Note: Descriptions are shown in the official language in which they were submitted.
13 41487
INFRARED DECEPTION COUNTERMEASURES SYSTEM
This invention relates to a countermeasures system for detecting and diverting
an
attacking unit.
When penetrating enemy territory under conditions of limited warfare, bombers
suffer
attack from enemy aircraft vectored by radar. These aircraft attack the
bombers with
air-to-air missiles which employ both microwave and infrared homing systems.
Currently bombers of this class have increased their penetration capability by
employing electronic countermeasures system to deny the attacking missiles
accurate
radar position data. In the past, infrared countermeasures systems have been
employed in an attempt to deceive infrared homing systems by employing the use
of
flares or decoys, which provide the homing system with an incorrect angle of
attack.
These approaches suffer shortcomings such as having an insufficient power in
the
right portion of the spectrum and lacking a sufficient duration of burning
time coupled
with inherent break-away problems from the launching aircraft to be defended.
Other
problems have been encountered with the use of decoys because of their limited
time
of flight and the absence of an exact knowledge as when to launch, plus the
over-all
problem of carrying sufficient quantities of such decoys.
The purpose of this invention is to obviate the problems that have arisen in
the prior
art. This countermeasures system is basically comprised of an enemy attack
detection
device which may be an active or passive system. The countermeasures system in
responding to the presence of the attacking enemy produces a radiation as for
example
light which illuminates the attack unit, which, in turn, reflects a portion of
the
radiation. The reflected radiation is received by the countermeasures system,
analyzed
and information in signal form so received is used to control the
characteristics of a
radiation directed at the attack unit to thereby deceive the homing controls
in the
attack unit and divert the attack unit's direction.
Specifically, the basic function of the system in a preferred embodiment is
the
location of an attacking aircraft which is the missile carrier by passive
electronics
countermeasures or infrared techniques followed by illumination of the
attacking
aircraft with a continual laser beam. The next step requires the examination
of the
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frequency pattern of the light reflected from spinning reticle or scanner in
the missile
head while the missile is on the plane and the last step requires the
modulation of the
laser beam with the appropriate frequency pattern and phase shift so that a
false target
is seen by the missile. The missile, accordingly, will attack this false
target when it is
launched. As soon as it has turned sufficiently to move the false target out
of its field
of view, the missile will also have lost the airplane. Since it cannot
reacquire and has
limited turning rates, the missile will wander and appear erratic thus
aborting its
mission. The attacking aircraft being unable to see the modulated infrared
laser beam
will conclude that the missile was defective. It is therefore seen that this
new system
is capable of acting as a continuously operable countermeasures system capable
of
denying angular information to infrared seekers employing spinning reticle
direction
fording techniques.
An object of this invention, therefore, is to deceive homing type attacking
missiles by
illuminating the missile with a false target signal.
Another object of this invention is to deceive an aircraft carrying a missile
into
believing there has been some malfunction in the missile by using a target
error signal
which is invisible to the aircraft's pilot.
Yet another object of this invention is to establish a compact countermeasures
system
incorporating a modulatable electromagnetic generator as a target error signal
source.
Yet another object is to provide defense for aircraft against attacking
missiles
employing homing guidance as described that is completely automatic and does
not
require an operator.
Yet another object of this invention is to provide an efficient lightweight
countermeasure system requiring relatively low power drain from the aircraft
power
supply uniquely adapting it for airborne use.
Yet another object of this invention is to provide angle deception for passive
guidance
systems of the type generally known to those skilled in the art as LORD (lobe
on
receive only).
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Other objects, features and advantages will become apparent after
consideration of the
following detailed specification together with the appended drawings, in which
FIG. 1 illustrates an aircraft being pursued by aircraft carrying an attack
missile.
FIG. 2 is a schematic of a multislit scanner.
FIG. 3 depicts a typical multislit scan element.
FIG. 4 shows a schematic of a countermeasures system in its preferred
embodiment.
FIG. 5 represents a missile infrared video output.
FIG. 6 illustrates a missile integrated error signal from its infrared video
output.
FIG. 7 shows a missile reference generator signal.
FIG. 8 depicts a countermeasures displaced oscillator signal.
FIG. 9 illustrates a laser modulator output.
Referring now to FIG. 1 where there is illustrated an aircraft 11 which is
being
pursued by an attacking aircraft 13 (partially shown), directly beneath the
attacking
aircraft i3 is illustrated a missile 14 of the air-to-air type which has just
been
launched from attacking aircraft 13. Located in the rear of the defendant
aircraft 11 is
a deceptive infrared countermeasures system 12 which responds to the presence
of the
attacking aircraft 13 and emits a signal which is received by the missiles
target signal
generator 15 which causes the missile 14 to follow a doted path 10 into a
diverted
position 16.
