FR2885737A1 - Antenna system for mast of ship, has antennas with sub-antennas arranged to form simultaneous reception lobes having planes, and calculation device generating detection signal when one of correlation coefficients exceeds preset limit - Google Patents

Abstract

The system (1) has antennas with linear sub-antennas (2, 3) each comprising sensors (21-2M, 31-3N) arranged to form respective line portions and generating a basic signal. The sub-antennas are arranged in a manner to form simultaneous reception lobes having planes. A calculation device (8) calculates coefficients of time or frequency correlation between relevant combined signals of one portion and relevant combined signals of another portion of the same antenna. The device generates a detection signal when one of the correlation coefficients, for detecting a target, exceeds a preset limit.

Description

2885737 1

  SYSTEM OF CRUCIFORM ANTENNAS WITH LINEAR SUB-ANTENNAS

AND ASSOCIATED TREATMENT

  The invention relates generally to antennas for ships, and in particular to the structure of the antenna and the architecture of the processing of data from the sensors of such antennas when they are used in reception.

  It is known in the field of radar to use areal antennas with beamforming by calculation, intended to detect, locate and classify targets or sources. Such an antenna is generally made of a matrix comprising up to several thousand sensors arranged to form a rectangular flat surface. These sensors generally have an identical directivity diagram. This elementary directivity diagram does not have the resolution sufficient for the required performance of the antenna in localization. A beam generation device performs a combination (e.g., a linear combination) of signals generated by the sensors to form the required site and bearing directivities.

  Such an integrated antenna on a naval vessel has a very directional main reception lobe and is thus pivotally mounted to perform 360 detection around the building.

  Such an antenna has disadvantages. For a given precision of the location in site and in the field, this antenna is very expensive, does not have optimal monitoring capabilities, is difficult to integrate on the ship and significantly reduces its stealth.

  There is therefore a need for an antenna system solving these disadvantages. The invention thus relates to an antenna system comprising: a plurality of antennas each having: a first and a second linear sub-antennas: each having a plurality of sensors arranged to form first and second portions of lines respectively; each sensor generating a basic signal; the angle between respective directional vectors of the first and second tangents in the middle respectively of the first and second line portions being between 30 and 150; the sub-antennas of the different antennas being arranged so as to form a simultaneous reception lobe substantially including a plane; an antenna processing device forming several combined signals for each line portion, this signal being a combination of the basic signals of the sensors of this portion of line; a signal processing device generating combined signals useful in filtering the noise of the combined signals from each line portion; a device for calculating the correlation coefficients between the useful combined signals of the first line portion and the useful combined signals of the second line portion of the same antenna; a device generating a detection signal when a correlation coefficient exceeds a predetermined threshold.

  Alternatively, the system comprises a device for transmitting an electromagnetic signal capable of forming a simultaneous emission lobe substantially including said plane.

  According to another variant, the system further comprises a target detection device, comparing each calculated correlation coefficient with a predefined threshold when an associate. according to

  comprises associated detection, detecting and locating a correlation coefficient target exceeds the threshold another variant, the antenna system a signal processing device and correlation coefficients generating information relating to the detected target, said generated information including the distance, the site, the deposit or the speed of the target.

  According to yet another variant, the system comprises a device displaying the generated information.

  According to one variant, the sensors consist of emissive elementary sensors; the data processing device processing the combined signals according to the signal emitted by each sensor or each combined signal, this processing comprising for example a pulse compression.

  According to another variant, the system further comprises a transmitter, the data processing device processing the combined signals as a function of the signal emitted by the transmitter, this processing comprising, for example, pulse compression.

  The invention also relates to a mast including such an antenna system, the mast having a maximum dimension along an axis, the linear sub-antennas being disposed on surfaces of the mast so that said axis is perpendicular to said plane included in the receiving lobe of the sub-antennas.

  The invention also relates to a vessel incorporating such a mast.

  It can be provided that the emission device emits several electromagnetic beams simultaneously. It can be expected that the emission device emits a very wide beam in site and in deposit. It can be provided that the two antenna portions are rotated by plus or minus 45 in the plane of the two antenna portions.

