EP1463954A1

EP1463954A1 - Device and method for the suppression of pulsed wireless signals

Info

Publication number: EP1463954A1
Application number: EP02806033A
Authority: EP
Inventors: Estelle Thales Intellectual Property KIRBY; Alain Thales Intellectual Property RENARD
Original assignee: Thales SA
Current assignee: Thales SA
Priority date: 2002-01-08
Filing date: 2002-12-20
Publication date: 2004-10-06
Also published as: CA2439563A1; US7161528B2; WO2003058270A1; FR2834563A1; AU2002365019A1; US20050104767A1; FR2834563B1

Abstract

The invention relates to a method for suppressing pulsed signals, particularly signals of types DME or TACAN, occurring in the wireless signals received (Ue) by a radio frequency receiver. The reception frequency band of the receiver is divided into frequency sub-bands corresponding to the transmission channels of the pulsed signals. The pulsed signals and the transmission channel of said pulsed signals are detected in the frequency sub-bands. The frequency sub-band containing the detected pulsed signals is filtered during the time that the pulsed signals last in order to eliminate said pulsed signals. The inventive method protects wireless receivers of types GPS or GALILEO from interference by pulsed DME/TACAN-type signals.

Description

DEVICE AND METHOD FOR SUPPRESSING PAULS RADIO SIGNALS

The invention relates to a method for suppressing in a radio signal received by a receiver, pulsed signals, in particular those emitted by civilian distance measurement equipment, "Distance Measuring Equipment or DME" in English, between an aircraft and a ground beacon.

The use of the L5 radio frequency band for the global radiodetermination system or GPS (English name for

"Navstar Global Positioning System") in the civil sector or, for the E5 band for the European GALILEO system, there is the problem of compatibility with the signals transmitted by the DMEs.

In fact, one of the problems posed by the transmission of data by radio spectrum signals, such as those of GPS signals or those of GALILEO, is the sensitivity of the receivers to the jammers taking into account the low powers involved and the large distances separating the transmitters and receivers.

The signal in the frequency bands for civil GPS III is obtained by the modulation of a carrier by a first signal, "pilot channel", producing a modulation spectrum with a width of the order of 20 MHz and by a second quadrature signal modulated by the navigation message, "given channel", producing a second modulation spectrum of 20 MHz.

FIG. 1 represents an example of a pulsed signal emitted by a distance measuring equipment (DME) between a beacon on the ground and an aircraft. We will call this signal thereafter, DME signal. Figures 2a and 2b respectively represent the frequency aspect of a pulse and a pair of pulses.

The DME signal comprises two pulses spaced apart by a predetermined time t0 depending on the mode (ARINC 709 A standard). For example, in the X mode of ground beacons, this time t0 of spacing between the pulses is of the order of 12 μS. The pair of pulses is repeated with a frequency of the order of 2700 pairs of pulses per second. Each pulse has a Gaussian shape envelope and a carrier frequency depending on the tag.

Each beacon on the ground is recognized by the aircraft interrogating the beacon by the frequency of the pulse transmission channel. A database in the aircraft makes it possible to obtain the position and the frequency of each beacon. The DME transmission channels for the various beacons are in 1 MHz steps.

Due to its large coverage, the GPS receiver can receive in its frequency band a multitude of DME pulsed signals from several beacons located on the ground. These signals emitted at random times and at frequencies spaced at 1 MHz, cause a deterioration of the signal to noise ratio of the GPS receiver and, consequently, measurement errors.

In the general case, the effects of the interference of the signals drawn on the receivers depend on many factors among which one can quote, the peak power, the duty cycle, the width of the pulse.

GPS receivers in particular work with very low wanted signal levels of the order of -130dBm, the level of the noise floor of the receiver. The type of coded digital modulation used in GPS receivers allows extraction of the signal useful for these low reception levels. Under these conditions, with a very low useful signal power received, a low level pulse at the input of the GPS receiver, of the order of 10 dB above the noise floor, can produce saturation of the input stages radio frequency and saturation of the receiver's analog-to-digital converter. Analog solutions can be developed to avoid these saturations. To be able to develop a digital solution, the radiofrequency input stages and the analog-to-digital converter must be dimensioned in order to avoid this saturation taking into account the interference environment. To avoid this type of disturbance by DME a simple solution would consist in reassigning the frequencies of DME, but if this solution is possible in the United States of America, it is not possible in Europe or in Japan because of the high density radio beacons.

There are currently many devices and methods using anti-interference treatments to detect and to eliminate interference from a received signal so as to recover a clean signal usable by a standard GPS receiver at the output of the receiver. Among these methods, one can cite that of modifying the radiation pattern of the receiving antenna to create a receiving trough (or zero) in the direction of the disturbers, a technique known under the name in English of "Controled Reception Pattern Antenna ”or abbreviated CRPA.

