Anti-interference device for satellite radio navigation signal receiver

Abstract

Anti-interference device for satellite radionavigation signal receiver The invention relates to an anti-interference device (10, 30, 40, 50) comprising: - an analog-to-digital converter (12) configured to transform a received analog radionavigation signal into a digital signal comprising a plurality of samples;- at least one pre-cumulative filter (22) adapted to a modulation of the transmitted signal, each filter corresponding to an impulse response filter, substantially equal to a waveform of the transmitted signal, allowing for the accumulation of samples and operating at the sampling frequency of the received signal, and delivering at the output a filtered complex signal, said device further comprising at least one first amplitude processing module (24) configured to process the amplitude of said filtered complex signal, said first amplitude processing module being both downstream of said pre-cumulative filter and upstream of at least one signal demodulation channel. Figure for the abbreviation: Figure 1;

Description

Title of the invention: Anti-interference device for satellite radionavigation signal receiver

[0001] The present invention relates to an anti-interference device configured to be embedded within a satellite radionavigation signal receiver(s), the device comprising an analog-to-digital converter configured to transform a received analog radionavigation signal into a digital signal comprising a plurality of samples, and comprising at least one pre-cumulative filter adapted to signal modulation, the pre-cumulative filter corresponding to an impulse response filter, substantially equal to the waveform of the signal emitted by a satellite, allowing the accumulation of samples of the received signal to be carried out, operating at the sampling frequency of the received signal for all the receiving channels, and delivering at the output a filtered complex signal.

[0002] The invention also relates to a receiver comprising such an anti-interference device.

[0003] The invention also relates to a vehicle comprising the aforementioned receiver, in particular an aircraft.

[0004] The invention also relates to an anti-interference method implemented by such anti-interference device.

[0005] The invention also relates to a computer program comprising software instructions which, when executed by a computer, implement such an anti-interference method.

[0006] The present invention relates to protection against interference in GNSS (Global Navigation Satellite System) receivers, in particular certified GNSS receivers used in civil aviation.

[0007] More specifically, the present invention relates to the protection of such certified GNSS receivers against PPD (Personal Privacy Devices) jamming devices intended to protect privacy by preventing the operation of "spyware" intended to record the trajectory of vehicles, such jammers being available for purchase, particularly on the Internet and inexpensive, and consequently very widespread, some of these jammers having repeatedly caused disruptions at airports.

[0008] The integrity of a GNSS receiver represents the degree of confidence that can be placed in the accuracy of the information it provides. Such integrity is susceptible to being affected by an event such as intentional or unintentional interference, in particular, Those caused by anti-jamming treatments against the aforementioned privacy-protective jamming devices (PPDs). Such anti-jamming treatments serve to maintain service continuity in the event of sudden interference but also distort satellite signals and generally result in an error in the location information provided by the GNSS system, also known as a "positioning failure." In applications such as commercial civil aviation, where the reliability of positioning information is crucial for flight safety, such positioning failures in a navigation system must be anticipated as much as possible.

[0009] Moreover, the major constraint in the case of certified aeronautical receivers is that satellite signals must not be distorted in order to avoid introducing bias into the measurements at the risk of compromising the integrity of the estimated position.

[0010] Conventional anti-jamming treatments are performed on the signal amplitude by applying a non-linear memoryless function, meet this requirement of no distortion, and offer interesting anti-jamming performance in the case of a constant envelope jammer, because such amplitude processing does not present the risk of distorting satellite signals.

[0011] Such Amplitude Data Processing (ADP) processes are applied, according to the state of the art, to the output signal of an analog-to-digital converter, the converter digitizing the received analog radionavigation signal.

[0012] A first known ADP amplitude processing technique applied at the output of an analog-to-digital converter consists of:

[0013] - estimate the argument 0 and the modulus p of each digitized signal sample in baseband, notably using a CORDIC algorithm (from the English CO-ordinate Rotation Digital Computer), each sample being represented by the complex number I + jQ = px ei°, with I and Q representing respectively the in-phase channel and the quadrature channel of the signal received and digitized by the receiver,

[0014] - smooth the modulus p by a low-pass filter, then subtract the number from the samples complex pUssé x eJu.

[0015] A second known ADP amplitude processing applied at the output of an analog-to-digital converter is described in application FR 3 075 975 in the name of the Applicant, and consists of determining a real gain k to be applied to complex received signal samples I + jQ, this gain k being determined as a function of the modulus p = I2 + Q2 ) according to non-linear functions tabulated in memory (i.e. entered in a table), for different values ​​of the ratio J / N of the interference power J to the estimated thermal noise power N.

[0016] However, such amplitude treatments are only effective if the interference power J (at constant envelope) is greater than the received noise power N (Gaussian).

[0017] In particular, for the first amplitude treatment mentioned above, it is necessary to position a decision threshold on the J / N ratio from which the correction is applied, arbitrarily while taking care not to apply the correction if there is no interference or if the J / N ratio is not high enough because this would further degrade the C / NO signal ratio (C from the English Carrier) to NO noise after demodulation, by correlation.

[0018] For the second amplitude treatment mentioned above, the non-linear function giving the gain k = f(p) is adapted to the interference-to-noise ratio J / N estimated by tabulating several profiles, ranging from a constant gain profile (i.e., no effect) to a gain profile adapted to an infinite J / N ratio, in order to avoid a threshold effect and to maximize the efficiency for each value of J / N.

[0019] In summary, the two aforementioned ADP amplitude treatments, classically applied directly to the signal at the output of an analog-to-digital converter, are generally effective against continuous wave (CW) interference that sweeps across the entire RF radio frequency band because the associated flat frequency response allows the envelope to remain constant, which is particularly the case for the signals used by the aforementioned PPD jamming devices.

