U.S. patent application number 11/177782 was filed with the patent office on 2008-01-03 for lookdown and loitering ladar system.
This patent application is currently assigned to Lockheed Martin Corporation. Invention is credited to Nicholas Krasutsky.
Application Number | 20080002176 11/177782 |
Document ID | / |
Family ID | 38876265 |
Filed Date | 2008-01-03 |
United States Patent
Application |
20080002176 |
Kind Code |
A1 |
Krasutsky; Nicholas |
January 3, 2008 |
Lookdown and loitering ladar system
Abstract
A LADAR system and with lookdown and loitering capabilities is
disclosed. In one aspect, an apparatus includes a LADAR sensor and
a gimbal. The LADAR sensor is mounted to the gimbal, which is
capable of scanning in azimuth sufficient to provide a look down
and loitering capability. In another aspect, a method includes
flying an airborne vehicle through an environment; and scanning a
LADAR signal forward and to at least one side into a field of
regard.
Inventors: |
Krasutsky; Nicholas;
(Carrollton, TX) |
Correspondence
Address: |
WILLIAMS, MORGAN & AMERSON
10333 RICHMOND, SUITE 1100
HOUSTON
TX
77042
US
|
Assignee: |
Lockheed Martin Corporation
|
Family ID: |
38876265 |
Appl. No.: |
11/177782 |
Filed: |
July 8, 2005 |
Current U.S.
Class: |
356/4.01 ;
356/5.01 |
Current CPC
Class: |
G01S 7/4813 20130101;
G01S 17/42 20130101; G02B 23/16 20130101; G01S 7/4811 20130101;
G01S 7/4817 20130101; G01S 7/484 20130101 |
Class at
Publication: |
356/004.01 ;
356/005.01 |
International
Class: |
G01C 3/08 20060101
G01C003/08 |
Claims
1. An apparatus, comprising a LADAR sensor; and a gimbal to which
the LADAR sensor is mounted, the gimbal being capable of scanning
in azimuth sufficient to provide a look down and loitering
capability.
2. The apparatus of claim 1, wherein the gimbaled transceiver is
incapable of scanning in azimuth through at least 180.degree..
3. The apparatus of claim 2, wherein the gimbaled transceiver is
capable of scanning through at least .+-.90.degree. off a
boresight.
4. The apparatus of claim 1, wherein the gimbaled transceiver is
capable of transmitting at least 90.degree. off a boresight.
5. The apparatus of claim 1, further comprising: a platform
defining a chamber in which the LADAR sensor and gimbal are housed;
and a window in the platform closing the chamber.
6. The apparatus of claim 5, wherein the platform is a vehicle.
7. The apparatus of claim 6, wherein the vehicle is an airborne
vehicle.
8. The apparatus of claim 7, wherein the airborne vehicle is a
flying submunition, a guided weapon system, a reconnaissance drone,
or a manned aircraft.
9. The apparatus of claim 5, wherein the window is segmented.
10. The apparatus of 9, wherein the segments comprise flat
facets.
11. The apparatus of claim 9, wherein the segments are
hemispherical.
12. The apparatus of claim 5, wherein a fuselage of the platform
mating with the window is shaped.
13. The apparatus of claim 5, wherein the window is
hyperspherical.
14. The apparatus of claim 1, further comprising a laser designator
mounted on-gimbal with the LADAR sensor.
15. The apparatus of claim 14, wherein the laser designator and the
LADAR sensor are co-aligned.
16. The apparatus of claim 14, wherein the laser designator is
pointed independently of the gimbal pointing direction.
17. The apparatus of claim 14, wherein the laser designator
receives pointing and targeting information from the LADAR
sensor.
18. The apparatus of claim 1, wherein the LADAR sensor is capable
of laser designating.
19. The apparatus of claim 1, further comprising: an off-gimbal
LADAR laser; and a large mode area fiber over which the beam
generated by the LADAR laser is transmitted to the LADAR
sensor.
20. A wide-angle LADAR system, comprising: a platform defining a
chamber; a faceted window closing the chamber; and a gimbaled LADAR
sensor housed in the closed chamber capable of scanning in azimuth
substantially through 180.degree. and housed in the chamber to scan
through the faceted window.
21. The LADAR system of claim 20, wherein the platform is a
vehicle.
22. The LADAR system of claim 21, wherein the vehicle is an
airborne vehicle.
23. The LADAR system of claim 20, wherein the window is
segmented.
24. The LADAR system of 23, wherein the segments comprise flat
facets.
25. The LADAR system of claim 20, further comprising a laser
designator mounted on-gimbal with the LADAR sensor.
26. The LADAR system of claim 20, wherein the LADAR sensor is
capable of laser designating.
27. The LADAR system of claim 20, further comprising: an off-gimbal
LADAR laser; and a large mode area fiber over which the beam
generated by the LADAR laser is transmitted to the LADAR
sensor.
28. An apparatus, comprising: an airborne vehicle defining a
chamber in the forward end thereof, a faceted window closing the
chamber; a gimbaled LADAR sensor housed in the closed chamber
capable of scanning in azimuth substantially through 180.degree.
and housed in the chamber to scan through the faceted window; and a
laser designator mounted on-gimbal with the LADAR sensor.
29. The apparatus of claim 28, wherein the gimbaled transceiver is
incapable of scanning in azimuth through at least 180.degree..
30. The apparatus of claim 28, wherein the airborne vehicle is a
flying submunition, a guided weapon system, a reconnaissance drone,
or a manned aircraft.
