GB2101440A - Direction finding - Google Patents

Abstract

A direction finding system has radio receivers 10 and 12 receiving. respectively. the difference between the signals picked up by antennas N and S, and the signals picked up by antennas E and W. The resultant analogue signals, proportional to the sine and cosine of theta are converted into digital form and divided by the digital value of the signal picked up by a sense antenna C. A divider 26 then divides the resultant outputs one with the other, to produce an output proportional to tan theta and with no sense ambiguity. The use of digital techniques minimises receiver matching problems. The narrow aperture ( lambda /4) of the system as described is widened in another system which includes an approximate-bearing unit for resolution of sense ambiguity. Figures 2, 3 (neither shown). <IMAGE>

Description

SPECIFICATION Improvements in and relating to direction finding systems The invention relates to direction finding and more specifically to systems and methods for determining the direction of received radio signals.

Various novel features of the invention will be apparent from the following description, given by way of example only, of direction finding systems and methods according to the invention, reference being made to the accompanying diagrammatic drawings in which: Figure 1 is a block diagram of one of the systems using a narrow-aperture antenna arrangement; Figure 2 diagrammatically shows a wideaperture arrangement of antennas for use with another of the systems; and Figure 3 is a block diagram of a wide-aperture system using the antenna arrangement of Figure 2.

More specifically to be described below is a direction finding system for assessing the bearing of a received radio signal, comprising first and second antenna means having respective direction-sensitive patterns arranged orthogonally to each other and for respectively producing first and second electrical analogue signals respectively dependent on the magnitude of the received radio signal and its sense with respect to their response patterns, means for producing a third signal dependent on the magnitude of the received radio signal and independent of its sense, means operative to convert the analogue signals into first, second and third digital signals respectively, means operative to measure the ratio of each of the first and second digital signals to the third digital signal so as to produce respective outputs respectively dependent on the sine and cosine of the bearing angle of the received signal, and output means responsive to the said outputs to assess the bearing angle.

Advantageously, the output means includes means for measuring the ratio of the said outputs and to derive therefrom a bearing signal having a value dependent on the bearing angle.

Such a system may include analogue display means in the form of a cathode ray tube to the opposite pairs of the plates of which are respectively applied first and second re-converted analogue signals which are produced by reconverting the first and second digital signals into analogue form.

According to another aspect, there will be disclosed in more detail below a direction finding system for assessing the bearing of a received radio signal, comprising first antenna means defining a pair of first measuring points separated along a line by at least half the wavelength at the frequency of the radio signal, second antenna means defining a pair of second measuring points separated along an orthogonal line by at least half the said wavelength, first input means, including analogue-to-digital converting means, responsive to the sum of and the difference between, the radio signal as respectively received at the.said first two measuring points to produce a first output signal dependent on the amount by which the phase difference between the radio signal as received at each of the said first two measuring points exceeds an integral number of cycles at the frequency of the received radio signal, second input means, including analogue-to-digital converting means, responsive to the sum of, and the difference between, the radio signal as respectively received at the said two second measuring points to produce a second output dependent on the amount by which the phase difference between the radio signal as received at each of the said two second measuring points exceeds an integral number of cycles at the frequency of the received radio signal, modifying means responsive to the frequency of the radio signal and operative in dependence thereon and on the said separation distance and on a signal approximating to the bearing to be assessed, to determine the approximate value for each said phase difference whereby to modify if necessary the integer value in each said output signal, means connected to measure the ratio of the modified output signals whereby to determine the said bearing, and means responsive to the approximating signal for resolving any ambiguity in the said bearing.

For example, each input means may comprise a summing circuit connected to sum the phases of the radio signal as received at each of the respective two measuring points whereby to produce a cosine signal dependent on the cosine of half the said phase difference, a differencing circuit connected to measure the difference between the phases of the radio signal as received at each of the two measuring points whereby to produce a sine signal dependent on the sine of half the phase difference, means for quadrature phase-shifting one of the sine and cosine signals with respect to the other, means for taking the ratio between the cosine and sine signals whereby to produce an output dependent on the tangent of the phase difference, and means responsive to the said tangent signal to produce the respective said output signal.The sum and differencing circuits may either be preceded by, or followed by, respective analogue-to-digital converting circuits.

