Method for determining position and velocity of targets from signals scattered by the targets

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PATENT INVENTOR INVENTOR'S OFFICE NAME (56) (57)

Total Defense Research Institute, 172 90 Stockholm SE Magnus Herberthson, Linköping SE Defense materiel plant

Ways to determine the locations and speed of the targets with the spread of signals

CALLED PUBLICATIONS: - - SUMMARY:

The present invention relates to a method of determining signals and the velocity of said target from the target. A first and a second station comprising transmitter / receiver (s, s) are used₂) of electromagnetic or acoustic signals and performs partly monostatic measurements (Mi, M₂) from each station, and partly a bistatic measurement between the stations (B_1Z), and first calculates through the two monostatic measurements a number of target candidates with 2-dimensional position and 2-dimensional velocity, whereupon these are tested against the result of the bistatic measurement and the target candidates are retained with the appropriate error margins found in all the measurements.

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The present invention relates to a method of determining the location and speed of the respective targets by means of signals spread from one or more targets. In particular, the invention is useful in the event that a very large number of targets are to be determined in space and assigned a velocity vector. The invention is, inter alia, intended to be used in the system for determining the state of the target for which patent applications have been filed today and which has Hans Hellsten as the inventor, to which patent application 0101661-7 is hereby referred for more detailed reasoning on the system of transmitters and receivers themselves, in continuation stations.

The invention assumes a 3-dimensional position space and a 3-dimensional velocity space, which together form a 6-dimensional state space of up to ~ 10²⁰ cells, where in principle each cell should be assigned the value 0 (no target) or 1 (target). The large number of cells in the state room places special demands on the signal processing method, one of which is possible addressed in the cited patent application by Hellsten. In the present patent application, an alternative method is proposed.

The large number, - 10²⁰, cells in the state space make known projection methods unmanageably slow. The simplest known method is to first create all possible target positions for three stations and N targets. They will be fewer than N³ to the number, and it is easy to determine at each possible target position both the location and the velocity vector for the target. You get ~ N³ candidates who are each supported or rejected by further independent measurements. This provides a processing method that requires the CN³ operations. If K is large, this method may become too slow for interesting values ​​of N.

A specific feature of the systems for which the present method is intended to be used is, as above, that the information in an initial detecting phase can be considered binary, which can be utilized. Furthermore, input data is initially distributed between different stations. This means that a local, distributed processing that calculates location and speed parameters for the targets gives an advantage, since it then gets a parallelizing effect at the sensory level. Extensive communication between stations is then necessary.

The stumbling block becomes the association problem, ie. to correctly pair data from different radar stations. An association where it is unclear whether it is correct or not

519 088 can be called a candidate Afterwards it becomes clear if a candidate is a correct association, ie. a goal or a wrong association, sometimes called a ghost. The problem is thus to deal with the initially very large number of ghosts, ie to find the goals among all candidates. in⁵i!

The object of the invention is to solve this problem, which is accomplished by the invention being given the design as set out in the following independent patent claim. Suitable embodiments of the invention are apparent from other claims;

IN

The invention will be described in more detail below with reference to;

The accompanying drawing, wherein 'i

Fig. 1 shows how the geometry around two stations on one plane and how rotationally symmetric target locations when measured from two stations can be transferred to points in a half plane; and Fig. 3 shows how to determine location and speed by combining measurements from two pairs of stations having a station in common.

If you study Figure 1 with two stations, p_?, s₂, on the ground, which sees one and the same target, ti, in monostatic measurements, it is immediately recognized that they place the target along a semicircle in the position room above the ground. Bistatic measurements between the two stations share this symmetry, so that a semicircle can be accepted or discarded in its entirety. For each point on the semicircle, an estimated velocity v is determined up to a line in the velocity space. In total, two stations thus assign a candidate (target or ghost) a two-surface in the state room. A family of such two-surfaces in the state room forms a relatively complicated subset of the state space. It is therefore desirable to have an initial calculation where candidates can be represented by points in a reduced state space, rather than surfaces. In the case of studying both {mode and speed, as described above, the reduced state space becomes!

consequently, 4-dimensional. Furthermore, some type of candidate reduction should be done in this reduced state space, so that the final processing in the full state space is simplified.

The basic idea of ​​the invention is to work in low-dimensional, &lt;6, state rooms that are common to several sensors. Optimally, surfaces are in the full state 519 088 HHMUl · / ::? ·.

the room, corresponding to certain sensor registrations, represented by points in the camp dimensional dito. Because the reduced state space is common to several sensors, it is possible under certain conditions to obtain a sensory and local elimination of many ghosts by calculations that are not processing heavy. Such a glazed batch of candidates then together with other, similarly glazed candidate batches, provides input to a final and complete association in the entire permit room. This final calculation is then, by the relative smallness of the input quantities, also affordable processing intensive.

