GB2214025A

GB2214025A - Object location

Info

Publication number: GB2214025A
Application number: GB8729916A
Authority: GB
Inventors: Alan Frederick Dadds
Original assignee: Philips Electronic and Associated Industries Ltd
Current assignee: Philips Electronics UK Ltd
Priority date: 1987-12-22
Filing date: 1987-12-22
Publication date: 1989-08-23
Also published as: GB8729916D0

Abstract

To locate an object emitting a 360 degree scanning signal, and more particularly to resolve ambiguity of position when two or more such objects emit non-synchronised but similar signals circles representing the locus of possible positions of emitters are obtained by measuring the time t between reception at detectors 10, 12 and T between consecutive receptions at one detector. All the possible fixes in the field of view of the detectors 10, 12 are determined as circles which pass through the signal detectors 10, 12 and each of the possible object positions are computed and those fixes which do not correlate with the circles calculated are treated as ghosts. Each circle is calculated by determining the angle A=360to DIVIDED T that an arc of a circle bounded by the detectors 10, 12 subtends at an object 14 and using the angle A and the known distanced between the detectors 10, 12 to calculate the radius R of the respective circle. <IMAGE>

Description

DESCRIPTION A METHOD OF AND SYSTEM FOR LOCATING AN OBJECT The present invention relates to an object locating method and system.

Object location by triangulation is well known from military and civil applications. Essentially the method requires two geographically separated observers to take magnetic compass bearings on an object which emits a signal which may be a sound, light or a radio emission and these bearings are plotted on a map and the position of the object is determined by intersection of the lines plotted. Such a method is accurate when there is only one object but if there are several objects in close proximity all emitting substantially the same signal then it can be difficult to distinguish one object from another or for that matter false fixes or ghosts from the correct fixes.

This is particularly a problem in electronic surveillance where receiving equipments monitor emissions from say radar carried by moving objects, such as ships, over a relatively wide viewing angle. If the objects emit distinctive signals then fixes can be obtained relatively easily. However many objects such as ships of various sizes are equipped with rotating search radars which are mass produced and have emission frequencies and scanning rates which are generally the same. Accordingly in a closely arranged group of objects whose relative positions may be changing, their electronic emissions can be detected and the approximate position of the group can be determined using normal direction finding techniques. However such measurements will give rise to a plurality of phantom or ghost fixes as well as the correct fixes and the problem occurs how to distinguish them.

According to one aspect of the present invention there is provided a method of locating the position of an object emitting a rotating scanning signal, comprising detecting signal emissions from said object at two geographically separate locations, the distance (d) between which locations is known, determining the time (t) between the detection of the same signal emission at the two locations, determining the time (T) of a 360 degree scan by said signal emission, calculating the included angle (A) which the object makes with the two locations in accordance with the equation: A = 360t/T and calculating the radius (R) of a circle passing through the object and said two geographically separate locations in accordance with the equation R = d/2 sin A.

According to another aspect of the present invention there is provided an object location system for use with locating the position of at least one object emitting a rotating scanning signal, the system comprising first and second receivers having respective signal detectors disposed at geographically separate locations a known distance (d) apart, means coupled to the first and second receivers for determining the time (t) of transit of the scanning signal between the signal detectors of the first and second receivers, means for determining the time of a 360 degree scan by said signal, means for calculating the angle (A) subtended at the object in accordance with the equation: A = 360t/T and means for calculating the radius (R) of a circle passing through the locations of the object and the signal detectors in accordance with the equation: R = d/2 sin A.

The present invention is based on an application of the classical geometrical theorem that the angle subtended by an arc of a circle or a chord at its circumference is half that subtended by the same arc or chord at the centre of the circle.

Thus the object Locating method and system in accordance with the present invention is able to determine the angle A by measuring the time elapsed between the receipt of the object's scanning signal at the two locations and knowing the time of a complete scan. Also by knowing the distance between the two locations, the radius (R) of a circle passing through the object and the two locations can be determined.

When there are two or more objects in close proximity and having similar scanning radar equipments which are not synchronised then it is possible to determine their correct positions from phantom ones by obtaining fixes of all the possible real and phantom object positions in the normal way and then computing the radii of the respective circles passing through the two monitoring locations and each of the possible object positions. By correlating the fixes with the radii of the circles constructed it is possible to determine the actual object positions.

In one embodiment of this method the correlation is carried out by calculating the possible object positions as X, Y coordinates on an arbitrary grid, calculating the x,y coordinates of the centres of all the possible circles and their radii (R), calculating the distance (D) between all possible combinations of X, Y and x, y, choosing for each x, y, the X, Y giving a value of D most similar to the radii calculated and selecting as the real object locations those items having the best D/R match.

In another embodiment of the method in accordance with the present invention, the signal emissions from said object are detected at three geographically separate locations, the distances (dl and d2) between first and second of the three locations and between the second and third of the three locations being known, and wherein the included angles (Al and A2) which the object makes with the respective pairs of locations and the radii of the respective circles passing through the object and the first and second locations and the object and the second and third locations are determined, the intersection of said circles representing the position of the object.

