U.S. patent application number 12/434453 was filed with the patent office on 2011-07-28 for precision guided munitions.
This patent application is currently assigned to EMAG Technologies, Inc.. Invention is credited to Karl F. Brakora, Jack H. Thiesen.
Application Number | 20110180654 12/434453 |
Document ID | / |
Family ID | 44308241 |
Filed Date | 2011-07-28 |
United States Patent
Application |
20110180654 |
Kind Code |
A1 |
Thiesen; Jack H. ; et
al. |
July 28, 2011 |
PRECISION GUIDED MUNITIONS
Abstract
A guidance system for actively guiding a projectile, such as a
bullet after it has been fired from a gun. The guidance system
includes a radar unit that includes a plurality of receiver arrays.
An optical scope is also mounted to the gun for optically sighting
a target. An inertial measurement unit provided on the gun locks
onto the target after it has been sighted by the scope, and
provides a reference location at the center of the receiver arrays
from which the bullet can be directed. The receiver arrays receive
radar monopulse beacon signals from the bullet. The signals from
the bullet are used to identify the position of the bullet and the
roll of the bullet. The signals sent to the bullet provide flight
correction information that is processed on the bullet, and used to
control actuators that move steering devices on the bullet.
Inventors: |
Thiesen; Jack H.; (Plymouth,
MI) ; Brakora; Karl F.; (Dexter, MI) |
Assignee: |
EMAG Technologies, Inc.
Ann Arbor
MI
|
Family ID: |
44308241 |
Appl. No.: |
12/434453 |
Filed: |
May 1, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61049601 |
May 1, 2008 |
|
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|
61058097 |
Jun 2, 2008 |
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Current U.S.
Class: |
244/3.14 |
Current CPC
Class: |
F42B 10/64 20130101 |
Class at
Publication: |
244/3.14 |
International
Class: |
F42B 15/01 20060101
F42B015/01 |
Claims
1. A guidance system for guiding a bullet to a target after it is
fired from a gun, said guidance system comprising: a radar unit
including a plurality of receiver arrays, said receiver arrays
emitting radar signals to the bullet and receiving position signals
from the bullet; an optical scope mounted to the gun for optically
sighting the target; an inertial measurement unit provided on the
gun, said inertial measurement unit identifying a center location
of each receiver array in the radar unit; an RF transceiver on the
bullet, said RF transceiver including an antenna; and at least one
flight actuator on the bullet that controls the direction of the
flight of the bullet, wherein the position signals from the bullet
received by the radar unit identify the position of the bullet and
the radar signal from the radar unit to the bullet provides flight
path guidance to change the trajectory of the bullet in response to
its position so that the at least one flight actuator directs the
bullet towards the target.
2. The system according to claim 1 wherein the inertial measurement
unit references the center of the array with respect to the target
after it has been optically sighted and wherein the radar unit
controls the flight of the bullet using the difference between the
position of the bullet relative and the center of the receiver
arrays.
3. The system according to claim 1 wherein the position signals
from the bullet include signal polarizations that identify the roll
of the bullet where the roll of the bullet is determined by
comparing amplitudes of polarizations referenced to a linear
polarization of a receiver on the bullet.
4. The system according to claim 3 wherein the radar unit
calculates flight corrections of the bullet using the roll of the
bullet and the position of the bullet.
5. The system according to claim 1 where the radar unit employs sum
and difference monopulse tracking to locate the projectile and
compute course corrections that control the flight of the
bullet.
6. The system according to claim 1 wherein the signals transmitted
between the gun and the bullet are encoded.
7. The system according to claim 6 where the encoding is chosen so
that a line-of-sight signal and delayed reflected signal have low
correlation.
8. The system according to claim 1 further comprising a switch on
the gun that activates the inertial measurement unit to track
relative motion between the target and the radar before the bullet
is fired from the gun and after the target has been acquired.
9. The system according to claim 8 wherein the switch is on a grip
of the gun.
10. The system according to claim 8 wherein the switch is provided
by a half trigger pull of a trigger on the gun.
11. The system according to claim 1 wherein the radar unit is
mounted to the gun.
12. The system according to claim 1 wherein the radar unit is
separate from the gun.
13. The system according to claim 1 wherein the bullet includes a
fusible switch that causes power to be provided to circuitry on the
bullet after it is fired.
14. The system according to claim 1 wherein the bullet is a .50
caliber bullet.
15. The system according to claim 1 wherein the plurality of
receiver arrays is two arrays.
16. The system according to claim 1 wherein the plurality of
receiver arrays is four arrays.
17. The system according to claim 1 wherein the at least one flight
actuator is a plurality of flight actuators that control the
trajectory of the bullet.
18. A guidance system for guiding a bullet to a target after it is
fired from a gun, said guidance system comprising: a radar unit
mounted to the gun, said radar unit including a plurality of
receiver arrays, said receiver arrays emitting radar signals to the
bullet and receiving position signals from the bullet; an optical
scope mounted to the gun for optically sighting the target; an
inertial measurement unit provided on the gun, said inertial
measurement unit tracking the motion of the center location of each
receiver array in the radar unit, wherein the inertial measurement
unit corrects for motion between the center of the receiver arrays
and the target after it has been optically sighted; an RF
transceiver on the bullet, said RF transceiver including an
antenna, wherein the position signals from the bullet include
signal polarizations that identify the roll of the bullet where the
roll of the bullet is determined by comparing amplitudes of
polarizations referenced to a linear polarization of a receiver on
the bullet; and at least one flight actuator on the bullet that
controls the direction of the flight of the bullet, wherein the
position signals from the bullet received by the radar unit
identify the position of the bullet and the radar signal from the
radar unit to the bullet provides flight path guidance to change
the trajectory of the bullet in response to its position so that
the at least one flight actuator directs the bullet towards the
target and wherein the radar unit controls the flight of the bullet
using the difference between the position of the bullet relative to
the center of the receiver arrays, said radar unit calculating
flight corrections of the bullet using the roll of the bullet and
the position of the bullet and said radar unit employing monopulse
radar tracking to compute trajectory information to be sent to the
bullet.
