U.S. patent application number 13/479088 was filed with the patent office on 2013-05-02 for compact fixed-source array test station for calibration of a semi-active laser (sal) seeker.
This patent application is currently assigned to Raytheon Company. The applicant listed for this patent is Michael K. Burkland. Invention is credited to Michael K. Burkland.
Application Number | 20130110440 13/479088 |
Document ID | / |
Family ID | 48173254 |
Filed Date | 2013-05-02 |
United States Patent
Application |
20130110440 |
Kind Code |
A1 |
Burkland; Michael K. |
May 2, 2013 |
COMPACT FIXED-SOURCE ARRAY TEST STATION FOR CALIBRATION OF A
SEMI-ACTIVE LASER (SAL) SEEKER
Abstract
A fixed-source array test station provides a compact
cost-effective high-throughput test bed for testing optical sensors
that require stimulus at fixed angular positions. An array of fixed
collimated sources at different angular positions in the sensor's
FOV are positioned on a surface of a focal sphere at the effective
focal length of a spherical lens and aligned along respective
radial lines to the center of the spherical lens so that each said
divergent optical beam is collimated by the spherical lens to form
a collimated optical beam that overlaps the entire entrance pupil
of the optical seeker. The sources are activated in accordance with
an activation profile in order to calibrate or otherwise test the
sensor.
Inventors: |
Burkland; Michael K.;
(Tucson, AZ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Burkland; Michael K. |
Tucson |
AZ |
US |
|
|
Assignee: |
Raytheon Company
|
Family ID: |
48173254 |
Appl. No.: |
13/479088 |
Filed: |
May 23, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12947234 |
Nov 16, 2010 |
8392143 |
|
|
13479088 |
|
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Current U.S.
Class: |
702/104 ;
702/116 |
Current CPC
Class: |
F41G 7/002 20130101;
G01S 3/7803 20130101; G06F 17/00 20130101; F41G 7/226 20130101;
F41G 7/2293 20130101 |
Class at
Publication: |
702/104 ;
702/116 |
International
Class: |
G06F 17/00 20060101
G06F017/00 |
Claims
1. A test station for calibrating a semi-active laser (SAL) seeker
including a SAL detector, comprising: a fixture for mounting the
SAL seeker, said seeker including an optical system having an
entrance pupil that gathers optical energy over a field of view
(FOV) and focuses the optical energy into a spot on the SAL
detector; a spherical lens having a first side positioned at a
location in front of where the entrance pupil of the seeker would
be located, said spherical lens comprising one or more materials
each having an index of refraction greater than one and less than
or approximately equal to two; an array of fixed optical sources
that generate respective temporally pulsed divergent optical beams,
said sources positioned on a surface of a focal sphere at an
effective focal length of the spherical lens offset from a second,
opposite side of the spherical lens over the entire FOV of the
seeker and aligned along respective radial lines to the center of
the spherical lens so that each said divergent optical beam is
collimated by the spherical lens to form a collimated optical beam
that overlaps the entire entrance pupil of the seeker; a memory
that stores a first source activation profile to generate a
transfer function for the SAL detector, said transfer function
mapping a spatial displacement .DELTA.X and .DELTA.Y in orthogonal
axes of a centroid of the spot to azimuth and elevation angles for
the source of the spot; a controller responsive to the first source
activation profile to activate the sources one at a time in a phase
delayed manner to implement the activation profile; and a computing
device responsive to a measured spatial displacement .DELTA.X and
.DELTA.Y of the spot on the SAL detector for each source as the
source is activated to create a table entry for the transfer
function in a calibration table that maps the measured .DELTA.X and
.DELTA.Y to the known azimuth and elevation angles of the activated
source.
2. The test station of claim 1, wherein the seeker's optical system
is a non-imaging optical system and the SAL detector comprises a
segmented non-imaging detector.
3. The test station of claim 1, wherein the seeker's optical system
is an imaging optical system and the SAL detector comprises an
imaging detector having an array of detector elements.
4. The test station of claim 1, wherein the spherical lens
comprises only one unitary material having an index of refraction
greater than one and less than or approximately equal to two.
5. The test station of claim 1, wherein the spherical lens
comprises a plurality of different materials each having an index
of refraction greater than one and less than or approximately equal
to two that form a gradient index spherical lens.
6. The test station of claim 1, wherein the radius of the spherical
lens is greater than or equal to 2.54 cm and less than or equal to
15.24 cm.
7. The test station of claim 1, wherein each optical source
approximates a point source on the surface of the focal sphere.
8. The test station of claim 7, wherein each optical source
comprises: a non-point optical source that emits an internal
divergent beam; a collimator that collimates the divergent beam to
form an internal collimated beam; and a negative lens that projects
the internal collimated beam into the divergent beam to approximate
the point source.
9. The test station of claim 7, wherein an array of optical sources
comprises: an array of non-point optical sources formed on a first
substrate conformal with the surface of the focal sphere, each said
source emitting an internal divergent beam; and an array of
lenslets formed on a second substrate aligned to array of non-point
optical sources and conformal with the surface of the focal sphere,
each said lenslet projecting the internal divergent beam into the
divergent beam to approximate a point source.
10. The test station of claim 1, wherein a maximum distance between
one of the sources and the entrance pupil is less than
approximately 61 cm and a maximum distance between any two of the
sources is approximately 15.25 cm.
11. The test station of claim 10, wherein the effective focal
length is less than approximately 38 cm.
12. The test station of claim 10, wherein a distance between the
first side of the spherical lens and the entrance pupil is less
than approximately 5.8 cm.
