Radiant energy detection system for the angular location of a light-radiating object

A radiant energy detection system which allows angular location in bearing and elevation by using a simple detector element and a static mask preferably formed by a PLZT strip between two intersecting polarizers. A control circuit operates the mask to provide a configuration having a transparent zone equal to n measurement quadrants (n being a whole number equal to 1, 2 or 3) and an opaque zone equal to the remaining 4-n quadrants, and to bring about four successive operating states which are distinguished one from the next by an angular displacement of .pi./2 radians around the optical axis. The control circuit operates further a switching circuit synchronously with the mask to connect the output of the detector successively to four reception channels each of which includes a high speed memory. A measuring circuit calculates the angular location from the four detected signal values stored in the memory.

BACKGROUND OF THE INVENTION 
The present invention relates to a radiant energy detection system for the 
angular location of a light-radiating object. 
The invention finds a particular application in the field of the 
opto-electrical detection and the tracking of a target which emits light 
or is remotely illuminated and it is also applicable to automatic means 
for guiding a missile such as a semiactive homing head. 
In such systems, an optical receiver focuses the useful radiation from the 
light-radiating object to be located onto a detecting device. The detector 
is usually associated with a moving mask which has transparent areas on an 
opaque background in various forms such as grids, optical tracks, sectors, 
etc. The combination of the detector and mask is calculated to allow the 
requisite location data to be obtained by processing the detected signals. 
These data are in general the co-ordinates of the image of the object in 
the detection plane relative to two cartesian reference axes X and Y, that 
is to say the elevation and bearing of the object, which correspond to the 
aiming error between the optical line of sight of the receiver and the 
direction in which the remotely situated object lies in the observed 
field. 
There exist various embodiments of such systems, in which the detection 
device generally consists of a plurality of detecting elements. For 
example, an array of detectors lies in one of the reference directions X 
and Y and an array of slots or an optical track is movable in the second 
direction of measurement. The detection device may contain a single 
optical element provided that the mask incorporates a plurality of 
different optical tracks. 
According to other embodiments, the detector device comprises four 
detection quadrants and there is no associated mask, processing on the 
four detection channels being by sum and difference formation to enable 
the requisite divergence values to be obtained. 
An object of the invention is to provide a location system of low cost and 
fairly simple design which can be installed on board a moving object, such 
as a homing head for example, so as to operate in particular on a 
light-radiating object which may be formed by a source emitting light 
pulses. In this case, the system includes a detector which has only one 
photosensitive element and which is associated with a particular mask 
produced in a static form. 
In accordance with a feature of the invention, there is provided a radiant 
energy detection system for the angular location of a light-radiating 
object, combining: an opto-electrical receiver comprising an optical 
arrangement for focussing the radiation from the said object onto a 
photosensitive detector element centered on the optical axis through a 
mask device which is controlled by an associated electrical control 
circuit. There are also provided circuits for processing the detected 
signal to generate signals representing the divergences in bearing and 
elevation for the object being aimed at; the said mask being a static 
device controlled to present a configuration having a transparent area 
corresponding to n measurement quadrants (n being a whole number at least 
equal to 1 and at most equal to 3) and an opaque area formed by the 
remaining 4-n measurement quadrants. The control circuit provides four 
successive operating states which are distinguished from one another by a 
rotation of .pi./2 radians of the configuration around the optical axis, 
and the processing circuits including a switching circuit operated 
synchronously with the mask by the control circuit to connect successively 
the output of the detector element to four reception channels which supply 
a divergence measuring circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the simplified diagram shown in FIG. 1, which shows the main 
components forming a locating system according to the invention, the 
light-radiating object 1 is assumed to be situated in the reception field 
and to be radiating light waves in this field either directly or by 
reflection. These waves, or that fraction of the waves which is received 
by the optical receiver, form the useful radiation to be detected. The 
optical receiver consists of an optical focussing device represented by 
the objective 2. The detecting device 3 is positioned parallel and close 
to the corresponding focal plane so that the image of the object is formed 
on it as a spot of predetermined diameter. 
In accordance with the invention, the detector 3 consists of a single 
photosensitive element. This element is connected to pre-amplifier and 
amplifier circuits indicated at 4. 
The expected useful radiation, or at least the wave band in which it lies, 
is generally known. Consequently, a selection operation is performed on 
the received radiation so as to eliminate ambient interference radiation, 
or at least the major proportion thereof; this selection operation usually 
being performed by optical filtering using a filter device 5 inserted in 
the optical path. 
The detector 3 is preceded by a mask device 6 controlled by an arrangement 
7. The combination of 6 and 7 is so calculated as to cause sequentially at 
the detector 3, a law of discontinuous illumination determined from the 
four measurement quadrants. The receiver circuits downstream of the 
amplifier 4 being arranged accordingly, the principle of operation of 
elements 3 and 6 will first be explained. 
