U.S. patent number 3,731,103 [Application Number 05/128,628] was granted by the patent office on 1973-05-01 for adaptive arrays.
This patent grant is currently assigned to Hughes Aircraft Company. Invention is credited to Thomas R. O'Meara.
United States Patent |
3,731,103 |
O'Meara |
May 1, 1973 |
ADAPTIVE ARRAYS
Abstract
Herein are disclosed multiple beam laser systems with adaptive
phase control for establishing, at a target, an in-phase condition
between the corresponding electromagnetic fields of all the beams.
Phase modulation at different frequencies or differing waveforms is
applied in the transmission paths of selected radiating elements of
the array; and modulation components in the received energy are
utilized to control phase shifters in the transmission paths so as
to maintain the cophase condition at the target.
Inventors: |
O'Meara; Thomas R. (Malibu,
CA) |
Assignee: |
Hughes Aircraft Company (Culver
City, CA)
|
Family
ID: |
22436238 |
Appl.
No.: |
05/128,628 |
Filed: |
February 24, 1971 |
Current U.S.
Class: |
250/203.2;
342/370; 250/201.9; 342/81; 398/151; 398/140 |
Current CPC
Class: |
G01S
7/42 (20130101); G01S 17/02 (20130101); H01S
3/2383 (20130101); G01S 17/87 (20130101) |
Current International
Class: |
G01S
17/02 (20060101); H01S 3/23 (20060101); G01S
7/42 (20060101); G01S 7/02 (20060101); G01S
17/00 (20060101); G01S 17/87 (20060101); H04b
009/00 () |
Field of
Search: |
;250/199
;325/154,159,180,368,369,56 ;343/7A,7.5,17.5,1SA,1TD,854,208
;356/4,5,28,152 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Bentley; Stephen C.
Claims
What is claimed is:
1. The method of transmitting a plurality of beams of optical
energy such that the plurality of beams are substantially in phase
at a target, said method comprising the steps of:
providing a plurality of beams of coherent optical energy;
modulating each said beam at a different frequency by varying the
transmission path delay of each of said beams at the modulating
frequency of that beam;
transmitting said plurality of modulated beams at a target;
receiving a portion of the energy reflected from the target;
and
controlling the mean phase of each of said beams to cause the
amplitude modulation components in the received energy, which are
at approximately the same frequency as the modulating frequency of
that beam, to be nulled.
2. A system for transmitting a plurality of beams of optical energy
and for controlling their relative phase so that the beams are
substantially in phase at a remotely located target, said system
comprising:
a laser;
an array of electronically controllable optical phase shifters;
a laser power amplifier;
lens means for applying the output beam from said laser through
said array of optical phase shifters to said laser power
amplifier;
identifying modulation means for applying modulation drive signals
at a different frequency to each of said electronically
controllable optical phase shifters, whereby each of the output
beams from said phase shifters are phase modulated at different
modulation frequencies;
telescope means for transmitting the output beams from said laser
power amplifier towards the target;
receiving means for receiving a portion of the transmitted energy
reflected from the target; and
control means responsive to modulation signal components in the
received energy for controlling the mean phase of each of said
electronically controllable optical phase shifters so as to null
the modulation signal components in the received energy which are
at the frequency of the modulation drive signal applied to the
respective optical phase shifter;
whereby the phase of said plurality of beams is adaptively
controlled so that said beams are substantially in phase at the
target.
3. The system of claim 2 wherein said identifying modulation means
including a different reference oscillator for supplying the
modulation drive signal to each of said electronically controllable
optical phase shifters; and said control means including a
plurality of control circuits, with each control circuit coupled
for controlling the phase of a different one of said beams and
comprising a filter having a passband centered at the frequency of
the modulation drive signal of the associated beam, and means for
applying amplitude modulation signal components of the received
energy to said filter, a phase sensitive detector having a signal
input coupled to the output of the filter of that control circuit,
a reference input coupled to the output of the reference oscillator
which supplies the modulation drive signal for the associated beam,
and an output coupled to the electronically controllable optical
phase shifter disposed in the transmission path of the associated
beam.
