EP3311185A1

EP3311185A1 - Method and device for locating an electromagnetic emission source and system implementing such a method

Info

Publication number: EP3311185A1
Application number: EP16728700.2A
Authority: EP
Inventors: Jean-François Grandin; Hugo SEUTE; Raphaël Sperling; Laurent Ratton
Original assignee: Thales SA
Current assignee: Thales SA
Priority date: 2015-06-17
Filing date: 2016-06-13
Publication date: 2018-04-25
Also published as: US10852388B2; FR3037659A1; WO2016202748A1; US20180149729A1; FR3037659B1

Abstract

The invention relates to a method which uses a network of ESM receivers (21, 22). The method includes: a step of determining a first locus from a first measurement providing information on the difference in the angle of arrival of said emission beam on two receivers (21, 22), said first locus including the points of space that provide the first measurement on said two receivers (21, 22); a step of determining a second locus from a second measurement providing information on the direction of arrival on at least one of the receivers (21, 22), said second locus including the points of space that provide the same second measurement on said receiver (21, 22); and a step of determining a third locus in which the position of the source (1) is located, said third locus being the intersection of said first locus and said second locus.

Description

METHOD AND DEVICE FOR LOCATING AN ELECTROMAGNETIC TRANSMISSION SOURCE

AND SYSTEM IMPLEMENTING SUCH A METHOD. The present invention relates to a method for locating an electromagnetic source. The invention also relates to a treatment device capable of implementing this method. It also relates to a system implementing this method. The invention can be applied in many situations. It can in particular be applied for the elaboration of a tactical situation that is to say for the location of emission of fixed or mobile radars, slow or fast, in terrestrial, maritime or airborne contexts starting from a network of Radar Detectors (a Radar Detector being hereinafter referred to in the document as ESM for Electronic Support Measure) fixed or mobile on land, sea, drone, aircraft or helicopter vehicles. In particular, the invention applies:

- In a maritime context, for monitoring from the coast by a network of ESM stations;

- In a terrestrial context, for example for surveillance by a swarm of drones;

- In an air context, for example for the trajectory of airborne threats by a network of ESMs embedded in an airplane. In traditional methods, the localization is based on iterative methods by gradient, by the maximum likelihood estimation, requiring calculations of step of the gradient often consuming resources in terms of computational power, or methods requiring to digitize the data. research space.

An object of the invention is especially to estimate the location of the emission by a direct calculation and therefore not iterative.

For this purpose, the subject of the invention is a method for locating an electromagnetic emission source from an ESM receiver network, said source emitting an emission beam scanning the space, said method comprising:

a step of determining a first geometric locus from a first measurement giving an arrival angle difference information of said emission beam on two receivers, said first locus having the points of the space giving the same first measurement on said two receivers;

a step of determining a second geometric locus from a second measurement giving arrival direction information on at least one receiver, said second locus having the points of the space giving the same second measurement on said receiver;

a step of calculating the third geometric locus on which the position of said source is located, said third locus being the intersection of said first geometric locus and said second locus.

In a particular mode of implementation, said method comprises:

a step of measuring an arrival angle difference (ΔΘ) of said emission beam on two receivers, called ADOA measurement;

a step of calculating a so-called ADOA cylinder from said ADOA measurement, said iso-ADOA cylinder corresponding to points in the space giving the same ADOA measurement for said two receivers;

a step of measuring the arrival time difference of said transmission beam on said two receivers, called TDOA measurements;

a step of calculating a hyperboloid said iso-TDOA from said TDOA measurements, said iso-TDOA hyperboloid corresponding to points giving said same TDOA measurements;

a step of calculating the intersection of said iso-ADOA cylinder and said iso-TDOA hyperboloid, said intersection comprising the position of said source.

In another particular embodiment, said method comprises a step of measuring the time difference of passing said transmission beam on two receivers, called the DTPL measurement;

a step of calculating a cylinder called iso-DTPL from said measurement DTPL, said iso-DTPL cylinder corresponding to points in the space giving the same measurement DTPL for said two receivers;

a step of calculating the intersection of said iso-ADOA cylinder and said iso-TDOA hyperboloid, said intersection comprising the position of said source. The calculation of said cylinder is for example carried out in two dimensions, corresponding to a given altitude, corresponding to the calculation of a circle, the position of said source being the intersection of said circle and said iso-TDOA hyperbola.

