Note: Descriptions are shown in the official language in which they were submitted.
~ 33~3~
Indicator apparatus for determining the miss distance of
a projectile in relation to a fixed or moving target.
The present invention relates to an indicator appar-
atus for determining the miss distance of a projectile to
a fixed or moving target, comprising pressure-sensitive
transducers intended to sense the pressure wave generated
by a projectile at at least four points, and means for
generating electrical signals there~rom, adapted for ap-
plying to computer circuits which, ~rom the pressure wave
characteristic, give the distance from one of the pressure
transducers to the source of the pressure wave, i.e. the
so-called "bang generation point".
When a projectile passes through the atmosphere at
- supersonic speed, a backward pressure wave is generatçd
from the tip of the projectile. The wave has the form of
a conical surface with a vertex angle depending on the speed
o~ the projectile in relation to the speed of sound.
In miss distance calculators known up to now, the dis-
tance between the bang generation point and the point con-
tacted by the pressure wave is calculated. ~his distance is
~- however not equal to the miss distance, i.e. the shortest
distance between projectile and target, since the target
;~ has moved during the time the pressure wave, i.e. the coni-
cal surface/ has been propagated from the bang ~eneration
point to the target or a sensing point equivalent thereto.
A known miss distance calculator is taught by the
25 British Patent 902 756, and it comprises two pairs of micro-
phones placed in such a way that the approximate distance
to the bang generation point can be determined with the aid
:;
'~
,
,
. :
~33~
of one pair, and the space quadrant the target in which
the pressure wave is generated by the other pair. With
this system it is thus only possible to determine an ap-
proximate distance to the bang generation point and to
give a source area for the pressure wave. Since the point
source of ~he pressure wave is not exactly known, the move-
ment of the target during the time the pressure wave is
propagated from the bang generation point to one of the
microphones cannot be taken into consideration. Further-
more, not knowing where the projectile passes through theappropriate space quadrant is extremely unsatisfactory from
the point of view of miss distance calculation.
Both the United States patents 3 217 290 and 2 925 582
relate to miss distance calculators, the latter of which is
15 based on essentially the same principle of determining the -
position of the bang generation point as the device accord-
ing to the British patent, while the former utilizes the
pressure wave attenuation from the bang generation point to
the receiver for calculating the distance, since attenuation
is different for different frequencies in the pressure wave
frequency spectrum. However, the latter patent discloses a
very accurate method of calculating the distance to the bang
generation point, and this method applied in the apparatus
~` in accordance with the present invention.
The object of the present invention is to enable cal-
culation of the distance between the projectile and target,
even when the latter has very high speed, simultaneously as
information is supplied as to the position of the projectile
and target at every instant, and thus the position of the
projectile in relation to the target when the projectile is
at its shortest distance to the target.
; The indicator apparatus in accordance with the inven-
tion is intended for use in all conceivable cases of firing
at targets, i.e. firing from fixed ground weapons towards
moving arial targets, from airborn movable weapons to mov-
ing targets, from airborn movable weapons to fixed ground
targets and finally from fixed surface weapons to fixed
surface targets.
,
3~33ia3~
In accordance with the invention these objects are re-
alized by that at leas-t four pressure transducers are part
of a first pressure transducer system, and are arranged in
the corners of a polyhedron with at least four corners and
computer circuits which comprise means for measuring the
time differences between the passage of the pressure wave
past the transducers in the first transducer system and
where the computer circuits which are adapted to give the
position of the bang generation point and thereby a first
point on the projectile trajectory from these time differ-
ences as well as the measured distance to the bang gener-
ation point. The indicator apparatus also comprises a se-
cond transducer system adapted either to sense the height
of the first transducer system above a horizontal plane, or
the passage of the pressure wave at a further instant, said
circuits being adapted for giving the direction of the tra-
jectory, which together with the first point gives the actual
trajectory. While taking in account the positions of the tar-
get and projectile, the computer circuits are adapted to give
the distance between the projectile and the target at every
instant. In cases where firing takes place against a fixed
target, the second transducer system will sense the passage
` of the pressure wave at a further instant, the computer cir-
cuits being adapted to give a second point on the trajectory
which, together with the first point, gives the actual tra-
jectory.
