Method for extracting superimposed SIF mode and / or S mode IFF responses

Abstract

Method for extracting superimposed SIF and / or S-mode IFF responses The present invention relates to a method for extracting superimposed SIF and / or S-mode IFF responses in a basic signal received by an IFF interrogator, the method comprising the steps of: - detecting, in the basic signal, signals having predefined coding modes, to obtain a set of detected signals, - selecting, from the set of detected signals, the signal having the largest amplitude, - decoding the selected signal to obtain a bitstream for the selected signal, - reconstructing the selected signal, and - subtracting the reconstructed signal from the basic signal to obtain a restricted basic signal, repeating the previous steps by replacing the basic signal with the restricted basic signal until the set of detected signals is empty. Figure for abstract: 1

Description

Title of the invention: Method for extracting superimposed SIF mode and / or S mode IFF responses

[0001] The present invention relates to a method for extracting IFF responses of SIF mode and / or S mode superimposed in a basic signal received by an IFF interrogation equipment. The present invention also relates to an associated electronic extraction device. The present invention also relates to an associated computer program product.

[0002] IFF (from the English “Identification Friend or Foe” translated into French as “Identification Ami ou Ennemi”) designates all the systems allowing aerial surveillance and friend / foe identification for anti-fratricide.

[0003] IFF interrogation equipment, or IFF interrogators, is equipment capable of transmitting a signal, called an interrogation signal, to an aircraft, and of receiving in return a response signal from the aircraft, transmitted by its IFF responder. The interrogation signal consists, for certain modes, of a sequence of pulses, the time separation between two pulses defining a coding mode for the response signal. The response signal transmitted by the aircraft is then coded according to the coding mode defined by the interrogation signal. The coding modes commonly used are the SIF (Selective Identification Feature) modes which include military modes 1, 2 and 3, as well as civil modes A and C. Mode S is also widely used in civil use and modes 4 and 5, not covered by this invention, are for military use. The response signals corresponding to these coding modes are transmitted on the 1090 MegaHertz (MHz) frequency channel.

[0004] Nevertheless, the density of air traffic, as well as the coexistence of different modes on the same frequency channels, are the cause of interference in the signals received by an IFF interrogator. Such interference, called FRUIT (from the English "False Replies Un-synchronised In Time" translated into French as "False responses unsynchronised in time"), affects both signals having the same coding mode and signals of different coding modes. This interference reduces the probability rate of reception of the response signals.

[0005] Methods have been developed to reduce interference in signals received by fixed linear multi-antenna systems with electronic beam shaping. Such methods consist of applying digital processing to the signals received by each antenna of the antenna system. However, these methods are not suitable for systems receiving signals on the same reception path, and therefore in particular for the traditional rotating antenna system known as "Monopulse".

[0006] There is therefore a need for a tool for improving the probability of reception of a response signal by an IFF interrogator.

[0007] For this purpose, the present description relates to a method for extracting IFF responses of SIF mode and / or S mode superimposed in a basic signal received by an IFF interrogator, the method being implemented by an electronic extraction device and comprising the steps of:

[0008] - detection, in the basic signal, of signals having coding modes predefined, to obtain a set of detected signals, each detected signal being characterized by a coding mode, an estimated amplitude and an estimated start time instant,

[0009] - selection, from the set of detected signals, of the signal having the greatest amplitude,

[0010] - decoding of the selected signal according to the signal coding mode, the estimated amplitude for the signal, the estimated start time instant for the signal and the base signal, to obtain a bitstream for the selected signal,

[0011] - reconstruction of the selected signal as a function of the signal coding mode, the estimated amplitude for the signal, the estimated start time instant for the signal, and the obtained bit stream for the signal, and

[0012] - subtraction of the reconstructed signal from the base signal to obtain a base signal restricted,

[0013] repeating the steps of detecting, selecting, decoding, reconstructing and subtracting by replacing the basic signal with the restricted basic signal until the set of detected signals is empty.

[0014] According to particular embodiments, the method comprises one or more of the following characteristics, taken in isolation or in all technically possible combinations:

[0015] - the predefined coding modes are mode 1, mode 2, mode 3, mode A and mode C, all included in the name SIF mode, and mode S corresponding to an IFF interrogator.

