U.S. patent application number 10/105081 was filed with the patent office on 2003-07-24 for hybrid optical correlator/digital processor for target detection and discrimination.
Invention is credited to Hasson, Victor, Nordstrom, Gerald A..
Application Number | 20030137647 10/105081 |
Document ID | / |
Family ID | 26802244 |
Filed Date | 2003-07-24 |
United States Patent
Application |
20030137647 |
Kind Code |
A1 |
Hasson, Victor ; et
al. |
July 24, 2003 |
HYBRID OPTICAL CORRELATOR/DIGITAL PROCESSOR FOR TARGET DETECTION
AND DISCRIMINATION
Abstract
An optical correlator is used to detect the presence of a target
in a laser detection and ranging (LADAR) system by analyzing the
return signal, and to provide initial estimates of the targets
range and velocity to the LADAR receiver. The optical correlator
includes an acoustic optical Bragg cell that deflects an input
laser signal using the received and down-converted LADAR signal. A
the integrating optical detector is disposed to receive the optical
outputs of the acoustic optical Bragg cell and an optical processor
analyzes and processes the integrating optical detector data. The
integrating optical detector integrates the optical outputs of the
Bragg cell over time and the integrating optical detector output is
sampled over a sampling period so that target detection is
uncorrelated to noise. A the integrating optical detector data
point that exceeds a predetermined threshold is considered to be a
valid detected target and the location of the output of the optical
correlator on the integrating optical detector is indicative of the
Doppler shift and hence the velocity of the detected target. The
time of detection of the target is indicative of the range to the
target. This allows the range-Doppler-amplitude of the target to be
estimated and provided to the receiver to allow for more accurate
processing of the receiver data. In addition, the optical
correlator can be used to provide whole body Doppler and range
estimates of the target and can also inherently averages the
speckle data.
Inventors: |
Hasson, Victor; (Winchester,
MA) ; Nordstrom, Gerald A.; (Truxton, NY) |
Correspondence
Address: |
WEINGARTEN, SCHURGIN, GAGNEBIN & LEBOVICI LLP
TEN POST OFFICE SQUARE
BOSTON
MA
02109
US
|
Family ID: |
26802244 |
Appl. No.: |
10/105081 |
Filed: |
March 22, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60351556 |
Jan 24, 2002 |
|
|
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Current U.S.
Class: |
356/5.01 |
Current CPC
Class: |
G01S 17/50 20130101;
G01S 7/481 20130101; G02B 27/46 20130101; G01S 13/587 20130101;
G06E 3/001 20130101; G01S 7/487 20130101 |
Class at
Publication: |
356/5.01 |
International
Class: |
G01C 003/08 |
Claims
What is claimed is:
1. A hybrid processor for detecting a target in an intermediate
frequency (IF) signal, the hybrid processor comprising: a
correlator RF drive module receiving the IF signal, the correlator
RF drive module configured and arranged to up-convert the IF signal
into a correlator drive signal having a correlator frequency and to
bandlimit the correlator drive signal; an optical spectrum
analyzer/correlator including a laser source providing a laser
beam; an acousto-optical Bragg cell having a modulator input
coupled to the correlator RF drive module and receiving the
correlator drive signal therefrom; collimating optics disposed
between the laser source and the acousto optical Bragg cell the
collimating optics configured and arranged to collimate the laser
beam and to provide a collimated laser beam, the acousto-optical
Bragg cell disposed to receive the collimated laser beam; the
acousto-optical Bragg cell responsive to the correlator drive
signal by diffracting the collimated laser beam, the
acousto-optical Bragg cell providing a plurality of diffracted
output light beams and an undiffracted output light beam; an
optical system configured and arranged to receive the plurality of
diffracted light beams and to Fourier transform the plurality of
diffracted light beams and further configured and arranged to image
the plurality of the diffracted light beams onto an image plane; an
integrating optical detector array including a plurality of
photodetectors, the optical photodetector array disposed within the
image plane and providing a plurality of output signals each
corresponding to the output of one of the plurality of
photodetectors; an optical processor coupled to the integrating
optical detector array and receiving the plurality of output
signals therefrom, the optical processor operative to process the
plurality of output signals to detect a target.