ESM sensors are for example worn by an aircraft. Said network comprises for example a single ESM sensor, the measurements being made in two consecutive positions of said sensor.

Said source being mobile, the step of calculating the position of said source at a given instant is for example followed by a Kalman filtering step and prediction of the positions of said moving target.

The invention also relates to a processing device able to communicate with an ESM sensor network, said device being able to implement the method as described above.

The invention also relates to a system for locating an electromagnetic emission source comprising at least one ESM sensor network and processing means implementing said method. Other characteristics and advantages of the invention will become apparent with the aid of the description which follows, given with regard to appended drawings which represent:

- Figures 1a and 1b, an illustration of contexts of application of the invention;

FIG. 2, an illustration of the type of measurements used;

FIG. 3, an illustration of a measurement of the DTPL type, measure of time difference of transmission lobes;

FIG. 4, a representation of a scanning radar lobe intercepted by two ESM receivers;

FIG. 5, a representation of the positions of a target and of two ESM receivers in an orthogonal reference frame;

FIGS. 6a and 6b, an example of a geometric locus used in the implementation of the invention, a cylinder in this example;

FIG. 7, an example of a change of reference allowing the calculation of a second locus used in the method according to the invention.

Figures 1a and 1b illustrate contexts of application of the invention. FIG. 1a shows a target 1 producing a transmission 10. In the example of FIG. 1a, the target 1 is a rotating radar. Two ESMs are embedded on two mobile carriers 1 1, 12, each carrier being equipped with an ESM. FIG. 1b illustrates the same configuration but with two fixed ESMs 13, 14. The invention deals with the location of a fixed transmission by two fixed or mobile ESM stations. However, it can be applied to a single ESM station in motion.

The invention can also be applied to a mobile emission, slow or fast, insofar as the estimation being performed on a lobe passage, the successive positions can then be filtered by many conventional methods including a Kalman filter. We then obtain a sequence of positions integrated by the filtering with a reconstitution of the speed of the target.

Figure 2 illustrates the two types of measurements used. Two types of measures are considered: - The ADOA (Angular Difference Of Arrival) type measurement measuring the arrival angle difference ΔΘ of the radar emission on two remote ESM receivers 21, 22;

- FDOA (Time Difference Of Arrival) type measurement, giving a direction of arrival.

The two ESMs 21, 22 can be called ESM1 and ESM2 respectively.

The measurement of the ADOA type can in particular be carried out by the following different methods:

- The measurement of DTPL (Difference of Lobes Passage Time) for radars with constant scanning speed ω on the passage between the two ESMs;

The difference between two direct (sufficiently precise) angular measurements obtained from goniometry on each ESM, between the two ESMs, for example from the measurement of AOA obtained on a single aircraft by a goniometer device measuring amplitude, phase or TDOA short base;

- The measurement of angular variations of LBPDE (Long Baseline Phase Difference Evolution) type on a single plane in motion using a large interferometric base;

- The arrival frequency Doppler frequency measurement known as FDOA (Frequency Difference Of Arrival) carried out on a single moving ESM or between two ESM subject to relative movements with the source. The DOA measurement can in particular be carried out by measuring the arrival time difference of the radar pulses on two remote ESM receivers 21, 22 corresponding to two carrier aircraft.

These two measurements, although not responding to the same equations, give the first-order equivalent to a DOA measurement, the TDOA measurement being a priori more accurate than the AOA-type measurement.

For certain measurement combinations envisaged, the method applies to particular type of radar emissions. For example, the DTPL measurement applies to constant-scan radars. The measurement of the single-receiver ESM FDOA applies to coherent waveforms. The invention will be described in the following particular context:

- The ADOA measurement results from the measurement DTPL (lobe passing time difference) and the measurement or knowledge of the scanning speed ω of the radar emission assumed to be almost constant during the passage between the remote MSs. The measurement of DTPL can be obtained by considering the difference of the TPL (lobe passing time) of the same lobe on the two ESMs;

- The TDOA measurement results from the difference in radar reset arrival times on the two remote ESMs.