31. The apparatus of claim 28, wherein the laser designator and the
LADAR sensor are co-aligned.
32. The apparatus of claim 28, wherein the laser designator is
pointed independently of the gimbal pointing direction.
33. The apparatus of claim 28, wherein the laser designator
receives pointing and targeting information from the LADAR
sensor.
34. The apparatus of claim 28, further comprising: an off-gimbal
LADAR laser; and a large mode area fiber over which the beam
generated by the LADAR laser is transmitted to the LADAR
sensor.
35. A method, comprising: flying an airborne vehicle through an
environment; and scanning a LADAR signal forward and to at least
one side into a field of regard.
36. The method of claim 35, wherein flying the airborne vehicle
includes flying a flying submunition, a guided weapon system, a
reconnaissance drone, or a manned aircraft.
37. The method of claim 35, wherein scanning the LADAR signal
includes scanning in azimuth through 180.degree..
38. The method of claim 37, wherein scanning in azimuth through
180.degree. includes scanning through .+-.90.degree. off a
boresight.
39. The method of claim 35, wherein scanning the LADAR signal
includes transmitting 90.degree. off a boresight.
40. The method of claim 35, further comprising: loitering over an
area within the field of regard; and scanning the LADAR signal into
the area while loitering.
41. The method of claim 40, further comprising banking the airborne
vehicle while loitering.
42. The method of claim 41, further comprising tracking a target
while loitering.
43. The method of claim 40, further comprising tracking a target
while loitering.
44. The method of claim 40, further comprising designating a target
from aboard the airborne vehicle.
45. The method of claim 35, further comprising designating a target
from aboard the airborne vehicle.
46. The method of claim 45 wherein the designation employs LADAR
information extracted aboard the airborne vehicle.
47. The method of claim 45, wherein the designation is performed by
the LADAR transmitter.
48. The method of claim 45, wherein the designation is performed by
a laser designator separate from the LADAR transmitter.
49. The method of claim 48, where the designation is performed
on-gimbal.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention pertains to a laser detection and
ranging ("LADAR") system and, more particularly, to a LADAR system
with lookdown and loitering capabilities.
[0003] 2. Description of the Related Art
[0004] A need of great importance in military and some civilian
remote sensing operations is the ability to quickly detect and
identify objects, frequently referred to as "targets," in a "field
of regard." 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] Remote sensing 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 and
receive radio waves to and reflected from objects, LADAR systems
transmit laser beams and receive reflections from targets. Because
of the short wavelengths associated with laser beam transmissions,
LADAR data exhibits much greater resolution than RADAR data.
Typically, a LADAR system creates a three-dimensional ("3-D") image
in which each datum, or "pixel", comprises an (x,y) coordinate and
associated range for the point of reflection.
[0007] LADAR systems used for small missile applications are
generally mounted at the front of the missile to maximize the
collection area for the receiver while maintaining the missile
cross section. These LADAR systems use an optical dome which limits
the field of regard ("FOR"). Some of these LADARs use a strap-down,
staring configuration. In others, gimbals are used to provide
stabilization, scan the LADAR transmit beam, and increase the
sensor FOR. Traditional gimbal configurations place the gimbal
supports along the side of the missile body, using the outer
support for elevation and the inner ring for azimuthal motion. Such
an arrangement is described in, for example: [0008] (i) U.S.
Letters Pat. No. 5,200,606, entitled "Laser Radar Scanning System,"
on Apr. 6, 1993, to LTV Missiles and Electronics Group as assignee
of the inventors Nicholas J. Krasutsky et al.; [0009] (ii) U.S.
Letters Pat. No. 5,224,109, entitled "Laser Radar Transceiver," on
Apr. Jun. 29, 1993, to LTV Missiles and Electronics Group as
assignee of the inventors Nicholas J. Krasutsky et al.; and [0010]
(iii) U.S. Letters Pat. No. 5,285,461, entitled "Improved Laser
Radar Transceiver," on Feb. 8, 1994, to Loral Vought Systems
Corporation as assignee of the inventors Nicholas J. Krasutsky et
al. Each of these patents is now commonly assigned herewith.
[0011] Some gimbaling arrangements where the outer gimbal provides
the azimuthal motion have been used in belly-mounted
configurations. However, this is not compatible with a missile
attack scenario and greatly limits the conditions under which the
missile can be launched. As LADAR systems and missile weapons
systems become more sophisticated, this mission scenario is
becoming more common.
[0012] Furthermore, providing missiles with designator capabilities
has also proved problematic because, unless the missile can loiter
and keep the designator beam on target, the time available for
designation is very limited. With a front mounted sensor and a
gimbal with a limited FOR, the missile can designate as it is
flying toward the target. The designator loses sight of the target
as the missile passes over it and the missile must turn around to
begin designation again. This intermittent designation time is
incompatible with the normal operation of a designator which must
keep the beam in the target so the attacking missile can track on
it.
[0013] The present invention is directed to resolving, or at least
reducing, one or all of the problems mentioned above.