[0020] However, in the case of fixed frequency CW continuous wave interference, such amplitude treatments classically applied directly to the output signal of an analog-to-digital converter in the entire RF band only begin to be effective if the interference power exceeds that of the total noise in the RF band, whereas the degradation of the C / N0 signal (C for Carrier) to noise N0 ratio after demodulation, by correlation, begins from the moment when the interference power is no longer negligible compared to the noise power in the band reduced to the main lobe of the satellite signal spectrum.

[0021] When applying the aforementioned amplitude processing, there is therefore a region of intermediate power values ​​for which the amplitude processing has no effect and where the degradation of the signal-to-noise ratio C / N0 after demodulation is significant. Such a region is associated with a "dip" in the signal-to-noise ratio curve C / N0 after demodulation as a function of the interference power, i.e., as a function of the interference power J.

[0022] Such a "dip" is likely to cause the tracking loops of the satellite signals to lose contact, whereas by increasing the power of the interference the loops would have remained connected, which makes the aforementioned amplitude processing unusable in the context of certified aeronautical receivers.

[0023] The aim of the invention is therefore to provide a protection solution against interference that protects GNSS receivers, particularly those of civil aviation, for which certification requirements demand that received signals not be distorted.

[0024] To this end, the invention relates to an anti-interference device configured to be embedded within a satellite radionavigation signal receiver, the device comprising:

[0025] - an analog-to-digital converter configured to transform a signal of ra analog navigation received as a digital signal comprising a plurality of samples;

[0026] - at least one pre-accumulation filter adapted to a modulation of the emitted signal, the filter of pre-cumulative filters corresponding to an impulse response filter, substantially equal to a waveform of the signal emitted by a satellite, allowing the accumulation of samples of the received signal, operating at the sampling frequency of the received signal for all reception channels, and delivering at the output a filtered complex signal,

[0027] said device further comprising at least one first amplitude processing module configured to amplitude process said complex signal filtered by said at least one pre-accumulation filter, said at least one first amplitude processing module being both downstream of said at least one pre-accumulation filter and upstream of at least one signal demodulation channel configured to provide at least one satellite measurement as output.

[0028] Thus the anti-interference device applies amplitude processing to the signal after linear bandpass filtering adapted to the modulation of useful satellite signals emitted, which advantageously allows attenuation of interferences whose power is less than that of the total noise in the digitized radio frequency band.

[0029] In particular, due to its position, downstream of the pre-accumulation filter, in the signal reception and processing chain, such amplitude processing is effective from the moment when the power of the interference exceeds that of the noise after filtering by said pre-accumulation filter.

[0030] In other words, such an anti-interference device makes it possible to react to a lower level of interference power than a conventional anti-interference device applying amplitude processing at the output of an analog-to-digital converter because the noise at the output of the pre-accumulation filter is lower.

[0031] Thus, the anti-interference device according to the present invention is effective against constant envelope interference whose power is less than that of the noise in the radio frequency band, and whose frequency remains centered in the band of the main lobe of the modulation.

[0032] When dealing with a jammer that sweeps in frequency according to an arbitrary profile, the amplitude processing implemented by the anti-interference device allows it to be filtered effectively without significantly distorting the useful signals coming from the satellites.

[0033] According to other advantageous aspects of the invention, the anti-interference device comprises one or more of the following features, taken individually or in any technically possible combination:

[0034] - the received radio navigation signal results from the use, within the same band radio frequency bandwidth, with distinct signal modulations,

[0035] and wherein the device comprises, at the output of said analog-to-digital converter, in parallel, for each of said distinct signal modulations:

[0036] - a pre-accumulation filter adapted to said signal modulation, and

[0037] - a first amplitude processing module associated at the output of said pre-filter cumulative;

[0038] - said device is configured to receive modulated radionavigation signals BOC;

[0039] - said analog-to-digital converter is configured to transform the signal analog radionavigation received, in a complex digital signal sampled in baseband,

[0040] and in which for each assembly, associated with a modulation, comprising a pre-cumulative filter adapted to said signal modulation and a first amplitude processing module associated at the output of said pre-cumulative filter, said device further comprises a second amplitude processing module configured to process in amplitude said complex baseband sampled digital signal supplied at the output by said analog-to-digital converter, said second amplitude processing module being both downstream of said analog-to-digital converter and upstream of said pre-cumulative filter;

[0041] - said analog-to-digital converter is configured to transform the signal of analog radionavigation received, in a real digital signal sampled at intermediate frequency;

[0042] - the device further comprises, for each set, associated with a modulation, including a pre-accumulation filter adapted to said signal modulation and a first amplitude processing module associated at the output of said pre-accumulation filter:

[0043] - a template filter connected to the output of said analog-to-digital converter, and

[0044] - a baseband switching unit, said unit comprising:

[0045] - a digitally controlled carrier oscillator at a fixed frequency inter median;

[0046] - a sine and cosine table producing a complex signal whose argument is the phase produced by said oscillator, and

[0047] - a multiplier configured to multiply the signal supplied by said template filter by the said complex signal generated via the said table, the output of the said multiplier being provided as input to the said pre-accumulation filter;

[0048] - the device further comprises, for each set, associated with a modulation, comprising a pre-accumulation filter adapted to said signal modulation and a first amplitude processing module associated at the output of said pre-accumulation filter, a baseband conversion unit connected to the output of said analog-to-digital converter, said unit comprising:

[0049] - a digitally controlled carrier oscillator at a fixed frequency inter median;

[0050] - a sine and cosine table producing a complex signal whose argument is the phase produced by said oscillator, and

[0051] - a multiplier configured to multiply the signal supplied by said converter analog-to-digital conversion by said complex signal generated via said table

[0052] the output of said multiplier being provided as input to a second amplitude processing module configured to amplitude process the baseband sampled complex digital signal provided as output by said multiplier, said second amplitude processing module being connected as output to said pre-cumulative filter;

[0053] - the device further comprises, at the output of the baseband conversion unit, a anti-aliasing filter configured to divide the sampling frequency by two;

[0054] - the device further comprises, at the output of the baseband conversion unit, a anti-aliasing filter associated at the output with an automatic gain control module.