19. The system according to claim 18 wherein the signals
transmitted between the gun and the bullet are encoded.
20. The system according to claim 19 where the encoding is chosen
so that a line-of-sight signal and delayed reflected signal have
low correlation.
21. The system according to claim 18 further comprising a switch on
the gun that activates the inertial measurement unit to lock onto
the target before the bullet is fired from the gun.
22. The system according to claim 18 wherein the bullet includes a
fusible switch that causes power to be provided to circuitry on the
bullet after it is fired.
23. The system according to claim 18 wherein the bullet is a .50
caliber bullet.
24. A guidance system for guiding a projectile to a target, said
guidance system comprising: a radar unit including a plurality of
receiver arrays, said receiver arrays emitting radar signals to the
projectile and receiving position signals from the projectile; an
inertial measurement unit identifying a center location of each
receiver array in the radar unit; an RF transceiver on the
projectile, said RF transceiver including an antenna; and at least
one flight actuator on the projectile that controls the direction
of the flight of the projectile, wherein the position signals from
the projectile received by the radar unit identify the position of
the projectile and the radar signal from the radar unit to the
projectile provides flight path guidance to change the trajectory
of the projectile in response to its position so that the at least
one flight actuator directs the projectile towards the target.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional Patent
Application Ser. No. 61/049,601, filed May 1, 2008, titled
Monopulse Active Guidance for Independently Controlled Bullets and
to U.S. Provisional Patent Application Ser. No. 61/058,097, filed
Jun. 2, 2008, titled Precision Guided Munitions.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates generally to guided munitions and,
more particularly, to a system for guiding a projectile, where the
system provides monopulse radar active guidance for independent
control.
[0004] 2. Discussion of the Related Art
[0005] Snipers and sharp shooters are valuable for both their
lethality and their disproportionate ability to limit the
maneuverable battle space of hostile infantry. The ability of a
sniper to selectively engage and kill an enemy at distances over
one mile has a paralyzing effect on an adversarial combat force.
Given the tempo of operations common in asymmetric warfare, it is
often too late to deploy support by the time an engagement has
begun, and a commander must depend on assets already in place. One
way to address the issue of sniper availability is to provide a
squad-level weapon that can give any war fighter the range and
killing ability of a sniper.
SUMMARY OF THE INVENTION
[0006] In accordance with the teachings of the present invention, a
guidance system is disclosed for actively guiding a projectile,
such as bullet after it has been fired from a gun. The guidance
system includes a radar unit having a plurality of receiver arrays.
An optical scope is also mounted to the gun for optically sighting
a target. An inertial measurement unit provided on the gun locks
onto the target after it has been sighted by the scope, and
provides a reference location at the center of the receiver arrays
from which the bullet can be directed. The arrays receive radar
monopulse beacon signals from the bullet. The signals received by
the radar unit from the bullet are used to identify the position of
the bullet and the roll of the bullet. The signals sent to the
bullet from the radar unit provide flight correction information
that is processed on the bullet, and used to control actuators that
move steering devices on the bullet.
[0007] Additional features of the present invention will become
apparent from the following description and appended claims, taken
in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is an illustration of a soldier shooting a guided
bullet that is being adaptively steered to its target;
[0009] FIG. 2 is an illustration of a sniper rifle equipped with a
system for providing bullet guidance after the bullet has been
fired, according to an embodiment of the present invention;
[0010] FIG. 3 is a broken-away, rear perspective view of a radar
unit and scope mounted rifle shown in FIG. 2;
[0011] FIG. 4 is a broken-away perspective view of the stock of the
rifle shown in FIG. 2;
[0012] FIG. 5 is an illustration of a .50 caliber bullet including
an RF transceiver and flight actuators that provide bullet
guidance, according to an embodiment of the present invention;
[0013] FIG. 6 is a block diagram of an RF transceiver module
provided on the bullet shown in FIG. 5;
[0014] FIG. 7 is a block diagram of a bullet guidance system
providing a closed control loop between a processor on the sniper
rifle and a guidance control system on the bullet, according to an
embodiment of the present invention;
[0015] FIG. 8 is an illustration showing multipath reflections from
the ground between a radar unit on a rifle and a bullet in
flight;
[0016] FIG. 9 is a front, perspective view of a radar unit for the
rifle shown in FIG. 2 including multiple receivers, according to
another embodiment of the present invention;
[0017] FIG. 10 is an illustration of a radar system tracking and
guiding an indirect fire projectile;
[0018] FIG. 11 is a broken-away, perspective view of a precision
guidance module within a guided projectile;
[0019] FIG. 12 is a schematic block diagram of a radar processing
system for a radar guided projectile;
[0020] FIG. 13 is a schematic block diagram of a forward
communications system for a guided projectile;
[0021] FIG. 14 is a schematic block diagram of electronics in the
guided projectile;
[0022] FIG. 15 is an illustration of a bullet being guided by a
radar signal with a radar unit that is not attached to the
rifle;
[0023] FIG. 16 is a schematic diagram of a classic four-aperture
monopulse system; and
[0024] FIG. 17 is a block diagram showing RF and control
electronics in a guided bullet.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0025] The following discussion of the embodiments of the invention
directed to a radar system for guiding a projectile is merely
exemplary in nature, and is in no way intended to limit the
invention or its applications or uses.