Figure 3 illustrates the measurement of DTPL. This Lobes Passage Time Difference measurement is the difference in transit time T of a radar that sweeps at speed ω on two fixed stations. The example of Figure 3 is given without loss of generalities and the same equations apply to the first order even for fast mobiles. Assume the following situation:

- A radar 1 emits by scanning the space at a rotation speed ω assumed fixed and known;

- The passage of the main lobe 31 is observed on two ESM stations 21, 22. The time of passage of the main lobe is t ₁ on the first, ESM1, and t ₂ on the second, ESM2;

The difference in lobe passage time (DTPL) is given by in the following equation:

where Δα is the arrival angle difference ΔΘ of the radar emission on the two remote receivers 21, 22, r ₁ and r ₂ being the respective distances from these two receivers to the radar, n ₁₂ is a supposed Gaussian noise of standard deviation of 1 to 5 ms. The date of passage of lobe can be obtained with this precision by integration on the successive lobes.

The measurement of the difference in arrival time between the two remote observers 21, 22 is also considered. The measurement techniques used in radar and EW have notably been described in the article by Quazi, AH. in active and passive Systems for target localization "IEEE ASSP-29, No. 3, June 1981, pages 527-533. The technique used consisting in the search for the peak of the intercorrelation function is also presented in the article by Piersol, AG "Time delay Estimation Using Phase Data", IEEE ASSP-29, No. 3, June 1981, pages 471 -477. There are several other competing techniques including a technique using the Fourier transform phase of the intercorrelation function.

We notice :

The TDOA (Time Difference Of Arrival) corresponds to the time difference during the course of r ₁ and r ₂ by the radar waves.

Given that the order of magnitude of c is 3.10 ⁸ m / s, which corresponds to a distance traveled of 300 km in 1 ms, the term is negligible

before n ₁₂ in the equation (1) of the DTPL, because n ₁₂ is of the order of 1 ms and the distance between the stations 21, 22 is of the order of 3 km. The term

can therefore be neglected in the resolution.

Finally at the first order, the difference in passage time of the lobe is:

We therefore have two measurements given by the following equations

Measure the difference in arrival time

Measurement of the difference in passage time of the lobes

The localization is obtained by finding the values of the position of the emission which simultaneously satisfy these two equations.

Subsequently, a "set of pulses" resulting from a so-called "deinterleaving" operation performed in parallel on each ESM which consists of seeking to group all the pulses from the same radar and corresponding to a single lobe passage.

From the pulses of each lobe, for each ESM station 21, 22 is calculated:

- A quadruplet of measures {TOA _k , TPL _k , LLA _k , Lv _k ). summarizing the geometric parameters of the lobe, assuming otherwise known the precise position of the ESM 21, 22;

- A radio description of the perceived waveform (List of PRIs, frequencies and Dl mainly).

These lobes are then tracked locally on each ESM by a method using:

- The similarities between the radio descriptors;

- The proximity of TDOAs;

- The regularity of the dates of passage of the TPL lobes.

The present invention relates neither to the deinterlacing function, nor to the function of characterizing the lobes, nor to the function of association of the lobes in the monoplate-form, nor to the function of identifying the emission. These operations are known and assumed to be performed elsewhere.

Without loss of generality the elemental localization provided on a lobe passage may be integrated by any tracking of the transmission lobe sequences over time.

FIG. 4 represents the lobe 41 of the scanning radar 1 intercepted by the two ESMs, ESM1 and ESM2.

These two ESMs intercept the same ImpX pulses but with different levels, related to the radar illumination function and with a TDOA related time offset. In the general case, the temporal sequence of the pulse pattern is independent of the temporal evolution of the radar lobe. This amounts to the independence between the DTPL and TDOA measurements.

In this application, the location method performs the following steps: - For each ESM i. o For each intercepted lobe k, compute a quadruplet of measures {TOA _k , TPL _k , LL _k , Lv _k ). summarizing the geometric parameters of the lobe 41, this lobe being also characterized by a summary of the characteristics of the FO (PRI, Frequencies, Dl, intra-pulse codes ...).