FIG. 2 shows the image of the object in the detection plane at C, the 
photosensitive area of the detector 3 defining the image of the observed 
field. This area may be circular as shown or may be of some other shape, 
such as rectangular for example. Point O represents the location of the 
optical axis Z of the system and OX and OY represent the reference axes 
for measurement. The co-ordinates ES and EG of the centre of the spot C 
represent the amounts of aiming error in elevation and bearing 
respectively. 
The mask 6 is designed to produce one of the configurations 6A, 6B and 6C 
shown in FIGS. 3, 4 and 5. To simplify the explanation, it will be assumed 
that the configuration 6A of FIG. 3 is produced which has a transparent 
zone corresponding to one measurement quadrant and an opaque zone 
corresponding to the three remaining quadrants. Mask 6A is operated 
sequentially by the control circuit 7 so as to assume four successive 
operating states which are distinguished from one another by a rotation of 
.pi./2 around the optical axis. These states are shown in FIG. 6 at the 
successive times to, to+T, to+2T, to+3T, the initial state being repeated 
at time to+4T and so on. 
If the corresponding amounts of detected light energy are called E1, E2, 
E3, E4 respectively, the divergences ES and EG are given by: ES = (E1 + 
E4) - (E2 + E3), and EG = (E1 + E2) - (E3 + E4), assuming that the useful 
radiation received does not vary, i.e. the sum ET = E1 + E2 + E3 + E4 
remains substantially constant. 
Consequently, the receiver circuits are arranged to produce four reception 
channels and to allow the aforementioned measurements ES and EG to be 
made. They include a switching device 8 which is supplied with the 
detected signal after it has been amplified in circuit 4, and which has 
four outputs. The switch 8 is operated by the control circuit 7 in 
synchronisation with the mask 6 so as to switch the detected signals S1, 
S2, S3 and S4, corresponding to the above values E1, E2, E3 and E4 onto 
successive ones of the four output channels. These signals are applied, 
via high-speed memory circuits 9 to 12 which may consist of circuits of 
the sample-and-hold kind controlled by the circuit 7, to a circuit 13 
termed a divergence measuring circuit to measure the divergences, which 
circuit produces signals representing the aiming error in elevation ES and 
in bearing EG. 
Block 14 represents the ancillary user unit which may consist of a display 
device or of tracking means to slave the sighting axis Z to the direction 
in which the object lies. Also shown are an automatic gain control circuit 
at 15 to control a variable gain amplifier at 4 from signals representing 
the total amount of detected energy ET, and a remotely situated emitter at 
16 which illuminates the object 1 when the latter does not have a source 
to emit useful radiation to be detected. 
It will readily be appreciated that the configurations 6B (FIG. 4) and 6C 
(FIG. 5) allow divergence measurements ES and EG to be made in the same 
way. In comparison with configuration 6A, twice as much energy is received 
in the case of 6B and three times as much as in the case of 6C in the 
course of a given sequence 4T. Configuration 6C is thus the most 
advantageous of the three. 
It will be clear from what has just been said and from the sequential mode 
of operation of the mask 6 that when, in addition, the useful radiation is 
emitted in a pulsed fashion with a period of T, the system is particularly 
beneficial if reception is synchronised with the incident light pulses. 
The mask 6 is produced in the form of a static device produced by means of 
ceramics which exhibit transparent or birefringent properties when 
subjected to an electrical field. Such ceramics cause the plane of 
polarization of incident light to be rotated by an angle which is a 
function of the locally established electrical field. The electrical 
fields are obtained by applying predetermined voltages to a circuit 
deposited on the surface of the ceramic. Ceramics known by the 
abbreviation PLZT have these properties. If a ceramic 20 of this nature, 
in the form of a PLZT strip for example, is assumed to be arranged between 
two polarizers 21 and 22 which intersect at .pi./2, the resulting 
operation is that briefly described below with reference to FIGS. 7 and 8. 
In the case of FIG. 7 there is no applied electrical field and the 
incoherent incident light 23 is polarized by element 21, the resulting 
polarized light 24 is not affected by the PLZT strip and is stopped by 
polarizer 22. By applying an electrical field E of predetermined magnitude 
as in FIG. 8, the plane of polarization of the light 24 is rotated by .pi. 
/2 and the light then passes through the second polarizer 22. Such an 
arrangement is described inter alia in the journal "Applied Optics" volume 
14, No. 18 of August 1975, on pages 1866 to 1873 on which appears an 
article "PLZT electro-optic shutters application" by J. Thomas Cutchin, 
James O. Harris and George R. Laguna. 