[0016] - the basic signal is a sampled signal, the detection step comprising: • determining an adaptive threshold for detecting signals having an SIF coding mode, the adaptive threshold being determined for each sample of the basic signal as a function of the basic signal, • the detection, in the basic signal, of signals having an S coding mode, • updating the adaptive threshold based on the estimated amplitude for each detected signal of coding mode S, and

[0023] - q _ D[n] = apn • the detection, in the samples of the basic signal whose amplitude is greater than or equal to the updated adaptive threshold, of signals having an SIF coding mode.

[0017] - the decoding of each detected signal having a coding mode S, comprises: • the determination, for each bit expected for the coding mode S, of scores established on the basis of Rayleigh and Rice distributions applied to the basic signal on different temporal portions of the basic signal, and • assigning a value to each bit by comparing the scores determined for the bit so as to obtain the binary stream of the signal.

[0018] - the reconstruction step comprises estimating an estimated frequency for the selected signal, the reconstructed signal being obtained as a function of the estimated frequency, the estimation of the estimated frequency comprising: • the calculation, for each portion of the selected signal corresponding to a bit of the decoded binary stream of the selected signal, of the derivative of the instantaneous phase of the selected signal, • the deletion of calculated derivatives greater than a predetermined threshold,

[0019] the estimated frequency being the average of the remaining derivatives.

[0020] - the reconstruction step comprises the estimation, for each bit of the binary stream, of an estimated origin phase, the reconstructed signal being obtained as a function of the estimated origin phase for each bit, the estimation of the origin phases of the bits comprising: • compensation of the estimated frequency in the selected signal to obtain a frequency-compensated selected signal, • determining, for each bit, a weighted average for the selected frequency-compensated signal, the weightings being established on the basis of Rice distributions applied to the selected frequency-compensated signal over a time portion of the selected frequency-compensated signal specific to each bit of the selected signal,

[0021] the phase at the origin of each bit being the argument of the corresponding weighted average determined.

[0022] - the reconstructed signal associated with the selected signal is expressed in the form next: • for an S coding mode: +[ n ~(k + 8)N s ] • for a SIF coding mode: [002 4 ] [ n ] _ [ t -1.4 5kN s ] ex 2n ^

[0025] Or: i. ii. iii. iv. a denotes the estimated amplitude of a selected signal s[n], p'Fjjl denotes the estimated preamble of a selected signal s[n] having an S coding mode, where ps[n] = pj p0[n-3Ns] + po[u4À\]eÂ2^'0 / \| N denotes the number of bits corresponding to a selected signal s[n] having a coding mode S, fa denotes bit k of the selected signal s[n], P fi denotes the time shape of the signal corresponding to the bit g[n] denotes the time shape of a pulse of a selected signal s[n] having a SIF coding mode, vi. Ns denotes the number of samples of the basic signal y[n] in 1 microsecond, vii. f denotes the estimated frequency for the selected signal s[n], and viii. 0 denotes the phase at the origin of bit b. KK

[0026] - before each repetition, when the time interval corresponding to the signal reconstructed intersects the time interval of one or more previous reconstructed signals, the method comprises an additional processing step comprising: the selection of the reconstructed signal, called the weak signal, having the lowest amplitude among all the reconstructed signals cross-referenced, subtracting, in the restricted base signal from which the weak signal was obtained, the current reconstructed signal to obtain a doubly restricted base signal for the weak signal, repeating the decoding and reconstruction steps for the weak signal by replacing the restricted base signal of the weak signal with the doubly restricted base signal,

[0027] the reconstructed signal obtained replacing the weak signal.

[0028] The present description further relates to an electronic device for extracting superimposed S-mode and / or SIF-mode IFF responses in a basic signal received by an IFF interrogator, the device comprising:

[0029] - an IFF interrogator having a reception channel suitable for receiving the basic signal, And

[0030] - a processing unit suitable for implementing the steps of a method such as described previously.

[0031] The present description also relates to a computer program product comprising a readable information medium, on which is stored a computer program comprising program instructions, the computer program being loadable onto a data processing unit and causing the implementation of a method as previously described when the computer program is implemented on the data processing unit.

[0032] The present description also relates to a readable information medium on which a computer program product as previously described is stored.

