Patent ID: 12228664

In all the figures similar elements bear identical reference numbers.

DETAILED DESCRIPTION

In relation toFIG.1, a system for locating1a transmitter3of a radio frequency signal comprises at least two receiving stations2a,2b,2c.

The receiving stations2a,2b,2care geographically remote, separate and distant and are in a link with a processing station6which is used to process the signals from these receiving stations2a,2b,2cto determine time offsets and frequency offsets of one and the same signal received by each of these stations and, on the basis of these offsets, deduce therefrom a trajectory of the transmitter3and therefore its location. These time and frequency offsets are the TDOA and FDOA explained in the introduction. The processing station6is, inFIG.1, separate from the receiving stations but in a particular embodiment, one of the receiving stations can also be the processing station.

The transmitter3to be located is for example a satellite but it can be any object as long as it transmits a radio frequency signal which can be received by at least two receiving stations.

InFIG.1, three stations are shown, two stations may be enough but the greater the number of stations the better the accuracy of the computations.

In relation toFIG.2, each receiving station2x(x=a or b or c) comprises a first receiver21xconfigured to acquire signals from the transmitter3to be located and a second receiver23xconfigured to acquire signals from one or more satellites of one or more GNSS constellations5.

Furthermore, the first receiver21xis configured to acquire signals from the known object4. Specifically, since the processing of the known object serves to evaluate the characteristic parameters of the imperfections inherent to the receiving stations and degrading the accuracy of location of the transmitter3, it is consequently essential that the process of receiving the signals coming from the known object4undergoes the same degradations and does not go via a dedicated receiving line.

The signals from natural or artificial celestial objects make it possible to palliate the absence of the GNSS signal as will be seen further on.

These celestial or artificial objects are for example stars or else geostationary satellites. In the following description the expression “known object” will be used to denote these objects. These known objects have the advantage that their ephemerides are well-known and that their trajectory can therefore be computed reliably and the TDOAs and/or FDOAs can be predicted. Specifically, since the TDOA and FDOA are predictable it is possible, by comparing values computed on the basis of the measurements and values resulting from the predictions, to evaluate any errors in the computations of the TDOA and/or FDOA of the transmitter to be located.

Furthermore, a signal emitted by a radiating celestial body is noise related to its equivalent temperature which is therefore detectable if its temperature is relatively high in relation to the cosmic radiation at 3 Kelvins (Sun, moon, quasar, etc.). Since the terrestrial stations are relatively close by comparison with their distance from these celestial bodies, they see this celestial body from a virtually identical angle and therefore receive the same thermal noise, but at times that are slightly shifted due to the difference in mutual separation of the stations.

Hence, the correlation of the two signals will be at a maximum when these two signals have been realigned: the autocorrelation function of wideband white noise is indeed a “Dirac” pulse at time 0. Thus, it will be understood that a noise has correlation properties (the Fourier transform of the autocorrelation function of a noise gives its spectrum by definition).

Returning toFIG.2, each receiving station2a,2b,2cfurther comprises a local clock h2xconfigured to provide a local time base tlocal2x. Furthermore, the receivers21x,23xare clocked to this clock. The term “local clock” is understood to mean an oscillator providing a stable frequency signal which makes it possible, on its rising or falling edges, to trigger and clock the sampling of the acquisitions by each receiver. Moreover, the counting of the clock edges provides a common timestamping of the samples of each acquisition.

When the GNSS signal is available, the local time base is synchronized on an absolute time base resulting from the demodulation of the GNSS signal. In this regard, the second receiver23xis configured to acquire signals from a satellite positioning system5, and is further configured to demodulate the acquired signals to extract therefrom an absolute time base in order to correct each local time base of each receiving station.

Thus, the receiving stations2xare mutually synchronized by means of the signal from the satellite positioning system using this absolute time base. Note that when the GNSS signal is not available, the local time base which is no longer slaved to the absolute time will drift weakly but independently for all the receiving stations such that the separation in synchronization increases with time.

Each receiving station2a,2b,2calso comprises a receiving antenna A1x, connected to each receiver21x,23x. Furthermore, the receiving stations each comprise a communication interface (not shown) to communicate with the processing station6.

As regards the acquisition, each receiver is composed of a conventional radio frequency receiving unit. This receiving line includes a frequency converter slaved to the frequency reference, a multi-channel digitization line deriving from an analog-to-digital converter slaved to the frequency reference. This receiving line well-known to those skilled in the art will not be described in further detail here.

This concerns the case where the GNSS signal is not available such that the local time base is no longer reliable and provides an erroneous date, which slowly drifts as soon as the GNSS signal becomes unavailable.

In this particular situation, a method for locating a transmitter3is described hereinafter in relation toFIG.3. Such a method is implemented in a processing unit7of the processing station6.

At least two receiving stations2a,2b,2cproceed to the acquisition (step E1) and the timestamping (step E2) of the portions of signals from the transmitter3to be located and of at least one known object4. In particular, one obtains for the transmitter a signal Semetteur_x and for the object a signal Sobjet_x. These signals are dated using the local time base of each receiver station2a,2b,2c.

These signals are transmitted (step E3) to the processing station6which will, after receiving these signals (step REC) for example correlate pairwise the signals from several stations in order to be able to compare identical portions of signals to deduce therefrom the TDOAij and/or FDOAij i.e. the time-domain and frequency-domain separations of identical signal portions (the indices i and j denote the stations a, b, c) determined for two stations i,j.

One of the objectives expected by the use of an object with a known ephemeris is to be able to correct the local time bases of the receiving stations, as soon as the TDOA of the known object or else the FDOAs of the known object are used.

