FR2971340A1 - METHOD FOR MEASURING TRANSMISSION FREQUENCIES USING A ROTARY INTERFEROMETER - Google Patents

Abstract

The subject of the invention is a method for measuring the carrier frequency of the radio signal emitted by a remote transmitter by means of an antenna comprising at least two broadband sub-antennas situated in the same plane and whose phase centers are separated. from a distance d. These sub-antennas are associated so as to form an interferometry base. The radio signals received by the two sub-antennas are processed together to form a video signal, characterizing the phase difference existing between the radio signals sensed by these sub-antennas. The frequency of the video signal is measured and the carrier frequency of the radio signal emitted by the transmitter is deduced therefrom. The measurement of the carrier frequency of the radio signal emitted by the transmitter is equal, to a scale factor, to the measurement of the frequency of the formed video signal.

Description

The invention relates to the general field of radar reception and more particularly broadband frequency analysis of the received signals.

The technical problem considered in the context of the present invention is that of the measurement, by means of an inexpensive device, of the carrier frequency of a signal which may be continuous or pulsed (non-continuous duration), for example radar signals. The present emission frequency measurement systems have average quadratic accuracies, rms (i.e. "root mean square" according to the English name) of the order of Megahertz. They mainly use two techniques, depending on whether the received reception is instant broadband or not. In the case of broadband reception, ie when the relative instantaneous band covered extends over several octaves, the technique used always amounts to obtaining measurement accuracy by converting the carrier frequency of the signal into a signal. phase shift linearly dependent on a delay i (A (p = 27cfi), and by measuring this phase shift using a device acting as a phasemeter.This principle makes it possible to obtain the desired measurement accuracy, but leads to a ambiguous measurement, the phase difference being measured only to 21cc, this ambiguity must be removed, which complicates the structure of the measurement means implemented In the case where the reception is not a broadband reception that is, if the relative instantaneous band is much below the octave, the technique used is generally based on the use of a spectral analysis whose resolution offers much of the precision sought. , the band e instant being very below the total frequency coverage, it is resorted to superheterodyne-type reception means necessarily complex from a hardware point of view (double frequency change, preselection filter bank, last frequency filter intermediate very stiff, etc ...).

An object of the invention is to propose an alternative technical solution to current solutions. These offer good performance, but at the cost of hardware additions that can reach high levels of complexity, such as spectral analyzers necessarily having to be associated with superheterodyne receivers with wide frequency coverage. Another object of the invention is to take advantage of a reception architecture that may exist in addition, interferometer architecture in rotation in particular, without adding hardware to perform this frequency measurement function. In other words, a goal of ~ o the invention is to use already existing signals to propose a solution favoring simplicity much more than the technical performance and, therefore, less expensive to implement.

To this end, the subject of the invention is a method for measuring the carrier frequency of the radio signal emitted by a remote transmitter, which implements an antenna comprising at least two broadband sub-antennas situated in the same plane and whose Phase centers are separated by a distance d, the sub-antennas being associated to form an interferometry base. The radio signals S1 and S2 received by the two sub-antennas are processed together to form a video signal S, characterizing the phase difference existing between the radio signals S1 and S2 picked up by these sub-antennas, whose frequency is measured. The measurement of the frequency of the radio signal emitted by the transmitter being equal, to a scale factor, to the measurement of the frequency of the video signal S formed.

In a simple implementation mode, the formed video signal is obtained by directly demodulating the radio signal S1 or S2 received by one sub-antenna by the radio signal S2 or S1 received by the other sub-antenna.

In a particular embodiment, the method according to the invention implements a monopulse phase antenna, comprising two sub-antennas whose phase centers are spaced a distance d substantially equal to the width L d a sub-antenna.

In a particular mode of implementation, the estimation of the frequency of the formed video signal S is obtained by spectral analysis of this signal. According to a preferred variant of this mode of implementation, the spectral analysis of the video signal S formed is carried out by FFT.

In another particular embodiment, the spectral analysis of the formed video signal S is performed by correlation with a replica of the emitted signal defined a priori.