[0033] Other characteristics and advantages of the invention will appear on reading the following description of embodiments of the invention, given by way of example only and with reference to the drawings which are:

[0034] [Fig.l], [Fig.l], a schematic view of an example of an electronic device for extracting superimposed S-mode and / or SIF-mode IFF responses,

[0035] [Fig.2], [Fig.2], a schematic view of an example of a method for extracting superimposed S-mode and / or SIF-mode IFF responses,

[0036] [Fig.3], [Fig.3], an example of a signal having an S coding mode, and

[0037] [Fig.4], [Fig.4], an example of a signal having an SIF coding mode.

[0038] Figure 1 illustrates an electronic device 10 for extracting IFF mode S and / or SIF mode response signals superimposed in a basic signal yj received by an IFF interrogator.

[0039] The electronic device 10 comprises an IFF interrogator 12 and a processing unit 14. The IFF interrogator 12 and the processing unit 14 are, for example, close to each other in the same block. Alternatively, the IFF interrogator 12 and the processing unit 14 are remote from each other and communicate via wireless links.

[0040] The IFF interrogator 12 is typically on the ground. The IFF interrogator 12 has a receiver with at least one signal reception channel. The signals that the present method proposes to extract are received on the same reception channel of the IFF interrogator 12.

[0041] The processing unit 14 comprises, for example, a calculator interacting with a computer program product. The processing unit 14 is, for example, a computer.

[0042] The calculator comprises, for example, a processor comprising a data processing unit, memories and an information medium reader, as well as possibly a human-machine interface and a display.

[0043] The computer program product comprises an information medium.

[0044] The information medium is a medium readable by the computer, usually by the data processing unit. The readable information medium is a medium suitable for storing electronic instructions and capable of being coupled to a bus of a computer system.

[0045] For example, the information medium is a USB key, a floppy disk or flexible disk (also known as a "Floppy disk"), an optical disk, a CD-ROM, a magneto-optical disk, a ROM memory, a RAM memory, an EPROM memory, an EEPROM memory, a magnetic card or an optical card.

[0046] The computer program comprising program instructions is stored on the information medium.

[0047] The computer program is loadable onto the data processing unit and is adapted to cause the implementation of a method for extracting superimposed SIF mode and / or S mode IFF responses, when the computer program is implemented on the processing unit of the computer. Such an extraction method will be described in the remainder of the description.

[0048] The operation of the electronic device 10, implementing a method for extracting superimposed SIF mode and / or S mode IFF responses, will now be described with reference to the flowchart of [Fig. 2], as well as to the examples of FIGS. 3 and 4.

[0049] The extraction method comprises a step 100 of receiving a basic signal y{nl on the same reception channel of the IFF interrogator 12.

[0050] The basic signal y[n] is a complex signal reported in baseband sampled with a sampling period Ts, Ns designating the number of samples over a period of 1 microsecond. Generally, in the remainder of the description, the notation x[n] designates a sample n of the signal x.

[0051] The IFF interrogator 12 operating in reception at the frequency of 1090 MHz, it is capable of receiving signals having an S or SIF coding mode. The received signals are potentially superimposed (also called interference) in the basic signal y[n]. Other parasitic signals (receiver noise) are also likely to be superimposed in the basic signal y[n].

[0052] An example of a signal having a coding mode S is illustrated in Figure 3. Such a signal has a preamble ps(t) extending over 8 ps and a payload whose extent is such that the sum of the extent of the preamble and the payload is equal to 64 ps (short response) or 120 ps (long response). The payload comprises different signal sections, namely a DF portion between 8 and 13 ps (coded on 5 bits), a main portion between 13 and 40 ps for short responses (coded on 27 bits) or between 13 and 96 ps for long responses (coded on 83 bits), and a CRC code between 40 and 64 ps for short responses (coded on 24 bits) or between 96 and 120 ps for long responses (coded on 24 bits). The time of a bit is 1 ps. Each bit is digitally modulated by Pulse Position Modulation (or PPM). Thus, the waveform pd A 1 corresponds to a pulse of width 500 ns followed by a 500 ns silence. The waveform p[j2 of a bit at 0 corresponds to a 500 ns silence followed by a 500 ns wide pulse. Furthermore, the preamble ps(t) is composed of two Pp pulses a 1 ps silence, followed by two Pq pulses followed by a 3 ps silence.

