LONG-RANGE, SMALL TARGET RANGEFINDING
The field of the invention is that of laser telemetry.
A telemeter allows the measurement of the distance between it and a target.
An optical telemeter uses the propagation of light as measuring means, it is
5 composed of an emitter and of a receiver. It emits light directed toward the
target and detects a fraction of this light returned by the target. The distance
is obtained on the basis of the time required for the light to propagate out to
the target and for the light to return to the receiver. The emission is
temporally modulated. The emitted light transports this modulation to the
10 target. The target reflects or backscatters this light. A fraction of this returned
light transports the modulation to the receiver of the telemeter. Measurement
of the time elapsed between the identification of the starting modulation of
the telemeter and the identification of the modulation of its return by the
receiver makes it possible to calculate the distance between the telemeter
1s and the target on the basis of the speed of propagation of light in the media
traversed.
Typically, a telemeter comprises an emission device, comprising an emitter
and its optic for shaping the laser beam, a reception device comprising an
optic for collecting and focusing on the focal plane laser echoes
20 backscattered by the target, and a processing device for driving the emission
and reception and allowing computation of the distance.
The optical echo of the target is converted into an electrical signal by the
detector, the electrical signal being superimposed on the noise. The filtered
and amplified signal obtained at the end of the detection chain is digitized.
25 A frame consists of a series of data sampled over the duration counted from
the emission of the pulse and over the time of an outbound-return journey
corresponding to the maximum distance of use or over the duration
corresponding to the distance sub-domain sought. The sampling frequency is
chosen so as to optimize the extraction of the signal of the echo from the
30 noise and allow the expected resolution and precision in terms of distance.
For example, a sampling frequency of 59.94 MHz would allow a distance
increment of 2.5 m.
Various solutions have been implemented for improving the range of a laser
telemeter.
5 A first solution consists in increasing the energy emitted per pulse. But, the
increase is limited by constraints of ocular safety and by the increase in
volume and in energy consumption of the emission device.
Another solution consists in increasing the surface area of the reception
pupil. This solution is, likewise, limited by telemeter bulkiness and weight
10 constraints.
In the case where the dimensions of the target are smaller than the
dimensions of the spot made by the laser at the level of the target, only the
fraction of light deposited on the target contributes to the telemetry. This
fraction is dependent on the quality of the laser beam determining the size of
15 the spot, and on the way the beam is pointed toward the target.
Under ideal conditions, the laser beam is very slightly divergent and perfectly
pointed toward the target, all of the emitted light contributes to the telemetry.
However, the sighting line is rarely directed toward the part that is most
contributory in the telemetry sense, the most contributory part being a zone
20 which returns the largest fraction of the emission by reflection or
backscattering toward the reception device. To avoid significant losses of
performance, as soon as the sighting line is not directed toward the most
contributory part of the target, it is necessary to increase the divergence of
the beam, to the detriment of the telemeter's range.
25 In the case of non-cooperative targets, the laser emission is usually pulsed. A
target is cooperative when the target favors the return of the l~ght in the
direct~ono f the emitter with the aid of a cubic wedge for example.
In the case of pulsed telemetry, the signal arising from the detector is
composed of the noise of the detection chain, of the optical noise collected in
the reception field and of the echo of the expected target. When the signal is
sufficiently significant, the detection of the moment of arrival of the echo is
5 done by thresholding. Stated otherwise, a target is detected if the intensity of
the echo is greater than a threshold fixed beforehand above the level of the
noise.
The signal at the moment of the thresholding is the sum of the amplified
signal coming from the detection of the echo and the optical and electronic
lo noise. For a target, the signal will have an amplitude varying from one pulse
to the next. For a signal in the vicinity of the level of the threshold, the signal
will not always exceed the threshold. When the signal is below the threshold
there is no detection. If a signal does not at any moment exceed the
threshold, the echo is absent or too weak.
15 The observation can be done from the start of the pulse over a duration
corresponding to the maximum distance sought, for example 533 ps for a
maximum distance of 80 km. The observation can also be done over a
duration corresponding to a distance sub-domain, for example over a
duration corresponding to the sub-domain lying between 40 and 50 km.
20 Another possibility for improving the probability of target detection over a
given time interval is to increase the pulse repetition rate.
To improve the detection of the echoes of the target in relation to noise, it is
possible to combine the detection signals subsequent to several pulses. The
combining of several detection signals can be undertaken according to a
25 post-integration method. This procedure is old, it has been implemented with
analog methods but it is still in vogue in the digital age.
Post-integration processing is a way of combining the frames of signals
detected subsequent to each pulse.
For a given telemeter, the post-integration step makes it possible to improve
the gains appreciably when the distance between the telemeter and the
target is sufficiently stable over the duration of the measurement.
In the case where the telemeter is properly pointed at the target the
5 probability of the presence of the echo of the target in each frame is 1. If the
distance between the telemeter and the target varies little in the course of the
post-integration phase, at each distance increment, the data of frames are
added up. The expected signal S is added up linearly, it is therefore
proportional to N, N being the number of frames, zsms.O n account of its
,r
lo nature, the detection noise B is summed quadratically, the amplitude of the
noise is proportional to the square root of the number of frames, z~ \m.
;? ..r
The ratio of the intensity of the expected signal to the intensity of the noise
will be proportional to the square root of the number of frames, -xc,: v 5 e: dX.
