WO1998055821A1

WO1998055821A1 - Method for detecting a projectile point or moment of impact

Info

Publication number: WO1998055821A1
Application number: PCT/FR1998/001111
Authority: WO
Inventors: Charles Dussurgey; Alain Renard; Bernard Maillet
Original assignee: Sextant Avionique; Secapem
Priority date: 1997-06-03
Filing date: 1998-06-03
Publication date: 1998-12-10
Also published as: IL133103A0; DE69810621T2; FR2764057B1; FR2764057A1; EP0986728A1; DE69810621D1; EP0986728B1

Abstract

The invention concerns a method for detecting a projectile point or moment of impact and a projectile for implementing the method. In order to determine the point of impact, for example, the projectile is equipped with a device designed to receive GPS signals and re-transmit them to a remote receiver. The method consists in determining in the remote receiver: the position of the mobile periodically; the moment (td) when the signals transmitted by the mobile re-transmission device disappear; and from at least one position measurement and the moment measurement (td), the position of the impact by extrapolation.

Description

METHOD FOR THE DETECTION OF THE POINT OR TIME OF IMPACT OF A PROJECTILE

The invention relates to a method and a system for detecting the point or instant of impact of a projectile. It is often useful to know the position of the point of impact of a projectile. This point of impact is usually the ground. However, it can be constituted by another mobile.

The position of the point of impact can be calculated from the characteristics of the projectile and the launch conditions. However, in some cases, this calculation is not easy or does not provide satisfactory results. For example, the trajectory of a bomb dropped from an airplane is difficult to calculate. In any case, it is always useful to know, preferably in real time, the exact position of the point of impact. For the training of personnel, a system has already been proposed for detecting the point of impact of a bomb dropped from an airplane which uses acoustic means. However, this system is not always reliable, in particular because the noise from the aircraft risks disturbing the detection of the noise caused by the impact.

COPY OF CONF-R AT-ÛI-J Goniometric location systems have also been proposed, using for example three rotating directional antennas, to locate from each antenna the direction from which a radio source seems to be emitting, and to locate the emission point by calculating the point of ^• intersection of the three directions identified. These direction finding methods are very imprecise and require several reception facilities.

The invention allows the realization of a detection system, in real time, of the point of impact of a projectile which does not have the drawbacks mentioned above and which is of good accuracy.

The system according to the invention is characterized in that the projectile comprises a device for the reception of GPS or similar signals and for the transmission of the received GPS signals to a remote receiver, and in that the receiver, for example on the ground , comprises means for periodically determining the position of the mobile from the GPS or similar signals received and for detecting the position of the impact by extrapolation from, on the one hand, the determination of the instant when the signals from the projectile, and, on the other hand, from the previous position measurements disappear.

Such a detection system is of course not sensitive to impact noise. The measurement period is for example between 100 s and 1 second. By GPS or similar signals is meant here signals which come from satellites or beacons, these signals being used to determine the position and the speed of mobiles by measuring the distance between this mobile and satellites or beacons whose positions are known. Each distance is measured from the time separating the instant of emission of a signal by a satellite or beacon, from the instant of reception of the same signal by the mobile. The signal transmitted by each satellite or beacon includes a carrier frequency, the value of which constitutes a parameter used for determining the speeds. It should be noted that today there are two satellite networks, one of which is assigned to the GPS system proper ("Global Positioning System") and the other to the GLONASS system ("Global Orbiting Navigation Satellite System"). The use of geostationary satellites and fixed ground beacons is also envisaged.

In each of these systems, the satellite emits a signal at a specific time, this instant being a datum transmitted by the satellite. The distance is proportional to the time separating the instant of transmission, provided by the signal from the satellite, from the instant of reception by the mobile.

In the GPS system, to which reference will be made hereinafter, each satellite transmits on a carrier frequency 1, ^ equal to 1 575.42 MHz as well as on an auxiliary carrier frequency 1-2 at 1227.06 MHz.

