Method for detecting a missile equipped with surface elements made of metamaterial or "frequency-selective surfaces" materials

Abstract

The invention relates to a method for detecting a missile (1) equipped with surface elements (2) made of metamaterials or "frequency-selective surfaces" materials, a transmission pulse (4) being transmitted by means of a transmission antenna (8), a check being carried out as to whether the transmission pulse (4) is scattered by the missile (1), in the event of backscattering a radar pulse (4) being backscattered with a changed signal (SC) by means of controlling the surface elements (2), characterised in that a measurement signal is received by means of a receiving antenna (10), and by means of a power distributor (34) two receive signals (34) are generated from the measurement signal wherein: the receive signals are sampled and samples n=l..N of the receive signals are generated; the receive signals A(n)+C(n) and B(n)+C(n) each have a noise component A(n) and B(n), respectively, and each have, in the event of backscattering, an additional signal component C(n) influenced by the surface elements; the samples of the receive signals are first multiplied (24) and the multiplied samples are summed over a sliding time window, and a signal (formula I) is determined; an expected value (formula II) is determined; the expected value E{S} is compared with a threshold value; and it is established whether or not a common signal (SC) is present in both receive signals.

Description

[0001] “METHOD FOR DETECTING A MISSILE EQUIPPED WITH SURFACE ELEMENTS MADE OF METAMATERIAL OR FREQUENCY SELECTIVE SURFACES”

[0002] The invention relates to a method having the features of patent claim 1 and a radar system having the features of patent claim 15.

[0003] For several years, research and development in the field of frequency selective surfaces (FSS) and metamaterials (MM) has been conducted in electrodynamics. Applications include antennas and radar cross-section (RCS) modification. To date, the corresponding proposals for FSS and MM have tended to be static and passive. This means that the structures are designed for specific applications and cannot change their properties during operation. However, efforts are now emerging to modify the properties of FSS and MM during use, or to adapt them to a changing environment. Initial approaches in the civilian sector can be seen in the specialist literature. The specialist literature indicates that the reflection behavior can also be controlled using an applied control voltage, allowing larger areas to be covered.In addition, it can be seen from the specialist literature that the production costs for producing a unit of area will not be that high.

[0004] Many areas of application can be identified from the specialist literature, particularly in the military field. The problem can now be explained using Fig. 1 and Fig. 2. Fig. 1 shows a missile 1 with a surface element 2 made of adaptively controllable metamaterial. Furthermore, a search radar 3 is present, which emits transmission pulses 4 with a pulse duration 5, TP. The transmission pulse 4 is scattered by the missile 1 with the surface elements 2, producing a radar pulse 6 scattered by the flying object 1 with a signal SC with a signal value C as a random variable. The signal SC 6 has an expected value E{C] = 0 or E{C] # = 0 and is statistically independent and uncorrelated with the noise signals NA 22 and NB 23, which is the normal case in reality. The detection distance RD is designated by the reference symbol 7.Assuming that the surface of missiles 1, such as aircraft, drones, or rockets, is coated with a configuration of the aforementioned surface units 2, and that each surface unit 2 is supplied with a control voltage, the RCS can be changed during flight. Thus, depending on an approach scenario, the relevant RCS can be reduced in order to shorten the detection distance RD 7 to a search radar 3. Such measures to reduce RCS are generally referred to as "stealth" measures. If the RCS properties of the missiles are not changed, this can be referred to as a passive "stealth" principle. If, on the other hand, the RCS properties are adaptively changed, this can be referred to as an active "stealth" principle.

[0005] With the active "stealth" concept, i.e. the reflection behavior of the individual area units 2 is changed by control voltage, in addition to influencing the RCS, a further effect of reducing the detection distance can be achieved. By means of a varying control voltage, the reflection can be changed in amplitude and phase within the radar pulse duration TP 5 of the transmitted pulse 4. If all area units 2 are now controlled with noise-like control voltages, whereby there is no correlation between the individual control voltages, the scattered radar pulse 6 of the search radar 3 experiences modulation in amplitude and phase. In extreme cases, the scattered radar pulse 6 loses its similarity to the transmitted pulse 4. The search radar 3 thus receives the scattered radar pulse 6 and attempts to detect it from the device noise with the help of signal-adapted filters, so-called "matched filters".As long as the received radar pulse 6 is similar to the expected received pulse, the signal-to-noise ratio (SNR) at the output of the matched filter can be increased. This enables the search radar 3 to detect even small signal components from the noise.