In order to execute an accurate calculation of the miss
distance, it is required that the distance from a pressure
transducer to the bang generation point can be calculated
exactly. According to a known method, this distance can be
calculated by determining when the maximum amplitude of the
~ pressure wave passes the transducer. This method is uncer-
-~ tain however, inter alia because of heavy background noise
- being superimposed on the pressure wave, making it difficult
to determine with analogue measurements when the maximum
pressure wave amplitude passes the receiver. In accordance
with the invention the method described in the US Patent
2 925 582 is utilized, according to which any projectile
moving at supersonic speed generates a ballistic pressure
wave in which the air pressure increases rapidly from a sta-
-tic pressure P0 to an excess pressure P0 + Pl, to attenuate
linearly in both time and space to a subpressure P0 - P2,
subsequently to return just as quickly to the static press-
ure P0. The amplitude on the leading edge and trailing edge
of the wave, and the time distance between these edges de-
pends on the projectile characteristics, such as caliber
and speed, and on the perpendicular distance from the bang
generation point to the point where the pressure wave is
sensed. The apparatus in accordance with the present inven-
tion also utilizes the distance in time between the leading
edge and trailing edge of the wave to determine the distance
to the bang generation point. The content of the abovemen-
tioned patent thus constitutes documentation of the known
technique utilized in the apparatus according to the inven-
tion for measuring the distance to the bang generation point.
Some embodiments, selected as examples, of apparatus
in accordance with the invention are described in detail
below while referring to the accompanying drawings in which
igure 1 shows firing from the ground to a target towed by
an aeroplane, comprising an indicator apparatus with four
; pressure transducers according to the invention,
Figure 2 shows an indicator apparatus according to Figure 1,
and the passage of the pressure wave past the transducers,
Figure 3 shows a vector diagram of the quantities measured
by the transducers in Figure 2,
Figure 4 shows both conceivable projec-tile trajactories ob-
tained in one embodiment of the indicator apparatus in ac-
cordance with the invention,
Figure 5 shows the vector diagram and the quantities
measured in the embodiment according to Figure 4,
Figure 6 shows an alternative embodiment of the indicator
apparatus with a further pressure transducer at some dis-
tance from the four transducers,
Figure 7 shows the vector diagram for measuring the projec-
tile trajectory in the embodiment according to Figure 6,
'' " , '
Figure 8 shows a vector diagram for an alternative measure
ment of the projectile trajectory,
Figure 9 shows a vector diagram for firing from the air
towards a fixed ground target, utilizing two pressure-
sensing transducer arrays in accordance with the invention,Figure 10 shows a circuit diagram for a microprocessor in
a transducer array in accordance with the invention,
Figure 11 shows a block diagram of an indicator apparatus
according to Figure 6 with the circuit elemen-ts associated
with the pressure transducers.
Figure 1 illustrates very clearly the conditions pre-
vailing when firing towards targets towed by aeroplanes,
moving at such high speed that the speed is not negligible
in relation to the speed of sound and that of the projectile.
When a projectile is fired by a weapon towards a sleeve tar-
get 2, the weapon is aimed to hit a point on the target, in
this case the centre 6 of the target 2. This point of aim
or sight has been calculated with reference to the distance
and speed of the target in relation to the firing point. At
the firing instant, the target is in a position not shown
on the Figure. In the position shown in full lines on the
~ Figure, the projectile 3 is nearest to the target 2, and
r~ at a distance from it denoted by dM, which is the miss dis-
tance. In front of the target 2 t:here is a pressure wave -
sensing transducer means 5, at a specific distance from the
~- centre 6 of the target 2. When the projectile is at the
point PMp at the least distance from the target it is at
the distance dMK from the transducers.