- For each pair of ESM stations: o Association of lobes of the same emission between the platforms; o Estimation of radar ou radar rotation or antenna rotation period (ARP) speed par by joint estimation, or knowledge of the rotation period (PRA) through the ESM identification; o Calculation of the resulting DTPL circle; o TDOA estimation by integrating the TDOA of the pulses received on the 2 ESMs, this integration being able to be carried out by different methods including the correlation of the pulse trains received by the two receivers or the simple average of the arrival time differences (TOA ) impulses received by the 2 MSBs; o Calculation of the resulting TDOA hyperboloid.

- For all ESM stations: o Calculation of the intersection location of DTPL circles and TDOA hyperbolas; o If two stations are used, calculate the intersection of the DTPL circle and the TDOA hyperbola.

It is at this level of location processing, for all ESM stations, that the invention comes into play. The determination of the location according to the invention is described only in the context of TDOA and DTPL measurements, but the invention is also applicable to all the systems described above. These systems all perform localization based on a measurement of angular type or direction (here the TDOA) and a measurement of angular difference type (here the DTPL).

5 shows the target 1 and the two ESM receivers 21, 22 in a mark O, x, y, z at the time of an i ^th extent, i ranging between 1 and N. In fact, two observers 21, 22 perform N TDOA measurements (difference in arrival time of the same signal between the two remote sensors 21, 22) and DTPL (difference in lobe passing time) on the fixed target 1. The measurement of TDOA and the measurement of DTPL are recorded between the first receiver 21 and the second receiver 22. R _{1, i} and R _2, respectively, denote the respective distances of the first and second receivers to the target. In the coordinate system 51, the coordinates of the target, the first receiver and the second receiver to the ^ith measurement are respectively:

The DTPL measurements respond to the following explicit measure equations:

TDOA measurements respond to the following explicit measurement equations:

According to the Theorem of the Inscribed Angle, the points giving the same measurement of DTPL for given receiver positions are located on a cylinder 61 called the iso-DTPL circle shown in Figure 6a. This cylinder contains the positions Ρ-ι, P ₂ of the receivers 21, 22 and the position Pe of the target 1.

Referring to FIG. 6b, where C is the center of the base circle 62 of the cylinder, according to the Theorem of the angle at the center, the angle is equal to

double the angle ΔΘ obtained from the measured DTPL between the receivers

Moreover, by definition of the radius R of the circle, we have equal distance between C and P _! and between C and P ₂ , i.e. C is on the segment mediator

noted med

We thus obtain the following system, noting M the middle of the segment

We deduce the radius R from the base circle 62 of the iso-DTPL cylinder, where r ₂ is the distance between the positions Pi and P ₂ in the x, y plane:

Finally we get the equation of the iso-DTPL cylinder With

The points giving the same measurement of TDOA for given receptor positions are located on a hyperboloid called iso-TDOA hyperboloid, which is obtained by the calculations presented hereinafter from the representation of FIG. 7. The position Pe of the target 1 and the positions P ₁ , P ₂ of the receivers 21, 22 are calculated in a new orthogonal coordinate system Ο ', x', y ', z' where O 'is equal to Pi and the axis x' passes through P ₁ and P ₂ . In this reference, the position Pe is indicated by the coordinates (x ' _e , y' _e , z ' _e ). We denote by R _1, R ₂ the respective distances of the first and second receiver 21, 22 to the target, and the distance ₁₂ is noted between these two receptors.

Noting φ the angle between the axis x 'and the projection of Pi Pe in the plane x', y 'and noting Θ the angle between the axis z' and P ₁ Pe, it comes:

By the way is the difference in arrival time between

the receivers 21 and 22.

According to the Pythagorean Theorem: either:

We deduce the polar equation of the iso-TDOA hyperboloid:

noting

We then obtain the equation of the hyperboloid in Cartesian coordinates in the coordinate system using the formulas

After calculations, we obtain the equation:

noting

is

For the continuation, it is necessary to express the formulas of passage of the reference to the mark (and vice versa). After calculations, he comes:

and vice versa

noting

In order to simplify the expression of the intersections, we write the equations of the iso-DTPL cylinder and the iso-TDOA hyperboloid in the reference (

previously defined. The following system is obtained after calculation:

In order to simplify the resolution, we try to determine the intersection between the iso-DTPL cylinder and the iso-TDOA hyperboloid in the constant z plane, which is reduced by the reference change formulas, noting C = z - z ₁ , to have:

