GB2047038A

GB2047038A - Vehicle position fixing

Info

Publication number: GB2047038A
Application number: GB8006385A
Authority: GB
Original assignee: Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1979-03-06
Filing date: 1980-02-26
Publication date: 1980-11-19
Also published as: GB2047038B

Abstract

In the Specification of our copending Application for Letters Patent No. 79/07,829 (Serial No. 2044033) (I/6259/M) there is described and claimed radio direction-finding apparatus for use as the heading sensor in a vehicle location system, which apparatus comprises a two-directional antenna (preferably crossed loops) coupled (alternatively) via a radio receiver to computing means which adjusts the receiver's gain such that the resultant of its two outputs is at a predetermined level, and (from the degree of adjustment and the two original antenna outputs) calculates the angle between the chosen radio source's direction and one or other of the antenna directions, so giving a relative value for the vehicle's heading. This earlier invention uses a radio source at a known bearing to provide information regarding the heading of the vehicle; it does not by itself fix the position of the vehicle. The present invention provides a vehicle position-fixing device in which, using the basic concept of the direction- finding device of the earlier invention, but with receiving means which is tuneable to the frequency of any one of a number of possible radio sources of known position, additional computing means are employed to produce as outputs (i) a signal which may be used to tune the direction-finding device to the frequency of a chosen radio source, and (ii), after operating upon the directional data obtained in respect of at least three chosen radio sources, a signal corresponding to the position of the vehicle.

Description

(ii) values for

fed to the receiving and adjusting means, (iii) values for

and (iv) after operating upon the obtained values fore, a value for the position of the vehicle itself.

The antenna system's two outputs are preferably fed to the receiving and adjusting means via an electronic switch (or flip-flop) operating about 200 times a second.

The receiving and adjusting means is preferably a radio receiver with automatic gain control (AGC - the effect of this is to divide the received signal by an applied control factor), and the adjustment is made by applying to the gain control a signal corresponding to the desired factor A such that the sum of the receiver's squared outputs for the two antenna directions has the predetermined value Z' (the magnitude of which is decided upon in accordance with various parameters which are described hereinafter). AGC receivers are conventional items of equipment, and need no further comment.

The receiving and adjusting means is a radio receiver tunable, by an applied tuning control signal (from the additional computing means), to any selected radio source. Such tunable receivers are conventional items of equipment, and need no further comment. However: The type of radio receiver employed for the receiving and adjusting means will naturally depend upon the prevailing circumstances, but in general it will be a super-heterodyne receiver tunable, by the applied control signal, to radio transmissions in the long wave or medium wave parts of the spectrum (roughly from 200KHz to 1.8MHz), and will preferably be tunable into any one of the standard radio sources (for example, the Non-Directional Beacons used for aircraft navigation) available around the world.In the European area seven such sources are: ANTWERP - "ONW" 355 KHz BIRMINGHAM - "GX" 347 KHz " - "GM" 334 KHz PALMA - "SJJ" 307.5 KHz " - "ADX" 384 KHz ROME - "FIU" 345KHz " - "URB" 285 KHz These NDB sources broadcast their call-sign in morse for 15 seconds every minute.

Multi-conversion superhets (with at least two IF stages) are well-known in the art; typical ones are those available from Marconi as the H254D and from Racal as the Type 1784 or the Type 1792.

The computing means to which is fed the two signal outputs of the receiving and adjusting means is most preferably a microprocessor, constrained to operate in the desired way by virtue of pre-programming during its construction. This pre-programming will enable the microprocessor: (i) to be aware of the position and frequency of a number of selected radio sources, and to produce the tuning control signals necessary sequentially to tune the receiving and adjusting means to a series of some or all of those sources; (ii) for each chosen radio source, to use its two inputs (the two outputs from the receiving and adjusting means) to produce the signal corresponding to the factor A by which the antenna system's two outputs X and Y are to be adjusted to produce values of X' and Y';; (iii) to use the same two outputs to produce a signal corresponding to the angle o between the direction of that chosen radio source and one or other antenna system direction; and (iv) having obtained 0 values for at least three radio sources, to operate upon the values in such a way as to derive the position of the vehicle.