4. The system of claim 3 wherein each of said electronically
controllable phase shifters is a piezoelectrically driven mirror
electrically coupled to the associated reference oscillator for
applying phase modulation, and is electrically coupled to the
output of the associated phase detector for adjusting the mean
phase value of said beam.
5. The device of claim 3 wherein each said phase shifters is an
electro-optical device electrically coupled to said reference
oscillator of the associated beam for providing phase modulation of
said beam, and is electrically coupled to the output of said phase
detector for correcting the mean phase value of said associated
beam.
6. The system of claim 2 wherein said receiving means comprises a
plurality of receiving circuits; means for sensing a phase
difference between signals in said plurality of receiving circuits;
and means for adjusting the phase delay within said plurality of
circuits to null the phase difference.
7. The system of claim 3 wherein said receiving means comprises a
plurality of receiving circuits; means for sensing a phase
difference between signals in said plurality of receiving circuits;
and means for adjusting the phase delay within said plurality of
circuits to null the phase difference.
Description
BACKGROUND OF THE INVENTION
This invention relates generally to adaptive arrays and
particularly to such arrays wherein the relative phase of energy
transmitted by a plurality of radiating elements is controlled to
establish an in-phase condition at the target.
The distribution of electromagnetic energy radiated by an aperture
immersed in the atmosphere differs from the ideal diffraction
limited behavior (assumes a vacuum) due to refractive index
inhomogeneities caused by variations in atmospheric density. The
angular width of the main lobe, its direction, and the intensity
distribution within the lobe are all affected by these density
variations. Among the principal sources of density variation are
atmospheric turbulence and the heating caused by absorption of the
radiated energy. In the case of atmospheric turbulence, the effect
of the inhomogeneities depends on the strength of the turbulence
and on the path length. As the strength of the turbulence and/or
the path length increases, the first noticeable effects are changes
in the direction of the beam (beam wander) and those associated
with the random phase shift introduced across the beam (loss of
coherence). The distribution of the radiated energy departs
appreciably from the ideal when the energy radiated from different
parts of the aperture is no longer phase coherent at the receiving
point.
For example, the performance of very narrow beam width (large
aperture) coherent laser systems operating through the atmosphere
is seriously degraded by atmospheric turbulence and also in some
cases by nonlinear propagation effects. Three turbulence related
propagation effects are: the width of the main lobe of the
radiation pattern is increased, reducing both resolution capability
and power on target; the direction of the main lobe deviates from
that predicted under free space conditions, and is not constant in
time; and the shape of the radiation pattern may become highly
irregular and time varying.
It is possible to reduce the deleterious effects of the atmosphere
on the radiation pattern of a large aperture by utilizing instead,
an array of smaller apertures whose phase is adaptively controlled.
If each of the individual elements in the array are small enough
that their radiation pattern is diffraction limited, near
diffraction limited performance from the entire array may be
obtained by adaptively changing the relative phase of the
excitation sources driving various radiation elements, in such a
manner that the atmospheric effects are compensated. The
implementation of this concept requires the capability of sensing
and changing the relative phase of the radiated energy at the
target.
One adaptive compensation technique uses the phase difference
between signals from multiple receiving channels, as determined by
phase comparison of the optical carrier frequencies (after
heterodyne conversion), to control the required phase adjustments
of the transmitted beams. Although this technique may be a marked
improvement over nonadaptive systems, it does not provide direct
confirmation of a cophase condition at the target. For example, if
a phase measurement error due to a path unbalanced exists in the
system, then a cophase condition will not be established at the
target and hence atmospheric conditions are not fully compensated
for by this technique. Additionally, this "phase comparison between
receiving channels" approach requires heterodyne detection that
introduces mechanization difficulties, especially when targets of
high doppler signatures are involved; and further problems are
encountered with targets at such ranges that backscattered energy
is comparable to energy reflected from the target.
SUMMARY OF THE INVENTION
It is therefore an object of the subject invention to reduce the
deleterious effects of the atmosphere on the radiation pattern of a
large aperture by utilizing instead an array of smaller apertures
which are adaptively controlled to maintain the plurality of
transmitted beams "in-phase" at the target.
Another object of the subject invention is to provide an adaptive
array which directly senses the establishment of the cophase
condition of all the transmitted beams at the target.