To enable the four measurement quadrants to be selected in space, the 
circuit deposited on the PLZT strip may be produced in the form shown in 
FIG. 9 by means of electrodes forming interlocking arrays. The electrical 
field in the quadrant or quadrants concerned is produced by a DC source 30 
and a combination of gate circuits 31 to 34 controlled by a circuit 35. 
The connections shown correspond to operation with the configuration shown 
in FIG. 3. A simple permutation of the inputs to gates 31 to 34 would 
produce the preferred mask configuration of FIG. 5. The four states shown 
in FIG. 6 are produced by operating the gate circuits 31 to 34 in 
succession by means of circuit 35 at times to, to+T, etc. The combination 
30 to 35 corresponds to the control circuit 7 in FIG. 1. The switching 
circuit 8 may be produced in a similar fashion by means of four gate 
circuits 41 to 44 which are operated in succession by circuit 35. The 
control outputs are shown separately from those intended for gate circuits 
31 to 34 to indicate that there are delays due to the upstream circuits 6, 
3 and 4. The gate circuits may for example be produced from field effect 
transistors. 
An embodiment of the control circuit 35 is shown in FIG. 10 and FIG. 11 
relates to the operating waveforms. Emission is assumed to be of the 
pulsed type. Synchronization between the actuation of the mask 6 and the 
reception of the useful signal at emission period T is achieved by 
applying the detected signal S5, after suitable amplification, to the 
control circuit 7 (the connection shown as a broken line in FIG. 1). In 
circuit 35, the signal S5 (FIG. 11a) is compared with a predetermined 
positive threshold VS1 in particular to allow for the noise level. The 
comparison takes place in a comparator 45 of the logic-output kind which 
emits a signal S6 (FIG. 11b). In addition, the signal S5 is applied to a 
differentiating circuit 46 whose output signal S7 (FIG. 11c) is compared 
with a negative threshold - VS2 in a second logic-output comparator 47. 
The comparison outputs S6 and S8 (FIG. 11d) are applied to an AND circuit 
48 whose output signal S9 (FIG. 11c) is applied to a first delay circuit 
49. The latter is calculated to produce a delay T1 equal to the emission 
period T less the response lag T2 resulting from the combination of the 
immobile mask 6, the detector 3 and the amplifier 4, the lag T2 being 
mainly due to the mask device 6. The value of T2 is also adjusted in such 
a way that the resultant signal S10 (FIG. 11f) is approximately central to 
the useful signal S5 subsequently obtained. A matching circuit, such as a 
monostable device, may be provided to adjust the length of this signal to 
that Ti of the emitted pulse. The signal S10 is applied to a logic circuit 
50 which counts up to four, followed by a four-output decoding circuit 51 
to identify the four successive values of count. The outputs of the 
decoder 51 control respective ones of the gate circuits 31 to 34 (FIG. 9). 
The signal S10 is also applied to a second delay circuit 52 where it is 
subjected to a delay equal to the aforementioned lag T2. The resulting 
signal S15 (FIG. 11g) is processed in the same way by means of a 
count-up-to-four circuit 53 followed by a decoding circuit 54 to produce 
the outputs for controlling the gate circuits 41 to 44 (FIG. 9) and the 
sample-and-hold circuits 9 to 12 (FIG. 1). In fact, additional delay 
circuits which are not shown may be inserted in the control connections to 
circuits 9 to 12 to allow for the delay due to gate circuits 41 to 44 and 
to ensure that the sample-and-hold circuits select the peak value of the 
useful signal S5. 
The form taken by the circuits 9 to 12 of FIG. 1 depends upon whether the 
mode of emission is continuous or discontinuous. The function of these 
circuits is to store the value of the detected signal until the next 
operating sequence, each sequence lasting for a period of 4T during which 
the mask assumes its four successive operating states. 
FIG. 12 is a diagram of an embodiment of the measuring circuit 13 of FIG. 1 
which, from the signals S11 to S14 representing the respective detected 
levels in the four operating states, produces signals representing the 
co-ordinates for the bearing divergence EG and elevation divergence ES of 
the object 1. These signals are produced by calculating the ratios: 
##EQU1## 
when the optical configuration of the mask is that shown either in FIG. 3 
or FIG. 5. These ratios are calculated by means of elements which, as 
shown, consist of differential amplifying circuits 70, 71 and 72, adding 
circuits 73 and 74, and dividing circuits 75 and 76. Elements 70 to 76 may 
easily be formed by means of integrated circuits. In fact the values of 
the aforementioned ratios when obtained, have been multiplied by a 
coefficient which corresponds to the gain of the systems represented by 
70, 71, 73 and 75 in the case of bearing. 