Stated otherwise, for a post-integration step on N frames having a probability
IS of presence of the echo of the target in a frame of 1, the ratio of the intensity
of the signal to the intensity of the noise SIB is proportional to v%.
During a difficult pursuit of a mobile target, notably when the dimensions of
the target are smaller than the dimensions of the spot of the laser at the level
of the target, certain frames do not contain any information relating to the
20 presence of an echo of the target. The post-integration applied to all the
frames does not have the expected effectiveness. Frames which contain only
noise are thus added to the frames which also contain an echo of the target.
The probability of an echo of the target in a frame therefore directly affects
the gain expected by the post-integration step.
25 When the probability of presence of the target on the spot of the laser at its
level is lla, that is to say only one frame out of a frames comprises an echo
of the target, the ratio o-f th e intensity of the signal to the intensity of the noise
SIB is proportional to E.Th erefore a 2 .f~ra mes are necessafy for the same
n
ratio of the intens~ty of the signal to the intensity of the noise \6 as that
obtained in n frames when the probability of presence of the echo is 1. To
obtain for example a gain of 10 ($8 = 10) subsequent to the post-integration
step, this requires the summation of 100 frames. If the probability of presence
of the echo of the target in a frame is 1/2 then a gain of 10 will be obtained by
5 summing 22.100=400 frames.
An aim of the invention is to improve the performance of a telemeter using a
step of post-integration, using the invention previously described in patent
application EP 2364455. This patent application EP 2364455 proposes a
telemetry reception device capable of detecting temporally and spatially the
lo echo provided by the target illuminated by the laser pulse.
The temporal detection allows the distance to be measured by measuring the
time of flight of the pulse, it can be done by means of one or more detectors.
The spatial detection can be obtained by means of one or more detectors.
This detection, on the basis of one or more pulses, makes it possible to label
1s the direction from which the maximum of light backscattered by the target.
comes or the absence of target. This maximum of light results from the
interaction of the target with the spot of the laser pulse. it is thus possible to
recenter the direction of emission so as to maximize the effectiveness of the
telemeter.
20 The passband required for the temporal detection is very large in comparison
to that of the spatial detection, thus increasing the noise of the temporal
detection chain. Consequently, the spatial detection is much more sensitive
than the temporal detection.
According to one aspect of the invention, there is proposed a device for
25 measuring a distance of a target by means of a telemeter comprising:
- a laser pulse emitter,
- a receiver of the laser echoes backscattered by the target,
comprising
a spatial detection device which comprises at least one
photodiode set up as integrator and is able to provide a socalled
spatial signal, and
a temporal detection device which comprises at least one
photodiode coupled to a transimpedance circuit and is able
to provide a so-called temporal signal,
- means for processing the spatial signal and the temporal signal,
comprising a unit for calculating the distance of the target, the
temporal signal being in the form of a data frame which is the
recording of data detected over a predetermined duration.
It is principally characterized in that the processing means comprise:
- means of post-integration of temporal signals, linked at output to
the unit for calculating the distance of the target,
- linked to the spatial detection device and to the temporal
detection device, means for selecting the temporal signals to be
transmitted to the post-integration means, as a function of the
spatial signal.
Stated otherwise, the proposed telemeter comprises a laser pulse
emitter, a receiver making it possible to provide spatial information and data
20 frames, and means for processing the spatial information and data frames so
as to carry out a selective post-integration of these data frames.
The telemeter according to the invention is configured to select and adapt the
processing of the frames on the basis of the spatial information.
The capacity of the telemeter to detect the target spatially and to mark the
25 frames arising from the temporal detection containing a priori an item of
information relating to the presence of an echo of the target makes it possible
to limit the drawbacks of the laser pulses emitted but which do not reach the
target.
According to a first embodiment, the means for selecting the temporal signals
comprise a switching control linked at output to the temporal detection device
via a switch, and linked at input to the spatial detection device and able to
switch the temporal detection device via the switch as a function of the
5 spatial signal, and in that the post-integration means are linked at input to the
temporal detection device. According to this embodiment, all the frames
arising from the temporal detector are transmitted to the post-integration
means, since selection has occurred upstream of the temporal detection.
According to a variant of this embodiment, the switching control is
lo furthermore linked to the post-integration means.
According to a second embodiment, the means for selecting the temporal
signals are discrimination means which are linked at input to the spatial
detection device and to the temporal detection device and at output to the
post-integration means. According to this embodiment, only certain frames
Is arising from the temporal detector are transmitted to the post-integration
means, since the selection occurs downstream of the temporal detection.
Advantageously, the processing unit comprises means of temporal labeling
or date-stamping of the frames, thereby allowing processing by postintegration
when a mobile target is detected.
20 Advantageously, the receiver furthermore comprises a protection flap so as
to protect the receiver when no detection is necessary.
Advantageously, the telemeter furthermore comprises a device for aligning
the direction of emission and the reception pathway.
Advantageously, the telemeter furthermore comprises means for detecting a
25 presence of a target for distances smaller than the minimum telemetry
distance. Depend~ngo n the telemeters, this minimum distance may be from
50 to 500 m. These detecting means make it possible to deactivate the
operation of the laser emission and to ensure the ocular safety of the device
from the zero distance.