13. The test station of claim 1, wherein spherical lens and
position of the sources on the focal sphere are prescribed by the
following set of equations: .theta. = ArcTan [ 2 ( n - 1 ) Cos [
Tan [ F O V 2 ] ] ( A 2 + ( R + .delta. ) Tan [ F O V 2 ] ) n R ] ,
d = 2 Cos Tan [ F O V 2 ] ( A 2 + ( R + .delta. ) Tan [ F O V 2 ] )
, E F L = n R 2 ( n - 1 ) , and ##EQU00004## N A = n Sin .theta. =
n Sin [ ArcTan [ d / 2 E F L ] ] ##EQU00004.2## where .theta. is
the divergence of the divergent beam, R is the radius of the
spherical lens, .delta. is the distance from the from side of the
spherical lens to the entrance pupil, d is the width of the
collimated beam, A is the aperture of the entrance pupil, EFL is
the effective focal length and NA is the numerical aperture of the
source.
14. The test station of claim 1, wherein, said memory stores a
second activation profile to generate as video signal or a
two-dimensional scene, said controller responsive to the second
activation profile to simultaneously active multiple sources for
each frame of the video signal to implement the second activation
profile, said computing device responsive to data from said SAL
detector to characterize the response of the SAL seeker to the
scene.
15. A test station for a semi-active laser (SAL) seeker including a
SAL detector, comprising: a fixture for mounting the SAL seeker,
said seeker having an entrance pupil to detect optical energy over
a field of view (FOV) and focus the optical energy into a spot on
the SAL detector; a spherical lens having a first side positioned
at a location in front of where the entrance pupil of the seeker
would be located, said spherical lens comprising one or more
materials each having an index of refraction greater than one and
less than or approximately equal to two; an array of fixed optical
sources that generate respective temporally pulsed divergent
optical beams, said sources positioned on to surface of a focal
sphere at an effective focal length of the spherical lens offset
from a second, opposite side of the spherical lens over the entire
FOV of the seeker and aligned along respective radial lines to the
center of the spherical lens so that each said divergent optical
beam is collimated by the spherical lens to form a collimated
optical beam that overlaps the entire entrance pupil of the seeker;
a memory that stores one or more source activation profiles to
perform different tests; a controller responsive to one of the
source activation profiles to activate the sources to implement the
activation profile; and a computer that selects the source
activation profile for a particular test, retrieves it from memory
and provides it to the controller and is responsive to recorded SAL
detector data to perform the test.
16. The test station of claim 15, wherein a maximum distance
between one of the sources and the entrance pupil is less than
approximately 61 cm and a maximum distance between any two of the
sources is approximately 15.25 cm.
17. The test station of claim 15, wherein spherical lens and
position of the sources on the focal sphere are prescribed by the
following set of equations: .theta. = ArcTan [ 2 ( n - 1 ) Cos [
Tan [ F O V 2 ] ] ( A 2 + ( R + .delta. ) Tan [ F O V 2 ] ) n R ] ,
d = 2 Cos [ Tan [ F O V 2 ] ] ( A 2 + ( R + .delta. ) Tan [ F O V 2
] ) , E F L = n R 2 ( n - 1 ) , and ##EQU00005## N A = n Sin
.theta. = n Sin [ ArcTan [ d / 2 E F L ] ] ##EQU00005.2## where
.theta. is the divergence of the divergent beam, R is the radius of
the spherical lens, .delta. is the distance from the from side of
the spherical lens to the entrance pupil, d is the width of the
collimated beam, A is the aperture of the entrance pupil, EFL is
the effective focal length and NA is the numerical aperture of the
source.
18. A test station for an electro optic (EO) sensor, comprising: a
fixture for mounting the EO sensor, said seeker having an entrance
pupil to detect optical energy over a field of view (FOV) and focus
the optical energy onto an EO detector; a spherical lens having a
first side positioned at a location in front of where the entrance
pupil of the seeker would be located, said spherical lens
comprising one or more materials each having an index of refraction
greater than one and less than or approximately equal to two; an
array of fixed optical sources that generate respective temporally
pulsed divergent optical beams, said sources positioned on a
surface of a focal sphere at an effective focal length of the
spherical lens offset from a second, opposite side of the spherical
lens over the entire FOV of the seeker and aligned along respective
radial lines to the center of the spherical lens so that each said
divergent optical beam is collimated by the spherical lens to form
as collimated optical beam that overlaps the entire entrance pupil
of the seeker; a memory that stores one or more source activation
profiles to perform different tests; a controller responsive to one
of the source activation profiles to activate the sources to
implement the activation profile; and a computer that selects the
source activation profile for a particular test, retrieves it from
memory and provides it to the controller and is responsive to
recorded EO detector data to perform the test.
19. The test station of claim 18, wherein a maximum distance
between one of the sources and the entrance pupil is less than
approximately 61 cm and a maximum distance between any two of the
sources is approximately 15.25 cm.
20. The test station of claim 18, wherein spherical lens and
position of the sources on the focal sphere are prescribed by the
following set of equations: .theta. = ArcTan [ 2 ( n - 1 ) Cos [
Tan [ F O V 2 ] ] ( A 2 + ( R + .delta. ) Tan [ F O V 2 ] ) n R ] ,
d = 2 Cos [ Tan [ F O V 2 ] ] ( A 2 + ( R + .delta. ) Tan [ F O V 2
] ) , E F L = n R 2 ( n - 1 ) , and N A = n Sin .theta. = n Sin [
ArcTan [ d / 2 E F L ] ] ##EQU00006## where .theta. is the
divergence of the divergent beam, R is the radius of the spherical
lens, .delta. is the distance from the from side of the spherical
lens to the entrance pupil, d is the width of the collimated beam,
A is the aperture of the entrance pupil, EFL is the effective focal
length and NA is the numerical aperture of the source.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims benefit of priority under 35 U.S.C.