[0055] - for each assembly, associated with a modulation, comprising a pre-filter accumulations adapted to said signal modulation and a first amplitude processing module associated at the output of said pre-accumulation filter, said first and / or said second amplitude processing module comprises:

[0056] - an interference rejection filter comprising a multiplicative gain applied res respectively to an in-phase channel (I) and a quadrature channel (Q) of said radionavigation signal received and digitized by the analog-to-digital converter (12), the value of the multiplicative gain being determined by a non-linear function K;(p) of the magnitude p of the received signal, the function K;(p) being determined so as to maximize the signal-to-noise ratio after application of the multiplicative gain and demodulation of the received signal, and

[0057] - an estimator of a J / N ratio between the power of an interfering signal and the power of thermal noise, the non-linear function K(p) depending on the ratio J / N, the value of the multiplicative gain being calculated from said ratio J / N,

[0058] and wherein said device further comprises another automatic gain control module connected between said pre-accumulation filter and said first module of amplitude processing downstream of said pre-accumulation filter.

[0059] The invention also relates to a receiver comprising such an anti-interference device.

[0060] The invention also relates to a vehicle comprising the aforementioned receiver, in particular an aircraft.

[0061] The invention also relates to an anti-interference method implemented by said anti-interference device embedded within a satellite radionavigation signal receiver(s) and comprising at least the following successive steps:

[0062] - analog-to-digital conversion of the analog radionavigation signal received in a digital signal comprising a plurality of samples;

[0063] - filtering by means of at least one pre-accumulation filter adapted to a modulation of signal, the pre-cumulative filter corresponding to an impulse response filter, substantially equal to the waveform of the signal emitted by a satellite, said filtering allowing the accumulation of samples of the received signal, operating at the sampling frequency of the received signal for all reception channels, and delivering at the output a filtered complex signal,

[0064] - amplitude processing of said complex signal filtered by said at least one filter of pre-cumuls, said amplitude processing being applied both downstream of said at least one pre-cumul filter and upstream of at least one signal demodulation channel configured to provide at least one satellite measurement as output.

[0065] The invention also relates to a computer program comprising software instructions which, when executed by a computer, implement such an anti-interference method as defined above.

[0066] These features and advantages of the invention will become clearer upon reading the following description, given solely by way of non-limiting example, and made with reference to the accompanying drawings, in which:

[0067] - [Fig. 1] [Fig. 1] is a diagram illustrating an anti-interference device according to a first basic variant of the present invention;

[0068] - [Fig.2] [Fig.2] is a diagram illustrating an anti-interference device according to a second variant of the present invention;

[0069] - [Fig.3] [Fig.3] is a diagram illustrating an anti-interference device according to a third variant of the present invention;

[0070] - [Fig.4] [Fig.4] is a diagram illustrating an anti-interference device according to a fourth variant of the present invention adapted to the presence of different modulations in the same radio frequency band;

[0071] - [Fig. 5] [Fig. 5] illustrates the effect of the anti-interference treatment by treatment amplitude according to the present invention on the curve representing the ratio C / N0 signal (C from the English Carrier) to noise N0 after demodulation, by correlation in function of the power J of the interference.

[0072] Fig. 1 is a representation of an anti-interference device according to a first "basic" variant of the present invention, such an anti-interference device being configured to be embedded within a satellite radionavigation signal receiver.

[0073] According to this first basic variant, the anti-interference device 10 includes first of all an analog-to-digital converter 12 configured to transform a received analog radionavigation signal into a digital signal comprising a plurality of samples.

[0074] Following the example of [Fig.1], said analog-to-digital converter 12 is configured to transform the received analog radionavigation signal into a real digital signal sampled at an intermediate frequency.

[0075] Furthermore, according to this example, the anti-interference device 10 further comprises a template filter 14 connected to the output of said analog-to-digital converter 12, and a baseband conversion unit, said unit comprising: a digitally controlled carrier oscillator 16 at the intermediate fixed frequency (residual before digitization), a sine and cosine table 18 producing a complex signal whose argument is the phase produced by said oscillator 16, and a multiplier 20 configured to multiply on the one hand the signal supplied by said template filter 14 by said complex signal generated via said table 18 on the other hand, the output of said multiplier 20 being supplied as input to a "pre-cumulative" filter 22.

[0076] In other words, when the signal is digitized in intermediate frequency by the analog-to-digital converter 12, the aforementioned elements 16, 18 and 20 allow switching to baseband before applying the amplitude processing according to the present invention as described below.

[0077] According to one embodiment, as illustrated later in relation to [Fig. 2] relating to a second embodiment, the analog-to-digital converter 12 is directly configured to transform the received analog radio navigation signal into a complex digital signal sampled in baseband. According to this embodiment, the oscillator 16 would be digitally controlled by the carrier at a stub frequency corresponding to a residual frequency before digitization.