In order to obtain an understanding of the countermeasures system 12 and its
effect on
the target signal generator 15, a study of a typical target signal generator
will be made
with reference now to FIG. 2, in which there is shown one form of a target
signal
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generator. A complete and definitive description of this type of target signal
generator
may be had by a study of U.S. Pat. No. 3,034,405, titled "Multislit Scanner".
This type
of target signal generator consists of a modified Cassegrain telescope having
a
spherical reflector 23 provided with an opening 24 in the center and a plane
reflector
21 which is mounted on reflector support 31 by narrow supports 22. Spherical
reflector 23 is likewise secured to the reflector support 31. A rotor 29 is
supported for
rotation about axle 28 by anti-friction bearings 32. Axle 28 is mounted on
reflector
support 31 which in turn is mounted in a conventional manner. Scanner 26 which
is
alternately referred to a reticle is mounted on rotor 29 at the focal plane of
the
telescope and photosensitive detector 27 is mounted on axle 28. The Cassegrain
telescope comprising spherical reflector 23 and plane reflector 21, scanner
26, and
photosensitive detector 27 constitute the target signal generator 15.
Photosensitive detector 27 in a preferred example is formed of lead sulphide
the
resistance of which varies inversely with the intensity of incident radiation.
The
Cassegrain telescope focuses radiation from sources within the view of the
telescope
onto scanner 26. The scanner 26 rotates with the rotor 29 at a spin frequency
determined by a driving spin motor and reference generator 36, which is
illustrated as
driving the rotor 29 via the drive shaft 34 and a drive member 33. The
incident radiant
energy falling on scanner 26, is chopped by the scanner in a manner to be
described
and variations in the intensity of the incident radiation falling on detector
27 are
transmitted to amplifier 37.
The infrared missile seeker 15 noted above is of the homing type and
represents a
serious threat because of its high accuracy to aircraft in moderately clear
weather
conditions. The homing mechanisms of these seekers operate near the region of
near
infrared and their detectors, e.g., 27 are most sensitive at wavelengths of 1
to 3
microns.
Because of the short wavelengths used it is apparent from FIG. 2 that the
optics of this
system are small in size and offer high resolution accounting for the high
performance
obtained. The seeker described is purely passive and requires no transmitter
because it
homes on the exhaust heat of its target's aircraft, in this case, as shown in
FIG. 1,
defense aircraft 11. As noted above when infrared energy is reflected by a
primary
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1341487
reflector, namely, spherical reflector 23, to a plane reflector 21, it then
passes through
a central portion 24 of the spherical reflector. The reflected infrared energy
simultaneously passes through a spinning scanner 26 such as that illustrated
in FIG. 3.
This spinning reticle or scanner 26 affords discrimination against clutter
such as
clouds and sunlight and provides the basic direction finding information. The
scanner
26 is spun by the spin motor reference generator 36 by the arrangement
described
above. After the infrared energy has passed through the scanner 26, the energy
has
been chopped because of the scanner's structural configuration. This chopped
energy
strikes the photosensitive detector 27. The key to the operation of this
system is the
spinning scanner 26 which is driven by the spin motor reference generator
combination 36.
Referring now to FIG. 3, where there is shown a typical scanner 26, which
consists of
an infrared transparent material upon which is evaporated a metallic film
pattern such
as an opaque sector 42. In the scanner 26 depicted it is seen to consist of
two semi-
circular sectors 41 and 46. Sector 41 is for target sensing and is comprised
of a
plurality of slits 43, each slit 43 consisting of a transparent sector 44 and
an opaque
sector 42. Sector 46 is for indicating the phase of the signal generated by
the target
sensing sectors and is semi-transparent so as to permit one half of the
radiant energy
falling on the phasing sector to be transmitted. It should be noted that the
pattern on
the scanner 26 may take a variety of designs. In its operation at any instant
of time, a
true target will consist of a point lying wholly within one segment of the
scanner 26
while clutter would intersect more than one segment of the scanner 26. In this
manner
the seeker can discriminate between a true target and clutter.
The system is typically a null seeking system and when the target is in the
center of
the scanner 26 no chopped signal gets through. However, as the target moves
further
away from the center of the scanner an increasing chopped signal passes into
the
photosensitive detector 27 and to an amplifier 37 to yield an error signal.