[0053] An example of a signal exhibiting an SIF coding mode is illustrated in [Fig.4]. Such a signal has 15 pulses of 450 nanoseconds (ns) each, spaced 1 ps apart for a total duration of 20.75 ps. The end pulses, denoted F1 and F2, are always at one. The data is carried by the remaining 13 bits.

[0054] The extraction method comprises a step 110 of detecting, in the basic signal yfn], signals having predefined coding modes, to obtain a set of detected signals. At the end of the detection, each detected signal is characterized by a coding mode, an estimated amplitude a, and an estimated start time instant y. The estimated start time instant y typically corresponds to a sample number among the samples forming the basic signal y[n]-

[0055] Preferably, the predefined coding modes are the S or SIF coding modes described previously.

[0056] In a preferred embodiment, the detection step 110 comprises a sub-step of determining an adaptive threshold p[n] for the detection of signals having a SIF coding mode. The adaptive threshold p[n] is determined for each sample n of the basic signal y

[73] and as a function of the basic signal yfn]-

[0057] The adaptive threshold p[ n] was determined so as to filter signals having an S coding mode when detecting SIF coding mode signals. This makes it possible to reduce the risk of mistaking an S coding signal for an SIF coding signal.

[0058] The adaptive threshold p[ n ], for sample n, is, for example, given by the formula next: 100591

[0060] Where: * M p [n] = max niji} i^n-2UV S +1, n]

[0061] mn[n]= min P[f] , 1 j^nfn¥2XNs+^

[0062] 2 r n2 P[n]=^2 J I =n ^ * y[JÎ] denotes the base signal for sample n, and • Ns denotes the number of samples of the basic signal y[jj] in 1 microsecond.

[0063] In this embodiment, the detection step 110 then comprises the detection, in the basic signal yjn] of signals having a coding mode S. The detection of signals having a coding mode S is, for example, carried out by an algorithm for recognizing the shape of the preamble of the mode S signals in the basic signal ^221- An example algorithm is described in the following article: François Le Neindre, Guillaume Ferré, Dominique Dallet, Emilie Boulanger, Frankie Letellier. Aircraft Signal Detection in Heavy Co-Channel Interference Environment. 2020 IEEE Latin-American Conference on Communications (LATINCOM), Nov 2020.

[0064] In this embodiment, the detection step 110 comprises updating the adaptive threshold as a function of the estimated amplitude a for each detected signal of coding mode S. The update makes it possible to readjust the threshold in the case where the detected mode S signal has not been filtered by the initial threshold. The updating of the threshold consists, for example, in adjusting the amplitude of the adaptive threshold with the amplitude of a detected mode S signal when the amplitude of such a signal multiplied by a predefined coefficient is greater than the initial threshold.

[0065] The updated adaptive threshold p '

[12] , for sample n, is, for example, given by the following formula:

[0066] p[n] = max{p[n];cil} (2)

[0067] Where: • iT J 1 denotes the function relating to the samples included in the interval [ ^, <5f + ( N + 8 ) Ns - 1 ] ' • $ denotes the estimated start time instant of the preamble of the signal of S mode detected, and • Ct denotes the estimated amplitude for the detected mode S signal, • N denotes the number of bits of the detected mode S signal, and • Ns denotes the number of samples of the basic signal y[n] in 1 microsecond.

[0068] In this embodiment, the detection step 110 comprises the detection, in the samples n of the basic signal y

[12] whose amplitude is greater than or equal to the updated adaptive threshold p'[n], of signals having a coding mode SIF. Detection is, for example, carried out on the basis of a correlation, in particular by exploiting the end pulses of a SIF mode.

[0069] The SIF mode detection step is performed on the complex modulus of the basic signal j^î] • This modulus is also forced to zero when it is below the updated adaptive threshold p'[n]. This is translated by the following formula:

[0070] .iM^ll si|y[«]l >P'[n] yp[n] = 0 otherwise (3)

[0071] Then, a normalized correlation is performed by considering two spaces of the width of a pulse of a SIF mode, on each side of a pulse. This correlation p[ n] is given by the following formula:

[0072] i M (4) if=n4+1w

[0073] Consequently, to detect a pair of F1-F2 (end pulses), the geometric mean between two instants of V [ n] for the offset Fl-F2, i.e. 20.3 ps, is calculated. The geometric mean p[u] is given by the following formula:

[0074] p,[n] = ^[n]p[n+[20.3NsJ] (5)

[0075] Where: • [J denotes the integer part of a real number.