  It can be provided that the correlation coefficient calculating device performs correlation calculations between useful combined signals of the first line portion and useful combined signals of the second line portion derived from basic signals measured simultaneously by the sensors. electromagnetic.

  Other features and advantages of the invention will become clear from reading the following description which is given by way of non-limiting example and with reference to the figures. These figures show: FIG. 1, a schematic representation of an example of an antenna and architecture of data processing from the sensors; Figures 2 and 3 are diagrammatic representations of ship masts including an antenna system according to the invention provided with several pairs of linear sub-antennas.

  A sensor will subsequently designate a device comprising one or more elementary sensors. A sensor having a plurality of elementary sensors generates a basic signal from the signals of the elementary sensors in a manner known per se.

  In order to improve the performance of a sensor, it is common to use a module comprising several sensors. The term sensor used in the following also covers a sensor module, because a sensor and a sensor module are functionally identical for the antenna processing.

  The antenna processing will designate a signal processing of the sensors which forms, by combining the signals of the sensors, signals called channels or beams which privilege a direction of propagation in the space of the physical quantity. The signal combinations mentioned later will for example be linear combinations of these signals.

  The invention proposes an antenna system comprising several antennas. Each of these antennas comprises at least two linear sub-antennas, each provided with sensors forming a line portion. The two line portions are defined as follows: the tangents are formed in the middle of each line portion. The angle between vectors of these tangents must then be between 30 and 150. The orientations of the portions of lines are thus sufficiently distinct for the antenna to retrieve sufficient information along two distinct axes considered as orthogonal. The antenna system has an antenna processing device that generates one or more combined signals. The antenna system has a signal processing device applied to the combined signals, which provides one or more combined signals useful. These useful combined signals are the combined signal processing results for extracting the noise, and are generated prior to the correlation processing. The antenna system further comprises a device for calculating the correlation coefficients between the useful combined signals of a linear sub-antenna with the useful combined signals of the other linear sub-antenna of the same antenna. Information in resolution is obtained by calculation rather than by increasing the number of sensors.

  A simplified example of operation of an antenna of the system will be described with reference to FIG. 1. The antenna 1 of FIG. 1 comprises two linear sub-antennas 2 and 3. The linear sub-antennas 2 and 3 each comprise several sensors, respectively 21 to 2M and 31 to 3N. Sensors 21 to 2M are arranged to substantially form a first line portion. The sensors 31 to 3N are arranged to substantially form a second portion of line.

  The first and second line portions of FIG. 1 satisfy the previously defined orientation condition: these line portions are in this case straight line segments orthogonal and in the same plane. The angle between the director vectors may be in a suitable range chosen by the skilled person. It can be envisaged that this angle is included in the following ranges: [400; 1400], [500; 1300], [600; 1200], [700; 1100], [800; 1000], [85; 95], or [89; 91]. The sensors 21 to 2M are in this case used for the determination of the site of a source or a target, while the sensors 31 to 3N are used to determine its deposit.

  These sensors comprise one or more elementary sensors, not illustrated, of the appropriate type. A sensor having a plurality of elementary sensors generates a basic signal from the signals of the elementary sensors in a manner known per se. Each sensor therefore generates a basic signal that can undergo a particular signal processing before the antenna processing. The sensors of a portion of line may have an identical directivity and be equidistributed on this portion of line. The sensors 21 to 2M respectively generate the basic signals S1 to SM illustrated by Si '. The sensors 31 to 3N respectively generate the basic signals G1 to GN illustrated by Gj '. Subsequently, the index i 'designate all the signals or numbers associated with a sensor 2i'. Thus the signal S4 is associated with the sensor 24. Similarly, the index j 'designates all the signals or numbers associated with a sensor 3j'.

  Thus, the signal G2 is associated with the sensor 32.

  An antenna processing device 4 forms a combined signal of the sensors of a line portion, in a manner known per se. The antenna processing device 4 thus generates the combined signals VSi associated with the signals Si '. An antenna processing device 5 forms a combined signal of the sensors of the other line portion, in a manner known per se. The antenna processing device 5 thus generates the combined signals VGj associated with the signals Gj '. The combined signals are intended, among other things, to form directivity lobes of the antenna used in reception.