As the scanner spins a cyclic chopped pattern is detected. The phase of the
cyclic
chopped pattern from the scanner depicted is compared by a phase comparator 38
when the phase of the reference generator signal to produce an angle
correction
voltage, in the same manner as the error signal is compared with a reference
generator
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13~~~87
signal and a conical scanning radar. This angle correction voltage is fed to a
control
surface actuator 39 which steers the missile. In this manner the missile knows
in
which direction to correct its aiming error in order to hit the target. In
this case,
defense aircraft 11. The relationship of the target signal generator and the
signals
produced therein with the signals sent from a countermeasures system 12 will
be
described more fully hereafter.
Referring now to FIG. 4 where there is schematically illustrated a
countermeasures
system that represents one embodiment of applicants' invention. This system 12
is
located in the rear of defense aircraft 11 and in order that this embodiment
of the
system be described a number of presumptions must be kept in mind throughout
the
study of the system, namely, that the attacking aircraft will be located
somewhere
within a solid angle 0, FIG. 1, centered dead astern. This, of course, does
not preclude
the location of another system of the same type in the forward part of the
aircraft to
detect missiles approaching from that direction or for that matter the system
may be
located at any of a number of positions on the defense aircraft 11. It should
also be
kept in mind that in this embodiment the attacking aircraft 13 will be within
some
reasonable range, for example, 5,000 yards, when the enemy aircraft decides to
launch
the missile. At launch time the attacking aircraft will be emanating microwave
radiation such as that from an aircraft ranging only radar or an airborne
intercept
radar. It is to be understood that while the system will be described in terms
of a
passive detection of enemy aircraft, the system may operate with the use of an
active
enemy detection system of either an infrared or microwave type.
In view of the foregoing examples, we can now turn to FIG. 4 in which the
countermeasures system employs a small microwave parabolic antenna 51 which
function is to receive both aircraft ranging only radar signals 50 and
simultaneously
receive infrared signals in a manner to be described more fully hereafter. As
the
attacking aircraft 13 approaches, the microwave parabolic antenna 51 receives
the
aircraft ranging only radar signals 50 and reflects them in a manner shown to
a
conically scanned element such as a triscanner 53, which, in turn, permits
their
passage via radiator 54, rotary joint 68 to a detector 73. In order that
microwave
parabolic antenna 51 be capable of receiving both aircraft ranging only
signals and
infrared signals, there is supported on the boresight axis of the reflector an
infrared
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lens with a microwave grating 56 supported by support rods 57. The infrared
lens
with a microwave grating permits the passage of infrared energy while
reflecting the
microwave energy back to the tri-scan element. These reflected radar signals
50 must
pass through the rotating tri-scan element 53 which tri-scan element 53
receives its
rotary drive from a spin motor and reference generator 62 via drive shaft 63
and drive
elements 64, 66 and 67.
The rotary joint 68 permits radiator 54 and its integrally attached tri-scan
element 53
to rotate independently of microwave conduit 71 and the rest of the system.
The
microwave energy reflected from parabolic reflector 52 and microwave grating
56
passes through the radiator 54 and into a microwave detector 73, which in turn
feeds
the information to scan video receiver 74. The microwave parabolic antenna 51
is
continually conically scanning and searching the aforementioned cone in the
stern
direction due to the rotary drive of tri-scan element 53 brought about by spin
motor 62
whose operation was noted above.
In order that the antenna 51 continually search and track the output of the
scan video
receiver 74 is fed to phase comparators 76 and 77, which are simultaneously
receiving
the output of the spin motor and reference generator 62, the phase comparators
76 and
77 compare the phase of the error signal from the scan video receiver 74 with
the
phase of a signal from the reference generator which is directly coupled with
the spin
motor which conically scans the antenna.
The output of the phase comparators 76 and 77 are fed to an antenna servo
search and
track system 78 which has a search and track programmer and suitable
amplifiers to
increase the voltage from the phase comparators 76, 77 to control respectively
the up-
down slew motor 59 and the right-left slew motor 61, which maintain the
microwave
antenna 51 in a continuous search and track path of the attacking aircraft 13.
It is
therefore apparent that this arrangement will permit the system to accurately
track the
enemy aircraft in angle by tracking an aircraft ranging only signal.
Upon reception and tracking of this aircraft ranging only signal, this system
would
assume that the enemy was preparing to launch an infrared homing missile and
the
infrared deceptive jamming would be then initiated in the following manner. As
soon
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13 ~1~87
as the microwave energy of the aircraft ranging only radar signal 50 is
detected by
microwave detector 73 and fed to the scan video receiver 74, an output from
the scan
video receiver 74 would instantly activate laser switch control 81 whose
output signal
would pass through a normally closed switch 82 to activate a laser power
supply, the
output of which would activate a continually operable laser.