[0076] For example, a threshold / -sucst is set for P'[II] for the detection of SIF mode signals, this threshold making it possible to avoid excess false detections, while maintaining a satisfactory correct detection rate. The threshold XSiFest, for example, equal to 0.93.

[0077] Furthermore, the amplitudes of the end pulses F1-F2 are estimated by taking the average of the amplitudes of their corresponding samples. If the ratio between the two obtained amplitudes is not between 10 and 10, the detection is rejected, which makes it possible to reduce false detections.

[0078] In a variation of this preferred embodiment, the SIF or S mode signals are detected without determining a prior adaptive threshold for the SIF modes, for example, by cross-correlations such as those described previously.

[0079] The extraction method comprises a step 120 of selecting, from the set of detected signals, the signal having the greatest amplitude.

[0080] The selection step 120 is performed on the basis of the estimated amplitude and the estimated start time instant y for the signals detected during the selection step detection 110. It consists of selecting the detected signal having the largest estimated amplitude a. This signal is then called the selected signal s[n].

[0081] The extraction method comprises a step 130 of decoding the selected signal s[n] as a function of the coding mode of the signal, the estimated amplitude a for the signal, the estimated start time instant y for the signal and the basic signal to obtain a binary stream for the selected signal s[n].

[0082] The binary stream designates the set of values ​​of the bits coding the signal in a function of the signal coding mode (15 bits for SIF mode, 56 bits for the useful signal in short S mode, 112 bits for the useful signal in long S mode). Thus, at the end of the decoding step 130, a sequence of zeros and ones is obtained for each signal.

[0083] Preferably, the decoding is performed differently depending on the coding mode of the signal. In particular, the decoding of an S mode signal is different from the decoding of an SIF mode signal.

[0084] In an exemplary implementation, the decoding of a SIF mode signal is performed on the basis of an on-off keying demodulator. The estimated amplitude a of the signal considered is in this case obtained by averaging the amplitudes of the end pulses F1 and F2. Then, at each time position of a pulse, the amplitude of the pulse is estimated by taking the average of the corresponding samples. If the estimated amplitude is greater than Q'2O', the corresponding bit is estimated at the value 1, and otherwise at the value 0.

[0085] An example of decoding a SIF mode signal is given in M. Stevens, Secondary Surveillance Radar, ser. Artech House radar library. Artech House, 1988.

[0086] In an exemplary implementation, decoding of a Mode S signal is performed in accordance with the method presented in the RTCA standard document, “DO-260B (2009) Minimum Operational Performance Standards for 1090 MHz Extended Squitter Automatic Dependent Surveillance-Broadcast (ADS-B) and Traffic Information Services-Broadcast (TIS-B).

[0087] In another exemplary implementation, decoding an S-mode signal comprises the following substeps.

[0088] A first sub-step concerns the determination, for each bit bk expected for the coding mode S, of scores established on the basis of Rayleigh and Rice distributions applied to the basic signal yjn] on different time portions of the basic signal yfn] - The expected bits bk are determined according to the coding mode of the signal. The different time portions of the signal are defined due to

[0089]

[0090]

[0091]

[0092]

[0093]

[0094]

[0095]

[0096] the specific pulse shape of the useful part of a mode S signal (due to binary PPM modulation). In an example, the scores for each bit bk are given by the following equations: S o W = ; + [ i ] & cr) (6) C(z[i]^ aj+^Mi]; a) 1—lî. l V yi - (7) Or: z[n] = | * yM denotes the base signal for sample n, • a denotes the standard deviation of the complex white Gaussian noise, specific to the receiver 12, • a denotes the estimated amplitude of the mode S signal, * R(z [ 11 ] ; O') denotes the Rayleigh distribution of Z [ H ] for the parameter a, with: K(z[n];d) = ^exp(-^J-)' • C(z[n];a, a) denotes the Rice distribution of Z