When a projectile 3 passes through the atmosphere at
supersonic speed, a pressure wave 4 is generated, indicated
on the figure by lines going backwards from the tip of the
projectile. This pressure wave has the form of a cone with
the projectile at its vertex, and moves at- the speed of
sound, the wave front forming a conical surface where the
cone angle is decided by the speed of the projectile in re-
lation to the speed of sound. At every point on its trajec-
tory the projectile will thus generate a pressure wave mov-
ing at the speed of sound in a direction at right angles to
~L33~3~
the wave front. In the case shown in Figure 1, the target
2 is in a position outside the paper when the projectile
was at position PB, from which a wave front 4 has been
propagated in a direction parallel to the line dB. When
the projectile first arrives at a pOSitiOIl 3 on the fi-
gure the wave front 4 has arrived at the pressure trans-
ducers 5 in front of the target, which is then in posi-
tion 2 7 The point PB is thus the point at which -the wave
generated mee-ts the transducers in position 5 .
If, as has been the practice up to now, it is con-
sidered satisfactory to calculate the distance dB between
the bang generation point PB and the transducers in posi-
tion 5 , it is quite clear that this distance is in no way
a measurement of the miss distance dM, i.e. the shortest
distance between projectile and target, since the target
has naturally moved from position 2 to position 2- during
the time the conical surface has moved from position 4 to
position 4 . The problems the present invention intends to
solve are thus to enable calculation of the miss distance
dM and also position PMp of the projectile 3 when it is
nearest to the target 2.
In order to calculate the miss distance dM -the follow-
ing quantities must be taken into account:
1. The distance dB between the hang generation point PB
and the posi-tion 5', of the pressure transducer array (here-
-~ inafter termed "array") during the passage of the pressure- wave 4 .
2. The pressure wave propagation direction from the bang
generation point to the array, i.e. the vertex angle of the
conical pressure wave.
3. The direction of the trajectory in relation to the path
of the target.
4. The projectile speed.
5. The target speed.
6. The prevailing speed of sound.
7. The distance from the array to the point on which
sights are set.
'
, . ~
.
7 ~
The distance dB between the bang generation point PB
and the transducer array when the pressure wave 4 passes
it can be calculated, starting from the time between the
front and trailing edges of the pressure wave, in accord-
S ance with US Patent 2 925 582. The direction from the array
to the target, and the direction of the trajectory in re-
lation to a conceived coordinate system with its central
point in the array can be calculated starting from the di-
rection of the pressure wave in relation to the array. This
distance can in turn be calculated starting from the time
interval for the conical surface to pass the transducers in
the array.
Figure 2 shows in detail how the conical pressure wave
4 meets two transducers M2 and Ml, in that order, the por-
tion of the conical surface meeting all the transducers in
~; the array having been drawn with perspective curves in the
Figure. The transducers Ml-M4 are arranged at the corners
of a tetrahedron having congruent sides and moving along
the x axis at a speed of VM, the transducers Ml, M2 and
M3 being in the x-y plane. In the array there is a coor-
dinate system with its centre at the point where the per-
- pendicular from the vertex M4 meets the side surface Ml-
' M2-M3- '
The coordinate system also moves at a speed of VM
along the x axis. In Figure 2 there is also plotted the
point PMp, at which the projectile 3 is when the distance
between the target 2 and projectile is at a minimum and
which is thus the miss distance dM.
According to the invention, the direction to the bang
generation point PB can be calculated with knowledge of
the times for the passage of the conical surface past each
of the transducers Ml M4. The curvilinear surface must
thereby be approxi~ated to a flat surface as illustrated
in Figure 3. In this figure, the transducer M2 is met by
the conical surface first, and by the generatrix A-A which
is thus common for the curvilinear conical surface and the
approximated flat surface, as shown in Figure 3. The vec-
tors extendirlg from the coordinate system origin to the
;' , ' ' `
', ~ .