Advantageously, the laser emitter comprises means for adapting the
divergence and for collimating the laser beam at infinity.
Advantageously, the telemeter furthermore comprises means for orienting
the telemetry axis.
5 Advantageously, the telemeter comprises means for measuring deviometry
between the telemetry axis and the position of the target, the measuring
means being connected to the spatial detector.
The telemetry axis orienting means associated with the deviometry
measurement allow pursuit of the mobile target.
10 Advantageously, the telemeter has the capacity for pursuit of the target, on
the basis of the spatial detection information, by modifying the spatial
arrangement of the spatial and temporal detectors, in accordance with the
invention previously described in patent application EP 2364455.
According to another aspect of the invention, there is proposed a method for
I5 measuring the distance of a target by means of a telemeter such as
described previously and comprising:
- a spatial detection step comprising a sub-step of emission of a
laser pulse by the emission device, a sub-step of detecting the spatial
signal Ss and of acquiring a value I of integration of Ss,
- a temporal detection step comprising a sub-step of emission of
laser pulses by the emission device, and a sub-step of acquiring a
temporal signal ST in the form of data frames,
. a step of post-integration of the data frames ST as a funct~on of
the spatial signal Ss,
- when the result of the post-integration is above a threshold, a
step of calculating the distance.
According to a first mode of operation of the invention, the spatial
detection and the temporal detection are sequential and the method for
measuring the distance of a target is ensured by means of a telemeter
according to the first embodiment; it comprises the following sequential
5 steps:
. a spatial detection step comprising a sub-step of emission of a
laser pulse by the emission device, a sub-step of detecting the spatial
signal Ss corresponding to the laser echo of said pulse and of
acquisition of a value I of integration of Ss by the switching means,
and when the value I is below a predetermined threshold S1, the
previous step is repeated,
- otherwise a target then having been detected, a temporal
detection step is implemented comprising a sub-step of emission of
other laser pulses by the emission device, and a sub-step of acquiring
a temporal signal ST in the form of data frames, corresponding to the
laser echoes of these other pulses,
- a step of post-integration of the data frames ST obtained during
the temporal detection step.
According to a variant to this mode of operation of the invention, the
20 spatial detection and the temporal detection are sequential and the method
for measuring the distance of a target is ensured by means of a telemeter
according to the second embodiment comprises the following sequential
steps:
- a step of 1'' spatial detection comprising a sub-step of emission
25 of a laser pulse by the emission device, a sub-step of detecting the
spat~asl ignal Ss corresponding to the laser echo of sa~dp ulse and of
acquiring a value I of integration of Ss, and when the value I is below a
predetermined threshold S1, the previous sub-steps are repeated,
- otherwise a target then having been detected, a temporal
detection step is implemented comprising a sub-step of emission of
other laser pulses by the emission device, and a sub-step of acquiring
a temporal signal ST in the form of data frames termed group A of
frames, corresponding to the laser echoes of these other pulses,
- a step of post-integration of this group A of data frames ST
obtained during the temporal detection step,
- a temporal detection step is implemented comprising a sub-step
of emission of other laser pulses by the emission device which differ
from those of the group A, a sub-step of acquiring a temporal signal ST
in the form of data frames termed group B of frames, corresponding to
the laser echoes of these other pulses, and a sub-step of placing this
group B of frames in memory,
. a step of 2nd spatial detection comprising a sub-step of
emission of a laser pulse by the emission device, a sub-step of
detecting the spatial signal Ss corresponding to the laser echo of said
pulse and of acquiring a value I of integration of Ss,
- when the value I is above a predetermined threshold S1 right
from the first pulse, the spatial detection being confirmed, a step of
post-integration of the group B of data frames is activated. The
previous step of temporal detection of the group A is repeated,
- when the value I is below a predetermined threshold S1, the
previous steps from the spatial detection are repeated.
According to another mode of operation of the invention, the spatial
25 detection and the temporal detect~on are simultaneous and the method for
measuring the distance of a target is ensured by means of a telemeter
accord~ngto the third embodiment; it comprises the following steps:
- a spatial detection step comprising a sub-step of emission of a laser
pulse by the emission device, a sub-step of detecting the spatial signal Ss
corresponding to the laser echo of said pulse and of acquisition of a value I of
integration of Ss by the selection means and a simultaneous sub-step of
5 detecting a temporal signal ST corresponding to the same laser echo of said
pulse,
-when the value i is below a predetermined threshold Sf, the previous
step is repeated,
- otherwise a target then having been detected, a step of postintegration
of the corresponding data frames ST, by the postintegration
means.
These steps are repeated until the measurement of the distance between the
target and the telemeter is obtained.
The method according to the invention makes it possible to weight the data
15 frames as a function of the intensity level of the spatial signal.
Preferably, the method furthermore comprises a step of temporal labeling (or
date-stamping) of the frames prior to their post-integration.
Optionally, the target to be telemetered is mobile. In this case, the steps can
be repeated for various assumptions of relative speed between the target
20 and the telemeter.
The invention will be better understood on studying a few embodiments
described by way of wholly non-limiting examples, and illustrated by
appended drawings in which:
- Figures 1 schematically represent examples of telemeter according to
25 the invent~onw, hich vary according to the selection means used,
- Figures 2 represent operating schematics of a telemeter according to
the invention, when the spatial detection alternates with the temporal
detection (fig 2a and 2b), when the spatial detection is simultaneous with the
temporal detection (fig 2c),
5 - Figure 3 represents frames ST containing an item of information
representative of the presence of the echo of a target, the relative speed of
the target being constant, as a function of time.
