FR2542874A1 - Determining incident direction of EM waves, esp. microwaves - Google Patents

Abstract

The incidence angle is determined from the extreme values of signals derived from the received signals of a rotatable aerial arrangement. Two identical aerials are mounted parallel to and adjacent to each other with an intervening space. The aerials are rotated together about an axis perpendicular to the aerial direction. One of the aerial's signals is delayed by 90 degrees in phase and at least one signal is time switched by 180 degrees. The resulting signals are combined by vector summation and the result of that demodulated synchronously with the switching clock modulation signal to produce a direction dependent indication voltage.

Description

 The invention relates to a method for determining the direction of incidence of electromagnetic waves, in particular microwaves, using an antenna device which can pivot by definable angles, the reception signals of the latter. making it possible to obtain, by derivation, indicator signals whose extreme values are associated with a determined angle of rotation of the antenna device corresponding to the direction of incidence sought. The invention further relates to a device for implementing this method.

 Processes of this type are part of direction finding procedures.

The systems used in these processes can be divided into two categories:
- Adcock direction finders, Doppler effect, crossed frame, etc., used for omnidirectional direction finding procedures; and
- pivoting sector direction-finders, such as scanning, single-pulse and interferometer direction finders.

 Omnidirectional direction finding methods have the advantage of using a fixed antenna system, but they are sensitive to parasitic waves falling into their environment through reflectors.

The resulting multi-channel reception can cause significant detection errors.

 To eliminate such interference to a large extent, it is possible to use the pivoting sector direction finders provided with orientable narrow beam antennas, mentioned above, and thus determine the direction of incidence from which the information d the servo for the antenna system is deducted.

 A scanning direction finder belonging to this category works with an antenna lobe which can rotate mechanically or electrically periodically, and the differences in reception amplitude appearing in the pivot sector are used to determine the orientation error.

 In the case of mono-pulse direction finders, the antenna system has antenna lobes slightly angularly offset, and the orientation error is determined by forming differential signals and signals simultaneously for the antenna signals. summons. For their operation, the amplitudes and phases of these differential and summation signals are compared. This process is very cumbersome to implement, but it is very suitable for pulse radar systems, for which the direction of incidence of different echoes must be determined.

 In the case of interferometry direction finders, the phase difference or propagation time difference is determined for an incident wavefront by referring to two spaced apart antennas, for example by means of a high phase difference measurement frequency or a measurement of the difference in propagation time in the range of nano-seconds, from which the direction of incidence can be deduced. This process allows very high measurement precision, but it is normally technically very heavy.

 The direction-finding methods making it possible to determine the direction of incidence of electromagnetic fields in an extremely precise manner are of the greatest importance essentially for measurement needs. In the case of geodetic surveys at sea, for example, it is thus necessary to determine with great precision the direction relative to a target.

Similar requirements also apply to geophysical exploration work at sea, in which, for example, one must permanently determine with great precision the bearing angle of a trailing body in relation to the towing vessel.

 The invention therefore aims to develop a method which can be implemented with the necessary high precision, while being simplified compared to known methods.

 This object is achieved according to the invention by a method of the type specified in the preamble which is characterized in that two identical antennas fixed parallel to each other with a certain spacing, can rotate together around an axis so that the axis is substantially perpendicular to the orientation of the antennas and in that, among the signals picked up by the antennas during their common rotation, the signal from one of the antennas is delayed in phase by 900 and that at least one of the antenna signals is shifted by 1800 according to the rhythm of a modulation signal, the two signals then being added by vector summation, while the resulting signal is demodulated in synchronism with the rhythm of the modulation signal, the voltage thus obtained depending on the orientation being used as an indicator signal. In an advantageous embodiment of this process, the indicator signal is sent to the drive system of the antenna system to control it, the value e xtreme used then being a minimum value.

 According to another characteristic of the invention, the two signals are respectively offset by 1800 according to the half-frequency of the same modulation signal.

 Preferably, the modulation signal is in the range of sound frequencies.

 In addition, the method according to the invention may include an amplitude demodulation of the signal obtained by addition.

 Furthermore, a lower intermediate frequency can be formed in the channel leaving each antenna from the received high frequency signals, this lower intermediate frequency then being processed to generate the indicator signal.