120 as a continuation-in-part application of co-pending U.S. patent
application Ser. No. 12/947,234 entitled "Fixed-Source Array Test
Station for Calibration of a Semi-Active Laser (SAL) Seeker" filed
on Nov. 16, 2010, the entire contents of which are incorporated by
reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates to guided projectiles that engage
targets by detecting and following laser light scattered from the
targets, and more particularly to a test station for calibration of
the spatial transfer function (STF) of the semi-active laser (SAL)
seeker and other testing, as well as testing other electro-optic
(EO) sensors.
[0004] 2. Description of the Related Art
[0005] Laser guided ordinance is commonly used to engage point
targets with a high probability of success and minimal collateral
damage. Such ordinance includes guided artillery projectiles,
guided missiles, and guided bombs, all of which will be referred to
herein as "projectiles".
[0006] A laser guided projectile's guidance system typically
includes a semi-active laser (SAL) seeker, fixed-post or gimbaled,
to detect pulsed laser electro-magnetic radiation (EMR) scattered
from the intended target and to provide signals indicative of the
target bearing and a flight controller that processes the signals
to manipulate one or more control surfaces (e.g. fins or canards)
to guide the projectile to the target. The SAL seeker includes
either a non-imaging or imaging optical system that captures and
focuses the scattered laser EMR into a spot onto a segmented
non-imaging detector (e.g. a quad-cell detector) or an imaging
detector. As the target bearing changes the position of the spot on
the detector changes (e.g. a focal plane array). The detector
compares the integrated EMR incident on each cell (segment) to
calculate a spatial displacement of the centroid of the spot. The
effective field-of-view (FOV) is dictated by the central monotonic
region of the detector's spatial transfer function (STF) in which
the spot is incident on all four cells, which is in turn determined
by the spot size. The detector's central monotonic region is
commonly referred to as the "linear" region. The seeker maps the
spatial displacement .DELTA.X and .DELTA.Y along orthogonal axes to
Azimuth (Az), Elevation (El) angles in a calibration table to
select an angle measurement as an estimate of the bearing to
target.
[0007] Referring now to FIG. 1, a laser guided projectile 100 may
engage a target 190 by detecting and following scattered laser
radiation 195 from the target 190. In FIG. 1, the target 190 is
represented as a tank, but may be another type of vehicle, ship,
boat, or a structure, building or other stationary object. The
target 190 may be illuminated with laser radiation 185 from a laser
designator 180. The laser designator 180 may be located on the
ground, as shown in FIG. 1, or may be located in a vehicle, ship,
boat, or aircraft. The laser designator could be located on the
projectile itself. This is typically referred to as an active laser
seeker. The scattered laser radiation 195 is a portion of the
illumination laser radiation 185.
[0008] The laser-guided projectile 100 may include a projectile
body 115, control surfaces 125, and a guidance system. The guidance
system may include a SAL seeker, of which only a transmissive dome
132 is visible in FIG. 1. The guidance system may include a flight
control system to control the flight of the laser guided projectile
100 by manipulating one or more control surfaces 125 based on at
least one guidance signal from the SAL seeker. In the example of
FIG. 1, the control surfaces 125 are shown as canards, but may be
fins, wings, ailerons, elevators, spoilers, flaps, air brakes or
other controllable devices capable of affecting the flight path of
the laser guided projectile 100.
[0009] FIG. 2a shows a frontal view of a detector 250 and a focused
laser spot 245. The detector 250 may comprise a "quad-cell"
detector including four quadrants or "segments" A, B, C, D. Other
detector configurations including multiple segments may be used.
Each quadrant may produce a corresponding signal A, B, C, and D in
response to the integrated laser power incident upon each quadrant.
Guidance signal .DELTA.X may indicate an imbalance between the
laser power incident upon the left (quadrants A and B) and right
(quadrants C and D) halves of the detector 250. Guidance signal
.DELTA.Y may indicate an imbalance between the laser power incident
upon the top (quadrants A and C) and bottom (quadrants B and D)
halves of the detector 250. The terms "left", "right", "top", and
"bottom" refer to the detector 250 as shown in FIG. 2a and do not
imply any physical orientation of the detector 250 within a
projectile 100. When the laser spot 245 is centered on the detector
250, the signals A, B, C, D may be essentially equal and the
guidance signals .DELTA.X and .DELTA.Y may both be zero or nearly
zero.
[0010] More particularly, the detector 250 may effectively measure
the centroid of the incident EMR on the detector 250. The spatial
transfer function (STF) 255 is a ratio of the laser power on the
different quadrants of the detector. When laser power in spot 245
is hitting all four quadrants A-D, the guidance system operates in
a linear region (or more generally a "monotonic" region) 260 of the
transfer function 255. Within the linear region
.DELTA.X=((A+D)-(B+C))/(A+B+C+D) and
.DELTA.Y=((A+B)-(C+D)/(A+B+C+D) where A, B, C and D are integrated
laser power incident on the respective cells. The transfer function
255 in the linear region 260 determines via a calibrated look-up
table (LUT) the Az, El angles of the guidance system from the
target (e.g. target bearing). When laser power is hitting only two
quadrants, the guidance system operates outside the linear region,
where the transfer function nears +/-1. The guidance system only
knows the direction towards the target, but not its true angle.
[0011] The SAL seeker is calibrated offline to generate the
calibration LUT that maps the measured and calculated .DELTA.X and
.DELTA.Y to the Az, El angle pairs over a field-of-view (FOV) and
with an angular resolution required for a mission. The SAL detector
is mounted on a high precision 3-axis stage. A single Q-switched
laser and collimator are mounted in a static fixture to direct a
pulsed beam to simulate a spot reflected off a target. A controller
rotates the SAL sensor on the 3-axis stage to specified locations
to detect the stationary target. A computer records the
measurements and maps each .DELTA.X, .DELTA.Y pair to the Az,El
pair for each location of the stage to generate the LUT. This test
station and methodology is both expensive and slow. Currently, the
total cost of a test station is a few to several hundred thousand
dollars for the Q-switched laser and focusing optics, the
collimator and the 3-axis stage. The cost is driven by the required
angular resolution of both the source and rotation of the stage. A
typical medium-fidelity calibration may involve 400 measurements.