SUMMARY OF THE INVENTION
[0014] The present invention includes, in its various aspects and
embodiments, a laser detection and ranging ("LADAR") system and
with lookdown and loitering capabilities. In one aspect, the
invention includes an apparatus comprising a LADAR sensor and a
gimbal. The LADAR sensor is mounted to the gimbal, which is capable
of scanning in azimuth sufficient to provide a look down and
loitering capability. In another aspect, the invention includes a
method comprising flying an airborne vehicle through an
environment; and scanning a LADAR signal forward and to at least
one side into a field of regard.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] 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:
[0016] FIG. 1 illustrates a LADAR system in one particular
embodiment constructed and operated in accordance with the present
invention in an assembled view;
[0017] FIG. 2 illustrates the LADAR system of FIG. 1 in an exploded
view;
[0018] FIG. 3 is an exploded view of several components of an
optical train of one particular embodiment of the sensor in the
LADAR system of FIG. 1-FIG. 2;
[0019] FIG. 4 shows LADAR sensor of FIG. 1-FIG. 2, which is
alternative to that shown in FIG. 3 mounted to the gimbal ring;
[0020] FIG. 5A-FIG. 5D show several views of the LADAR system of
FIG. 1-FIG. 2 positioned at various angles in azimuth and
elevation;
[0021] FIG. 6A-FIG. 6C illustrate alternative embodiments for the
window of the LADAR system of FIG. 1-FIG. 2;
[0022] FIG. 7A-FIG. 7D depict the LADAR system of FIG. 1-FIG. 2 in
operation in a lookdown and loitering scenario;
[0023] FIG. 8A-FIG. 8C illustrate an on-gimbal laser designator,
first shown in FIG. 4, from different perspectives;
[0024] FIG. 9A-FIG. 9B illustrate in a cross section and a plan
view, respectively, the sensor of FIG. 1 with a scan mirror in the
LADAR position for LADAR operations; and
[0025] FIG. 10A-FIG. 10B illustrate in a cross section and a plan
view, respectively, the sensor of FIG. 1 with the scan mirror in
the SAL position for SAL operations.
[0026] While the invention is susceptible to various modifications
and alternative forms, the drawings illustrate specific embodiments
herein described in detail by way of example. 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 spirit and scope of the invention as defined by the
appended claims.
DETAILED DESCRIPTION OF THE INVENTION
[0027] 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.
[0028] This present invention comprises a LADAR gimbaling and
windowing technique that allows LADAR systems to be used in
loitering aerial platforms, although the invention is not so
limited. The gimbal and window arrangement described herein allows
the LADAR system to be mounted in the front of an aerial platform
and used over azimuth angles exceeding .+-.90.degree.. This
capability allows the LADAR system to be used to both track and
accurately designate targets continuously while the missile moves
above in a circular loitering pattern. This capability is provided
by reversing the gimbal axes, using window arrangements that cover
a very wide azimuth angle and designing the sensor head and gimbal
to move freely through wide angles without hitting the windows or
the sides of the missile. This arrangement also facilitates the use
of laser designators in conjunction with the LADAR so that
designation can be performed on moving targets while the missile
loiters above the target area.
[0029] A large mode area fiber was used to move the transmitter
output to the gimbal. This frees the laser from the size and weight
constraints associated with being located on the gimbal and
facilitates the placing of the designator on the gimbal. The
designator laser outputs much larger pulses than the laser used for
the LADAR so it was placed on the gimbal rather than also being
fiber coupled to the sensor head. This arrangement is the preferred
embodiment but fiber coupling both the LADAR and designator lasers
as well as putting the LADAR laser on the gimbal is considered to
be covered by this disclosure.
[0030] FIG. 1 and FIG. 2 illustrate a LADAR system 100 in one
particular embodiment constructed and operated in accordance with
the present invention in assembled and exploded views,
respectively. In general, the LADAR system 100 includes a sensor
103 mounted in a gimbal ring 106. The assembled sensor 103 and
gimbal ring 106 are housed in a chamber 109, as shown in FIG. 1,
defined by a forward end 112 of a platform 115. In the illustrated
embodiment, the platform 115 is an aerial vehicle, and more
particularly a missile or an airborne guided submunition, but this
is not necessary to the practice of the invention.
[0031] The platform 115 includes a faceted window 118 that closes
the chamber 109, as will be discussed further below. The faceted
window 118 provides a wide Field of Regard ("FOR"). It also
protects the sensor 103 and gimbal ring 106 from environmental
conditions and, in this particular embodiment, aerodynamic forces.
The faceted window 118 also contributes to the aerodynamic
performance of the platform 115 as a whole, as will be recognized
by those skilled in the art having the benefit of this disclosure.
Note that the fuselage of the forward end 112 is shaped to match
the faceting of the window 118. This also is not necessary to the
practice of the invention, but enhances the aerodynamic performance
of the platform 115 in this particular embodiment.
[0032] The sensor 103 may be any suitable LADAR sensor known to the
art. Suitable LADAR sensors are disclosed in the aforementioned
patents-namely: [0033] (i) U.S. Letters Pat. No. 5,200,606,
entitled "Laser Radar Scanning System," on Apr. 6, 1993, to LTV
Missiles and Electronics Group as assignee of the inventors
Nicholas J. Krasutsky et al. ("the '606 patent); [0034] (ii) U.S.
Letters Pat. No. 5,224,109, entitled "Laser Radar Transceiver," on
Apr. Jun. 29, 1993, to LTV Missiles and Electronics Group as
assignee of the inventors Nicholas J. Krasutsky et al. ('the '109
patent); and [0035] (iii) U.S. Letters Pat. No. 5,285,461, entitled
"Improved Laser Radar Transceiver," on February 8, 1994, to Loral
Vought Systems Corporation as assignee of the inventors Nicholas J.
Krasutsky et al. Applicant hereby incorporates by reference for all
purposes those portions of the cited patents disclosing the
composition and operation of the sensor as if set forth verbatim
herein. Note that the sensors of the cited patents are gimbaled in
accordance with what is now conventional practice. Applicant does
not incorporate by reference those portions of the cited patents
addressing the gimbaling of the sensors.