 A device for implementing this method, comprising an antenna device which can pivot by means of a controlled drive member and comprising switching elements which form, from the received signals, an indicator signal having an extreme value in the direction sought, is characterized in that the antenna device comprises two identical antennas held parallel to each other with a certain distance, in that the output of one of the antennas is connected to a phase delay element of 900 and, on the two output lines leaving the phase delay element and the other antenna respectively, at least one is connected to a phase shifter of 1800 whose output, like the another output line, leads to an adder element to which is connected an AM receiver associated with a synchronous demodulator, and in that a clock signal generator is connected to at least one of the phase shifters and to the demodulator s synchronous to control them synchronously, the output of this synchronous demodulator being connected to a display device for the indicator signal.

 Advantageously, the output of the synchronous demodulator is connected to the control terminal of the antenna drive member.

 Preferably, the clock signal generator is a code generator which delivers a rectangular modulation signal.

 According to a particular embodiment, each antenna comprises a receiving cavity comprising a detecting diode, and the receiving cavities are arranged symmetrically with respect to a common heterodyne oscillator and receive a coherent oscillator signal by means of coupling elements arranged symmetrically.

 In this case, between each receiving cavity and the corresponding 180-phase dephaser, a buffer amplifier designed for an adjustable intermediate frequency is mounted for the adaptation of the phase-shifters.

 Preferably, the phase delay element of 900 is located between the buffer amplifier and the phase shifter of 1800 of the branch associated with one of the antennas and is designed for the intermediate frequency.

 In an advantageous embodiment, the device which is the subject of the invention comprises microwave antennas connected to waveguides, the frequency of the heterodyne oscillator being coupled to two mixing circuits by means of coupling loops directional, and one of the waveguides having a greater length than the other waveguide to generate the phase delay of 900.

 In this case, a microwave antenna serving as a transmitting antenna is connected by a waveguide to the heterodyne oscillator.

 The method which is the subject of the invention must be classified between the methods of scanning direction-finding and of direction-finding with interferometer. It is distinguished by a high precision which can be obtained by simple technical means. The use of the method for electromagnetic waves in the field of ultra-short waves and microwaves is particularly advantageous.

Other advantages and characteristics of the invention appear from the claims as well as from the description and the appended drawings, in which are commented and illustrated, only by way of nonlimiting examples, preferred embodiments of the invention. In these drawings:
- Figure 1 is a simplified block diagram of a basic embodiment of the device for implementing the method according to the invention;
- Figure 2 shows the signal forms used by the method:
- Figures 3 and 4 are partial block diagrams of practical embodiments of the invention;
- Figure 5 is a view turned 90 of the antenna system used in the device of Figure 4; and
- Figure 6 is a simplified view of a calibration device.

A device suitable for implementing the method, according to the figure
1, has two antennas 1, 2, which are fixed parallel to each other with a certain spacing on an axis of rotation lii. The spacing of the antennas 1, 2 perpendicular to each direction of detection, indicated in the figure by two parallel arrows to the left of the antennas, is preferably about 1.6 wavelength. The larger the base, that is to say the spacing of the antennas, the higher the resolution, the result being however affected by a corresponding ambiguity when the difference between the rays represents half a wavelength or a multiple of this value.

In the case of the spacing indicated, a wave difference of 1800 corresponds to a mechanical pivoting of the antennas of 180. FIG. 6 shows another possibility for placing two antennas 38, 39 parallel to one another. such that they can be oriented together by the rotation of the axis 14 by means of a servo-motor 12.

 The antennas 1, 2 receive signals from a transmitter or a reflector, among which the signal received by the antenna 1 is delayed in phase by a quarter wavelength, or 900, by means of a phase shifter or delay element 3. In the simplest case, the delay element is a quarter wave offset line or a broadband hybrid-900 component. In addition, two controllable phase shifters of 1800 4, 5 are disposed respectively in each of the two antenna lines. These switching elements can be simple ring mixers. Two-phase modulators can also be advantageously used, which cause a phase shift of + 900 as a function of a control signal.