For each measurement, the 3-axis stage must rotate to the specified
location and settle, which takes approximately 2.5 seconds. Data
acquisition requires another 0.5 seconds to detect a sufficient
number of pulses (e.g. 50) at the Q-switched laser's maximum
operable PRF (e.g. 100 Hz) for a high SNR measurement. The
medium-fidelity calibration of a single SAL seeker requires
approximately 20 minutes. A high-fidelity calibration may take up
to 50 minutes. Furthermore, as both the FOV and fidelity
specifications increase the calibration time will increase. In a
manufacturing environment that must calibrate thousands of seekers
the time requirement is burdensome and expensive. Their remains a
long felt need for a more cost-effect and time-efficient method to
calibrate SAL seekers.
SUMMARY OF THE INVENTION
[0012] The following is a summary of the invention in order to
provide a basic understanding of some aspects of the invention.
This summary is not intended to identify key or critical elements
of the invention or to delineate the scope of the invention. Its
sole purpose is to present some concepts of the invention in a
simplified form as a prelude to the more detailed description and
the defining claims that are presented later.
[0013] The present invention provides a compact test station for
calibration of SAL seekers with a high unit throughput. The test
station may also be used to calibrate and otherwise test other
types of EO sensors.
[0014] In an embodiment, a test station for calibrating a
semi-active laser (SAL) seeker including a SAL detector comprises a
fixture for mounting the SAL seeker. This fixture may be fixed or
allow some range of motion to adjust the alignment of the seeker
but does not require a 3-axis stage. The seeker has an optical
system with an entrance pupil that gathers optical energy over a
field of view (FOV) and focuses the optical energy into a spot on
the SAL detector. The SAL detector may comprise a segmented
non-imaging detector such as the convention quad-cell detector or
an imaging detector such as a FPA.
[0015] An array of fixed optical sources generates respective
temporally pulsed divergent optical beams. A spherical lens having
a first side is positioned at a location in front of where the
entrance pupil of the seeker would be located. The spherical lens
comprises one or more materials each having an index of refraction
greater than one and less than or equal to two. The sources are
positioned on a surface of a focal sphere at the effective focal
length of the spherical lens offset from a second, opposite side of
the spherical lens over the entire FOV of the seeker and aligned
along respective radial lines to the center of the spherical lens.
The spherical lens collimates each divergent optical beam to form a
collimated optical beam that overlaps the entire entrance pupil of
the seeker. The spherical lens reduces the overall scale of the
test station, reducing both the distance from the sources to the
entrance pupil of the seeker and reducing the distance between
sources to cover the FOV.
[0016] A memory stores one or more source activation profiles to
perform different tests. The profile may activate the sources one
at a time in a phase delayed manner generate a transfer function
mapping a spatial displacement .DELTA.X and .DELTA.Y in orthogonal
axes of a centroid of the spot to azimuth and elevation angles for
the source of the spot to "calibrate" the SAL seeker. The profile
may activate the sources to "verify" the transfer function. The
profile may activate the sources with a two-dimensional video
signal to project a video scene onto the SAL seeker to
"characterize" the performance of the seeker. A controller
responsive to one of the source activation profiles activates the
sources to implement the activation profile. A computer that
selects the source activation profile for a particular test,
retrieves it from memory and provides it to the controller and is
responsive to recorded SAL detector data to perform the test.
[0017] These and other features and advantages of the invention
will be apparent to those skilled in the art from the following
detailed description of preferred embodiments, taken together with
the accompanying drawings, in which:
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1, as described above, is a schematic illustration of a
laser-guided projectile engaging a target;
[0019] FIGS. 2a and 2b, as described above, are an embodiment of a
quad-cell detector and its spatial transfer function;
[0020] FIG. 3 is a diagram of a compact fixed-source array test
station using a spherical lens for calibration of a SAL seeker in
accordance with the present invention;
[0021] FIG. 4 is an optical diagram of the fixed-source array
positioned on a surface of a focal sphere at the back focal length
of the spherical lens;
[0022] FIGS. 5a-5c are design flow and optical diagrams and a
three-dimensional plot of the solution space of an embodiment of
the process for designing the compact fixed-source array test
station;
[0023] FIGS. 6a-6b are diagrams of an embodiment of a source
comprising a Light Emitting Diode (LED);
[0024] FIGS. 7a and 7b are diagrams of a VCSEL with bare die and
chip on submount packaging options;
[0025] FIG. 8 is diagram of a point source projector for either the
LED of VCSEL;
[0026] FIG. 9 is a diagram of an embodiment of a high-resolution
fixed-source array using a lenslet array;
[0027] FIGS. 10a and 10b are electronics and opto-mechanical block
diagrams for the fixed-source array; and
[0028] FIG. 11 is a flow diagram for the fixed-source array test
station to calibrate a SAL seeker.
DETAILED DESCRIPTION OF THE INVENTION
[0029] The present invention provides a compact cost-effective
high-throughput test station for testing optical sensors that
require stimulus at fixed angular positions over a FOV. For
example, SAL seekers require stimulus one point at a time over the
FOV to perform the STF calibration to generate the transfer
function. The station may also be used to perform verification of
the transfer function, characterization of seeker performance in
response to a video signal stimulus and to calibrate the boresight
alignment between the SAL and infrared (IR) sensors in a dual-mode
system. A dual-mode sensor system typically consists of co-aligned
SAL and long wave infrared (LWIR) sensors, but in general the IR
imaging sensor can operate in any desired band. The system may also
be used to calibrate the distortion in an IR imaging system and to
calibrate the gimbal orientation in a gimbaled system. These tests
may require source activation profiles with temporal and/or spatial
patterns. The different tests may be conductor for either
non-imaging detectors (e.g. quad-cell) or imaging detectors (e.g. a
FPA).