  Each of the linear sub-antennas has a signal processing device processing signals from the antenna processing. This signal processing device provides one or more combined signals useful at the output of each linear sub-antenna.

  The signal processing devices 6 and 7 extract the useful signal from the noise, in a manner known per se. Devices 6 and 7 thus process respectively the combined signals VS 1 and VG 1 to generate the combined useful signals TS 1 and TG 1. The signal processing devices 6 and 7 may also be coupled to the antenna transmitting device if it is of the transceiver type or of another antenna if the antenna is of the receiver type only, in order to perform a processing taking into account the signals emitted in a manner known per se, such as pulse compression.

  The computing device 8 calculates the temporal or frequency correlation coefficients (depending on whether the processing has been carried out in the time or frequency domain) between the useful combined signals TSi of the first line portion and the useful combined signals TGj of the second portion. line. The matrix [Cij] of the correlation coefficients is thus formed. Details concerning the calculation of these coefficients are given later. The computing device 8 also exploits the correlation coefficients [Cij] in order to detect a target and generate a detection signal. One possible operation is as follows: a detection device (included in the calculation device 8 in the example) compares each correlation coefficient with a respective predefined threshold. When a given correlation coefficient is below its predefined threshold, we consider that no source or target is at the intersection of the two directivity lobes VSi and VGj, in the site i and the deposit j. When a correlation coefficient exceeds its predefined threshold, we consider instead that a source or target is at the intersection of the two directivity lobes, in the site i and the deposit j. A detection signal associated with the result of the comparison can thus be generated in the form of a binary value. The set of signals can then be arranged in a matrix [Rij]. The threshold is defined according to the desired performance of the antenna and the associated data processing device (including antenna processing, signal processing and information processing), in terms of probability of detection and false alarm.

  In the case of antenna processing known to those skilled in the art, if the antenna of FIG. 1 is of the transmission / reception type, the directivity diagram at the emission of the antenna is that of a lobe in FIG. cross shape and reciprocally the directivity pattern at the reception is the same as the emission. With the antenna structure presented, the combination of antenna and signal processing makes it possible to obtain the same information as that obtained by a surface-oriented, for example planar, antenna whose reception directivity lobe would also be fine. that the center of the cross formed by the directivity lobe. Moreover, again in the case of antenna processing known to those skilled in the art, if the antenna of FIG. 1 does not carry out correlation processing between the signals coming from the linear sub-antennas, the detection performances are those of sub-antennas alone. These performances are significantly lower than those obtained by the antenna of the invention.

  The processing device 9 may perform additional information processing steps, for example to improve the false alarm probability performance or to determine the speed, the distance of a target or any other useful information. The processing device 9 thus aims to make the information usable by an operator or a processing device. This device 9 receives input data such as the matrix [Cij], the matrix [Rij] or any similar data. All the determined information can be returned to the users by a suitable display device 10, known per se.

  According to the invention, the antenna system has several antennas as described above, arranged to form a simultaneous reception lobe substantially including a plane. The antennas can thus be fixed relative to the ship while covering the entire horizon around the ship. The horizon is covered simultaneously, which increases the ship's watch capabilities. For this, the skilled person can provide an adequate number of antennas each covering a sector, so that the meeting of these sectors include said plan. The antennas can in particular be distributed homogeneously around an axis and be included in respective separate planes. The planes preferably form angles greater than 30 between them. For example, it is possible to use 2, 3, 5 or 6 antennas in a system.

  In addition, the use of sub-antennas described above reduces the size of the mast supporting the antennas. Thus, the stealth of the ship can be improved.

  FIG. 2 illustrates a mast equipped with an example of antenna system according to the invention. The system has several antennas attached to respective faces of the mast. The mast 20 has 4 peripheral faces, each having two linear and rectilinear sub-antennas 132 and 133 as defined above.

