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BACKGROUND OF THE INVENTION
The invention relates to the measuring of distance between an active
installation, which will be called hereafter "hunter" and a member passive
in the measurement, which will be called hereafter "target". It finds a
particularly important, although not exclusive, application for measuring
the distance and position of the target with respect to the hunter,
particularly for allowing the meeting and mooring of two space vehicles.
In the latter, case, it is necessary for the hunter to be provided with a
device for:
measuring the distance of the target and its position with respect to the
hunter, as soon as the distance separating them is less than about 100 km,
also determining the attitude of the target, but solely for much shorter
distances, not exceeding a few tens of meters.
Measuring devices have already been proposed for use for this purpose
including, on the target, several reflectors spaced apart in a given
geometrical pattern and, on the hunter, a monochromatic light pulse source
and a matrix detector for forming an image of the reflectors. The detector
is generally formed by a matrix of charge coupled sensors or CCD. The
attitude of the target with respect to the hunter is then determined by
comparing the image obtained at a short distance with the known
distribution pattern of the reflectors. At a short distance, it is also
possible to measure the distance separating the hunter and the target by
analysis of the image, but, as soon as the distance becomes great, it can
no longer be determined by measuring the size of the target for this image
is practically a pin point for the detector. In this case, it is known to
determine the distance by measuring the flight time of a pulse delivered
by the light source. The flight time is in effect equal to twice the
distance divided by the speed of light.
But, when the distances are great, the power which returns to the hunter is
very low and the problem arises of identifying the echo in the image,
where parasites appear due to sources situated in the field, particularly
when the sun is situated therein.
This problem is all the more serious since the conventional method of using
the CCD includes a phase of accumulation of the charges generated in each
of the sensors by the incident photons. The permanent sources, such as the
sun, create charges during the whole duration of the integration phase,
very much greater than the duration of the pulses delivered by the source.
It has already been proposed to overcome this problem by interposing, in
the return path of the light echo to the CCD, a mechanical or
optoelectronic shutter having an opening time almost equal to the time of
the pulse delivered by the source. By moidifying the delay in opening of
the shutter with respect to the emission of the pulse, the moment is
sought when a maximum output signal is obtained. From the delay, which
corresponds to the flight time, the distance may be derived.
This solution is unsatisfactory. The mechanical or optoelectronic shutter
is expensive. Its use is complex. The measurement involves proceeding by
trial and error so as to obtain coincidence between the return time of the
pulse and the opening of the window, that is to say synchronizaton.
The invention aims at providing a measuring method and device answering
better than those known heretofor the requirements of practice,
particularly in that they allow distance measurements to be made without
adding any mechanical or electromechanical member to the means which are
in any case required for position and/or attitude measurement.
SUMMARY OF THE INVENTION
For this, the invention provides more particularly a method for the optical
measurement of the distance between a hunter and a target, in which
substantially monochromatic light pulses are emitted repetitively from the
hunter in the direction of the target and the light flux reflected by the
target is collected on a matrix detector with line by line or point by
point transfer reading, characterized in that the transfer is carried out
at a given frequency during the return of the reflected echo and in that
the emission, collection and transfer sequence is repeated for at least
two different transfer frequencies, the distance being derived from the
shift between lines or points having received the echo for the two
different frequencies.
The invention will be better understood from reading the following
description of a particular embodiment, given by way of non limitative
example, using as detector a frame transfer CCD of a type already used for
short distance position and attitude measurements between space vehicles.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a general diagram showing the distance and position measurements
to be made in view of a rendez-vous in space;
FIG. 2 is a general diagram showing the construction of a frame transfer
CCD;
FIG. 3 shows, from left to right, the successive transfers undergone by the
pixels of a column of the CCD (the hatched squares designating the pixels
which do not give rise to reading by the output register);
FIG. 4 shows the succession in time of an illumination pulse, of the
beginning of the frame transfer and of the echo return time;
FIG. 5 is a diagram showing the difference in position of the apparent
image as a function of the return time of the echo with respect to the
transfer sequence;
FIGS. 6 and 6A are respectively a general diagram and a functional diagram
of a device using a frame transfer CCD;
FIG. 7 is a block diagram of the sequencer of the device shown in FIG. 6A;
FIG. 8 is a timing diagram of the signals which appear in the block diagram
of FIG. 6;
FIG. 9 shows an interline transfer CCD also for implementing the invention;
and
FIG. 10 shows schematically the use of a linear CCD strip for forming a
distance measuring device.
