METHOD OF FIRE CONTROL FOR AIRCRAFT
The field of the invention is that of fire control for military aircraft when shooting at a moving target, and especially an air-to-air gun shooting at a target in flight.
In certain configurations of aerial engagement, a military aircraft is apt to use its gun to destroy a target consisting of another aircraft.
To be able to process the target correctly, the pilot must ascertain its distance so as to determine whether the target is within range, its position and its trajectory. Part of this information is generally provided by the radar detection or optronic systems of the aircraft. However, a certain number of aircraft are not furnished with these detection and acquisition means. The pilot must then on the basis solely of the piloting and navigation information and visual information given by his Head Up display acquire the target so as to engage the firing sequence.
United States patent US 4 020 324 describe a device of this type. It essentially comprises:
a weapon system comprising a fixed gun and a supply of
munitions;
an inertial platform making it possible to determine the attitude
of the aircraft in space;
aerodynamic probes making it possible to ascertain the speed
and the incidence of the aircraft relative to the ambient air;
means of aerodynamic command such as control surfaces
making it possible to direct the aircraft in space;
manual means of supervision disposed in the cockpit such as
sticks, levers, joy-sticks, etc.,
a Head Up display conventionally comprising a symbols
generator, a source of images displaying said symbols, a
collimation optic making it possible to form an image at infinity
of said symbols and a superposition optic (in the simplest case,
a semireflecting flat plate) making it possible to superimpose
the image at infinity of the symbology on the outside
landscape. For the application concerned, the symbols concerned are an acquisition reticle racq and a firing reticle rfire also called a piper;
• electronic means of processing and of command making it possible to process the various data emanating from the inertial platform, the aerodynamic probes and the manual means of supervision, and to generate the information necessary for the symbols generator, for the supervision means and for the weapon system.
The fire control described in this patent comprises the following
steps:
a. visual acquisition of the target C by the pilot of the aircraft A in
the visual field of the Head Up display;
b. determination of the distance of the target by superposing on
the target viewed the acquisition reticle racq of circular shape
of a diameter about equal to the apparent maximum dimension
of the target, so as to determine the distance separating the
aircraft from the target;
c. determination of the flight time of the munitions and of the
position of the points of impacts PI of the munitions by the
electronic processing means;
d. calculation of the position of the firing reticle rfire by the
processing means, firstly in the frame of the aircraft and then in
the frame of the Head Up display;
e. superposition of the firing reticle rfire on the target by the pilot
by acting on the means of command during an acquisition time;
f. finally, engaging of the firing sequence by the pilot.
The device indeed describes the method making it possible to determine the distance separating the aircraft from the target and consequently to know whether it is within firing range, on the other hand it does not specify how the calculation of the position of the firing reticle takes account of the speed of the target and more generally the conditions necessary for a successful shot at target.
The invention proposes a novel method of acquisition of the target and a novel method of calculation of the position of the firing reticle making it possible to implement the gun with a high probability of destruction of the target without using complex means of simulation. It can therefore be implemented on relatively unsophisticated combat aircraft.
More precisely, the subject of the invention is a method of fire control for military aircraft for firing at a moving target, said aircraft possessing at least:
a gun secured to the fuselage of the aircraft capable of
delivering munitions;
navigation sensors making it possible to deliver at least the
information regarding the speed of the aircraft relative to the
ground and the attitudes of the aircraft in a frame parallel to the
geographical reference frame;
sensors necessary for piloting and making it possible to deliver
the aerodynamic information such as the aircraft speed, and
temperature and pressure data;
means of command of the aircraft making it possible to control
its position in space, such as control surface commands;
collimated means of display of Head Up display type
comprising means of generating a firing symbology comprising
at least a first circle-shaped collimated reticle and a second
collimated firing reticle also called a "piper",
means of supervision by the pilot of the firing symbology
displayed that may be a command post or the control stick;
a database comprising information of ballistic type on the
munitions used;
means of processing the data emanating from the means of
piloting and of navigation, from the means of supervision of the
symbology and from the database;
said method comprising the following steps:
a. visual acquisition of the target by the pilot of the aircraft in the visual field of the Head Up display;
b. determination of the distance of the target by superimposing on
the target viewed an acquisition reticle of circular shape of a
diameter about equal to the maximum dimension of the target, so
as to determine the distance separating the aircraft from the
target, said diameter being adapted by the pilot in real time to the
dimensions of the target viewed;
c. determination of the flight time of the munitions and of the vector
representing the position of the points of impacts of the munitions
by means of the acquisition reticle and of the processing means;
d. calculation of the position of the firing reticle by the means of
processing, firstly in the frame of the aircraft and then in the frame
of the Head Up display;
e. superposition of the firing reticle on the target by the pilot by
acting on the means of command during an acquisition time;
f. engaging of the firing sequence by the pilot;
characterized in that, during the acquisition time, the position of the target is kept constant by the pilot acting on the means of command, that the acquisition time lasts at least one second and that the vector position of the firing reticle in the frame of the aircraft is equal to the product of the inverse of a transfer matrix times the vector representing the position of the points of impacts, said transfer matrix being equal to the sum of the identity matrix and of the product of the flight time of the munitions times a matrix representing the instantaneous rotation of the aircraft.