 The output lines of the phase shifters 4.5 lead to an adder element 6, in which the signals from the phase shifters are added. The summation signal of the adder element 6 is sent to a receiver system 7 with amplitude demodulator mounted at its output. A known AM receiver, commonly used for radio or radiotelephone reception, can be used as the receiving system. The LF output of the receiver system demodulator 7 is connected to a synchronous demodulator 8, for which a four-quadrant multiplier is used which will be simply designated below by the term "multiplier". Instead of a four-quadrant multiplier, a simple FET switching matrix can also be used.

 Control signals for the multiplier 8 and for the two phasers 4, 5 are generated in a code generator 9 and are sent to these switching elements by the lines shown in the figure. The clock frequency of these control signals is in the range of sound frequencies, preferably in a range around about 1 kHz.

 The control signals are shown diagrammatically in FIG. 2. The sign for the multiplication is constantly inverted, with the clock signal coming from 9. The two two-phase modulators 4, 5 of this example are then phase shifted in the opposite direction from one pulse each time with the half clock frequency. It follows that the two modulators are in phase if the sign intended for the multiplier is positive, namely that they are both switched to 0 or 1800. If the sign is negative, they are in phase opposition , that is to say that the phase shift of one modulator is 0 while that of the other is 1800.

 The output signals of the two modulators 4 and 5 are added vectorially in the adder element 6. For the case where the input signals of the two modulators 4, 5 have the same phase and the same amplitude, and the two modulators are identical, that is to say both switched to 0 or 1800, there is at the output of the adder element 6 double the input voltage. If the two modulators are different, that is to say if one is switched to 0 and the other to 1800, the voltages at the output of the adder element are canceled due to the phase opposition. When one of the input voltages of the modulators 4, 5 is phase shifted by 900 with respect to the other, after summing vector in the adder element 6, a summing voltage equal to 1.4 times the input voltage is obtained . The absolute value of this summation voltage is independent of the reciprocal switching state of the modulators 4, 5, since the sign of the phase shift of 900 is not taken into account. This is what happens when the two antennas receive signals of the same phase, the input signal of the modulator 4 being phase shifted by 900 by the delay element 3 relative to the input signal of the modulator 5.

 In FIG. 2, A shows the sign for the multiplication and the resulting phases for the two modulators 4 and 5, are respectively represented by B and C. The diagrams D, E, and F relate to the output voltage of the receiver 7 , and more precisely, according to the combination of B and C, the output voltage relates to the input voltages of the two-phase modulator which are in phase for D, phase-shifted by 900 for E and in phase opposition for F.

 When the antennas 1, 2 are precisely oriented on the detected transmitter, there are at the inputs of the modulators 4, 5 signal voltages phase shifted by 900, so that at the output of the multiplier 8 there is no DC voltage . When the orientation of the antennas 1, 2 undergoes an angular variation, the phase shift deviates from 900. At the output of the multiplier 8, there then appears a direct voltage whose sign is a function of the orientation while its absolute value is proportional to the angular variation. This DC voltage is used as the control voltage to orient the antennas 1, 2 precisely. This control voltage is sent to the servo-motor 12, which orients the antennas 1, 2 according to the set value, via an analog or digital filter 10, which provides a stable regulation characteristic (PID behavior), and a servo amplifier 11. An angular encoder 13 connected to the servo motor 12 converts the angle of rotation into an electrical quantity, which can be registered for further processing and exploitation.

 The device can be built very compactly when working in the microwave field. In the case of use at very high frequencies, of the order of 9 GHz and more, two-phase modulators are however very expensive components.