The present invention will now be described, by way of example, with reference to the accompanying drawings, wherein: Figure 1 is a diagram illustrating the principle of the method in accordance with the present invention, Figure 2 is a timing diagram illustrating how the times are obtained in order to calculate the angle A, Figure 3 illustrates an example of an object locating method based on two intersecting circles, Figure 4 is a timing diagram in which signals from three objects are being received, Figures 5 to 10 illustrate six different arrangements of 3 objects in a group, the objects being shown as black spots and ghost positions are shown as open rings, and Figures 11 and 12 illustrate how the locations of real and phantom objects can be distinguished.

In the drawings corresponding features have been given the same reference numerals.

Referring to Figures 1 and 2, two ESM receivers 10, 12 (or at least their antennae) are located at a fixed distance d apart. An object 14 equipped with a scanning search radar is located at an unknown distance and bearing from the receivers 10, 12. At each of the receivers 10, 12, the electromagnetic beam is detected at times T1 and T2, respectively, Also the duration T of a complete scan is detected by either one of the receivers 10 or 12 measuring the time between two successive 360 degree scans of the object's radar the duration T may be regarded as a constant for that object's radar. Assuming that the propagation times between the object 14 and the receivers 10 and 12 are negligible compared to the time difference t between T1 and T2 then an included angle A of the triangle formed by the receivers 10, 12 and the object 14 can be calculated in a block 16 in Figure 1 by the equation: A = 360t/T.

Knowing the value of the angle A then by applying the theorem that the angle subtended by an arc at the circumference of a circle is half that subtended by the same arc at the centre, the radius R of a circle passing through the locations of the receivers 10, 12 and the object 14 can be calculated in the block 18 (Figure 1) in accordance with the equation: R = d/2 sin A.

If required a bearing can be obtained by a separate operation from the site of the receiver 10 or 12 to confirm the actual point on the circle at which the object 13 is located.

Figure 3 shows a first practical application of the method in accordance with the present invention. In the embodiment shown three receivers (or their detectors) 10, 12 and 20 are located at fixed sites and the distances dl, d2 between the pairs of receivers 10 and 12 and 12 and 20 are measured. The included angles Al, A2 subtended by the pairs of receivers 10, 12 and 12, 20 with the object 14 are determined in accordance with the method described with reference to Figures 1 and 2 and then the radii R1 and R2 are calculated. The intersections of the circles having the radii R1, R2 denote the locations of the receiver 12 and of the object 14. Hence the position of the object 14 can be determined from a map or grid on which the circles are drawn.

Referring now to Figures 4 to 10, another practical application of the method in accordance with the present invention is to resolve the relative positions of a group of objects which are in the joint field of view of the receivers 10, 12.

Figure 4 illustrates the times of arrivals of signals at the receivers 10 and 12, the time T of one scan of the radar carried by one of the objects and the times t1, t2 and t3 measured with respect to a signal 22 which is treated as an arbitrary reference. The times t1, t2 and t3 may be misleading as to actual positions of the objects because they are influenced by not only the relative geometrical positions of the objects but also by their distance from the receivers 10, 12. Accordingly when considering the times t1 t2 and t3 one has to take into account all the possible relative positions of the objects. Once this information has been derived then it has to be examined to eliminate the ghost or phantom locations of the objects.In a general case if the group comprises n objects or identical emitters then within the group there are n2 possible combinations of bearings or fixes. If circles are calculated for a whole group of n2 possible fixes, the correlation between the circle and the fix can be used to help distinguish an object or emitter location from a ghost or phantom location.

Figures 5 to 10 assume that there are three emitters El, E2 and E3 located in the field of view of the receivers 10, 12. If bearings are taken from the locations of the receivers 10, 12, because the emissions are substantially identical then the emitters El, E2 and B could be at the intersections of the bearings B1 to B3 and B4 to B6. Now assuming that the emissions from the emitters are not synchronised then the receivers 10, 12 can, as described with reference to Figures 1 and 2, determine the times t and T for all the real and imaginary positions of the emitters El1 E2 and E3 and thereby calculate the respective included angles. The radii of circles passing through the locations of the receivers 10, 12 and each of real and phantom locations of the emitters El, E2 and E3 can then be calculated.

By a technique such as curve fitting the respective circles can be constructed as shown in the Figures 5 to 10. For the sake of clarity the circles passing through the real locations are shown by continuous lines and those passing through the phantom locations by broken lines. Any of the bearings or fixes taken previously which do not lie on one of the continuously drawn circles are treated as ghosts and are eliminated from further consideration. In Figures 7 and 9, the emitters E2 and E3 will lie on one and the same continuous circle and in Figure 8 the emitters El, E2 and E3 will lie on substantially the same continuous circle. Thus by a process of elimination the actual bearings of the emitters El, E2 and E3 can be distinguished from ghosts.

This elimination can be carried out in software, reference being made to Figures 11 and 12. Figure 11 shows two objects or emitters El and E2 and two ghost or phantom object locations G1, G2 formed by intersections of the bearing lines drawn from the sites of the receivers 10 ,12. The respective circles are drawn, their centres lying on the perpendicular bisector 24 of the imaginary line interconnecting the receivers 10, 12.