A further object is to provide an adaptive array which does not
require a phase-matched, multi-channel, heterodyne receiver
system.
Still another object is to provide an adaptive array which may be
used with systems having a single receiving channel that need not
have any common paths with any one of the transmitting
channels.
Yet another object is to provide an adaptive array wherein the
defocusing effect of moving targets is substantially reduced.
In accordance with one preferred embodiment of the subject
invention, a dithering of variable phase shifters associated with
the radiating elements of an optical transmission array is employed
such that each of the transmitted beams is phase modulated at a
separate characteristic dither frequency. On reception of the
energy from a target, envelope detectors and filters centered at
each of the dither frequencies provide signals for controlling a
compensating phase shifter in each transmitting channel to provide
an in-phase condition of all radiated fields at the reflecting
target. For each transmitting beam, the magnitude of the amplitude
modulation components in the received energy at the associated
dither frequency is indicative of the deviation of the phasing of
its electromagnetic fields from a cophase condition at the target.
The phase of the received amplitude modulated signals is indicative
of the polarity of the phase error of the associated transmission
channel.
The subject invention eliminates problems encountered by other
systems for adaptively controlling arrays in that the cophase
condition at the target is measured directly. Since the radiated
field from each aperture element of the array is characterized by
its own signature -- its dither frequency -- separation of the
received information into path error components associated with one
and only one path is easier to mechanize and it does not require
heterodyne detection or complex computations.
BRIEF DESCRIPTION OF THE DRAWINGS
The novel features of this invention, as well as the invention
itself, will be better understood from the accompanying description
taken in connection with the accompanying drawings in which like
reference characters refer to like parts and in which:
FIG. 1 is a block diagram of a laser transmitting and receiving
system having an array adaptively controlled to establish a cophase
condition at the target, in accordance with the principles of the
subject invention;
FIG. 2 is a diagram of the composite electromagnetic field at the
target, for explaining the phase to amplitude conversion process
utilized by the adaptive arrays of the subject invention;
FIG. 3 is a block diagram of a laser transmitting and receiving
system wherein each transmitting element of the array is adaptively
controlled for establishing an "in-phase" condition of the energy
at the target;
FIG. 4 is a block diagram of a portion of the laser system of FIG.
3 with additional circuitry for maintaining an accurate phase
reference independently of target range;
FIG. 5 is a block diagram of a phase compensated receiver that may
be incorporated into the systems of FIGS. 1, 3 or 4; and
FIG. 6 is an optical array system which utilizes a single laser
power amplifier and telescope for transmitting a plurality of
adaptively controlled beams.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The subject invention may be best understood by first considering
the basic two element array of FIG. 1. As there shown, the laser 12
excites transmitting aperture elements 14 and 16 by means of
transmission paths 18 and 20, respectively. Path 18 includes a
power or beam splitter element 22 and an electronically
controllable phase shifter 24. Path 20 includes beam splitter 22, a
mirror 26 and a beam splitter 28.
Transmitting apertures 14 and 16, which may include focusing optics
such as telescopes, radiate beams 30 and 32 respectively, so as to
illuminate a target 34. A portion of the total energy reflected
from target 34, shown as beam 36, is received by aperture 16 and is
applied by means of beam splitters 28 and 38 to a mixer photodiode
40. This received energy is heterodyned in mixer 40 with an optical
frequency signal supplied by a laser 42, and is amplified in an IF
amplifier 44. The output signal from amplifier 44 is processed by
an envelope detector 46 which produces an output signal that varies
in amplitude in accordance with the envelope of the IF signal from
amplifier 44.
The signal from detector 46 is processed by a passband filter 48
centered at a frequency .omega..sub.m, and the output therefrom is
applied as one input to a phase sensitive detector 50. The signal
from phase detector 50 has an amplitude A sin .phi., where A is a
function of the magnitude of the signal applied from filter 50 and
.phi. is the phase angle between this last mentioned signal and a
reference signal .omega..sub.m applied from a reference oscillator
52.