[0026] The present invention proposes a monopulse radar system for
providing active guidance of a bullet after it has been fired from
a gun. In one embodiment, the invention is a monopulse active
guidance independently controlled bullet. FIG. 1 is an illustration
of a sniper 10 firing a bullet from a rifle at a target 12 some
distance away. As will be discussed below, the sniper 10 will
acquire the target 12 optically using a scope on the rifle or an
optical sighting system nearby, and a monopulse radar system will
track the bullet along a flight path 14 towards the target 12. The
bullet will make adjustments to its flight so that it will hit the
target 12 with a high-degree of accuracy. The guided bullet of the
invention provides sniper-like capabilities to any shooter.
[0027] In one non-limiting embodiment, the guided bullet is a .50
caliber round that is guided up to ranges of 2 km with a better
than 20 cm degrees of accuracy. This accuracy will be accomplished
using high-resolution radar tracking and an adaptive communications
link to transmit flight correction data to the bullet that is
capable of continuously adjusting its trajectory. The bullet
guidance system can employ advanced phased array systems.
[0028] The proposed guided bullet system of the invention includes
six main sub-systems. These sub-systems include a .50 caliber rifle
capable of firing guided munitions, a radar unit integrated into
the optical sighting system used to acquire the target, a guided
bullet that includes the ability to be steered, the ability to
communicate, and the ability to provide a beacon, a back-end
processor that collects information from an optical range finder,
an inertial measurement unit (IMU) to provide positional correction
information to the radar, and an integrated power supply for both
the rifle and the bullet.
[0029] The proposed guided bullet system allows the position and
range-to-target to be sighted optically and locked into the
targeting system prior to firing. Afterwards, a three-axis inertial
measurement system (IMU), integrated into the radar, will begin
measuring the pointing deviations from a locked position. Data from
the IMU will maintain the scope reference position during firing,
recoil and recovery even in the case where the shooter is acquiring
a new target. Immediately after firing, the array transceiver
system will initiate communications with the bullet, providing a
phase coded mask that will be used to mitigate the effects of
intentional and unintentional jamming, such as radio frequency
interference with other guided bullets. After initialization, the
bullet responds using its assigned phase coding. The radar
estimates the on-bullet clock to synchronize beacon operation and
flight correction data transfer. During guided flight, RF
operations at the rifle alternate between beacon monopulse
measurement and communication. In the beacon monopulse mode, the
bullet transmits an encoded set of predetermined polarizations
referenced to a particular flight control surface. The use of a set
of polarizations eliminates the effects of amplitude variance
between the transmitter and receiver, basically enabling
differential measurement.
[0030] The radar receiver on the rifle uses the beacon in a passive
monopulse detection scheme to locate the bullet with an accuracy
better than 0.020.degree. in both elevation and azimuth. The roll
of the bullet is measured by comparing the amplitudes of the
encoded polarization sequence referenced to the linear polarization
of the receiver. Absolute roll is determined by knowing the initial
orientation of the bullet and tracking changes over the course of
the flight path. Based on the roll and position measurement, flight
correction is calculated and transmitted back to the bullet,
closing the control loop. Range information is derived by measuring
the two-way time-of-flight for a request to transmit from the radar
and a transmission from the projectile.
[0031] FIG. 2 is a perspective view of a sniper rifle 20 that fires
and then guides a guided bullet using radar tracking, as discussed
above. The rifle 20 includes a barrel 22, a receiver 24, a stock
26, a grip 28, a trigger 30, a magazine 32 and a stand 34. A switch
44 on the grip 28 activates the guidance system to acquire the
target before the bullet is fired. The rifle 20 also includes a
radar transceiver unit 42 and a scope 36 mounted to the top of the
receiver 24. The transceiver unit 42 includes two transceiver
antenna arrays 38 and 40 mounted on opposite sides of the scope 36,
as shown. The antenna arrays 38 and 40 include a plurality of
antenna elements 46, here patch antenna elements, although other
types of antenna elements may be equally applicable. FIG. 3 is a
broken-away, perspective view of the rifle 20 showing a back view
of the scope 36 and the transceiver array 42.
[0032] FIG. 4 is a broken-away perspective view of the rifle 20
showing the stock 26. In this non-limiting embodiment, the stock 26
houses various parts of the guidance system including batteries 50,
an inertial measurement unit (IMU) 52 and processing circuitry
54.
[0033] FIG. 5 is a perspective view of a guided bullet 60 of the
type discussed above that is fired and guided by the rifle 20,
according to an embodiment of the present invention. The bullet 60
includes a projectile portion 62 at the front of the bullet 60 and
guidance fins 64 at a rear of the bullet 60. The guidance fins 64
are moveable on actuators 66, 68 and 70 that can be controlled by
the guidance system in the bullet 60, as will be discussed in more
detail below. The actuators 66, 68 and 70 that move the fins 64 can
be any suitable actuator for the purposes described herein, such as
piezoelectric actuators. The bullet 60 includes a dual-polarized
patch antenna array 72 having patch antenna elements 58 at the rear
of the bullet 60 between the fins 64 that receive and transmit the
RF signals consistent with the discussion herein. A battery 76
provides power to the various electrical devices on the bullet 60.
The bullet 60 also includes processing circuitry 74 for processing,
power management and flight control. The bullet 60 also includes a
fusible switch 78 that turns on the circuitry 74 when the bullet 60
is fired.
[0034] FIG. 6 is a block diagram of the processing circuitry 74 as
one non-limiting embodiment. The processing circuitry 74 includes a
state machine 80 that is powered by a power source 82 representing
the battery 76. The circuitry 74 also includes a power management
device 84 that provides power to a transmitter 86 and a
linearly-polarized receiver 88 controlled by the state machine 80.
The transmitter 86 includes a vertically polarized antenna 90 and
the receiver 88 includes a horizontally polarized antenna 92.
