Guidance law to improve the accuracy of tactical missiles

A missile guidance system in a canard controlled airframe with means to mure the angle of attack as well as the pursuit angle by the use of a two degree of freedom roll-free reference gyroscope or other inertial reference device so as to move the canard control surfaces, where the canards direct the missile to the target.

BACKGROUND OF THE INVENTION 
At the present time proportional navigation is used to guide most tactical 
missiles to high speed maneuvering air targets. This technique has also 
been used against much slower surface targets, although in this latter 
case there are two good reasons for using pursuit guidance instead. 
First, implementation of pursuit guidance using a velocity-vane or 
weathercock stablized seeker is very simple: certainly much simpler than 
any known implementation of proportional navigation. Second, in the 
presence of gravity pursuit guidance yields a straighter flight path to 
the target. 
The velocity-vane implementation of pursuit guidance has been available a 
number of years in commercial guidance equipment sold by and marketed 
throughout the world by the Texas Instrument Company. This "Weathercock or 
Velocity-vane Seeker System" has been sold to as many as 50 countries and 
comprises generally speaking a statically stable seeker head on the front 
of a rotary pivoted member called a boom. The weathercock or velocity-vane 
contains a four-quadrant laser detector. By means of a laser beam 
scattered from the target the Texas Instrument weathercock seeker system 
would in effect guide a missile or bomb onto a stationary target on a 
still or windless day. It is well known in the art however that against a 
moving surface target, or in the presence of a crosswind, pursuit guidance 
as implemented by the weathercock system developed by the Texas Instrument 
Co. has fundamental limitations. Specfically, the bomb or missile will 
always miss the designated target under these conditions by distances 
typically as high as forty (40) feet. 
SUMMARY OF THE INVENTION 
The invention involves the use of a two-degree-of-freedom roll-free 
reference gyroscope, or other inertial reference device performing the 
same function, in combination with a velocity-vane seeker, the seeker 
modified from that currently commercially available to measure the angle 
of attack as well as the pursuit angle, the outputs from the inertial 
reference and seeker combined electronically or otherwise to form a 
control input to automatically move the canard control surfaces on a 
missile airframe, where the canards direct the missile to the target. 
Accordingly, one object of the invention is to improve the accuracy of 
tactical missiles employing velocity-vane seekers against surface targets. 
It is another object of the instant invention to provide a modification to 
the classical pursuit law heretofor employed with the velocity-vane or 
weathercock stablized seeker. 
It is a still further object of the invention to implement the use of this 
modified pursuit guidance law by utilizing an inertial reference device, 
e.g. a two-degree-of-freedom roll-free gyroscope. 
It is another important object of this invention to disclose a method of 
utilizing an inertial reference device and velocity-vane seeker to measure 
the missile flight path angle.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 illustrates in cutaway perspective view the Texas Instrument prior 
art pursuit guidance system called a "weathercock or velocity-vane laser 
seeker guidance system." The invention described and claimed in the 
instant patent application is a modification of and an improvement of the 
Texas Instrument weathercock laser seeker guidance system. 
In FIG. 1 the velocity-vane seeker 10 is fastened to the nose of the 
missile 14 by a solid rod member 12 with a forward connection means known 
as a "hooks joint" 13. The hooks joint is a completely rotatable pivot 
means that is well known in mechanical design that allows the 
velocity-vane laser seeker to rotate and move in response to the airflow 
caused by the missiles' velocity. A cooled four quadrant laser detector 
11, well known in the art, or any means capable of responding to energy 
reflected or emanating from a target, is located in the nose of the 
seeker. This device transmits a signal created by the incident energy into 
the nose of the guided missile 14 as has been done for many years by the 
Texas Instruments laser seeker system, sold on the market both in the U.S. 
and to approximately 50 foreign countries. A servomotor responding to 
electrical signals from the four guadrant detector, moves each of the 
canard pairs numbered 16 and 17 in FIG. 1. FIG. 3 indicates in some detail 
how the pursuit guidance system is implemented in two dimensions. 
Orthogonal components of the pursuit error angle epsilon (.xi.) are 
measured in two "control planes"; canard deflections delta (.delta..sub.1) 
and delta (.delta..sub.2) proportional to these measurements then produce 
steady-state yaw angles alpha (.alpha.) and beta (.beta.) relative to 
missile velocity V which cause the missile to accelerate laterally in the 
direction of the target. 
Against stationary targets, and with no appreciable crosswind or flow bias 
effect, the weathercock seeker guidance system thus utilized in accordance 
with the classical pursuit guidance law was very effective. However, 
against targets moving even at 20 to 30 miles per hour, or stationary 
targets in the presence of significant crosswind, pursuit guidance as 
implemented by the Texas Instrument System was ineffective and will always 
result in a miss. For instance, FIG. 2 illustrates the situation where the 
missile 14 has a practical 4g acceleration limit; that is to say the 
missile is a low performance vehicle having a maximum lateral 4g 
acceleration capability. This Figure illustrates that for high velocity 
surface targets, pursuit guidance faced with this practical acceleration 
limit may cause the miss to be as high as 60 ft. 