In a preferred form, all these may basically be done as follows: (i) The microprocessor will have in its memory data regarding the position and frequency of a number of selected radio sources. Either these will all be within range, and so useable, or the microprocessor will select for use those that are within range; in either event the microprocessor will select a series of in-range sources, and will issue a sequence of tuning control signals by which the receiving and adjusting means may be tuned to one after another of the selected sources.

(ii) For each radio source, to produce a value for the factor A, the microprocessor determines X and Y, squares each and adds the squares, square-roots the sum, and divides the result by the known (predetermined) value Z'.

(iii) For each radio source, in order to produce a value for 8 the microprocessor either multiplies A by Z', divides the result into both X and Y, selects the largervalue, and calculates the angle for which that value is the sine (if it was a Y value) or the cosine (if it was an X value), or it simply - and more preferably - divides Y' by X' and calculates the angle for which the result is the tangent.

(iv) Finally, having obtained values for 0 for at least three radio sources, the microprocessor operates SPECIFICATION Vehicle position fixing This invention concerns vehicle position-fixing systems, and relates in particular to such systems involving the use of radio direction-finding apparatus.

In the Specification of our copending Application for Letters Patent No. 79/07,829 (Serial No. 2044033) (1/6259/M) there is described and claimed radio direction-finding apparatus for use as the heading sensor in a vehicle location system, which apparatus comprises a two-directional antenna (preferably crossed loops) coupled (alternately) via a radio receiver to computing means which adjusts the receiver's gain such that the resultant of the two outputs is at a predetermined level, and (from the degree of adjustment and the two original antenna outputs) calculates the angle between the chosen radio source's direction and one or other of the antenna directions, so giving a relative value for the vehicle's heading.More specifically, the invention of our aforementioned copending Application provides a radio source direction-finding device, suitable for use (in co-operation with a chosen radio source) as a heading sensor in a vehicle location system, which device comprises: a) a radio antenna system adapted to be directional alternately in two different directions so as to give two respective outputs (X, Y) which are fed alternately to b) receiving and adjusting means by which each of the antenna system's two ouputs (X, Y) is adjustable by a factor (A) such that their resultant when adjusted is a predetermined value (Z'), which means' output (X', Y') is fed to c) computing means adapted to produce as outputs (i) a signal corresponding to the factor (A) by which the antenna system's two outputs are to be adjusted as aforesaid, which signal is fed to the receiving and adjusting means, and (ii) a signal corresponding to the angle (0) between the direction of the chosen radio source and one or other antenna system direction.

This earlier invention uses a radio source at a known bearing to provide information regarding the heading of the vehicle; the inventive device does not by itself fix the position of the vehicle. Moreover, if the distance moved by the vehicle is large relative to the distance from the vehicle to the chosen radio source, so that the change in bearing of the radio source from the vehicle becomes significant, then the described device cannot accurately derive the vehicle's true heading.

The present invention seeks to provide an improved vehicle position-fixing device in which, using the basic concept of the direction-finding device of the earlier invention, there is obtained a true and accurate fix of the vehicle's actual position.

In one aspect, therefore, this invention provides a vehicle position-fixing device comprising: a) a radio source direction-finding device, as described and claimed in our aforementioned Application No.79/07,829, the receiving means of which is tuneable to the frequency of any one of a number of possible radio sources of known position; and b) additional computing means adapted to produce as outputs (i) a signal which may be used to tune the direction-finding device to the frequency of a chosen radio source, and (ii), after operating upon the directional data obtained in respect of at least three chosen radio sources, a signal corresponding to the position of the vehicle.

The radio source direction-finding device is as described and claimed in our aforementioned Application No..79/07,829 (Serial No. 2044033).

Most conveniently, therefore, the antenna system is two loop antennae the planes of the loops corresponding to the two chosen directions, and a typical practical set-up might use a cruciform ferrite core bearing windings tuneable to the chosen radio source. The two different directions in which the antenna system is adapted to be directional are most advantageously at 90 to each other (in a horizontal sense), and thus the resultant is the square root of the sum of the squared outputs from the antenna system (the resultant when adjusted therefore being the square root of the sum of the squared respective outputs from the receiving and adjusting means). It is convenient if one of the two directions be aligned with the vehicle's heading.