[0035] The operation of the guided bullet 60 breaks down into three
functions, namely, receive correction data, correct flight path and
transmit a radio frequency beacon. The communication functions of
the bullet 60 require that a full transceiver module be packaged in
the bullet 60. The correction data will be received in a single
polarization, down-converted to IF, and demodulated according to a
phase coding mask stored in the state machine. Flight control
information is then decoded and written to data registers. Flight
surfaces, such as the fins 64, are actuated using level shifted
control signals from the state machine 80. In the beacon-mode, the
bullet 60 will transmit a sequence of three predetermined
polarizations, such as -30.degree., 0.degree. and 30.degree., which
allows the linearly-polarized receiver 88 to accurately determine
the bullet's orientation. This scheme makes it possible to account
for signal strength variations of rising from part-to-part
tolerance and to accurately track the absolute roll of the bullet
60.
[0036] A target is acquired through the normal optical sighting
process using the scope 36. It is assumed that the active
range-finding is provided by the optics that will be operated
through controls on the grip 28 of the rifle 20. When the target
has been acquired and the range determined, the guidance system is
locked. The current orientation of the rifle 20 is set by pressing
the switch 44 on the grip 28, or possibly by a switch that is
closed by a half-pull of the trigger 30. This establishes bias
power to the radar and initiates the position/orientation tracking
function of the 3-axis IMU 52 integrated in the stock 26 of the
rifle 20. In this manner, the orientation of the rifle 20 relative
to the target is known at all subsequent times and this information
is used to provide guidance. The IMU 52 must provide sufficient
accuracy of the rifle's angular orientation at a rate that allows
for correction of rifle motion. Thus, when the shooter is ready and
the bullet 60 is fired, the bullet 60 will home to the position
initially sighted regardless of the subsequent motion of the rifle
20.
[0037] Projectile acquisition masking is generally provided between
time 0-4 ms. The fusible switch 78 in the bullet 60 is tripped by
the concussion of firing, powering up the circuitry 74. The purpose
of the fusible switch 78 is to preserve battery power over the
storage life of the bullet 60. After firing, the rifle 20
establishes initial communications with the bullet 60 and measures
polarization and roll of the bullet 60. In order to prevent
detection and the initiation of countermeasures, the transmitting
array module will initially operate in a low-directivity, low-power
mode. The initial communication is a low-data rate transfer that
contains the phase-coding that will be used by the bullet 60 during
the remainder of its flight. This coding protects the radar from
seduction and jamming while spreading the power spectrum of the
transmitted signal to prevent detection. This coding also mitigates
RF interference arising from independently operated, co-located
guided bullets. When the guided bullet 60 has received and
processed the initialization data, it transmits a beacon pulse to
the rifle 20 using its assigned code. The beacon signal from the
bullet 60 is used to estimate the frequency of the on-bullet clock.
This estimation process allows the coordination of flight
correction and beacon modes during guided flight.
[0038] During the time from 4 ms to 100 ms after firing, the bullet
60 will experience its most rapid deceleration and turbulence.
During this time, it will not be possible to correct errors in the
roll or flight path.
[0039] During time 100 ms to 4500 ms after firing, the bullet 60 is
in a stable guided flight as its transceiver toggles between
beacon-mode and receiving-mode. Initially, the radar unit 42 on the
rifle 20 operates in a low-directivity, low-power transmission mode
to prevent detection or the initiation of countermeasures. As the
range increases, the directivity and transmitted power of the
transmit array module increases as more elements are engaged. This
is a significant benefit of the proposed system. Because the bullet
60 will be transmitting a beacon back to the rifle 20 with a
significant on-target pattern null and because the transmit beam at
the rifle 20 can be adaptively shaped, keeping transmit power at
the minimum level required to maintain an acceptable bit error rate
and signal detection becomes practically unworkable. At short
range, the total coherent integration time required by the radar
unit 42 is comparatively small. As the range increases,
increasingly long beacon intervals are required. By the last phase
of the bullet fight, total beacon-mode intervals will be on the
order of milliseconds.
[0040] At time 5000 ms, the electronics of the bullet 60 go
silent.
[0041] FIG. 7 is a block diagram of a bullet guidance system 100 of
the type discussed above, according to an embodiment of the present
invention. The system 100 includes a unit 102 representing the
guided bullet electronic controls on the bullet 60 and a targeting
and tracking unit 104 representing the targeting and tracking
controls at the rifle 20. The unit 102 receives encoded directional
information from a radar beacon 106 provided by the unit 104.
Transmitted trajectory update commands in the beacon 106 are
received by a command and control receiver 108 on the unit 102 that
provides some front-end processing, such as frequency
down-conversion, and provides the signal to an RF integrated
circuit and state machine 110. The RFIC and state machine 110
provide power management, decoding, encoding and polarization
control, as will be discussed in further detail below. The decoded
guidance information is sent to a flight control processor 112 that
controls actuators 114 on the unit 102. Power management signals
are provided to a battery and signal conditioning circuitry 116
that powers the actuators 114. Further, the RFIC and state machine
110 generate a PN encoded beacon signal that is sent to a beacon
transmitter 118 that generates a signal that is transmitted back to
the tracking and targeting unit 104 identifying the bullet's
position.
[0042] An encoded beacon signal 120 from the unit 102 is received
by a radar array 126 on the tracking and targeting unit 104, where
it is sampled and detected at box 128, integrated at box 130 and
its vertical polarization is measured at box 132 in a processor
122. From the vertical polarization, the roll and angular position
of the unit 102 is determined at box 134. The position of the unit
102 is determined relative to the center of the arrays 38 and 40.