As regards the TDOAij it is exactly the difference in propagation time taken by identical portions of the signal of the transmitter3to reach the station i and to reach the station j. Of course these time separations are measured in relation to the local time bases which are inaccurate given the absence of the GNSS signal.

Thus, on the basis of the timestamps of the received signals from at east two receiving stations2a,2b,2cthe processing station determines (step DET1) measured time separations TDOA_objet_ijMES, TDOA_emetteurijMEScorresponding to the received signals relating to the transmitter to be located and to the known object (or objects). Of course, similar processing is possible on the basis of the FDOAs.

Next, on the basis of known ephemerides and determined in an absolute time base in relation to at least one known object, theoretical time and/or frequency separations TDOA_objet_ijTH, FDO_Aobjet_ijTHrelating to the known object are determined (DET2).

By taking the difference of the separations measured and the theoretical values, one determines (step DET3) a time-domain error RES_TDOAij(or TDOA residual) affecting the TDOAs and which makes it possible to correct (step E4) the time bases of the receiving stations. In a similar manner a frequency error (or FDOA residual) could be computed on the bases of the measured FDOA values and the theoretical values.

According to an embodiment, on the basis of the residual errors affecting the measured TDOAs and/or FDOAs data of correction of the local time bases are determined (step DET5).

Then, these correction data are transmitted to each receiving station (step TRANS) which resets the date of its local clock.

According to an embodiment, the processing station keeps the residual and makes the timestamping corrections of the signal portions received from each station (step CONS).

Finally the locating (step LOC1) of the transmitter3using measured TDOAs and/or FDOAs once the receiving stations have corrected their local time base is carried out.

As regards the TDOA, this gives the following expression
TDOA_objet_ijMES=TDOAijréel+CorrNoise+ΔErrGNSSij+ΔErrTshortij+ΔBiasCal
WithTDOAijRéelthe actual physical value, that one is seeking to measure;CorrNoise; the correlation noise, typically AWGN (Additive White Gaussian Noise) (which is white, Gaussian, of zero mean and predictable energy and determined by the channel);ΔBiasCal: the error incurred by the offsets of the physical device of the station and which can be calibrated (time of propagation through the equipment, uncertainty on the actual geographical position of the receivers), They are considered very stable on the scale of several weeks and are therefore considered as known since they are estimated by a calibration process;ΔErrGNSSij: the difference in timestamping error obtained by the use of the GNSS signal (typically low, tending to be of AWGN type).ΔErrTshortij: the difference in short-term timestamping error (which is not compensated for by the GPS correction process, thus the short-term clock jitter).

When the known object is tracked by the receiver stations (nominal rating) its position and therefore the actual TDOAijRéelvalues are known to the nearest error ERR_PROPAG_TDOAijof the propagator which allows it to compute the theoretical value TDOAijTH. By eliminating the terms that are assumed to be known, one therefore defines the TDOA residual by:

RES_TDOAij=TDOAijMes−TDOAijTH=ERR_PROPAG_TDOAij+CorrNoise+ΔErrGNSSij+ΔErrTshortij. (Here, it has been considered that ΔBiasCalis known and has been removed).

Most of these terms are of negligible intensity by comparison with the drift that one is seeking to estimate and fall in the category of noise that can be approached by a low zero-mean noise. Once the stations are in nominal mode (start-up phase finished, time base slaved for the first time to the GNSS, then continuous slaving to the GNSS, etc.) the drift of REF_FREQi(and therefore the term ΔErrTshortij) and the associated time base depends only on the characteristics of REF_FREQiin non-slaved mode. Its characteristics are chosen to be of very good quality. In nominal mode (namely when the time-domain synchronization making use of the GNSS constellations is operational), the tracking of a known object therefore makes it possible to estimate the short-term timestamping error (incurred by the short-term jitter in the local clocks) of the system which can be used to also correct the local clocks of the receiving stations. In one implementation of the invention, the processing loops using the known objects are active and used even when the GNSS synchronization is active and operational. These loops are then used solely in order to correct the short-term jitter in the local clocks.

In the event of the GNSS synchronization no longer being possible, the invention compensates both for mid-term and short-term drifts in the local clocks.

As described, the obtainment of the time-domain error is based on the tracking of a known object. The reliability of the measurements concerning it is therefore critical.

Specifically, when these are geostationary satellites, these latter can be in maneuvering phases such that their trajectories are not predictable on the basis if the ephemerides. The method of location of the transmitter to be located supposes the prior (and where applicable simultaneous) location of the reference artificial objects (step LOC2).

Hence, the locating method comprises a step of determining (step DET4) on the basis of the measured TDOAs and/or FDOAs relating to the known object, an indicator of reliability of the measured time separations, said reliability indicator having the aim of determining whether or not the ephemerides of the known object can be used for the local clock correction. This reliability indicator in particular makes it possible to determine whether or not the known object is in the process of maneuvering.

Of course the known object, when it is a celestial object (for example the sun) is not concerned by these concepts of reliability. Specifically, these known objects are classified and easily identifiable and extremely accurate ephemerides are available.

However, the classified natural objects may not be in permanent visibility (for example if one uses the sun as the known object, its visibility is of course subject to day/night alternation) for the high-reliability resetting measurements (not dependent on the propagation error) and the use of the artificial objects (for example geostationary) in constant visibility can systematically be used in relative resetting (subject to propagation error) in the phases of invisibility of the natural objects.

Furthermore, and advantageously, the reliability of the measurements resulting from the known objects consists in comparing several TDOA residuals obtained for several objects which are known but different to check the alignment of these time-domain residuals with one another since they are not meant to depend on the known object. If some of these objects diverge too far from the others then it can be deduced therefrom that their ephemeris is not reliable and the object can then be removed from the list of reference objects that can be used for maintaining the synchronization between the stations.