One of the measuring points of the first antenna means may constitute one of the measuring points of the second antenna means.

Advantageously, the means for producing the said approximating signal comprises first and second antenna means having respective direction-sensitive patterns arranged orthogonally to each other and having apertures of less than a quarter of the said wavelength and for respectively producing first and second electrical analogue signals respectively dependent on the magnitude of the received radio signal and its sense with respect to their responsive patterns, means for producing a third signal dependent on the magnitude of the received radio signal and independent of its sense, means operative to convert the analogue signals into first, second and third digital signals respectively, means operative to measure the ratio of each of the first and second digital signals to the third signal so as to produce respective outputs respectively approximately dependent on the sine and cosine of the bearing angle of the received signal, and output means responsive to the said outputs to produce the said approximating signal.

The foregoing are exemplary of and not exhaustive of the various novel features of the invention now to be described in detail.

The system shown in Figure 1 has two antenna pairs made up of antenna elements N, S, and E, W which are fixedly arranged in relation to each other according to the points of the compass.

Antenna pair N-S produces an output feeding a radio receiver 10 via a subtractor 11, an antenna pair E-W produces an output feeding a radio receiver 12 via a subtractor 13. The system being described is a narrow-aperture system in which the aperture of each antenna pair is less than a quarter of a wavelength of the frequency of the signal whose direction is to be assessed. In Figure 1, the received signal is shown at R and is assumed to making a bearing angle 6 with respect to the N-S axis.

The system also has a sense antenna C which is positioned at the centre of the antenna array and feeds a third radio receiver 1 4. It is assumed that the three receivers 10, 12 and 14 are matched as to gain and phase. They may, for example, have matched front ends and be driven by a common local oscillator.

It will be apparent that the output, Sa1, of receiver 10 will be given by Sa,=K1 . sin 6. cos wt, where w is the angular frequency of the reference carrier.

Similarly, the output, Sa2, of the receiver 12 is given by Sa2=K, . cos 0. cos ot.

The central antenna, antenna C receives a signal which is independent of the bearing angle 0, and therefore the output of receiver 14, Sa3, is given by Sa3--K2 . cos ot The outputs Sa,, Sa2 and Sa3 are fed to respective analogue-to-digital converters 16, 18, 20 which therefore produce respective outputs Sd1, Sd2 and Sd3 given by Sd1=K3 . sin 0. cos wt, Sd2=K3 . cos 0. cos wt, and Sd3=K4 . cos wt The output Sd1 of the analogue-to-digital converter 1 6 is fed into a divider 22 where it is divided by the output Sd3 from the analogue-todigital converter 20.Similarly, the output Sd2 of the analogue-to-digital converter 1 8 is fed into a second divider 24 where it is divided by the output Sd3 of the analogue-to-digital converter 20.

Therefore, divider 22 produces an output 0, where O,=Sd,/Sd3 =K3/K4 . sin 0 Similarly, divider 24 produces an output 02 where 2=Sd2/Sd3t -KMK4. cos 0 The outputs 0, and 02 are fed into a third divider 26 which produces an output St where St=O 1/ 2 tan 0 The output St is fed into an output unit 28 which processes the signal S2 to produce the corresponding value for 0, the required bearing.

For example, the unit 28 may be in the form of a look-up table storing the value of 0 for each value of tan 0.

In this way, therefore, the system produces an output representing the desired bearing and with no sense ambiguity.

The system described can be compared with the basic Watson-Watt type of direction finding system. The advantage of that system was that it gave very rapid direction finding even in the presence of interfering signals. However, the system described and illustrated in Fig. 1 has the added advantage that the receiver matching problems are minimised by the use of digital techniques, particularly in respect of the l.F.

filters.