[0030] The test station includes an array of optical sources at
fixed angular positions relative to the center of a spherical lens
positioned in front of the entrance pupil of the seeker's optical
system. The number of sources depends on the extent of the FOV and
required angular fidelity for the test. For typical SAL detectors
this may range, for example, from 100 to 1,000 points, hence
sources. For this fixed-source array test station to replace the
conventional single-source 3-axis stage sensor approach the test
station should markedly reduce overall system cost and
significantly improve test throughput while maintaining
performance, and do so in a compact footprint and physical
configuration that is practical. For example, it is desirable that
the entire test station fit on an optical bench.
[0031] The problems to overcome as apparent to one skilled in the
art were many. To reduce system cost below a hundred thousand
dollars, the per source cost had to be very low. The combination of
the sources and spherical lens had to satisfy the collimation
specification at a reasonable distance (e.g. within a few feet)
from the sensor. The sources had to be small enough that their
placement on the focal sphere offset from the back of the spherical
lens at the required angular resolution at that distance did not
interfere with each other mechanically or optically (vignetting the
beam of the neighboring source). Each source had to be capable of
generating its pulsed beam with a sufficiently high pulse
repetition frequency (PRF) such that the measurement time for a
single point is sufficiently short. The fixed-source array had to
be capable of switching from one source to another source very fast
to maintain high throughput for point-by-point tests over the FOV
such as SAL calibration or to perform more sophisticated activation
patterns such as for SAL verification or characterization. Each of
these challenges were overcome to provide a compact fixed-source
test station that is on the order of a factor of ten less expensive
and capable of a throughput that is at least three orders of
magnitude (1,000.times.) faster than the conventional test station
for calibrating a SAL STF.
[0032] Without loss of generality, an embodiment of a test station
configured to calibrate the STF for a SAL seeker having a quad-cell
detector will be described. One of ordinary skill in the art will
appreciate that the test station may be configured to perform other
tests such as verification and characterization of the SAL seeker
or may be configured to perform such tests with an imaging
detector. The test station may also be configured to perform tests
that require stimulus at fixed angular positions over the FOV for
any EO sensor.
[0033] Referring now to FIGS. 3 and 4, a test station 300 includes
a sensor fixture 302 for mounting a SAL seeker 304 (the unit under
test). Fixture 302 may be a static fixture that provides for no
movement, or it may be a fixture that provides some limited range
of motion to align the SAL seeker prior to calibration and is then
fixed throughout the test procedure or may provide for simple
rotation about an axis if used in combination with a linear array
of fixed sources. SAL seeker 304 comprises an optical system 306
having an entrance pupil 308 that gathers optical energy over a FOV
and focuses the energy into a spot on a SAL detector such as shown
in FIGS. 2a and 2b. The optical system may comprise a diffuser that
spatially homogenizes the incident light to effectively increase
the FOV. The SAL detector is mounted inside the seeker behind the
optical system. The optical system may be fixed-post or mounted on
a 1- or 2-axis gimbal behind a transmissive dome 310. A power
supply 312 provides power for fixture 302 (if needed) and for SAL
seeker 304 to drive the SAL detector and possibly a gimbal.
[0034] A spherical lens 313 having a first side is positioned at a
location directly in front of the entrance pupil 308 of the seeker.
The spherical lens comprises one or more materials each having an
index of refraction n greater than one and less than or
approximately equal to two. The lens may comprise a single unitary
material or multiple materials of differing indices of refraction,
known as a "gradient index spherical lens" or a "Luneberg lens".
The radius R of the spherical lens will typically be between 2.54
cm and 15.24 cm. The effective focal length (EFL) of the lens is
greater than its radius R for 1<n<.apprxeq.2 (e.g. n less
than or approximately equal to 2).
[0035] An array of fixed optical sources 314 generates respective
temporally pulsed divergent optical beams 315. The sources are
positioned on a surface of a focal sphere 316 at the effective
focal length of the spherical lens offset from a second, opposite
side of the spherical lens over the entire FOV of the seeker at
different fixed angular positions aligned along respective radial
lines to the center 317 of the spherical lens. The surface of the
focal sphere 316 is suitably a partial shell 318 that is mounted on
and offset from the side, opposite side of the spherical lens. The
spherical lens collimates each divergent optical beam 316 to form
respective collimated optical beams 320 that each overlap the
entire entrance pupil 308 of the seeker.
[0036] The spherical lens may introduce a small spherical
aberration if the entrance pupil is illuminated towards the edge of
the collimated beam. Options are to accept the spherical
aberration, to either add a secondary curvature to the base curve
of the focal sphere or modify the optics of the optical sources to
compensate for the spherical aberration, or to characterize the
aberration and compensate the waveform numerically.
[0037] The spherical lens reduces the overall scale of the test
station, reducing both the distance from the sources to the
entrance pupil of the seeker and reducing the distance between
sources to cover the FOV. The distance between any source and the
entrance pupil is typically less than approximately 61 cm. The
distance between any two sources at opposites sides of the FOV is
typically less than approximately 15.25 cm. The EFL is typically
less than approximately 38 cm. At this scale, the entire test
station can be positioned on the top of a typical optical bench
322.
[0038] A computer device 330 (e.g. a computer) controls the
operation of and processes data from the SAL seeker and selects an
activation profile 332 stored in memory 334 and sends the
activation profile to control electronics 321 to activate the
sources in accordance with the profile. To calibrate the STF for
the SAL detector, the sources are activated in a phase delayed
manner (mechanically or electronically switched) to measure the
spatial displacement of the spot for each individual source. The
known angular position (Az, El) of each fixed source is paired with
the measured spatial displacement (.DELTA.X, .DELTA.Y) to create a
table entry for the transfer function in a calibration table.
