Abstract:
The invention relates to a method and device for producing an optical link using light flashes. According to the invention, an arrangement ( 13 ) comprising a plurality of capacitors (C 1  to C 5 ) is provided in order to power a flash lamp ( 3 ) and the connection of said capacitors within the arrangement is controlled in a programmed manner such that the resulting capacitance thereof increases from one flash in the next.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a method and a device for producing an optical link by light flashes between a flashlamp and a receiver that receives said light flashes, most particularly a link to be employed in devices for locating and guiding missiles, such as for example as described in document U.S. Pat. No. 4,710,028 (FR-2 583 523). 
   BACKGROUND OF THE RELATED ART 
   In known devices of this type, the flashlamp may be mounted onboard said missile or else placed in a fixed station, the optical link then including a mirror mounted onboard the missile and sending said light flashes back toward said receiver. 
   To be able to provide very long optical links, it is necessary for said flashlamp to be powerful, and it therefore consumes a large amount of energy. As a result, such a flashlamp is necessarily bulky and has to be provided with means capable of discharging the heat that it generates to the outside. 
   SUMMARY OF THE INVENTION 
   The object of the present invention is to remedy these drawbacks by providing an optical link by light flashes of low energy consumption using a flashlamp that is not very bulky and does not require any heat dissipation. 
   For this purpose, according to the invention, the method for producing an optical link, formed by a series of light flashes between a flashlamp and a device for locating a missile moving away from said locating device, each light flash being generated by an electrical discharge produced by capacitive means and applied to said flashlamp, is noteworthy in that:
         said capacitive means are produced in the form of an arrangement of a plurality of capacitors; and   the connection of said capacitors within said arrangement is controlled in a programmed manner in such a way that the resultant capacitance of said capacitive means increases from one flash to the next.       

   Thus, thanks to the present invention, it is possible to vary the power of said flashlamp as a function of time so that the power is small when the missile is close to the locating device and progressively increases with the flashlamp/locating device distance, the power acquired at the maximum range of the missile being delivered only at the end of the range. Such a variation therefore reduces the energy consumption by the flashlamp, thereby reducing the heat generated in the flashlamp and therefore its volume. 
   To implement the method according to the invention it is advantageous, on the one hand, for said capacitive means to comprise a plurality of circuit branches each comprising a capacitor and a controlled switch, said circuit branches being connected in parallel to one another between the anode and the cathode of said flashlamp, and, on the other hand, for the device to include:
         a high-voltage generator for supplying said capacitors; and   means for controlling said controlled switches, making it possible, on the one hand, to charge said capacitors from said high-voltage generator and, on the other hand, to discharge said capacitors into said flashlamp so that the electrical discharge corresponding to a flash is larger than the electrical discharge corresponding to the preceding flash and smaller than the electrical discharge corresponding to the next flash.       

   Preferably, said capacitive means include an additional capacitor connected in parallel to said circuit branches between the anode and the cathode of said flashlamp. 
   In one advantageous embodiment:
         the capacitors of said circuit branches which, from the first to the last, have different capacitances, are classified in order of increasing capacitance;   the additional capacitor has a lower capacitance than said first capacitor having the smallest capacitance among the capacitors of said circuit branches;   the first flash of the series results from the discharge of just the additional capacitor;   the second flash of the series results from the combined discharge of said additional capacitor and said first capacitor of said circuit branches;   the third flash of the series results from the combined discharge of said additional capacitor and said first and second capacitors of said circuit branches;   in general, the intermediate flash of index i of the series (i being an integer smaller than the total number of flashes in the series) results from the combined discharge of said additional capacitor and the i−1 first capacitors of said circuit branches; and   the last flash of the series results from the combined discharge of said additional capacitor and all of said capacitors of said circuit branches.       

