GB1593286A - Direction finding - Google Patents

Description

(54) IMPROVEMENTS IN AND RELATING TO DIRECTION FINDING (71) We, RACAL COMMUNICATIONS EQUIPMENT LIMITED, a British Company, of Western Road, Bracknell, Berkshire, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed.

to be particularly described in and by the following statement: The invention relates to direction finding systems and methods for sensing the bearings of sources of radio signals.

According to the present invention, there is provided a direction finding system for assessing the bearing of a received radio signal, comprising an aerial arrangement operative to have a predetermined response pattern which rotates about an axis with which it is asymmetric whereby to produce a succession of first signal components each having a level dependent on the instantaneous angular position of the response pattern with respect to the direction of the received radio signal, means for producing a succession of second signals each corresponding in time to one of the first signals but having a value dependent on the level of the received signal averaged over all directions, means for comparing the value of each first signal with the corresponding second signal whereby to produce a succession of resultant outputs whose mutual relationship is compensated for the effect of changes in level in the received radio signal, and means responsive to the levels of the resultant outputs for producing an output signal representing the bearing of the received signal.

According to the invention, there is further provided a method of direction finding by assessing the bearing of a received radio signal, comprising the steps of rotating a radio aerial response pattern of predetermined form about an axis with which it is asymmetric whereby to produce a succession of first signal components each having a level dependent on the instantaneous angular position of the response pattern with respect to the direction of the received radio signal, producing a succession of second signals each corresponding in time to one of the first signals but having a value dependent on the level of the received signal averaged over all directions, comparing the value of each first signal with the corresponding second signal whereby to produce a succession of resultant outputs whose mutual relationship is compensated for the effect of changes in level in the received radio signal, and producing an output signal representing the bearing of the received signal in response to the levels of the resultant outputs.

Direction finding systems and methods according to the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings in which: Figure 1 is a block circuit diagram of the system; Figure 2 shows waveforms occurring in the system; Figure 3 is a diagram showing one form in which the bearing information can be displayed; and Figure 4 is a block circuit diagram of one form of circuitry for implementing the display of Figure 3.

The system to be described is primarily for finding the direction or bearing of a source of high frequency (HF) radio signals, but can be used for other frequency ranges.

As shown in Figure 1, the system comprises four omnidirectional aerials N, E, S and W, respectively arranged at the North, East, South and West points of the compass. It is assumed that the received signal whose bearing is to be sensed is arriving in the direction of the arrow A making the angle 0 with the North/South axis as shown. The outputs of aerials N and S are connected to an adding/subtracting unit 10 whose difference output (that is, the output carrying the difference between the signals received by the aerials N and S) is fed via a digital multiplier 12 to a summing unit 14.

The aerials E and W are connected to a second adding/subtracting unit 16 whose difference output is fed via a second digital multiplier 18 to the second input of the adding unit 14.

The "sum" outputs of the adding/subtracting units 10 and 16 are both connected to an adding unit 20 which therefore produces an output on a line 22 representing the sum of the signals received by all four aerials. This signal is fed through a 90" phase-shift unit 24 to the third input of the adding unit 14. The unit 24 is required in order to compensate for the 90" phase difference between, on the one hand, the sum outputs from the units 10 and 16 and, on the other hand, the difference outputs from those units.

The output of the adding unit 14 is fed into a first processing channel, Channel 1, and is received by a signal processing unit 26 which takes the form of a radio receiver (the input signal being fed to its antenna input).

The amplitude modulation output from the receiver 26 is detected in a detector 27 and fed through a narrow-band band pass filter 28, and then used to control a voltage controlled oscillator (VCO) 29 whose frequency is counted into a resettable counter 30.

In a second channel, Channel 2, the signal on line 22 is fed into a second signal processing unit, again taking the form of a radio receiver, 32, and the amplitude modulation output is detected in a detector 33 and fed through a narrow-band band-pass filter 34, and then used to control a second VCO 35 whose frequency is counted into a resettable counter 36.