2. The hybrid processor of claim 1 wherein the optical processor in
processing the received plurality of output signals is operative to
compare each of the plurality of output signals to a predetermined
threshold, wherein a target is detected when one of the output
signals exceeds the predetermined threshold.
3. The hybrid processor of claim 1 wherein the optical processor is
further operative to correlate the plurality of output samples
modulo the number of the plurality of photodetectors divided by the
number of the plurality of diffracted light beams.
4. The hybrid processor of claim 1 wherein the optical processor is
further operative to estimate the Doppler shift of the target as a
function of the position of the photodetector corresponding to the
detected output signal within the plurality of photodetectors.
5. The hybrid processor of claim 1 wherein the optical processor
further receives an indicia of the transmission of a pulse, and
estimates the target range as a function of the time delay between
the transmission to the target detection.
6. The hybrid processor of claim 1 wherein the optical processor
includes: a timing and control module operative to provide a
plurality of control signals; an accumulator coupled to the the
integrating optical detector array and receiving the plurality of
output signals therefrom, the accumulator configured and arranged
to accumulate the plurality of output signals over a predetermined
period of time; a optical processor memory coupled to the
accumulator and configured and arranged to store the plurality of
accumulated values wherein each of the plurality of accumulated
values corresponds to a respective photodetector; a threshold
detector coupled to the optical processor memory, the threshold
detector configured and arranged to compare each of the plurality
of accumulated values to a predetermined threshold, wherein in the
event that one of the plurality of accumulated values exceeds the
predetermined threshold a threshold signal is provided to the
timing and control module; the timing and control module responsive
to the threshold signal by providing a cue signal output.
7. The hybrid processor of claim 6 wherein the timing and control
module is further responsive to threshold signal by estimating the
Doppler shift of the detected target as a function of the position
of the photodetector corresponding to the detected output signal
within the plurality of photodetectors.
8. The hybrid processor of claim 6 wherein the timing and control
module is further responsive to the threshold signal and further
receives an indicia of the transmission of a pulse, and estimates
the target range as a function of the time delay between the
transmission to the target detection.
9. The hybrid processor of claim 1 wherein the optical processor
provides a cue signal upon the detection of a target, the optical
processor is further operative to estimate the Doppler shift of the
target, and to estimate the range of the target.
10. The hybrid processor of claim 9 further including: a first
sample module configured and arranged to sample the IF signal and
provide a plurality of sampled IF signals; an analog to digital
converter coupled to the first sample module and configured and
arranged to digitize the plurality of sampled IF signals into a
plurality of digitized IF data; a first in first out buffer coupled
to the analog to digital converter and configured and arranged to
store a predetermined amount of the digitized IF data and dumping
excess data when overfilled; a digital signal processor coupled to
the optical processor and to the first in first out buffer; the
first in first out buffer responsive to the cue signal by not
accepting new digitized IF data and providing the stored data to
the digital signal processor; the digital signal processor
responsive to the cue signal, the estimate of the Doppler shift of
the target, and the estimate of the range of the target by
processing the stored data provided by the first in first out
buffer to determine at least one target characteristic.
11. A method for instantaneous all-range cueing, said method
comprising the steps of: providing an intermediate frequency (IF)
signal; up-converting and bandlimiting the IF signal into a
correlator drive signal having a correlator frequency; applying the
correlator drive signal to an acousto-optical Bragg cell; applying
a collimated laser beam to the acousto-optical Bragg cell;
detecting an plurality of diffracted light beams from the
acousto-optical Bragg cell; comparing the detected plurality of
diffracted light beams with a predetermined threshold; in the event
that one of the detected plurality of diffracted light beams
exceeds the predetermined threshold providing a cue signal
indicative of a detected target.
12. The method of claim 11 further including the step of estimating
the Doppler shift of the detected target.
13. The method of claim 11 further including the step of estimating
the range of the detected target.
14. The method of claim 11, further comprising the step of
receiving the cue signal in a digital signal processor.