Other methods are based on estimates which are all the more precise when they are carried out over relatively long periods during which interference is present. These methods have major drawbacks, on the one hand, they are ill-suited to intermittent interference and, on the other hand, they require to operate, in the latter case, that the interference is observed over a sufficiently long time interval. long, therefore in particular, that it is geographically stable which is not the case for pulsed interference.

Other methods such as "adaptive filtering in the frequency domain" or that of detecting the pulse and cutting the receiver for the duration of the pulse ("puise blanking" in English) are potential solutions. The drawback of the first solution based on digital filtering techniques is linked to the fact of the need for too much computing power.

In the “puise blanking” method, which consists in cutting off the receiver for the duration of the DME pulses when they are detected in the received signal, the interference pulse is easily detectable by the fact that the signal normally received by the receiver GPS is located at the level of thermal noise. For example, the detection of the pulse can be carried out by the relative analog measurement of radio signal strength or, more simply, when the received signal is digitized by an analog-digital converter, by an analysis of the histogram of the digital levels. in real time. In the latter case, the pulses are deleted by setting the digital output of the converter to zero at the time of the pulses.

The “blanking” method is a simple technique but which has a major drawback, in fact, it produces a degradation important of the signal-to-noise ratio of the receiver because the useful signal is completely cut off for the entire duration of the pulses. In the case of receivers of the GPS or GALILEO type, the useful signal cuts can be numerous at high altitude because of the large number of DME beacons on the ground for aircraft and the repetition frequency of the DME pulses.

The same problems arise for wideband receivers receiving pulses used in the military field, such as those of tactical air navigation equipment "Tactical Air Navigation or TACAN" in English.

In order to overcome the drawbacks of anti-interference devices of the prior art in radio receivers, in particular those of GPS III or GALILEO, the invention proposes a method of suppressing pulsed signals in particular of DME or TACAN type present in radio signals. received by a radio frequency receiver, characterized in that the receiver reception frequency band is divided into frequency sub-bands corresponding to the channels of transmission of the pulsed signals, in that the presence of the pulsed signals is detected and the transmission channel of said signals drawn from the frequency sub-bands, and in that the frequency subband comprising the detected pulsed signals is filtered, for the duration of the pulsed signal, in order to eliminate said pulsed signals. This invention can use the knowledge of the interference to be treated.

DMEs are modulated by an integer frequency spaced in steps of 1 MHz: for example, 1174 MHz, 1175 MHz, 1176MHz .... The number of DME channels that can be considered as annoying in the L5 band is a maximum of twenty . It is possible to consider fewer channels: the disturbance of the central frequencies of the spread radio navigation signal has much more consequences than the disturbance of the lateral frequencies. Twenty channels will be taken as an example. Figure 3 shows a block diagram of the invention.

The first operation of the process is to determine the unknown parameters of the pulsed signals and then eliminate them.

The invention also relates to a device for implementing the method according to the invention. In an implementation of the method of suppressing the pulsed signals, the cutting of the reception frequency band into twenty sub-bands is carried out by a battery of detection filters FD1, FD2, ... FD20 adapted to the form d 'wave. For example, this battery of detection filters can also be a battery of band-pass filters if no knowledge is used a priori on the form of the interference or a battery of filters with impulse response adapted to the form of the interference in each band, which amounts to making an intercorrelation. The signal Ue to be processed enters this filter bank via a common input E1, it comes out at the respective outputs S1, S2 .... S20 of the filters, twenty bandwidth signals approximately 0.3MHz.

Each signal at the output of the band pass filter FD1, FD2, .... FD20, attacks a respective detection device DD1, DD2, .... DD20 for each sub-band. The detection device can be based on a simple measurement of the power of the signal at the output of the detection filter. This power measurement is filtered and then compared to a threshold for each sub-band. The decision of the detection device is determined for each sub-band.

Advantageously, the detection device can be adapted to the shape of the signal in order to improve the detection performance. From the information coming from the detection devices DD1, DD2, .... DD20, a calculation Cg of the frequency mask to be eliminated and a calculation Cf of the rejector filter to be used are carried out. The signals received Ue, after having undergone a delay Tr, are processed by an adaptive filter Fa of parameters originating from the calculation Cf of the rejector filter. The invention will be better understood with the aid of exemplary embodiments of devices for suppressing pulsed radioelectric signals according to the invention, with reference to the appended drawings, in which:

- Figure 1, already described, shows an example of a pulsed signal;

- Figures 2a and 2b, already described, respectively represent the frequency aspect of a pulse and a pair of pulses.

- Figure 3, already described, shows a block diagram of the invention;

- Figure 4 shows a possible embodiment of the invention;

- Figure 5 is a first variant of the device of Figure 4; FIG. 6 constitutes a second variant of the device of FIG. 4.