According to a variant of this mode of covering, the replica used corresponds to the phase variation expected a priori between the signals S1 and S2 received by the two sub-antennas for a given frequency.

According to another variant of this mode of implementation, the replica used corresponds to the phase and amplitude variations expected a priori for a given frequency and for the corresponding antenna pattern 20 for each sub-antenna at this frequency.

In another embodiment of the method according to the invention, the antenna used is an antenna comprising a plurality of broadband sub-antennas arranged to form two-by-two interferometry bases, at least one of which gives an ambiguous measure of the frequency of the radio signal emitted, the ambiguity of the measurement being raised by associating this measurement with the measurements made by the other bases.

In another embodiment of the method according to the invention, a plurality of interferometry bases are used, arranged in a 360 ° plane about a central axis and rotating around this axis.

The invention advantageously constitutes an inexpensive device which does not necessarily aim at obtaining a mean square-frequency precision of the order of one MHz, insofar as, according to the application considered, a few tens of MHz may suffice. In addition, this low cost requirement makes it possible to look for solutions that can take advantage of a device performing another function in its own right.

The invention lies precisely in this context. Indeed, it can be grafted free, that is to say without adding material, to a particular direction finding device.

Advantageously, the device according to the invention can be used both to detect a signal, to estimate its frequency and its direction of arrival. It can also be applied to other areas of frequency measurement, such as the field of acoustic waves.

The characteristics and advantages of the invention will be better appreciated thanks to the description which follows, a description which is based on the appended figures which represent: FIG. 1, the basic diagram of the interferometry base implemented by the process according to the invention; FIG. 2 is a schematic diagram of a simple embodiment of the reception chain used by the method according to the invention; - Figure 3, a timing diagram of the signal measured by the method according to the invention from two sub-antennas arranged to form an unambiguous interferometry base; FIG. 4, the spectrogram of the signal measured by the method according to the invention, obtained by spectral analysis by FFT of the signal S formed from the signals received by the sub-antennas constituting an unambiguous interferometry base; FIG. 5, the spectrogram of the signal measured, obtained by multi-replicative correlation spectral analysis of the signal S formed from the signals received by the sub-antennas constituting an unambiguous interferometry base; FIG. 6, the schematic illustration of an antenna forming an unambiguous interferometry base; FIG. 7 is a diagrammatic illustration of an antenna forming an ambiguous interferometry base; FIG. 8, a timing diagram of the signal measured from two sub-antennas arranged to form an ambiguous interferometry base such as that of FIG. 7; FIG. 9, the spectrogram of the measured signal, obtained by multi-replicative correlation spectral analysis of the signal S formed from the signals received by the sub-antennas constituting an ambiguous interferometry base such as that of FIG. 7; FIG. 10, the schematic illustration of an antenna consisting of two omnidirectional sub-antennas 101 and 102 in the plane of rotation, forming a particular ambiguous interferometry basic case; FIGS. 11 and 12, respectively the timing diagram and the spectrogram relating to the measurement of the emitted signal obtained from two sub-antennas arranged to form an ambiguous interferometry base such as that of FIG. 10; - Figures 13 and 14, schematic illustrations of antennas implemented by the method according to the invention and to form ambiguous and unambiguous interferometry bases; FIG. 15 is an illustration of a particular interferometry base arrangement enabling instantaneous 360 ° measurement of signals emitted by remote sources. FIG. 16, the block diagram of an application implementing the method according to the invention for making a coarse frequency measurement of the received signal, an unambiguous measurement associated with a precise but ambiguous measurement of the frequency of the received signal; to obtain in a simple way a precise and unambiguous frequency measurement; FIG. 17 is a graph illustrating how, as a function of the true frequency of the received signal, the ambiguous frequency F 'obtained from the phasemeter associated with the means embodying the method according to the invention varies.