15. A method of cueing for signal processing, said method
comprising the steps of: receiving a detected intermediate
frequency (IF) signal; converting said IF signal to a first radio
frequency (RF) signal and a second RF signal; up-converting said
first RF signal, thereby producing an up-converted signal; causing
said up-converted signal to propagate through an acousto-optic
cell; directing a probe laser signal through said acousto-optic
cell; detecting one or more diffracted output signals from said
acousto-optic cell; producing a cueing signal in response to said
detecting one or more diffracted output signals; determining one or
more characteristics of an object from said second RF signal in
response to said cueing signal.
16. The method of claim 15 further comprising the step of:
converting a wide bandwidth optical signal pulse to said IF signal
in a heterodyne detector.
17. The method of claim 16 further comprising the step of producing
said wide-bandwidth optical signal pulse from a mode-locked
laser.
18. The method of claim 15 wherein said step of converting said IF
signal is up-converting said IF signal.
19. The method of claim 18, said step of converting said IF signal
further comprising the step of filtering said IF signal, thereby
producing a filtered signal.
20. The method of claim 19 wherein said step of filtering is
match-filtering.
21. The method of claim 19 further comprising the step of
down-converting said filtered signal and thereby producing a
baseband signal.
22. The method of claim 21 further comprising resolving said
baseband signal into an in-phase components and an in-quadrature
component.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority under 35 U.S.C.
.sctn.119(e) to U.S. Provisional Patent Application Serial No.
60/351,556, filed Jan. 24, 2002.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] N/A
BACKGROUND OF THE INVENTION
[0003] Coherent long-range laser radar (LADAR) systems rely on
externally supplied data to cue the RF receiver processor to
examine received data at a known time and frequency. Such
externally supplied data can include target angle sensor data,
target range, target range rate, and target velocity. This reliance
on externally supplied target data reduces the utility of this type
of long range LADAR as a stand-alone autonomous system. The
externally supplied data is needed in these types of systems to
limit the volume of space to be searched for long-range fast-moving
targets to keep the amount of data to be stored and processed to a
manageable level.
[0004] In addition, LADAR systems having an all range capability
would require multiple pulses to overcome speckle using pulse tone
acquisition waveforms. Speckle occurs due to the surface roughness
of the target having a greater dimension than the wavelength of the
laser radar used to detect the target. This leads to interference
in which a speckle null may occur and a target missed or lost until
the target orientation relative to the the LADAR receiver has
changed sufficiently to shift the interference from the null.
Speckle is typically overcome by the use of multiple pulses of
laser radar energy to ensure that the probability of detection of a
target is nearly 100%.
[0005] Range to a target can be found by removing the Doppler shift
of the return pulse and correlating the return pulse with the
transmitted pulse to determine the phase shift of the return pulse.
This allows the time the pulse took to travel to and from the
target to be accurately determined and therefore the range of the
target may be accurately estimated. This process requires the
identification and removal of the Doppler shift from the return
pulse.
[0006] Typically to remove the Doppler shift from a signal, the
spectrum of the signal is electrically analyzed. One method of
analyzing the spectrum of a signal is to measure the energy of the
signal of interest in each of a plurality of narrow frequency
bands. The sensitivity and accuracy of this method is dependent
upon the number and the width of the selected frequency bands of a
corresponding plurality of band-pass filters. This process
typically uses a superheterodyne receiver and is referred to as a
swept spectrum analysis. However, swept spectrum analysis does not
monitor or measure all frequencies at all times. As the sensitivity
of the spectrum analyzer is increases by increasing the number of
bandpass filters and reducing the bandwidth of each of the filters,
the time required to sweep through all of the resulting frequency
bands is increased. Accordingly, the probability of intercepting a
given signal is less than 100%. In an environment in which
frequency-hopping systems are used, some signals are likely to be
missed at least some of the time.
[0007] Another method of spectrum analysis also uses narrow
bandpass filters, but in this method, a non-linear device is
coupled to the output of the corresponding bandpass filter. The
non-linear device provides an output that is dependent on the
energy contained within the passband of the corresponding filter
channel. The outputs of each of the non-linear devices are
integrated over a time interval consistent with the passband width
of the bandpass filter, and the outputs are multiplexed and sampled
at rate to ensure a probability of intercept near 100%. However,
since the frequencies of interest are typically in the radio
frequency (RF) region the component values for each of the required
bandpass filters can be awkward. In addition to ensure that there
is no corruption of the filtered signals, each bandpass filter must
have sufficient stop-band attenuation to prevent crosstalk signal
interference from adjacent bands. Accordingly, the bandpass filters
must be of a sufficiently high order to provide the necessary
stop-band attenuation to suppress these adjacent signals. The
number of components may also increase with If the order of the
filters so that these filters increase in size and the rate of
power consumption increases with the order of the filters.