Depending on the pattern of the fixed-source array, measured values
may not exist for all table entries. These values may be calculated
by interpolating from measured values. The table entries may also
be interpolated to provide entries at regular intervals of spatial
displacement. To characterize the performance of the SAL seeker,
the sources can be sequentially activated in an arbitrary spatial
pattern, or can be are activated in combinations to project a
two-dimensional video signal onto the SAL detector. The computer
records and then evaluates the response of the SAL seeker to
different scenes.
[0039] An embodiment for designing the test station is illustrated
in FIGS. 5a through 5c. The SAL seeker dictates the aperture size A
of the entrance pupil as a constraint. (step 350). The FOV and
angular resolution of the FOV by the sources are specified design
inputs (steps 352 and 354).
[0040] Equation 1 generates a solution space 356 as shown in FIG.
5c (step 358).
.theta. = ArcTan [ 2 ( n - 1 ) Cos [ Tan [ F O V 2 ] ] ( A 2 + ( R
+ .delta. ) Tan [ F O V 2 ] ) n R ] ( 1 ) ##EQU00001##
[0041] where .theta. is the divergence angle of the optical source,
n is the index of refraction, A is the aperture size, R is the
radius of the spherical lens and .delta. is the SAL offset between
entrance pupil 308 and the surface of spherical lens 313 as shown
in FIG. 5b. Solution space 356 includes three-dimensional surfaces
360 in (n,R,.delta.) for different divergence angles .theta. such
as that might be available for commercial off-the-shelf
sources.
[0042] The designer selects a surface 360 corresponding to a
particular divergence angle .theta. (step 362). This selection may
be influenced by the divergence angles that are supported by
optical sources that satisfy other criteria such as cost or size.
The designer selects a specific solution set of the lens
prescription (n, R) and the SAL offset .delta. (step 364). The
selection of a specific solution set may be influenced by, for
example, the available and expense of a spherical lens with a
particular radius R and index of refraction n or by the need to
satisfy a minimum SAL offset dictated by the seeker.
[0043] The next step is to calculate the diameter d of the
collimated beam 320 produced by spherical lens 313 in accordance
with equation 2 (step 366).
d = 2 Cos [ Tan [ F O V 2 ] ] L , where , L = A 2 + ( R + .delta. )
Tan [ F O V 2 ] ( 2 ) ##EQU00002##
[0044] where L is the distance measured vertically from the center
of entrance pupil 308 of aperture size A to the center of
collimated beam 320 for an optical source 314 at the edge of the
FOV. The solution set ensures that the diameter d of the collimated
beam 320 is such that the collimated beam for any source 314 in the
FOV will overlap the entire entrance pupil 308.
[0045] The designer designs the optical source 314 in accordance
with equations 3-7 (step 368).
E F L = n R 2 ( n - 1 ) ( 3 ) N A = n Sin .theta. = n Sin [ ArcTan
[ ( d / 2 ) / E F L ] ] ( 4 ) # Sources = F O V / Angular
Resolution ( 5 ) Source Spacing = E F L F O V / # Sources ( 6 )
Source Dimension <= Source Spacing ( 7 ) ##EQU00003##
[0046] For 1<n<.apprxeq.2, the EFL is greater than R
according to equation 3. This offset is referred to as the back
focal length.
[0047] The numerical aperture NA of the source determines the
amount of light from the source that is coupled into the entrance
pupil. The NA is related to the beam width d, which is in turn
related back to the divergence angle .theta.. The requirement for a
certain NA can effect the selection of the divergence angle in step
262.
[0048] The number of sources (#Sources) is the ratio of the FOV
over the angular resolution specified in step 354. The source
spacing is the ratio of the FOV to the number sources multiplied by
the effective focal length (EFL). The source dimension or
"footprint" of the source must be less than or equal to the source
spacing in order to position the array of optical sources on the
focal sphere to achieve the specified angular resolution.
[0049] If an acceptable design for optical source 314 is determined
that satisfies all of the different constraints, inputs and other
practical considerations (step 370) the design is completed (step
372). If not, the designer can select a different solution set at
step 364, and possibly a different divergence angle at step 362 and
repeat the optical source design. Once complete, equations 1-4
prescribe the design of the spherical lens 313, optical source 314
and positioning of the array of sources 314 on the focal sphere 316
such that the spherical lens collimates the divergent beams 314
into respective collimated beams 320 that each overlap the entire
entrance pupil 308.
[0050] A representative solution space for an Aperture size A of
2.54 cm and a FOV of .pi./4 for equations 1-7 is depicted in Table
1 for EFL as a function of Radius R and index n, Table 2 for Beam
Diameter d as a function of offset 6 and Radius R and Tables 3a and
3b for optical source spacing for optical source design as function
of Radius R and Index n. In Tables 3a and 3b the empty cells
represent solutions that are not physically realizable because the
source dimension is greater than the specified angular resolution
and source spacing that would be required.