The telemeter 1 represented in Figures 1, comprises an emission device 2
for emitting a laser beam 21, a reception device 3 for receiving the
10 backscattered echoes 31 and a processing unit 4.
The device 2 for emitting a laser beam 21 comprises a laser pulse emitter 5
allowing weak divergence. Preferably, the laser pulse emitter 5 is high
frequency. Advantageously, the telemeter 1 comprises a device comprising a
secondary emitterlreceiver 6 for safety, configured for the detection of the
Is presence of a target situated at a distance below the minimum telemetry
distance and adapted for stopping the operation of the laser emitter 5 in case
of detection of a target at a distance below the minimum telemetry distance
so as to ensure the ocular safety of the telemeter 1. Typically, minimum
telemetry distance is intended to mean a distance lying between 0 and 500
20 m.
Advantageously, the emission device 2 can furthermore comprise an optic,
not represented, capable of shaping the laser beam. Stated othenvise, on its
exit from the emitter 5, the laser beam 21 is at the expected divergence and
collimated at infinity.
25 Advantageously, the telemeter 1 comprises means 7 for or~enting, the
emissions making it possible to modify the direction of emission relative to
reception.
According to the invention, the telemetry of a target is performed as follows.
The emission device 2 emits a pulsed laser signal 21 and optionally transmits
the date of emission of the pulse 21 to the processing unit 4.
The reception device 3 for receiving the backscattered echoes 31 comprises
5 an optic 8 which focuses the backscattered echoes 31 on a focal plane 18.
As described in patent application EP 2364455, this focal plane can be
subdivided into several elementary detection zones. At each elementary
zone, the detection can be spatial or temporal. The reception device 3 can
comprise means for steering the backscattered echoes 31 toward the spatial
lo detector 10 or the temporal detector 11, as described in patent application
EP 2364455. The spatial 10 and temporal 11 detectors can be situated in the
focal plane 18, or the light can be transported from the focal plane to the
detectors 10, 11 by optical means such as mirrors or optical fibers. The
spatial and temporal detections can be done either by a single detector
Is associated with two distinct reading circuits, one dedicated to spatial
detection and the other to temporal detection, or by two distinct detectors,
one for temporal detection and the other for spatial detection. A spatial
detector 10 is typically a photodiode set up in an integrator circuit. The setup
is low noise. During the integration period, the charges created are stored
20 and then, after the integration period the reading circuit converts these
charges into a signal proportional to the number of charges collected. The
duration of the integration is suited to the distance domain or to the subdomain
in which the target may be situated. The detection of a very weak
signal of the order of a few tens of photons is possible. A temporal detector
25 11 is typically a photodiode of PIN type, an avalanche photodiode or a
photodiode with large passband optimized for the temporal labeling of the
echoes and to allow the measurement of the distance between the target and
the telemeter. Its performance is principally limited by the inherent noise of
the detector and associated circuits of transimpedance and amplification
30 type. The resolution of the temporal modulation allows precise measurement
of the distance. An avalanche photodiode makes it possible to improve the
ratio of the intensity of the signal to the intensity of the noise. A few hundred
or indeed thousand photons are required in order to obtain a signal-to-noise
ratio sufficient to limit the risks of false alarm. The level of the signal
detectable by the temporal detector 11 is much higher than that detectable by
the spatial detector 10.
Advantageously, the telemeter 1 comprises an orientation device 12 for
orienting the telemetry axis comprising the emission pathway and the
5 reception pathway. This device can be dynamic so as to allow fine pursuit of
the target on the basis of the spatial detection deviometry information.
Advantageously, the reception device furthermore comprises a retractable
protection flap 13 to protect the receiver when no detection is necessary.
Advantageously, the reception device 4 comprises means for shaping the
lo data for their digital processing. The sampling frequency would be 59.958
MHz for a distance measurement increment of 2.5 m. However, a higher
sampling frequency may be adopted for better representatwity of the pulses
during the processing operations.
The processing unit 4 comprises the following means:
- Selection means 14 which establish the value I of integration of
the spatial signal Ss arising from the spatial detection and select
the frames arising from the temporal detector which are to be
post-integrated,
- Means 16 of post-integration of the selected temporal frames,
Means 17 for calculating the distance between the target and
the telemeter, on the basis of the result provided by the postintegration
means.
According to a flrst embodiment shown in Figure la, these selection means
activate the spatial detection or the temporal detection, as a function of the
25 value I of integration of Ss. These selection means are for example ensured
by switching means allowlng the steerlng of the backscattered echoes 31
toward the spattal detector 10 or the temporal detector 11. These switching
means comprise a switching control 14b able to establish a decision as a
function of I and a switch 14a able to execute this decision. The switching
means 14a, 14b steer toward the spatial detector 10 during the spatial
detection. When the spatial detection is ensured, the switching means 14a,
s 14b toggle to the~positionfo r steering toward the temporal detector 11, for the
acquisition of a group of frames. According to this embodiment, all the frames
of the group which arise from the temporal detector 11 are transmitted to the
post-integration means 16, since selection has occurred upstream of the
temporal detector.
lo According to a second embodiment shown in Figure Ib, a variant of the first
embodiment, the switching control 14b is furthermore linked to the postintegration
means 16.
