.about.5 ps). The high
frequency response of QWIPs has been measured by microwave
rectification and optical heterodyne techniques. Rectification
frequency of 33 GHz has been measured, see Liu et al., 32 IEEE J.
Quant. Electron. 1024 (1996), for QWIPs with 16 wells. With optical
heterodyne techniques on a 100 well QWIP a frequency response of 83
GHz has been measured, see Liu et al., 67 Appl. Phys. Lett. 1594
(1995).
[0047] Other detector technologies may be also employed. The
individual detectors 520 may be implemented as practically any kind
of avalanche photodiode, Indium Antimonide, or quantum well
infrared photodetector ("QWIP") technology. QWIPs, in particular,
are well known in the art and may be employed. QWIPs suitable for
passive infrared, multi-spectral detection in the present invention
are, for example, disclosed in the following:
[0048] U.S. Pat. No. 6,452,187, entitled "Two Color Grating Coupled
Infrared Photodetector," issued Sep. 7, 2002, to Lockheed Martin
Corporation as assignee of the inventors Lewis T. Claiborne, et
al.; and
[0049] U.S. Pat. No. 6,180,990, entitled "Hyper spectral Radiation
Detector," issued Jan. 30, 2001, to Lockheed Martin Corporation as
assignee of the inventors Lewis T. Claiborne, et al.
[0050] Note that these technologies may employ alternative
materials and fabrication techniques. QWIPs, for instance,
frequently are fabricated in Gallium Arsenide (GaAs). Thus, the
present invention is not limited to APDs fabricated in
Mercury-Cadmium-Tellurium (HgCdTe).
[0051] FIG. 6 and FIG. 7 depict two particular, alternative
structures for the detectors 520 in FIG. 5. Note that, in the
illustrated embodiment, the detectors 520 all have the same
construction, although this is not necessary to the practice of the
invention. FIG. 6 and FIG. 7 illustrate individual pixels 600, 700,
respectively, as may be used to implement the detectors 520 in
section views. The pixels 600, 700 can be replicated to produce a
complete two-dimensional imager detector array 460 of practically
any desired size, such as 64 pixels by 64 pixels or larger. It is
to be noted that both pixels 600, 700 are simultaneously sensitive
to both passive and active detection. The detector subsystem 120
uses a single focal plane array 420 sensitive to both passive and
active detection as opposed to two separate imagers each sensitive
to only the passive or active detection.
[0052] Referring now to FIG. 6, the pixel 600 includes an upper
layer array common contact 605, one element of which is represented
by the contact 605a. This forms a common electrical contact for the
signals detected by the pixels 600 of the detector array 460. This
design is based on III-V semiconductor materials and is preferably
composed of either Gallium Arsenide/Aluminum Gallium Arsenide
(GaAs/AlGaAs) or Indium Gallium Arsenide/Indium Phosphorous
(InGaAs/InP). In a pixel 600 using either of these materials, the
passive portion of the imager comprises a photoconductor based on
inter-subband transitions. This type of detector is a QWIP. The
absorbing transition may be in either the conduction or valence
energy bands. Due to the absorption mechanism, an optical coupler,
typically a diffraction grating, is used to increase sensitivity.
This grating may be either a surface grating or the much deeper
cavity-based grating as used in an enhanced QWIP or EQWIP, such as
described in U.S. Pat. No. 5,539,206, entitled "Enhanced Quantum
Well Infrared Photodetector," issued Jul. 23, 1996, to Lockheed
Martin Corporation as assignee of the inventor Thomas R.
Schimert.
[0053] The MWIR or LWIR absorbing region 615 comprises a stack of
multiple quantum wells as described in U.S. Pat. No. 5,539,206.
They are placed between the array contact 605 and an intermediate
contact 610. The signal produced by absorption of the MWIR or LWIR
radiation is generated between the contacts 605 and 610. In one
particular implementation, the GaAs quantum wells are 30-50
angstroms wide and separated by 300-600 angstrom AlGaAs barriers.