Advantageously, the incidence of the aircraft is calculated on the basis of the speed and attitude information, without the use of incidence probes on the aircraft.
The invention will be better understood and other advantages will become apparent on reading the description which follows given with no limitation implied and by virtue of the appended figures among which:
figure 1 represents the respective positions of the aircraft and of
the target;
figure 2 represents the changes of frame between the terrestrial,
aircraft and Head Up display frames.
• Figures 3a, 3b and 3c represent the images viewed by the pilot through the superposition optic of his Head Up display at the commencement of the target acquisition phase and before the firing engagement phase.
Figure 1 represents the respective positions of the aircraft A and of the target C. They are separated by a distance D at the instant t. With respect to a fixed three-dimensional terrestrial frame rt with center O, the aircraft is imbued with a speed represented in figure 1 by the vector VCRAFT and the target has a speed VTARGET in this same frame. The point PI
represents the forecast point of impact of the munitions that can be fired with the target C.
Figure 2 represents the three frames necessary for the representation of the various data relating to the fire control. These are:
the fixed three-dimensional terrestrial frame rt with center O;
the three-dimensional frame rcraft tied to the aircraft with
center og;
the three-dimensional frame rshud tied to the Head Up display
SHUD with center Os. The acronym SHUD stands for the
expression Smart Head-Up Display.
The changes of frame are made by means of switching matrices pxx as indicated in figure 2. Thus, the switching matrix making it possible to switch from the terrestrial frame rt to the aircraft frame rcraft is called pag, the switching matrix making it possible to switch from the aircraft frame rcraft to the display frame rshud is called psa and the switching matrix making it possible to switch from the terrestrial frame RT to the display frame rshud is called psg- Also, the inverse matrices of those above are denoted pga, pas and pcs- Of course, the matrices pag and psg vary over time as the aircraft moves around whereas pas is a constant matrix insofar as the display is secured to the aircraft.
Also, the position of the target in space is denoted mtarget- When the firing reticle is positioned on the target, these two positions are then merged within the field of the Head Up display. Likewise, the time necessary for the munition to reach the target is denoted tflight-
Regardless of the frame chosen, we can write:
Equation 0 OGMTARGET (t + TFLIGHT) = OGMTAKGET (t) + TFLIGHT.(VTARGEr - VCRAFT)
Equation 0 assumes that the speeds remain constant during the flight time tflight- This approximation is justified insofar as the flight time is very short, the munitions being fired within range distance, that is to say within a few hundred meters and their average speed being very high, of the order of several hundred meters-second.
The vector KFfKE representing the position of the center of the
firing reticle satisfies, when it is superimposed with the target:
UU IX)
Equation 1 RFIRE = OGMTAKGET (t)
The possible symbologies of the firing reticle are varied. They all have the characteristic of possessing a center which is easily identifiable by the pilot.
And of course, if the munition reaches the target, this being the
goal sought, then the vector representing the position of the points of impact
that is denoted PI satisfies:
Equation 2 OGMTARGET (t + TFLIGHT) = pf i.e.:
?1 =O itf (t} + T (ft -$ }
I 1 UG1VL TAKGET V) ^ *- FLIGHT -\v TARGET v CRAFT >
PI is obtained by a ballistic calculation. The main parameters necessary for this calculation are the coordinates of the munition on ejection, its speed, its altitude relative to the ground, the air speed, the attitude of the aircraft, the density of the air, the static pressure which depends on the attitude of the aircraft, the temperature of the air.