The two sets of computing means are preferably one and the same device.

As will be appreciated, the preferred invention as so far described may alternatively be defined as: apparatus for determining the position of a vehicle in relation to at least three chosen radio sources S,, S2, S3 of known position and of receivable signal strength Z1, Z2, Z3 which sources are at bearings 0 2, 03....from the vehicle's heading, which apparatus comprises a) two 900-crossed loop antenna attachable to the vehicle so that one of them is aligned with the vehicle's heading, the outputs X (= Z cos 0) and Y (= Z sin 0) of the loops for each source being alternately fed to b) receiving and adjusting means by which each loop's output is receivable and adjustable by a factor A to a level X' (=at) or Y' (= - respectively such that the sum of the squared adjusted signals ((X')2 + (y1)2) is a predetermined value (Z')2 (where Z' =i, the adjusted signals then being fed to c) computing means by which there can be calculated and given as outputs (i) the appropriate series of frequency control signals for tuning or re-tuning the receiving and adjusting means for a, or the next, selected radio source so that the device is tuned to a sequence Si, S2, S3...of at least three such sources giving rise to a corresponding sequence Oi, 02, 03...of values for 0, upon those values in such a way as to determine the position from which were taken the "sightings" resulting in those 13 values.

The manner in which this last operation is effected corresponds to a trial-and-error system of aligning bearing lines for each 0 value Ha, 02, 03...;with the corresponding source Si, S2, S3....(whose positions are known) so as to obtain alignment for at least three such sources simultaneously; where the bearing lines cross is the point from which the sightings were taken.In rough outline, a particular sequence of steps for carrying out this "alignment" system is as follows: 1 The microprocessor selects two sources (usually adjacent) which, together with the vehicle's position, define a triangle, and for each of which it has taken a bearing (01 and 02), operates upon the two bearings to give the joint angle subtended by the two sources, and then "plots" all the points at which the two sources subtend this angle. These points fall on a line defining the circumcircle of the two sources and the vehicle's position (the locus of a point giving a constant angle subtended by two fixed sites is a circle with the two sites at the ends of a chord, the radius of the circle equaling half the length of the chord divided by the sine of the subtended angle).

2) The microprocessor then selects another two such sources (one of which may be one of those in the first pair of sources), and repeats the procedure to obtain a second circle.

3) This process is repeated until circles have been plotted for all available/chosen pairs of sources.

4) Where all the circles intersect, there is the vehicle's position. However, because of inevitable inaccuracies in taking the bearings, and plotting the possible positions, the intersect will normally be a small polygon (a triangle if there are only three sources) rather than a point; the microprocessor then calculates the in-centre of the polygon (where intersect the lines bisecting the internal angles) to give the required position.

The position-fixing device of this invention operates upon the 0 values obtained for at least three radio sources. At least three are required because with less (with only two or one, and with no other data, it is impossible to determine a point position. On the other hand, more than three sources can be used, and so doing will improve the accuracy with which the position is fixed. Thus, because of inherent inaccuracies in measuring source direction the best that can usually be expected is that three bearing lines will "cross" to form a small triangular area, known as the "cocked hat", somewhere within which is the true position; as the number of sources used, and bearing lines "plotted", increases not only does the resulting defined area become more like a circle but as it becomes smaller so it more accurately fixes the position.

The computing means contains stored in its memory data on both the frequency and the position of a whole series of radio sources covering the area within which the vehicle is likely to be. It may select or reject sources from the whole series on a number of bases. For example, it may reject sources because they are too far away to allow a useful bearing to be taken (the computing means may arrive at this conclusion by estimating the vehicle's position and deducing that certain sources are unlikely to be of any value, or by initially using all sources but rejecting those for which the received signal is below some minimum strength value).In addition, it may reject a source because its bearing (0) is too similar to that of a source that has already been selected - the two together would give position results of very poor accuracy (again, it may decide upon this rejection either by estimating the vehicle's position, and deducing the bearing, or by actually using the sources and calculating the bearing).

