Method and device for laser-optical measurement of cooperative objects, more especially for the simulation of firing

A method for the laser-optical measurement of cooperative objects at an urtain distance by emission of a laser beam and evaluation of the echo reflected at the object, more especially for simulation of firing for practice purposes. In order to be able to adapt the size of the beam cross-section to the target conditions within limited distance ranges, within a coherent measurement process a plurality of similar individual measurements are undertaken with differing beam divergences, which are associated in each instance with differing distance ranges. If echos which come from a distance substantially greater than the distance associated with the set divergence are received, then these are excluded from the evaluation.

DESCRIPTION 
The invention relates to a method and a device for the laser-optical 
measurement of cooperative objects at an uncertain distance, more 
especially for the simulation of firing for practice purposes, in which a 
laser beam is emitted and its echo reflected at the object is received and 
evaluated. 
In known arrangements for shooting practice, there are provided on the 
weapon a laser emitter and receiver, with which the target cooperates by 
means of a retroreflector, which sends back the laser beam in its 
direction of origin. The determination of whether the simulated firing 
would have struck the target is undertaken in the form of a measurement 
process in which the direction from which the echo arrives is compared 
with the direction in which the simulated hit is disposed; in this 
connection, the last-mentioned direction is represented, for example, by 
the optical axis of the system. The measurement process can be based on a 
scanning movement of the laser beam; in this case, the direction in which 
the target is situated, as seen by the marksman (target line) may be 
determined as that direction in which the beam is emitted at that instant 
within its scanning movement at which an echo reflected by the target is 
received. The prerequisite for this is a laser beam of limited divergence 
and having a cross-sectional magnitude in the region of the target which 
is np greater than appears to be expedient for the simulation of firing. 
However, at a given divergence, the beam cross-section increases with 
increasing distance. Accordingly, at a typical divergence of one mrad, the 
lobe formed by the laser beam has a width of 0.2 m at a distance of 200 m 
and a width of 3 m at a distance of 3 km. This leads to the paradoxical 
result that with known firing simulators in most cases better hit rates 
are achieved at a greater target distance than at close range. It would be 
possible to contemplate the idea that in order to avoid this error the 
beam divergence should be set in each instance in relation to the 
measurement distance in such a manner that the laser beam lobe possesses a 
predetermined width in the target region. However, in the case of firing 
simulation and in the case of the measurement of rapidly moving objects, 
such as aircraft during landing control, there is not sufficient time 
available for such a setting. 
The measurement of the object can also be based on the principle that the 
target field is illuminated by a stationary laser beam of relatively large 
divergence and that the direction from which the echo reflected by the 
target comes is determined at the device. This has the advantage that the 
laser beam scanning movement, which is costly in terms of engineering and 
which is time-consuming in the measurement process, can be dispensed with. 
In order even at close range to be able to guarantee a width of the 
illuminated field of for example 4.times.4 m, the beam must on the other 
hand possess a divergence which is so great that, on account of the energy 
density of more distant targets which decreases quadratically with 
distance, no echo signals capable of evaluation can be received any 
longer. Furthermore, the energy density cannot be increased without 
further ado, because at small distances it must not exceed the threshold 
for damage to the eyes. 
Accordingly, the object of the invention is to provide a method and a 
device of the initially mentioned kind, which lead to results which are 
more faithful to reality and which also permit the application of the 
last-explained measurement principle for objects at differing distances, 
more especially for the simulation of firing. 
The solution according to the invention consists in that the divergence of 
the beam is set in such a manner that the beam exhibits a predetermined 
size of cross-section at the object. 
In this connection, the beam is to be understood as referring in general to 
the laser beam as such. In consequence of its being set to a predetermined 
size of cross-section, its energy density at the object (apart from the 
losses caused by the atmosphere) is invariably the same, so that reliable 
signals are also obtained from the most widely varying distances. 