~ 3~
corner points on the tetrahedron, at which the transducers
are arranged are also shown on the figure. According -to
the premises, the transducers Ml, M2 and M3 are in the x-y
plane and the tetrahedron moves at the speed VM along the
x axis. The conical surface approximated to a flat surface
moves at a speed of VK in the standardized vector direction
-nk which is counter to the direction to the bang genera-
tion point PB.
Since the array and the wave plane move at a certain
speed in relation to each other, the total speed between
the plane and array will contain a speed vector in a di-
rection nk perpendicular to the plane. ~hen the array moves,
the perpendicular dlrection of the plane is not affected,
while the speed of the plane in relation to the array, due
15 to the movement of the latter, is given by the expression: ~:
-VM Q . nk . The speed of the plane relative to the array
will thus be: -VK . nk.
For the case where the transducer M2 is in the plane,
as shown in Figure 2, general expressions can be set up
for the perpendicular distance from the plane to the re-
maining transducers Ml, M3 and M4, these distance being
: positive as seen in the direction of movement nk of the
plane. The following is applicable to these distances.:
-
~i = (r2 - ri) nk, where i = 1, 3, 4;
If it is assumed that the pressure wave plane meets
the transducer M2 at the time t = 0, the time taken for
the plane to meet the other transducers wil]. be:
~ i nk
ti = V~ = (r2 ~ ri) (V )~ where i = 1, 3, 4;
The following equation system can now be set up:
(r2 ~ rl) (V ) = tl (t2 = 0)
(r2 ~ r3) ~VK~ t3
nk
(r2 ~ r4) ~VK~ t4
9 113313b~
If it is assumed that the side of the tetrahedron is
two length units, the vectors (r2 - ri) can be written as
follows:
r2 ~ r~ r, 1, O)
r2 ~ r3 = (0, 2, 0)
~ r2 ~ r4 = (- 3 ~ ~ 3
! If these expressions are introduced into the above
equation system the following equation system is obtained
:~ in matrix form: :
~' 10 ~- ~ 1 nxl ~tl
:~ I 0 2 0 n'~ = ~ t3
~nzJ lt4
which becomes~ n' = (nx, ny, nz) tl VK 2 ;~.
side) nK = constant nK
, 15 VK and the size of the tetrahedron are thus insignificant. .;~
ny = 12 t3
i::
. nx 2~3 t3 7~- tl
3 ~ nz ~ 6 t3 ~ 3 tl t 2 t3 - t4
nz = ~ t3 ~ t4 ~ t
.~ 20 or:
x = ~ t3 - 2V~ t
ny = V~ t3
nz = t3 - 3 t4 ~ tl
If the array is rolled an angle ~ the following ex-
pressions are obtained with respect to the flight direc-
tion:
n = n'
ny = nycos ~ - nzsin ,~
',
:: ..
- : ' ' ~ - . :: .:... : ~ ,
3~3~
n = (nx~ ny, nz)
nk - :
Inl
From the above equation, it is apparent that the di-
rection to the bang generation point PB can unambiguously
be expressed by the vector nkO
The distance dB is calculated starting from the inter~
val in time T between the leading and trailing edges of the
; pressure wave. The premise here is that the projectile has
supersonic speed, i.e. it has a Mach number M > 1. If the
wavelength of the pressure wave is ~, then ~ ~ k fl(M)
. rnl providing that r>~L, where r is defined in Figure 2
and L is the length of the projectile. The wave length ~
- can be measured by measuring the time T between the flanks
on the leading and trailing edges of the pressure wave.