  Figure 3 further illustrates a fixed antenna system mounted on the mast of a ship. The upper part 22 of the mast has a rectangular parallelepiped shape. Each side face of the upper portion 22 has a horizontal straight line portion 142. The lower portion 23 of the mast has four side faces of the same shape. Each side face of the lower portion 23 has a vertical rectilinear line portion 143. The lower side 23 of each face of the lower portion 23 may have a length of between 2 and 3 meters.

  There may be different limitations on the shape of the portions of lines. In particular, it can be envisaged that at least one portion of a line has a curved shape. It can be expected that such a curve has no point of inflection. It can also be envisaged that the variation of curvature is limited.

  It is thus possible to limit the curvature close to the middle of the portion of the line. We define L the length of the line portion and the curvilinear distance between a point and the middle of the line portion. For any point such that d / L <0,1, we can predict that the angle between a direction vector of the tangent at this point and a direction vector of the tangent in the middle is not included in the range [45; ].

  It is possible to provide for a portion of a line to be conformal, that is to say, to have a shape that conforms to the non-rectilinear shape of its support, and that a processing of the signals of the modules makes this portion of the line equivalent to a portion of straight line. In particular, such treatment may be applied to a portion of line fixed to the surface of the cabin, wing or tail of an aircraft. The treatment of compliant antennas is a technique known to those skilled in the art.

  The two line portions may be separated by any distance provided that the target or source is in the far field of the two sub-antennas which is defined by those skilled in the art for each subantenna as the ratio of the square of the rectilinear length of the antenna by the lowest wavelength used by the antenna.

  The two portions of lines can be arranged at a sufficient distance between them so that a coupling between their sensors is weak. But the two portions of line may be intersecting, there may be; either a sensor common to both portions of lines: this implies that the correlation coefficient for this sensor is reduced to its autocorrelation coefficient; or a hole in one of the two portions of the line: this case corresponds to lacunary antennas known per se to those skilled in the art.

  Although only these types of antennas have been illustrated in the different figures, it is also possible to envisage applying the invention to an antenna having a matrix of sensors, for example of rectangular shape. The matrix is then fractionated into subantennial portions as defined above. In particular, it is possible to delimit several rows and columns and to calculate correlation coefficients for several row-column pairs. We can also consider more than two portions of sub-antennas having orientations as defined above and not forming a matrix and calculate correlation coefficients for several pairs of these portions of sub-antennas. Correlation coefficient calculations for different couples can be cross-checked to improve antenna performance.

  In a sonar application, it is possible to use a passive antenna whose sensors are hydrophones or an active antenna whose sensors are transducers. The processing device forming the combined signal notably performs a channel formation function.

  In an application of the antenna to a radar, an antenna is used in reception and the sensors of the modules are adapted for capturing radar signals. The processing device forming the combined signal notably performs a beam forming function.

  To calculate the temporal correlation coefficient of complex video signals (for example TSi and TGj in the example of FIG. 1), which is particularly suited to a radar application, the coefficients of [Cij] can be calculated as follows: Let X (t) and Y (t) be complex, non-periodic, centered and stationary random signals of the second order. The correlation function of the two signals is defined as the expected expectation of the product of X (t) by the conjugate complex of Y (t-t), where i is the time difference between the two signals.

  correlationXY (r) = E [X (t) Y * (t - z)] = LX (t, w) Y * (t -r, w) dP (w) In the case of ergodic signals, the correlation function check the following equality: Itcorrelation (r) = limT * (t - r) dt T In practice the integral is calculated over a finite time interval that corresponds to the integration time.

  Those skilled in the art will be able to adapt the formulas to the case of periodic, non-centered signals or not verifying all the statistical properties mentioned previously.

  We define the normalized correlation function between the two signals: correlationxy (r) Jcorrelationxx (0). Jcorrelation ,, Y (0) The use of normalized correlation coefficients makes it possible to perform a target detection without worrying about the differences in levels between X and Y. Since the correlation function tends to zero when t, tends to zero. infinity, we consider in practice that the time shift t is bounded. For example, if t is within the time interval [- t max, t max], then there exists a value of t for which the normed correlation function reaches its maximum CxY, the maximum correlation coefficient between the two subtotals. Linear antennas.