The proposed solution consists, in principle, of using a rotating antenna comprising a plurality of distinct radiating subassemblies, at least two subassemblies, spaced apart from each other, configured to pick up the emitted signals, each subset being configured to give rise to a separate receive channel. The subsets form a base for interferometry in rotation about an axis so as to cover a given angular range. In the simplest embodiment, very inexpensive with respect to conventional solutions, the antenna according to the invention comprises two broadband channels, which give rise to obtaining after demodulation to a video signal in a band. narrow.

In its general form, as illustrated in FIG. 1, the interferometry base is constituted by an antenna rotating around an axis of rotation 15 and comprising at least two sub-antennas 11 and 12 having phase centers distant between they have a given length d.

Thus, assuming that the two elements of the antenna forming the base of interferometry are illuminated by a monochromatic plane wave of wavelength λ, the phase difference Acl). between the signals S1 and S2, represented by the arrows 13 and 14, respectively received on the elements (i.e. the sub-antennas) 11 and 12 of the antenna is written: A0 = 2nd / to sin (0t) [1]

Where 0 is the rotational speed of the antenna constituting the base and t the time.

As a result, the frequency measurement method according to the invention mainly consists of knowing 0 moreover, to measure over time the phase difference between S1 and S2 to deduce therefrom and therefore the frequency of the received signal.

The invention is thus based on the constitution and the use of a base of interferometry in rotation about an axis 15 so as to cover a given angular range. The interferometry base consists of at least two broadband sub-antennas whose phase centers 11 and 12 are separated by a distance d. Each sub-antenna separately delivers a signal to the associated reception means.

The invention then consists, in principle, of amplifying the radio signals from the two sub-antennas, of transposing each signal separately with respect to the same reference oscillator, and then of transposing the demodulated signals into video frequency. The phase difference between the two video signals obtained is then extracted to deduce the frequency of the received signal.

In a simple embodiment, illustrated in FIG. 2, the radio signals S1 and S2 received from the two sub-antennas 21 and 22 are directly demodulated 23 together, one demodulating signal the other, in analog form, in radiofrequency (RF ), so that a signal S is obtained in video frequency representative of the phase difference between S1 and S2.

As a result, a simple spectral analysis of the signal S by Fourier transform for example, makes it possible to make a rough estimate of the measurement of the wavelength, of the frequency, of the signal received by the antennas 21 and 22.

The advantage of this method is that it makes it possible to estimate the frequency of a signal whose frequency can be in a very wide range, for example between 2 and 18 GHz, without electronic means or important calculation means, estimation of the frequency of the RF signal received by the sub-antennas being carried out in video frequency on the signal S.

The frequency Fv of the video signal S thus obtained from the signals S1 and S2 is related to the frequency F of the radio signal received by the sub-antennas by a scale factor which, for a given antenna, depends only on the speed of rotation of the antenna: F '= F d / c sin (Ot) / t [2]

where c represents the propagation speed of the waves (typically 3 108 m / s). Therefore, using the small angle approximation, valid since the direction of the radiation source is located in the aperture of the antenna, one can write: F '= F S2 d / c [3 ] ûd / c representing the scale factor.

Thus, for example, for a distance d between the two centers of ~ o phase sub-antennas 11 and 12 equal to 30cm, a frequency F of 10GHz, and an angular velocity of 1tour / s (ie 4 = 2rr radians / s ), Fv = 63Hz is obtained. The scale factor is then equal to 6.3 10-9.

It should be noted that, if the antenna is relatively directive, the signal S, resulting from the demodulation of the signals S1 and S2 by each other, corresponds to a sinusoid section whose amplitude is modulated by the shape of the antenna lobe. FIG. 3 shows the shape 31 that the temporal signal S takes in this case, and FIG. 4 the result 41 of the frequency estimation of the signal S, obtained in the same case. The signal S, considered here by way of example, is a signal resulting from the demodulation, by the other, of the signals S1 and S2 received, for a sinusoidal emission at 9.87 GHz, by two sub-antennas. length 30cm whose phase centers are 30cm apart, the overall antenna being rotated at 1 rpm. Moreover, the spectral analysis of S is here carried out by FFT. In alternative embodiments of the invention, which are more efficient in terms of sensitivity and estimation accuracy but less immediate implementation, the received signals S1 and S2 can undergo, before processing, before combination. , several frequency transposition, filtering and amplification operations. Similarly, the signals S1 and S2 may, according to a particular implementation form, be separated into different subbands so as to increase the frequency discrimination capacity of the analysis performed. In a preferred embodiment of the invention, illustrated in FIG. 5, the frequency estimation process is performed, after digitization of the signal S, by multi-replica correlation 51 (in digital form). according to a suitable treatment.