[0078] The "pre-cumulative" filter 22 is adapted to at least one modulation of the emitted signal, the pre-cumulative filter 22 corresponding to an impulse response filter substantially equal to a waveform of the signal emitted by a satellite, allowing the accumulation of samples of the received signal, operating at the sampling frequency of the received signal for all the receiving channels, and delivering at the output a filtered complex signal.

[0079] Such a "pre-cumulative" filter 22, also called a modulation-matched filter, is in particular described in application FR 2 931 600 on behalf of the Applicant and

[0080] corresponds to a sliding window accumulation filter suitable for accumulating samples of the received signal. More specifically, to demodulate the signal, each satellite channel Ci, C2, C3 must perform at least one correlation between the received baseband signal S(t) and a local code Code(t) specific to the tracked satellite, such a correlation being expressed in continuous time in the following form: " " . f " , z À \ , , which is equivalent to considering a space Correlation = J Codent ).S(t)M discrete temporal at:

[0081] Correlation— lL^àk2Code (tk + qTchip) .Sic) which by demonstration is equivalent to Correlation = E ^dq + n\a(n ) with a ni v 1 ■ v 7 u ( n ) — E Si p), ^in) being the pre-accumulation function implemented by The "pre-cumulative" filter 22, q the lead of the local code Code (tk + q.Tchip) relative to the received signal S(tk), kl the index of the first signal sample S(tki) integrated into the correlation, k2 the index of the last signal sample S(tk2) integrated into the correlation, ni the index of the first pre-cumulative o(nl) integrated into the correlation, n2 the index of the last pre-cumulative o(n2) integrated into the correlation, kln the index of the first signal sample S(tk[n]) integrated into the pre-cumulative o(n), k2n the index of the last signal sample S(tk2n) integrated into the pre-cumulative o(n). The correlation implemented according to this equation Correlation = E , . + n).a( n ) presents The advantage of requesting a calculation at the frequency Fcode of the code with index n, rather than at the frequency Fe of the sampled signal with index k in a discrete space, is that it allows the use of an adder operating at frequency Fe to perform several correlations in parallel within the same channel. Multiple correlations with staggered versions of the same local code accelerate the search for synchronization on the received signal during acquisition. They also allow monitoring for the presence of multipath propagation, decoys, or multiple correlation peaks.

[0082] The reduction in computational workload is then real, provided that the calculation of pre-cumulative values ​​between correlations is factored, since a pre-cumulative calculation per correlation is not advantageous. This is why the "pre-cumulative" filter 22 performs the calculation upstream of the satellite channels Ci, C2, and C3. For this purpose, the "pre-cumulative" filter 22 corresponds to a sliding sum filter on N consecutive samples of received signal, N corresponding to the number of signal samples per code slot (or "chip"). The "pre-cumulative" filter 22 is common to the satellite channels Ci, C2, and C3, which use pre-cumulative values ​​as needed, by subsampling, in a way that synchronous with their local codes.

[0083] The "pre-cumulative" filter 22 is applied to the received baseband signal (i.e., after carrier demodulation via the aforementioned elements 16, 18, and 20). It should be noted that the demodulation of the Doppler effect on the satellite signal carriers is subsequently performed by the Cb channels C2 and C3. The Doppler effect is not problematic in the pre-cumulative stage because, over the duration of a code slot corresponding to the sliding filter horizon, the Doppler induces a negligible carrier phase variation. To perform the sliding summation, a delay line with N samples is used, for example, and combined with an adder and a subtractor as described in French patent application FR 2 931 600, thus avoiding the need for N adders.

[0084] According to the example in [Fig. 1], the anti-interference device 10 further specifically comprises, according to the present invention, a first amplitude processing module 24 configured and specifically positioned in the chain of constituent elements of the device 10, for amplitude processing said complex signal filtered by said pre-cumulative filter 22, said first amplitude processing module 24 being both downstream of said at least one pre-cumulative filter and upstream of at least one signal demodulation channel configured to provide at least one satellite measurement as output. In particular, in the example in [Fig. 1], three channels C1, C2, and C3 are shown.

[0085] In the context of the present invention, the positioning of the first amplitude processing module 24 at the output of the pre-cumulative filter 22 within the electronic chain of device 10 is advantageous because the pre-cumulative filtering implemented by the pre-cumulative filter 22 has the particular advantage of providing the smallest possible equivalent noise bandwidth compatible with the tracking of satellite signals by the signal processing channels without signal distortion. Minimizing the bandwidth reduces the noise at the filter output and thus improves the sensitivity of the subsequent amplitude processing by more readily highlighting noise interference (i.e., by exhibiting a higher J / N ratio).

[0086] Thus, thanks to the amplitude processing implemented by the module 24 after pre-accumulation filter 22, it is possible to reduce the effect of a jamming signal regardless of its frequency, and without distorting the satellite signal.

[0087] The amplitude processing implemented by the module 24 after pre-accumulation filter 22 generally consists of applying a non-linear and memoryless function to the signal from the pre-accumulation filter 22, which is very effective against constant envelope interference, i.e. those whose baseband magnitude is constant (i.e. describing a circle on the I & Q diagram).

[0088] Not shown, according to a first embodiment, the first amplitude processing module 24, specifically positioned according to the present invention at the output of the pre-cumulative filter 22 (i.e., the input of the first amplitude processing module 24 is connected to the output of the pre-cumulative filter 22), is configured to:

[0089] - estimate the argument 0 and the modulus p of each digitized signal sample in baseband (sample represented by the complex number I + jQ = px ei0), notably using a CORDIC algorithm (from the English CO-ordinate Rotation Digital Computer),

[0090] - smooth the modulus p using a low-pass filter, then remove the modulus p from the samples complex number pUssé x ei0. This method can use the Cordics technique to determine the modulus p and angle 0 of each received signal sample.