  CxY = Cxy (r = ro) I = maxt-r man, r mac l 4CxY (Z) I] The time offset to is determined by the geometry of the antenna. In the case of two identical linear subantials intersecting at their center, the maximum CxY is reached for to = 0.

  CxY (r) = The maximum correlation coefficients Cij are obtained by replacing the random signals X (t) and Y (t) by the useful combined signals as previously defined TSi and TGj. The correlation coefficients Cij thus form a matrix [Cij] whose values are between 0 and 1.

  A maximum correlation coefficient value Cij greater than a predefined correlation threshold implies that at least one source or target is detected at the virtual intersection of the directivity lobes of the two linear sub-antennas 2i and 3j. In the case of FIG. 1, the presence of a source or target at the intersection of site i and deposit j is determined.

  Another calculation method, based on the exploitation of real combined signals, makes it possible to simplify the calculation step. The correlation coefficients are then determined, considering the correlation function in the following way: correlation x Y (r) _ (E bX (t) + Y (t -) I2 1- E bX (t) 12] - E bY (t) 12 Or again correlation x Y (r) = 4 (E bX (t) + Y (t - r) I2] - EX (t) - Y (t - r) IZ This method makes it possible to obtain the correlation coefficients directly from the signal powers by simply summing or subtracting.

  On the other hand, it is conceivable to exclude signals that are too weak from the detection. Thus, we can first calculate the denominator of the normed correlation function mentioned above, and compare it to a minimum threshold. When the denominator of the normalized correlation function is less than the minimum threshold, the corresponding correlation coefficient is not taken into account for the detection, which amounts to giving it a zero value. This can significantly reduce the integration time required for similar performance. Alternatively, one can also compare each threshold of the denominator to a respective threshold.

  To ensure an optimal result, it is desirable that the acquisition of the signals used for the correlation calculation is synchronous.

  Although a correlation calculation solution has been described in the time domain, it is also possible to perform the correlation coefficient calculations in the frequency domain, for example for an application of the antenna to a sonar. The correlation coefficients in the frequency domain can be determined from the coherence function defined as follows.

  The Fourier transforms of the correlation functions of two previously defined signals X and Y are the inter-spectral densities (or the spectral density of interaction).

  2885737 18 Fourier transform (xy correlation) (f) = S xy (f) Similarly, the Fourier transforms of the previously defined X and Y signal correlation functions are the power spectral densities of the X and Y signals. Fourier (correlationxx) (f) = SXX (f) Fourier transform (correlationyy) (f) = Syy (f) The coherence function of between X and Y is defined by cX ,, (f) = coherence (f) = The calculation of the coherence coefficients is generalized for all the analysis frequency bands Bf. In this case, the calculation of the coherence function becomes I, S XY (I, S). f) df cm, (f) = coherence XY (B f) = S x X (f) df S ry (f) df ff Antenna processing devices 4 and 5 can be expected to weight the basic signals of the sensors as a function of differences in directivity or sensitivity, before making the combination (for example linear) of these signals.

  Antenna processing devices may also include adaptive processing which has the function of eliminating a spurious signal, such as that from a scrambler or other processing that improves the functionality and performance of the antenna and associated data processing.

  The signal processing devices 6 and 7 of the combined signals can perform: bandpass filtering, Doppler or MTI filtering, pulse compression processing, or deviation measurement or other processing which improves the features and performance of the antenna and associated data processing.

  Although not shown, the antenna may include adequate data processing stages, providing information appropriate to the operators. In general, the calculation of the correlation coefficients will preferably be performed after an antenna processing step and a signal processing step. The calculation of the correlation coefficients will generally be followed by a step of thresholding and information processing.

  The information processing stages corresponding to the devices 8 to 10 in FIG. 1 serve, for example, to detect, locate or display the presence of a source or a target.

  In the case of discrete signals, the calculation of the correlation coefficients can be performed on a number N of samples of the useful combined signals. Those skilled in the art will determine the number of samples required based on the desired probabilities of detection and false alarm.