Although two digital back-end channels (i.e. at the outputs of the receivers 10 and 12) are referred to in Fig. 1, it may be practicable to use one common digital channel and to time-share between the two receivers 10 and 12.

If desired, the receivers 10 and 12 can be completely digital receivers at least at lower radio frequencies (and possibly at higher frequencies as permitted by available digital techniques). This avoids the front-end matching problems of analogue receivers.

The need for closely matched front-ends can be reduced by injecting a standardizing signal, from time to time and at the received frequency, into all three inputs in parallel, via switches 36, 37 and 38 from a source 39. The injected signal is used to calibrate the receivers for small differences in gain and phase in the front ends.

Tan 0/2 will be 1.0 if no corrections are required and deviations from 1.0 can be used to compute the errors which can be corrected using corrections signals stored in a digital memory. If all three receivers have a common first oscillator, the injection signal can be synthesised by mixing this oscillator output with a frequency equal to the first l.F. Therefore, when the receiver frequency changes, the injected frequency will change in sympathy.

The digital sides of the analogue-to-digital converters 16, 1 8 and 20 can include filters and automatic gain control operative at the intermediate frequency of the receivers and these enable phase and amplitude differences between the three channels to be held at negligible values.

The system described above is shown as using three radio receivers. However, it may be modified to use only two receivers by time sharing. The use of three receivers 10, 12 and 14 has the advantage of continuous sense resolution.

As so far described, the system produces a digital output which may of course be displayed by means of an appropriate digital display.

However, in addition the system may be arranged to produce an analogue output on a cathode ray tube display unit 30. For this purpose, the digital output Sd, of the analogue-to-digital converter 16 is branched off and fed into a digital-toanalogue converter 32 so as to be re-converted back into analogue form and then applied across the X plates of the CRT 30. Similarly, the output Sd2 of the analogue-to-digital converter 1 8 is branched off and re-converted into analogue form by a digital-to-analogue converter 33 and applied across the Y plates of the CRT 30.

A narrow ellipse will therefore be produced on the CRT screen, representing the angle fl. This display will have sense ambiguity and this may be resolved in known fashion by applying a signal to the control grid of the CRT in dependence on the output of the sense antenna C. For this purpose, therefore, the output Sd3 of the analogue-to-digital converter 20 is branched off and re-converted to analogue form by a digital-toanalogue converter 34 and then applied to the control grid of the CRT.

The CRT display has the advantage of enabling the bearing to be obtained even in the presence of interference. If the interference is such as to cause the CRT to show two traces, one due to the true bearing, the other due to the interference, the bearing of the wanted signal can be determined if the bearing of the interfering signal is known.

As indicated above, the system described with reference to Figure 1 is a narrow-aperture system, that is, the antenna elements of each antenna pair are separated by less than one quarter of the wavelength at the received frequencies, that is, it assumes that the incident signal strength is proportional to cos 0. Figures 2 and 3 show a wide aperture system which gives greater accuracy.

As shown in Figure 2, the system has three antennas A, B and C arranged as shown, with the distance AB equal to the distance BC, each such distance being equal to D, where D is greater than half the wavelength A at the frequency of the received signal. The received signal is shown at R and is assumed to make an angle 0 with the axis BC.

The system also includes two further antennas a and c, respectively lying on the north-south and east-west axes. The distance AB is equal to the distance Bc and each such distance is substantially less than half the wavelength at the frequency of the received signal R.

Therefore, it will be apparent that, for the pair of antennas A and B, the total phase difference, j1, in terms of phase cycles, between the signals which they respectively receive is represented by the distance B-X in Fig. 2 and is therefore given by D 1--.sin0.cos8 (1) where A is the wavelength at the angular frequency zc of the incident wave R, and a is the angle of elevation of the wave R with respect to the horizontal.