[0006] If the scattered radar pulse 6 is now modulated by the control voltages to the point where it resembles a noise signal, the matched filters become ineffective. This prevents the SNR from being improved, thus reducing the probability of detection. This results in a reduction in detection distance, which gives the attacking missile 1 an advantage. Search radar systems 3, which use the pulse compression method, are particularly affected by this type of active "stealth," as they have a relatively long pulse duration TP 5. The same active "stealth" principle can also be applied to tracking radar systems.

[0007] From the generic document WO 2022 / 074 218 A1, a method for detecting a missile equipped with surface elements made of metamaterials or frequency selective surfaces is known, wherein a transmission pulse is transmitted by means of a transmission antenna, wherein it is checked whether the transmission pulse is scattered by the missile, wherein in the case of backscattering, a radar pulse with a modified signal is backscattered by means of a control of the surface elements. Two reception signals are received by means of two reception antennas, wherein the reception signals are sampled and samples n=1..N of the reception signals are generated, wherein the reception signals A(n)+C(n) and B(n)+C(n) each have a noise component A(n) andB(n) and in the case of backscattering each have an additional signal component C(n) influenced by the surface elements, whereby the samples of the received signals are first multiplied and the multiplied samples are summed over a sliding time window and a signal S = where is an expected value. C(n)} + E[A(n) ■ B(n)}, where the expected value E{S} is compared with a threshold value, determining whether a common signal (SC) is present in both received signals or not. The respective noise of the receive paths of the two antennas are uncorrelated and statistically independent.

[0008] This design has the disadvantage that decoupling the signals from the two antennas is complex. Furthermore, the two antennas require additional space and double the cost. Implementing this concept often involves limited space. Furthermore, mutual coupling can occur between two closely placed antennas. The antennas must be positioned directly adjacent to ensure that the signal can be received equally by both antennas. This coupling can be reduced with technical measures. This leads to greater design effort for the antenna construction.

[0009] The invention is therefore based on the object of providing a method and a radar system, wherein the search radar or the tracking radar, in addition to its previous capabilities, is capable of detecting the signal scattered by a target, which no longer has any similarity to the original transmitted signal, and of determining the location of the scattering body, while at the same time saving costs and installation space.

[0010] This problem underlying the invention is now solved by a method having the features of patent claim 1 and a radar system having the features of patent claim 15.

[0011] The method is used to detect a missile with surface elements made of metamaterials or "frequency selective surfaces" materials, wherein a transmission pulse is sent by means of a transmission antenna, wherein it is checked whether the transmission pulse is scattered by the missile, wherein in the case of backscattering, a radar pulse with a modified signal (SC) is backscattered by means of a control of the surface elements. A measurement signal is received by means of a reception antenna, wherein two reception signals are generated from the measurement signal by means of a power splitter, wherein the reception signals are sampled and samples n = 1...N of the reception signals are generated, wherein the reception signals A(n) + C(n) and B(n) + C(n) (20) each have a noise component A(n) andB(n) and in the case of backscattering each have an additional signal component C(n) influenced by the surface elements, whereby the samples of the received signals are first multiplied and the multiplied samples are summed over a sliding time window and a signal is determined, whereby an expected value. is determined, whereby the expected value E{S} is compared with a threshold value, determining whether a common signal is present in both

[0012] reception signals are present or not.

[0013] Only one antenna is used. This has the advantage of eliminating the effort required to decouple the two antennas. To still maintain two reception paths, Pa and Pb, the received measurement signal is divided equally between the respective reception paths using a power divider. This generates two reception signals. This eliminates the need for a complex construction for decoupling the two antennas, as is required in the prior art.

[0014] It is important that the power splitter exhibits low mutual coupling between the two paths Pa and Pb. The power splitter is an electrical component whose design and construction, due to its mode of operation and size, are better suited to reducing mutual coupling between the paths.

[0015] In a preferred embodiment, microwave isolators are integrated into the power divider. This improves the decoupling of the two power paths.