Even though the measurement result of the simulation of firing is dependent 
upon the cross-sectional dimensions of the beam, it is evident from the 
setting of the beam cross-section that the assessment is independent of 
the distance. For example, if for the hit/miss distinction it is merely 
determined whether a narrow laser beam simulating firing strikes or does 
not strike a detector or retroreflector at the object, the setting of the 
divergence according to the invention, of the beam ensures that the result 
of this determination is valid at any distance, while in the case of 
conventionally non-adjustable beam divergence an excessively low hit rate 
is measured at a small object distance in consequence of the beam 
cross-section which is then small, and an excessively large hit rate is 
measured at a large distance in consequence of the beam cross-section 
which is then large. If a beam passing through a scanning pattern is 
employed in this connection, then the term "beam" can be understood as 
referring to the entire scanning beam system; in these circumstances, the 
scanning angle which is run through takes the place of the divergence, so 
that the size of the scanning pattern at the target invariably has a 
pre-determined size. The divergence of the laser beam forming the scanning 
system can likewise be appropriately adjustable. 
Advantageously, within a coherent measurement process a plurality of 
similar measurements are undertaken with different beam divergences, which 
are associated with different distances so as to correspond to a 
predetermined beam cross-section. In this connection, those echos which 
come from a substantially greater distance are expediently eliminated from 
the evaluation. A device for carrying out this method possesses, in the 
beam path of the laser beam emitter, an objective of variable focal length 
together with a drive for rapid adjustment during the measurement. 
If very rapidly adjustable arrangements for varying the focal length are 
available, a discontinuous operation enters into consideration, in which 
each measurement process is composed of a plurality of measurement steps, 
during which the divergence is fixedly set in each instance and with which 
specific distances are in each instance associated. However, in general a 
continuous adjustment is to be preferred, during which a sequence of 
individual measurement or a continuously coherent measurement takes place. 
On account of the high speed of laser range finding and its electronic 
evaluation, a multiplicity of individual measurements may be carried out 
without further ado within a measurement process lasting for a period of 
time below the human reaction threshold of 0.1 sec, for example for a 
distance of 4 km a total of 20 measurements, in each instance for mutually 
adjacent partial distances of equal length or of equal divergence 
difference. It is also possible within a measurement process in the first 
instance to carry out a distance measurement and subsequently to set the 
divergence to the measured distance for a second measurement step; in this 
case, the angular measurement is merely based on the results of the second 
measurement step. 
It forms part of the teaching of the invention that only those echos which 
originate from a greater distance than the respectively appropriate 
distance must be excluded from the evaluation, while the echos originating 
from a smaller distance can be separated out but do not need to be 
separated out. 
This becomes comprehensible when consideration is given to the fact that 
only those echos which originate from an imaginary corridor, proceeding 
from the measurement device, of constant cross-section, are to be 
evaluated. If the laser measurement beam is adjusted in such a manner that 
it completely fills this corridor at the distance of the object to be 
measured, then at any smaller distance it has a cross-section which is 
smaller than that of the corridor. Consequently, each echo which comes in 
from a smaller distance also originates from a place located within the 
corridor and is consequently desired. Beyond the set distance, the laser 
beam cross-section does however exceed the corridor cross-section, so that 
it is not certain from echos coming in from a greater distance whether 
they originate from a location situated inside or outside the corridor. 
Accordingly, only the echos originating from a greater distance must be 
excluded from the evaluation. 
According to the invention, in many cases it is not necessary to provide a 
particular arrangement which excludes the echos originating from a greater 
distance, if it is ensured that the sensitivity of the receiving 
arrangement is set in such a manner that echos coming in from a greater 
distance are below the response threshold. This concerns both those 
receivers which are disposed at the measuring device (firing simulator) 
and also those on the object side, which can be provided there for the 
reception of specific information. This setting is easy, because the 
energy density of the echo decreases superproportionally with the 
distance. In order to be able to utilize this phenomenon on an optimal 
basis, it can be expedient that the sensitivity of the reciever or the 
intensity of the laser beam is adjustable. However, this is also not 
absolutely necessary. If the response threshold of the receiver transmits 
echos from smaller or greater distances, depending upon the respective 
transparency of the atmosphere, the only result of this is that the width 
of the prescribed corridor from which the echos can be received is 
correspondingly larger or smaller. This can be taken into consideration in 
the evaluation of the results. 