  For example, in the time domain, we consider 30 N temporal samples of the complex signals X and Y and it is assumed that the maximum CXY is reached for zo = 0.

NOT

E X (t) EY * (t) r = i

CXY __ N N

  IY (t) 2 t_, t_1 If the weak signals are eliminated by testing the denominator as described previously, then the number of N samples necessary for similar performance in probability can be significantly reduced. false alarm and detection.

  From the theoretical point of view, the calculation of the correlation coefficients is similar to a non-coherent integration that differs from the coherent integrations usually performed on the antennas. Non-coherent detection can be extended over a longer time than coherent integration. The secondary lobes associated with the treatment of the antenna of the invention are thus distributed randomly on the plane perpendicular to the central lobe and not deterministically.

  The method of testing the denominator of the correlation coefficient has made it possible in practice to reduce by 3 the number of samples required for a given level of performance.

  In order to ensure that the presence of a jammer at the same location as the target does not reduce the location performance of the antenna, the antenna can perform the following steps: locate the jammer and point to the jammer, measure the signal from jammer, subtract this signal from the signals measured later by the modules. The inclination of the linear sub-antennas, for example at 45 relative to their initial axis, also makes it possible to reduce the influence of a jammer on the measurements.

  Although the invention has proved particularly advantageous for radar sensors, it is of course conceivable to apply this invention to antennas whose elementary sensors are hydrophones, microphones, transducers, radioelectric, electromagnetic, ultrasonic sensors, accelerometers, optical or infrared.

  The invention can also be used in the underwater field to detect obstacles or underwater objects, or to provide an image thereof.

  For example, the invention can be used in the maritime field to detect or even provide an image of surface buildings.

Claims (9)

  1. Antenna system (1) characterized in that it comprises: -a plurality of antennas each having: -a first (2) and a second (3) linear subantennes 5.   each having a plurality of sensors (21-2M, 31-3N) arranged to respectively form first and second portions of lines, each sensor generating a basic signal (Si ', Gj'); the angle between respective directional vectors of the first and second tangents in the middle respectively of the first and second line portions being between 30 and 150; the sub-antennas of the different antennas being arranged to form a simultaneous reception lobe substantially including a plane; an antenna processing device (4, 5) forming several combined signals (VSi, VGj) for each line portion, this signal being a combination of the basic signals of the sensors of this line portion; a signal processing device (6, 7) generating useful combined signals (TS 1, TG 1) by filtering the noise of the combined signals from each line portion; a calculating device (8) of the correlation coefficients ([Cij]) between the useful combined signals of the first line portion and the combined signals 2885737 23 useful of the second line portion of the same antenna; a device (8) generating a detection signal ([Rij]) when a correlation coefficient exceeds a predetermined threshold.   An antenna system according to claim 1, comprising a device for transmitting an electromagnetic signal capable of forming a simultaneous transmission lobe substantially including said plane.   The antenna system of claim 1 or 2, further comprising a target detection device, comparing each calculated correlation coefficient with an associated predefined threshold, detecting and locating a target when a correlation coefficient exceeds the associated threshold. .   An antenna system according to claim 3, comprising a sensor signal processing device (9) and correlation coefficients generating information relating to the detected target, said generated information including the distance, the site, the deposit or the speed of the target.   Antenna system according to claim 4, comprising a device (10) displaying the generated information.   Antenna system according to any one of the preceding claims, wherein: the sensors consist of emissive elementary sensors; the data processing device processing the combined signals as a function of the signal emitted by each sensor, this processing comprising, for example, pulse compression.   Antenna system according to any one of claims 1 to 5, further comprising a transmitter, the data processing device processing the combined signals according to the signal transmitted by the transmitter, said processing comprising for example a compression pulse.   A mast including an antenna system according to any of the preceding claims, the mast having a maximum dimension along an axis, the linear sub-antennas being disposed on surfaces of the mast so that said axis is perpendicular to said plane. included in the receiving lobe of the sub-antennas.   9. A vessel including a mast according to claim 8.