DESCRIPTION OF THE INVENTION
The invention will be described in its application to the measurement of
the distance and of the position of a target 10 with respect to a hunter
12, both formed by space vehicles. The device carried by the hunter 12
must allow the distance D to be determined as well as angles .alpha. and
.beta. which define the orientation of the target with respect to a main
axis of the hunter 12 (tangential to the trajectory for example). When the
measuring device carried by the hunter must also determine the attitude of
the target 10, this latter carries several back folding reflectors (not
shown) distributed in accordance with a pattern recognizable by the
device.
Measurement devices are already known comprising, on the hunter 12, a
monochromatic pulse source (generally one or more laser diodes) and a
solid state detector including a matrix of sensors. This detector is
generally of the charge coupled type or CCD, having the construction shown
in FIG. 2.
For bringing out the advantages of the invention, the conventional
construction and operating mode of a CCD should first of all be recalled.
The photosensitive element 14 of the CCD, called image zone, is formed by
a semiconductor material wafer having a two dimensional matrix network
formed of a large number of photosensitive sites 16, grouped in lines and
columns. Each photosensitive site 16 forms a potential well which traps
the electrons generated by interaction of the incident photons which it
receives. During conventional use of a CCD, the electron packets are
trapped in a potential well for a constant time, called integration time,
and are then transferred to an output register 18 whence they are
transformed sequentially into an analog voltage by a preamplifier 20
operating at video frequency. Transfer takes place pixel by pixel in the
same line, under the control of a clock 22.
It can be seen that the photosensitive sites also form shift registers.
But, since these registers in the image zone 14 are photosensitive, it has
been thought up to now that the number of electrons generated during the
reading phase should be negligible with respect to the number generated
during the integration phase, so as not to disturb the image. Now, the
reading stage, including the output register 18, is slow for it has a very
large number of pixels to read.
For this reason, in a so called frame transfer CCD, a memory zone 24 is
inserted between the image zone 14 and the output register 18. The memory
zone is formed of elements distributed in a matrix identical to that of
the image zone, but not photosensitive. Each of the elements of the memory
zone stores the packet of electrons created in the corresponding
photosensitive site of the image zone 14, without disturbances. Transfer
of the contents of the image zone 14 into the memory zone 24 takes place
at a rate fixed by a line transfer clock 25, one line at a time.
The CCD shown in FIG. 2 is too slow for measuring the flight time of a
light pulse with good resolution: it must consequently be associated with
a distance measurement device, formed for example by a mechanical or
optoelectronic shutter and a measuring chain. In addition, the fraction of
incident light which returns to the hunter is very low. To be able to
distinguish the echo, a very powerful light source is required which can
in practice only be a pulsed source. But the possibility of obtaining a
very short integration time in the image zone 14 of a CCD makes it further
necessary to add the shutter which is an expensive component and, in
addition, only supplies a measurement after a period of distance
searching.
By way of example, CCDs are available at the present time whose image zone
14 includes photosensitive site distributed in a matrix of 384 columns and
290 lines. The memory zone 24 includes the same number of components.
Conventionally, the sequence of operations during use in a CCD of the kind
shown in FIG. 2 is as follows:
unloading the image zone 14 by transferring electron packets from one line
to the next and from the last line of the image zone to the first line of
the memory zone 24, at a rate which may be 1M-line per second;
integration phase, during which the charges generated in the image zone by
the incident photons are accumulated, the sensitivity of the detector
depending on the duration of this phase (generally going from 1 ms to a
few seconds);
transfer of the charges accumulated in the image zone 14 to the memory zone
24, by shifting of the lines, caused by 290 successive transfer pulses
delivered by the clock 25;
reading of the memory zone 24, by shifting by one line at a time in the
output register 18, then shifting this register, at a speed which may be
1M pixel, or 2.5 klines per second.