The wells and contact layers are doped in the range of
5.times.10.sup.17 to 5.times.10.sup.18. Alternatively, the
structure may be composed of InGaAs wells and InP barriers.
[0054] Still referring to FIG. 6, a SWIR absorbing region 625
preferably may comprise InGaAs. The region 615 is positioned
between the intermediate contact 610 and a contact 620. A SWIR
radiation signal is produced between contacts 610 and 620. The SWIR
absorber region 625 is preferably composed of
In.sub.0.53Ga.sub.0.47As that is lattice matched to InP. This
composition provides a spectral response that will strongly absorb
radiation at the preferred LADAR laser wavelength of 1.55 .mu.ms. A
typical thickness for the SWIR absorber region 625 is 2-4 .mu.m.
The SWIR absorber region 625 in conjunction with intermediate
contact 610 and contact 620 can form a photoconductor, a
photodiode, or an avalanche photodiode.
[0055] Turning now to FIG. 7, the pixel 700 is illustrated. This
implementation is composed of the II-VI semiconductor material
HgCdTe, and uses a double stack for absorbing the two different
bands of IR radiation. The incident IR radiation must be incident
on the SWIR absorbing stack first with the longer wavelength
radiation (MWIR or LWIR) passing through to be absorbed in the
second stack. Both the SWIR and the MWIR or LWIR stacks are formed
to comprise P-n junctions, preferably hetero-junctions. The sensor
700 has a supporting substrate 710 comprising CdZnTe. A SWIR
receiver comprises a P region 714 and an n region 720, both
comprising HgCdTe. The longer wavelength incident IR radiation is
absorbed by regions 725 and 730, both of which comprise HgCdTe.
[0056] Further referring to FIG. 7, the short wavelength IR
radiation is absorbed by the Pn junction comprising regions 714 and
720. The signal is conducted away from the pixel through a
conductor 735 and a conductor 740. For the SWIR radiation, the
region 725 functions essentially as a conductor. A longer
wavelength IR radiation signal is produced by the structure
comprising regions 725 and 730 with the longer wavelength IR
radiation signal generated between the conductor 740 and a
conductor 745. Typical compositions for the various layers are
Hg.sub.0.8Cd.sub.0.2Te for the LWIR absorbing layer,
Hg.sub.0.7Cd.sub.0.3Te for the MWIR absorbing layer, and
Hg.sub.0.35Cd.sub.0.65Te for the SWIR absorbing layer.
[0057] FIG. 8A illustrates one particular embodiment of the ROIC
450, first shown in FIG. 4, within focal plane array 420 for
collecting passive IR data and active LADAR data. The passive image
signal from a particular detector 520 (shown in FIG. 5A)
representing a pixel 802 is transferred through a line 804, through
an amplifier 806, to a charge integrator 808. The passive image
signal is integrated for a predetermined integration time. The
integrated signal for a particular pixel 802 is readout when the
address decoded by pixel address decoder 810 corresponds to the
particular pixel 802. The integrated signal is amplified by output
amplifier 812 that drives passive column data bus 814. The analog
amplified integrated signal is then digitized by analog-to-digital
converter 816. The resultant digital signal is corrected for
passive imager non-uniformity and other possible system
non-uniformities in passive non-uniformity corrector ("NUC") 818.
The corrected passive digital signal corresponding to pixel 802 is
then stored in data store 820 creating a two-dimension passive
image data set. The passive image data set is then transferred to
processor 125 via line 145.
[0058] Several input amplifier circuit designs can be used for the
amplifier 806. These include direct injection, buffered direct
injection, and capacitive transimpedance amplifiers, among others.
Each of these designs has advantages and disadvantages, primarily
based upon the detector array 460 design, the operating temperature
of the imager, and allowable power dissipation.
[0059] The charge integrator 808 likewise has several design
possibilities. These include charging a capacitor to a
predetermined voltage and draining the capacitor charge through an
imager pixel during the integration time if the device is a
photoconductor, charging a capacitor if the device is photodiode,
and others that handle high or low dark currents, subtraction of
dark currents, etc. As with the input amplifier 806, selection of
the charge integrator design is based upon the detector array 460
design, the operating temperature of the imager, and power
dissipation.