Atype of microprocessor computing means able to operate in the desired manner is one of those micro-computer devices available from Intel under the family name MC586.

Since the available computing means are generally binary digital devices, capable only of handling data that arrives in the form of bits (binary digits) strung together to make "words", whereas the signals picked up by the antenna system, and received and adjusted by the receiving and adjusting means, will be analogue signals giving rise to data (X, Y, X', Y') in analogue form, the computing means needs to be associated with a suitable analogue-to-digital (and, subsequently, digital-to-analogue) converter. Moreover, since the computing means will be so constructed as to be capable of operating only upon words that do not exceed a pre-specified maximum number of bits, the capacities of converter and computing means need to be matched.It is preferred that the computing means be able to handle words up to 10 bits long (that is, words defined by a maximum of 10 binary digits, giving a maximum possible number of words of 210, or 1024), so the converter should conveniently be capable of providing at least 10 bit words.

In addition, of course, the converter must provide sufficient bits per word to ensure that the fraction of analogue signal associated with each bit is satisfactorily small; for the purpose of the present invention a converter of 10 bits per word (that is one that is capable of measuring the size of the analogue signal to the nearest 210th - or 1024th - part) is quite acceptably accurate.

The antenna system's two outputs X and Y are adjustable (in the receiving and adjusting means) by a factor A such that their resultant when adjusted is a predetermined value Z'. In the preferred form, the two loops' outputs X, Y are adjustable by a factor A such that their adjusted resultant, the square root of the sum of the squared adjusted receiving means outputs (Y-(x')2+(Y')2), is the predetermined value Z'. In a general case the particular predetermined value Z' is somewhat arbitary, but in a more limited case it is rather less so, for the value chosen will be constrained by the capabilities of the computing means employed.Thus, where the computing means is a microprocessor/converter combination of 10-bit word capacity, capable of dividing the receiving means output into 1024 parts, and where the size of any such part is fixed, it is clearly necessary so to choose the predetermined value of Z' such that the maximum possible receiving means output (X' or Y') is less than the maximum analogue signal that can be handled by the converter and processed into digital form to be operated upon by the microprocessor. On the other hand, it is naturally desirable to use the converter/microprocessor combination to the limits of its accuracy, so that the predetermined value of Z' should be chosen to make the maximum possible receiving means output (X' or Y') ciose to the maximum handleable value.By way of example, with a 10-bit converter/microprocessor combination in which each of the 1024 parts represents a 1 millivolt fraction of the analogue signal, giving a maximum handling capacity of 1.024 volts analogue input, it would be satisfactory to set the predetermined value Z' such that the maximum possible receiving means output (X' or Y') is 1.000 volts - that is, 1,000 millivolts, which is below, but close to, 1,024 millivolts.

The antenna system's two outputs are alternately feedable to the receiving and adjusting means, while the computing means needs to be able to distinguish which of its inputs relates to which of these two outputs. It is most convenient, therefore, if the computing means produces, as a further output, a signal useable to control - in the preferred use referred to hereinbefore, via the switch itself - which of the antenna system's two outputs is fed to the receiving and adjusting means.

An embodiment of the invention is now described, though only by way of illustration, with reference to the accompanying drawings in which: Figure 1 is a schematic block diagram of a circuit and apparatus for the device of the invention; Figure 2 is a vector diagram representing the outputs of the antenna system of Figure 1 for a single radio source; and Figures 3a and 3b are plotting diagrams representing the calculation of position using the device of the invention.

The apparatus and circuit of Figure 1 comprises two 90 -coiled loop antenna (10,11) connected, via an electronic double4hrow single pole switch (12), to an appropriately-tunable AGC super-heterodyne radio receiver RR (13) which is in turn connected, via an analogue-to-digital converter A-D (14), to a microprocessor M/P (15) which provides as one of its outputs the required position information. The microprocessor 15 has a second output connection to the switch 12, and third and fourth output connects, via a digital-to-analogue converter D-A (16), to the automatic gain control section (AGC) and to the tuning section (T) of the receiver 13.