[0016] The power splitter can comprise a stripline arrangement. The stripline arrangement comprises at least two striplines with the lowest possible coupling. A stripline (or "microstrip") is a specific class of electrical waveguide. All striplines have one thing in common: they consist of one or more thin, conductive strips applied to a dielectric. Stripline structures can, for example, consist of conductive strips arranged in a plane. They are often arranged insulated in or above a metallic surface.

[0017] Furthermore, it is important to note that the power splitter should preferably be located, or in particular installed, directly at the base of the receiving antenna. The noise components of both the receiving antenna and the power splitter are kept as low as possible. This has the advantage that the noise components of the two receiving paths dominate.

[0018] The single-antenna design also reduces the installation space for the front end and saves the cost of an antenna. It is expected that the power splitter will cost less than a second antenna, as such power splitters are also produced in larger quantities for other applications.

[0019] The radar system is used to carry out the method and has a transmitting antenna, a receiving antenna, a power splitter, and an evaluation circuit. Transmission pulses can be transmitted by means of the transmitting antenna, and a measurement signal can be received by means of the receiving antenna. Two reception signals are generated from the measurement signal by means of the power splitter. The reception signals can be evaluated by means of the evaluation circuit as follows: the reception signals can be sampled and samples n=1...N of the reception signals can be generated. The reception signals A(n)+C(n) and B(n)+C(n) each have a noise component A(n) and B(n) respectively, and in the case of backscattering, each have an additional signal component C(n) influenced by the surface elements. The samples of the reception signals can first be multiplied and the multiplied samples can be summed over a sliding time window, and a signal S = value can be determined, whereby the expected value E{S} is comparable to a threshold value, whereby it can be determined whether a common signal (SC) is present in both received signals or not.

[0020] Only one receiving antenna with a power divider is used. Multiple such receiving antennas, each with a power divider, can be used. In particular, each direction finder can be equipped with a receiving antenna with a power divider. It is conceivable that the search radar's receiving antennas could also be equipped with a power divider.

[0021] A transmitting antenna, in particular an omnidirectional one, transmits pulses with a specific duration and at a specific carrier frequency. The carrier frequency and its bandwidth must be selected so that no external radar systems, radio services, or other artificial sources transmit in this frequency range. Furthermore, background radiation must also be taken into account in frequency planning. The pulse duration is calculated by multiplying a predetermined number of samples (N) 25 by a predefined sampling rate. These transmitting pulses can be coded or uncoded. The transmitting antenna is preferably located at the location of the search radar. The transmitting antenna can be part of the search radar. The transmitting pulses can thus be transmitted by a transmitting antenna of the search radar. Alternatively, an additional transmitting antenna can be arranged at a different location than the search radar.

[0022] The received signals generated by the power divider are preferably filtered and amplified separately and converted from analog to digital (A / D) 15, 16. The electronic components in the respective receive paths introduce noise into the received signals. This noise reduces the detectability of a radar pulse signal.

[0023] The A / D conversion is preferably operated by a common sampling frequency generator 17 so that jitter in the sampling clock does not influence the operation.

[0024] It can be assumed that the additive noise signals in the respective receive paths are statistically independent of one another, uncorrelated, and without DC components. This means that the respective expected values ​​of the N samples of the noise voltage values ​​A(n) and B(n), which are random variables, are zero: E{A(n)} = E{B(n)} = 0 , where n = 1 ... N. This is the normal case in radar receivers. After the A / D conversion 15, 16, the corresponding samples of the two paths, i.e., simplified for the nth sample Pa: [C(n) + Ä(n)] and Pb: [C(n) + B(n)], are first multiplied by one another 25 and summed over a sliding time window. The length of the time window corresponds to the number of samples N. With each summation process, the time window is shifted by one clock cycle, ie the oldest value is subtracted from the sum and the current value is added to the result.

[0025] The output signal of the summation is then fed to a threshold detector 18, which then decides whether a common signal SC of the scattered radar pulse was present in both received signals. The signal SC 6 is assumed to be statistically independent and uncorrelated with the noise signals. Furthermore, the expected value of the signal voltage C, which is also a random variable, is E{C] = 0 or E{C] = 0 .

[0026] If a signal SC 6 is received via antenna 10, a DC component is generated during multiplication. However, the formation of a constant DC component in the uncorrelated and statistically independent noise voltages of the two receive channels A and B is suppressed by the absence of the individual DC components in the noise.