   Thus, it is possible to obtain, at the end of the range of the missile, a high power for said flashlamp without the use of high-capacitance capacitors. 
   Preferably, the device according to the present invention includes programmed means that are programmed according to the trajectory of said missile and drive said means for controlling said controlled switches and determine the time interval between two successive light flashes. 
   Advantageously, such drive means are initialized, possibly with a delay, by the launch of said missile. 
   The time interval between two successive light flashes may be fixed and the same for all the flashes of the series. However, it may be varied, for example according to an exponential law. 
   In the usual case in which said flashlamp is of the type that includes a trip electrode, the device according to the invention includes a very high-voltage generator for supplying said trip electrode and this generator is controlled by said means for controlling said switches. 
   The figures of the appended drawing will clearly explain how the invention can be realized. In these figures, identical references denote similar elements. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates schematically a method of locating a missile. 
       FIG. 2  is the block diagram of one embodiment of the light-pulse emitter mounted onboard said missile. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows a device  1  capable of locating a missile  2  relative to a reference axis X-X (for example a line of sight), said missile  2  moving away at a constant speed from the locating device  1 . The latter is, for example, of the type described in document U.S. Pat. No. 4,710,028 (FR-2 583 523). 
   For the purpose of locating the missile  2  by the device  1 , said missile includes a flashlamp  3  capable of emitting light pulses  4  toward said device  1 , which includes a receiver for receiving said light pulses (device not being represented otherwise). 
   As shown in  FIG. 2 , the flashlamp  3  includes a grounded cathode  5 , an anode  6  supplied by a control device  7 , and a trip electrode  8  supplied by a very high-voltage generator  9  (for example one operating at the order of 10 kV). The flashlamp includes a cavity  10  that forms a reflector  11  and is closed off by an optical window  12 . In addition, the cavity  10  is filled with a rare gas, for example xenon. 
   The control device  7  mounted onboard the missile  1  comprises capacitive means  13 , a high-voltage generator  14  (for example operating at 500 V) capable of charging said capacitive means  13 , a control device  15  for controlling said capacitor means ( 13 ) and the very high-voltage generator  9 , a programmed synchronization-pulse generator  16  for driving the control device  15 , and an electrical power supply  17 , for example a thermal battery, for supplying the generator  14 , the device  15  and the generator  16 . 
   In the embodiment shown in  FIG. 2 , the capacitive means  13  comprise five capacitors C 1  to C 5  connected in parallel, the anodes of said capacitors being commoned, on one side, to the high-voltage generator  14  and, on the other side, to the anode  6  of the flashlamp  3 . 
   Moreover, the cathode of the capacitor C 1  is connected directly to ground, whereas the cathodes of the capacitors C 2  to C 5  are respectively connected to ground via switches I 2  to I 5  (for example MOS transistors) controlled by the control device  15 . 
   Preferably, the capacitance of the capacitor C 5  is higher than that of the capacitor C 4 , which is itself higher than that of the capacitor C 3 , which is itself higher than the capacitance of the capacitor C 2 , which is itself higher than that of the capacitor C 1 . For example, if the capacitance of the capacitor C 1  is equal to c 1 , the capacitances c 2  to c 5  of the capacitors C 2  to C 5  may be equal to 2c 1 , 3c 1 , 4c 1  and 5c 1 , respectively. 
   Since the capacitor C 1  is permanently linked to the generator  14 , it is charged by the latter. 
   Upon launching the missile  2 , a control command is sent to the programmed pulse generator  16  via a control line  18  in which a delay device (not shown) may optionally be interposed. In response to this control command, the generator  16  drives the device  15  so that it closes the switches I 2  to I 5  so that the capacitors C 2  to C 5  are charged by the generator  14 . When a switch is closed, it remains in that state. 
   Next, the operation of the device  7  and the flashlamp  3  takes place as follows:
         at time t 1 , the generator  16  emits a first pulse that is sent to the control device  15 . The latter actuates the very high-voltage generator  9  so that the trip electrode  8  is raised to this very high voltage. Since the voltage across the terminals of the capacitor C 1  is also applied between the cathode  5  and the anode  6 , it follows that an electric arc is generated between these electrodes, and said capacitor C 1  discharges. The power of this brief arc, which forms a first light flash, is therefore proportional to the electric charge accumulated in just the capacitor C 1 , which charge is itself dependent on the capacitance of this capacitor C 1 . At the end of said first light flash, the control device  15 , driven by the generator  16 , deactuates the very high-voltage generator  9  and closes the switch I 2  so that the capacitors C 1  and C 2  are recharged by the generator  14 ;   at time t 2 =t 1 +Δt 1  after t 1 , the generator  16  emits a second pulse and the control device  15 , which receives it, actuates the very high-voltage generator  9 . This results in the flashlamp  3  generating a second light flash, the power of which is proportional to the sum of the electric charges accumulated in the capacitors C 1  and C 2 . At the end of the second light flash, the control device  15  deactuates the very high-voltage generator  9  and closes the switch I 3 . The capacitors C 1 , C 2  and C 3  are then charged by the generator  14 ;   at time t 3 =t 2 +Δt 2  after t 2 , the generator  16  emits a third pulse and, in response, the control device  15  actuates the very high-voltage generator  9 . Thus, in a similar way to what was described above, the flashlamp  3  generates a third light flash, the power of which is proportional to the sum of the electric charges accumulated in the capacitors C 1 , C 2  and C 3 . At the end of the third light flash, the control device  15 , driven by the generator  16 , deactuates the very high-voltage generator  9  and closes the switch I 4 . The capacitors C 1 , C 2 , C 3  and C 4  are then recharged by the generator  14 ;   at time t 4 =t 3 +Δt 3  after t 3 , the generator  16  emits a fourth pulse and, in response, the control device  15  actuates the very high-voltage generator  9 . Thus, in a similar way to what was described above, the flashlamp  3  generates a fourth light flash, the power of which is proportional to the sum of the electric charges accumulated in the capacitors C 1 , C 2 , C 3  and C 4 . At the end of the fourth light flash, the control device  15  deactuates the very high-voltage generator  9  and closes the switch I 5 . The capacitors C 1 , C 2 , C 3 , C 4  and C 5  are then recharged by the generator  14 ; and   at time t 5 =t 4 +Δt 4  after t 4 , the generator  16  emits a fifth pulse and, in response, the control device  15  actuates the very high-voltage generator  9 . Thus, in a similar way to what was described above, the flashlamp  3  generates a fifth light flash, the power of which is proportional to the sum of the electric charges accumulated in the capacitors C 1 , C 2 , C 3 , C 4  and C 5 .       

   Thus it may be seen that, flash after flash, the power of said flashes increases up to a maximum when the missile  2  reaches its maximum range. 
   The times t 1  to t 5  and the time intervals Δt 1  to Δt 4  are programmed in the generator  16 . Said time intervals Δt 1  to Δt 4  may be constant and equal. However, they may be variable. 
   From the foregoing, it is readily understood that, thanks to the present invention, said flashlamp  3  is operated in a stable manner, with a programmed power variation, capable of preventing the flash detector (a CCD or CMOS sensor) from saturating at the launch of the missile. In addition, a substantial amount of energy is saved, enabling the range of the missile  2  to be increased for the same power. The substantial reduction in the heat generated by the flashlamp  3  that results therefrom makes it possible to reduce the size of the optical block associated with said lamp and to produce the latter from materials that are less critical, in respect of high-temperature resistance, than those currently used.