The desirability of using a laser light source resides in the fact that such
lasers offer
the property of emitting essentially monochromatic, phase coherent light
energy in the
near infrared portion of the spectrum. Monochromatic light output known as
stimulated emission of radiation makes the infrared beam emerge from the laser
with
phase coherence so that a collimated beam is obtained without the use of
auxiliary
optics. Because the beam is essentially monochromatic and collimated, power
densities per solid angle may be obtained which are many times higher than can
be
obtained with any other known type of optical frequency generator. A
continually
operable laser that may be used in the instant application relies on trivalent
neodymium in calcium tungstate. The laser is fully described in the following
publication: "Physical Review" May 15, 1962, Vol. 126, No. 4, by L. F.
Johnson, on
pages 1406 to 1409. A modulatable xenon lamp suitable for modulating the
aforementioned laser is described in the September, 1962, issue of
Illuminating
Engineering, at pages 589-591. Laser modulation techniques are further
discussed in
the publication, Electronics for Nov. 10, 1961, at pages 83-85. Other types of
lasers
which are modulatable to perform the function stated herein are the diode type
laser as
described in the publication, Electronics for Oct. 5, 1962, at pages 44-45.
It should be noted that while one laser light source is illustrated the system
could
function with two lasers. One laser to give continual operation and a second
to give a
modulated output. The discussion while directed to lasers as a light source is
not
meant to exclude other light sources of sufficient power and having frequency
components at the correct wavelength.
The laser 84 in its now activated condition would emit a collimated beam of
monochromatic infrared energy 85 aimed at the attacking aircraft and its
infrared
homing missile. The laser or laser beam director 84 is integrally attached by
laser
support member 86 to parabolic reflector 52. Because of the integral physical
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relationship of the laser its beam will inherently follow the search and track
function
of the conically scanning microwave antenna 51, and accordingly illuminate the
attacking aircraft and missile simultaneously with the microwaves antenna
tracking
operation. Because of the early detection ranges of the microwave detector 73
this, of
course, occurs prior to the attacking aircraft 13 launch of its air-to-air
missile 14. The
infrared beam 85 emitted by the laser 84 is received by the target signal
generator 15
in the missile 14 head. This beam is chopped and reflected by the spinning
scanner
26, recollimated by the spherical reflector 23 and transmitted back to the
parabolic
reflector 52 of the microwave antenna 51. This collimated reflected and
chopped
beam of infrared energy is then reflected by the parabolic reflector 52 and
detected by
the photosensitive detector 58 mounted on support rods 57. It is therefore
apparent
that the signal produced by the photosensitive detector 58 will represent the
frequency
of modulation of the infrared beam as reflected by the rotating scanner. This
output
signal from the photosensitive detector 58 is amplified by audio amplifier 87
and fed
through a normally closed switch 88 to generator 89 which has a scan audio
filter 91.
The scan audio filter may be a comb filter of resonant reeds in which the reed
which
is resonant at the scanners spin frequency gives an output from the scan audio
filter 91
at the correct spin frequency which starts at a random initial phase with
respect to the
attacking missiles scanner phase. The scan audio filter 91, in the example
given, being
of the comb filter type having resonant reeds in which the reed which is
resonant at
the scanner's spin frequency has the inherent characteristic of maintaining an
output
signal for a definite period of time after its input signal is removed.
Digital and analog
devices to determine frequency may also be used. This phase shifted scanner
spin
frequency signal is fed to a triggered oscillator 92. This triggered
oscillator 92, for
example, may be controlled by a sawtooth generator 93 and an amplitude control
device 94. The amplitude control device may be a Schmitt trigger, which has
the
property that an output of constant peak value is obtained for the time period
that the
input wave form exceeds a specific voltage. It is important for reasons to be
explained
hereafter that the output from the triggered oscillator 92 function for a
distinct period
of time, then cease its output for another distinct period of time to provide
look-
through period for a check of the scanners spin frequency, before repeating
the signal.
As mentioned above this is controlled by the sawtooth generator 93 and the
amplitude
control device 94, which controls the oscillator 92 so it is turned ON and OFF
for the
proper intervals. This check of the scanners spin frequency is needed to
determine any
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changes in the spin frequency and also to prevent ring-around between the
laser 84
and the detector 58 or the system from locking up on its own modulation. This
action
takes place because the receiver is deactivated during transmission by the
look-
through process just described. The sawtooth generator 93 which is activated
by the
output from the scan audio filter produces a signal whose voltage increases
with the
passage of time until the Schmitt trigger of the amplitude control device 94
is
activated at which time an output is noted from the amplitude control 94 which
in turn
triggers the oscillator 92 to pass the phase shifted scanner frequency
detected by the
scan audio filter 91. The output from the oscillator 92 is illustrated in FIG.