[0027] The transmitter 9 transmits pulses with the selected pulse duration TP via the transmitting antenna 8 at specific time intervals. The pulse repetition frequency PRF is selected so low that the direction finders can reliably detect a pulse at all times. With this measure, the direction finders do not have to distinguish between pulses. Since only the target generates a modulated scatter signal SC6, a distinction can be made between the pulses of normal operation according to DE 102016015 107 B3 and the pulse scattered by the target. Therefore, such modulated pulses indicate the presence of an aircraft with an active "stealth" application. If the transmitting antenna 8 is placed at the same location as the search radar antenna from DE 10 2016 015 107 B3, the direction finders can distinguish between clutter and flying object by simultaneously evaluating the coded radar pulses known from DE 10 2016 015 107 B3 and the received signal SC 6.Radar pulses scattered by clutter objects, such as forests, mountains, or other obstacles, are still similar to the transmitted radar pulse. Clutter is present when both the method from DE 10 2016 015 107 B3 and the method presented here result in detection.

[0028] The direction of arrival of the signal SC, 6 is determined by each direction finder and communicated to the search radar via the existing information path described in DE 102016015 107 B3. The location of the flying object can be determined using beam methods as described in DE 10 2016 015 107 B3.

[0029] In the following, important properties of the process known from DE 10 2016015 107 B3 may be explained again, since these features are or may be part of the present process.

[0030] The method uses a search radar and at least one direction finder. Preferably, multiple direction finders are present. The direction finders and the search radar are not located at the same location, but rather at different locations. This allows the target to be located from multiple directions. The search radar transmits multiple pulses spaced apart in time with different codes and / or different signal shapes. The codes are randomly generated. The signal shapes are randomly varied.

[0031] The codes are generated by a random number generator. Codes can be N-digit binary numbers, for example. The search radar uses a random code and modulates the radar transmission pulse with this code. Specifically, it consists of 2 NFrom a possible N-digit binary number, one is randomly selected and the radar transmission pulse is modulated with this number using a digital modulation method. Thus, search radar has 2 N-M binary numbers to choose from. M is the number of binary numbers that have already been chosen within a certain time period. All codes within M pulse repetition intervals are preferably different, where M is greater than or equal to 2. Thus, if a binary number has been chosen and is used for a pulse repetition interval, it should only be selectable again after a certain time. If N is chosen to be large, e.g. N=7 and M=2, this results in 126 selectable binary numbers for each pulse repetition interval PRI. The already selected number is blocked for the duration of M=2 pulse repetition intervals, but can be selected again after the second pulse repetition interval. If the number N is sufficiently large, it can even be publicly known, i.e. the adversary is allowed to know it. On the other hand, M can be a secret fixed value or even determined using a random generator in the search radar. However, M > 1 must apply.With this measure, the pulse repetition interval is always assigned a unique number and all pulses belonging to this pulse repetition interval can be assigned to this pulse repetition interval and thus to a temporal validity.

[0032] As a further variant, each number can be assigned a specific signal shape. These signal shapes should preferably be chosen so that they are orthogonal to each other. This allows the receiver, with the help of a matched filter, to extract the specific, current signal shape from a multitude of other received signal shapes. This also allows multipath propagation effects to be processed to such an extent that the desired received signal can be separated in time from the other received signals.

[0033] A constant carrier frequency can be used for all transmitted pulses. If a change and / or fluctuation in the radar return cross-section requires the use of multiple transmitted frequencies to increase the probability of target detection, the number of carrier frequencies can be increased. A number of K=4 should be considered a typical nominal value. The use of multiple frequencies is also known as frequency diversity and serves solely to increase the probability of target detection in the case of strongly fluctuating radar return cross-sections. Since today's jammers are capable of simultaneously jamming a broad frequency band with a noise signal, frequency changes are no longer applicable as an electronic countermeasure. In this sense, only one transmitted frequency is sufficient to keep the radar system simple.The search radar generates a burst consisting of Q pulses, each of which is preferably modulated onto the carrier frequency with a different randomly selected binary number using a digital modulation method. The pulse repetition interval can be constant or can be changed from pulse to pulse within the burst duration. The decoy system, referred to below as the DRFM system, can only copy and modify a received radar transmission pulse when it arrives at the location of the DRFM system. If a DRFM system wants to simulate a search radar with a decoy signal or false target pulse that is spatially ahead of the actual target location of the target before the actual radar transmission pulse arrives, it must resort to the transmission pulses already received. If the search radar now changes the pulse parameters from radar transmission pulse to transmission pulse, the DRFM system is no longer able to simulate false targets at a distance shorter than the true distance to the target.This limits the simulation of false target distances to the spatial area behind the real target up to the instrumented range, ie the maximum clearly determinable distance.