In a simple form of implementation the correlation is carried out on the phase only. The operation performed for each replica considered is then: EN 1S; exp ((2njdFk / c) sin (OiAt)) I ~ = o 1 o Yk = [4] where:

k is the index of the correlation filter (i.e. the index of the replica 15 considered); Fk is the frequency corresponding to the index filter k; N is the total number of samples of the signal used for the correlation calculation; At is the time interval between two successive samples; If is the value of the signal demodulated at the instant i'At; Yk is the amplitude at the output of the rank correlation filter; this is the speed of propagation of the waves.

Advantageously, such a treatment considerably reduces by comparison a conventional non-adapted processing method, of the FFT type, by means of the frequency estimation. Thus, in the example considered, the received frequency measurement (10 GHz) by FFT (9.3 GHz), 42, appears to be different from the more faithful measurement, 52, obtained by correlation (9.9 GHz).

It should be noted that the resolution (or the separating power) of the frequency estimator thus implemented can be evaluated by considering that the observation window of the signal is limited by the angular aperture at -3 dB of each sub. -antenna.

Thus, if the angular aperture of each sub-antenna is A6 = c / (FL), where L is the length of the sub-antenna, the observation time 'robs is then equal to: Tobs = M / D = c / (F'LD) [5] So that, for the video signal, the analysis resolution is then given by the relation: Efv = FL «O / c, [6] while for the hyper signal we have , from the relation [3]: [7] δF = FL / d Thus, if, as in the example illustrated in FIG. 6, the width L of each of the sub-antennas 11 and 12 is equal to the distance between their phase centers 61 and 62, which is the classic case of a monopulse phase antenna, the resolution of the device can be estimated as being substantially equal to the frequency of the observed signal.

For example, considering a frequency of F = 10 GHz, a rotation speed of 1 revolution / s, an antenna length L = 30 cm, a distance between the two phase centers d = 30 cm, the analysis resolution is δF '. = 63Hz for the video signal and bF = 10GHz for the microwave signal.

It should also be noted that the estimation precision of the frequency 6F (ie the standard deviation of the measurement made according to the invention) is, for its part, related to the analysis resolution by the signal-to-ratio. -noise. o.F is given by the following relations: 6F = S F / 2.5, / 25 / B [8] or again35 Il aF = (FL / d) / 2,5V2S / B [9]

Thus, in the above example, for a signal-to-noise ratio of 20 dB (ie a linear scale ratio 100), the estimation accuracy of the video frequency is 1.8 Hz, which corresponds to a theoretical accuracy. to estimate the frequency of the received signal by 285 MHz, taking into account the scale factor. For a signal-to-noise ratio of 40dB, the accuracy is 28.5MHz. It should be noted here that to the extent that the signal-to-noise ratio S / B in reception varies proportionally to the antenna gain G = 4rrS / A2, it is found that for a given antenna surface, the accuracy is advantageously independent of the frequency. As a result, since the requirements concerning the accuracy of the estimation of the direction of arrival are generally independent of the frequency of the signal, it is advantageously possible to cover with the same antenna device a large frequency band, provided that said antennas are able to receive signals throughout this frequency band.

It can also be seen that the resolution and the analysis accuracy are independent of the rotation speed O. However, an error in the value of the rotational speed Q results in an error in the frequency measurement itself.