The signal from phase detector 50 is summed with the reference
signal .omega..sub.m by means of a transformer device 54; and this
summed signal is applied to an electronically controllable phase
shifter 24. Phase shifter 24, which may be a movable mirror or an
electro-optical device, for example, varies the effective
transmission path length (or net phase shift) 18 in response to the
signal applied thereto.
The optical signals which excite aperture elements 14 and 16 are of
a common frequency but generally are not in phase. These two
aperture elements radiate energy to a common target 34, and the
fields from each of these elements generally experience a further
differential phase shift at the target because of path length
differences. The composite signal at target 34 may be expressed
as,
E.sub.r = A.sub.r [cos (.omega..sub.c t) + cos (.omega..sub.c t +
.beta..sub.o) ] (1)
or
E.sub.r = 2A.sub.r cos .beta..sub.o cos (.omega..sub.c t +
.beta..sub.0/2) (2)
where .omega..sub.c id the frequency of laser 12 and .beta..sub.o
is the composite differential phase error. The constructive (or
destructive) interference of the two field components yields a
spatial interference pattern with a sinusoidal envelope 56 (FIG.
2). For those cases where .beta..sub.o varies with time, the
spatial interference pattern wanders back and forth over the target
region.
The modulation of electronically controllable phase shifter 24
produces a phase excursion,
.psi..sub.ps (t) = -.beta..sub.c + .beta..sub.m sin .omega..sub.m t
(3)
where .beta..sub.c is a corrective (unmodulated) phase shift
applied from detector 50 to provide the desired phase adjustment.
The net phase error .beta..sub.o at target 34 is also dithered over
this same range; that is,
.beta..sub.o = .beta..sub.a - .beta..sub.c + .beta..sub.m sin
.omega..sub.m (t) (4)
where .beta..sub.a is the atmospheric (or other) phase error to be
corrected by .beta..sub.c. As a consequence, the interference
pattern dithers back and forth over the target. Thus, the phase
modulation produced by phase shifter 24, introduces an amplitude
modulation at the dither frequency, in the composite signal on
target and hence in the received return signal.
From another viewpoint, that of a fixed point .beta..sub.o, the
envelope modulation process of Equation 3 is also illustrated in
FIG. 2. For a phase error .beta..sub.1 to the right of the envelope
peak (.beta..sub.a = .beta..sub.c) of phase error curve 56, the
modulation envelope 58 is in phase with the dither source
(oscillator 52 in FIG. 1). For a phase error .beta..sub.2 to the
left of the modulation envelope, the modulation envelope 60 is
180.degree. out-of-phase. For .beta..sub.a = .beta..sub.c the
fundamental component of the modulation envelope vanishes. Thus,
the array of FIG. 1 has the required characteristics for a feedback
control system whereby the mean value of phase shifter 24,
.beta..sub.c, is controlled such that .beta..sub.a - .beta..sub.c
is driven to zero, thereby establishing the cophase condition at
the target 34. In particular it is noted that the magnitude and
plurality of the output signal of phase detector 50 is such as to
force the mean phase value of path 18 to establish the cophase
condition at the target.
The extension of the target cophasing concept of the subject
invention to more than one phase controlled transmission path is
illustrated in FIG. 3. In FIG. 3, each of the elements of the
various transmission paths is given the same numeral designation as
the corresponding element of transmission path 18 of FIG. 1 with
the letter a, b and c identifying the elements associated with
transmission paths 62, 64 and 66 respectively. Also in FIG. 3 laser
amplifier 25 has been coupled in each of the transmission paths
between the phase shifters 24 and the aperture elements 14.
Additionally, the received channel or signal path 68 is illustrated
(for generality) in FIG. 3 with a separate receiving aperture 70
and beam directing mirror 72 associated therewith. In the operation
of the system shown in FIG. 3, the phase of each electronically
controllable phase shifter (24a, 24b and 24c) is dithered at its
own characteristic frequency .omega..sub.m. The corresponding
amplitude modulation component in the reflected return signal (as
explained above relative to FIG. 2) is separated, after detection,
from the IF carrier by envelope detector 48; and from the other
modulation frequency components by means of bandpass filters
centered on frequency .omega..sub.m. Therefore the phase correction
offset, .DELTA..sub.c, introduced by each of the electronically
controllable phase shifters is a function of the modulation caused
by the dither of that phase shifter only. The output signal of IF
amplifier 46 is also coupled to a utilization device 47 which may
be a display or computer unit, for example, and which utilizes the
target information contained therein.