If this arrangement is used, it can be seen that, during the phases of
unloading the memory zone 24 and transfer to the memory zone, the
electrons which continue to be generated in the photosensitive sites of
the image zone 14 are located at a different position from that which they
would have in the image formed during the integration phase. So that they
do not disturb the measurements, in the conventional method of using a
CCD, the integration phase must have a duration very much greater than the
duration of the unloading and transfer phases.
The invention uses a method which may be considered as totally opposite,
consisting in suppressing the integration phase, or at least in reducing
it to the duration of the transfer time of a line. The image delivered by
the memory zone 24 is then formed of columns in which each pixel
represents the sum of the elementary charges cumulated each one during the
duration of the transfer pulse delivered by clock 25 for a position in the
respective column, during travel through the image zone 14.
If the scene to be observed has an illumination which is constant in time,
all the pixels of the same column will have received the same number of
photons and they will all have the same level. If the scene does not have
homogeneous illumination, the image will be formed by spectral lines whose
levels correspond to the mean illumination of respective columns;
If, on the contrary, one of the photodetector sites of the image zone 14
receives a brief light pulse, of a duration less than that of the transfer
pulses, the charges are only generated in this site for the duration of
the light pulse; since there is no transfer during this time interval, the
image will be sharp.
With this practice, it is possible:
to eliminate the parasite illuminations, for example by subtracting the
signals corresponding to two successive pixels of the same column;
because the detector is now very fast, to measure the flight time of the
light pulse, which results in a shift of the representation of the echo on
the image with respect to the representation which would have been
obtained by working in accordance with a traditional method.
Since, moreover, there has been no modification of the position of the echo
in the direction of the lines of the matrix, the new method of working in
no wise prevents measurements from being made in the scanning direction,
that is to say determining for example the angle .alpha. of FIG. 1. As for
angle .beta., it may be determined by making two successive measurements
with different transfer frequencies.
FIG. 3 shows the staggering of the transfers with respect to the time t of
the light pulse delivered by the source of the hunter 12. The time T is
that required for the successive transfers of 290 lines, fixed by the
frequency of clock 25. If it is assumed that the image of the echo is
formed on the photosensitive site 26 situated in the middle of the image
zone 14 (FIG. 5), because the echo returns to the CCD at a time when the
transfer has brought this image zone to 28, the representation appears at
26a during reading. If we designate by t.sub.0, t and t' the times of the
end of the first transfer, of sending the illumination pulse and of the
echo return, respectively, and by f.sub.1 and f.sub.2 two successive
transfer frequency values, which lead to heights of the representation in
the images h1 and h2, we have:
h1=h+.alpha..multidot.f.sub.1 (t'-t.sub.0)
h2=h+.alpha..multidot.f.sub.2 (t'-t.sub.0)
from which we derive the time of flight .delta.t=t'-t, equal to twice the
distance divided by the speed of light
.delta.t=t'-t=[(h1-h2)/.alpha.(f1-f2)]+(t.sub.0 -t)
and:
h=h1-f.sub.1 [(h1-h2)/(f.sub.1 -f.sub.2)]
It can be seen that thus, on the one hand, the time of flight may be
determined from which the distance is derived, and, on the other hand, the
position of the target in the direction of the columns for instance .beta.
which is a direct function of h.
The representation of the illumination pulse is formed by a spot. Using
illumination barycenter search techniques, spatial resolution of
measurement may be obtained reaching about 0.02 pixel. Since the maximum
transfer frequency is about 1 Mline per second, the resolution obtained
may be 0.02 .mu.s with respect to the flight time, namely 3 meters with
respect to the distance.
It is possible, by advancing or retarding the time t of the illumination
pulse with respect to the time t.sub.0, to create an upwards or downwards
shift of the image and, therefore, not to reduce the field of view of the
device with respect to that of the image zone 14.
The general construction of the measurement device may be that shown
schematically in FIG. 6. The device shown in this Figure includes an
optical head 32 and an electronic unit 34. Head 32 includes an objective
lens 36 with an optical input filter 38 whose pass band corresponds to the
emission spectral ray of the laser diode 40 forming the pulse source. The
optical head further contains the CCD detector 42 on which the image
delivered by objective 36 is formed. This CCD is provided with a Peltier
cooler 44 having a supply circuit 68. The optical head 32 further contains
an interface box 46 receiving clock signals from unit 34, as will be seen
further on. By means of an optical guide 48, the optical transmission of
the laser diode 40 is transmitted to the axis of the objective 36. The
purpose of interface 46 is essentially to match the clocks and the video
signal retransmitted over a line 50 to the electronic unit 34.