According to a third embodiment shown in Figure Ic, the selection means are
discrimination means 14 which are linked at input to the spatial detection
1s device 10 and to the temporal detection device 11 and at output to the postintegration
means 16. The two detectors, spatial and temporal, are active for
all the pulses. As a function of the value I of integration of Ss, there is
discrimination of the frame arising from the temporal detection device 11, so
as to determine whether it has to be transmitted to the post-integration
20 means 16. The apportionment is static between the spatial detection and the
temporal detection. This entails for example a semi-transparent plate.
According to this embodiment, only certain frames of the group which arise
from the temporal detector 11 are transmitted to the post-integration means
16, since selection has occurred downstream of the temporal detector 11.
25 The processing unit 4 advantageously comprises means 15 of temporal
labeling of the frames ST, which associate with them the date-stamps of the
emissions provlded by the emission device 2 (steps 206 1, 302); they are
situated at the output of the emission device 2 and at the input of the postintegration
means 16. They may optionally be integrated into the selection
30 means. Thls temporal labeling is indispensable in the case of a mob~leta rget,
as described further on, but not for a fixed target; but when it IS not known a
priori whether the target is fixed or mobile, this default temporal labeling is
carried out.
The telemeter 1, according to one aspect of the invention, makes it possible
to locate the most contributory part of the target, to finely orient the emission
5 axis with this part and to telemeter it. It thus makes it possible to limit the
waiting duration before the display of the distance of the target when laser
pulses are fired alongside the target.
Several modes of operation of the telemeter 1 can be used, all based on
post-integration of laser echoes which are detected by the temporal detector
lo and are previously selected as a function of the signal arising from the spatial
detector: either the spatial detection alternates with the temporal detection,
optionally with a variant, or the spatial detection is simultaneous with the
temporal detection.
Figure 2a represents a mode of operation of the telemeter according to the
Is first mode of use, when the spatial detection alternates with the temporal
detection. The telemeter used is that described in conjunction with Figure la.
In a first step 200, a spatial detection is activated until detection of a target for
which the measurement of the distance between the target and the telemeter
may be desired, this spatial detection step comprises sub-steps 201, 202 and
20 203. Optionally, the detection of the presence of a target situated at a
distance below the minimum telemetry distance is activated, thereby making
it possible to ensure the ocular safety of the device.
In a sub-step 201, a laser pulse is emitted by the emlssion device 2. In a substep
202, the spatial detection is in integration for a duratlon corresponding to
25 the outbound-return journey time of the emissions in the domain or subdomain
of use of the telemeter, so as to acquire an Integration value I. For
example, for a target situated in a sought-after distance sub-domain lying
between 40 and 50 km, the integration window will lie between 266 ps and
333 ps, the instant of emission of the pulse corresponding to time zero, the
30 domain corresponding to the interval between the telemeter and its maximum
emission distance, by between 0 and 80 km. In a step 203, the integration
value I is compared with a threshold value ST, fixed previously. If the
integration value I is less than the threshold value S1, the echo of the target
is not sufficient. It is then useless to activate the temporal detection and sub-
5 steps 201 to 203 are repeated. In this case, either the direction of the
telemetry axis or the time window is changed, or it is decided that there is no
detectable target. Steps 202 and 203 are ensured by the spatial detector 10
and the switching means 14a, 14b.
If the integration value I is greater than the threshold value S1, this
10 necessarily signifies that a target is present, the following steps of temporal
detection will use frames comprising the signal arising from the target in
addition to noise in the distance domain or sub-domain sought. The switching
in a step 204 toggles to allow temporal detection. A group of K frames is
acquired, this step comprising sub-steps 205 to 207.
Is in sub-step 205, the telemeter emits a new laser pulse by means of the
emission device 2. In sub-step 206, a data frame ST is recorded during the
return time window of the echo of the target corresponding to the distance
domain or to the sub-domain sought; and optionally dated during a sub-step
206.1, as will be seen further on in the case of a mobile target. Depending on
20 the characteristics of the system and the type of target that it is desired to
detect, the direction of emission of the laser is considered to be stable on the
target for a given number K of emissions. Sub-step 207 manages the
acquisition of a group of K frames by repeating steps 205 to 206. These K
frames are transmitted to the post-integration means 16 with a view to the
25 post-integration step 208. According to this alternate (or sequential) mode of
operation, all the frames arising from the temporal detector are taken into
account for the post-integration, since selection has occurred upstream of the
temporal detection.
In a step 209 ensured by the post-integration means 16, it is verified whether
30 the result of the step 208 of post-integration of the K frames allows the
publlcatlon of a distance with an acceptable probab~lltyo f false alarm, stated
otherwise whether the result of the post ~ntegrat~oins above a second
threshold value S2. If it is not possible to extract a distance through the
calculation means 17, that is to say if the post-integration is not yet sufficient
to publish the distance (=test of step 209 is negative), a new cycle takes
place: the switching means relaunch a phase of spatial detection of the
5 presence of the target and then K new frames are aggregated so as to be
added to the post-integration before.