The carrier frequency, which is also used for determining the speeds, is modulated by a repetitive pseudo-random binary sequence according to a BPSK type phase modulation ("Binary Phase Shift Keying"). Each sequence constitutes a code which, in the case of GPS, has a length of 1023 bits. These codes are issued at a frequency of 1.023 MHz. They coexist with other codes issued at a frequency ten times higher: 10.23 MHz. The codes, called C / A, at 1.023 MHz allow in principle an approximate position determination while the codes, called P, at the frequency ten times higher, allow a precise localization.

The role of each code is threefold: on the one hand, it makes it possible to identify the transmitting satellite, on the other hand, it facilitates the detection and demodulation of the signals which are drowned in very high noise, the detection being carried out by correlation between the code received from the satellite and an identical code produced locally, and on the other hand finally, it is used to measure time shifts for the calculation of distances.

The carrier frequency Lτ_ is also coded by other binary data which represent, in particular, the date signal transmission. The satellite being identified and the date of emission of the signal being known, the position of this satellite at the time of the emission of the signal is then known with great precision. For the record, we will indicate here that in the system

GLONASS the carrier frequency L ^ constitutes the identification of the satellite. It therefore differs from one satellite to another. The frequencies L ^ range from about 1600 MHz to 1615 MHz. A pseudo-random code is also provided in this GLONASS system, but its usefulness is limited to extracting the signal in noise and determining time differences for measuring the distances between receiver and satellite.

The invention also relates to the determination of the instant of impact of a non-destructible projectile. As for the detection of the position of the point of impact, a device is provided on the projectile for receiving GPS or similar signals and for retransmitting these signals to a remote receiver, the remote receiver comprising means for determining, periodically, the position of the projectile. As this projectile and / or the device for receiving and re-transmitting GPS signals is not destructible during impact, the instant of impact is extrapolated from the position of the point of impact and at least one position previously measured. The extrapolation is, in all cases, necessary because the position determinations are not carried out permanently but periodically, the period being at least of the order of 100 ms. This sampling period is either fixed or variable. It is a function of the speed and / or acceleration characteristics of the projectile.

On the projectile, two antennas are preferably provided, namely an antenna for receiving GPS signals at the rear and a transmitting antenna arranged in such a way that the radiation pattern is preferably directed towards the front. The projectile being able to take orientations variables, in one embodiment, the antenna diagram is strongly omnidirectional.

In an embodiment intended for a training projectile, the latter has a plastic shell and the latter is shaped to participate in the emission radiation diagram. During experiments carried out within the framework of the invention, it has been found that with a transmitting antenna arranged at the rear, the plastic fuselage - which constitutes a dielectric - can increase the o ni-directionality of the diagram of radiation.

D • other characteristics and advantages of 1 • invention will appear with the description of some of its embodiments, this being carried out with reference to the accompanying drawings in which: - Figure 1 is a diagram of a projectile with an antenna for receiving and re-transmitting GPS signals,

FIG. 2 is a diagram similar to that of FIG. 1, but for two variants, FIG. 3 is a diagram showing an interpolation carried out to determine the position of the impact of a projectile,

- Figure 4 is a diagram of a device intended to equip a projectile whose position of impact is to be detected, and - Figure 5 is a diagram for the reception of signals supplied by a device of the type of that of the figure 4.

The example which will be described, in relation to the figures, relates to the detection of the position of the impact of a training projectile such as a bomb dropped from an airplane (not shown).

This drive bomb 10 (FIG. 1) has the general shape of a rocket with an elongated body 12 narrowed in the shape of a point 14 towards the front and fins 16 towards the rear. Since the path of the bomb 10 is from top to bottom, the antenna 18, intended to receive the signals from the GPS satellites, is arranged at the rear, that is to say at the top. In this example, a quadrifilar antenna made up of helically wound wires is provided. We know that such an antenna has good omni-directionality qualities. As a variant, an antenna of the "PATCH" type is used, formed of one or more conductors on a support. The GPS antenna is also used to retransmit signals to a ground station (not shown in Figure 1). Although the transmitting antenna is placed at the rear of the bomb 10 whereas it must transmit towards the front, this antenna gives, despite everything, good results because it has been found that the insulating plastic stiructure of the bomb 10 participates in the radiation pattern of the transmitting antenna. These results are due to a dielectric coupling.