FIG. 4 represents a possible embodiment of the invention. The battery of detection filters FD1, FD2 ... FD20 comprises a common input E1 attacked by the received signal Ue, of bandwidth 20MHz, interfered with by the DMEs. At the outputs S1, S2 .... S20 of the detection filters, there are the channels containing the useful signal, the thermal noise and any DME interference. Each channel output S1, S2 .... S20 attacks its respective detection device DD1, DD2, ..., DD20. Each detection device DD1, DD2, ..., DD20 makes it possible to measure and filter the power of the signals on each channel. The comparison with respect to a threshold makes it possible to determine the presence or not of DME interference in the twenty channels observed. The decisions of the detection device make it possible to determine the outputs of the isolation filters FI1, FI2, .... FI20 respective to each channel to be used in the result of the filtering through the respective switches 11, ..., I20 controlled by their respective detection devices DD1, DD2, ..., DD20. The input signal Ue is delayed by the delay Tr before attacking the isolation filters. The result of the isolation FI1, FI2 ..... FI20 is the sum, by the summator Sm, of the signals at the output of the isolation filters.

The isolation filters FI1, FI2 ..... FI20 applied to the signal Ue to be processed are a battery of 20 bandpass isolation filters. The size of the isolation filters is adapted to the spectrum of the signal to be eliminated. The decisions of the detection devices make it possible to determine whether or not to use the output of each isolation filter. If the useful signal is not disturbed, the result of the isolation is the sum of the signals at the output of the isolation filters. If the detection device has seen one or more disturbed frequencies in the useful signal, the result of the isolation is the sum of the filters for isolating the undisturbed sub-bands. In order not to deteriorate the signal in the absence of interference, the sum of the outputs of all the filters (signal Us) is equal to the input signal Ue. The transfer functions of the insulation filters must therefore be complementary.

The set of devices FD1, FD2, ..., FD20 of FIG. 3 is a battery of twenty filters, each filter having a bandwidth Bs adapted to the DME / TACAN signal substantially equal to 0.3 MHz. Figure 5 is a first variant of the device of Figure 4. In this first variant, the detection devices are also used for insulation. In this case, this device is of the blanking type.

In the variant of FIG. 5, the input signal Ue being applied to all of the detection filters FD1, FD2, ..., FD20, each output S1, S2 .... S20 of the filters attack on the one hand , a respective detection and isolation device DDI1, DDI2 ..... DDI20 and, on the other hand, the respective inputs es1, es2, ... es20 of a summator Sm through the delay Tr and a respective switch 11, 12, ..., 120 controlled by the associated detection and isolation device DDI1, DDI2, .... DDI20.

FIG. 6 constitutes a second variant of the device of FIG. 4. The isolation filter bank is replaced by a convolutional adaptive filter Fad of the RIF type whose impulse response is the sum of the impulse responses corresponding to each frequency band, except those disturbed.

In the embodiment of the second variant, shown in FIG. 6, each output S1, S2 .... S20 of each detection filter FD1, FD2, ..., FD20 attacks its respective detection device DD1, DD2, .. ., DD20. The input signal Ue is applied on the one hand to the inputs of the detection filters and, on the other hand, to the input of the convolutional adaptive filter Fad of the RIF type through the delay Tr. Each impulse response RM, RI2. .... RI20, corresponding to each channel strip, is applied through its respective switch 11, 12 I20 controlled by its respective detection device DD1, DD2, ..., DD20 to a summator Sp performing the sum of the impulse responses corresponding to each frequency band, except those disturbed, the impulse response of the filter Fad being that provided by the summator Sp.

The interference suppression method according to the invention is optimized due to the taking into account of the characteristics of DME / TACAN. One of the advantages of the method and of the devices for implementing the method according to the invention lies in the fact that a small part of the received band is suppressed in the presence of the pulses, of the order of 1 MHz (DME channel) on the 20 MHz bandwidth of the received signal and only for very short times corresponding to the durations of the DME / TACAN pulse (a few microseconds), the signal received in the sub-band considered between two consecutive pulses not being cut. In this way, the signal is received by the receiver between the pulses, therefore for almost all of the time.

With a random modulation code as used in GPS or GALILEO type receivers, these holes in the short reception band practically produce no measurement error in the receiver. The probability of completely blocking the received signal is almost zero compared to the prior art "sink blanking" device which suppresses all of the signal received by the receiver during the duration of the pulsed signals.