An IMU 136 is used to null motion of the center of the arrays 38
and 40 so as to provide a reference for the received signal. The
IMU 136 is sampled at box 138 and the IMU data is buffered at box
140. The orientation of the unit 104 is determined at box 142, and
the orientation of the unit 104 and the roll angular position of
the unit 102 are then sent to box 144 that computes the flight data
to steer or guide the unit 102. The flight control data is then
encoded at box 146 and transmitted by transmitter 148. The proposed
beacon-communication frequency for the guided bullet concept can be
selected to be near 30 GHz. It may be desirable however to shift to
higher frequencies in future systems to reduce the likelihood of
counter measures being developed.
[0043] Each element in the arrays 38 and 40 is a completely
independently weighted transceiver that provides excellent
watt-to-watt efficiency required by battery-powered operation and
the ability for multi-beam and null-steering known as an active
electronically steered array (AESA). Highly-miniaturized RF
electronics allows extremely compact transceiver modules for use in
the harsh environment of a supersonic bullet. Beamforming
technology allows the guided bullet's flight controller to be
integrated directly into the rifle 20. These levels of integration
provides a fire and forget capability that guides the bullet 60
along its most natural ballistic trajectory while the shooter is
free to engage new targets or respond to other threats.
[0044] Using an AESA for millimeter/wave communications, passive
monopulse radar has distinct advantages over optical illumination
and beam-riding methods. The use of beam steering allows the radar
to quickly adjust to maintain target tracking even if misaligned or
undergoing violent acceleration during recoil. This allows the
tracking and guiding system to be integrated directly with the
rifle 20 without the need for mechanical stabilization. There is no
need for precise alignment after the initial sighting, making this
a fire-and-forget weapon, where radar alignment must be maintained
to .+-.45 g. Using radar tracking rather than optical guidance
allows the bullet 60 to follow its optimal ballistic trajectory
rather than a flat trajectory to the target such as required by a
beam rider guidance system. This reduces the requirements of the
flight control system and provides higher impact energy because the
bullet 60 only needs to correct deviations from its ballistic
course rather than sacrifice air speed to overcome the force of
gravity. Unlike RF systems that paint the target during the entire
flight, a millimeter/wave system can use adjustable power levels
and spread spectrum pulse compression to hinder detection and the
initiation of counter measures.
[0045] By assigning each bullet a unique address and communication
coding, the bullet 60 is protected from jamming and seduction.
Further, radar has all-weather capability. Based on a single
optical sighting and range, the bullet 60 can be guided through
rain, fog, snow, smoke, dust or haze without the signal degradation
of optical systems subjected to these complicating environmental
factors.
[0046] The use of AESA technology also has distinct advantages over
similar fixed-aperture monopulse systems. Waveguide fed horn
antennas, a common monopulse architecture, are inherently large and
heavy, and must be mechanically steered to maintain SNR as the
target moves with respect to the rifle boresite. By contrast, the
AESA technology is only 15-20 mm deep regardless of the total
aperture size. In a 64-element array configuration, each quadrature
antenna will have 16 independent receivers, providing protection
against multi-point failure and improvement in noise figure.
Instantaneous electronically controlled beam-pointing makes it
possible to keep a projectile optimally in-beam even if the radar
antenna has moved off target or as the projectile arcs over a
ballistic trajectory.
[0047] The present invention proposes applying a multi-use AESA
architecture to establish both a radar and communication link with
the bullet 60, and to design and construct highly miniaturized
on-bullet RF transceivers. The RF electronics required to provide
the bullet control will need to withstand approximately 40,000 Gs
of acceleration at the time of firing. To survive these conditions,
the RF electronics must be highly-miniaturized, low-mass and
packaged in a low-thermal conductivity potting material. An
advantage of using a compact integration scheme is survival of
firing accelerations. The low mass of highly compact modules
reduces forces and allows for compliant potting around the
electronics. A firmly-isolated, local ground plane will be created
in the potting of the electronics on the bullet 60 to support the
common-ground requirements of the transceiver electronics. Battery
power to the circuit 74 will be established at the time of firing
by a miniature inertial switch, such as the switch 78.
[0048] The position of the bullet 60 in azimuth and elevation can
be determined by beacon monopulse radar. Monopulse radar is a
high-resolution method of determining a point-like target's angular
position with only a single-pulse. Monopulse radar is capable of
providing much higher angular resolution than scanning methods
while maintaining a substantially lower data rate. Beacon monopulse
radar is the passive radar implementation of monopulse radar in
which the target emits a signal that is detected by the radar. In
beacon-mode, the design variables that govern the SNR are the
beacon to power P.sub.t and the compressed-pulse integration time
N.tau..sub.c where N is the number of coherently summed pulses and
.tau..sub.c is the pulse compression time. From this, the SNR can
be given as:
S N R = P t N .tau. c G t G r .lamda. 2 ( 4 .pi. R ) 2 kT 0 F
##EQU00001##
Where G.sub.t and G.sub.r are the gains of the bullet's antenna and
the receive array, respectively, .lamda. is the wavelength, R is
the range and kT.sub.0F is the input equivalent noise density of
the receiver.
[0049] Higher pulse compression ratios or more coherent averages
can be used to increase the SNR of the link, particularly at
greater ranges where maintaining resolution becomes more difficult.
Angular resolution as a fraction of the antenna beamwidth is
inversely proportional to the square root of the SNR. The
approximate angular error in a given direction .sigma..sub.x for
phase-sensing monopulses can be given as:
.sigma. x = M 2 .beta. x .pi. S N R ##EQU00002##
Where SNR is the signal to noise ratio and .beta..sub.x is the
sectoral pattern beamwidth.
[0050] In order to achieve the 0.1 milliradian accuracy required to
guide a projectile to within 20 cm at 2000 m, SNR of 40 dB or
higher can be required. This indicates integration times on the
order of milliseconds at maximum range. At maximum range, long
integration times or higher beacon power may be necessary to
achieve the desired accuracy.