The circuits 20 associated with the antenna 18 are located in the same housing 22 as the antenna 18, at the rear of the bomb 10, substantially along its axis.

In the variant shown in FIG. 2, which is preferably provided for transmission frequencies higher than approximately 2 GHz, the antenna 18 ′ for receiving GPS satellite signals is arranged in the same way as the antenna 18 for Figure 1 but the transmitting antenna, 24 or 26, is separate from the receiving antenna 18 '.

In a first embodiment, the antenna 24 has the shape of a belt surrounding the shell 10 at the front of the antenna 18 ', between the latter and the middle zone 28. Such an antenna allows an almost omnidirectional emission without variation phase or level even if the projectile rotates around its axis. One can also use a plurality of elementary antennas (or "patch") coupled.

In a second embodiment, an antenna 26 is provided near the nose 14 of the bomb 10, around the hull. This antenna 26 is formed of several elementary antennas coupled. You can also use a belt type antenna. In both cases, the layout of the antenna on the front conical part favors forward transmission. To determine, on the ground, the location of the impact of the bomb 10, using the signals retransmitted by the antenna 18, or by the antenna 24 or 26, the procedure is as follows:

A GPS signal receiver, which will be described later in connection with FIG. 5, periodically determines, for example every 200 ms, the position of the mobile and, as a function of the positions measured, the trajectory 32 of the mobile. In the example shown in Figure 3 the trajectory is in a vertical plane, the axis Oz being vertical and the axis Ox horizontal. For each point of the trajectory 32 calculated, the corresponding time is kept in memory as well as the speed and the acceleration.

The impact of the mobile on the ground will generally result in destroying the electronics for receiving and re-transmitting GPS signals. The instant of impact is thus determined by the disappearance of the signals received on the ground.

The detection of the instant of disappearance of the GPS signals is carried out using a circuit located either downstream of the processing circuits supplying the GPS signals or upstream of the processing, on the carrier of the received signal . In the first case (downstream of the processing circuits), a means is provided for permanently measuring the signal to noise ratio and the instant of impact corresponds to the instant when the signal to noise ratio drops to below a determined threshold. In the second case (upstream of the processing circuits), a circuit is provided which, before the processing, and before correlation, determines the level of the signal, for example the level of the automatic gain control and the instant of l impact is when the signal drops below a threshold predetermined, or when the automatic gain control exceeds a predetermined limit.

From the trajectory calculated during reception of the GPS signals on the ground and other calculated parameters (speed and acceleration), we can extrapolate the trajectory between the last instant t_ of measurement and the instant tçj of the disappearance of the GPS signals . This extrapolation therefore provides the position of the point of impact of the mobile on the ground. Of course the calculation and the extrapolation are carried out in real time. In a variant, when the receiver-transmitter circuits of GPS signals of the mobile are resistant and are therefore not destroyed by the impact on the ground, the invention makes it possible to determine in a simple manner the instant of impact. Indeed, in this case, the position of the impact is known. The extrapolation will therefore relate to the determination of time and not to the determination of position.

FIG. 4 represents an exemplary embodiment of reception and retransmission circuits on board the mobile.

In this example, the antenna 18 for receiving GPS signals supplies a signal to the amplifier 112. This amplified signal is applied to the first input 114 ^ of a member 114, such as a frequency change mixer, of which the second input 1142 receives a local signal supplied by the output 116 ^ of a generator 116. The local signal applied to the input 1142 has a frequency of 1580 GHz while the signal detected by the antenna 18 is the signal of the GPS carrier at the frequency 1575.42 MHz.