[0034] The decoy system, or DRFM system, is typically installed on an aircraft and has a receiving and a transmitting antenna. This means that the DRFM system can physically only be located at a single, fixed location in space. The DRFM system can attempt to fool the search radar into thinking the target is at a different azimuth location through side-lobe jamming. Side-lobe jamming means that the search radar's receiving antenna receives a decoy signal via its antenna side lobes, but the radar assumes that the received signal was received via the antenna's main lobes. However, since the DRFM system's transmitting antenna can physically only be located in one place, a decoy attempt can be detected by determining the location of the source of the electromagnetic field. By combining the data, the search radar recognizes that the false target cannot actually be in the main lobe of the search radar's receiving antenna.As a side effect, it is also possible to distinguish between ground clutter and the target if the ground clutter is located in the same resolution cell as the target. In this case, too, the scattering sources are spatially separated in azimuth and elevation.

[0035] A particular advantage of the invention is the use of at least one, and in particular several, direction finders. These direction finders can be passive. The adversary may even know the location of the direction finders. Due to the physical condition that the source of the decoy signal, namely the DRFM transmitting antenna, is attached to an aircraft, the location can be unambiguously determined. Even if the DRFM system attempts to manipulate the phase front using antennas distributed throughout the aircraft, spatial deception is not possible or only possible under theoretical conditions due to the finite size of the aircraft and the use of several spatially distributed direction finders. In practice, these theoretical conditions are never achieved because many parameters, such as the locations of the direction finders or the orientation of the DRFM antennas, will never be coordinated.

[0036] A monostatic search radar is used, in particular, as a search radar. The direction finders operate asynchronously to the monostatic search radar. Precise time synchronization is not necessary, as the direction finders do not need to measure the propagation time of the radar pulse from the radar antenna via the target to the direction finder antenna. The direction finders and the search radar receive the received pulses scattered by the target or emitted by the decoy in the form of the measurement signal.

[0037] Through several queries, it can now be determined whether the received pulses are the desired useful signal or a decoy signal. First, it is checked whether the received pulses were received by the direction finder within a specific time window after transmission. The propagation time of a measurement signal between at least one direction finder and the search radar is determined, with the time window for receiving the received pulse being calculated based on the propagation time and the size of the area to be monitored. Received pulses that reach the respective direction finder outside the time window are not considered a useful signal. This time window is determined for each transmitted pulse and for each direction finder.

[0038] It is also checked whether the received pulses contain the corresponding code. If these two conditions are met, the direction of reception of the received pulse is determined using the direction finder, and the direction of reception is further determined using the search radar. By comparing the directions of reception, it can be checked whether the received pulses originate from a common target area. If the received directions can all be assigned to a target area, then it is not a decoy signal, but a useful signal. By three-dimensionally tracking an incoming electromagnetic field—namely, the corresponding received pulses and the bearing values ​​measured from them—the search radar can also distinguish between ground clutter and the target by combining all the bearing values.

[0039] Another option for the transmission signal is the use of a continuous wave (CW) signal. This allows for a longer integration time and thus increased sensitivity. However, all distance information is lost.

[0040] The disadvantages mentioned above are therefore avoided and corresponding advantages are achieved.

[0041] There are now numerous possibilities for designing and developing the method and radar system according to the invention. For this purpose, reference is made first to the claims subordinate to claim 1. The invention is explained in more detail below with reference to the drawing and the accompanying description. The drawing shows:

[0042] Fig. 1 shows a schematic representation of a missile and a search radar, in which a transmission pulse is scattered on surface elements made of metamaterial on the missile and received in the form of a scattered radar pulse by the search radar,

[0043] Fig. 2 shows a circuit arrangement for determining whether a

[0044] missile scattered radar pulse has been detected with a signal SC by a receiving antenna, and

[0045] Fig. 3 is a diagram showing a simulation result, where a

[0046] Signal S is recorded as a family of curves over a number of realizations.