As was stated, during the acquisition time preceding firing, the pilot maintains the position of the target in his display constant. We then write: Equations (OSMTAKGET}SHUD = Cst. The subscripted term of a vector
indicates in this and the following equations the frame of reference in which the vector lies. Differentiating with respect to time:
Equation 4 — (osMTARGET )SHUD = O
In the terrestrial frame, this equation 4 may be written with the notation defined earlier:
Equation 5 — PASPGA (osMTARGET )GEO = O
We know that pas is a constant matrix insofar as the display is secured to the aircraft. Consequently, we have:
Equation 6 — PCA(osMTAKGET )GFO = O, i.e. ot
Equation 7 I —pga \(osMTARGET )GEO + PGA .—(osMTAKGET )GEO = O
Now, we can write:
Equation 8 OxMTAKGE.r = OSOG + OCO + OMTARGET, which yields:
d P d P 3 P 5 P 3 P P P
Equation 9 -OsMTARGEr =—OsOG +-OGO+~OMTAKGET = -OSOG+VTAKGET-VCRAFT
a a a a a
In equation 7, we change frame by applying the switching matrix p'qa, the transpose of the matrix pga, and we then obtain equation 10:

) +p. jlpi 1(0(5) +6? . -i? ) =o
~. - ua r- ua \~ H' TARGET /c KAH o/i-[ ~,
/CRAFT GA' -3 G,4 \ S G ICRAFT \ TARGtT CRAFT /CRAFT
that can also be written in the form of equation 11:
d „, "!„ / S> \ .pt(: angle of roll
T: angle of sideslip,
these three angles being provided by the inertial platform;
if we also put:
TAS standing for True Air Speed: true speed of the aircraft relative
to the ground,
Vz vertical component of TAS, if we define three parameters a, b
and c such that:
a = -sin(6); b = cos(0).cos(cp) and c = -Vz/TAS, then
the incidence I is:
/ = arc sin
- arc tan 2

CLAIMS
1. A method of fire control for military aircraft (A) for firing at a moving target (C), said aircraft possessing at least:
a gun secured to the fuselage of the aircraft capable of
delivering munitions;
navigation sensors making it possible to deliver at least the
information regarding the speed of the aircraft relative to the
ground and the attitudes of the aircraft in a frame parallel to the
geographical reference frame;
sensors necessary for piloting and making it possible to deliver
at least the aerodynamic information;
means of command of the aircraft making it possible to
oversee its position;
collimated means of display (SHUD) of Head Up display type
comprising means of generating a first circle-shaped collimated
reticle (racq) and a second collimated firing reticle (rfire) also
called a "piper",
means of supervision by the pilot of the firing symbology
displayed that may be a command post or the control stick;
a database comprising information of ballistic type on the
munitions used;
means of processing the data emanating from the means of
piloting and of navigation, from the means of supervision of the
symbology and from the database;
said method comprising the following steps:
a, visual acquisition of the target by the pilot of the aircraft in the
visual field of the Head Up display;
b. determination of the distance of the target by superposing on
the target viewed the acquisition reticle (racq) of circular shape of
a diameter about equal to the maximum dimension of the target,
so as to determine the distance separating the aircraft from the
target, said diameter being adapted by the pilot in real time to the
dimensions of the target viewed;
c. determination of the flight time of the munitions and of the vector
representing the position of the points of impacts of the munitions
by means of the acquisition reticle (racq) and of the processing
means;
d. calculation of the position of the firing reticle (rfire) by the
processing means, firstly in the frame of the aircraft and then in
the frame of the Head Up display;
e. superposition of the firing reticle (rfire) on the target by the
pilot by acting on the means of command during an acquisition
time;
f. engaging of the firing sequence by the pilot;
characterized in that, during the acquisition time, the position of the target is kept constant by the pilot acting on the means of command, that the acquisition time lasts at least one second and that the vector position of the firing reticle in the frame of the aircraft is equal to the product of the inverse of a transfer matrix times the vector representing the position of the points of impacts, said transfer matrix being equal to the sum of the identity matrix and of the product of the flight time of the munitions times a matrix representing the instantaneous rotation of the aircraft.
2, The method of fire controt as claimed in claim 1, characterized in that the incidence of the aircraft is calculated on the basis of the speed and attitude information, without the use of incidence probes on the aircraft.