8. The
output from the triggered oscillator 92 simultaneously actuates a laser
modulation
switch 96 and solenoid 90 which opens normally closed switches 82 and 88 which
act
to turn off the laser power supply 83 and the related laser 84. It will be
seen that as the
circuit between laser switch control 81 and the laser power supply 83 is
broken by the
opening of switch 82, the laser power supply is simultaneously activated by
the
actuation of laser modulation switch 96 which results in the emission of a
modulated
infrared beam 85 from laser 84. Laser modulation switch 96 produces a square
wave
shown in FIG. 9, which modulates the laser 84 at the spin rate of the missiles
scanner.
The laser modulation switch may include for example a rectifier to obtain only
one
polarity to be delivered to a power amplifier which controls a grid which in
turn
controls the laser modulation. This phase shifted modulated signal from the
laser is
now directed at the enemy's target signal generator 15 and brings about an
angle
deception by interchannel cross-coupling which will be discussed more fully
hereafter.
The transmitted beam of monochromatic infrared energy 85 illuminates a volume
of
space much larger than the attacking aircraft. Energy will be reflected from
portions
of the airplane and from the reflected portions of the scanner in the target
signal
generator 15. The energy reflected by the rotating scanner will be modulated
at a rate
determined by the number of reflected segments, their width and the spin rate
of the
spin motor 36, FIG. 2. Energy will also be reflected from the missile's
detector 32
since it is coated to be nonreflected in the wavelength region of maximum
detector
performance and will consequently be more reflective than it otherwise would
be at
the wavelength of the laser beam. The difference in reflectivity between the
detector
and the scanner comprises the signal source of the ac signal received at the
microwave
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antenna 51 in the defending aircraft. The photosensitive detector 58 in the
countermeaures system will have incorporated therein a narrow band filter
placed in
front of it (not shown). Hence, because only a narrow wavelength region is
used and
because the signal to be detected from the missile is chopped, strong do
signals from
clouds, sunlight, attacking aircraft itself and exhaust from the defending
aircraft will
be reduced to negligible portions.
The signal from detector 58 which contains the modulation components from both
halves of the scanner 26, FIG. 3, is passed to amplifier 87 which filters out
the high
frequency components from the upper half of the scanner 41, leaving an error
signal
from the lower half of the scanner 46, thus the output of the video amplifier
87
contains signal information directly related to the missile scanner rotational
frequency.
As noted earlier there arises a relationship between an error signal in the
target signal
generator 15, FIG. 2, and the spin motor reference generator 36.
Referring now to FIGS. 5, 6, and 7, there is shown in FIG. 5, a typical
missile infrared
video output which is shown in its integrated form as a sine wave in FIG. 6,
and
represents an integrated error signal from the infrared video output.
FIG. 7 illustrates the missile reference generator signal from reference
generator 36.
The reference generator 36 may either be a sine wave generator or an impulse
generator. For purposes of convenience, a sine wave output has been shown in
FIG. 7.
Since the infrared seeker compares the phases of FIG. 6 vs that shown in FIG.
7, and
uses the output of its phase comparator 38 to activate the control surfaces of
air-to-air
missile 14 in order to give false information to such a system all that has to
be done is
to shift the phase of the error signal with respect to the reference generator
signal. As
described above, the laser beam 85 will be a square wave modulated at the
scanner
spin frequency and at some random phase with respect to the true target error
signal
phase of the spinning scanner. Since the laser beam 85 represents a strong
signal
which is displaced in time phase as compared to the target signal, the jamming
signal
represented by a square wave depicted in FIG. 9, will be shifted as shown in
FIG. 9
with respect to the true error signal when compared with phase of the
reference
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generator signal as shown in FIG. 7. Hence false angle information is
presented to the
missile seeker system as a large error signal and will cause the missile
threat to veer
off from the true heading at some random false heading.
It should be clearly understood that the invention is not limited to the
infrared portion
of the electromagnetic spectrum, but is broadly applicable to any system using
guidance systems employing spinning scanning direction finding techniques
regardless of what portion of the electromagnetic spectrum is involved.
While there has been hereinbefore described what are considered preferred
embodiments of the invention, it will be apparent that many and various
changes and
modifications may be made with respect to the embodiments illustrated, without
departing from the spirit of the invention. It will be understood, therefore,
that all
changes and modifications as fall fairly within the scope of the present
invention as
defined in the appended claims are to be considered as part of the present
invention.
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