The signal appearing on the output 114 ₃ of the mixer 114 is at a frequency equal to the difference between the frequencies of the signals applied to the inputs 114! and 114 ₂ . This signal is applied to a second amplifier 118, then is transmitted to a filter 120 and, from there, is connected to the transmitting antenna 18, directed towards the GPS receiver on the ground.

Figure 5 shows the installation on the ground. It includes a conventional GPS receiver. Processing means allow the calculation of the trajectory and the instant of impact and, therefore, the location of this point of impact.

In the example shown, the installation on the ground comprises a reception antenna 142 whose signal is applied to a telemetry receiver 144 which performs conventional filtering and amplification functions. The output of the receiver 144 is connected to a circuit 146 for calculating the trajectory of the mobile. Furthermore, the installation comprises a so-called reference receiver 148 having its own antenna 150. In a manner known per se, the reference receiver allows, at each instant, the comparison between, on the one hand, the distance measured between this receiver on the ground and the satellite and, on the other hand, the distance, known a priori, between this receiver and the satellite. This comparison is used to correct the measurements provided by the mobile. This type of correction, which allows great accuracy in the location of the mobile, is called a differential GPS system. The reference receiver 148 has a clock output 148! connected to a corresponding clock input 146! of circuit 146. It also includes an output 148 ₂ providing the reference data on a corresponding input of a circuit 152 for calculating position, speed, acceleration and time, which, with circuit 146, makes it possible to calculate path. The circuit 146 further comprises an input 146 ₂ receiving a help signal (inertial for example), to improve the performance of attachment and tracking of GPS signals.

Claims

1. Method for determining the position of the impact of a projectile (10), characterized in that the projectile being equipped with a reception-emission device for, on the one hand, receiving positioning signals by satellite, emitted by a set of satellites, which are used to determine the position of the projectile by measuring the distance between it and the satellites whose positions are known, and, on the other hand, transmit the signals received to a remote receiver (142 - 152), the position of the mobile is determined in the remote receiver: - periodically,

- the instant (td) of the disappearance of the signals transmitted by the mobile transmission device, and,

- from at least one position measurement and from the measurement of the instant of disappearance of the signals, by extrapolation, the position of the impact.

2. Method according to claim 1, characterized in that the determination of the positions is carried out over a period of between 100 ms and 1 second.

3. Method according to claim 1 or 2, characterized in that the extrapolation used to determine the position of the impact also involves speed and acceleration.

4. Method according to any one of claims 1 to 3, characterized in that the instant of disappearance of the received signals is determined by comparison of these signals with a threshold, the instant of disappearance corresponding to the moment when a signal received falls below the threshold.

5. Method according to any one of the preceding claims, characterized in that the position determination signals are of the GPS type or the like.

6. Projectile for implementing the method according to any one of claims 1 to 5, characterized in that it comprises an antenna (18, 18 ') for receiving satellite signals which is arranged at the rear of this projectile.

7. Projectile for implementing the method according to any one of claims 1 to 5, characterized in that it comprises an antenna (18; 24; 26) for transmitting signals arranged in such a way that it emits signals towards the front of the projectile.

8. Projectile according to claim 7, characterized in that the transmitting antenna (18) is arranged at the rear and is coupled at the front by a dielectric.

9. Projectile according to claim 8, characterized in that the dielectric constitutes the shell of this projectile.

10. Projectile according to claim 7, characterized in that the transmitting antenna forms a belt (24) or a crown surrounding the projectile.

11. Projectile according to claim 10, characterized in that the transmitting antenna (26) is arranged at the front of the projectile.

12. Method for determining the instant of impact of a projectile or mobile, characterized in that the projectile or mobile comprising a reception-emission device for, on the one hand, receiving positioning signals by satellites, transmitted by a set of satellites, which are used to determine the position of the mobile by measuring the distance between the latter and the satellites whose positions are known, and, on the other hand, transmitting the signals received to a remote receiver, in this remote receiver we determine:

- periodically, the position of the mobile, and,

- from these periodic measurements and from the position of the immobilized mobile, by extrapolation, the instant of impact.