[0036] For the sake of clarity and to further an understanding of
the present invention, a portion of '606 patent describing the
optics of the sensor therein is reproduced below somewhat modified.
Turning now to FIG. 3, selected portions of the optics 300 of that
sensor are shown in an exploded view. A gallium aluminum arsenide
("GaAlAs") laser 330 pumps a solid state laser 332. The solid state
laser 332 emits the laser light energy employed for illuminating
the target. The GaAlAs pumping laser 330 produces a continuous
signal of wavelengths suitable for pumping the solid state laser
332, e.g., in the crystal absorption bandwidth. Pumping laser 330
has an output power, suitably in the 10-20 watt range, sufficient
to actuate the solid state laser 332.
[0037] The pumping laser 330 and the solid state laser 332 are
fixedly mounted on the housing of the forward end 112. The output
of the solid state laser 332 is transported to the gimbal by means
of a high power optical fiber 333. Since the solid state laser 332
is fiber-coupled to the gimbal, many laser types can be used, e.g.,
side pumped lasers and fiber lasers, provided they can be coupled
into the fiber. In the case of fiber lasers it is also possible to
use the lasing fiber directly to connect to the sensor head. Thus,
alternative embodiments may use lasers other than solid state
lasers. Output signals from the high power optical fiber 333 are
transmitted through a beam input lens 331 and a fiber optic bundle
334. The fiber optic bundle 334 has sufficient flexibility to
permit scanning movement of the LADAR system 100 during operation
as described below.
[0038] Still referring to FIG. 3, the solid state laser 332 is
suitably a Neodymium ("Nd") doped yttrium aluminum garnet ("YAG"),
a yttrium lithium fluoride ("YLF"), or a Nd:YVO.sub.4 laser. The
solid state laser 332 is operable to produce, in this particular
embodiment, pulses with widths of 10 to 20 nanoseconds, peak power
levels of approximately 10 kilowatts, at repetition rates of 10-120
kHz. The equivalent average power is in the range of 1 to 4 watts.
The preferred range of wavelengths of the output radiation is in
the near infrared range, e.g., 1.047 or 1.064 microns.
[0039] The output generated by solid state laser 332, in the
present embodiment, is carried to the gimbaled head by the high
power fiber 333, as mentioned above. The high power fiber 333 has
sufficient flexibility to permit scanning movement of the LADAR
system 100 during operation as described below. The output end of
the high power fiber 333 is mounted on the gimbaled head so that
the laser beam emerging from it passes through the beam expander
340. The beam expander 340 comprises a series of (negative and
positive) lenses which are adapted to expand the diameter of the
beam to provide an expanded beam 342, suitably by an 8:1 ratio,
while decreasing the divergence of the beam.
[0040] The expanded beam 342 is next passed through a beam
segmenter 344 for dividing the beam into a plurality of beam
segments 346 arrayed on a common plane, initially overlapping, and
diverging in a fan shaped array. The divergence of the segmented
beams 346 is not so great as to produce separation of the beams
within the LADAR system 100, but preferably is sufficiently great
to provide a small degree of separation at the target, as the
fan-shaped beam array is scanned back and forth over the target (as
will be described below with reference to output beam segments
348). Beam segmentation can be accomplished by using a series of
calcite wedges, a holographic diffraction grating or a phased
diffraction grating. The preferred method is using a phased
diffraction grating because of its predictable performance and
power handling capability.
[0041] As shown in FIG. 3, the resultant segmented beams 346 are
then reflected from a third turning mirror 354, passed through an
aperture 356 of an apertured mirror 358, and subsequently reflected
from a scanning mirror 360 in a forward direction relative to the
platform 115. The aperture 356 is located off the center of the
aperture mirror 358. The scanning mirror 360 is pivotally driven by
a scanning drive motor 362, which is operable to cyclically scan
the beam segments 346 for scanning the target area. In a preferred
embodiment, the beam segments 346 are preferably scanned at a rate
of approximately 100 Hz. The turning axis of the scanning drive
motor 362 is aligned in parallel with the segmenter 344 axis
whereby the resultant beam array 346 is scanned perpendicularly to
the plane in which the beams are aligned.
[0042] An afocal, Cassegrainian telescope 362 is provided for
further expanding an emitted beam 364 and reducing its divergence.
The telescope 362 includes a forward-facing primary is mirror 366
and a rear-facing secondary mirror 368. A lens structure 372 is
mounted in coaxial alignment between the primary mirror 366 and the
scanning mirror 360, and an aperture 374 is formed centrally
through the primary mirror in alignment with the lens
structure.
[0043] The transmitted beams which are reflected from the scanning
mirror are directed through the lens structure 372 for beam
shaping, subsequently directed through the aperture 374 formed
centrally through the primary mirror, and subsequently reflected
from the secondary mirror 368 spaced forwardly of the primary
mirror and is then reflected from the front surface of the primary
mirror 366. The resultant transmitted beam 376, is a fan shaped
array which is scanned about an axis parallel to its plane. The
beam array 378 illustrates the diverged spacing of the beam
segments as they reach the target, wherein the beams are in
side-by-side orientation, mutually spaced by a center-to-center
distance of twice their diameters.
[0044] The telescope 362 receives laser energy reflected from a
target that has been illuminated by the array of transmitted beams.
This received energy is then reflected successively through the
primary mirror 366 and the secondary mirror 368, the lens assembly
372, and the scanning mirror 360, toward the apertured mirror 358.