[0008] The above prior art methods are primarily analog in nature,
so that the output of the various systems is nearly instantaneous.
However in some applications, analog systems are inherently less
accurate than digital systems although analog systems can provide a
speed advantage over digital systems. Digital systems have been
employed in these systems requiring high accuracy in order to
provide the required highly accurate data. In particular, Fast
Fourier transforms (FFTs) can be used to determine the spectral
energy within one or more channels of interest. However, searching
a three-dimensional volume of space for targets and attempting to
detect and estimate the range and Doppler shift of a target
requires the storage and processing in real-time of an extremely
large amount of data.
[0009] Therefore, what is needed is a system that allows for the
detection, estimation of the range, and estimation of the Doppler
shift of a target that is simple and does not require complex
analog filters or the storage and real-time processing of large
amounts of data.
BRIEF SUMMARY OF THE INVENTION
[0010] An optical correlator is used to detect and discriminate the
presence of a target in a laser detection and ranging (LADAR)
system by analyzing the return signal, and to provide initial
estimates of the targets range and velocity to the LADAR receiver.
The optical correlator includes an acoustic optical Bragg cell that
diffracts/deflects an input laser beams using the received and
down-converted LADAR signal. A set of integrating photodetectors is
disposed to receive the diffracted/deflected laser beam output of
the acoustic optical Bragg cell. The set of integrating
photodetectors integrates the diffracted/deflected laser beam
output of the Bragg cell over time and the integrated output is
sampled at a predetermined sampling period to uncorrelate the
target from noise. An optical processor analyzes and processes the
data output from the set of integrating photodetectors. If one of
the integrated outputs exceeds a predetermined threshold, a valid
target has been detected. The physical location of the particular
integrating photodetector that has received the
diffracted/deflected laser beam output of the Bragg cell is
indicative of the Doppler shift and hence the velocity of the
detected target. The time of detection of the target is indicative
of the range to the target. This allows the range-Doppler-amplitude
of the target to be estimated and provided to the receiver to allow
for more accurate processing of the receiver data. In addition, the
optical correlator can be used to provide whole body Doppler and
range estimates of the target and can also inherently average the
speckle data.
[0011] In one embodiment, a hybrid processor for detecting a target
at an intermediate frequency (IF) signal includes a correlator RF
drive module that receives the IF signal and is configured and
arranged to up-convert the IF signal into a correlator drive
signal. The correlator drive signal has a correlator frequency and
further is a band limited signal. The correlator drive signal is
coupled to an optical spectrum analyzer/correlator.
[0012] The optical spectrum analyzer/correlator includes a laser
source providing a laser beam, an acousto-optical Bragg cell having
an input receiving the correlator drive signal, and collimating
optics disposed between the laser source and the acousto optical
Bragg cell. The collimating optics are configured and arranged to
collimate the laser beam and to provide the collimated laser beam
to be incident on the acousto-optical Bragg cell. The
acousto-optical Bragg cell is responsive to the correlator drive
signal by diffracting/deflecting the incident collimated laser
beam, and wherein the acousto-optical Bragg cell provides a
plurality of diffracted/deflected output laser beams and an
undiffracted/undeflected output laser beam. An optical system is
configured and arranged to receive the diffracted/deflected laser
beam and to perform a Fourier transform on the diffracted/deflected
laser beam and to image the plurality of the diffracted Fourier
transformed laser beams onto an image plane. An optical integrating
photodetector array that includes a plurality of integrating
photodetectors that are disposed within the image plane provide a
plurality of output signals, each signal corresponding to one of
the plurality of integrating photodetectors. An optical processor
is coupled to the optical photodetector array and receives the
plurality of output signals therefrom, and is operative to process
the plurality of output signals to detect a target and to provide
an output cueing signal indicative of a detected target. In
addition, the optical processor can analyze the received data to
estimate the Doppler shift of the target and the range of the
target.