It is noted that the single reference channel technique of the
system of FIG. 1 uses one of the transmission paths as an
uncontrolled reference and the phase of the controlled channel is
adjusted to establish the cophase condition at the target. When
this approach is extended to a large group of adaptively controlled
radiating apertures, the extraction of the desired error
information associated with each of the adaptive channels by simple
filtering is complicated. This is due to intermodulation products
resulting from the interaction of the phase modulation applied to
each of the plurality of transmitted beams. One approach for
avoiding this intermodulation problem is a sequential switching
technique such that only the reference element and one other
element then being established in phase synchronism, are radiating.
After proper phasing is established for a particular controlled
channel it is turned off and the same operation is repeated with
the other channels. This switching sequence is performed at a
repetition frequency high enough that the sequence may be completed
before there is any substantial change in the required phase
corrections. Since each transmitting channel is cophased to the
reference before a next element is turned on, it would not be
necessary to turn off the preceding elements. Therefore each
element of the array may be phase adjusted by a sequence which
activates each of the controlled transmitting channels one at a
time.
As an alternative to the technique wherein the phase of each
controlled transmission path is determined with respect to a
reference channel, the system of FIG. 3 measures and corrects the
average phase error in each transmission path as compared to all
other channels considered simultaneously. In the mechanization of
FIG. 3, the synchronously detected component at a frequency
.omega..sub.m is a measure of the average of the sine of the phase
errors of channel m compared to all other channels. Each error
signal S.sub.Dm (from the m.sup.th phase sensitive detector) can
then be employed to correct the phase error in the m.sup.th channel
(feeding the m.sup.th radiating aperture). The distrubuted
reference syst em of FIG. 3 has one important advantage in that the
loss of any one signal component (A.sub.m = 0) as a result of
propagation or equipment failure does not destroy the operation
effectiveness of the remaining channels. The operation of the
distributed reference system may be compared to the common homodyne
detection process. For this comparison the signal input is
analogous to the phase modulated signal from a particular
transmitting channel, while the function of a reference oscillator
is performed by the phasor sum of the ensemble of the remaining
channels, as returned from the target.
The subject invention may be utilized to compensate for frequency
errors as well as phase error corrections inasmuch as frequency
errors may be considered as phase errors which increase
approximately as a linear function of time. Such frequency errors
could result from oscillator drifts or differential doppler shifts,
for example. As used herein the term "differential doppler shift"
means the difference in the doppler frequencies between transmitted
beams.
Although in the transmission systems shown in FIGS. 1 and 3, the
energy radiating from each of the transmitting apertures originates
from a common laser source, in an alternate embodiment a separate
laser oscillator may be used in each of the transmitting channels.
The operation of the system would remain unchanged except that the
correction voltages, S.sub.Dm, would be applied to a frequency
controlling element, such as a mirror internal to the laser cavity,
rather than a phase controlling element. This technique circumvents
the accumulation of excess phase shift by changing the transmitted
frequency of each array element to compensate for frequency errors.
An increase in cost may be associated with this mechanization due
to the need for a plurality of laser oscillators, although the beam
splitting elements would be eliminated. Also, each laser oscillator
would have to be supplied with its own "self-stabilizing loop" to
control its frequency during search and acquisition of targets. The
frequency stability of the plurality of lasers must be such that
they stay within the "capture range" of the control loops.
If the dither phase modulation frequencies (.omega..sub.m) are
selected too low then there is the possibility of interference from
target or atmospheric scintillation modulations. On the other hand,
if the labeling (tagging) modulations employed are too high a
problem with the phase synchronization between the dither signal
reference and the return signal may result. Hereinabove for the
purposes of explaining the fundamental principles of the invention
it has been assumed that either the beam labeling frequencies
(.omega..sub.m) were sufficiently low, or that the propagation
paths are sufficiently short that for all practical purposes the
phase detection operation could be considered to be synchronous.