Because the reflected beam is of substantially larger
cross-sectional area than the transmitted beam, it is incident upon
the entire reflecting surface of the apertured mirror 358, and
substantially all of its energy is thus reflected laterally by the
apertured mirror 358 toward collection optics 380.
[0045] The collection optics 380 includes a narrow band filter 382,
for filtering out wavelengths of light above and below a desired
laser wavelength to reduce background interference from ambient
light. The beam then passes through condensing optics 384 to focus
the beam. The beam next strikes a fourth turning mirror 86 toward a
focusing lens structure 388 adopted to focus the beam upon the
receiving ends 390 of a light collection fiber optic bundle 392.
The opposite ends of each optical fiber 392 are connected to
illuminate a set of diodes 394 in a detector array, whereby the
laser light signals are converted to electrical signals which are
conducted to a processing and control circuit (not shown).
[0046] The fiber optic bundle 392 preferably includes nine fibers
393 (only one indicated), eight of which are used for respectively
receiving laser light corresponding to respective transmitted beam
segments and one of which views scattered light from the
transmitted pulse to provide a timing start pulse. Accordingly, the
input ends 390 of the fibers 392 are mounted in linear alignment
along an axis which is perpendicular to the optical axis. The
respective voltage outputs of the detectors 394 thus correspond to
the intensity of the laser radiation reflected from mutually
parallel linear segments of the target area which is parallel to
the direction of scan.
[0047] However, the invention admits wide variation in the
implementation of the sensor 103. FIG. 4 shows an alternative LADAR
sensor 103 mounted to the gimbal ring 106. As is best shown in FIG.
2, the sensor 103 includes a pair of trunions 200 (only one shown)
that are rotatably mounted within a pair of bores 203 (only one
shown) in the gimbal ring 106. The bores 203 include mechanical
assemblies such as bearings, bushing, etc. (not shown) to
facilitate rotation of the trunions 200 in the bores 203 in a
manner known to the art. The LADAR sensor 103 is a variant of the
sensor described in the '109 patent referenced above and employs an
optical train similar to that described above relative to FIG. 3. A
servo-drive motor 204 drives the sensor 103 through the trunions
200 to scan the sensor in elevation. In the illustrated embodiment,
the sensor 103 is scanned in elevation approximately .+-.300
relative to the axis 206 defined by the trunions 200 and shown in
FIG. 2 in broken lines. However, the amount of elevational scan is
implementation specific and may differ in alternative
embodiments.
[0048] Returning to FIG. 4, the sensor 103 is mounted through the
gimbal ring 106 from the top 401 and bottom 402 so that extended
travel and scanning in azimuth is possible. Note that "top" and
"bottom" are defined relative to the nominal orientation of the
platform 115 relative to the Earth's field of gravity or the ground
surface. As the platform 115 changes this orientation, so, too,
will the orientation of the "top" 401 and "bottom" 402 relative to
these references. The sensor 103 is mounted through the gimbal ring
106 using a trunion/bore approach and bearing/bushing approach
similar to that described immediately above and as is conventional
in the art. The sensor 103 and gimbal ring 106 are driven in
azimuth by servo-motor 405 about an axis 403 shown in FIG. 4 in
broken lines. The sensor 103 is driven in elevation by the servo
motor 204 about an axis 206 shown in FIG. 2 in broken lines.
[0049] The position of the gimbal in elevation and azimuth is
measured by position sensing devices located on the opposite sides
of the gimbal ring across from each of the servomotors 204 and 402.
The azimuthal position sensor 401 is shown in FIG. 4 along with the
corresponding azimuthal gimbal servo-motor 405. Position can be
sensed by a number of devices including potentiometers, electrical
encoders and optical encoders, or other techniques known to the
art, with the preferred method being optical encoders.
[0050] In the illustrated embodiment, the gimbaled sensor 103 is
capable of scanning in azimuth substantially past 180.degree.. In
the illustrated embodiment, the goal is a full 210.degree. scan and
the term "substantially" is a recognition that sometimes
manufacturing variances or tolerances or sometimes operational
conditions impair achievement of a full 210.degree. azimuthal scan.
The illustrated embodiment achieves the 210.degree. scan by
scanning .+-.105.degree. from the boresight 406, or longitudinal
axis of the platform 115, shown in broken lines in FIG. 4.
[0051] However, this is not necessary to the practice of the
invention in all embodiments. One intended purpose of the present
invention is application in a lookdown and loitering mode, as is
discussed further below relative to FIG. 7A-FIG. 7D. Thus, all that
is required is that the gimbaled receiver 103 be able to scan
sufficiently far in azimuth to one side of the platform 115 so as
to enable this functionality. An embodiment capable of scanning a
full 210.degree. by scanning .+-.105.degree. off boresight is more
versatile. However, this functionality can be achieved by scanning
off to only one side 90.degree. off boresight. In general, any
given embodiment should be able to scan at least 90.degree. off
boresight to at least one side of the platform 115.
[0052] Referring again to FIG. 3, in the illustrated embodiment,
the LADAR transmitter has been moved off the gimbal and its output
is coupled to the sensor head 103 by means of an optical fiber 333.
This simplifies the packaging of the sensor 103. Off-gimbal laser
configurations have been used in gimbaled systems in the past but
they generally used complicated mirror configurations to maintain
alignment between the transmit and receive paths. Recent
developments in Large Mode Area ("LMA") optical fibers have allowed
high peak powers to be transmitted while maintaining good beam
optical quality. These fibers can emit directly as part of a fiber
laser or amplifier, alternatively, they can be used to transmit the
output from any laser up to the gimbaled platform.