Since the distance dB =cos~' the following expression on
dB is obtained:
, . ~
dB = klf2 (M~ T
If supersonic streaming is generally prevalent in the vi-
cinity of the projectile then we have:
f2 (M) = 3 and n = 4
M
For small values of dB and Mach number close to 1, special
calibration must be done for the projectile, e.g. for M <
1,3 and dB ~ 20-39 L.
After calculating the position of the bang generation
point in relation to the coordinate system in the array,
the direction of the projectile trajectory can be calcu-
lated in relation to the direction in which the tar~et is
towed, i.e. along the x axis. Different methods can be ap-
plied here, and Figure 4 illustrated one method of calcu-
lating the direction of the trajectory starting from thefiring point. Apart from the transducers Ml-M4 in the array,
there is a fifth pressure transducer M5 arranged at the
firing point, according to this embodiment. After calcu-
`: . ., :
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lating the position of the bang generation point P3, thetime tB it takes for the pressure wave to go from the bang
generation point PB to the transducer (in this case M2)
first met by the conical surface can then be calculated.
Knowing the distance dB the time tB is obtained from:
, dB
tB = r where c is the speed of sound.
c
Furthermore, the time taken from firing the projec-
tile until the pressure wave hits the transducer M2 is
measured. This time is denoted Q t5.
The distance from the firing point to the transducer
M5 and to the bang generation point PB will then be:
db
dk = Vp (~t5 c )
where Vp is the projectile speed.
Half the conical angle of the pressure wave, i.e.
the Mach angle will thus be
= arc sin M
where Mp is the Mach number for the projectile.
The conceivable trajectories meeting the conditions
above form the surface area of a cone, with its v~ertex at
the bang generation point and a conical angle of 2 - ~.
Furthermore, the axial direction of the cone is paral-
lel to the direction for the unit vector nk, and its base
circle is determined by the distance dk from PB. If it is
assumed that the weapon is mounted on a horizontal ground
plane, then this plane intersects the base circle o~ the
cone at two points coinciding on the figure with the trans-
ducer points M5 and M5-. These two points thus give two con-
ceivable trajectories, constituting both cone generatrices
starting from the intersection points with the ground plane.
These points are also symmetrical in relation to a vertical
plane through the cone axis.
The unit vector nb is directed along the path from PB
towards the weapon.
. .
~331~
n n = cos ( - - ~) = sin~
nk nb nkY by kz bz
bl = 1 = > nb = ~ (n b + n b )
The distance from PB to the x-y plane is positive if PB
is under (nk ~ 0)
'~ z
a = -d n
If nb is determined so that dk nb is the vector from the
bank generation point to the firing point, then the z-com
ponent of this vector is equal to -(h-a), where h is the
flying altitude, i.e. dk nb = -(h-a).
h-a
nb = ~
nk nb + nk nb = sin~ + nk (h-a) =~ nb ~ nb
x x y y z k x y
nb2 + nb2~ (h a~ 2
This equation generally gives two real solutions, cor-
responding to the symmetrically situated firing points M5
and M5 , except for the case where the plan of the cone
base is parallel to the hori~ontal plane, which gives an
infinite number of solutions. ~owever, if corresponding 60-
lutions are made for two consequent firings, one of the two
conceivable solutions will be eliminated, and in the present
case this means that the firing point M5 can be eliminated.
Starting from the calculations above, the miss distance
; dB kan now be calculated (the miss distance being the least
; distance between the projectile and the target) using the
vectors shown in Figure 5.
The movement of the array along the x axis at the op-
tional time t can be given as:
rM = VM t x
At an optional time t the position of the projectile
in the coordinate system with its origin in the array will
~ -- 13 ~33~3~
thus be:
r (t) = dB(nk ~ Mpnb) (vMQ p b
The vector between the projectile and the array at
time t = 0, i.e. when the conical surface meets the trans-
ducer M2, will then be:rp(0) = dB(nk ~ Mp nb)
The vector from the array to the aiming point S at
which sights are set is rs and it is always on the x axis.