That is, it was assumed that there was no time slip or phase error
between the received input signal to the phase sensitive detector,
such as detector 50a of FIG. 3, for example, and the reference
signal applied from the reference oscillator, such as 52a.
In the more general case, where there exists a substantial round
trip delay .tau. in the received signal, the control voltage
S.sub.Dm from the m.sup.th phase detector is a function of cos
(.omega..sub.m .tau.). If .omega..sub.m .tau. becomes as large as
.pi./4 the signal S.sub.Dm vanishes independently of phasing errors
at the target. If .omega..sub.m .tau. equals .pi.I, where I is an
odd integer, the sign of the error signal S.sub.Dm reverses and the
system will "lock on" the minima rather than the maxima of the
composite beam (56 of FIG. 2). Therefore, in an uncompensated
system the value of .omega..sub.m .tau. must be restricted. Such a
limitation on the value of the term .omega..sub.m .tau. determines
the highest permissible value of .omega..sub.m for a given maximum
range. Since high modulation frequencies are sometimes desirable to
minimize noise interference problems or to accommodate large
frequency errors it may be desirable to incorporate techniques to
compensate for the round trip delay time (.tau.).
One method of compensating for the time displacement between the
reference signal to the synchronous detector (phase sensitive
detector 50, for example) and the return modulated envelope
(waveform 58 of FIG. 2) would be by knowing or measuring the range
to the target and computing the associated phase delay value. This
phase shift could then be compensated by introducing a phase delay
correction in the path of the reference signal to the synchronous
detector.
A more generally applicable method of compensating for the loss of
synchronism between the reference to the control loop phase
detector and the modulated return signal is to transmit the
reference dither frequency (.omega..sub.m) in the form of
modulation on some carrier and to process the associated return
signal so that this transmitted reference may be used as the
reference for the synchronous detector. Since both the input signal
to the phase sensitive detector and the reference thereto would
then experience the same path delays, a purely synchronous
detection operation can be obtained independently of the value of
the term .omega..sub.m .tau.. For the system such as shown in FIG.
3, the most convenient choice for carrier and modulation types
would be those which are already present, that is, phase or
frequency modulation of the optical carriers. FIG. 4 shows one
transmit channel and the received channel of FIG. 3 modified to
compensate for the above described synchronization problem. The
transmitted channel in FIG. 4 is designated generally by the
reference numeral 62' and elements thereof corresponding to like or
similar element of channel 62 of FIG. 3 are, in FIG. 4, identified
by like reference numerals. Only one modified transmission channel
is illustrated in FIG. 4 for explaining the transmission path delay
compensation technique. However, in practice the compensation
modifications would be incorporated into each of the transmission
channels such as channel 64 and 66 of FIG. 3, in a manner identical
to that to be described for channel 62'.
Referring now primarily to FIG. 4, optical energy from laser 12
(FIG. 3) is modulated by electronically controllable phase shifter
24a and transmitted by aperture 14a. The energy transmitted from
aperture 14a along with the energy from the other transmitting
apertures (not shown in FIG. 4) is reflected from target 34. A
portion of this reflected energy is intercepted by antenna 70 and
processed by means of mirror 72, beam splitter 38, photodiode mixer
40 and laser local oscillator 42 in the same manner as described
relative to FIG. 3. The output of mixer 40 after being processed by
IF amplifier 46 is applied in parallel to an envelope detector 48
and a limiter 78. The limiter 78 removes the envelope modulation
effects and the output signal therefrom is demodulated by means of
a conventional frequency discriminator 80. The output from
discriminator 80 is applied in parallel to a bandpass filter 82 at
a frequency .omega..sub.1, associated with the transmission channel
62', as well as to similar filters which are at the frequencies
associated with the remaining transmitting channels (not shown).
For example, if a channel 64' and 66' were shown the output of
frequency discriminator 80 would also be processed by bandpass
filters at frequencies .omega..sub.2 and .omega..sub.3,
respectively.