[22] for the parameters a and ° with: * b / gMa\ C(ZLnj;a7 a; — — exp^--”^^ I Q (x)=if Q e xcose of- A second substep involves assigning a value to each bit bk by comparing the scores determined for bit bk so as to obtain the estimated bit stream of the signal. The assignment of the values ​​0 or 1 to each bit bk is, for example, performed on the basis of the following conditions: , fo si S Q (k) > S^k) (8) b k =\ i ■ 1 smon. The extraction method comprises a step 140 of reconstructing the selected signal s[n] as a function of the coding mode of the signal and depending on the estimated amplitude a for the signal, the estimated start time instant y for the signal, the binary stream obtained for the signal. At the end of the reconstruction, it is obtained a reconstructed signal Such a reconstructed signal corresponds to the envelope estimated complex of the selected signal s[n] being processed.

[0097] In an exemplary implementation, the reconstruction step comprises estimating an estimated frequency for the selected signal s[n], the reconstructed signal being obtained as a function of the estimated frequency

[0098] In this example, the estimation of the estimated frequency £ comprises the calculation, for each portion of the selected signal s[n] corresponding to a bit bk of the decoded binary stream of the selected signal s[n], of the derivative of the instantaneous phase of the selected signal s[n].

[0099] Then, the calculated derivatives greater than a predetermined threshold are deleted. The predetermined threshold is for example greater than or equal to 1 MHz for an S mode and greater than or equal to 3 MHz for an SIF mode.

[0100] The estimated frequency f is then the average of the remaining derivatives.

[0101] In an exemplary implementation, the reconstruction step comprises estimating, for each bit bk of the bit stream, an originally estimated phase 0. The signal aa reconstructed is obtained based on the originally estimated phase for each bit bk.

[0102] In this example, the estimation of the phases at the origin 0 of the bits bk includes the compensation of the estimated frequency in the selected signal s[n] to obtain a frequency-compensated selected signal §

[22] . The frequency-compensated selected signal §

[21] is, for example, expressed by the following formula:

[0103] s[n] = s[n]e-j2n^n (9)

[0104] Then, for each bit bk, a weighted average Sfr is determined for the selected frequency-compensated signal gfu]- The weights are established on the basis of Rice distributions applied to the selected frequency-compensated signal §

[22] on a time portion of the selected frequency-compensated signal §

[22] specific to each bit bk of the selected signal s[n]. The weighted average -¾ is, for example, expressed by the following formula:

[0105] (10)

[0106] Where: • a denotes the estimated amplitude of the selected signal s[n], and • a denotes the estimate of the standard deviation of the IFF 12 interrogator receiver noise, C denotes the probability distribution function of a Rice distribution.

[0107]

[0108]

[0109]

[0110] [YES]

[0112] The phase at the origin of each bit bk is then the argument (the angle) of the corresponding -¾ weighted average determined. In the context of a signal with an S coding mode, the phases of the four preamble pulses are also estimated and are noted 0^00- -4^ -2' “1 Thus, the reconstructed signal is suitable for being expressed as a function of the estimated amplitude a for the selected signal s[n], the estimated start time instant y for the selected signal s[n], the bit stream obtained for the selected signal s[n], the estimated frequency for the selected signal s[n] and the phase at the origin of each bit bp. For example, the reconstructed signal / Yn] is expressed in the following form: a. for coding mode S: D[n] = ap n+^2^ {n-(k + Z)N (H) a. for SIF coding mode: Q [ n ] = a^^^q [ n -1.4 5kN s ] ex 2n ^2+ $J] (12) Or: • a denotes the estimated amplitude of the selected signal s[n], * ^s^ne 'c Estimated Pæam^u'c of a selected signal s[n] presenting an S coding mode, where p~s [ n ] = P1 [ n 1 + pj n - Ns ] J+pQ [ n-3Ns ] =) + pQ [ n-4Ns ] J • N denotes the number of bits corresponding to a selected signal s[n] having a coding mode S, * lh denotes the estimated bit k of the selected signal s[n], • P^ denotes the temporal form of the signal corresponding to the bit of a selected signal s[n] having a coding mode S, * qr[n] denotes the time shape of a pulse of a selected signal s[n] having a SIF coding mode, • denotes the number of samples of the basic signal (y[n]) in 1 microsecond, • f denotes the estimated frequency of the selected signal s[n], and

[0113]