The electronic unit 34 is connected to the optical head 32; it is also
connected to the power bus 52 and the data bus 54 of the space vehicle
which carries the device. This unit has an internal bus 56 to which the
different circuits are connected providing:
sequencing of the CCD,
analog processing of the video signal from the optical head,
digital processing of the signal after digitization (particularly
substraction of the brightnesses corresponding to two successive points of
the same column),
controls and computations,
power supply of the different circuits of the electronic unit 34 and of the
optical head 32,
control and power supply of the Peltier effect cooler 44,
control and power supply of the laser diode 40.
The general construction of unit 34 may for instance be as shown in FIG. 6
which includes the following components, connected to the internal bus 56:
an image memory 58, with an output connected to a transceiver for radio
connection with the ground, and transmission of measurement data,
processor 60,
an analog unit 62 for analog processing of the video signal and A/D
conversion,
a real time processing unit 64,
a sequencer 66, which will be described in detail further on, delivering
clock signals to the interface 46.
a power circuit 68 containing a DC-DC converter and also providing
temperature regulation by controlling circuit 44.
Many of the components of such a device are identical to those which are
found in distance measurement devices using an electroptical shutter.
However, the functional diagram is of the kind shown in FIG. 6A. The
sequencer 66, in response to inputs 70 and 72 representing the transfer
frequency to be adopted and the delay before triggering of the laser
diode, delivers a control pulse to the laser diode 40, clock pulses to the
interface 46 and the addresses of the pixels during a transfer, in the
form of X and Y coordinates in the matrix (corresponding to angles .alpha.
and .beta. of FIG. 1).
The sequence may particularly have the construction shown in FIG. 7:
compared with a sequencer for star sensing device, the additional
components are those situated in the dash dot frame. The sequencer 66
includes a programmable divider 74 receiving signals from a local clock 76
and feeding a pixel counter 78 in cascade with a line counter 80. Both of
the counters may be 9 bit counters delivering at their output the X and Y
coordinates of each pixel in turn. Divider 74 is programmed by a register
82 from data delivered by the local bus 56. This bus also allows a
register 84 to be loaded for programming the triggering delay of the laser
diode 40 defining the time t. The triggering of 40 itself is caused by a
comparator 86 comparing the contents of the register and the output of the
pixel counter 78. The two counters 78 and 80 are each associated with a
decoder 88 or 90. The decoder 90 drives control logic 92 which causes a
network of gates 94 to be enabled supplying a synchronization circuit 96
delivering the different control signals required to the CCD.
FIG. 8 is a timing diagram of the different signals generated. The lines
identified by .psi.m (the bottom line representing on a larger scale a
part of the top line) indicate the succession in time of the transfer
signals inthe memory zone. The lines .psi.1 show the reading signals in
the memory zone. The video signal may have the form shown by lines V. The
first seven reading pulses may form a prereading which is not used in
processing the image. Finally, the signals delivered by gates 94 further
include a pulse 98 for synchronizing the laser diode. The flight time due
to the distance is compensated for by programming the register 84 from the
local bus 56.
The invention is not limited to the particular embodiment which has been
described above by way of example. Numerous variants are possible. In
particular, instead of a frame transfer CCD, a line transfer CCD may be
used. In such a CCD, the photosensitive sites 100 are disposed in lines
each adjacent to a shift register 102, as is shown in FIG. 9. All the
registers are connected to an output register 104 which itself drives a
video preamplifier 20. The registers are not photosensitive and, in a
conventional operation of the CCD, all the electron packets generated in
the photosensitive sites are transferred at the end of integration into
the shift registers and these latter are read through the output register
104. Such a type of register may also be adapted for implementing the
invention.
Finally, when a measurement of the position of the target in direction X is
not desired, the CCD may be reduced to a simple strip including an image
zone 14 and a memory zone 24. In this case, the optical image forming
system 106 is provided for forming a linear image.
* * * * *
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