[0051] Monopulse radar is intended to track a single target with
high accuracy, and is particularly susceptible to the effects of
clutter and multipath. FIG. 8 is a representation of a radar unit
160 on the rifle 20 and a bullet 162, where the bullet 162 travels
along a multipath surface 164. A direct transmission between the
radar unit 160 and the bullet 162 is shown by path R and a
multipath reflection off of the surface 164 is shown by path R'.
Many methods have been developed in the art to cancel the effects
of multipath, but each makes strong assumptions about the
scattering surfaces that cannot be made for the general case of
varying environments and terrains certain to be found in the
operation of tactical projectile guidance system.
[0052] Multipath error is the primary impediment to accurately
determine the elevation and azimuth position of the beacon. It is
caused by a beacon's signal reflecting from the terrain, buildings,
walls, power lines, or other features. Conceptually, the simplest
type of multipath error results from specular reflection from a
relatively flat surface. In the case of the guided bullet 60,
multipath induced signal degradation largely arises from scattering
off of both rough and/or volumetric scatters. At Ka-band
frequencies, most natural terrains, such as tall grass, brush,
uneven desert, rocky or gravel surfaces, are probabilistic scatters
and do not produce a coherent image. Over enough distance, these
scatters can be expected to act as a zero mean noise source and
therefore are not troubling. On the other hand, man-made surfaces
and objects, such as asphalt, buildings, walls, and a few natural
surfaces, such as water, dense snow, and ice, produce specular
reflections.
[0053] A number of methods can be used to reduce the effects of
multipath error. A high-directivity radar receiver antenna helps to
mitigate multipath effects. High directivity at the receiver
reduces the requisite SNR and narrows the beamwidth so less
indirect multipath clutter is received. The 32.times.32-element
receive array proposed for this system will have a beamwidth of
approximately 4.degree.. Multipath from scatters more than
.+-.2.degree. from the target line-of-sight are thus substantially
attenuated. The effects of high antenna directivity are most
beneficial when the shooter is very near to the ground and the
specular multipath reflection point is near the shooter.
[0054] Using a large modulation bandwidth is another technique to
eliminate multipath error. Since signals at different frequencies
have different phase delays, waveforms decorrelate as a function of
increasing bandwidth. Another benefit of higher bandwidth
techniques can be particularly successful in reducing the effects
of multipath signals if the total time delay can be resolved and
range gated. In one embodiment, a spread spectrum coded beacon
signal using a phase-modulated pseudorandom noise code (PN-code) is
employed. In this case, if a multipath signal has been delayed by
more than one chip period of the PN code, it is decorrelated from
the direct signal after demodulation. A PN code is generated by
switching the phase between from 0.degree. and 180.degree.. The
switching rate, or chip rate, determines the bandwidth of the
signal.
[0055] The multipath signal can be isolated and rejected because a
reflected signal has a different path length than the line-of-sight
signal. Assuming a flat specular surface and low elevation angle,
which is the worst case scenario, the difference in path length is
given by:
R 1 - R .apprxeq. 2 h 1 h 2 R ##EQU00003##
[0056] If a signal is delayed by more than on modulation chip and
the delayed code is orthogonal with the undelayed code, then the
multi-path signal is strongly decorrelated in the demodulation and
its effects greatly reduced. For instance, a 2 GHz bandwidth
corresponds to 15 cm of additional path length to delay the
reflected signal by one chip. Thus, if the PN code is orthogonal
with its shifted image, a shooter 1 m away from a flat multipath
surface could resolve a beacon if it is 15 m from the same surface
at 2000 m, 7.5 m at 1000 m, and 3 m at 500 m. Given the actual
trajectories of the .50 caliber bullet, the use of a higher
modulation frequency makes it possible to uniquely track the beacon
over many flat terrain features. In the case when the specular
reflection angle is out of the radar beam the effects of multipath
are significantly reduced.
[0057] FIG. 9 is a perspective view of a radar unit 170 that can
replace the radar unit 42 in an effort to help with multi-path
errors. In this embodiment, the radar unit 170 includes four
receiver phased-arrays and one transmitter phased-array 172
including patch antenna elements 174 that form the AESA. The unit
170 provides separate apertures that have the effect of increasing
the radar sensitivity, decreasing the beamwidth and increasing the
directivity at the expense of creating grating lobes. Higher
directivity at the beacon and the radar receiver are advantageous.
Higher directivity at the beacon produces more radiation in the
direct line-of-sight, and less in stray multipath directions. High
directivity at the receiver reduces the requisite SNR and narrows
the beamwidth so less indirect multipath clutter is received. In
this system, the beamwidth can easily be narrowed by using multiple
discrete apertures at the expense of creating grating lobes.
However, the grating lobes can be set to angles where the
contribution of multipath is likely to be small and which can be
easily range-gated.
[0058] There are several methods to estimate the multipath induced
error in a monopulse system. Using Bayesian estimation of the
current position based on the certainty of the measurement and the
prior trajectory, it is possible to maintain sub-milliradian
accuracy even with high uncertainty during some phases of bullet
flight. It is possible to avoid many multipath effects and errors
by using a higher trajectory than the optimal ballistics path.
During the terminal phase of flight and in some low-elevation
scenarios, it may become necessary to operate in what is known as a
Low-E mode in which elevation tracking and course correction is
disabled. Azimuth detection and correction would remain unaffected.
The effects of operating in this mode at the end of controlled
flight should not be problematic since it is expected that course
corrections to the bullet 60 will necessarily become smaller as the
bullet 60 nears the target.