TABLE-US-00001 TABLE 1 Effective Focal Length (EFL) in cm Index (n)
1.43 1.50 1.52 1.54 1.72 Radius (R) cm 5.08 8.46 37.62 5.13 7.24
6.02 10.16 16.89 15.24 14.86 14.48 12.04 15.24 25.35 22.86 22.28
21.74 7.11 20.23 13.30 12.00 11.69 11.41 18.06
TABLE-US-00002 TABLE 2 Beam Diameter, d in cm Radius, R in cm 5.08
10.16 15.24 20.23 Offset, .delta. cm 2.54 8.10 11.96 15.80 19.66
3.81 9.07 12.93 16.76 20.62 5.08 10.03 13.89 17.73 21.59 6.35 11.00
14.83 18.69 22.55
TABLE-US-00003 TABLE 3a Source Dimension of 0.95 cm Resolution
(rad/proj) = 0.12 Resolution (rad/proj) = 0.06 # of sources = 13 #
of sources = 26 Index, n Radius 1.43 1.5 1.52 1.54 1.71 1.43 1.5
1.52 1.54 1.71 5.08 10.16 1.02 15.24 1.52 1.37 1.35 1.32 1.09 20.23
2.03 1.83 1.80 1.75 1.45 1.02
TABLE-US-00004 TABLE 3b Source Dimension of 0.13 cm Source
Dimension of 0.0051 cm Resolution (rad/proj) = 0.015 Resolution
(rad/proj) = 0.0008 # of sources = 100 # of sources = 200 Index, n
Radius 1.43 1.5 1.52 1.54 1.71 1.43 1.5 1.52 1.54 1.71 5.08 10.16
0.127 0.127 0.127 0.08 0.05 0.05 0.05 0.05 15.24 0.20 0.18 0.18
0.18 0.15 0.10 0.10 0.08 0.08 0.08 20.23 0.25 0.23 0.23 0.23 0.18
0.127 0.127 0.127 0.10 0.10
[0051] To appreciate the impact of the spherical lens, consider the
following representative example. For an aperture A of 2.54 cm, a
FOV of .pi./4, an optical material of n=1.52 (i.e., crown glass,
BK7), a spherical lens of radius 15.24 cm, SAL offset of
.delta.=1.02 cm and an EFL of 22.35 cm, the maximum distance from a
source to the entrance pupil is only 45.72 cm and the maximum
distance between two sources at opposite sides of the FOV is only
11.68 cm. This provides a compact footprint for the test station
that can be placed on a typical optical bench.
[0052] Depending on the FOV of the SAL Seeker and the required
fidelity of the fixed angular positions of the sources on the focal
sphere, the number of sources may be 100 to 1,000 or even higher.
To be cost-effective, the per source cost must be low. This must be
achieved without sacrificing angular fidelity and a test system
footprint and geometry that is practical. We use SAL sources such
as LEDs or VCSELs that have a large divergence angle. Initially it
was thought that such sources could not be collimated in a manner
that was cost-effective or physically practical; the sources would
have to be placed at long distances from the sensor to appear in
focus and not impact the calibration performance. These types of
less-expensive sources are notorious for not having a repeatable
divergence angle and can require highly sophisticated optics to
collimate accurately. Furthermore, it was thought that the size of
the optics required to overfill the entrance pupil would make the
geometric construction of the array impossible, adjacent sources
would either physically interfere with each other or would vignette
the optical beam of the neighboring sources. We found that we could
collimate all of the rapidly diverging beams with a single
spherical lens in a compact footprint at low cost. Optical
throughput could be sacrificed and still have sufficient power to
project a spot of enough intensity onto the SAL detector.
[0053] The throughput of the test system is largely determined by
the dwell time of a given source needed to reliably detect a spot
and the lag time to switch from activation of one source to
activation of another source. The dwell time is a function of the
number of pulses the sensor must detect to reliable detect a spot
and the PRF of the source to deliver those pulses. Optical sensors
such as the SAL detector can generally detect pulses over a certain
pulse width range. Within that range the sensor may exhibit
non-uniform noise properties. In lower noise regions of the pulse
width range, the sensor detects incident pulses with a higher SNR
and thus requires fewer received pulses to measure a spot position
to the fidelity required for calibration. Both LEDs and VCSELs are
capable of generating pulsed beams at very high PRFs, 10 kHz and
higher. Furthermore, LEDs and VCSELS can be controlled via exterior
electronics allowing for direct user control of the pulse width.
The pulse width may be selected in a low noise region of the
sensor. As a result, the dwell time at each source is at most
approximately 100 ms, more typically approximately 1 ms and as low
as approximately 0.1 ms. Because we are not slewing the sensor and
do not have to wait for its response to settle, the lag time to
switch is very short. The maximum time would be approximately 100
ms using a mechanical switch and a typical time would be
approximately 100 micro seconds using electronic switching. In
general, this time could be reduced even further until the point at
which either the sensor under test is unable to resolve the
separation of pulses, or the rise and fall time of the
electronically activated sources limits pulse separation. The
sensor under test can be designed to operate at a variety of
different clock rates with a particular electronic bandpass, while
typical LED and VCSEL rise and fall times are physically limited to
approximately 10 ns,
[0054] Referring now to FIGS. 6a-6b, an embodiment of an optical
source comprises a Light Emitting Diode (LED) 400. The LED consists
of an anode 402, a cathode 404 and a semiconductor p-n junction
406. When the diode is forward biased by application of a voltage,
current flows from the anode 402 to the cathode 404, the charges
carriers (electrons and holes) flow into the semiconductor p-n
junction 406 (the active region) and combine. This lowers the
energy level of the electron. The excess energy is released as a
photon 408 through the transparent window 410.
[0055] The wavelength of the photon emitted by an LED depends on
the band gap in the semiconductor. With proper selection of
materials and/or doping concentration, the band gap can be
engineered to produce photons anywhere from the near-UV to the
near-IR, making it an excellent candidate for a calibration source
for SAL systems. In addition, when the LED is properly activated,
rise and fall times of approximately 10 ns are possible, allowing
pulse widths from 20 ns to continuous wave. This functionality
allows SAL calibration to be performed at a pulse width that
minimizes the noise in amplitude estimation. A typical complaint of
LED sources is that the divergence angle makes them difficult to
collimate. However, if power levels are sufficient, a higher f/#
optic can be used for collimation, making this a moot point. In
addition, the large divergence provides a hidden advantage, by
allowing a single optic to act as both a beam expander and
collimator.