[0091] Not shown, according to a second preferred embodiment, the first amplitude processing module 24, specifically positioned according to the present invention at the output of the pre-accumulation filter 22 (i.e., the input of the first amplitude processing module 24 is connected to the output of the pre-accumulation filter 22), comprises:

[0092] - an interference rejection filter comprising a multiplicative gain applied res respectively to a phase I channel and a quadrature Q channel of said radionavigation signal received and digitized by the analog-to-digital converter 12, the value of the multiplicative gain being determined by a non-linear function K;(p) of the magnitude p of the received signal, the function K;(p) being determined so as to maximize the signal-to-noise ratio after application of the multiplicative gain and demodulation of the received signal, and

[0093] - an estimator of a J / N ratio between the power of an interfering signal and the power of a thermal noise, the non-linear function K;(p) depending on the ratio J / N, the value of the multiplicative gain being calculated from said ratio J / N.

[0094] In other words, the first amplitude processing module 24 consists of determining a real gain k to be applied to the complex received signal samples I + jQ, this gain k being determined as a function of the modulus p = I2 + Q2 ) according to non-linear functions tabulated in memory (i.e. entered in a table), for different values ​​of the ratio J / N of the interference power J to the thermal noise power N estimated as described in the aforementioned application FR 3 075 975.

[0095] Such preferential amplitude treatment is optimal, self-adaptive, and advantageously uses very few material resources and in a completely autonomous manner.

[0096] Optionally, not shown, said device 10 further includes an automatic gain control module connected between said pre-accumulation filter 22 and said first amplitude processing module 24, (i.e. downstream of said pre-accumulation filter 22).

[0097] Such an implementation option amounts to using an automatic gain control loop at the output of the pre-cumulative filter 22 in order to encode the pre-cumulative signals using a number of bits precisely adapted to limit the computational cost in the channels. In other words, an automatic gain control module located between said filter and pre-cumuls 22 and said first amplitude processing module 24 allows the amplitude of the pre-cumuls to be automatically adapted to the number of bits kept, in particular to compensate for the attenuation of the jammers obtained thanks to the pre-cumuls filter 22.

[0098] Furthermore, such an automatic localized gain control between said pre-accumulation filter 22 and said first amplitude processing module 24 downstream of the pre-accumulation filter 22 makes it possible to make applicable the amplitude processing first amplitude processing module 24 in accordance with the aforementioned FR 3 075 975 request by ensuring a constant average power at the input of the amplitude processing module 24.

[0099] According to an embodiment not shown, the anti-interference device 10 according to the present invention comprises a processing unit formed for example of a memory and a processor associated with the memory, and the anti-interference device 10 is at least partly realized in the form of software, or a software component, executable by the processor, in particular the analog-to-digital converter 12, the template filter 14, a baseband conversion unit comprising the aforementioned elements 16, 18 and 20, the pre-cumulative filter 22, and the first amplitude processing module 24. The memory of the anti-interference device 10 is then capable of storing such software or software components, and the processor is then capable of executing them.

[0100] According to another implementation also not shown, the analog-to-digital converter 12, the template filter 14, a baseband switching unit comprising the aforementioned elements 16, 18 and 20, the pre-cumul filter 22, and the first amplitude processing module 24 are each implemented in the form of a programmable logic component, such as an FPGA (Field Programmable Gate Array), or in the form of a dedicated integrated circuit, such as an ASIC (Application Specified Integrated Circuit).

[0101] When a portion of the anti-interference device 10 according to the present invention is implemented in the form of one or more software programs, i.e., in the form of a computer program, this portion is also capable of being stored on a computer-readable medium (not shown). The computer-readable medium is, for example, a medium capable of storing electronic instructions and being connected to a bus of a computer system. By way of example, the readable medium is an optical disc, a magneto-optical disc, a ROM, a RAM, any type of non-volatile memory (e.g., EPROM, EEPROM, FLASH, NVRAM), a magnetic card, or an optical card. A computer program containing software instructions is then stored on the readable medium.

[0102] It should be noted that the aforementioned implementation is also applicable to the variants detailed below in relation to Figures 2 to 4.

[0103] Figure 2 is a diagram illustrating an anti-interference device according to a second embodiment of the present invention, in which said converter analog-to-digital converter 12 is configured to transform the received analog radionavigation signal into a baseband sampled complex digital signal, and wherein said device further comprises a second amplitude processing module 32 configured to amplitude process said baseband sampled complex digital signal supplied at output by said analog-to-digital converter 12, said second amplitude processing module 32 being both downstream of said analog-to-digital converter 12 and upstream of said pre-cumulative filter 22.

[0104] Such a second embodiment avoids the "dip" in the signal-to-noise ratio curve C / NO after demodulation as a function of the jamming power, i.e. as a function of the interference power J, and this without losing the interest of a wideband amplitude processing implemented by the upstream amplitude processing module 32.

[0105] In other words, the second embodiment of the present invention consists of advantageously applying two amplitude processing in series, namely, on the one hand, an amplitude processing implemented by the module 32 on the signal received digitized by the analog-to-digital converter 12 in the entire radio frequency band and upstream of the "pre-cumuls" filter 22, followed, on the other hand, by an amplitude processing implemented by the module 24 on the signal after the filtering adapted to the modulation of the "pre-cumuls" filter 22, and carried out upstream of the signal processing channels Ci, C2, C3 used to demodulate the satellite signals and produce satellite measurements.