The counter outputs are fed to a ratio unit 48 which produces a quotient signal fed to a logic unit 50 for calculation of the bearing.

The digital multipliers 12 and 18 are fed with square wave signals on lines 38 and 40 respectively, and these signals are also used to control the resetting of the counters 30 and 36, being fed to the counters on lines 42 and 44 respectively, all in a manner now to be described.

As is apparent from Figure 1, the difference output from the adding/subtracting unit 10 (that is, the difference between the signals received by the aerials N and S) will be given by P = k1.cos 0.cos, (1) where kl is a constant and coswct represents the received signal.

Similarly, the difference output from the adding/subtracting unit 16 will be given by Q = k1 sin 0, coswct, (2) Figure 2A shows the square wave signal presented to unit 12 on line 38, while Figure 2B shows the square wave signal presented to unit 18 on line 40. The square wave signals form a switching sequence comprising four cycles I, II, III, IV. Therefore, during cycles I and IV, unit 12 passes the signal P (Equation 1) to adder unit 14 while during cycles II and III, it passes the signal -P. Correspondingly, unit 18 passes the signal +Q (Equation 2) to the adder unit 14 during cycles I and II and passes the signal -Q during cycles III and IV.

In effect, therefore, the difference outputs from the adding/subtracting units 10 and 16 are being respectively multiplied by the square waveforms shown in Figures 2A and 2B. If only the fundamental components of these two square waveforms are considered, taking the fundamental components of the waveform of Figure 2A as cosot and that of Figure 2B as sinot, then the corresponding signals respectively fed into the two inputs of the adder unit 14 are R = k1 cos B.coso,t. cost, and (3) S = kl sin 0.cosct. into. (4) Therefore, the output of the adder unit 14 is given by T = coswoct [1 + k1 (cos #.cos#t + sin #.sin#t)] = coseoct [1+kl cos (ot -6)] (5) Therefore, the received signal w, has been effectively amplitude-modulated and the amplitude modulation enables the desired bearing 0 to be extracted, being simply the phase lag or lead with respect to the reference or switching frequency o. In other words, the bearing can be extracted by extracting the amplitude modulation from the output of the unit 14 and measuring its phase difference with respect to the reference or switching frequency #.

Accordingly, the amplitude modulation is extracted by the detector 27 and the narrow-band band pass filter 28 (whose band is centred on the switching frequency of the square wave signals applied to the lines 38 and 40). Figure 2C shows a representation of the amplitude modulation (assuming an ideal case in which the received signal is continuous at a constant level).

In Channel 1, during each cycle I, II III and IV, of the switching sequence, the frequency of the VCO 28 is counted into the counter 30, the count of the counter being reset to zero at the end of each cycle of the switching sequence by means of the line 42. The counter therefore produces a succession of digital signals whose values respectively represent the modulation superimposed by the system on the received signal. The digital signals successively produced during the cycles I, II, III and IV can be stated thus: S1 = k3 + k4 (cos 0 + sin 0); S11 = k3 - k4 (cos 0 - sin 0); S111 = k3 - k4 (cos 0 + sin 0); = = k3 + k4 (cos 0 - sin 0); where k3 and k4 are constants.

Ignoring the effect of Channel 2 for the time being, it will be apparent that the logic unit 50 successively receives the digital signals S1, SIl, SIll and SIV during the four cycles I, II, III and IV. In the logic unit, the value for cycle III is subtracted from that for cycle I, giving M = k3 (cos 0 + sin 0), (6) and the value for cycle IV is subtracted from that for cycle II, giving N = k3 (cos 0 - sin 0) (7) In the logic unit, the ratio M/N is taken, giving cos # + sin # M/N = cos 0 + sin 0 - (cos 6 - sin O) 1 1 + tan 6 1 - tan 0 = tan (0 + 45"). (8) In this way, 0 can be easily determined.It will be appreciated that in practice the switching sequence would be repeated over and over again on a particular received signal and each switching sequence would produce a signal of the form shown in Figure 2C. The logic unit 50 would therefore produce a succession of values for 0.