Making the assumptions that antenna loop 10 is aligned with the heading of the vehicle (not shown), and that a radio signal of receivable strength Z (see Figure 2) is being picked up from a source (not shown) in a direction at an angle (3 to the vehicle's heading (and thus to antenna loop 10), the operation of the device is as follows: 1) The source's signal Z induces in the loops 10 and 11 received signals of strength X (= Z cos 0) and Y (= Z sin 0) respectively, and these signals are fed alternately (about 200 times a second), by switch 12 operating underthecontrol of the microprocessor 15, to the receiver 13.

2) The receiver 13 amplifies and demodulates the antenna signals (though for clarity this is not represented here), and, the microprocessor applying modifying signals to the receiver's automatic gain control, passes them in adjusted form (X' = Y = A)t via the converter 14, to the microprocessor 15.

3) The microprocessor's job is to tune and retune the receiver 13, to control the switch 12, to calculate from X', Y' and Z' the adjustment factor A (and to feed it, via the converter 16, to the AGC of the receiver 13), to calculate from X' and Y' the appropriate value for 0, and, having obtained 0 values for at least three radio sources, to determine therefrom the vehicle's true position.

Tuning (or retuning) the receiver 13 is effected by the microprocessor selecting from its data store information relating to the frequency of a chosen source, and then outputting a suitable tuning control signal (T) to the tuning section (T) of the receiver 13 so as to tune the latter to the required frequency.

Controlling the switch 12 is effected by supplying to the switch's energising means (not shown) suitable pulses at the appropriate rate (about 200 times per second), the pulses being generated by the microprocessor in accordance with its internal clock system.

Calculating the adjustment factor A is effected by means of the microprocessor operating upon its inputs (X', Y' and the known, predetermined, value Z'). Thus, the microprocessor knows X' and Y' (its present inputs), knows the present value of the factor A (and so can calculate X = AX' and Y = AY'), and so for any new values of X and Y can work out the factor A such that

Equals Z'.

Calculating 3 is also effected by means of the microprocessor operating upon its inputs. Knowing X, Y and Athe microprocessor can work out X' and Y' (as before), it can then work out Y'IX' and finally it can work out tan Y'/X', the answer being 0.

Finally, calculating the position from the 0 values for a series of at least three sources, is also effected by the microprocessor operating upon its inputs. With reference to Figures 3a, b, and in general terms, this may be done in the following manner: Assume that the vehicle is at position (X,x) travelling in the general direction of line 0-0, with its fore-and-aft line, against which it takes bearings, in that direction. Assume also that it has taken the bearings of four suitable sources at co-ordinates (A,a), (B,b), (C,c) and (D,d), and has obtained H values for these of -a, -p, +y and +â respectively. The microprocessor then calculates the subtended angles for the source pairs (A,a)-(B,b),(B,b)-(C,c), and (C,c)-(D-d) (not (A,a)-(D-d), because (X,x) is too near to the line joining these two), getting (an3), (p+y) and (a-y) respectively, and in each case it plots the circumcircle through the three points (as (A,a)-(B,b)-(X,x), and so on). Where all these intersect is the point (X,x).

If the microprocessor already knows roughly where the vehicle is - say, from the last position fix a few seconds earlier - then it need only plot a limited length of each circle, as indicated by the heavy line portions around (X,x).

The three circles are unlikely to intersect at appoint - and more usually they will cross to define a triangle (as in Figure 3b) with the vehicle's position (X,x) at the centre.

Claims

1. A vehicle position-fixing device including a radio source direction-finding device, as described and claimed in our aforementioned Application No.79/07,829, Serial No. 2044033 and comprising: 1 a) a radio antenna system adapted to be directional alternately in two different directions so as to give two respective outputs (X,Y) which are fed alternately to b) receiving and adjusting means tuneable to the frequency of any one of a number of possible radio sources of known position, by which means each of the antenna system's two outputs (X,Y) is adjustable by a factor (A) such that their resultant when adjusted is a predetermined value (Z'), which means' two outputs (X', Y') are fed to c) computing means adapted to produce as outputs (i) a signal corresponding to the factor (A) by which the antenna system's two outputs are to be adjusted as aforesaid, which signal is fed to the receiving and adjusting means, and (ii) a signal corresponding to the angle (0) between the direction of a chosen radio source and one or other antenna system direction; and 2) additional computing means adapted to produce as outputs (i) a signal which may be used to tune the direction-finding device to the frequency of a chosen radio source, and (ii), after operating upon the directional data obtained in respect of at aleast three chosen radio sources, a signal corresponding to the position of the vehicle.