We now calculate the intersection between these three geometries. We thus obtain the system consisting of the three preceding equations, namely:

Subtracting the third equation to the second, we get The previous system then becomes:

To solve this system, it suffices to solve by known methods a polynomial equation of degree 4 in x, of the form

Up to degree 4, several known explicit resolution methods can be used. On the other hand, it is shown that from the 5th degree no explicit method of resolution exists. At this stage according to the invention, there is therefore a location obtained by direct explicit calculation of the intersection of the cylinder Iso-DTPL and the hyperboloid TDOA in the altitude plane 0, corresponding to the plane O, x, y , for a chosen point for example 100 km in the direction of the radar emission 1 on the bisecting plane of the base formed by the two ESMs. Advantageously, the method according to the invention forms, from the measurements, a polynomial equation of degree 4 which is then solved by using methods of explicit resolution of this type of equation. Several known methods of resolution can be used, among them we can mention the resolution of a degree equation of 4 by the method of Ferrari or the resolution of an equation of degree 4 by the method of Lagrange.

According to the invention, the resolution algorithm produced is therefore an explicit algorithm and does not use conventional gradient resolutions, for example MLE (Maximum Likehood Estimation) type algorithms, also called EMV (Maximum Likelihood Estimation).

In the explicit methods, it is sought to geometrically construct the solutions as intersections of certain curves or surfaces, for example the iso-DTPL cylinder, the hyperboloid TDOA or the altitude plane 0 in the case of the invention. The solution is then obtained without iteration and without having to evaluate it on multiple positions, hypothesis of a digitized search space.

Not only is there a huge gain in computing power resources, but more:

an execution time can be calculated according to the number of input data;

- the same memory occupation is predictable;

there is no risk of divergence since the method uses a direct and non-iterative calculation.

The method according to the invention is therefore suitable for real-time execution. Indeed, it uses a memory size and requires a calculation time both of which are predictable. It applies advantageously to many types of ESM measurements.

Advantageously, the altitude error, in Z, has no influence on the iso-DTPL cylinder. It may then be advantageous to eliminate the variable Z in the resolution by setting the value of the altitude to avoid artificial indeterminacy in the system of equations. Indeed, given that the altitude error has a weak influence, it may be better not to try to to estimate it to avoid this artificial indeterminacy. If one wishes to obtain a greater precision, after a calculation of localization estimating a first localization, one specifies the altitude with this point of localization using the numerical model of the ground at disposal. The process is then repeated from this altitude using the same explicit resolution according to the invention.

The measurement qualities envisaged for the DTPL type (accuracy of 1 ms) and for the TDOA type (precision of 10 ns) already give a precise location in mono-measurement. In the context of a fixed or mobile target, quasi-instantaneous measurements of two-to-two intersection of a number P of distant ESMs lead to P (P-1) / 2 intersections. A barycenter estimator of the exact intersections is then produced. This estimator is biased because there are only P independent measurement pairs but in the case of DTPL, TDOA the measurements are so precise that this bias is negligible. It is always possible to use the estimate obtained to initialize an asymptotically unbiased method that converges without difficulty because initialized very close to the real solution.

For fixed targets, and fixed ESMs, DTPL and TDOA measurements are constants and can be integrated, leading to even more accurate performance.

In the context of a moving target, the instantaneous locations obtained can be processed by Kalman filtering to estimate the evolution of the target. From a position calculation at a given moment, it is thus possible to predict the positions of the target over time.

The invention has been described for measurements of the DTPL type, measuring differences in transit time of the transmission beam of the source, and for TDOA type measurements, giving differences in arrival time of the beam of transmission, the transmission beam being intercepted on two remote fixed or mobile ESMs.

Other types of measurements giving information on the angle of arrival difference and the direction of arrival of the emission beam can be used, all these measures making it possible to geometrically construct the solution according to the invention as intersection of some geometric places obtained from these measurements. These geometric places are sets of space points that would give the same measure if the source was arranged at these points. It is thus possible to construct iso-ADOA curves corresponding to points in the space giving the same ADOA measurement for two receivers 21, 22, or else altitude plans 0.