As stated above, Figure 2C represents an ideal case in which the received signal is continuous at a constant level. In practice, however, the signal may be discontinuous particularly at HF, and its amplitude may vary. Since the determination of the bearing 6 depends on the actual levels of the modulation, these effects would cause considerable error. The second channel, Channel 2, of the system, is intended to deal with this problem as will now be explained.

In the second channel, the overall signal from the four aerials is processed in the receiver 32 to produce an output dependent on the level of the received signal. This output is used to modify the output of Channel 1 to compensate for discontinuities or reduced levels in the received signal or, if the received signal is such that compensation is not possible, to temporarily inhibit the system to prevent the production of misleading results.

In order to carry out this function, in Channel 2, the detected output from receiver 32 controls the frequency of the VCO 34 and this frequency is counted in the counter 36 over each cycle I, II. III and IV of the switching sequence, the counter 36 being reset to zero at the end of each cycle. Therefore, at the end of each cycle, the counter 36 contains a count which is dependent on the received signal over that switching cycle. At the end of each switching cycle, when the counters 30 and 36 are reset, their counts are passed on to the ratio unit 48 where the ratio is taken between them.

From Equation (5), and representing the varying signal level of the received radio signal by modulation Ml, the output of the adder unit 14 can be rewritten as T = cosu,t.(M,).[1 + kl cos (t - 0)] (9) Putting [1 + k, cos (ot - 0)] as M2, then T = coscot.(M1).(M) (10) Likewise, the output of the adder 20 (being the omnidirectional signal) can be represented by U = cossoct.(M) (11) The counters 30 and 36 effectively take the moduli of the modulations on the two signals T and U, and the ratio unit 48 therefore measures the quotient V, where

The resultant output of the ratio unit 48 is therefore compensated for the effects of reducing levels or discontinuities in the received signal - because, if, for example, the received signal is falling during a particular switching cycle, not only with the resultant count in the counter 30 be less but so also will be the count in the counter 36, and the quotient will therefore tend to be unaffected by the reducing level. Similarly, if there is a discontinuity in the received signal during a switching cycle, there will be a resultant reduction in the final counts of the counters 30 and 36 at the end of that switching cycle and again the quotient will tend to be unaffected. The result is, therefore, that a compensated signal is fed to the logic unit 50 for accurate measurement of the bearing. Therefore, the integrating functions performed by the counters 30 and 36 and the taking of the ratio between their counts at the end of each switching cycle enables accurate bearing measurement to be made even in the event of complete discontinuities in the received signal - which will of course cause the output level from the receiver 26 to fall suddenly to zero and, if used directly to assess the bearing, will give misleading information. If the discontinuities or reductions in level of the received signal are too great in length or amount, the output of the ratio unit 48 will fall below a predetermined minimum and the logic unit 50 will detect this and produce no bearing output at all for that switching sequence.

If desired it is of course possible for the ratio between the two channels to be taken at the outputs of their receivers (that is, at intermediate frequency), the resultant signal then being passed through a band pass filter, and used to control a VCO feeding a counter in the manner described. In such a case, of course, the ratio unit would take the ratio between the signals T and U of Equations (9) and (10) above, giving a quotient W where W costoct (MI).(M2) COSWct. (M1) = (M2) The switching sequence used and the feeding in of the omnidirectional signal to the adder 14 from the line 22 causes the output from the adder 14 to be equivalent to that which would be produced by an aerial or aerial system having a rotating polar response curve of cardioid pattern.It will be appreciated that it is not necessary for the aerial array to be limited to four aerials. There may be advantages in having more than four aerials, symmetrically arranged with respect to the points of the compass and with corresponding alteration in the switching sequence.

The digital bearing information produced by the logic unit 50 can be displayed in a variety of ways.

First, it may simply be displayed by means of a digital display unit as a digital number.

Secondly, it may be displayed in analogue form on a cathode ray tube, for example with reference either to- a polar co-ordinate system or to a Cartesian co-ordinate system.