[0060] The passive non-uniformity corrector 818 can implement
several different non-uniformity correction techniques. For
devices, such as QWIPs and EQWIPs, which have a degree of linear
uniformity across the detector array 460, a simple offset
correction can suffice. For an detector array 460 that has devices
with varying linearity across the detector array 460, both offset
and gain correction can be implemented. For applications requiring
greater non-uniformity correction, a table look-up process can be
implemented. Possible system non-uniformity sources include a
cosine.sup.4 term due to the optical train 320.
[0061] The active LADAR image signal from a particular pixel 802 is
transferred through a line 822, through an amplifier 824, to an
array of sample and hold (S&H) circuits 826. The timing of each
S&H circuit is triggered by a counter 828, which itself is
triggered by the start of a pulse from laser 110. The S&H
circuits 826 therefore each correspond to a range in the imaged
scene 150. Each S&H circuit 826 for a particular pixel 802 is
readout when the address decoded by pixel address decoder 810
corresponds to the particular pixel 802. The signal from each
S&H circuit 826 is sequentially amplified by output amplifier
830 that drives active column data bus 832. The analog amplified
S&H signal from each S&H circuit 826 is then digitized by
analog-to-digital converter 834. The resultant digital signal is
corrected for active imager non-uniformity, and other possible
system non-uniformities in active non-uniformity corrector 836. The
digitized corrected active image signals corresponding to each
S&H circuit 826 from pixel 802 are passed through a threshold
detection circuit 838. The threshold detected S&H signals, and
their corresponding times from counter 828 are then stored in data
store 820 creating an active image data set. The active image data
set is then transferred to processor 125 via line 145. The
processor 125, using the corresponding times, calculates the range
to an object for each pixel 802 thus creating a three-dimension
active image data set.
[0062] Issues relating to the active channel amplifier 824 and the
non-uniformity corrector 104 are similar to those of the passive
portion of the ROIC 450. Further sources of active system
non-uniformity are laser-speckle and any effects associated with
the polarized laser radiation.
[0063] The array of S&H circuits 826 is subject to several
limitations. The number of S&H circuits 826 will be a trade off
between the space available for each pixel 802 and the desire for a
large depth (range) of image collection with high resolution. For
military type targets, a resolution of less than a meter can be
required. Coupled with potentially large targets, 32 S&H
circuits 826 are the minimum with more than 32 being desired for
greater range resolution. The absolute range resolution also limits
the arrays of S&H circuits 826 as the time between each
sampling command is limited. An upper limit to the sampling rate is
approximately 500 MHz using current integrated circuit processing
technology. This results in a range resolution of one-third
meter.
[0064] The requirements for the portion of the ROIC 450 that
processes the active LADAR signals are driven by a
burst-illumination LADAR. A burst illumination LADAR system
requires only a single laser pulse. The return pulse waveform is
sampled at predetermined times creating a step-wise continuous
waveform. The difference in time between each sampling .tau..sub.1
determines the range resolution. Sampling the return pulse waveform
every two nanoseconds (500 MHz) provides one-third meter range
resolution. Due to a limited number of S&H circuits, the
starting time for the sampling process is determined by the range
to the potential target, the number of S&H circuits 826 in the
S&H circuit array 826 for each pixel within the semiconductor
detector array 460, and the required range resolution. Preferably,
half the S&H circuits 826 for each pixel will be used on either
side of the optimal time To based upon the range to the potential
target. This will center the potential target within the active
image data set. The sampled waveform for each pixel within the
semiconductor detector array 460 is read out for subsequent
processing to produce the active image data set. Alternatively, a
preliminary laser pulse may be used in a range finding mode. This
allows higher resolution sampling of the burst illumination LADAR
pulse as the optimal time To is more accurately known. If a system
is only used for targets within a given fixed range, then To may be
established a priori.