[0053] Returning to FIG. 1 - FIG. 2, the sensor head 103 and gimbal
ring 106 are sized to fit within the chamber 109 defined by the
platform 115 as the sensor 103 is scanned. Several views of the
LADAR system 100, positioned at various angles, are shown in FIG.
5A-FIG. 5D. More particularly, using the intersection of the
boresight 406, shown in FIG. 4, and the axis 206, shown in FIG. 2,
as the origin, FIG. 5A-FIG. 5B show the sensor 103 positioned at
55.degree. azimuth, -30.degree. elevation in perspective and front,
plan views, respectively. FIG. 5C shows the sensor 103 positioned
at 90.degree. azimuth, -30.degree. elevation in a side, perspective
view. FIG. 5D shows the sensor 103 positioned at 90.degree.
azimuth, -13.degree. elevation in a side, plan view.
[0054] Returning to FIG. 1-FIG. 2 again, the flat window segments
121 (six of which are shown in FIG. 2, but only one of which is
indicated) of the faceted window 118 provide a wide FOR. The window
segments 121 are fabricated from a material that transmits the
LADAR signal but can also withstand applicable environmental
conditions. In the illustrated embodiment, one important
environmental condition is aerodynamic heating due to the velocity
of the platform 115. Another important environmental condition for
the illustrated embodiment is abrasion, such as that caused by dust
or sand impacting the window 118 at a high velocity. Thus, for the
illustrated embodiment, BK-7 glass is a highly desirable material,
but alternative embodiments may employ fused silica. ZnSe,
Al.sub.2O.sub.3, Ge, and Pyrex.
[0055] Using the flat window segments 121 rather than a spherical
dome (not shown) also reduces the cost of the window 118, allows
wide azimuth angles, and allows more freedom in the placement of
the gimbal trunions 200. There is no significant degradation on
image quality provided the window facets 121 do not have any wedge
angle between their surfaces. However, the faceted window 118
increases the overall length of the front end 112, has more
aerodynamic drag and flow asymmetry, and requires seams. It also
has the potential for reflection losses if the output beam meets
any window surface at near grazing incidence.
[0056] The specific window arrangement chosen depends on trades
specific to the missile and its mission but some representative
arrangements are shown in FIG. 6A-FIG. 6C. These show arrangements
using different numbers of facets and different angles between the
facets. They were picked because they have significantly different
overall length and drag characteristics e.g. FIG. 6A has the
shortest overall length but also has the most aerodynamic drag. All
arrangements utilize a small number of facets and each facet is
supported at the top and bottom by the airframe structure. As the
number of facets is increased, a low drag aerodynamic shape can be
approximated but this requires a large number of facets and a
complicated support structure, adding significantly to the cost.
Such shapes may be required for high speed or for very long range
applications.
[0057] Note, however, that the faceted window 118 is not necessary
to the practice of the invention in all embodiments. Alternative
embodiments may instead employ, for instance, a single
conventional, spherical hypersphere (not shown) or spherical
segments (also not shown) if the aerodynamic requirements for a
given application are sufficiently important. Alternatively, one
compromise uses a spherical segment in front and one or two others
out at right angles to the missile axis. If tone one side is domed,
loitering must be down in the direction that places that segment
towards the ground. Thus, the window 118 may also be spherical or
spherically segmented in alternative embodiments.
[0058] The LADAR system 100 will also include electronic circuitry
(not shown) for generating the scan signals that drive the
servo-motors, laser, detectors, and scanning drive motor and to
capture the information in the detected signals. Scan signal
generation can be performed by first using the scanning drive motor
362 to drive the scan mirror 360 in elevation. This produces
multiple rows of pulses as shown in FIG. 7B. Scanning the entire
sensor in azimuth using the servo motor 405, shown in FIG. 4, then
produces a scan of the target area. Suitable information capture
and processing techniques are disclosed in: [0059] (i) U.S. Letters
Pat. No. 6,115,113, entitled "Method for Increasing Single-Pulse
Range Resolution," on Sep. 5, 2000, to Lockheed Martin Corporation
as assignee of the inventor Stuart W. Flockencier; [0060] (ii) U.S.
Letters Pat. No. 5,243,553, entitled "Gate Array Pulse Capture
Device," on Sep. 7, 1993, to Loral Vought Systems Corporation as
assignee of the inventor Stuart W. Flockencier. Both of these
patents are commonly assigned herewith. Note, however, that any
suitable technique known to the art may be employed.
[0061] The electronic circuitry and detection electronics are
fixedly mounted relative to the housing or other suitable
supporting structure aboard the platform 115. The scanning and
azimuth translations of the LADAR system 100 therefore do not
affect corresponding movement of the detection system. Accordingly,
the mass of the components which are translated during scanning is
substantially lower than would be the case if all components were
gimbal-mounted. These benefits are amplified in the case of the
embodiment shown in FIG. 3 since the laser is also off-gimbal.
[0062] Since the LADAR system 100 is capable of looking out at over
.+-.90.degree. to both sides of the platform 115, it can be used
over a wide swath as the platform 115 moves through its
environment. Consider FIG. 7A, which shows the potential for target
examination out to the range 700 of the LADAR system 100 on both
sides of the flight path 703, shown in broken lines. The
surveillance area 706 includes the area 709 that has already been
reconnoitered and the area 712 currently under surveillance. The
area 712 currently under surveillance is determined by the position
of the platform 115, the range 700 of the LADAR system 100, and the
extent of the azimuthal scan of the LADAR system 100.