The vector t between the point S and the trajectory for the
time t = 0 will be:
t p(0) rS dB(nk Mpnb) rS
The scalar product between the vector t and the vector
g = VMQ + Vpnb then gives the angle ~ according as the fol-
lowing:
. _
1~ ~ = arccos t . ~
It~.~g!
The vector dM from the point S to the miss point PMp
will then be:
dM = ! t ! . sin~
dM = t - Itlcos~ . q
In accordance with an alternative embodiment of the in-
vention, the direc~ion of the trajectory can be calculated
using a further transducer M5, spaced from the array of four
pressure transducers Ml-M4 arranged at the corner points of
a tetrahedron. The mathematical model on which this calcu-
lation is based is apparent from Figure 6. In this figure a
coordinate system fixed in relation to the ground is used,
in which the array Ml-M4 is at origin at the time t = 0,
i.e. when the conical surface of the pressure wave meets
the transducer M2. The position of the bang generation point
P~ and the conical surface giving rise to this point is de-
termined as before. The extra transducer M5 is on the x axis
and at a disstance 15 in front of the array Ml-M4. The extra
transducer M5 and the array Ml~M4 are moved along the x axis
from the origin after the time t = 0, and at the time t = t5
the conical surface reaches the transducer M5 from a second
:
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14
~3~L38
~
bang generation point P5 on the trajectory. The distance
d5 can then be calculated, but not the position of the
bang generation point P5, since the direc-tion of this
point is not known. However, it is known that the point
must lie on a sphere having its centre at the point M5
and radius d5, and since the projectile speed and the
time t = t5 are known, the distance P5 PB between both
bang generation points can be calculated. It is also
known that the second bang generation point P5 must lie
on the base circle of the cone, the generatrix of which
corresponds to the distance between both bang generation
points.
Both geometrical figures thus obtained, i.e. -the sphere
with its centre in M5 and radius d5, and the base circle to
a cone with its vertex at PB and cone angle = ~2 ~ ~, and the
generatrix with the length from P5 to PB will intersect each
other at two points, P5 and P5. Both these trajectories are
symmetrical with relation to a plane through the cone axis
and x axis. By making a corresponding calculation for a se-
cond projectile trajectory one of both lntersection poin~s
can be eliminated, and it is thus possible to determine the
trajectory direction, calculation of the miss distance then
taking place in the same way as that in conjunction with Fig-
gures 4 and 5.
The projectile speed at the time t = 0 is obtained by
placing a pressure transducer M6 at the firing point, the
time dif~erence between the firing instant and the time t = 0
being thus obtained, and in turn used to obtain the projec-
tile speed from firing tables. The quantities used in the
mathematical treatment below are apparent from Figures 6
and 7, in which the following denotations are:
Mp - projectile Mach number
c = speed of sound
c . Mp = Vp= pro~ectile speed
MM = Mach number for the target, i.e. VM = c . MM
distance P5 - P = ds Mp
distance B B P
distance P - P - c . Mp . t5
distance M5 - M5 c . MM 5
a
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The mathematical solution is as follows:
Projectile position at t = 0:
rp = dBnk ~ (dBMp + Vp-t)nb
nb = trajectory direction, positive from PB towards the
weapon
. Cone along the trajectory: nb . nk = sinu; Inbl = 1
Position of transducer array:
A
rM ~- V t x
The transducer M5 is assumed to lie on the x axis at a dis-
tance 15 in front of the array. The position of M5 is then:
r5 = (Q5 + VM . t) x
Bang generation point for M5 = P5
Registration time in M5 = t5
Distance M5 - P5 = d5
The position of M5 for the time t = t5, i.e. when the press-
ure wave passes, is:
( 5 Vp ~ t5) x (= a5x)
The projectile position at t = t5 is:
p( 5) dBnk ~ (dBMp + Vpt5) nb
20 The distance along the trajectory from P5 to rp (t5) = ~;
d5 . Mp
The position of P5 will thus be:
P5 = rp(t5) + d5Mpnb (note that nb is positive backwards in
the trajectory)
The value of d5 is obtained from the following equation:
~ d = ~p5 - r5(t5)l or
: d25 = (p5 - rs(ts)) ~ (P5 r5(t5))
which gives:
5) dBnk (dBMp + Vpt5)nb+ d5M nb
-(15 + VMts) Q ~ dBnk b5nb 5
15 + VMt5 ; b5 = dgMp + Vpt5 - d5M
; d 2 = (dBnk - b5nb ~ a5Q) (dBnk b5nb 5 ¦ k b
sin~
:. . :, :
~33~8
dB dBb5sin~ a5dBnk - dgbsSin~ ~~ b5 + a5b5nb
- dBa5nk + asb5nb + a5
n = d~ - (d 2 + a52 + bs - 2a5dBdk ~ 2dBb5s ~)
,
nb ~ nb from !nbI = 1
; Y Z _ _ ~two solutions for nb
k b J
dM and dM are subse~uently calculated in the same manner as
before.