[0114]

[0115]

[0116]

[0117]

[0118]

[0119]

[0120] • denotes the phase at the origin of bit h. He has The extraction method comprises a step 150 of subtracting the signal reconstructs ^F / jl from the basic signal y{nl to obtain a restricted basic signal 1. iiy J / The subtraction is performed by placing oneself at the estimated start time instant y of the selected signal s[n]. The restricted basic signal 1 . is expressed, for example, under the following form: yjw-y] =y[ny]-4^]k ' The steps of detection 110, selection 120, decoding 130, reconstruction 140 and subtraction 150 are then repeated replacing the base signal yjn] by the restricted basic signal until the set of detected signals is empty. This means that all the superimposed signals have been extracted from the base signal y[n]. Optionally, the extraction method comprises, from the first repetition (at least one signal already reconstructed) and after each subtraction step 150, an additional processing step 160 when the time interval corresponding to the reconstructed signal intersects the time interval of one or more previous reconstructed signals. In this case, the additional step 160 comprises the selection of the reconstructed signal ^

[12] , called the weak signal, having the lowest amplitude among all the previous reconstructed signals cross-referenced. The current reconstructed signal is then subtracted from the restricted base signal Fl from which the weak signal was obtained, to obtain a base signal doubly restricted I) for the weak signal. The decoding steps 130 and reconstruction 140 are then repeated for the weak signal by replacing the restricted basic signal F 1 from which it was obtained the weak signal by the doubly restricted basic signal Fl. The new signal The resulting reconstructed signal then replaces the weak signal. Thus, the present method makes it possible to successively extract, on the principle of successive interference cancellation (SIC), signals superimposed in a basic signal y[n]- Such a method is particularly suitable for an IFF interrogator for which the basic signal y [ n] is received on the same reception channel.

[0121] Such a method thus makes it possible to improve the probability of reception of a response signal by an IFF interrogator. Resistance to wideband jammers is, moreover, increased.

[0122] Those skilled in the art will understand that the previously described embodiments and variants may be combined to form new embodiments provided that they are technically compatible.

Claims

Claims

1. Method for extracting superimposed SIF mode and / or S mode IFF responses in a basic signal (y[nl) received by an IFF interrogator (12), the method being implemented by an electronic extraction device (10) and comprising the steps of: • detecting, in the basic signal (yfn]), signals having predefined coding modes, to obtain a set of detected signals, each detected signal being characterized by a coding mode, an estimated amplitude (a) and an estimated start time instant (y), • selecting, from the set of detected signals, the signal having the greatest amplitude, • decoding the selected signal (s[n]) as a function of the coding mode of the signal, the estimated amplitude (a) for the signal, the estimated start time instant (y) for the signal and the basic signal (yjnl), to obtain a bit stream (^) for the selected signal (s[n]),• reconstructing the selected signal (s[n]) as a function of the signal coding mode, the estimated amplitude (a) for the signal, the estimated start time instant (y) for the signal, and the bitstream (^) obtained for the signal, and • subtracting the reconstructed signal (^jJ) from the base signal (y[n]) to obtain a restricted base signal (yJ / j X repeating the steps of detecting, selecting, decoding, reconstructing and subtracting by replacing the base signal (y[n]) with the restricted base signal (yj^) until the set of detected signals is empty.,

2. The method of claim 1, wherein the predefined coding modes are mode 1, mode 2, mode 3, mode A and mode C, all encompassed by the term SIF mode, and mode S corresponding to an IFF interrogator (12).

3. A method according to claim 2, wherein the base signal (y[nl) is a sampled signal, the detecting step comprising: • determining an adaptive threshold for detecting signals having an SIF coding mode, the adaptive threshold being determined for each sample (n) of the basic signal (y[n]) as a function of the basic signal • detecting, in the basic signal (yjn]), signals having an S coding mode, • updating the adaptive threshold as a function of the estimated amplitude (a) for each detected signal of S coding mode, and • detecting, in the samples (n) of the basic signal (y[n]) whose amplitude is greater than or equal to the updated adaptive threshold, signals having an SIF coding mode.