[0047] As already described, in Fig. 1, a transmission pulse 4 with a signal SC 6 is scattered by a missile 1, whereby surface elements 2 made of metamaterial on the missile 1 generate a noise signal from the transmission pulse 4. To detect this, a receiving antenna is now used, which receives a measurement signal SC, 6.

[0048] Each direction finder and, if applicable, the search radar 3 is equipped with an antenna 10.

[0049] In addition to normal operation, as described, for example, in DE 10 2016 015 107 B3, an omnidirectional transmitting antenna 8 transmits transmission pulses with a specific duration TP, 5. The pulse duration is calculated from the number of samples N, 25 and the predefined sampling rate, which is known from the sampling frequency generation 17. The transmission pulses are generated using signal conditioning 9. These transmission pulses can be coded or uncoded. Furthermore, the transmission frequency may differ from that for normal operation. The transmission frequency and its bandwidth must be selected so that no external radar systems, radio services, or other artificial sources transmit in this frequency range. Furthermore, background radiation should also be taken into account in frequency planning. The transmitting antenna 8 is preferably located at the location of the search radar 3. The signal curve is shown schematically in Fig. 2. The device noise NA

[0050] 22 and NB 23 are modeled as additive noise, as is common in radar technology. The corresponding random variables A and B are statistically independent and uncorrelated.

[0051] The respective received signals are filtered and amplified separately via paths Pa, 12 and Pb, 13, and converted from analog to digital (A / D) via paths 15, 16. The electronic components in the respective receive paths introduce noise NA, 22 and NB, 23 into the received signals. This noise reduces the detectability of the signal SC, 6.

[0052] The A / D conversion 15, 16 is operated by a common sampling frequency generator 17 so that jitter has no influence on the operation.

[0053] It can be assumed that the additive noise signals NA, 22 and NB,

[0054] 23 in the respective receiving paths Pa, Pb are statistically independent of each other, uncorrelated, and without DC components. This means that the respective expected values ​​of the N samples of the noise voltage values ​​A(n) and B(n), which are random variables, become zero: E{A(n)} = E[B(n)} = 0 , where n = 1 ...N. This is the normal case in radar receivers.

[0055] After the A / D conversion 15, 16, the corresponding samples of the two paths, i.e. simplified for the nth sample Pa: [C(n) + A(n)] and Pb: [C(n) + B(n)], are first multiplied with each other 24 and summed over a sliding time window 26. The length of the time window corresponds to the number of samples N, 25. With each summation process, the time window is shifted by one clock cycle, ie the oldest value is subtracted from the sum and the current value is added to the result.

[0056] The output signal of the summation is then fed to a threshold detector 18, which then decides whether a common signal SC was present in both received signals. The signal SC 6 is assumed to be statistically independent and uncorrelated with the noise signals NA 22 and NB 23. Furthermore, the expected value of the signal voltage C, which is also a random variable, is E[C} = 0 or E[C} = £ 0 .

[0057] If a signal SC, 6 is received via antenna 10, a DC component is generated during multiplication 24. However, the formation of a constantly occurring DC component in the uncorrelated and statistically independent noise voltages NA and NB is suppressed by the absence of the individual DC components in the noise.

[0058] The transmitter 9 transmits pulses with the selected pulse duration TP 5 at specific time intervals. The pulse repetition frequency PRF is selected so low that the direction finders can always reliably detect a pulse. With this measure, the direction finders do not have to distinguish between pulses. Since only the target generates a modulated scatter signal SC 6, a distinction can be made between the pulses of normal operation according to DE 10 2016 015 107 B3 and the pulse scattered by the target. Therefore, such modulated pulses indicate the presence of a flying object 1 with active "stealth" application. Another embodiment of the transmitted signal is the use of a continuous wave (CW) signal.

[0059] The direction of incidence of the signal SC, 6 or the radar pulse 4 scattered by the missile 1 is determined by each direction finder and communicated to the search radar 3 via the existing information path described in DE 10 2016 015 107 B3. The location of the missile 1 can be determined using beam methods as described in DE 10 2016 015 107 B3.

[0060] In the following, the principle will be explained in more detail mathematically using expected values. This discussion serves to clarify the previous description and is limited to the essentials.