[0106] As previously indicated in relation to [Fig. 1], the amplitude processing implemented by module 24 downstream of the "pre-cumuls" filter 22 allows it to react to a lower interference power level than the amplitude processing implemented by module 32 upstream of the "pre-cumuls" filter 22 because the noise at the output of the "pre-cumuls" filter 22 is lower there.

[0107] In other words, the amplitude processing implemented by module 24 downstream of the "pre-cumuls" filter 22 makes it possible to attenuate interferences whose power is less than that of the total noise in the digitized radio frequency band, and becomes effective from the moment when the power of the interference exceeds that of the noise after bandpass filtering.

[0108] In addition, the additional upstream amplitude processing according to [Fig.2] implemented by module 32 upstream of the "pre-accumulation" filter 22 begins to be effective when the interference power exceeds the noise power throughout the radio frequency band, and thus takes over from the downstream amplitude processing implemented by module 24 downstream of the "pre-accumulation" filter 22.

[0109] In other words, such additional upstream amplitude processing according to [Fig. 2] implemented by module 32 upstream of the "pre-accumulation" filter 22 is effective against constant envelope interferences that sweep across the entire sampled radio frequency band, but only if their power is greater than that of the total noise in the radio frequency band.

[0110] Such additional upstream amplitude processing according to [Fig.2] implemented by module 32 upstream of the "pre-accumulation" filter 22 is also effective against jammers that sweep the entire radio frequency band, unlike the downstream amplitude processing implemented by module 24 downstream of the "pre-accumulation" filter 22.

[0111] In other words, the anti-interference device thus consists, according to this second embodiment, of applying an anti-interference filtering by amplitude processing at two places in the receiving chain, namely classically on the baseband signal (i.e. complex signal) digitized in the entire radio frequency band at the output of the analog-to-digital converter 12, and specifically according to the present invention on the signal after linear bandpass filtering adapted to the modulation of useful satellite signals implemented by the "pre-cumuls" filter 22.

[0112] Thus, the anti-interference device makes it possible, according to this second embodiment, to combine the advantage of the two amplitude processing and to ensure a maximum gain regardless of the power of the interference.

[0113] Not shown, according to a first variant, the upstream amplitude 32 processing module is configured to:

[0114] - estimate the argument 0 and the modulus p of each digitized signal sample in baseband (sample represented by the complex number I + jQ = px ei0), notably using a CORDIC algorithm (from the English CO-ordinate Rotation Digital Computer),

[0115] - smooth the modulus p using a low-pass filter, then remove the modulus p from the samples complex number pUssé x ei0. This method can use the Cordics technique to determine the modulus p and angle 0 of each received signal sample.

[0116] Not shown, according to a second preferred variant, the upstream amplitude 32 processing module comprises:

[0117] - an interference rejection filter comprising a multiplicative gain applied res respectively to a phase I channel and a quadrature Q channel of said radionavigation signal received and digitized by the analog-to-digital converter 12, the value of the multiplicative gain being determined by a non-linear function K;(p) of the magnitude p of the received signal, the function K;(p) being determined so as to maximize the signal-to-noise ratio after application of the multiplicative gain and demodulation of the received signal, and

[0118] - an estimator of a J / N ratio between the power of an interfering signal and the power of a thermal noise, the non-linear function K;(p) depending on the ratio J / N, the value of the multiplicative gain being calculated from said ratio J / N.

[0119] In other words, the upstream amplitude 32 processing module consists of de to finish a real gain k to be applied to complex received signal samples I + jQ, this gain k being determined as a function of the modulus p = I2 + Q2 ) according to nonlinear functions tabulated in memory (i.e. entered in a table), for different values ​​of ratio J / N of the interference power J to the thermal noise power N estimated as described in the aforementioned application FR 3 075 975.

[0120] It should be noted that for the amplitude processing according to this second preferred variant, simpler and more optimal, to be applicable, a constant average input power must be ensured, a condition which is met for the upstream amplitude processing implemented by the upstream amplitude processing module 32 thanks to the automatic analog gain control which conventionally controls the gain of the analog chain upstream of the analog-to-digital converter 12. Such automatic analog gain control measures the power on the I and Q channels and ensures a constant level by controlling the gains of the radio frequency chain upstream of the analog-to-digital converter 12 (i.e. ADC 12).

[0121] Fig. 3 is a diagram illustrating an anti-interference device 40 according to a third embodiment of the present invention, in which the analog-to-digital converter 12 is configured to digitize the signal at an intermediate frequency.The anti-interference device 40, according to this third embodiment of the present invention, then comprises, for baseband conversion before applying amplitude processing by the post-pre-cumulative amplitude processing module 24 22, a baseband conversion unit connected directly to the output of the analog-to-digital converter 12, the unit comprising: a digitally controlled carrier oscillator 42 at the intermediate fixed frequency (residual before digitization), a sine and cosine table 44 producing a complex signal whose argument is the phase produced by the oscillator 42, and a multiplier 46 configured to multiply the signal supplied by the analog-to-digital converter 12 by the complex signal generated via the table 44.

[0122] Furthermore, the output of the multiplier 46 is intended to be supplied as input to the second upstream amplitude processing module 32 configured to process in amplitude the complex baseband sampled digital signal supplied, as output, by the multiplier 46, the second amplitude processing module 32 being upstream of the pre-cumulative filter 22 because connected at output to the input of the pre-cumulative filter 22, itself connected downstream by its output to the input of the aforementioned first amplitude processing module 24.