Similarly, the total phase difference 02' between the signals respectively received by the antennas B and C, is represented by the distance B-Y and this is given by D ,=--. cos 8 . cos (2) Therefore, 0 2=tan 0 (3) and this enables the value of 6, the required bearing, to be calculated.

It will be apparent that the value of 0 obtained is independent of the angle of elevation. From Equations (1) and (2) it will also be apparent that COS =(412 +422)1/2 (4) D There are two additional factors which have to be taken into account.

First, the value for 0 will have ambiguity of sense. Secondly, each of the values 4, and 02 is the total phase difference and, because of the wide aperture between each pair of antennae, may therefore (when represented in phase cycles) be made up of a fractional part plus n where n is an integer.

In the system now to be described in more detail, the following functions are carried out:- (a) the signals received by the antennas A and B are processed to produce the fractional value for 0,; (b) the signals respectively received by the antennas B and C are processed to produce the fractional value of 02; (c) the signals received by the narrow aperture antennas a and c are processed to produce an approximate value for î, including its sense;; (d) the approximate value for î is, in combination with Equations (1) and (2), used to convert the fractional values of 0, and 02 into total values for X, and 02 (if the angle of elevation, S, is not known, the sum of the fractional value and an integer should come within a factor, cos S, of the total phase cycles derived from Equation (1) or (2) assuming S=O, and in this way the value of the integer can usually be obtained by inspection unless the propagation paths direction is uncertain);; (e) the total value for 0, and 02 are used to produce the accurate value for E, using Equation (3), and to produce the value of S, using Equation (4).

The foregoing is also discussed in a Paper by C.

W. McLeish and N. Burnyk published in September 1961 by the Institute of Electrical Engineers (Paper No. 3678E). The system now to be described in more detail with reference to Figure 3 is a development of the arrangement discussed in that Paper.

The antennas A, B and C (Fig. 2) are respectively connected to feed their signals to three receivers 40, 42 and 44 (Figure 3) which are matched in gain and phase. The outputs of the receivers 40, 42 and 44, each of which is dependent on the magnitude and phase of the component of the signal R which it receives, are respectively fed to three analogue-to-digital converters 46, 48 and 50.

A summing circuit 52 sums the digital outputs of converters 46 and 48, and a difference circuit 54 takes the difference between the digital outputs of the converters 46 and 48.

Similarly, a summing circuit 56 sums the digital outputs of the converters 48 and 50, and a difference circuit 58 takes the difference between the digital outputs of the converters 48 and 50.

If the phase of the signal arriving at antenna A is 0,/2 with respect to the midpoint of the distance AB, then the phase of the signal arriving at antenna B will be -#1/2.

Therefore, the output, Os, of the summing circuit 52 will be given (in digital form) by Os1=K . cos wt . (e+1.#1/2+e-i.#1/2) (5) =2K . cos wt . cos #1/2 Similarly, the difference output, Od,, of the differencing circuit 54 will be given by Od,=K . cos wt . (e+1012~e-l01/2) =2K . cos #t.i.sin #1/2 (6) The output Os, is passed into a divider 60 where it is divided by the signal Od, after the latter has been shifted by 900 in a phase shifting circuit 62 (the output of circuit 62 therefore being given by 2K . cos wt. sin 0,/2).

Therefore, the output, Ot, of divider 60 is given by Ot=tan (X,/2 This output is passed into an output unit 64 which produces an output equal to 0,/2 on a line 66. Unit 64 may therefore comprise a look-up table storing values of 0,/2 for corresponding values of tan 0,/2.

Similarly, the phase of the signal arriving at antenna C will be 02/2 with respect to the midpoint of the distance BC and the phase of the signal arriving at antenna B will therefore be 02/2. By analogy with Equation (5) and (6), therefore, the output Os2 of the summing circuit 56 will be given by Os2=2K . cos wt. cos 02/2, (7) and the output Od2 of the difference circuit 58 will be given by Od2=2K . cos ot.i. sin 02/2. (8) The output Os2 is fed into a divider 68 and divided by the signal Od2 after the latter has been phase-shifted by 900 in a phase-shifting circuit 70.Therefore, the divider 68 produces an output t2 where 0t2=tan 452/2. An output unit 72 converts this into a signal representing a corresponding value for 02/2 on a line 74.