The trajectory component nb can be alternatively cal-
culated according to the following method, and with reference
to Figure 8.
The position of the array at the time t = t5 is:
Mt5 Q
The position of M5 at the time t = t5 is:
r5(t5) = asx = (Q5 + VMt5)
For t = t5, the vector h(t) from the projectile to M'5 is a
generatrix of the Mach cone:
_
h(t5) . nb = Ih(t5)I cos~
where
h(t5) = rS(t5) - rp(t5)
The position of the projectile is:
r (t5) = dBnk ~ (dsMp + Vpt5) b
=>quadratic equation for the x component of nb (nb )
c2nb - 2clnb + CO
2 2
c2 = Mp as
, cl = - a5V t5
cO = (Mp - 1)(2dsct5 + 2a5dBnk a5 ) p 5
.:: . .. , .: . .
: ' `". ', ,' ;~ , '
- . . .
': ,., '" , ",
. ~
17 ~ 31~
The advantage with this method is that the calculation of
the distance d5 is not entirely necessary, although the
method has the disadvantage that it gives two solutions for
nb for -the above equation. Information on the distance d5
shXould therefor be used to calculate nb according to the
previously described method. This value'of nb is then used
for selecting the righ-t nb for -the two solut~ons in the
alternative methodO
As will be apparent from the above mathematical treat-
ment of the problem of determining the direction from thearray to the bang generation point, it is absolutely essen-
tial to exactly determine the times at which the conical sur-
face of the pressure wave arrives at -the different pressure
transducers.
In the case where the indicator apparatus in accordance
with the invention is to be used for firing from airborn mov-
able weapons towards a fixed ground target TA, the premises
are applicable which are apparent from Figure 9.
The distances dBl and dB2 are calculated in the same way
2G as previously for a certain assumed Mach number, e.y. M = 1,
5-2. The distance b between the bang generation poin-ts is sub-
; sequently calculated with knowledge of the quantities dBl,
dB2,/nkl~ and 12-
The time ~ t for the passage of the pressure wave past
both arrays K2 and Kl is meas~lred, the Mach number Mp for the
projectile being subsequently calculated from the formula:
b
Mp = ~ t - c - dBl ~ dB2
where c = the speed of sound.
The values of the distances dBl and dB2 are corrected
with the calculated Mach number, whereafter the trajectory
through the bang generation points PB1 and PB2 can be cal-
culated.