The ADOA measurement and the DOA measurement can be performed in different ways as previously indicated. A system implementing the method according to the invention comprises at least one network of ESM sensors performing the various measurements and processing means for determining the location of an electromagnetic emission source from these measurements. The processing means are for example integrated in a device able to communicate with the ESM sensors. From the signals received by the sensors, the processing means determine the differences in arrival angles and arrival directions. For this purpose, the signals received by the sensors are recorded, in their I and Q forms, and dated. This information I and Q and the corresponding dates are then sent to the processing device which can implement the method according to the invention. . This device can be on board or can be on the ground.

The description has been made for a system comprising a network of two sensors.

It is possible to make a system reduced to a single ESM sensor. In that case :

the measurement of ADOA type considered is an integrated differential measurement during the evolution in the space of the carrier of the single ESM sensor, between two consecutive measurement positions, for example taking into account the phase difference measured over a large area. interferometric base or LBPDE, FDOA or DTPL;

the measurement of AOA can be a conventional angular measurement, obtained for example by an interferometric device.

Claims

1. A method of locating an electromagnetic emission source (1) from an array of ESM receivers (21, 22), said source (1) emitting a scanning beam (10) scanning the space, characterized in what it includes:

a step of determining a first geometric locus (62) from a first measurement giving an arrival angle difference information of said emission beam on two receivers (21, 22), said first geometric locus having the points of the space giving the same first measurement on said two receivers (21, 22);

a step of determining a second geometric locus from a second measurement giving arrival direction information on at least one receiver (21, 22), said second geometric locus comprising the points of the space giving the same second measurement on said receiver (21, 22);

a step of determining a third geometric locus on which the position of said source is located, said third geometric locus being the intersection of said first geometric locus and said second locus.

2. Method for locating an electromagnetic emission source (1) according to claim 1, characterized in that it comprises:

a step of measuring an arrival angle difference (ΔΘ) of said emission beam on two receivers (21, 22), called ADOA measurement; a step of calculating a cylinder (62) said iso-ADOA from said ADOA measurement, said iso-ADAO cylinder corresponding to points in the space giving the same ADOA measurement for said two receivers (21, 22) ;

a step of measuring the difference in arrival time of said transmission beam on said two receivers (21, 22), called TDOA measurements;

a step of calculating a hyperboloid said iso-TDOA from said TDOA measurements, said iso-TDOA hyperboloid corresponding to points giving said same TDOA measurements; a step of calculating the intersection of said iso-ADOA cylinder and said iso-TDOA hyperboloid, said intersection comprising the position of said source.

3. A method for locating an electromagnetic emission source (1) according to claim 1, characterized in that it comprises:

a step of measuring the time difference of passing said transmission beam on two receivers (21, 22), called DTPL measurement;

a step of calculating a cylinder (62) said iso-DTPL from said measurement DTPL, said iso-DTPL cylinder corresponding to points of the space giving the same measurement DTPL for said two receivers (21, 22) ;

a step of measuring the difference in arrival time of said transmission beam on said two receivers (21, 22), called TDOA measurements; a step of calculating a hyperboloid said iso-TDOA from said TDOA measurements, said iso-TDOA hyperboloid corresponding to points giving said same TDOA measurements;

4. A method for locating an electromagnetic emission source (1) according to claim 2 or 3, characterized in that a given altitude being chosen, the calculation of said cylinder (62) is carried out in two dimensions. at the given altitude, said calculation corresponding to the calculation of a circle (61), the position of said source being the intersection of said circle (61) and said iso-TDOA hyperboloid.

5. A method of locating an electromagnetic emission source (1) according to any one of the preceding claims, characterized in that the ESM sensors (21, 22) are carried by an aircraft.

6. A method of locating an electromagnetic emission source (1) according to any one of the preceding claims, characterized in that said network comprises a single ESM sensor, the measurements being performed in two consecutive positions of said sensor.

7. A method of locating an electromagnetic emission source (1) according to any one of the preceding claims, characterized in that said source (1) being mobile, the step of calculating the position of said source to a given instant is followed by a Kalman filtering step and prediction of the positions of said moving target.

8. Processing device capable of communicating with an ESM sensor network, characterized in that it is able to implement the method according to any one of the preceding claims.

9 System for locating an electromagnetic emission source (1), characterized in that it comprises at least one ESM sensor array and processing means implementing the method according to any one of Claims 1 to 7 .