[0013] Other forms, features and aspects of the above-described
methods and system are described in the detailed description that
follows.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0014] The invention will be more fully understood by reference to
the following Detailed Description of the Invention in conjunction
with the Drawing of which:
[0015] FIG. 1 is a block diagram of one embodiment of a hybrid
optical spectrum analyzer/correlator for an all range coherent
laser radar;
[0016] FIG. 2 is a more detailed block diagram of the
acousto-optical spectrum analyzer depicted in FIG. 1; and
[0017] FIG. 3 is a more detailed block diagram of an optical
processor depicted in FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION
[0018] The disclosure of U.S. Provisional Patent Application No.
60/351,556 is hereby incorporated by reference.
[0019] FIG. 1 depicts a block diagram of an embodiment of a hybrid
processor 100 using an analog optical spectrum analyzer/correlator
and digital processing. The hybrid processor 100 receives a laser
radar ("LADAR") optical return signal and the optical receiver 102
detects the LADAR optical return signal and provides the detected
optical signal to a mixer 104. The mixer 104 also receives a first
mixer signal 103 and up-converts the detected optical return signal
to an up-converted signal having a first frequency. In the
embodiment depicted in FIG. 1, the first frequency is 7.5 GHz. The
up-converted signal is filtered using a matched filter 106 having a
bandwidth that is a first order matched filter based upon the whole
body Doppler shift of the target, the pulse shape and the pulse
width. The pulse shape and pulse width are dependent upon the
transmitter characteristics and the estimate of the whole body
Doppler shift of the target. As will be explained in more detail
below, the whole body Doppler shift is estimated and provided by
hybrid processor described herein.
[0020] After being estimated by the hybrid processor the matched
filter 106 is adjusted appropriately to remove the whole body
Doppler shift, leaving only the target rotational Doppler
information. The filtered up-converted signal is mixed in mixer 108
with a second mixer signal 105 to down-convert the up-converted
signal to a baseband signal having a second frequency. In the
embodiment illustrated in FIG. 1 the second frequency is 500 MHz.
The base band signal is split at power splitter 109 with a first
output resolved into in-phase and quadrature-components ("I&Q
components")by I&Q module 110. This down-conversion and
resolution into I&Q components cuts the digital sampling rate
that is required in half, resolves frequency fold-over and removes
the imaginary value image. The range of the target determines the
sampling rate that is needed. In one embodiment, the sampling rate
is 1 Gigasample/sec. As will be discussed in more detail below, the
estimate of the target range is provided by the optical spectrum
analyzer/correlator described herein. The I&Q components are
sampled, and digitized by analog-to-digital converter 112 and
stored in FIFO buffer 114. DSP 116 is coupled to FIFO buffer 114
can control the FIFO buffer 114 and retrieve data therefrom as
well. As will be explained below, the DSP 116 is also coupled to
the optical processor 130 and receives from the optical processor
130 a cue signal indicating the presence of a target, the estimate
of the target range, and the target Doppler shift. The DSP 116 is
responsive to the cue signal by retrieving and analyzing the data
stored in the FIFO buffer 114 to determine preselected target
characteristics, such as the target velocity, range, range rate,
and rotational velocity.
[0021] As discussed above, the baseband signal provided by mixer
108 is split by a power splitter 109 and the second output of the
power splitter 109 is mixed with a third mixer signal 119 in mixer
120. Mixer 120 and the third mixer signal 119 are used to center
the output signal of mixer 120 at a third frequency that is at or
near the center of the operating frequency range of the
acousto-optical Bragg cell (206 in FIG. 2). The output signal from
the mixer 120 is the correlator drive signal 122 that is provided
to the acousto-optical spectrum analyzer ("AOSA") 124. The AOSA 124
provides a plurality of laser beamlets to an optical
receiver-&-electronics module 126.