A third method of displaying the bearing is shown in Figure 3 and comprises a histogram-type display, again with reference to Cartesian co-ordinates. In this type of display, the bearing reading produced by each scanning sequence is used to produce a spot 100 (Figure 3) at an appropriate position on a CRT screen 101 with reference to an axis 102 calibrated in degrees (of bearing), the spots produced by succeeding scanning sequences being successively displaced in the direction of the orthogonal axis 104. Therefore, the bearing information produced as a result of each scanning sequence is given due weight in building up an overall representation of the bearing.The result may be informative than merely taking an overall average of a succession of readings when displaying - because averaging may obscure the offsetting effect of a secondary received signal having a different bearing and being received simultaneously with the primary signal. In the display method of Figure 3, the bearing of the second signal will be separately displayed by means of further spots 100A.

Figure 4 shows a block circuit diagram of circuitry for producing the histogram-type display of Figure 3.

As shown the circuitry includes a random access memory (RAM) 120 having a plurality of storage locations each of which can be addressed by means of an address line 122. Each storage location corresponds to a respective range of bearing values. For example, the first location may correspond to a bearing in the range 0 to 1.99 degrees, the second to a bearing in the range 2 to 3.99 degrees, the third to a bearing in the range 4 to 5.99 degrees and so on up to the 180th location which corresponds to a bearing in the range 358 to 359.99 degrees.

In a manner to be described, each storage location stores a number whose value is equal to the cumulative total of the number of bearing readings lying in the respective range. The data in each storage location is read out in turn on a channel 124, in a manner to be described, and temporarily stored on latches 126.

In addition the circuitry includes an adder unit 128 which receives the data on the latches 126 and has a second input line 130 by means of which, in a manner to be described, the data is up-dated as necessary to take account of any new bearing reading lying in the bearing range corresponding to that data.

The adder output is connected to a digital to analogue converter 132 and thence to the Y plates of the CRT 134 and is also connected back to the input of the RAM 120 via a channel 136.

The operation of the circuitry is controlled by a counter 140 which is driven by clock pulses on a line 142 and has a maximum count equal to the number of storage locations in the RAM 120 which are assigned to respective bearing ranges. The counter 140 has an output line 143 carrying a number corresponding to its instantaneous count and this number is fed to the RAM 120 on the address line. In addition, it is fed to a gating unit 144 whose second input 145 is connected to the logic unit 50 (Figure 1) and carries a number representing the bearing value detected by the system. If this bearing value corresponds with the number on line 143 (that is, if the bearing value lies within the range represented by the count of the counter at that time), a pulse is fed to the input line 130 of the adder unit 128.Finally, the line 143 is fed to a digital to analogue converter 146 and thence to the X plates of the CRT 134.

In operation, the counter 140 is stepped through its count by the clock pulses and successively reads out the data in the storage locations of the RAM 120. In addition, a successively increasing signal is applied to the X plates of the CRT 134 via the digital to analogue converter 146 so as to deflect the CRT spot along the axis 104. At each position on the axis 104 (corresponding to a particular range of bearing values), the digital to analogue converter 132 deflects the spot with respect to the axis 102 by an amount dependent on the number of bearing values detected as lying in that range.

In addition, any fresh bearing value detected as lying in that range causes a pulse to be fed into the adder unit 128 on the line 130, thus updating the data in the appropriate storage location in the RAM 120 via the channel 136.

A reset unit 50 is provided for resetting the storage locations of the RAM 120 to zero when a new set of bearing readings is to be taken.

When a storage location in the RAM 120 reaches a maximum count, corresponding to a maximum vertical deflection on the CRT 134, the system is arranged to subtract a predetermined amount from each subsequently interrogated storage location so as to alter the scale of all following readings to allow their data to be properly displayed according to their proper reactive values.

Although Figure 4 describes how the histogram-type display may be implemented by hardware, it will be appeciated that it may also be implemented by means of software or by means of a microprocessor.

In order to produce signals for energising the X and Y plates of the CRT to produce the various form of display described, the digital signals in the logic unit 50 may be converted into suitable analogue form using known techniques.