[0065] In operation, the preliminary passive search culminates with
a laser range finding pulse in the illustrated embodiment. A
preliminary search passive image data set is updated concurrently
with a laser pulse for temporal alignment between the active and
passive image data sets. In a preferred embodiment, the
semiconductor detector array 460 comprises 64 pixels in both the x
and y coordinates for a total of 4,096 pixels for active imaging.
With 32 S&H circuits 826 per pixel and a data read out rate of
1 MHz, the data cycle time for a single pulse is approximately 160
milliseconds to read out the active image data set. The passive
image data set will also be available for subsequent processing at
the conclusion of the active data set's required 160 millisecond
readout.
[0066] As semiconductor processing advances, higher sampling and
readout rates will be possible. The present invention can then be
used to create real time, video rate three-dimensional imagery. The
three-dimensional imagery can be displayed with either the
magnitude of the IR emission, or the magnitude of the LADAR return
signal.
[0067] Note that the ROIC 450 admits variation in implementation.
Consider the circuit 850, which is an individual circuit for an
individual one of the pixels 802 (shown in FIG. 8A). A threshold
detector 852 activates a S&H circuit 854 when the magnitude of
the analog output 856 of the detector exceeds a predetermined
value. Multiple memory buffers 858 (only one indicated) allow
multiple return pulsed to be captured for each pixel 802, which
will be useful when imaging through obscurants that produce
multiple returns. The time at which the threshold detector 852
actuates the S&H circuit 854, as measured by the counter
(timer) 860, can also be stored in the registers 862 (only one
indicated). The ROIC 450 includes a master clock (e.g., the counter
860) that is accessible by each unit cell (i.e., circuit 850) of
the ROIC 450. When the threshold detector 852 activates the S&H
circuit 854, the content of the counter 860 is latched into the
registers 862. The latched value is the range offset and indicates
a rough estimate of when the return pulse is received. Further
processing of the sampled analog signal determines the fine range
and pixel reflectivity. The passive image can still be collected as
described above in connection with FIG. 8A.
[0068] Returning to FIG. 4, as was mentioned above, in the detector
array 460, one column 430.sub.x (e.g., 430.sub.0) is dedicated to
three-dimensional LADAR imaging while the remaining columns
430.sub.x (e.g., 430.sub.1-430.sub.n) are dedicated to passive
infrared, multi-spectral imaging. The one column 430.sub.x
dedicated to three-dimensional LADAR imaging operates at high speed
in a sample and hold mode. The remaining columns 430.sub.x permit
long integration times for the passive imaging. In one particular
implementation, the circuitry for the passive columns 430.sub.x
uses standard integrating unit cells. The active column 430.sub.x
uses the sampling technique discussed above.
[0069] The focal plane array 320 will more typically be implemented
using multiple sets of active/passive detector columns 430.sub.x in
the detector array. FIG. 9 illustrates one such detector array 900.
The detector array 900 includes a plurality of columns
430.sub.0-430.sub.n arranged in sets 910, each set containing the
one column (e.g., column 430.sub.0) of active LADAR detectors and
multiple columns of passive infrared detectors (e.g., columns
430.sub.1-430.sub.n). Thus, the focal plane array 320, in this
implementation operates with one column 430.sub.0 in every set 910
dedicated to three-dimensional LADAR imaging in conjunction with a
1.5 .mu.m or 2.1 .mu.m laser. The other four columns
430.sub.1-430.sub.4 of every set 910 are dedicated to passive
infrared imaging in four narrow bands with peak wavelengths at 2.3
.mu.m, 3.5 .mu.m, 4.2 .mu.m, and 4.6 .mu.m. As was noted earlier,
however, other wavelengths may be employed in alternative
embodiments.
[0070] The QWIPs 420 can be relatively large in this particular
implementation (e.g., 40 .mu.m by 80 .mu.m) where the pitch is 40
.mu.m in the spatial direction and 80 .mu.m in the spectral
direction. This pitch in the spatial direction permits high
resolution imaging while this pitch in the spectral dimension
self-compensates for beam jitter and allows for maximum circuitry
for signal processing the ROIC 350 unit cells. As will be
appreciated by those skilled in the art having the benefit of this
disclosure, however, the various values for
.lambda..sub.0-.lambda..sub.n will affect the dimensions of the
structures for the focal plane array 320.