2. A device as claimed in claim 1, wherein the antenna system is two loop antennae,, the planes of the loops corresponding to the two chosen directions.

3. A device as claimed in claim 2, wherein the two loops are provided by a cruciform ferrite core bearing windings tuneable to the chosen radio source.

4. A device as claimed in any of the preceding claims, wherein the two different directions in which the antenna system is adapted to be directional are at 900 to each other (in a horizontal sense).

5. A device as claimed in any of the preceding claims, wherein one of the two antenna directions is aligned with the vehicle's heading.

6. A device as claimed in any of the preceding claims, wherein the two sets of computing means are one and the same device.

7. A device as claimed in any of the preceding claims, which is apparatus for determining the position of a vehicle in relation to at least three chosen radio sources S1, S2, S3....of known position and of receivable signal strength Z1, Z2, Z3 which sources are at bearings f 2, 03 from the vehicle's heading, and which comprises a) two 90 -crossed loop antennae attachable to the vehicle so that one of them is aligned with the vehicle's heading, the outputs X (= Z cos 0) and Y (= Z sin (3) of the loops for each source being alternately fed to b) receiving and adjusting means by which each loop's output is receivable and adjustable by a factor A to a level X' (= wX) or Y' (=wiz respectively such that the sum of the squared adjusted signals ((X')2 + (Y')2) is a predetermined value (Z)2 (where Z' =hay, the adjusted signals then being fed to c) computing means by which there can be calculated and given as outputs (i) the appropriate series of frequency control signals for tuning or re-tuning the receiving and adjusting means to a, or the next, selected radio source so that the device is tuned to a sequence S1, S2, S3 of at least three such sources giving rise to a corresponding sequence 0 2, 03 ...of values for 0.

(ii) values for

fed to the receiving and adjusting means, (iii) values for

and iv) after operating upon the obtained values for 0, a value for the position of the vehicle itself.

8. A device as claimed in any of the preceding claims, wherein the antenna system's two outputs are fed to the receiving and adjusting means via an electronic switch (or flip-flop) operating about 200 times a second.

9. A device as claimed in any of the preceding claims, wherein the receiving and adjusting means is a radio receiver with automatic gain control, and the adjustment is made by applying to the gain control a signal corresponding to the desired factor A.

10. A device as claimed in any of the preceding claims, wherein the computing means to which is fed the two signal outputs of the receiving and adjusting means is a microprocessor, constrained to operate in the desired way by virtue of pre-programming during its construction.

11. A device as claimed in any of the preceding claims, wherein the computing means is a digital device, and is associated with a suitable analogue-to-digital (and, subsequently, digital-to-analogue) converter.

12. A device as claimed in any of the preceding claims, wherein the computing means produces, as a further output, a signal useable to control which of the antenna system's two outputs is fed to the receiving and adjusting means.

13. A vehicle position-fixing device as claimed in any of the preceding claims and substantially as hereinbefore described.

ERRATA SPECIFICATION NO 2047038A Page No. 2, Line No. 1 (ii) for

read

Page No. 2, Line No. 4 (iii) for cos -1X Page No. 2, Line No. 4 (iii) for cos z,, read cos z z -1Y -iY Page No. 2, Line No. 6 for sin z, read sin -1Y 1Y Page No. 2, Line No. 6 for tan - read tan X X -1X -iX Page No. 5, Line No. 55 (iii) for cos z, read cos -1Y . 1Y Page No. 5, Line No. 59 for sin z, read sin -1Y 1Y Page No. 5, Line No. 59 for tan X' read tan