The output signal of frequency discriminator 80 contains all of the
labeling frequencies (.omega..sub.m) with a time delay
corresponding to the transmission-reception path of the received
signal. The output signals from the bandpass filters, such as
filter 82 associated with transmission channel 62', could be used
to reference the phase sensitive detectors, such as 50a, of the
associated channel. In the embodiment of FIG. 4, however, the
output of the bandpass filter is utilized to control a "phase
locked loop" 84 which includes a phase detector 86, a loop filter
88 and a voltage controlled oscillator 90. The phase detector 86
compares the phase between the oscillator 90 and the output signal
from the filter 82; and the output signal of phase detector 86,
after being processed by loop filter 88, is used to adjust the
voltage controlled oscillator 90 such that the frequency and phase
of oscillator 90 tracks the signal from filter 82. The output
signal from the oscillator 90 is also applied to the phase
sensitive detector 50a wherein it is utilized as a reference for
the received, modulated signal associated with the frequency
.omega..sub.1 applied to detector 50a through the filter 48a.
The output signal of phase sensitive detector 50a is summed with
the signal of a reference oscillator 52a by means of a transformer
device 54a. This summed signal is applied to control electronically
controllable phase shifters 24a to apply phase modulation to the
transmitted signal at the labeling frequency .omega..sub.1 ; and to
adjust the mean value of the phase of the channel such that it is
in phase with the other transmission channels (not shown in FIG. 4)
at the target 34.
As is now evident, the systems in accordance with the subject
invention function equally well with a separate or even remotely
located receiving aperture. This characteristic is desirable in
certain applications for a number of reasons, not the least of
which are the elimination of back-scattering and crosscoupling
problems inherent in systems that transmit and receive from the
same apertures. However, a certain amount of economy is sometimes
realized by the transmitting and receiving systems sharing
apertures and the subject system is of course adaptable to a common
transmit and receive mechanization, if so desired.
In some mechanizations it may be desirable to extend the size of
the receiving aperture beyond its coherent length in order to
improve signal-to-noise ratios for distance targets. In such cases
adaptive control of a receiving array may be employed.
One approach to adaptive control of the receiving system, if common
transmission and receiving apertures are employed, would be to use
the phase error information, S.sub.Dm, extracted from the
transmitter control portion of a multi-dither system to correct (in
an open-loop manner) for the phase errors in the received paths.
However, this approach suffers from the problem that the
transmitter system corrects for all phase errors in the
transmission path including any laser power amplifiers that may be
utilized, as well as those in the post-aperture radiation paths.
Also, since such an approach requires media linearity it is
restricted to low levels of transmitter power.
Another approach is to employ an adaptively controlled receiving
system with a separate phase modulation labeling frequencies
associate with each receiving channel in a manner analogous to the
above described transmitter system. However, this method requires
the doubling of the number of dither frequencies on transmission
and thereby increases the problem of avoiding possible
intermodulations.
The method of adaptively controlling a plurality of receiving
channels which may be best adapted to a large variety of
applications involves measuring the phase differences between IF
receiver channels and using this difference to control phase
shifters in the received channels such that the phase differences
between channels are driven to zero. One such mechanization is
shown in FIG. 5 wherein a plurality of receiving apertures
corresponding to the aperture 70 of FIGS. 3 and 4 are designated by
reference numerals 70a, 70b, 70c and 70d. In like manner the IF
amplifiers for each channel which would correspond to amplifier 44
in the previously described embodiments are given the same
reference numeral with a postscript corresponding to the particular
channel. In the embodiment of FIG. 5 varactor diode phase shifters
are inserted following the IF amplifiers in each controlled channel
prior to a summation circuit 94. Summation circuit 94 forms the sum
of the received signals which have previously been translated to
the intermediate frequency zone and phase adjusted to be in phase.
The varactor phase shifters associated with each of the controlled
receiving channels are designated by the reference numeral 92 with
the postscript of the corresponding channel. Phase detector 96
compares the phase differences between channel d and channel a and
drives the phase shifter 92a to null this difference. Similarly,
phase detector 98 compares the phase difference between channels b
and d and controls phase shifter 92b in response thereto; and phase
detector 100 compares channels c and d and controls phase shifter
92c. Hence the circuit of FIG. 5 adjusts the phase of the received
signals to a cophase condition prior to their summation in circuit
94. The output signal from summation circuit 94 may be applied to a
utilization device 47, such as a display or computation system, as
well as to envelope detector 46 (FIG. 3) which feeds the
transmitter control loops.
