Abstract:
Radiodirection finding method and apparatus using Doppler effect frequency modulation for measuring the bearing of a source transmitting a high frequency signal, wherein either on the reception side or on the transmission side, several antenna strands are used in pairs diametrically spaced from each other and spaced evenly apart angularly about a circumference. A first pair of diametrically spaced antenna strands are energized and then the next pair of diametrically spaced antenna strands in turn are selectively energized so as to create, at all times, a mobile fictitious antenna moving diametrically and alternately between the two diametrically opposite antenna strands of the particular pair of antenna strands considered. Different pairs of diametrically opposite antenna strands are successively energized in an angularly rotating sequence from the first pair of antenna strands.

Description:
This application is a continuation of application Ser. No. 07/019,232 now abandoned, filed Feb. 26, 1987 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a radiodirection finding method and apparatus using Doppler effect frequency modulation. 
     Different systems of aid to air or sea navigation are already known using Doppler effect frequency modulation. Among the known systems using such modulation may be mentioned the Doppler VOR system operating for transmission and the Rohde and Schwarz direction finder at reception. These systems are based on the switching of a high number of antenna strands (39 antenna strands for the Doppler VOR system and 16 antenna strands for the Rohde and Schwarz system) which are placed about a circumference, at equal distances from each other. These antenna strands are energized in a rotating sequence, in other words one after the other about the circumference, each of the strands being energized for a predetermind lapse of time, which corresponds to the rotation of a fictitious antenna about the center of the circumference. The bearing of a source transmitting a high frequency signal is obtained by measuring the phase shift between an antenna rotation reference signal and the signal obtained from the fictitious rotating antenna, after demodulation by a frequency modulation receiver. 
     The accuracy of such a direction finding system depends on the number of antenna strands, on the form of the switching signals, on the reproducibility of the circuits associated with eacch antenna strand (switching, amplification, dimensional characteristics) and on the phase stability of the receiver or on a fixed phase shift which may be compensated for. 
     To overcome the difficulties related to the accuracy required of the devices switching the antenna strands and their controls, the simplest solution consists in increasing the number of antenna strands. In fact, the accuracy required for the shape of control signals and the switching devices is all the more reliable the higher the number of antenna strands. However, this results in a bulky apparatus, and yet does not solve the problem of the phase shift of the receiver. 
     SUMMARY OF THE INVENTION 
     The present invention aims at overcoming these drawbacks by providing an apparatus of particularly simple design, compact and inexpensive but still capable of delivering a high performance level. 
     For this, the direction finding method using Doppler effect frequency modulation for measuring the bearing of a source transmitting a high frequency signal, in which, on the reception side or on the transmission side, several antenna strands are used spaced evenly apart about a circumference and in which these antenna strands are energized in turn so as to create a fictitious mobile antenna, is characterized in that the antenna strands are energized selectively in pairs of diametrically opposite strands so as to create, at all times, a mobile fictitious antenna moving diametrically and alternately between the two opposite antenna strands of the pair of antenna strands considered, and the different pairs of diametrically opposite antenna strands are energized successively in a rotating sequence. 
     The invention also provides a direction finding apparatus using Doppler effect frequency modulation for measuring the bearing of a source transmitting a high frequency signal, including an even number 2n, at least equal to four, of antenna strands spaced apart evenly about a circumference, a circuit for switching the antenna strands connected to the different antenna strands, a two output switching control circuit which is connected to the switching circuit of the antenna strands so as to selectively energize these strands in turn, a frequency modulation receiver connected to the switching circuit of the antenna strands and to which the frequency modulated signal is applied by switching the antenna strands, means for producing an antenna rotation reference signal , and a module for processing the signal received for delivering, from the phase shift between the signal received and the antenna rotation reference signal, an indication of the bearing of the source emitting the high frequency signal, characterized in that it includes means for producing a sub-carrier frequency signal and for applying this signal, through the switching circuit of the antenna strands, successively to each pair of opposite antenna strands in a rotating sequence, and means for ensuring synchronous demodulation of the sub-carrier at the output of the frequency modulation receiver. 
     Thus, in the method and apparatus of the invention, instead of switching the antenna strands with a rotating frequency, these strands are switched diametrically by a sub-carrier frequency. The result is that a single proportion switching device is used and that the problem related to the dispersions of the characteristics of the different components is thus eliminated. Furthermore, the apparatus is insensitive to all disymmetries of the amplification and proportional switching device. In fact, if the two branches of the amplification and mixing circuit do not have the same phase shift, this is compensated for automatically by averaging over a revolution, since the inputs are permuted at each half revolution. 
     The invention allows a small size radiodirection finding antenna to be constructed which is a requirement for correct operation. In fact, so that the amplitude modulation caused by the switching of the diametrically opposite antenna strands is low (less then 10%), the diameter of the antenna must not be greater than a tenth of the wave length. 