When the configuration of the mask is that of FIG. 4, the divergence 
signals are given by simpler formulae, the numerators becoming S11-S13 for 
the bearing divergence and S14-S12 for the elevation divergence, and 
circuit 13 is simplified to the extent that elements 72 and 73 are not 
needed under these circumstances. 
The signal S16 which is intended for the AGC circuit 15 (FIG. 1) to allow 
the receiver gain to be controlled may be formed by the sum output. The 
AGC circuit is produced by known techniques and operates by threshold 
comparison for example to produce a signal for controlling the gain of 
amplifier 4. 
The outputs ES and EG may be applied to slaving circuits 77, 78 to produce 
a desired technical effect, for example automatic tracking by adjusting 
the line of sight Z in directions X and Y. In the application to a homing 
head which is envisaged, the slaving circuits 77, 78 control members such 
as ailerons 79, 80 to control the direction of the missile. This control 
will be exercised in particular as determined by the type of operation 
selected, such as proportional navigation or tracking navigation. 
The static mask arrangement has further advantages. The range of control 
provided by the applied voltages allows one or more quadrants to be 
rendered opaque or transparent; all the quadrants may thus be rendered 
opaque or transparent. This property is useful in the case where the 
application is to a homing head since the whole of the mask device may 
thus be made transparent to produce an initial locking-on or acquisition 
phase. 
By way of example, FIG. 13 shows an embodiment of a locking-on arrangement. 
A sample-and-hold circuit 60 receives the detected and amplified signal S5 
and is controlled by the output S6 of the previously mentioned comparator 
45. The output signal S20 from the sample-and-hold circuit is compared 
with a positive threshold VS3 in a logic-output comparator circuit 61. 
Threshold VS3 is made lower than the threshold VS1 for comparator 45. 
Comparator 61 is followed by an inverter circuit 62 whose output is 
applied simultaneously to first inputs of four OR circuits 63 to 66. These 
OR circuits have second inputs which are supplied by respective outputs of 
decoder circuit 51 (FIG. 2). 
Operation is as follows: If signal S5 is lower than the detection threshold 
VS1, sample-and-hold circuit 60 is not actuated. If it is assumed that the 
signal S5 also fails to reach the threshold VS3, the OR circuits then 
receive a "1" signal at their first inputs. The result is that gates 31 to 
34 are all actuated simultaneously and terminals 26 and 29 of the mask 
(FIG. 9) all receive a supply, the mask being made completely transparent. 
As soon as the level of S5 exceeds VS1, circuit 60 is actuated and the OR 
circuits are then operated in succession by the corresponding "1" outputs 
from the decoder to cause the mask to operate normally. 
In the event of the lock-on being lost, that is to say when the useful 
signal drops below VS1 for a number of cycles T, circuit 60 is no longer 
actuated and signal S20 gradually declines in a discharge process. As soon 
as the level of S20 becomes lower than VS3, all the gate circuits 31 to 34 
are again operated by the outputs of OR circuits 63 to 66. This general 
actuation ceases when lock-on again takes place. The arrangement is made 
such that a loss of lock-on is recognized after a delay of at least four 
periods T. It may in fact be that, in the case of a mask as shown in FIG. 
3, the image spot forms in only one quadrant and there is no useful signal 
during three successive periods. The locking-on arrangement is prevented 
from operating at the wrong time by fixing the following parameters: the 
emission period T, the threshold VS3 and the selection of the sample and 
hold circuit 60. 
For applications to automatic target tracking, the emitter may be 
mechanically attached to the receiver and may move conjointly therewith. 
Another possibility is a separate emitter which is trained on the target 
independently by suitable means. Likewise, applications may be envisaged 
in which the emitter is on board the target and emits in a virtually 
omnidirectional or low-directivity pattern. In other applications, the 
emitter may be positioned in isolation from the receiver near to or 
remotely therefrom, the receiver being on board a moving vehicle which is 
to be steered toward a predetermined target. 
In conclusion, the respective positions of the emitter section and the 
receiver section depend mainly on the application envisaged and may take 
various forms among which are those described and those mentioned above. 
In certain of these embodiments, the system may possibly include means for 
generating a range-finding window so that the target is only detected 
within a restricted range band as far as the receiver is concerned. 
The term light-radiating object should not be considered as a restriction 
to the visible spectrum and it also covers, in particular, the infra-red 
range. 
The choice ceramic material for use in producing the static mask depends in 
particular on the spectral waveband intended for operation. The PLZT 
ceramics which were taken as an example allow operation in the visible and 
near infra-red spectrum up to 2 to 3 microns. 
It is also understood that the embodiment described is not to be considered 
as exhaustive and that it is capable of modifications conforming to the 
features of the invention and which also form part of the invention.