[0071] Wavelength responsivity for different portions of the focal
plane array 320 employing the detector array 900 can be varied by
altering the physical dimensions of the detector's diffractive
resonant-structure for different sections of the focal plane array
320. One such technique, employed for hyperspectral detection of
passive IR radiation only, is disclosed in U.S. Pat. No. 6,180,990,
entitled "Hyper spectral Radiation Detector," issued Jan. 30, 2001,
to Lockheed Martin Corporation as assignee of the inventors Lewis
T. Claiborne, et al. However, although not apparent from this
reference itself, the principle of varying physical dimensions can
be extrapolated to active LADAR detection in addition to
hyperspectral passive IR detection. See also P. Mitra, et al.,
"Multispectral Quantum Well Infrared Photodetectors," Applied
Physics Letters, May 12, 2003.
[0072] The principle physical dimensions to be altered are the
pitch, which is the distance between the center lines of the
detectors 420, and the width of each of the detectors 420. By
changing these physical dimensions, each portion of the detector
array 900 having selected dimensions is tuned to be responsive to a
particular band of infrared radiation. By stepping the pitch to
greater values from one pitch segment (not shown) of the detector
array 900 to the next, and by varying the width of the detectors
420 within each pitch segment, the principle band of radiation
sensitivity can be shifted progressively so that a substantial
portion of a particular spectrum can be collected and each band of
information analyzed individually.
[0073] The x-dimension (rows) of the detector array 900 provide a
linear dimension for the field of view 225 (shown in FIG. 2). With
the movement of this linear dimension over the field of view 225, a
two dimensional image can be produced by periodically sampling each
of the detectors 420 within the detector array 900 dedicated to
passive infrared detection. As was mentioned above, interspersed
among the passive detectors 420 are active detectors 420, which
contemporaneously detect reflected LADAR radiation. Each column in
the detector array 900 essentially corresponds to a pixel element
of the scene 150, but with multiple contiguous bands of information
being collected substantially at one time, thereby providing much
greater information about what is present within the field of view
225, in contrast to collecting only a single band of radiation.
[0074] Returning to FIG. 3, the optically dispersive element 310 is
shown to be external to the focal plane array 320 in the
illustrated embodiment. However, as was discussed relative to the
detector of FIG. 6, a diffraction grating may be employed with the
QWIPs to increase sensitivity. These diffraction gratings may be
surface gratings or the much deeper cavity-based surface gratings.
Both types of gratings are known to the art, and the embedded
diffraction grating is more fully disclosed in U.S. Pat. No.
5,539,206, entitled "Enhanced Quantum Well Infrared Photodetector,"
issued Jul. 23, 1996, to Lockheed Martin Corporation as assignee of
the inventor Thomas R. Schimert. These diffraction gratings can
also be employed as the optically dispersive element 310 of FIG. 3,
and so the optically dispersive element 310 may be external to the
focal plane array, as shown, or embedded therein.
[0075] Once the data has been acquired, it can be processed in any
suitable manner known to the art. Note that, in some embodiments,
the processing may include treatments to, e.g., enhance the
resolution of the LADAR data. The acquired data may also be
employed for any suitable purpose. In a military context, the data
could be employed, for instance, in an automated target recognition
("ATR") system. Or, the data may be processed to enhance accuracy,
or for use in other techniques of target identification. The data
may also be processed in way suitable for civilian use, depending
on the particular implementation. In one particular embodiment,
however, the active LADAR data and the passive infrared data are
"fused," to generate a single data set.