 A device, despite everything, very simple for implementing the method in the case of microwaves, is shown in FIG. 3. In this case, the antennas are receiving cavities 15, 16, which are arranged in mutual symmetry with respect to an emitting cavity 17 comprising a Gunn-effect diode, and which are equipped with Schottky-effect detector diodes. The device 15, 16, 17 corresponds to a radar module supplemented by an additional detection cavity, of a type which is used in motion detectors, for example for door openers and for security systems. alarm, and which is in the eommerce under the name of radar modules with Doppler effect. The two receiving cavities 15, 16 provided with detector diodes for microwaves are arranged symmetrically with respect to the emitting cavity 17 which comprises the diode with Gunn effect. To achieve these, for example, component 16, 17 is separated from a module consisting of components 15 and 17 or components 16 and 17, the remaining component comprising the Schottky diode being applied to a complete radar module with a partition arranged accordingly. -The two receiver circuits receive a coherent oscillator voltage. The outputs of the receiving cavities 15, 16 are respectively connected to a buffer amplifier 18, 19. These buffer amplifiers 18, 19 are designed for a low intermediate frequency, which can for example be included in the range from 10 to 150 MHz approximately, and they allow optimal adaptation of the two-phase modulators 21, 22 which follow them. The 900 20 phase shifter is also designed for the intermediate frequency, and it is located between the buffer amplifier 18 and the modulator 21. The outputs of the modulators 21, 22 , which are controlled in a similar manner to that of the example in FIG. 1, are connected to an adder element 23, the other components of the device also corresponding to those of the example in FIG. 1. The receiver is designed for the intermediate frequency, for which an appropriate favorable value can be chosen. The cavities 15, 16 having a relatively large opening angle. The device can however be manufactured very economically and works with good precision over a range of about 5 km.

 When the simultaneous requirements of precision, range and low sensitivity to parasitic reflections are more severe, it is advisable to use a microwave system according to Figures 4 and 5, in which the antennas used are for example horn antennas 24, 25 connected to waveguides 24a and 25a. A transmitter oscillator 26a is connected to a waveguide 34a, comprising two symmetrical directional coupling loops 26 which ensure coherent coupling of the beat frequency generated in the transmitter oscillator on the mixing diodes 24b and 25b. Another horn antenna 34 can also be connected to the waveguide 34a of the transmitting oscillator. But normally, the waveguide 34a is closed without reflection at 33.

 The phase delay of 900 necessary for the process is in this case obtained by a defined difference in length. The waveguide 25a is longer than the waveguide 24a by a section 29. This section 29 has a length equal to a non-integer multiple of a quarter wavelength of the waveguide at the frequency of microwave work.

 As in the example in FIG. 3, intermediate frequency buffer amplifiers 27, 28 are connected to the waveguides 24a and 25a, and, by their output, are connected to two-phase modulators 30, 31.

 With the device described, a phase difference of 90 or 2700 is obtained between the signals received. The operating system is the same as that described for FIG. 1. To generate and maintain constant the intermediate frequency with the necessary precision, it is advantageous to generate, via the receiver which is connected following the 'adder element 32 as in the example of Figure 1, an additional regulation voltage for setting the microwave oscillator 26a,. A technique is then used which is generally usual and known for ultra-short wave radio and television receivers.

 The horn antenna 34, visible in FIG. 4, and shown turned by 900 in FIG. 5, can be connected, instead of being closed by the resistor 33, can be connected to the waveguide 34a, to radiate the signal from the heterodyne oscillator advantageously to the corresponding station. This provides a simple solution for transmitting data or for carrying out an additional bidirectional range measurement.

 The accuracy of the angular detection is determined by the stability of the phase shift of all the high frequency components from the antennas to the adder element. The most critical point for this is the realization of the 900 phase shifter, which is only mounted in one of the two high frequency circuits, since its temperature and frequency response is not compensated by a symmetrical behavior in the two circuits. It is therefore advisable to carry out occasional checks and recalibrations of the measurement system, in order to always carry out angular measurements with the greatest precision.

 An advantageous device for verifying the measurement system is shown in FIG. 6. On the pivoting axis 14 of the antennas 38, 39, in this example VHF antennas having a range of approximately 5 to 10 km and preferably provided for ranges of about 3 km, a directional telescope 35 is placed in such a way that it can be oriented by a micrometric screw 36 around the axis 14. A graduated scale 37, placed on the circular support of the telescope 35 , allows to read in fractions of degrees of angle, the angle of rotation of the telescope 35 around the axis 14 relative to an origin point. The graduated scale 37 is calibrated so that its zero point corresponds to the calibrated adjustment angle of the antenna with respect to the angular encoder 13. When the corresponding station is visible optically, it is thus possible to determine the deviation between the optical axis and the electrical axis of the measurement system. To do this, the reticle of the telescope 35 is placed in coincidence with the objective, by means of the micrometric screw 36, after the automatic engagement of the electronic servo-system. The angular error to be corrected can then be read on the graduated scale 37 and be entered in the form of a correction value.