When the telemetry axis and the target are in motion with respect to one
another, the information regarding presence of the target on the telemetry
axis through the stringing together of steps 202 and 203 may optionally serve
lo for the pursuit of the target.
To reduce the dead switching times, a variant of this mode of
operation can be implemented. The cycle described hereinabove can be
amended in the following manner, described in conjunction with Figure 2b.
The telemeter used is that described in conjunction with Figure Ib. According
IS to this variant, after the acquisition of a group A of frames and their postintegration,
a second group B of frames is acquired. Next, the selection
means activate the spatial detection. If right from the first pulse the spatial
detection detects the presence of the target, the group B is post-integrated.
Otherwise the group B is rejected and the spatial detection continues.
20 According to the result of the post-integration of the group A, if the distance is
not obtained, there is no modification of the selection. There is modification of
the selection to spatial detection position if the distance is obtained.
The spatial detection termed 1'' spatial detection is activated (steps 2011,
202', 203'). When the presence of the target is affirmed by the spatia!
25 detection (test of step 204' is positive), there IS acquisition of a group A of K
frames of temporal detection (steps 205, 206.11, 206', 207'). The post-
Integration (step 208') of this group A is done. Either the distance can be
published (since step test 209' is positive) and the spatial detection is then
activated for a next telemetry (we start again at a ISs'p atial detection). If the
30 test 209' for this group A is negative, a second group B of K frames is
acquired (steps 205, 206.1', 206', 207') This second group B is kept in
memory. The switching means switch to the spatial detector (step 200). A
spatial detection termed 2nd spatial detection is activated.
- If this 2nd spatial detection confirms the presence of the target
(positive test of step 203'), then this second group B of K frames of temporal
5 detection is post-integrated while being aggregated with group A (step 208').
If the test 209' is positive subsequent to this post-integration of group B, the
process is terminated with the publication of the distance. Otherwise, the
switching means switch to the temporal detector and the process resumes at
the acquisition of a group A of K frames (= group A temporal detection).
10 - If the 2nd spatial detection does not confirm the presence of the
target, the second group B of K frames of temporal detection is rejected. And
the process continues for a new lSsp'a tial detection of the target.
Figure 2c presents a mode of operation of the telemeter when the spatial
detection is simultaneous with the temporal detection, according to another
15 embodiment. The telemeter used is that described in conjunction with Figure
1 c.
Optionally, the detection of the presence of a target at a distance below the
minimum telemetry distance is activated, thereby making it possible to
ensure the ocular safety of the device.
20 The measurement of the distance is done according to the process
composed of cycles and described in Figure 2c. In a step 301, the telemeter,
according to one aspect of the invention, emits a laser pulse by means of the
emission device 2. The temporal detection is activated simultaneously with
the spatial detection. As long as a target is not located by spatial detection,
25 the telemeter emits a new pulse.
Each pulse is optionally dated in a step 302, as will be seen further on.
In a temporal detection step 303, a data frame ST is recorded during the
return time window of the echo of the target in the distance domain and the
sub-domain sought, by means of the temporal detector 11. This data frame
ST is potentially usable for the calculation of the distance between the target
and the telemeter.
In a step 304 simultaneous with step 303 and ensured by means of the
s spatial detector 10, the spatial detection is in integration during the same
return time window of the echo of the target in the distance domain or the
sub-domain sought, so as to acquire an integration value I arising therefore
from the same laser echo.
In a step 305 ensured by the discrimination means 14, the spatial information
10 Ss arising from the spatial detection and the data frame ST arising from the
temporal detection are associated for a subsequent processing of the data,
since they arise from the same laser echo. They are for example associated
in the form of a block of data comprising I, ST and the date of emission of the
pulse.
1s Step 306 of analyzing the spatial information, also ensured by the
discrimination means 14, concludes either that the echo of the target is not
sufficient and in this case the data block acquired in the course of this
iteration is rejected and the process resumes at step 301, or the echo of the
target is sufficient and in this case we go to step 307: if I > Sf, then ST and
20 the date are communicated to the post-integration means 16.
Step 307 carries out the post-integration of the temporal frame ST with the
data accumulated during the previous cycles. According to this simultaneous
mode of operation, certain frames arising from the temporal detector are
taken into account for the post-integration, but generally not all, since
25 selection has occurred downstream of the temporal detection.
In a step 308 ensured by the post-integration means 16, it is verif~edth at the
signal obtained on completion of the step of post-integration of the frames
allows the publication of a distance: the level of the signal extracted is
compared with a threshold 52 above which the probability of false alarm is
30 acceptable. If there is no distance publishable by the calculation means 17,
the process is repeated from step 301 so as to aggregate a new frame until a
distance is publishable. When the level of the signal extracted is greater than
a threshold S2 above which the probability of false alarm is acceptable the
process terminates with the publication of the distance.
5 Let us return to the example proposed in the preamble: for a gain of 10, the
post-integration will pertain to only 100 valid frames i.e. 200 recorded frames
instead of 400 frames when the telemeter is not used, according to one
aspect of the invention.
Let us now deal with the case of a mobile target.
10 Figure 3 represents the recording of 16 frames containing an echo of the
target, the relative speed between the target and the telemeter being known
and constant. A correction taking account of the date-stamping of the
emission of the pulse corresponding to each frame and of the relative speed
makes it possible to approach the nominal process to do the post-integration
1s step. The distance obtained between the target and the telemeter will be
valid only at a given instant. The distance obtained must therefore be dated.