[0076] In one particular embodiment, the data is processed as set
forth in U.S. Pat. No. 5,511,015, entitled "Double-accumulator
Implementation of the Convolution Function," issued Apr. 23, 1996,
to Loral Vought Systems Corporation as assignee of the inventor
Stuart W. Flockencier. In general, the data is continuously
digitized and processed. The largest return is stored. The stored
data is then passed through a convolution peak detector function to
extract pulse position from the data. The entire captured pulse is
used to minimize the effects of signal strength variation, noise,
and distortions. A programmed template, designed to match the
nominal pulse shape, is convolved with the data in steps of one
data sample. When the data most accurately matches the template,
the convolver output is maximum, giving the pixel range. The
maximum convolver output value at this point is proportion to the
return pulse energy, which yields the time of flight and intensity
for the returned pulse.
[0077] Thus, data extracted from the active detectors 420 can then
be fused with the data sampled from the passive detectors 420 to
generate a more detailed image of the field of view 225 within the
scene 150. The detection of multiple bands of infrared radiation
provides spectral resolution across a broad band of detectable
passive IR radiation permitting separate processing for the
particular wavelength component. Thus, multiple infrared data sets
can be generated that can then be fused with the spatial data
actively extracted from the LADAR radiation to provide a more
complete data set. The fused data set will not only combine the
advantages of passively detected IR data with those of actively
detected LADAR data, but will also provide additional detail in
that might not be available in all bands across the broad band of
detectable IR radiation.
[0078] Returning to FIG. 2, Note that the platform 110 is a flying
submunition in the illustrated embodiment. However, the invention
is not so limited. The platform 110 may be some other type of
airborne platform, such as a drone or an aircraft. Indeed, the
platform 110 is not limited to airborne platforms, but may also be
ground-based or even a vessel afloat in a body of water. The
present invention is not limited by the particular implementation
of the platform 110 in this manner so long as it implements the
invention as described above and claimed below.
[0079] A sequence of operations for the system 200 illustrated in
FIG. 2 are shown in a flow diagram illustrated in FIG. 10. Upon
initialization (at 1010), the system 200 collects data (at 1020)
according to collection commands which have been provided to the
system 200. The collection commands may be, e.g., received from the
ground control station 295a, received from the airborne platform
295b, or preprogrammed and stored. These commands may include
identification of particular spectral bands that are to be
aggregated by the processor 250 and to eliminate the spectral bands
statistically indicated as a signal of no signal of interest or
have a low signal to noise ratio. The command signals can also
include directions to perform target processing.
[0080] A determination is then made (at 1030) if the data currently
being collected is for calibration purposes. If so, correction
factors are calculated and stored (at 1040) for the received raw
data for later use. Such correction factors are determined, e.g.,
by the non-uniformity in response of the focal plane array 240.
Exemplary aspects subject to the correction factors are detector
response gain and offset. Control is then returned to the start (at
1010). If the data is not used for the purpose of calibration, it
is to be processed and analyzed. The raw data is therefore then
corrected (at 1050) with the previously stored correction
factors.
[0081] After the data has been corrected (at 1050), the system 200
determines (at 1060) whether a command has been made to select
particular data. If so, the desired data is selected (at 1070) for
the current collection operation. The system 200 then determines
(at 1075) whether the data should be processed for target
detection. If so, target detection processing is performed. Target
detection processing is performed (at 1080) in accordance with
conventional techniques. The system 200 then processes (at 1085)
the LADAR and infrared data obtained from the received radiation
210. This and other data individually, or in combination, is
utilized to produce control data used by the controller 260 to
direct the operations of the optical train 220 and the focal plane
array 240. In some embodiments, the data is then compressed and
sent (at 1090) via the data link 290 to a selected location such as
the ground control station 295a or to the airborne platform
295b.
[0082] This concludes the detailed description. The particular
embodiments disclosed above are illustrative only, as the invention
may be modified and practiced in different but equivalent manners
apparent to those skilled in the art having the benefit of the
teachings herein. For instance, although the illustrated embodiment
passively detects IR radiation, radiation of other frequencies may
also be passively detected. Furthermore, no limitations are
intended to the details of construction or design herein shown,
other than as described in the claims below. It is therefore
evident that the particular embodiments disclosed above may be
altered or modified and all such variations are considered within
the scope of the invention as claimed below. Accordingly, the
protection sought herein is as set forth in the claims.
* * * * *