Lines 66 and 74 feed their outputs to a signal processing unit 76.

The signal processing unit 76 also receives an output on a line 78 from an approximate bearing unit 80 to be described below in detail and which responds to the signals received by the antennas a and c to produce an output on line 78 representing an approximate value for 0 (and with no sense ambiguity).

In addition, unit 76 receives a signal on line 84 dependent on the frequency of the received radio signal R. Line 84 may for example be connected to a frequency-indicating output of the receiver 32.

Finally, the unit 76 receives a signal on a line 86 from a manually settable unit 88 and which represents the separation D between the antennas of each pair A and B, and B and C.

The processing unit 76 operates by reference to Equations (1) and (2). It will be apparent from these two Equations that by substituting in them the approximate value of 0 (line 82), a value for A (obtained from the frequency indicating signal on line 84), and a value for D (line 86), and by assuming the angle of elevation # to be zero, approximate values for the total phases (0, and 2 can be obtained.The signal processing unit 76 may, for example, be in the form of a microprocessor which responds to the inputs on lines 78, 84 and 86 by calculating the corresponding total value for Xl and the corresponding total value for 02. Instead, for example, it could comprise a store in the form of a look-up table, which would store, for each expected combination of signals on these lines, the corresponding table values for ja and 02 In either form, however, it operates to combine the approximate total value for 01 with the fractional value for 9, received on line 66, so as to output, on a line 90, an accurate total value for #1.

Similarly, it combines the approximate total value for 02 with the fractional value for 02 received on line 74 so as to output, on a line 92, an accurate total value for 02. Thus, for example, if the approximate value for #, which it calculates for a given set of inputs on lines 82, 84 and 86 is 3.6, and the fractional value on line 66 and 0.55, this indicates that the true total value for ft on line 90 will be 3.55.

The true total value for 4, and 02 are fed into a ratio unit 94 which takes the ratio 0 2 and therefore produces an output equal to tan 8 (see Equation (3) above). This signal is converted into a value for 8 by a processing circuit 96 (for example, a store which holds, for each value of tan 8, the corresponding value of 8). In this way, the correct and ambiguity-free value for 8 is output on line 98 and may be indicated or displayed in any suitable way.

The unit 80 for processing the signals from the antennas a, B and c to produce the output representing the approximate value for 0 may be similar to the unit described above with reference to Figure 1. In such a case, the difference between the signals respectively received by antennas a and B would be fed into receiver 10 (Fig. 1), the difference between the signals respectively received by antennas B and c would be fed into receiver 1 2 and a signal representing the sum of the signals received by antennas a, B and c would be fed into receiver 14. The unit 80 may'use the receivers 40, 42 and 44 and the analogue-to-digital converters 46, 48 and 50, by means of time-sharing. However, this will of course increase the direction finding time.

if desired, the summing and differencing circuits 52, 54, 56 and 58 (Fig. 3) may be analogue circuits and therefore placed before, instead of after, the analogue-to-digital converters 46, 48 and 50.

An analogue display can be provided for the system of Figure 3 if desired, using a cathode ray tube in a manner analogous to that shown in Figure 1, by using digital-to-analogue converters one switched in turn between the outputs of adder 52 and the phase-shifting circuit 62 and the other switched in turn between the outputs of adder 56 and the phase-shifting circuit 70.

The systems described are advantageous because they are very fast in operation and use digital techniques to overcome the severe practical difficulty in obtaining accurate gain and phase matching, over wide temperature vibration ranges, at the "back-end" of receivers using analogue l.F. amplifiers, particularly concerning the crystal filters and a.g.c. characteristics. The systems described can therefore obtain a d.f.

reading from a very short transmission.