As wi11 be seen from the circuit diagram according to
E'igure 10, the four microphones M1, M2, M3 and M4 are con-
nected to a microprocessor 10, adapted for calculating thetime interval TF between the leading and trailing edges of
~:~33~38
the pressure wave as it passes one of the microphones, and
the time intervals for the passage of the wave past each of
the subsequent three microphones~ Each of the microphones
is connected to an amplifier 2 and a Schmitt trigger circuit
3, generating a pulse with TTL adjusted amplitide when the
signal from the microphone exceeds a prede-termined limiting
value. The outputs of -the Schmitt trigger circuit 3 are con- -
nected to a "stop" input 4 on each o:E the connected counters
5 and to a common OR gate 6, the output of which is connected
to a "start" input 7 on all the counters 5. The counters 5
are also connected to an oscillator 8, generating a pulse
train with a predetermined repetition frequency. The passage
times of a pressure wave past the microphones Ml, M2, M3 and
M4 are determined in the following way.
When the wave passes a microphone, e.g. the microphone
Ml, a pulse is generated in the Schmitt trigger circuit 3,
giving a start signal via the OR gate 6 to all the counters
5. These then count the pulses generated in the oscillator 8.
The counter having its "stop" input connected to the micro-
phone Ml, which is first met by the wave, is given a stop
signal simultaneously as it obtains a start signal, and it
thus remains zeroed. While the wave passes each of the remain-
ing microphones M2, M3 and M4, a pulse is generated in the
associated circuits 3 which stops the appropriate counter 5,
which thus contains a count proportional to the time which
has passed since it was started. When a counter has stopped,
the level is also raised on an input to an AND gate 9. The
counters 5 are furthermore arranged so as not to start for
new pulses from the OR gate 6 before a resetting signal has
been obtained from a microcomputor 11. When all the micro-
phones have been passed by the pressure wave, the levels on
all the inputs to the AND gate 9 are high, which means that
its output is high, which in turn gives an interruption sig-
nal to the microcomputer 11 which then reads off the count
in each of the counters associated with the microphones, and
registers these counts for transmission to the main system
computer. The microcomputer 11 subsequently resets the coun-
. .
: : "
,. . , -.
,. : ; :.,
: .
-
19
ters 5 and the cycle can be repeated. Where there is a fifth
microphone M5, this is connected to the microcomputor 11 in
the same way as -the other microphones.
The -ti~e TF between the leading and trailing edges of
the pressure wave is measured in a counter 12. When the lead-
ing edge meets a microphone, e.g. the microphone Ml, the coun-
ter 12 is started and counts pulses from the oscilla-tor 8
which is also connected to the counter. The count in the coun-
ter is stored as long as pulses are obtained from the Schmitt
trigger circuit 3 associated with the microphone Ml. If no
pulse is detected within a certain predetermined time, which
is dependent on the length of the projectile, the last re-
ceived pulse is assumed to come from the trailing edge of
the wave. An interruption signal is then given via the wire
13 to the microcomputer 11, which reads off the value in the
counter 12. This value thus gives the time between the
trailing and leading edges of the pressure wave. After read-
ing it, the microcomputer 11 resets the counter 12 via the
wire 14 and the cycle can be repeated.
The microcomputer 11 subsequently transmits collected
data via a link 15 to the central system computer 18 where
the mathematical calculations are carried ou-t. The connection
to the central computer can either be done via a radio link
or by a cable, in the case where the micro processor 10 is
on the ground.
Figure 11 is a block diagram of an indicator apparatus
in accordance with the invention, preferably of the kind
shown in Figure 6, where the microcomputer 10 is connected
to the central computer 18 via a radio link, comprising trans-
mitter receiver units 16, 17 for two-way communication. The
ground-based microphone M6 is connected to the central com-
puter 18 via a wire 19 for registering the firing time. The
miss distances registered during firing, and the positions of
the "miss points" can be registered on different readout means
20 connected to the central computer, i.e. stylus recorders,
presentation screens or magnetic tapes.
The characteristic constants for each projectile type
and firing occasion, which are re~uired for calculating the
position of the bang generation point and the direction of
~: :
,~
,
- ~33~
the trajectory, can be obtained from tables stored in the
computor.
., :'
, ' ,', ' "
.