[0022] Referring now to FIG. 2 in which a more detailed block
diagram of the AOSA 124 and the optical receiver & electronics
module 126 is provided. The AOSA 124 includes a laser source 202
that provides a laser beam that is collimated by collimating optics
204, wherein the collimated laser beam provided by the collimating
optics 204 is incident on a acousto-optical Bragg cell ("Bragg
cell") 206. The Bragg cell 206 includes an electro-optical
transducer 205 that is coupled to the correlator drive signal 122,
wherein the incident laser beam is diffracted/deflected in response
to the correlator drive signal 122. The correlator drive signal
forms an acoustic wave within the Bragg cell 206 having a radio
frequency carrier, i.e. the third frequency in which the amplitude
affects the index of refraction of the Bragg cell 206. The Bragg
cell 206 also includes an acoustical absorber 207 on the opposite
side of the Bragg cell 206 from the electro-optical transducer 205
to absorb the correlator drive signal 122 to avoid unwanted
internal reflections of the drive signal 122 within the Bragg cell
206. The Bragg cell 206 diffracts/deflects the incident collimated
laser using the modulated acoustic wave, i.e. the correlator drive
signal 122. Accordingly, the modulated acoustic wave determines the
space modulation of the collimated laser energy incident on the
Bragg cell 206. The space modulated output of the Bragg cell 206 is
a plurality of laser beamlets that provide instantaneous spectral
information on the correlator drive signal 122.
[0023] The output of the Bragg cell 206, i.e., the plurality of
output laser beamlets, is imaged, Fourier transformed, and
spatially filtered by Fourier imaging and spatial filter optics
208. The Fourier imaging optics 208 perform a Fourier transform on
the plurality of laser beamlets and image the plurality of laser
beamlets into an imaging plane where a an optical receiver 210 that
includes a plurality of integrating optical photodetectors is
disposed. The spatial filtering is necessary to remove the
undiffracted/undeflected laser energy that is present on the output
of the Bragg cell 206.
[0024] In the embodiment depicted in FIG. 1, the plurality of
integrating optical detectors is a linear array of integrating
optical detector photodetectors. Other detectors can be used for
example, avalanche photodiodes, p-i-n diodes, charge coupled
devices (CCDs), and multi-channel photodetector arrays (i.e.,
channeltrons) are all suitable integrating optical photodetectors.
The Fourier transformed laser output of the Bragg cell forms a line
spectrum that is imaged by the imaging optics onto an image plane.
The linear array of photodetectors is disposed in the image plane
so that the line spectrum is incident on the linear array of
photodetectors. The position of the largest output spectral line
with respect to the center frequency of the AOSA 124 yields an
estimate of the whole body Doppler shift. As will be explained in
more detail below, the line spacing is dependent on the system
geometry and specifications. The outputs from the linear array of
photodetectors is scanned by the optical detector output
electronics 212 and provided thereby as a plurality of integrating
optical photodetector output signals. Each of the plurality of
plurality of integrated optical detector output signals corresponds
to one of the plurality of photodetectors. In general, the
frequency resolution of the photodetector array is given by the
Bandwidth of the Bragg cell divided by the number of photodetectors
in the linear array and consistent with the usual time-bandwidth
limitations of the Bragg cell.
[0025] Referring again to FIG. 1, the plurality of integrating
optical detector output signals is provided to an analog-to-digital
converter 128 that samples the plurality of the integrating optical
detector output signals at a predetermined sample rate. The
analog-to-digital converter 128 provides a digital representation
of the samples of the integrating optical photodetector output
signals. In the embodiment illustrated in FIG. 1, the predetermined
sample rate is 80 Megasamples-per-second and the digital
representation of each sample is 14 bits long. The digitized
samples are provided to the optical processor 130 for analysis.
[0026] FIG. 3 depicts one embodiment of an optical processor 130
suitable for use with the optical spectrum analyzer/correlator
described herein. The optical processor 130 receives the digitized
samples in an accumulator 302 that accumulates the respective
received digitized samples over one or more complete scans of the
integrating optical photodetectors. This allows the integration of
the integrating optical detector data over a predetermined period
of time. When the digitized data has been accumulated for the
predetermined period, the data is stored in a memory 304 and the
digitized stored values are compared in a threshold detector 306 to
a predetermined threshold value. If a stored value exceeds the
predetermined threshold value, a signal is sent to the timing and
control module 308 that is responsive to the signal received from
the threshold detector 306 by providing a cue signal output. In
addition to the cueing signal, the timing and control module 308
also provides the address control to the address register 310 and
also provides the necessary control for the analog-to-digital
converters 112 and 128 and the internal control signals. The mux
310 also receives a zero signal so that the accumulator 302 and the
memory 304 can be zeroed and initialized.