[0056] Referring now to FIGS. 7a-7b, an embodiment of an optical
source comprises a Vertical Cavity Surface Emitting Laser (VCSEL)
500 in both a bare die and chip on submount packaging options. The
VCSEL is a semiconductor laser diode with laser emission 502
orthogonal to the top surface. The component consists of quantum
well region 504 (gain media) in addition to a p and n type
semiconductor Bragg reflector region 506 that serves as the laser
cavity. The component may be mounted as a chip on a submount 508.
The wavelength of the photons emitted by a VCSEL is determined by
the cavity geometry and gain media. With available manufacturing
VCSEL wavelengths range from the visible to the short wave
infrared, again making them particular useful for SAL calibration.
While the divergence angle is smaller than a typical LED, it is
still large enough to allow for a single optic to expand and
collimate the source. However, due to the smaller divergence a
VCSEL can provide a higher energy density. The VCSEL emission area
is also smaller leading to a relaxation in the optic focal length
requirement for a particular angular target size.
[0057] Ideally each optical source is a "point source" on the
surface of the focal sphere. The closer the optical source is to a
true point source, the more collimated the image projection. The
area of a collimated projected point source image is determined by
the numerical aperture of a spherical lens and the illumination
source--the larger the angle of projection onto the spherical lens
surface, the larger the output collimated beam.
[0058] Inexpensive off-the-shelf LED or VCSEL are not point
sources. Point sources are more accurately created with an optical
focusing system producing a virtual image by a lens system. This
lens system may be provided by conventional discrete optical
components or by a lenslet array. The more expensive or new
technologies may provide optical sources that are close enough to a
point source to not require an additional focusing system.
[0059] Referring now to FIG. 8, an embodiment of an optical source
600 includes a source 602 such as an LED or VCSEL in a housing 603
that emits a diverging beam 604. A single optical component such as
a plano-convex lens 606 collimates the beam to form an internal
collimated beam 608. A single optical component such as a negative
lens 610 projects the internal collimated beam into a divergent
beam 612 to approximate a point source. Other configurations of
optical components can produce a point source projection from a non
point source such as an Off-The-Shelf (OTS) LED or VCSEL.
[0060] Referring now to FIG. 9, an embodiment of a high resolution
array of optical sources includes an array of non-point optical
sources 650 such as LEDs formed on a first substrate 652 conformal
with the surface of the focal sphere and an array of lenslets 654
formed on a second substrate 656 aligned to the array of non-point
optical sources and conformal with the surface of the focal sphere.
An intermediate substrate 658 including electrical traces 660 for
addressing optical sources 650 is suitably positioned between the
source and lenslet arrays. The substrates are suitably formed of a
compliant material to conform to the surface of the focal sphere.
Each element in the array projects a beam with a specified
divergence from an approximate point source. Integration of the
lenslet array with an array of optical sources provides a much
smaller per-source footprint, and the possibility for much higher
angular resolutions to project video signals onto the detector.
[0061] Referring now to FIGS. 10a and 10b, block diagrams for
embodiments of the control electronics 700 for driving the
fixed-source array and the opto-mechanical test bed 701 are
illustrated. Control electronics 700 include a power supply 702 and
a circuit card assembly (CCA) 704 that stores the activation
pattern to activate the sources one at a time with minimum lag time
to perform the STF calibration or simultaneously in a video signal
to perform other tests. An electronics switch tree 706 sends
control signals to activate the different sources 708 in accordance
with the specified phase delays to generate pulsed beams 710 in
sequence. Each source 708 is also provided with a CCA 712 that
controls the pulse width of the beam and provides adequate
termination to minimize reflections back to the switch tree.
[0062] Opto-mechanical test bed 701 includes a spherical lens 713
and a spherical back shell 714 that is a section of the focal
sphere at the EFL of the spherical lens whose center of curvature
coincides with the center of the spherical lens so that optical
sources positioned on and orthogonal to back shell 714 are aligned
along respective radial lines to the center of the spherical lens.
Divergent optical beams from each optical source are collimated by
the spherical lens to form collimated optical beams that overlap
the entire entrance pupil of the seeker. This greatly simplifies
alignment of the sources and reduces the footprint of the test
system. A precision positioner 716 provides focus and tip/tilt
adjustment for the LED or VCSEL source. An opto-mechanical assembly
718 holds the source and provides housing for the projection
optics. An optical source assembly provides a stimulus or "target"
at the fixed angular position for each source as a divergent beam
from a point source. A sensor fixture 722 aligns the sensor (SAL
seeker) to the fixed-source array. The sensor fixture may be static
or provide some range of motion to correct for any alignment
errors.
[0063] Referring now to FIG. 11, an embodiment of the procedure for
calibrating the SAL seeker's STF using the fixed-source array test
station is illustrated. The unit under test, or UUT (SAL Seeker),
is powered on to active the SAL detector and any other ancillary
systems (step 800). The computer selects the source activation
profile for SAL Seeker STF calibration and sends the profile to the
control electronics that loads the appropriate phase delay into the
CCA, and sends a message to the UUT to begin recording data (i.e.
spatial displacements .DELTA.X and .DELTA.Y) (step 802). The UUT
begins recording data (step 804) and the individual sources are
activated in a phase delayed manner one at a time to implement the
activation profile (step 806). The UUT measures and records the
spatial displacement for each source (step 808) and sends the
recorded data to the computer (step 810). The computer records a
data entry for the measured spatial displacement and known angular
position of the source (step 812). This is done for each source to
develop an STF calibration table 900 for the UUT. The computer may
perform an interpolation of the data to fill in missing entries or
to provide regularly indexed entries. The computer then sends the
calibration table to the UUT where it is saved in memory (step
814). Similar procedures are followed to activate the sources
either one at a time or with a video signal to perform verification
or characterization tests.
[0064] While several illustrative embodiments of the invention have
been shown and described, numerous variations and alternate
embodiments will occur to those skilled in the art. Such variations
and alternate embodiments are contemplated, and can be made without
departing from the spirit and scope of the invention as defined in
the appended claims.
* * * * *