At any other moment a correction of the distance will have to be made by
taking account of the relative speed.
If the speed is insufficiently known, several speed assumptions will have to
20 be tested. The number of speed assumptions n corresponds to the relative
speed v between the target and the telemeter multiplied by the time At
between the first and the last frame and divided by the distance resolution R,
n=v.At/R.
For example, assuming a duration between the first and the last frame of 1
25 second, a relative speed that may vary from -20 m.s-' to +20 m.s-' and a
distance resolution of 2.5 m, a minimum of 16 speed assumptions will have
to be tested.
Now, in order for the post-integration step to be effective the signal of the
target must be summed across all the frames.
Consequently, for each speed assumption, a calculation of the distance
between the telemeter and the target is carried out while taking account of
the temporal label of the emission of each frame and of the chosen speed
assumption.
5 The speed assumption whose calculation, subsequent to the post-integration
step, gives the strongest signal is the most probable. The distance is
provided together with a date and a probable speed.
Let us take the example of a telemeter at 100Hz and of a target where, with
respect to the detection limit in a single pulse, an ideal case of a gain of 10 is
lo necessary. In the case where the target is properly centered for the duration
of the telemetry, the probability of presence of the echo in each frame would
be 1. The post-integration will be done in 100 frames. The duration between
the first and the last frame is 1 second, the maximum relative speed +20 m.s?
and the distance resolution 2.5 m, the number of speed assumptions is 16,
I 5 as we saw previously.
If the probability of presence of the echo of the target in a frame is %, 400
frames are then necessary, as we saw previously, to obtain a signal-to-noise
ratio identical to that obtained with a probability of presence of 1. The number
of frames is therefore multiplied by a factor of 4.
20 The number of speed assumptions is also multiplied by 4, i.e. 64 because of
the increase in the time between the first and the last frame which goes from
1 to4s.
Consequently, the number of operations to be carried out during the postintegration
step is ultimately multiplied by a factor of 4X4=16.
25 In the case of the use of a telemeter, according to one aspect of the
invention, the reckoning is the following. The number of frames to be
recorded is multiplied by 2 because of the probability % of presence of the
target in the frame, the number of frames to be processed in post-integration
remains the same: 100 frames since all these frames contain the echo of the
target and the number of speed assumptions is multiplied by two since the
time between the first emission and the last emission is multiplied by 2. The
number of operations to be carried out during the post-integration step is
multiplied by a factor of 2 solely because of the doubling of the number of
s speed assumptions.
Thus, with the aid of this invention, in the case of a mobile target where the
probability of presence of the echo of the target in a frame is ?/a, the
acquisition time and the number of operations during the post-integration
step, taking account of the assumptions regarding relative speed, is
lo multiplied by a. Whereas without this invention, in the case of a mobile target
where the probability of presence of the echo of the target in a frame is l/a,
the acquisition time is multiplied by aZ and the number of operations during
the post-integration step, taking account of the assumptions regarding
relative speed, is multiplied by a4.
Number of frames for
post-integration
I Probability of presence
of the target in the
Fixed
target
Centered I Unstable mobile target
mobile target
1 Without 1 According tc
invention invention
frame
Duration of acquisition
Speed assumptions
Number of operations

Claims
1. A device for measuring a distance of a target by means of a telemeter
(1) comprising:
- a laser pulse emitter (2),
- a receiver (3) of the laser echoes backscattered (31) by the
target, comprising
a spatial detection device (10) which comprises at least one
photodiode set up as integrator and is able to provide a socalled
spatial signal, and
a temporal detection device (1 1) which comprises at least
one photodiode coupled to a transimpedance circuit and is
able to provide a so-called temporal signal,
- means of processing (4) of the spatial signal and of the
temporal signal, comprising a unit (17) for calculating the
distance of the target, the temporal signal being in the form of a
data frame which is the recording of data detected over a
predetermined duration,
characterized in that the means of processing (4) comprise:
- means (16) of post-integration of temporal signals, linked at
output to the unit for calculating the distance of the target,
- linked to the spatial detection device (10) and to the temporal
detection device ( I I ) , means (14) for selecting the temporal
signals to be transmitted to the post-integration means, as a
function of the spatial signal.
2. The device as claimed in claim 1, in which the means for selecting the
temporal signals comprise a switching control (14b) linked at output to the
temporal detection device (1 1) via a switch (14a), and linked at input to the
spatial detection device (10) and able to switch the temporal detection device
(1 1) via the switch (14a) as a function of the spatial signal, and in that the
post-integration means (16) are linked at input to the temporal detection
5 device (1 1).
3. The device as claimed in the preceding claim, in which the switching
control (14b) is furthermore linked to the post-integration means (16).
4. The device as claimed in claim 1, in which the means for selecting the
temporal signals are discrimination means (14) linked at input to the spatial
10 detection device (10) and to the temporal detection device ( I I ) and at output
to the post-integration means (16).
5. The device as claimed in one of the preceding claims, inwhich the means
of processing (4) comprise means of temporal labeling (15) of the frames.
6. The device as claimed in one of the preceding claims, in which the
15 emitter (2) has a direction of emission and the receiver (3) has a direction of
reception and furthermore comprises a device for aligning the direction of
emission and the direction of reception (12).