Since the system is handling the signals digitally at the point of assessment of the bearing, it is possible easily to carry out corrective action to take account of calibration errors and the like, such as caused by the conditions of a particular measurement site; such corrections can be stored in "look-up" tables.

It will be noted that the two radio receivers do not have to be matched, except that any delays which they impose on signals being processed should be substantially equal.

WHAT WE CLAIM IS: 1. A direction finding system for assessing the bearing of a received radio signal, comprising an aerial arrangement operative to have a predetermined response pattern which rotates about an axis with which it is asymmetric whereby to produce a succession of first signal components each having a level dependent on the instantaneous angular position of the response pattern with respect to the direction of the received radio signal, means for producing a succession of second signals each corresponding in time to one of the first signals but having a value dependent on the level of the received signal averaged over all directions, means for comparing the value of each first signal with the corresponding second signal whereby to produce a succession of resultant outputs whose mutual relationship is compensated for the effect of changes in level in the received radio signal, and means responsive to the levels of the resultant outputs for producing an output signal representing the bearing of the received signal.

2. A system according to claim 1, including means for integrating each first signal and the corresponding second signal.

3. A system according to claim 2, in which the said first and second signals are integrated by converting them into respective corresponding frequencies and counting the frequencies into respective counters.

4. A system according to any preceding claim, in which the said response pattern is cardioid-shaped.

5. A system according to claim 4, in which the aerial arrangement comprises an array of ommidirectional aerials for relative positioning at predetermined points of the compass, means for subtracting the component of the radio signal received by one aerial of each one of a plurality of pairs of the aerials from the component received by the other aerial of that pair to produce a plurality of difference signals corresponding respectively to different said pairs of the aerials, means for respectively multiplying the difference signals by respectively phase-displaced multiplying signals having a predetermined frequency to produce respective product signals, and means for adding the product signals together and to the said second output, whereby the total signals so produced correspond to the signal that would be produced by an aerial arrangement having a cardioid pattern response rotating at the said predetermined frequency.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (14)

**WARNING** start of CLMS field may overlap end of DESC **. representing the bearing value detected by the system. If this bearing value corresponds with the number on line 143 (that is, if the bearing value lies within the range represented by the count of the counter at that time), a pulse is fed to the input line 130 of the adder unit 128. Finally, the line 143 is fed to a digital to analogue converter 146 and thence to the X plates of the CRT 134. In operation, the counter 140 is stepped through its count by the clock pulses and successively reads out the data in the storage locations of the RAM 120. In addition, a successively increasing signal is applied to the X plates of the CRT 134 via the digital to analogue converter 146 so as to deflect the CRT spot along the axis 104. At each position on the axis 104 (corresponding to a particular range of bearing values), the digital to analogue converter 132 deflects the spot with respect to the axis 102 by an amount dependent on the number of bearing values detected as lying in that range. In addition, any fresh bearing value detected as lying in that range causes a pulse to be fed into the adder unit 128 on the line 130, thus updating the data in the appropriate storage location in the RAM 120 via the channel 136. A reset unit 50 is provided for resetting the storage locations of the RAM 120 to zero when a new set of bearing readings is to be taken. When a storage location in the RAM 120 reaches a maximum count, corresponding to a maximum vertical deflection on the CRT 134, the system is arranged to subtract a predetermined amount from each subsequently interrogated storage location so as to alter the scale of all following readings to allow their data to be properly displayed according to their proper reactive values. Although Figure 4 describes how the histogram-type display may be implemented by hardware, it will be appeciated that it may also be implemented by means of software or by means of a microprocessor. In order to produce signals for energising the X and Y plates of the CRT to produce the various form of display described, the digital signals in the logic unit 50 may be converted into suitable analogue form using known techniques. Since the system is handling the signals digitally at the point of assessment of the bearing, it is possible easily to carry out corrective action to take account of calibration errors and the like, such as caused by the conditions of a particular measurement site; such corrections can be stored in "look-up" tables. It will be noted that the two radio receivers do not have to be matched, except that any delays which they impose on signals being processed should be substantially equal. WHAT WE CLAIM IS:

1. A direction finding system for assessing the bearing of a received radio signal, comprising an aerial arrangement operative to have a predetermined response pattern which rotates about an axis with which it is asymmetric whereby to produce a succession of first signal components each having a level dependent on the instantaneous angular position of the response pattern with respect to the direction of the received radio signal, means for producing a succession of second signals each corresponding in time to one of the first signals but having a value dependent on the level of the received signal averaged over all directions, means for comparing the value of each first signal with the corresponding second signal whereby to produce a succession of resultant outputs whose mutual relationship is compensated for the effect of changes in level in the received radio signal, and means responsive to the levels of the resultant outputs for producing an output signal representing the bearing of the received signal.

2. A system according to claim 1, including means for integrating each first signal and the corresponding second signal.

3. A system according to claim 2, in which the said first and second signals are integrated by converting them into respective corresponding frequencies and counting the frequencies into respective counters.

4. A system according to any preceding claim, in which the said response pattern is cardioid-shaped.

5. A system according to claim 4, in which the aerial arrangement comprises an array of ommidirectional aerials for relative positioning at predetermined points of the compass, means for subtracting the component of the radio signal received by one aerial of each one of a plurality of pairs of the aerials from the component received by the other aerial of that pair to produce a plurality of difference signals corresponding respectively to different said pairs of the aerials, means for respectively multiplying the difference signals by respectively phase-displaced multiplying signals having a predetermined frequency to produce respective product signals, and means for adding the product signals together and to the said second output, whereby the total signals so produced correspond to the signal that would be produced by an aerial arrangement having a cardioid pattern response rotating at the said predetermined frequency.

6. A system according to claim 5, in which the multiplying signals comprise

phase-displaced square wave signals each of whose fundamental sinusoidal components has the said predetermined reference frequency.

7. A system according to claim 5 or 6, in which there are four such aerials intended for mounting at the four cardinal points of the compass.

8. A system according to any preceding claim, including a display screen, and means responsive to each said output signal for producing a respective visual mark on the display screen at a point which is related to a first axis thereon in dependence on the value of the bearing and which is related to an orthogonal axis thereon in dependence on the value of the bearing and which is related to an orthogonal axis thereon in dependence on the point in time of the bearing measurement, whereby to produce a histogram type of display.

9. A system according to claim 8, comprising storage means having a plurality of storage locations each corresponding to a respective range of bearing values, the ranges together covering the total possible number of bearing values and not overlapping each other, means for successively and regularly accessing all the storage locations, means operative while each location is accessed to determine whether a signal to be displayed represents a bearing value lying in the range corresponding to the location and, if it does, to feed into that location a data signal of predetermined value whereby to increase the total value of the data stored in that location by the predetermined value, a cathode ray tube providing the said display screen, means for driving one set of plates of the cathode ray tube in dependence on the total value of the data stored in each location as it is accessed, and means for driving the orthogonal set of plates of the cathode ray tube in dependence on a count signal whose value increases stepwise as each location is accessed.

10. A method of direction finding by assessing the bearing of a received radio signal, comprising the steps of rotating a radio aerial response pattern of predetermined form about an axis with which it is asymmetric whereby to produce a succession of first signal components each having a level dependent on the instantaneous angular position of the response pattern with respect to the direction of the received radio signal, producing a succession of second signals each corresponding in time to one of the first signals but having a value dependent on the level of the received signal averaged over all directions, comparing the value of each first signal with the corresponding second signal whereby to produce a succession of resultant outputs whose mutual relationship is compensated for the effect of changes in level in the received radio signal, and producing an output signal representing the bearing of the received signal in response to the levels of the resultant outputs.

11. A direction finding system substantially as described with reference to Figures 1 and 2 of the accompanying drawings.

12. A direction finding system substantially as described with reference to Figures 1, 2 and 3 of the accompanying drawings.

13. A direction finding system substantially as described with reference to Figures 1 to 4 of the accompanying drawings.

14. A method of direction finding, substantially as described with reference to Figures 1 and 2 of the accompanying drawings.