[0059] Environments in which it will be most difficult for the
guided bullet 60 to be used are the most cluttered cases. However,
these are also the cases where it is unlikely that a soldier can
attempt a 1-2 km shot. A soldier, for instance, is unlikely to find
1000 m of unobstructed view in a forest, along an alleyway or down
a city street. The best application of this technology is firing
from elevated positions, such as from a tower, rooftop or hill. It
is also worth noting that this technology can easily be adapted to
aircraft and UAV's with minimal alterations to the radar.
[0060] Although the discussion above and below is more specifically
directed to guided bullets and guided indirect projectiles, it will
be appreciated by those skilled in the art that the RF tracking and
guidance system of the invention will have application to other
guided projectiles. FIG. 10 shows a general representation 180 of a
mortar team 182 firing a mortar 184 that is tracked and guided by
radar systems 186 as one alternative projectile.
[0061] The baseline concept is that the guided indirect projectile
to be steered uses canards incorporated on the fuze. Piezo-actuated
flight control surfaces can provide sufficient control authority to
accurately and significantly steer/divert a mortar over the 5600 m
trajectory. The steered projectile is tracked using a
beacon-monopulse radar located near the mortar emplacement. In this
configuration, the projectile emits a signal of 10 mW or more, at
microwave frequencies up to 35 GHz signal beacon from the fuse
assembly. The RF beacon is tracked using a highly compact and
inexpensive phased-array operating as a passive mono-pulse radar
receiver. Flight path corrections are calculated by comparing the
measured trajectory with the ideal trajectory, and flight control
commands are transmitted from the emplacement to the steerable
round.
[0062] Indirect fire support for military operations in urban
terrain (MOUT) must have the capability to engage adversaries
hidden in urban canyons. The preset mortar round will have
sufficient flight control authority to permit a 200' straight down
trajectory at the terminal point of the ballistic flight path. An
additional benefit of the communication link is in-flight fuze
programming. Connection of the fire control computer to the
communication system/radar makes this method of fuze programming
easily realized. This also makes the unguided ballistic trajectory
the failsafe default, since the canards will not deploy until
communication is established.
[0063] One significant capability improvement that is within reach
using this approach is the simultaneous precision engagement of
multiple independent targets. By implementing an effective TDMA
channelization scheme and using electronically steered phased array
radar, the present system can track and simultaneously control
multiple in-flight projectiles.
[0064] Active guidance and flight control provides a means to
compensate for wind and other disturbances without the inherent
difficulties and limitations of atmospheric characterization. RF
guidance is superior to optical guidance in low visibility
environments where indirect fire is most useful because RF can
operate through smoke, dust, rain and snow. A local RF system also
provides immunity to the difficulties associated with operation in
GPS denied environments. Finally, if it is deemed tactically
important, a forward observer can be equipped with a smaller
version of the radar to guide the projectile to target with extreme
precision at extended ranges and perhaps even allowing for the
possibility of engaging moving targets.
[0065] In the case of mounted cannons, the use of an
electronically-steered phased array can enable shoot-and-scoot
operations greatly improving survivability and confounding
countermeasures. The use of PN coded RF channels makes seduction
and jamming nearly impossible. Passive radar and a standard
communication channel with a low-probability-of-detection waveform
for COMMS makes detection of the radar very difficult if not
impossible.
[0066] Prior to firing, the radar system is surveyed into position
in a manner that conforms with the current training practices of
mortar and artillery teams. The baseline system concept calls for
the polar target coordinates and range-to-target to be provided to
the radar from the fire control computer. This information may be
transmitted via a simple serial link to the radar prior to firing.
After launch, the radar acquires the projectile, establishes a time
domain multiple access PN coded communication, and begins tracking
and flight control. The control system alternates between
communication and radar tracking of the beacon. Communication
occurs on a low-duty cycle minimally powered UF channel which will
be made to appear as though it was a standard voice
communication.
[0067] FIG. 11 is a broken-away perspective view of a fuze 190
showing various electronics therein with antenna 192 connected to
beacon transmitter and guidance control circuitry 194. During the
course of flight, deviations from the predicted trajectory are
measured with an accuracy of better than 0.0003 radians and
corrections to the flight are calculated at the radar. The COMM
link updates the kinematic control of the projectile. The key
metric of kinematic control is control authority, where there must
be enough control authority to accurately steer the projectile to
the target. An example of an approach to guide the projectile is to
use nose-mounted canards 196 to control the normal acceleration of
the projectile.
[0068] FIG. 12 is a schematic block diagram of a radar sensor
circuit 200, similar to the array 126 discussed above. Quadrature
antennas 202 comprise a sum and difference monopulse radar
receiver. Each of the quadrature antennas 202 may be comprised of a
number of radiating elements 204 forming an AESA as previously
described. Signals received at the antennas 202 are amplified by an
amplifier 206 and down-converted by a down-converter 208 to an IF
and summed by a summer 210. The signal is filtered by a filter 212
and enters a sample/detect circuit 224, such as described above at
the box 128. The circuit 224 samples the signal with an
analog-to-digital converter 214. The signal is aligned with a mask
216 and demodulated by a demodulator 218 by inverting the mask 216.
Finally, a fast-Fourier transform is performed at box 220 and the
signal is integrated and analyzed. The phase and/or amplitude
information is then compared to find the direction of arrival at
box 222 and angular position information is provided. In the
digital phase comparison hardware, the phase of the four recovered
CW beacon signals is used to determine the position of the beacon
with a 1-.sigma. accuracy of less than 300 .mu.radians. Finally,
the projectile's position is communicated to the system
guidance.
[0069] FIG. 13 is a schematic block diagram of a communications
system 230 for the guided bullet being discussed herein. The
purpose of the communication sub-system is to transmit flight
correction commands to the projectile. It is important that the
communication system not betray the position of the operator. In
operation, data is transferred from the guidance block to a
standard serial connection to the communication block. The data is
buffered into an integrated modem where it is encoded and modulated
and provided as the input to the AESA-based 35 GHz upconverter.