[0061] The noise signal NA, 22 has an expected value E{X(n)} = 0, where A(n) describes a random variable of the noise signal voltage of the n-sample. The noise signal NB, 23 has an expected value E{B(n)} = 0, where B(n) describes a random variable of the noise signal voltage of the n-sample.

[0062] Signal SC 6:

[0063] Signal voltage of the n-sample C(n) as a random variable with

[0064] Expected value E{C] = 0 or E{C] 0

[0065] The following always applies: Random variables A(n), B(n), and C(n) are statistically independent and uncorrelated, which is true in real systems. Expected value of signal S after summer 26:

[0066] Without signal SC, 6 :

[0067] With signal SC, 6 :

[0068] The situation will be illustrated graphically using a simulation (Figure 3). For the sake of simplicity, the following probability density functions (PDF) are assumed for the random variables.

[0069] Noise NA 22:

[0070] Furthermore: 0 < A c < 1

[0071] Clearly visible in Figure 3 is a free zone 33 between the sets of curves 30 without signal SC and 31 with signal SC for N > 4000. Figure 3 applies to a SNR per channel of -10.5 dB. Thus, the threshold can be placed in the free zone 33, and it can be determined whether a signal SC, 6 was received or not. Also shown is the curve of the expected value 32: E[S} = N ■ E{[C] 2} is drawn.

[0072] List of reference symbols:

[0073] 1: Missile

[0074] 2: Surface element made of metamaterial that is adaptively controllable

[0075] 3: Search radar

[0076] 4: Transmission pulse

[0077] 5: Pulse duration TP

[0078] 6: (Radar) pulse SC scattered on the flying object with the signal value C as

[0079] Random variable

[0080] 7: Detection distance RD

[0081] 8: Omnidirectional transmitting antenna

[0082] 9: Transmission signal processing

[0083] 10: Receiving antenna

[0084] 11: Transmission pulse duration generation and calculation

[0085] 12: Path Pa: Receiving unit (bandpass filter, amplifier)

[0086] 13: Path Pb: Receiving unit (bandpass filter, amplifier, .... )

[0087] 14: Delay adjustment unit to adjust the signal delays in the paths Pa and Pb

[0088] 15: Path Pa: Analog / Digital Conversion

[0089] 16: Path Pb: Analog / Digital Conversion

[0090] 17: Sampling frequency generation

[0091] 18: Threshold detector

[0092] 19: Path Pa: Schematic representation for the coupling of the additive noise signal NA

[0093] 20: Path Pb: Schematic representation for the coupling of the additive noise signal NB

[0094] 21: not assigned

[0095] 22: Path Pa: Noise signal NA with voltage value as random variable A

[0096] The noise signal has an expected value F£4} = 0 and is

[0097] Noise signal NB and signal SC statistically independent and uncorrelated 23: Path Pb: Noise signal B with voltage value as random variable B

[0098] The noise signal has an expected value E{B] ​​= 0 and is

[0099] Noise signal A and signal C are statistically independent and uncorrelated

[0100] 24: Multiplier

[0101] 25: N: Number of samples to be summed 26: Summing over a time window with N samples with output signal S

[0102] 27: Result whether a signal SC is present or not

[0103] 28: Signal S after the summer

[0104] 29: N: Number of summations 30: Family of curves without signal SC with 300 realizations

[0105] 31: Family of curves with signal SC with 300 realizations

[0106] 32: Expected value of S: EfS]

[0107] 33: Free zone for determining the threshold

[0108] 34: Power divider

Claims

Patent claims:

1. Method for detecting a missile (1) equipped with surface elements (2) made of metamaterials or frequency selective surfaces materials, wherein a transmission pulse (4) is transmitted by means of a transmission antenna (8), wherein it is checked whether the transmission pulse (4) is scattered by the missile (1), wherein in the case of backscattering, a radar pulse (4) with a modified signal (SC) is backscattered by means of a control of the surface elements (2), wherein a measurement signal is received by means of an antenna (10) and two reception signals are generated from the measurement signal by means of a power splitter (34), wherein the two reception signals are sampled and samples n=1..N of the reception signals are generated, wherein the reception signals A(n)+C(n) and B(n)+C(n) each have a noise component A(n) and B(n)+C(n) respectively.B(n) and in the case of backscattering each have an additional signal component C(n) influenced by the surface elements, whereby the samples of the received signals are first multiplied (24) and the multiplied samples are summed over a sliding time window and a signal S = value. is determined, whereby the expected value E{S} is compared with a threshold value, whereby it is determined whether a common signal (SC) is present in both received signals or not.