From the symmetry of Figures 1 and 2 it can be seen that the points on the circular arc corresponding to a candidate G can be represented by a point P (ti) in a half plane P12 on one side of the axis of symmetry. Figure 2 shows how the velocity v based on monostatic and / or bistatic measurements from a pair of stations can be determined by Doppler measurement, such as when on a component in the direction of the semicircular arc. This means that the velocity is on the line L (Si, s₂, Ti, V). All velocities on this line will be represented by the same 2-dimensional velocity u, which is a vector in the half plane P_J2· Conversely, if the velocity of the half plane P₁₂ is known, it is known that the velocity is on the line L (si, p₂, ti, u), where by u is now the canonical representative of u in three dimensions.

Unlike the simple problem of finding the common line between measurements from two stations, ie. the arc from the intersection of two (half) spheres, or the common point between measurements from three stations, ie. intersection of three (half) spheres, common points from two or three bistatic measurements between the same stations are more complicated to find, since each measurement generates one (half) ellipsoid with the stations at the focal points. It is more difficult to calculate common points between two ellipsoids than between spheres.

Therefore, the invention utilizes monostatic measurements between pairs of stations to calculate candidates. Then the associated bistatic measurement is used to eliminate ghosts. In this way, you do not need to calculate common points based on the ellipsoids, but only look at the points where the monostatic measurements indicate that there are candidates and see if the bistatic measurement has one or more candidates at the current distance. This is much simpler than calculating common points based on bistatic measurements. In cases where the distance-based analysis does not provide sufficient elimination of the number of ghosts,

519 088 you can also use the collected doppier information. The Doppler measurements divide according to the symmetry of the distance measurements and by requiring compatible monostatic and bistatic Doppler measurements, a further reduction in the number of ghosts is obtained.

The following is a more detailed description of the method in connection with a plurality of distributed isotropic transmitters / receivers of electromagnetic or acoustic signals. In the concrete example, we imagine a number of radar stations. Each radar station transmits a signal that is received partly by the station itself and partly by about 10 neighboring neighbors. We refer again to the above-mentioned patent application by Hellsten for a more detailed description of how the system can be designed in general.

In a first step, a first Si and a second s are used₂ transmitter / receiver of 15 electromagnetic or acoustic signals and performs partly monostatic measurements Mi and M₂ from each transmitter / receiver, and a bistatic measurement S_t2 between the stations. First, the two monostatic measurements create a number of candidates with 2-dimensional position and 2-dimensional velocity. Then these are tested against the result of the bistatic measurement and the candidates are retained who are found in all the measurements with appropriate margins of error.

In a second step, corresponding calculations are performed with one of the two original stations, say Si, and a third station s₃. Same calculations performed for si and s₂ is performed for St and s₃. Then, for each calculation, possible targets are sorted by distance to the common transmitter / receiver, here Si, and the distance gaps in the two calculations that simultaneously contain one or more candidates are determined. For these candidates, the radial doppler velocity is studied in the two measurements. If the difference is less than a predetermined value, the candidates are retained. Otherwise, they are rejected.

Figure 3 shows how a target 6 is represented by a point in the half-plane P₁₂ and another in the half-plane P_3i. The 3-dimensional position is given by the intersection of the semicircles. The true velocity v of is represented by the velocity u_1: which is a vector in the key plane to the point P (ti) in the half plane P_&lt;2 and the corresponding vector u₂ in the half-plane P_3i. Each such velocity can be depicted on a line of the three-dimensional space as above. The true velocity v lies in the intersection of these lines L (p_1fs₂, ti, ui) and

519 088

In a third step, a third bistatic measurement B can be performed₂₃ between the two transmitters / receivers, here s₂ and S₃₁ which is not the common transmitter / receiver as above. The result can be used in conjunction with the candidates remaining after step two above to further reduce the number of ghosts.

The local treatment around three stations has, through this procedure, with initially pairwise calculations around two stations and further so that all three stations are included in the calculations, in an elegant and relatively computationally low number of 10 goals and possibly, due to imperfections in the measurement system and its application, some remaining ghost. The interesting thing is that, in parallel with this, in a larger system a number of other stations may also have set a number of targets. The parallel handling is very favorable and an absolute prerequisite as above for the processing to be possible at all in a larger system with many stations. 15

Of course, to improve the safety of the calculations and to stave off any remaining ghosts, one can of course include additional stations, which have a sufficient range, in the calculations. One can conceive of both further monostatic as well as additional bistatic calculations.

It has already been mentioned several times that the present invention is intended to form part of a larger system of stations, for example that presented in the aforementioned patent application by Hans Hellsten. It states that a number of stations may be located as grid points in a substantially equidistant grid on a surface which limits the monitored position space, e.g. a ground surface. The idea in Hellsten's patent application is that the range of the stations should be such that each target is detected by bistatic measurement of at least six independent sets of transmitters and receivers. For this reason, the range of the stations is 2d, if the distance between the stations is d, if we assume a substantially flat surface. If the area is not substantially planar, the ranges must be adjusted so that at least 6-fold bistatic overlap of the monitored state space is achieved.

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