Claims

1. Method for suppressing pulsed signals in particular of the DME or TACAN type present in the radioelectric signals received (Ue) by a radiofrequency receiver, characterized in that the reception frequency band of the receiver is divided into frequency sub-bands corresponding to the transmission channels of the pulsed signals, in that the presence of the pulsed signals is detected and the transmission channel of said pulsed signals in the frequency sub-bands, and in that one filters, during the duration of the pulsed signal, the frequency sub-band comprising the detected pulsed signals, to eliminate said pulsed signals.

2. Method for suppressing pulsed signals according to claim 1, characterized in that the cutting of the reception frequency band is carried out by a battery of detection filters (FD1, FD2 ..... FD20) adapted to the shape wave.

3. Method for suppressing pulsed signals according to one of claim 2, characterized in that each signal at the output of the detection filter (FD1, FD2,.-... FD20), attacks a respective detection device (DD1, DD2, .... DD20) for each sub-band.

4. Method for suppressing pulsed signals according to one of claims 2 or 3, characterized in that the battery of detection filters (FD1, FD2 ..... FD20) is a battery of bandpass filters.

5. Method for suppressing pulsed signals according to one of claims 2 or 3, characterized in that the battery of detection filters (FD1, FD2, .... FD20) is a battery of impulse response filters.

6. Method for suppressing pulsed signals according to one of claims 1 to 5, characterized in that the detection device is based on a simple measurement of the power of the signal at the output of the detection filter (FD1, FD2 ..... FD20), this power measurement being filtered then compared to a threshold for each sub-band, the decision of the detection device (DD1, DD2 DD20) being determined for each sub-band.

7. Method for suppressing pulsed signals according to one of claims 3 to 6, characterized in that from information coming from the detection devices (DD1, DD2, .... DD20) a Cg template calculation is carried out frequencies to be eliminated and a calculation Cf of the rejector filter to use, the received signals (Ue), after having undergone a delay Tr, being processed by an adaptive filter (Fa) of parameters resulting from the calculation Cf of the rejector filter.

8. A method ^" of suppressing pulsed signals according to one of claims 1 to 7, characterized in that the reception frequency band of the receiver is cut into twenty sub-bands.

9. Device for suppressing pulsed signals in particular of the DME or TACAN type present in the radioelectric signals received (Ue) by a radiofrequency receiver, the reception frequency band of the receiver being divided into frequency sub-bands corresponding to the transmission channels pulsed signals, characterized in that it comprises a battery of detection filters (FD1, FD2 ... FD20) comprising a common input E1 attacked by the received signal (Ue) and outputs (S1, S2 .... S20) of the channel, each channel output attacking a respective device making it possible to filter and measure the power of the signals on each channel, the comparison with respect to a threshold makes it possible to determine the presence or not of DME interference in the channels observed.

10. Device for suppressing pulsed signals according to claim 9, characterized in that each channel output (S1, S2 .... S20) attacks a respective detection device (DD1, DD2, ..., DD20), the decisions of the detection device making it possible to determine the outputs of the isolation filters (FI1, FI2 ..... FI20) respective to each channel to be used in the filtering result through respective switches (11, 12, ... , I20) controlled by their respective detection devices (DD1, DD2, ..., DD20), the received signal (Ue) being delayed by a delay Tr before attacking the isolation filters, the result of the isolation being the sum, by the summator Sm, of the signals at the output of the isolation filters.

11. Method for suppressing pulsed signals according to claim 10, characterized in that the transfer functions of the isolation filters (FI1, FI2 ..... FI20) are complementary.

12. Device for removing pulsed signals according to • claim 9, characterized in that each output (S1, S2 .... S20) of the detection filters channel attack on the one hand, a respective detection and isolation (DDI1, DDI2 DDI20) and, on the other hand, of the respective inputs (es1, es2, ... es20) of a summator Sm through a delay Tr and a respective switch (11, 12, ..., I20) controlled by the associated detection and isolation device (DDI1, DDI2, ..., DDI20).

13. Device for removing pulsed signals according to claim 9, characterized in that the respective output (S1, S2 .... S20) of each detection filter (FD1, FD2, ..., FD20) attacks its respective device detection (DD1, DD2, ..., DD20), the input signal Ue being applied on the one hand to the inputs of the detection filters and on the other hand to the input of a convolutional adaptive Fad filter of the type RIF through delay Tr, each impulse response (RI1, RI2 ..... RI20), corresponding to each channel strip, being applied through its respective switch (11, 12, ..., I20) controlled by its respective detection device (DD1, DD2, ..., DD20), to a summator Sp effecting the sum of the impulse responses, corresponding to each frequency band, except those disturbed, the impulse response of the filter Fad being that provided by the summator sp.

14. Device for removing pulsed signals according to one of claims 9 to 13, characterized in that the set of detection filters (FD1, FD2, ..., FD20) is a battery of twenty filters, each filter having a bandwidth Bs adapted to the DME / TACAN signal substantially equal to 0.3 MHz.