If it is desired to be independent of the atmospheric conditions, then 
according to the invention it is possible to measure the echo transmission 
time and to exclude from the evaluation those echos with an echo 
transmission time substantially greater than that echo transmission time 
which corresponds to the distance associated with the respective 
divergence. For this purpose, the evaluation arrangement can include a 
range finder, an arrangement for the generation of a signal reproducing 
the respectively determined distance value, an arrangement for the 
emission of a signal reproducing the distance limiting value corresponding 
to the respective setting of the objective, and an arrangement for the 
comparison of the distance signal with the distance limiting value signal. 
If the comparison shows that the echo under examination originates from a 
distance which is beyond the distance limiting value, then it is excluded 
from the further evaluation. If the device is designed in such a manner 
that for differing distance ranges in each instance separate measurement 
steps are carried out, then a particular distance limiting value can be 
stored in the evaluation arrangement for each one of these distance 
ranges. To do this, it is sufficient to establish a series of staggered 
distance limiting values, which are associated in each instance with a 
divergence setting. In place of this, it is also possible to determine the 
limiting value in each instance at a specific percentage rate above the 
respective distance range. This is in particular advantageous in 
circumstances in which no separate measurement steps take place for 
different distance ranges, but the multiplicity of measurements merge with 
one another into a continuous measurement process during continuous 
setting of divergence.

P According to FIG. 1, it is assumed that from the firing simulator 1 there 
proceeds a laser beam, which is bounded by the lines 2. Target objects 3 
to 7 are represented as black rectangles. They are equipped with a triple 
reflector, which serves as a retroreflector and which is represented by a 
white central spot. The target objects 3 and 5 are disposed on the optical 
axis with their retroreflector within the beam cross-section. The target 
object 7 which is set up at a large distance with a certain lateral 
displacement is also situated within the beam cross-section, while the 
target object 4, which exhibits an equal lateral displacement and which is 
situated in closer proximity to the firing simulator, is situated outside 
the beam cross-section 2. This illustrates the apparently greater hit rate 
at greater distance in the case of known devices with invariable beam 
divergence; in this connection, it should be added as a further 
disadvantage that the range of the beam remains limited, because its 
cross-sectional dimension oriented to a distance range between for example 
500 and 1,500 m results, at greater distances, in an energy output which 
is no longer adequate. 
According to the invention, in the same measurement process not only the 
beam 2 but also further beams with other divergences are used, of which in 
FIG. 1 those with boundaries 8 and 9 are indicated, which, having a 
greater beam width, are associated with smaller distance ranges. They also 
cover the target object 4. 
Target object 6 is situated so far from the central axis that it is not to 
be detected by the beam; more precisely stated, it is not to be relied 
upon for the purposes of a measurement evaluation. Although, as can be 
seen in FIG. 1, it is situated within the cross-section region of the 
beams 8 and 9, this is achieved as a result of the fact that its energy 
density on reaching this object has fallen off so greatly as a result of 
its large divergence that the reflected signal is below the response 
threshold of the receiver. As a result of this, for each one of these 
beams there is a limited range, which for the beam 8 if for example at the 
boundary 10 and for the beam 9 at the boundary 11. Thus it is achieved 
that echos capable of evaluation are received only from those objects 
which have a certain minimum closeness to the optical axis; in this 
connection, if the energy loss in the atmosphere is disregarded, this is 
approximately cOnStant Over the entire range of the device. The energy 
loss resulting from atmospheric influences has an effect such that the 
distance threshold, beyond which no echo signals capable of evaluation can 
be received any longer for a prescribed beam, is situated at a smaller 
beam cross-sectional area. As a result of this, at greater distances the 
objects must have a smaller distance from the optical axis in order still 
to be able to be detected. 
The space from which according to the invention echo signals capable of 
evaluation can be obtained is accordingly not a lobe of the conventional 
type, which becomes thicker at a larger distance, but rather a corridor 
with a constant cross-section which becomes narrower at a large distance. 