In an implementation more sophisticated than the form described above, the correlation can be performed on the amplitude and on the phase simultaneously. In this case, the replica used for the correlation processing takes into account the shape of the antenna pattern corresponding to each frequency Fk for which the correlation is performed. This variant advantageously makes it possible to reduce the secondary lobes present at the output of the correlation filter applied to the signal, the antenna diagram realizing an amplitude weighting function of the signal observation window. This advantageously makes it possible to reduce the estimation bias mentioned above as well as the measurement accuracy.

The correlation function is then written: N-1 Yk HI S; G. exp ((2njdFk / c) sin (S2iAt)) i = o 1, k where G;, k is the antenna gain in the pointing direction 6; = Q'i "At, for the frequency Fk.

The resolution is not affected by this estimate. In an alternative embodiment of the invention, the distance d separating the phase centers 71 and 72 of the sub-antennas 11 and 12 is chosen, as shown in FIG. 7, so as to be greater than the length L of a sub-antenna. In this way, the resolution and accuracy of the measurements made by the measuring method according to the invention can be advantageously increased. FIGS. 8 and 9 respectively show the shape 81 of the demodulated signal S and that of the correlation result 91, in the case where d = 10 L, for a sub-antenna of length 30 cm and a transmission signal at 19 , 87GHz.

It should be noted that the improvement of the resolution can also be obtained by decreasing the length of each sub-antenna, that is to say by opening its radiation pattern. The limit case of such a solution 25 is however that of an interferometry base consisting of two omnidirectional sub-antennas 101 and 102 in the plane of rotation, whose phase centers are separated by a distance d, as the FIG. 10 shows such a variant. In particular, this variant is advantageous when the range report does not require a large antenna gain. By way of example, FIGS. 11 and 12 respectively show the shape 111 of the demodulated signal S and that of the correlation result 121, for an antenna base of 30 cm consisting of two omnidirectional antennas in the plane of rotation. for an emission signal at 9.83GHz. [10] It should be noted that when the antenna is also used to estimate the angular position of the target, the angular estimate becomes ambiguous as soon as the distance between the two phase centers 11 and 12 is greater than the length. L of a sub-antenna. In this case, it may be necessary to use as an interferometry base an antenna comprising a plurality of sub-antennas 131, 132,..., 13n, in other words more than two sub-antennas, such as that illustrated by FIG. Figure 13, so as to remove ambiguities. For this purpose, it is possible, for example, to use an antenna comprising three sub-antennas, as illustrated in FIG. 14, two identical and contiguous sub-antennas 141 and 142 constituting a first interferometry base 144 producing a first measurement ambiguously, and a third sub-antenna 143 identical to the first two and associated with one of them, to form a second interferometry base 145 realizing an ambiguous but more accurate measurement.

In such a case of use, the previously described treatments are applied in parallel on the two pairs of sub-antennas 144 and 145. The estimate obtained using the first pair makes it possible to determine a first unambiguous but rough estimate of frequency. The second pair provides several possible estimates of the frequency, because of the ambiguity. The estimate closest to that made by the first pair is considered valid. The principle can be extended by increasing the number of sub-antennas, and by associating these sub-antennas two by two so as to develop several independent measurements. The different estimates are then correlated with each other to derive the most likely direction of arrival.

It should also be noted that, in the case where it is desired to increase the probability of interception of the device, it is possible, as illustrated in FIG. 15, to implement the invention using several antennal bases 151 comprising at least two sub-antennas, to form a polyhedron consisting of fixed bases, the assembly rotating about a central axis of rotation 152, the resulting system can advantageously cover up to 360 ° instantaneously. An example of such a system is shown in Figure 15, seen from above. In such a configuration of the invention, each interferometry base 151 processes identically without overlap or with little overlap, a portion of the angular range to be monitored.