 As a variant, the axis of the antennas can also be corrected manually relative to the axis of rotation. In addition, electrical correction is possible, thanks to an additional adjustable phase shifter located in one of the two antenna circuits. The telescope is also used to calibrate the device on a fixed installation, by determining, for example optically, the angles formed with respect to geodetic reference marks.

Claims (14)

 1. Method for determining the direction of incidence of electromagnetic waves, in particular microwaves, using an antenna device which can pivot on definable angles, the reception signals of the latter making it possible to obtain by derivation of the indicator signals whose extreme values are associated with a determined angle-of-rotation of the antenna device corresponding to the desired direction of incidence, characterized in that two identical antennas fixed parallel to each other with a certain spacing, can rotate together around an axis so that the axis is substantially perpendicular to the orientation of the antennas and in that, among the signals picked up by the antennas during their common rotation, the signal of the one of the antennas is delayed in phase by 900 and at least one of the antenna signals is shifted by 1800 according to the rhythm of a modulation signal, the two signals then being summed by vector summation while the resulting signal is demodulated in synchronism with the rhythm of the modulation signal, the voltage thus obtained as a function of the orientation being used as an indicator signal.  2. Method according to claim 1, characterized in that the indicator signal is sent to the drive system of the antenna system to control it, the extreme value used then being a minimum value.  3. Method according to claim 1 or 2, characterized in that the two signals are respectively offset by 1800 according to the half-frequency of the same modulation signal.  4. Method according to any one of the preceding claims, characterized in that the modulation signal lies in the range of sound frequencies.  5. Method according to any one of the preceding claims, characterized in that it comprises an amplitude demodulation of the signal obtained by addition.  6. Method according to any one of the preceding claims, characterized in that a lower intermediate frequency is formed in the channel leaving each antenna from the high frequency signals received, this lower intermediate frequency then being processed to generate the indicator signal.  7. Device for determining the direction of incidence of electromagnetic waves, in particular microwaves, comprising an antenna device which can pivot thanks to a controlled drive member and comprising switching elements which form, at from the received signals, an indicator signal having an extreme value in the desired direction, characterized in that the antenna device comprises two identical antennas (1,2; 15,16; 24,25) held parallel to one the other with a certain spacing, in that the output of one of the antennas (1; 15, 25) is connected to a phase delay element of 900 (3; 20; 29) and, on the two lines of output starting respectively from the phase delay element and the other antenna, at least one is connected to a phase shifter of 1800 (4; 2t; 30) whose output, like the other output line, leads to an adder element (6; 23; 32) to which is connected an AM receiver (7) associated with a synchronous demodulator, and in that a clock signal generator (9) is connected to at least one of the phase shifters and to the synchronous demodulator to control them synchronously, the output of this synchronous demodulator being connected to a display device for the indicator signal.  8. Device according to claim 7, characterized in that the output of the synchronous demodulator (8) is connected to the control terminal of the antenna drive unit (12).  9. Device according to claim 7 or 8, characterized in that the clock signal generator (9) is a code generator which delivers a rectangular modulation signal.  10. Device according to any one of claims 7 to 9, characterized in that each antenna comprises a receiving cavity (15,16) comprising a detector diode, and the receiving cavities are arranged symmetrically with respect to a common heterodyne oscillator (17 ) and receive a coherent oscillator signal via symmetrically arranged coupling elements.  11. Device according to claim 10, characterized in that between each receiving cavity (15,16) and the corresponding phase shifter of 1800 (21,22) a buffer amplifier (18, 19) designed for an adjustable intermediate frequency is mounted for adaptation of phase shifters.  12. Device according to claim 1f, characterized in that the phase delay element of -900 is located between the buffer amplifier (18) and the phase shifter of 1800 (21) of the branch associated with one of the antennas ( 15) and is designed for the intermediate frequency.  13. Device according to one of the preceding claims, characterized in that it comprises antennas for microwaves (24, 25) connected to waveguides (24a, 25a), the frequency of the heterodyne oscillator ( 26a) being coupled to two mixing receiver circuits by means of symmetrical directional coupling loops, and one of the waveguides having a length greater than the other waveguide to generate the phase delay of 900 .  14. Device according to claim 13, characterized in that a microwave antenna (34) serving as a transmitting antenna is connected by a waveguide (34a) to the heterodyne oscillator (26a).