[0027] The optical processor 130, in addition to providing the cue
signal to the DSP 116 indicating that a target is present due to
the exceeded threshold value, can also confirm the presence of a
target through a correlation process. The correlation process that
is used to to confirm the presence of a target also increases the
signal to noise ratio and concomitantly reduce the false alarm
rate. The optical processor correlates the integrating optical
detector data stored in the memory 304 in a modulo N manner. That
is, the optical processor correlates every N data points, where the
modulo parameter N is equal to the number of photodetectors in the
linear array divided by the number of output beam spots of the
Bragg cell. In one embodiment in which there are 2000
photodetectors in the linear integrating optical detector array and
there are 80 beam spots output of the Bragg cell, the correlation
would occur in a modulo 25 manner. Advantageously, the modulo N
correlation obviates the speckle interference in the return signal
by averaging the speckle over the broad spectrum return signal. The
optical processor 130 further analyzes the optical data to estimate
the Doppler shift of the target and the target range. The optical
processor 130 analyzes the digitized integrating optical detector
data retrieved from the memory 306 to determine the position of the
largest output spectral line with respect to the center frequency
of the Bragg cell. As discussed above this provides an estimate of
the Doppler shift of the target. In addition, the range can be
estimated by determining the difference in time between the
transmission of the transmitted pulse and the detection of the
target.
[0028] After the DSP 116 receives the cue signal indicating a valid
target, the DSP 116 stops the storage of data in the FIFO buffer
114. The DSP 116 retrieves the data stored in the FIFO buffer 114
for analysis to determine one or more preselected target
characteristics. In one embodiment, the IF signal is used to
provide a second RF signal that is stored in memory separately. A
cue signal received by the DSP 116 will cause the DSP 116 to
retrieve the stored second RF signal data and to determine one or
more characteristics in of the object using this data.
[0029] In one embodiment a hybrid processor includes a Bragg cell
having a center frequency of 3 GHz, a bandwidth of 2 GHz, a dynamic
range of 30 dB (linear) and 60 dB (compression), a sensitivity of
-53 dBm (cw) and -20 dBm (pulsed), and provides an output of 80
laser beamlets. The photodetector array is a Loral Fairchild CCD
181M line scan module that includes 2592 photodetector elements and
has an effective readout rate of 1 MHz (typical) and 10 MHz
(maximum). If a faster photodetector array is desired a Channeltron
array may be used. The laser source for the AOSA is a GaAlAs laser
having a wavelength of 690 nm. An integration time of 1 microsecond
(typical) and 275 microsecond (maximum) is used. A LADAR source
providing burst pulses lasting 6 microseconds wherein each burst
pulse includes 256 1 ns micropulses spaced 40 ns apart.
[0030] In this embodiment, the frequency resolution of the system
is approximately 2 GHz per 2000 photodetectors, or approximately 1
MHz per photodetector. The spacing of the micropulses, 40 ns, leads
to an output of a comb spectrum of 40 teeth separated by 25 MHz.
Thus, the Doppler shift of a target may be estimated to within 12.5
MHz.
[0031] The hybrid processor provides an increase in signal to noise
ratio due to the processing of the received data. In general, the
thermal noise for a system is given by KTB, where K is Boltzman's
constant 1.3*10{circumflex over ( )}(-23), T is the system noise
temperature in Kelvins 300, and B is a bandwidth of a matched
filter, 250 KHz in the current embodiment. This leads to a system
thermal noise -120 dBm. Shot noise is typically 3 dB above thermal
noise, so that shot noise is -117 dBm. A signal-to-noise ratio of
15 dB should provide for good detection thus the minimum detectable
signal is -102 dBm.
[0032] Those of ordinary skill in the art should further appreciate
that variations to and modification of the above-described methods,
apparatus and system for a hybrid optical spectrum
analyzer/correlator for an all range laser radar may be made
without departing from the inventive concepts disclosed herein.
Accordingly, the invention should be viewed as limited solely by
the scope and spirit of the appended claims.
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