[0123] As an optional addition, the anti-interference device 40 according to this third embodiment comprises, at the output of the baseband conversion unit, namely at the output of the multiplier 46, an anti-aliasing filter 48 configured to divide the sampling frequency by two, or, according to another option, said anti-aliasing filter aliasing 48 associated at output with an automatic gain control module not shown, the digital automatic gain control module at the output of the anti-aliasing filter 48 being configured to adapt the coding according to the output power which can vary depending on where the interference is located relative to the bandwidth of the filter.

[0124] Figure 4 illustrates an anti-interference device according to a fourth embodiment of the present invention adapted to the presence of different modulations in the same radio frequency band. Indeed, the signal emitted by a satellite consists of a carrier, and optionally a subcarrier in the case of BOC or double carrier shift modulation, modulated by a known spreading code and possibly by unknown data. All satellites transmit on the same frequencies, and the emitted signals differ only in their code.

[0125] This variant features an anti-interference device 50 configured to process a received radionavigation signal resulting from the use of distinct signal modulations within the same radio frequency bandwidth. To this end, the anti-interference device 50 comprises, at the output of said analog-to-digital converter 12, in parallel, for each of said distinct signal modulations:

[0126] - a pre-accumulation filter adapted to said signal modulation, and

[0127] - a first amplitude processing module associated at the output of said pre-filter cumulative.

[0128] In other words, the anti-interference device 50 according to this fourth variant includes in parallel at the output of several pre-cumulative filters adapted to different modulations in the same radio frequency band, in particular in the case of signals using multiple modulations on satellite signals, at least as many amplitude processing modules as there are pre-cumulative filters.

[0129] As illustrated in [Fig. 4], the anti-interference device 50 comprises two distinct assemblies associated respectively with different modulations, each assembly comprising, according to this example, a template filter 52A or 52B, a baseband crossover unit comprising a digitally controlled carrier oscillator at the intermediate fixed frequency 54F1, for example, or a digitally controlled subcarrier oscillator 54SP, and a sine and cosine table 56 producing a complex signal whose argument is the phase produced by means of the phases produced by said oscillators 54F1 or 54SP. Each assembly further comprises a pre-cumulative filter 58A or 58B adapted to said signal modulation and a downstream amplitude processing module 60A or 60B associated at the output respectively with said pre-cumulative filter 58A or 58B.

[0130] Optionally, not shown in [Fig.4], the anti-interference device 50 includes in parallel, for each of the said distinct signal modulations, also a second amplitude processing module upstream of the pre-accumulation filter. In other words, according to this option, the anti-interference device 50 includes a second ADP amplitude processing module 32 upstream of the pre-accumulation filter 58a, and a second ADP amplitude processing module 32 upstream of the pre-accumulation filter 58B.

[0131] In particular, said anti-interference device 50 is configured to receive BOC (Binary Offset Carrier) modulated radionavigation signals whose spectrum has two main lobes on either side of the carrier and uses for this purpose, for each lobe, one of the two pre-cumulative filters 58A or 58B for the two lobes and applies, via the two downstream amplitude processing modules 60A or 60B, ADP (Amplitude Data Processing) amplitude processing in parallel, namely one on each lobe.

[0132] In the case of a BOC type signal, the two lobes can be demodulated separately within each satellite channel Ch C2, C3.

[0133] Some navigation systems use BOC signals consisting of a radio frequency carrier, a square wave subcarrier, and a spreading code. The waveform (time pattern multiplied by +1 or -1 depending on the spreading sequence) is sine (square waves), resulting in a power spectral density with two main lobes and an autocorrelation function with multiple peaks.

[0134] The purpose of this BOC modulation is twofold: to free up the spectrum between the two lobes for other existing signals, and to improve the accuracy of measurements in the presence of thermal noise and multipath.

[0135] Also, demodulation is carried out within each satellite channel Ci, C2, C3 via two separate demodulation units, each containing: a digitally controlled Doppler carrier oscillator 54D or subcarrier 54SP, a sine and cosine table 56 producing a complex signal whose argument is the phase produced by means of the phases produced by said oscillators 54D or 54SP, a multiplier 62 configured to multiply the signal supplied by each amplitude processing module 60A or 60B by said complex signal generated via said table 56, the output of each multiplier 62 corresponding to each of the two lobes 64 right and 66 left separately, considering each lobe as a separate BPSK signal, with a carrier shifted from the center frequency by an amount equal to the subcarrier frequency or its opposite.

[0136] Fig. 5 is a representation of the effect of the anti-interference treatment by amplitude processing according to the present invention at the output of the pre-accumulation filter on the curve representing the ratio of the signal (C for Carrier) to noise N0 after demodulation, by correlation as a function of the interference power.

[0137] Indeed, there is a zone of intermediate power values ​​between the noise power 76 in the equivalent band of the main lobe of the modulation satellite and the power 78 of the noise in the radio frequency band, for which the classic amplitude processing at the output of the analog-to-digital converter 12 has no effect, and where the degradation of the C / NO after demodulation is significant as illustrated by curve 80. We thus observe a "dip" in the curve 80 of the C / NO after demodulation as a function of the interference power.

[0138] As illustrated by curve 82, the amplitude processing at the output of the pre-accumulation filter makes it possible to "smooth out" the hole (i.e. "hollow") in C / NO of the aforementioned curve 80.

[0139] A person skilled in the art will understand that the invention is not limited to the embodiments described, nor to the particular examples of the description, the embodiments and variants mentioned above being capable of being combined with each other to generate new embodiments of the invention.