To remove both the miss caused by the unrealistic acceleration requirements 
of the classical pursuit guidance law and the miss caused by the crosswind 
and flow bias effects intrinsic to the velocity-vane seeker implementation 
was the practical situation the inventor faced. This is a significant 
problem that can be and is effectively solved by the method and aparatus 
disclosed and claimed in this patent application, when implemented in 
accordance with the new modified pursuit guidance law. 
The modified pursuit guidance law of this invention is to be expressed as 
canard deflecton delta (.delta.)=Kp (.xi.+((.mu.) (.gamma.)), where Kp and 
.mu. are suitable constants; .gamma., as shown in FIG. 5 is the flight 
heading angle, and epsilon (.xi.) the pursuit angle. Implementation of 
this law using a modified Paveway pursuit guidance seeker is particularly 
simple. The old pursuit guidance using the velocity-vane seeker is 
employed following target acquisition until the error or pursuit angle is 
about 1.degree.. The missile canards are then nulled and the missile 
weathercocks rapidly into coincidence with its velocity vector V. At this 
time a two-degree-of-freedom roll-free reference gyroscope is uncaged. The 
gyroscope combined with potentiometers mounted in the seeker on the hooks 
joint pivots provides the subsequent flight heading angle gamma (.gamma.) 
of the missile. 
FIG. 4 illustrates in a partially cutaway view the detailed arrangement of 
the gyroscope and the supporting gimbals. The outer gimbal 18 is fixed to 
the frame of the missile and is pivotable around pivot points 19 and 21. 
Inner gimbal 20 is attached to the outer gimbal through the pivot points 
22 and 23. The gyro 24 is free to spin about the axis 25. Because of the 
pivoted mountings the gyro spin axis thus defines a spacially or 
inertially fixed reference vector. That is, the gyro spin axis 25 remains 
as a constant line and it should be considered a constant line of 
reference that ultimately allows the measurment of the angle gamma 
(.gamma.) best shown in FIG. 5 to be described and explained in more 
detail hereafter. 
In FIG. 5 the combination apparatus of this invention is disclosed. This 
also illustrates the prior art device, namely the weathercock seeker 10 
(FIG. 1). Because the gyro spin axis 25 (FIG. 4) remains inertially fixed 
in space, it defines an angle relative to the changing logitudinal axis of 
the missile. The symmetry axis of the velocity-vane seeker always lies 
parallel to the missile velocity vector V, and hence by means of 
potentiometers mounted on the hooks joint pivots, the missile angle of 
attack can be measured. The angle eta (.eta.) measured by the gyroscope is 
combined with the angle of attack alpha (.alpha.) measured by the seeker 
to give the flight path angle shown in FIG. 5 as gamma (.gamma.). It is 
one object of this invention to measure the angle gamma (.gamma.) and to 
transmit the measurement, that may be expressed as a electrical signal to 
a servomechanism that controls the canards. 
In FIG. 5 the target is illustrated with velocity V.sub.T and the angle 
between the velocity vector V and the target is illustrated as epsilon 
(.xi.). The angle epsilon (.xi.) is commonly designated in the art as the 
pursuit, or equivalently, the look angle. 
This angle is used in a special relationship with angle gamma (.gamma.) to 
implement the guidance law. 
In FIG. 6 the angle gamma (.gamma.) is measured by a flight path angle 
generator 26 which may be both a gyroscope mounted to the missile airframe 
and potentiometers mounted on the hooks joint pivots of the weathercock 
stablized seeker, the resulting electrical signal may be fed sequentially 
to an analog or digital computer 27. the computer 27 is designed to 
multiply this signal with a gain mu (.mu.), add the product to the 
electrical signal coming from the pursuit angle generator 28, linearly 
scale this new combination and issue the resulting signal as a control 
input to the servomechanism 29 to deflect the canards 17 and 16 (FIG. 1), 
in accordance with the new guidance law. 
The method of guiding the missile thus requires a measure of the angle 
gamma (.gamma.) defined as the angle subtended by the spin axis of the 
gyro and the velocity vector V which is in turn directly parallel to the 
longitudinal center line of the weathercock device. The angle gamma 
(.gamma.) is therefore measured electrically by electrical apparatus such 
as potentiometers and resolvers or other equivalent apparatus, as is well 
known in the art. This measurement is then translated into an analog or 
digital signal, fed to the analog or digital computer, which in turn 
produces a control signal that is then fed into a servomechanism to allow 
the adjustment of the canards 16 and 17 best shown in FIGS. 1 and 3 to 
make an automatic adjustment to the flight path of the missile so as to 
achieve its final destination. The performance of the new guidance law is 
illustrated in FIG. 5, for a crossing target having a velocity V.sub.T =60 
ft/sec. With gain .mu.=0, the classical pursuit case, the miss is 30 ft. 
This is reduced to zero with a .mu. of 0.5 as shown in FIG. 5. 
Many obvious modifications and embodiments of the specific invention, other 
than those set forth above, will readily come to mind to one skilled in 
the art and having the benefit of the teachings presented in the foregoing 
description and the accompanying drawings of the subject invention and 
hence it is to be understood that such modification are included within 
the scope of the appended claims.