FIG. 16 shows a block diagram in which the invention makes it possible to make a very precise frequency measurement, with the addition of a simple conventional line-to-delay frequency (LAR) stage 161, of well-known structure. According to this scheme, the radio frequency signal received is taken from one of the two channels S1 or S2, in this case S2, after RF amplification. After division, this signal is conveyed by two separate channels to the two inputs of a phasemeter 162, one of the channels is a direct channel, without delay, while the other channel is a delay inducing channel T. The result is a phase shift 4 '= 21IFi. As long as the delay i is sufficiently large, a small frequency variation F can cause a phase variation at the level of the measured signal, a variation whose value is large compared to the intrinsic phase measurement error. As a result, after converting the phase shift Acp 'to frequency F' using a table 163, a very precise frequency measurement is obtained. However, this measure of frequency is an ambiguous measure. Indeed, as shown in Figure 17, the phasemeter only measuring phase to 2kn, frequency F '171 folded over a range of 1 / i. In other words, F 'is defined modulo 1 / ti. Nevertheless, provided that the measurement accuracy 163 of the frequency F of the RF signal determined by means of the method according to the invention is sufficient to determine, with a sufficient probability, the frequency band Br, (the width of the band Bn being equal to 1 / ti) in which the RF frequency of the RF signal is located, the combination of the unambiguous frequency measurement produced by the method according to the invention with the frequency measurement carried out by means of the frequency counter, a precise but ambiguous measurement. , advantageously makes it possible to remove the ambiguities and consequently, to determine a frequency F "which is unambiguous and has the precision of F '.

Claims (11)

REVENDICATIONS1. A method for measuring the carrier frequency of the radio signal emitted by a remote transmitter, characterized in that it uses an antenna comprising at least two sub-antennas (11, 12) broadband in the same plane and whose centers of the phase (61, 62) are separated by a distance d, the sub-antennas being associated to form an interferometry base (144), the radio signals S1 and S2 received by the two sub-antennas being jointly processed to form a video signal S, a function of the phase difference existing between the radio signals S1 and S2 picked up by these sub-antennas (11, 12), the frequency of which is measured; the measurement of the frequency of the radio signal emitted by the transmitter being equal, to a scale factor, to the measurement of the frequency of the video signal S formed. 2. Method according to claim 1, characterized in that the formed video signal S is obtained by demodulating the signal S1 received by a sub-antenna by the signal S2 received by the other sub-antenna. 3. Method according to one of claims 1 or 2, characterized in that it implements a phase monopulse antenna comprising two sub-antennas (11, 12) whose phase centers (61, 62) are spaced apart. a distance d substantially equal to the width L of a sub-antenna. 4. Method according to one of claims 1 to 3, characterized in that the estimation of the frequency of the formed video signal S is obtained by spectral analysis of this signal. 5. Method according to claim 4, characterized in that the spectral analysis of the video signal S formed is performed by FFT. 6. Method according to claim 4, characterized in that the spectral analysis of the formed video signal S is performed by correlation with a replica of the emitted signal defined a priori. 7. Method according to claim 6, characterized in that the replica used corresponds to the expected phase variation a priori between the signals S1 and S2 received by the two sub-antennas (61, 62) for a given frequency. 8. Method according to claim 6, characterized in that the replica used corresponds to the phase and amplitude variations expected a priori for a given frequency and the corresponding antenna pattern for each sub-antenna at this frequency. 9. Method according to any one of the preceding claims, characterized in that it implements an antenna comprising a plurality of broadband sub-antennas (131, 132, ..., 13n), arranged to form two by two interferometry bases (144, 145), at least one of which (145) gives an ambiguous measurement of the frequency of the transmitted radio signal, the ambiguity of the measurement being raised by associating this measurement with the measurements made by the other bases ( 144). 10.Procédé according to any one of claims 1 to 7, characterized in that it covers a plurality of interferometry bases (151) arranged in a plane over 360 ° about a central axis (152) and in rotation around this axis. 11. A method for performing an accurate measurement F "of the frequency FWay of the signal emitted by a remote transmitter by means of an interferometry base (21, 22), characterized in that it carries out a measurement F 'of the frequency of the signal emitted from a measurement of the phase (162) of the RF signal received by the interferometry base (21, 22), this frequency measurement having a systematic ambiguity, and which it associates this ambiguous measurement with F 'to the measurement F carried out by implementing the method according to any one of the preceding claims to remove the ambiguity and to determine an unambiguous measurement F "of the frequency of the signal received by the interferometry base.