Starting with the assumption that one has already obtained the scan offset times and the bearings from the receiver sites 10, 12. A suitably programmed processor can then (1) calculate the X and r coordinates of each said locations based on an arbitrary grid using the bearing information. (2) Using the scan offset times, the centres of the circles as x, y coordinates on the same grid and the radii of said circles can be calculated.

As a consequence of (1) and (2) one has for N emitters, N2 sets of X and Y coordinates and N2 sets of x, y coordinates. (3) Calculate the distances D between all possible combinations of X, Y and x, y. (4) For each x, y choose the X,Y giving D most similar to R, this provides a list of N2 items. (5) Choose those N items from the list which have the best D/R match as being the emitter locations.

In order to illustrate steps (3), (4) and (5) Figure 12 illustrates the calculations in respect of the centre (x, y) of the emitter E2. The coordinates of El are X1, Y1, of G1 are X2, V2, of G2 are X3, V3 and of E2 are X4, V4. The distance between the centre (x, y) and the respective real and phantom locations are D1, D2, D3 (not shown) and D4. Since D4 equals the radius of the circle passing through receiver sites 10,12 and the location E2 and is unequal to the radii of the other circles shown in Figure 11, then D4 is regarded as being the correct location of an emitter. Similar drawings to Figure 12 can be constructed in which measurements from another of the centres can be compared. However in the interests of brevity they will not be described.

From reading the present disclosure, other modifications will be apparent to persons skilled in the art. Such modifications may involve other features which are already known in the design, manufacture and use of systems and component parts thereof and which may be used instead of or in addition to features already described herein. Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present application also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of- the same technical problems as does the present invention.

The applicants hereby give notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.

Claims

CLAIM(S)

1. A method of locating the position of an object emitting a rotating scanning signal, comprising detecting signal emissions from said object at two geographically separate locations, the distance (d) between which locations is known, determining the time (t) between the detection of the same signal emission at the two locations, determining the time (T) of a 360 degree scan by said signal emission, calculating the included angle (A) which the object makes with the two locations in accordance with the equation: A = 360t/T and calculating the radius (R) of a circle passing through the object and said two geographically separate locations in accordance with the equation R = d/2 sin A.

2. A method as claimed in Claim 1, wherein there are n objects in the field of view of a detector at each of the two signal detecting locations, where n is 2 or more, the signal emissions from each object being substantially the same but not synchronised, further comprising obtaining fixes on the possible real and phantom positions of said objects, calculating for each said possible object position the angle (A) the respective possible object position makes with the signal detecting locations and thereby calculating the radius (R) of a circle passing through the signal detecting locations and the possible object position, and correlating the radii of the circles with the possible object positions to eliminate those positions which appear to be phantom locations.

3. A method as claimed in Claim 2, wherein the correlation is carried out by calculating the possible object positions as X, Y coordinates on an arbitrary grid, calculating the x, y coordinates of the centres of all the possible circles and their radii (R), calculating the distance (D) between all possible combinations of X, Y and x, y, choosing for each x, y, the X, r giving a value of D most similar to the radii calculated and selecting as the real object locations those items having the best D/R match.

4. A method as claimed in Claim 1, wherein the signal emissions from said object are detected at three geographically separate locations, the distances (dl and d2) between first and second of the three locations and the second and third of the three locations being known, and wherein the included angles (Al and A2) which the object makes with the respective pairs of locations and the radii of the respective circles passing through the object and the first and second locations and the object and the second and third locations are determined, the intersection of said circles representing the position of the object.

5. An object location system for use with locating the position of at least one object emitting a rotating scanning signal, the system comprising first and second receivers having respective signal detectors disposed at geographically separate locations a known distance (d) apart, means coupled to the first and second receivers for determining the time (t) of transit of the scanning signal between the signal detectors of the first and second receivers, means for determining the time of a 360 degree scan by said signal, means for calculating the angle (A) subtended at the object in accordance with the equation: A = 360t/T and means for calculating the radius (R) of a circle passing through the locations of the object and the signal detectors in accordance with the equation: R = d/2 sin A.

6. A system as claimed in Claim 5, for use in determining the position of n objects, where n is 2 or more, each emitting substantially the same signal which is not synchronised with the signal emission(s) of the or the other objects, comprising means for computing all the possible positions of the objects, and means for correlating said possible positions with circles calculated for each of the possible object positions.

7. A system as claimed in Claim 5, comprising first, second and third receivers having respective detectors at geographically separate locations, the first and second receivers constituting a first object locating system and the second and third receivers constituting a second object locating system, each system having means for calculating the radius of a circle passing through the object location and the respective pair of receivers, and means for determining'the two points of intersection of the two circles whose radii have been calculated, one of the points coinciding with the location of the detector of the second receiver and the other of the points being that of the object.

8. A method of locating the position of an object, substantially as hereinbefore described with reference to the accompanying drawings.

9. An object location system substantially as hereinbefore described with reference to the accompanying drawings.