[0063] The operation of the gimbaled LADAR sensor 100 in scanning
is conceptually illustrated in FIG. 7B. The gimbaled LADAR sensor
100 transmits the LADAR signal 705 to scan the area 712. Each scan
is generated by scanning elevationally, or vertically, several
times while scanning azimuthally, or horizontally, once within the
FOR. FIG. 7B illustrates a single elevational scan 707 during the
azimuthal scan 708. Thus, each scan is defined by a plurality of
elevational scans such as the elevational scan 707 and the
azimuthal scan 708. The velocity, depression angle of the sensor
103 with respect to the horizon, and total azimuth scan angle of
the LADAR platform 115 determine the extent of the scan.
[0064] The LADAR signal 705 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
115 transmitting the LADAR signal 705. Each pulse of the single
beam is split, and so the LADAR signal 705 transmitted during the
elevational scan 707 in FIG. 7B is actually, in the illustrated
embodiment, a series 711 of grouped beamlets 713 (only one
indicated). The gimbaled LADAR sensor 103 transmits the LADAR
signal 705 while scanning elevationally 707 and azimuthally 708.
The LADAR signal 705 is continuously reflected back to the platform
115, where it is detected and captured.
[0065] The characteristics of the LADAR signal 705 will be a
function of the LADAR sensor 103, which will, in turn, be a
function of the mission in a manner known to the art. The LADAR
sensor 300, shown in FIG. 3A-FIG. 3B, 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.
[0066] Assume the LADAR system 100 identifies the target 710 as an
object of interest, and wishes to continue observing the object. As
is shown in FIG. 7C, the platform 115 flies a circular loiter
pattern 717 over the target area 715, including the current
surveillance area 712. In the illustrated embodiment, the loiter
pattern 717 is in a clockwise direction, but could alternatively be
counterclockwise. The LADAR system 100 can then look out to the
side and examine a portion 718, the constant track and surveillance
area, of the area 712 being circled. If the platform 115 flew
level, the loitering radius for the loiter pattern 717 would need
to be large enough to allow the LADAR system 100 look down to see
the ground at the maximum gimbal lookdown angle. If, however,
bank-to-turn guidance is used, the platform 115 will bank into the
turn, providing the sensor with additional lookdown capability.
[0067] The bank angle .THETA. of the platform 115, shown in FIG.
7D, is a function of the turn radius and the velocity of the
platform 115. For highly maneuverable platforms, the bank angle
.THETA. can exceed 60.degree.. The banking of the platform 115
rotates the LADAR system 100 and provides additional down-look
capability for the seeker relative to the ground. Depending on the
bank angle .THETA., the LADAR system 100 could look straight down
or even past vertical. This is evident from the indicated coverage
cone 721 in FIG. 7C.
[0068] More particularly, FIG. 7C shows two areas 712, 718 on the
ground below the flight path 703. The area 718 shows the portion of
the ground which is always visible to the LADAR system 100,
regardless of the position of the platform 115 along its flight
path 703. The area 712 is the additional area which can be seen by
the LADAR system 100, depending on the position of the platform 115
along its flight path 703. The circle 721 drawn on the ground below
the loiter pattern 717 of the flight path 703 shows the line where
the LADAR system 100 is looking straight down. If the radius of the
loiter pattern 717 is comparable to or smaller than the altitude
724 of the platform 115 much of the area 718 is viewed at steep
angles to the ground. This facilitates use in urban or forested
target areas where terrain masking is a problem for sensors working
at shallow depression angles.
[0069] In the illustrated embodiment, the altitude 724 is
approximately 300 m, the diameter of the loiter pattern 717 is
approximately 2 km, the diameter of the area 718 is 1.2 km, and the
track window of the target 709 is 200 m.times.200 m. Note, however,
that these dimensions are implementation specific, and that other
embodiments might operate with different dimensions. Thus, these
dimensions are not material to the practice of the invention.
[0070] Returning now to FIG. 4, the illustrated embodiment also
includes an on-gimbal laser designator 409 that provides a laser
designator mode of operation. The laser designator 409 and
associated turning prism 410 are better illustrated in FIG. 8A-FIG.
8C. More particularly, the laser designator 409 produces a pulsed
laser beam 800 that may be used for target designation. FIG.
8A-FIG. 8C illustrates the emission of the pulsed beam 800 from the
laser designator 409 through the turning prism 410 within the
chamber 109 and behind the window 118. Note that it is possible to
use the LADAR transmitter for designation but, since the power and
beam characteristics normally required for designation are
different from those required for LADAR operation, the laser design
will be an undesirable compromise between the two requirements. The
designator optics can be strap-down, as in the illustrated
embodiment, or equipped with scanning mechanisms (not shown).
[0071] More particularly, the laser designator 409 is located on
the sensor 103 and generates a laser beam 800. The laser beam 800
is directed off the sensor 103 by the turning prism 410 in a
direction parallel to the optical axis 406 of the telescope 803.
Referring now to FIG. 9A and FIG. 9B, the sensor 103 includes a
scan mirror 903 that may be moved between two positions, one for
use in LADAR operation and one for use in SAL, or designation,
operations. The scanning mirror 903 is shown in the LADAR position.
The scanning mirror 903 is mounted to and moved by elevation
scanner motor 806, shown in FIG. 8B.