[0070] To minimize the emission signature of the transmitted
signal, an extremely short data packet (no more that 15 bytes) will
be transmitted. Bursty time randomized data transfer is one of the
best means for reducing the probability of detection for a convert
transmitter.
[0071] The multi-projectile communication system architecture is a
simple master-slave time domain multiple access (TDMA) type, with
the radar tracker dynamically allocating time channels for each
projectile. This gives the system the greatest flexibility in
acquiring positional updates as well providing the most robust
method for managing multiple projectiles. Another advantage of this
architecture is that it allows the radar/COMM system to estimate
the variances in the projectile's onboard clock, which is critical
for the delta-time based range estimate. Finally this makes it
possible to randomize the transmission time to further inhibit
detection.
[0072] FIG. 14 is a block diagram of the projectile's guidance and
control package 240. Bullet electronics 242 provide five critical
functions. The bullet transmits a phase-modulated beacon by
generating PN modulation data in a controller 248 and upconverting
to the desired RF frequency in a transmitter 244. The electronics
242 receives and parses flight control updates at receiver 246,
determines the bullet orientation, and maintains roll
synchronization at a processor 250 within the controller 248 for
the resonant control of the flight actuators. Finally, the
electronics 242 provides power management at box 254 and voltage
conditioning at box 256, and drives actuators 260 which regulate
the attitude of the bullet's nose.
[0073] An alternative or addition to measuring polarization to
determine roll is to use a roll synchronizer that utilizes a 2-axis
magnetic sensor that determines the orientation of the bullet with
respect to the local magnetic field. The time-varying amplitude of
this signal is measured at a comparator input on the
microcontroller and an internal counter that is phase-aligned to
the rotation rate of the projectile. Up-being zero phase-is
referenced to a particular actuator that is aligned with the
magnetic sensor.
[0074] FIG. 15 also depicts another embodiment of a guidance system
270 of the present invention where an optical sighting system and
radar tracking/communication system 28 is not integrated into the
rifle 20, but is separate.
[0075] Another important circuit in the bullet electronics module
is one that latches the integrated 3F supercapacitor power supply
into the power-on state. This circuit conditions on a voltage
impulse from a piezo sensor when the projectile is fired. After
power-on, the microcontroller manages energy distribution. In the
off state, the supercapacitor is electrically floating and the only
energy dissipation is from internal leakage, which can be less than
5 .mu.A.
[0076] FIG. 16 is an illustration of an implementation of a
four-aperture monopulse structure 300. The use of a passive
monopulse radar with a beacon is a highly favorable topology
considering the extreme range, acute accuracy, harsh environmental
conditions, the expected man-portability and reliability
requirements of the system. Monopulse radar is a high-resolution
method of determining a point-like target's angular position with
only a single RF pulse, and is capable of providing much higher
angular resolution than scanning methods while maintaining a
substantially lower data rate. The basic principle of monopulse
radar systems is that the similarities and differences between the
signals received at distinct antennas are strong functions of the
impinging wave's direction of arrival (DOA). More particularly, the
sectoral DOA of a single point source can be uniquely determined by
the sum of two signals (.SIGMA.-channel) and a difference of those
signals (.DELTA.-channel).
[0077] Phase-sensing monopulse operates on a similar principle. A
phase-sensing monopulse uses several antennas whose radiation
patterns are as closely matched as possible and the phase
difference between the received signals determines the DOA. Since
the antennas are distributed in space, obliquely impinging waves
arrive at each antenna with different time delays, and therefore
different phase delays. The DOA is found by measuring the phase
progression between antenna channels. Phase monopulse is the
preferred embodiment for this effort as it offers greatest
sensitivity for least radar hardware complexity.
[0078] Beacon monopulse is the passive radar implementation of
monopulse in which the target emits a beacon signal that is
detected by the radar. It eliminates the statistical nature of the
radar cross-section from the tracking equation, it mitigates
against multi-path, it reduces the power required by the radar
since the radar is not active, and it inhibits detection since a
strong RF emission is not required to track the bullet. In
beacon-mode, the design variables that given the SNR at the radar
receiver are the beacon power P.sub.t and the compressed-pulse
integration time N.sub..tau.C, where N is the number of coherently
summed pulses and .tau..sub.c is the pulse compression time. Higher
pulse compression ratios or more coherent averages can be used to
increase the SNR of the link, particularly at greater ranges where
maintaining accuracy becomes more difficult. Angular resolution as
a fraction of the antenna beamwidth is inversely proportional to
the square root of the SNR.
[0079] FIG. 17 is a block diagram showing RF and control
electronics 310 in a guided bullet. The RF front-end is comprised
of an antenna 312, a transmit amplifier 336 and a receive amplifier
314, and is a miniaturized T/R module with a 1 GHz PN modulator 332
integrated into the transmit path and receiving PN coded data. The
values of the PN code are written to an encoder buffer by a
microcontroller 320 thereby PN coding the beacon. A VCO 318
provides the LO for a mixer 324 and in transmit mode this same
output provides the IF for a mixer 334. When the T/R module
switches to the receive operation, the RF signals are
down-converted, filtered and delivered to a modem 322. Demodulated
data from the radar tracker will be decoded by the microcontroller
320 and the commands parsed into action within the bullet system.
The microcontroller 320 handles communication data, drives
actuators 326, provides PN data, and controls the power state of
all electronics in the projectile. The entire system can be powered
by a supercapacitor or a battery of suitable size and capacity.
[0080] The foregoing discussion discloses and describes merely
exemplary embodiments of the present invention. One skilled in the
art will readily recognize from such discussion, and from the
accompanying drawings and claims, that various changes,
modifications and variations can be made therein without departing
from the spirit and scope of the invention as defined in the
following claims.
* * * * *