Claims (1)

1. Claims

   1. A direction finding system for assessing the bearing of a received radio signal, comprising first and second antenna means having respective direction-sensitive patterns arranged orthogonally to each other and for respectively producing first and second electrical analogue signals respectively dependent on the magnitude of the received radio signal and its sense with respect to their response patterns, means for producing a third signal dependent on the magnitude of the received radio signal and independent of its sense, means operative to convert the analogue signals into first, second and third digital signals respectively, means operative to measure the ratio of each of the first and second digital signals to the third digital signal so as to produce respective outputs respectively dependent on the sine and cosine of the bearing angle of the received signal, and output means responsive to the said outputs to assess the bearing angle.

   2. A system according to claim 1, in which the output means includes means for measuring tHe ratio of the said outputs and to derive therefrom a bearing signal having a value dependent on the; bearing angle.

   3. A system according to claim 1 or 2, including analogue display means in the form of a cathode ray tube to the opposite pairs of the plates of which are respectively applied first and second re-converted analogue signals which are produced by re-converting the first and second digital signals into analogue form.

   4. A direction finding system for assessing the bearing of a received radio signal, comprising first antenna means defining a pair of first measuring points separated along a line by at least half the wavelength at the frequency of the radio signal, second antenna means defining a pair of second measuring points separated along an orthogonal line by at least half the said wavelength, first input means, including analogue-to-digital converting means, responsive to the sum of, and the difference between, the radio signal as respectively received at the said first two measuring points to produce a first output signal dependent on the amount by which the phase difference between the radio signal as received at each of the said first two measuring points exceeds an integral number of cycles at the frequency of the received radio signal, second input means, including analogue-to-digital converting means, responsive to the sum of, and the difference between, the radio signal as respectively received at the said two second measuring points to produce a second output dependent on the amount by which the phase difference between the radio signal as received at each of the said two second measuring points exceeds an integral number of cycles at the frequency of the received radio signal, modifying means responsive to the frequency of the radio signal and operative in dependence thereon and on the said separation distance and on a signal approximating to the bearing to be assessed, to determine the approximate value for each said phase difference whereby to modify if necessary the integer value in each said output signal, means connected to measure the ratio of the modified output signals whereby to determine the said bearing. and means responsive to the approximating signal for resolving any ambiguity in the said bearing.

   5. A system according to claim 4. in which each input means comprises a summing circuit connected to sum the phases of the radio signal as received at each of the respective two measuring points whereby to produce a cosine signal dependent on the cosine of half the said phase difference, a differencing circuit connected to measure the difference between the phases of the radio signal as received at each of the two measuring points whereby to produce a sine signal dependent on the sine of half the phase difference, means for quadrature phase-shifting one of the sine and cosine signals with respect to the other. means for taking the ratio between the cosine and sine signals whereby to produce an output dependent on the tangent of the phase difference. and means responsive to the said tangent signal to produce the respective said output signal.

   6. A system according to claim 5, in which the sum and differencing circuits are either preceded by. or followed by, respective analogue-to-digital converting circuits.

   7. A system according to any one of claims 4 to 6 in which one of the measuring points of the first antenna means constitutes one of the measuring points of the second antenna means.

   8. A system according to any one of claims 4 to 7. in which the means for producing the said approximating signal comprises first and second antanna means having respective directionsensitive patterns arranged orthogonally to each other and having apertures of less than a quarter of the said wavelength and for respectively producing first and second electrical analogue signals respectively dependent on the magnitude of the received radio signal and its sense with respect to their response patterns, means for producing a third signal dependent on the magnitude of the received radio signal and independent of its sense, means operative to convert the analogue signals into first, second and third digital signals respectively, means operative to measure the ratio of each of the first and second digital signals to the third signal so as to produce respective outputs respectively approximately dependent on the sine and cosine of the bearing angle of the received signal, and output means responsive to the said outputs to produce the said approximating signal.

   9. A direction finding system, substantially as described with reference to Figure 1 of the accompanying drawings.

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