4. Method according to claim 2 or 3, in which the decoding of each detected signal having a coding mode S, comprises: • the determination, for each bit (¾) expected for the coding mode S, of scores established on the basis of Rayleigh and Rice distributions applied to the basic signal (yfnl) on different temporal portions of the basic signal (y[n]), and • the attribution of a value to each bit (¾) by comparison of the scores determined for the bit (¾) so as to obtain the binary flow (^) of the signal.

5. Method according to any one of claims 1 to 4, in which the reconstruction step comprises the estimation of an estimated frequency (^) for the selected signal (s[n]), the reconstructed signal (^72]) being obtained as a function of the estimated frequency Q), the estimation of the estimated frequency Q) comprising: • the calculation, for each portion of the selected signal (s[n]) corresponding to a bit (b^) of the decoded binary stream (^) of the selected signal (s[n]), of the derivative of the instantaneous phase of the selected signal (s[n]), • the suppression of the calculated derivatives greater than a predetermined threshold, the estimated frequency Q-) being the average of the remaining derivatives.

6. The method of claim 5, wherein the reconstructing step comprises estimating, for each bit (b^ of the bit stream (^.), of an estimated phase at the origin 1 the reconstructed signal (^n]) being obtained as a function of the estimated phase at the origin 1 for each bit (b^, the estimation of the phases at the origin (¢,) of the bits (b^ comprising: 2£7 • the compensation of the estimated frequency Q) in the selected signal (s[n]) to obtain a frequency-compensated selected signal (§[22]), • the determination, for each bit (b^ of a weighted average (¾) for the frequency-compensated selected signal (§[22]), the weightings being established on the basis of Rice distributions applied to the frequency-compensated selected signal (§[22]) on a time portion of the frequency-compensated selected signal (^21]) specific to each bit (¾) of the selected signal (s[n]), the phase at the origin ( of each bit (b^ being the argument of the corresponding weighted average (¾) determined.

7. Method according to claim 6, in which the reconstructed signal (^j2p associated with the selected signal (s[n]) is expressed in the following form: • for an S coding mode: D[ n] = a Ps n + [ 22- ( k + 8 ) 7VS] • for an SIF coding mode: X2[ n] = t -1 A5kNs] Where:

8. i. a denotes the estimated amplitude of a selected signal (s[n]), u- denotes the estimated preamble of a selected signal (s[n]) having a coding mode S, where Q [ n ] = [ n ] ehnï ab J+pj n. jvs ] +pQ [ n-3Ns ] eX21'^^J + pD [ n-4Ns ] eÛ2n'tn+ê J N denotes the number of bits corresponding to a selected signal (s[n]) having a coding mode S, iii. denotes the bit k of the selected signal (s[n]), iv. Pg denotes the temporal form of the signal corresponding to the bit , ^k v. qJzi] denotes the temporal shape of a pulse of a selected signal s[n] having a SIF coding mode, vi. Ns denotes the number of samples of the basic signal (y[n]) in 1 microsecond, vii. f denotes the estimated frequency for the selected signal (s[n]), and viii. designates the phase at the origin of the bit A method according to any one of claims 1 to 7, wherein before each repetition, when the time interval corresponding to the reconstructed signal intersects the time interval of one or more previous reconstructed signals, the method comprises an additional processing step comprising: • the selection of the reconstructed signal called weak signal, having the lowest amplitude among all the reconstructed signals ( / ^^) intersected, • the subtraction, in the restricted basic signal ( ' ) from which the weak signal was obtained, of the current reconstructed signal (^[^) to obtain a doubly restricted basic signal (y Jjjj) for the weak signal, • the repetition of the decoding and reconstruction steps for the weak signal by replacing the restricted basic signal ( ) of the weak signal by the doubly restricted basic signal y jyj (yjuj)' the reconstructed signal (obtained replacing the weak signal.

9. Electronic device (10) for extracting IFF responses of mode S and / or mode SIF superimposed in a basic signal (y[u]) received by an IFF interrogator (12), the device comprising: • an IFF interrogator (12) having a reception channel suitable for receiving the basic signal (y[u]), and • a processing unit (14) suitable for implementing the steps of a method according to any one of claims 1 to 8.

10. A computer program product comprising a readable information medium, on which is stored a computer program comprising program instructions, the computer program being loadable onto a data processing unit and causing the implementation of a method according to any one of claims 1 to 8 when the computer program is implemented on the data processing unit.

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