This provides a greater approximation to real conditions. 
FIG. 2 shows the manner in which the said corridor is composed of a 
multiplicity of individual beams. In the lower half of this Figure, it is 
provided that in each instance for equal distance ranges (for example from 
200 to 200 meters) special measurement beams are provided, the ranges of 
action of which, as indicated by thick lines, are adjacent to one another. 
With this arrangement, at a greater distance there is a better 
approximation to the ideal corridor transverse dimension d than at a small 
distance. If, in place of this, it is desired to keep the falling below 
and exceeding of the ideal corridor dimension by the beam cross-section 
equal in all distance ranges, then what is arrived at is the arrangement 
indicated at the top in FIG. 2, in which the more distant distance ranges 
are greater than the close ones. With an equally good approximation to the 
cross-section values aimed at in the close region, with this arrangement a 
smaller number of individual measurements are sufficient. For example, 10 
measurements over 4,000 m are sufficient if the distance limiting value of 
each individual measurement is 40% higher than that of the preceding one. 
As has been stated, the determination of the distance limiting value 
associated with each individual measurement or individual beam, above 
which limiting value no further echos are to be processed, can be left to 
a situation in which appropriate values fall below the response 
sensitivity of the receiver, which sensitivity is possibly set in such a 
manner that the desired mean width d of the corridor detected is the 
result. If it is desired not to accept the uncertainties associated 
therewith, the solution according to FIG. 3 presents itself. 
The laser 13 emits the laser beam 14, in the beam path of which a device 15 
influencing the focal length of the objective is disposed. This is 
indicated as a glass sphere oscillating under the influence of the drive 
arrangement 16 in the direction of the optical axis in a regular manner at 
a frequency between preferably 10 and 100 Hz. In place of this, an axially 
moving, so-called Selfoc lens can for example also be used, or a rapidly 
movable zoom objective. The optical properties of the overall arrangement 
at any time of the period of oscillation are known; thus, the divergence 
of the generated laser beam 14 is also known at any instant. If during a 
measurement process lasting for one half-period of the oscillation of the 
sphere it is desired to carry out individual measurement at specific 
times, which are associated with specific beam divergences, then these 
times can be predetermined within the arrangement 17, which, in order to 
predetermine the periodic cycle, is connected with the drive arrangement 
16. Thus, expressed in illustrative terms, the arrangement 17 takes over 
the function of a time-switch which runs synchronously with the 
oscillattion of the sphere 15 and which in each instance at specific times 
of the half-oscillation initiates an individual measurement by control of 
the range finder 18 by means of line 19. 
The signal originating from the laser echo 20 is fed to the range finder 18 
from the receiver 21 and is processed in a conventional manner. Via line 
22, it feeds a signal representing the determined distance to the 
comparator 23. To the latter there is moreover fed via line 24 from the 
arrangement 17 a signal, which represents the distance limiting value 
corresponding to the respective individual measurement and to the 
pertinent laser beam divergence. The comparator 23 compares the signals 
fed to it via the lines 22 and 24, and thus compares the determined 
distance value with the distance limiting value associated with the 
measurement. If the distance value is below the distance limiting value, 
then a signal is transmitted via line 25 for further evaluation, while in 
the opposite case it is suppressed and thereby excluded from further 
evaluation. 
The further evaluation can for example, consist in the generation of a hit 
signal, a distance indication or the like. The laser beam 14 can also be 
moved in a scanning manner; the further evaluation will then include a 
correlation between the signal and the respective beam direction as is 
known per se. 
A detailed description of the arrangements 17, 18 and 23 is not required, 
because these are known to an electronics specialist having experience in 
the relevant field from the prior art. 
The device 15 varying the focal length is expediently permitted to 
oscillate permanently, in order from time to time to extract individual 
oscillations for the performance of the measurement process. However, it 
is of course also possible to generate its movement in individual cases 
from time to time. 
The condition of this device during each individual measurement can be 
regarded as quasi-stationary, because the path which it traverses during 
the short laser-optical range finding process is negligibly small.