7. The device as claimed in one of the preceding claims, in which the
laser pulse emitter (2) comprises means for adapting the divergence and for
20 collimating the laser beam at infinity.
8. The device as claimed in one of the preceding claims, furthermore
comprising means for detecting a presence of a target for distances less than
the minimum telemetry distance, the detecting means being adapted for
deactivating the operation of the laser emission and for ensuring the ocular
25 safety of the device from the zero distance.
9. The device as claimed in one of the preceding claims, furthermore
comprising means for orienting the telemetry axis.
10. The device as claimed in one of the preceding claims, furthermore
comprising means for measuring deviometry between the telemetry axis and
the position of the target, the measuring means being connected to the
spatial detector.
s 11. A method for measuring the distance of a target by means of a
telemeter (1) as claimed in one of the preceding claims, characterized in that
it comprises:
- a spatial detection step comprising a sub-step (202, 301) of
emission of a laser pulse by the emission device (2), a sub-step (203,
304) of detecting the spatial signal Ss and of acquiring a value I of
integration of Ss,
- a temporal detection step comprising a sub-step of emission of
laser pulses by the emission device (2), and a sub-step of acquiring a
temporal signal ST in the form of data frames,
- a step (208, 307) of post-integration of the data frames ST as a
function of the spatial signal Ss,
- when the result of the post-integration is above a threshold
(step 209, 308), a step of calculating the distance.
12. The method for measuring the distance of a target as claimed in the
20 preceding claim by means of a telemeter (1) as claimed in one of claims 1 to
10 taken in combination with claim 2, characterized in that it conlprises the
following sequential steps:
- a spatial detection step comprising a sub-step (201) of emission
of a laser pulse by the emission device (2), a sub-step (202) of
25 detecting the spatial signal Ss corresponding to the laser echo of said
pulse and of acquiring a value I of integration of Ss, and when the
value I is below a predetermined threshold S1 (step 203), the previous
step is repeated,
- otherwise a target then having been detected, a temporal
detection step is implemented comprising a sub-step (205) of emission
of other laser pulses by the emission device (2), and a sub-step (206,
207) of acquiring a temporal signal ST in the form of data frames,
corresponding to the laser echoes of these other pulses,
- a step (208) of post-integration of the data frames ST obtained
during the temporal detection step.
13. The method for measuring the distance of a target as claimed in the
preceding claim by means of a telemeter (1) as claimed in one of claims 1 to
lo 10 taken in combination with claim 3, characterized in that it comprises the
following sequential steps:
- a step of 1' spatial detection comprising a sub-step (201') of
emission of a laser pulse by the emission device (2), a sub-step (202')
of detecting the spatial signal Ss corresponding to the laser echo of
said pulse and of acquiring a value I of integration of Ss, and when the
value I is below a predetermined threshold S1 (step 203'), the previous
sub-steps are repeated,
- otherwise a target then having been detected, a temporal
detection step is implemented comprising a sub-step (205') of
emission of other laser pulses by the emission device (2), and a substep
(206', 207') of acquiring a temporal signal ST in the form of data
frames termed group A of frames, corresponding to the laser echoes
of these other pulses,
- a step (208') of post-integration of this group A of data frames
25 S~obtainedd uring the temporal detection step,
- a temporal detection step is implemented comprising a sub-step
(205') of emission of other laser pulses by the emission device (2),
which differ from those of the group A, a sub-step (206', 207') of
acquiring a temporal signal ST in the form of data frames termed group
B of frames, corresponding to the laser echoes of these other pulses,
and a sub-step of placing this group B of frames in memory,
- a step of 2nd spatial detection comprising a sub-step (201') of
emission of a laser pulse by the emission device (2), a sub-step (202')
of detecting the spatial signal Ss corresponding to the laser echo of
said pulse and of acquiring a value I of integration of Ss, when the
value I is above a predetermined threshold S1 (step 203'), the spatial
detection being confirmed, a step (208') of post-integration of the
group B of data frames,
- and then acquisition of a new group A of frames,
- and when the value I is below a predetermined threshold S1
(step 203'), the cycle resumes at the level of the first spatial detection.
14. The method for measuring the distance of a target as claimed in claim
11 by means of a telemeter (1) as claimed in one of claims 1 to 10 taken in
1s combination with claim 4, characterized in that it comprises the following
steps:
- a spatial detection step comprising a sub-step (301) of emission of a
laser pulse by the emission device (2), a sub-step (304) of detecting the
spatial signal Ss corresponding to the laser echo of said pulse and of
20 acquisition of a value I of integration of Ss by the selection means and a
simultaneous sub-step (303) of detecting a temporal signal ST corresponding
to the same laser echo of said pulse,
- when the value I is below a predetermined threshold S1 (step 306),
the previous step is repeated,
25 - otherwise a target then having been detected, a step (307) of post-
Integration of the corresponding data frames ST, by the postintegration
means (16).
15. The method as claimed in one of claims 11 to 13, furthermore comprising
a step (206.1, 302) of temporal labeling of the frames prior to their postintegration.
16. The method as claimed in one of claims 11 to 14, in which the target to
5 be telemetered is mobile.
17. The method as claimed in the preceding claim, in which the steps are
repeated for various assumptions of relative speed between the target and
the telemeter.