In the disclosed embodiments heterodyne detection (mixer 40) was
utilized because in general the detection is better than if video
detection without prior IF amplification were used. However, the
subject invention is equally well adapted to noncoherent detection
and in some applications such as those involving high doppler
frequency shifts, for example, noncoherent detection may be
preferred. For example, heterodyne detection systems require either
that the IF circuitry has sufficient bandwidth to accommodate
target doppler shifts or else the doppler frequency must be tracked
out of the system prior to the IF circuitry - such as by a tuneable
local oscillator which is adjusted "closed loop" to center the
received IF spectrum at a selected frequency.
In the enclosed embodiments the labeling modulation (.omega..sub.m)
has been applied by the same unit, e.g. phase shifter 24, that
applies the phase corrections, .beta..sub.c, however it will be
recognized that these two functions need not be mechanized by a
single unit. Hence, the phase modulation may be applied by one
electronically controllable device and the phase correction to
maximize the power on target by another.
For applications involving targets having very slow angular rates,
the electrically controllable phase shifters such as element 24,
may comprise conventional piezoelectrically driven reflective
mirror type phase shifting devices. One such phase shifter,
constructed of a disc shaped piezoelectrically driven bimorph
material having a small thin mirror of approximately 5 millimeter
diameter, for example, bonded at its center was found to have an
extended frequency range before mechanical resonances were
bothersome. In applications requiring the phase shifters to have a
frequency response above those conveniently obtainable with
mechanically or piezoelectrically driven devices, electro-optical
phase shifters may be utilized.
In dynamic system applications, a target may be designated to the
optical array system at some precisely defined angle and angular
rate. Each of the radiating optical aperture elements in the array
may have a mechanical steering mechanism (not shown) capable of its
own autonomous search, acquisition and track functions. If
boresight accuracy is sufficiently high the preliminary search may
be performed by only one or two of the radiating channels. After
all the elements have separately acquired the target, and are
tracking it, the adaptive control loops may be activated and the
adaptive array pattern forming and tracking commenced.
In an alternate embodiment shown in FIG. 6, the energy from a laser
12 is applied to electronically controllable phase shifters 24a,
24b, and 24c by means of lenses 71 and 73. The output beams from
the phase shifter units are applied by lenses 75 and 77 to a laser
power amplifier 79. The amplified beams from amplifier 79 are
transmitted by a single telescope 81. Each of the beams are phase
modulated at separate frequencies W.sub.m and are adaptively
controlled, in a manner similar to that explained above relative to
FIG. 3, in response to signals applied to each of the phase
shifters from a control electronic unit 83. Unit 83 includes a
receiving channel such as 68 of FIG. 3, as well as the associated
processing devices such as 48, 50, 52 and 54 of FIG. 3. Also for
electronic scanning applications the phase scanning control signal
may be provided by unit 83 and superimposed on, or time shared
with, the signals applied to the phase shifter unit 24. Although in
the interest of clarity of the drawing only a single lead is shown
from unit 83, it is understood that in practice a pair of leads may
be coupled between each of the phase shifters and unit 83. The
embodiment of FIG. 6 reduces the number of high power laser sources
required and by use of a single transmitting aperture simplifies
boresighting and angle tracking problems.
Thus there has been described new and improved adaptive arrays
wherein multiple, time varying perturbations, for example, phase
dithers, are introduced on transmission. The effects of these
perturbations are sensed on reception and employed to control
feedback loops which adjust the phasing of the plurality of
transmitting channels such that the energy at the target is in
phase. Some of the advantages realized by systems incorporating the
principles of the subject invention are: phase coherency at the
target is directly measured rather than being inferred by multiple
receiving channel phase measurements; either noncoherent or
coherent detection systems may be employed; receiving optics may be
located anywhere, thereby avoiding backscattering and
cross-coupling problems; the ensemble reference mechanization
provides a fail safe system in the sense that the array functions
correctly with the remaining elements in the event of the failure
of one or more radiating channels; and the defocusing effects
induced by moving targets are substantially reduced.
* * * * *