[0140] The present invention thus proposes several architectural variants of an anti-interference device designed to protect GNSS receivers used in civil aviation, for which certification requirements mandate that signals not be distorted. Amplitude processing specifically located at least downstream of a pre-cumulative filter fulfills this condition and makes it possible to effectively combat certain types of interference, particularly constant-envelope interference, i.e., interference whose baseband magnitude is constant (i.e., describing a circle on the I & Q diagram).

Claims

Demands

1. Anti-interference device (10, 30, 40, 50) configured to be embedded within a satellite radionavigation signal receiver(s), the device comprising: - an analog-to-digital converter (12) configured to transform a received analog radionavigation signal into a digital signal comprising a plurality of samples;- at least one pre-cumulative filter (22) adapted to a modulation of the transmitted signal, the pre-cumulative filter (22) corresponding to an impulse response filter, substantially equal to a waveform of the signal transmitted by a satellite, allowing the accumulation of samples of the received signal, operating at the sampling frequency of the received signal for all the receiving channels, and delivering at the output a filtered complex signal, said device being characterized in that it further comprises at least one first amplitude processing module (24) configured to process in amplitude said complex signal filtered by said at least one pre-cumulative filter, said at least one first amplitude processing module being both downstream of said at least one pre-cumulative filter and upstream of at least one signal demodulation channel configured to provide at the output at least one satellite measurement.;

2. Anti-interference device (50) according to claim 1, wherein the received radio navigation signal results from the use, within the same radio frequency bandwidth, of distinct signal modulations, and wherein the device comprises, at the output of said analog-to-digital converter (12), in parallel, for each of said distinct signal modulations: - a pre-accumulation filter (58A, 58B) adapted to said signal modulation, and - a first amplitude processing module (60A, 60B) associated at the output of said pre-accumulation filter.

3. Anti-interference device (50) according to claim 2, wherein said device is configured to receive BOC modulated radionavigation signals.

4. An anti-interference device (30) according to any one of the preceding claims, wherein said analog- converter digital (12) is configured to transform the received analog radionavigation signal into a baseband sampled complex digital signal, and in which for each set, associated with a modulation, comprising a pre-cumul filter adapted to said signal modulation and a first amplitude processing module associated at the output of said pre-cumul filter, said device further comprises a second amplitude processing module (32) configured to amplitude process said baseband sampled complex digital signal supplied at the output by said analog-to-digital converter (12), said second amplitude processing module being both downstream of said analog-to-digital converter (12) and upstream of said pre-cumul filter (22).

5. Anti-interference device (10, 40) according to any one of claims 1 to 3, wherein said analog-to-digital converter (12) is configured to transform the received analog radio navigation signal into a real digital signal sampled at an intermediate frequency.

6. An anti-interference device (10, 50) according to claim 5 further comprising, for each assembly, associated with a modulation, comprising a pre-cumulative filter adapted to said signal modulation and a first amplitude processing module associated at the output of said pre-cumulative filter: - a template filter (14, 52A, 52B) connected to the output of said analog-to-digital converter (12), and - a baseband conversion unit, said unit comprising: - a digitally controlled carrier oscillator (16, 54) with a fixed intermediate frequency; - a sine and cosine table (18, 56) producing a complex signal whose argument is the phase produced by said oscillator (16), and - a multiplier (20) configured to multiply the signal supplied by said template filter (14) by said complex signal generated via said table (18), the output of said multiplier being supplied at the input of said pre-cumulative filter (22).

7. An anti-interference device (40) according to claim 5 further comprising, for each assembly, associated with a modulation, a pre-accumulation filter adapted to said signal modulation and a first amplitude processing module associated at the output of said pre- cumulatives, a baseband conversion unit connected to the output of said analog-to-digital converter (12), said unit comprising: - a digitally controlled carrier oscillator (42) with an intermediate fixed frequency; - a sine and cosine table (44) producing a complex signal whose argument is the phase produced by said oscillator (42), and - a multiplier (46) configured to multiply the signal supplied by said analog-to-digital converter (12) by said complex signal generated via said table (44), the output of said multiplier (46) being supplied as input to a second amplitude processing module (32) configured to amplitude process the baseband sampled complex digital signal supplied as output by said multiplier (46), said second amplitude processing module (32) being connected as output to said pre-cumulative filter (22).

8. Anti-interference device (40) according to claim 7 further comprising, at the output of the baseband conversion unit, an anti-aliasing filter (48) configured to divide the sampling frequency by two.

9. Anti-interference device according to claim 7 further comprising, at the output of the baseband switching unit, an anti-aliasing filter (48) associated at the output with an automatic gain control module.

10. An anti-interference device according to any one of the preceding claims, wherein, for each assembly associated with a modulation, comprising a pre-cumulative filter adapted to said signal modulation and a first amplitude processing module associated at the output of said pre-cumulative filter, said first and / or said second amplitude processing module comprises: - an interference rejection filter comprising a multiplicative gain applied respectively to an in-phase channel (I) and to a quadrature channel (Q) of said radionavigation signal received and digitized by the analog-to-digital converter (12), the value of the multiplicative gain being determined by a non-linear function K;(p) of the magnitude p of the received signal, the function K;(p) being determined so as to maximize the signal-to-noise ratio after application of the multiplicative gain and demodulation of the received signal, and - an estimator of a J / N ratio between the power of an interfering signal; and the power of a thermal noise, the non-linear function K(p) depending on the ratio J / N, the value of the multiplicative gain being calculated from said ratio J / N, and in which said device further comprises another automatic gain control module connected between said pre-accumulation filter and said first amplitude processing module downstream of said pre-accumulation filter.

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