[0072] When the sensor 103 is being used in the LADAR mode, light
from the LADAR laser is directed into the far field and falls on
the target area as discussed above relative to FIG. 7A-FIG. 7C.
Scattered light 901 from the target area is collected by the
telescope 803 which directs it onto the elevation scan mirror 903.
The scattered light 901 is then directed upward through the optical
train 904 by the elevation scan mirror 903, and is focused onto the
LADAR detector fiber array 905. The high speed scanner rotates the
elevation scan mirror 903 through a small angle center around
45.degree.. This provides the fiber array 905 with a view of the
target scene at different elevations. The external gimbal 106 is
used to provide stabilization and to scan the sensor 103 in azimuth
so the entire target area can be examined by the LADAR and a three
dimensional scene image can be formed.
[0073] FIG. 10A and FIG. 10B show the sensor 103 when it is being
used in the SAL mode. Moving from the LADAR mode to the SAL mode is
accomplished by flipping the elevation scan mirror 903 of the
telescope optical path. The final position of the scanning mirror
903, as shown in FIG. 10A and FIG. 10B, is not critical as long as
it is out of the way of the SAL detector optical aperture 1004 so
that the SAL detector 1002 has a clear view through the telescope
803. It is assumed that the target is being designated by a source
external to the platform 115 in this particular embodiment.
Scattered light 901 coming from the target falls on the telescope
803. The SAL detector 1002 does not utilize all of the light
falling on the telescope 803, but rather, only light 1001 which
falls on the shaded area 1003 shown in FIG. 10A. The SAL detector
input aperture 1004 is placed at the exit pupil of the telescope
803 and the shaded area 1003 represents the portion if the
telescope 803 input aperture subtended by the SAL detector aperture
1004 at the entrance to the telescope 803.
[0074] Since SAL mode detector and optics are located at the exit
pupil of the sensor telescope 803, the SAL optics have access to
the entire angular field of regard of the telescope 803 but utilize
only a specific, unmasked portion 1003 of the telescope 803 input
aperture for light collection. This allows the SAL mode to use the
optical magnification of the telescope 803 while having an optical
path which is unobstructed by the telescope 803 secondary supports
909. The tradeoff is that only a portion of the entire telescope
803 aperture is used by the SAL detector. This limits the effective
range of that mode but it preserves linearity and limits noise
induced by the telescope 803 supports 909. The SAL sensor range
should still be adequate for most missile applications, especially
where lock-on before launch capability is not required. The small
SAL mode optics make packaging easier and lower system cost. Both
of these benefits are significant in small missile
applications.
[0075] The scanning mirrors currently used in most LADARs are
driven by placing them on a motor shaft. The motor controller then
moves the mirror through the desired pattern needed for LADAR
operation. These are usually high torque motors and moving them
through large angles can be difficult because it involves moving
across different motor windings where the available torque is
limited. While the mirror 903 is being flipped from the LADAR
position to the SAL position and back, neither mode is operational
so the mirror can be driven open loop through the low torque region
using the rotor and mirror inertia. Alternatively, a small set of
secondary windings can be used to aid in the transition. Scanning
mirrors can be controlled in a number of ways the specific method
is not important, only the fact that it is used as part of the
optical train in the LADAR mode and is moved out of the way for the
SAL mode.
[0076] Moving back to the LADAR mode is accomplished in a similar
fashion by flipping the scanning mirror 903 back to the position
shown in FIG. 9A and FIG. 9B so that it can be used to direct the
light into the LADAR detectors. Moving back and forth between the
two modes can be done as often as the operational scenario
requires, but the two modes cannot be used simultaneously.
[0077] The LADAR system 100 can be used to locate and track the
targets, e.g. the target 710 in FIG. 7A, and the coordinate
information passed to the laser designator 409 in a number of ways.
For instance, coordinate information may be passed as the
coordinates of the target 710, derived from Global Positioning
System ("GPS") coordinates platform 115 or as a targeting direction
using an inertial measurement unit ("IMU") aboard the platform 115.
In the illustrated embodiment, the LADAR sensor 103 and the
designator 709 are aligned to allow pointing and targeting
information to be shared directly between the two. Thus, the LADAR
system 100 can be operated in LADAR mode as illustrated in FIG. 7A
to locate the target 710. The LADAR system 100 will yield
three-dimensional data describing the location of the target 710,
which can then be passed to the control of the laser designator
409. This information can then be used to designate the target
710.
[0078] If the laser designator 409 and sensor 103 wavelengths are
different, both can be operated simultaneously. If the laser
designator 409 and sensor 103 are at the same wavelength, then the
laser designator 409 might interfere with the LADAR operation of
the sensor 103 when it is actively pulsing. This can be easily
addressed because the duty cycle of the laser designator 409 is
very low so the LADAR detectors (not shown) can be turned off
during the designation pulse without significant loss in imaging
capability.
[0079] The LADAR detectors can even be gated to pick up the return
from the designation beam 800 so that the position of the
designation beam 800 relative to the LADAR target image can be
determined. This is an accurate way to maintain alignment between
the two modes if the laser designator 409 has its own on-board
steering mechanism. As the LADAR system 100 loiters, the laser
designator 409 can maintain a spot on the target 710 as long as the
target 710 remains in area 718 of FIG. 7A. Illuminating the top of
the target 710 would prevent masking of the designator spot as the
platform 115 executes its flight pattern. Alternatively, a nearby
spot could be designated and the relative coordinates passed on for
further use.
[0080] 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. 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 and spirit
of the invention. Accordingly, the protection sought herein is as
set forth in the claims below.
* * * * *