2. Method according to claim 1, characterized in that the power divider (34) is arranged, in particular installed, at a base point of the receiving antenna (10).

3. Method according to claim 1 or 2, characterized in that the length of the transmission pulses is determined from a predetermined number (N, 25) of samples and a predefined sampling rate by multiplication, and / or that an A / D conversion (15, 16) of the received signals is operated by a common sampling frequency generator (17).

4. A method for operating a radar system to prevent deception by third parties, wherein a connection is provided between at least one direction finder and a search radar, wherein the search radar (3) transmits a plurality of pulses with different codes and / or with different signal forms, wherein the direction finder and / or the search radar receive pulses (6) scattered at the missile (1), wherein it is checked whether these received pulses (6) are scattered at a target and thus form a useful signal or are formed by decoy signals, wherein reception directions of the received pulses are determined by means of the at least one direction finder and / or the search radar, wherein it is checked whether the reception directions can be assigned to a common target area and thus form a useful signal or are not assigned to a common target area and thus form a decoy signal,wherein the search radar (3) and / or the direction finder receivers are each equipped with a receiving antenna (10) and a power divider (34) for additionally carrying out the method according to claim 1.

5. Method according to claim 4, characterized in that the transmitting antenna (8) is part of the search radar (3).

6. Method according to one of the preceding claims 4 or 5, characterized in that the pulse repetition frequency (PRF) of the transmitting antenna (9) is selected to be so low that the direction finder receivers can always reliably register a radar pulse (6).

7. Method according to one of the preceding claims 4 to 6, characterized in that the codes are generated randomly.

8. Method according to one of the preceding claims 4 to 7, characterized in that all codes are chosen differently within M pulse repetition intervals, where M is greater than or equal to 2.

9. Method according to one of the preceding claims 4 to 8, characterized in that the signal shape is randomly selected and varied from transmission pulse to transmission pulse.

10. Method according to one of the preceding claims 4 to 9, characterized in that a constant carrier frequency is used for all transmission pulses.

11. Method according to one of the preceding claims 4 to 10, characterized in that it is checked whether the received pulse has been received by the direction finder and / or the search radar within a certain time window after the transmission of the transmitted pulse.

12. Method according to one of the preceding claims 4 to 11, characterized in that the propagation time of a signal between the at least one direction finder (21, 22, 23) and the search radar (20) is determined, the time window for receiving the received pulse being calculated on the basis of the propagation time and the size of the area to be monitored.

13. Method according to one of the preceding claims 4 to 12, characterized in that several direction finder receivers (21, 22, 23) are used.

14. Method according to claim 4, characterized in that a continuous wave signal (CW signal) is emitted by the search radar.

15. Radar system for carrying out the method according to one of the preceding claims, with a transmitting antenna (8), with a receiving antenna (10), with a power splitter (34) and with an evaluation circuit, wherein transmission pulses (4) can be transmitted by means of the transmitting antenna (8) and a measurement signal can be received by means of the receiving antenna (10), wherein two reception signals can be generated from the measurement signal by means of the power splitter (34), wherein the reception signals can be evaluated by means of the evaluation circuit, wherein the reception signals can be sampled and samples n=1..N of the reception signals can be generated, wherein the reception signals A(n)+C(n) and B(n)+C(n) each have a noise component A(n) and B(n)+C(n) respectively. B(n) and in the case of backscattering each have an additional signal component C(n) influenced by the surface elements, whereby the samples of the received signals are first multipliable (24) and the multiplied Samples can be summed over a sliding time window and a signal is worth can be determined, whereby the expected value E{S} is comparable to a threshold value, whereby it can be determined whether a common signal (SC) is present in both received signals or not.

16. Radar system according to the preceding claim, characterized in that the power divider (34) is arranged, in particular installed, at a base point of the receiving antenna (10).

17. Radar system according to one of the preceding claims, characterized in that the power divider (34) has a stripline arrangement.

18. Radar system according to one of the preceding claims, characterized in that microwave isolators are integrated into the power divider (34).

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