     With the apparatus of the invention, an accuracy better than + or -1° can be obtained on a correct master plan. The accuracy obtained is independent of the phase shift of the receiver. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     One embodiment of the present invention will be described hereafter, by way of non limitative example, with reference to the accompanying drawings in which: 
     FIG. 1 is a block diagram of a direction finding apparattus of the invention, 
     FIG. 2 is a diagram of the switching control circuit for the antenna strands, 
     FIG. 3 is a diagram illustrating the phase variation of a signal picked up as a function of the position of the mobile fictitious antenna, 
     FIG. 4 is a diagram illustrating the switching control signals for the antenna strands and the antenna rotation reference signal, 
     FIGS. 5 and 6 are diagrams illustrating the variation of the frequency of the signal received as a function of the mobile fictitious antenna speed, 
     FIG. 7 is a block diagram of the signal processing module, and 
     FIG. 8 is a diagram illustrating different forms of signals appearing in the processing module. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The direction finding apparatus of the invention, the general diagram of which is shown in FIG. 1, functions for reception and it is intended to be embarked on board an aircraft for detecting the bearing of a source on the ground S transmitting a high frequency signal, whatever the band used. This apparatus uses an even number 2n of antenna strands at least equal to four, these antenna strands being spaced apart evenly about a circumference. In the non limitative embodiment shown in the drawings, the apparatus uses eight antenna strands disposed diametrically opposite in pairs and spaced apart evenly about the circumference, namely the antenna strands A, B, C, D, E, F, G, and H. These eight antenna strands are connected respectively to a circuit 1 for switching the antenna strands which is itself driven by a control module 3. This control module includes, among other circuits, a switching control circuit 3a which is connected to the switching circuit of the antenna strands 1. The control module 3 also includes a clock 4 which delivers a periodic signal applied to a divider stage 5 whose output produces a sub-carrier of frequency FSP, which is applied to the antenna strands switching circuit 1. 
     The control module 3 also includes a signal generating stage 6 which is connected to the output of divider 5. This stage 6 produces binary signals of weights, 1, 2, 4 (FR, 2FR, 4FR) which are applied to the switching control circuti 3a for causing the energization sequence of the antenna strands as will be seen hereafter. The signal FR is the antenna rotation frequency signal. These signals FR, 2FR, 4FR are also applied to a processing module 7 which also receives the sub-carrier FSP and whose output is connected to a bearing indicator 8. 
     Circuit 1 for switching the antenna strands delivers at its output a frequency modulated signal which is applied to a phase or frequency modulation receiver 2. This receiver produces at its output a low frequency amplitude modulated signal which is applied to the processing module 7, for calculating the bearing of source S transmitting the high frequency signal. 
     In the method of the invention, the antenna strands A-H are not switched one after the other in a rotating sequence, in a conventional way, but on the contrary they are energized by pairs of diametrically opposite antenna strands. In other words, in a first time interval defined by a square wave signal ae (FIG. 4), the first pair of diametrically opposite strands A-E is energized, then during the next square wave signal bf, the second pair BF is energized and so on with a square wave signal cg for the pair C-G and a square wave signal dh for the pair D-H. After a half revolution, the two antenna strands of the first pair A-E are again enerigized but with a phase shift of 180° of the sub-carrier FSP, under the control of a square wave signal ea as can be seen in the diagram of FIG. 4. The square wave signal ea is followed by similar signals fb, gc and hd for the pairs of antenna F-B, G-C and H-B. There can also be seen in this diagram the signal FR which is a logic signal whose period T corresponds to a complete revolution or to the complete switching of the antenna strands, i.e. to the eight successive square wave signals ae, bf, cg, dh, ea, fb, gc and hd. 
     FIG. 3 illustrates the basic principle used in the method and apparatus of the invention. It will be assumed in this case that the pair of antenna strands A-E is energized and that the source S transmitting the high frequency signal is aligned with the direction AE. The alternate energization of the antenna strands A-E results in the creation of a fictitious antenna X reciprocating along the diameter AE. The vectorial diagram of FIG. 3 shows the variation of the phase φ of the signal received as a function of the position of the fictitious antenna X. This phase varies from a value -φ max when the fictitious antenna X is at position of the antenna strand E and a value +φ max when the mobile fictitious antenna X is at the position of the antenna strand A, while passing through a zero value when the fictitious antenna X is at the center O. 
     The diagram of FIG. 5 illustrates the variation of the speed of movement v of the fictitious antenna X, which is zero at the position of the antenna strands A and E and maximum at the center O, and that of the frequency deviation Δf of the received signal. These variations, generated by the sub-carrier frequency, may be sinusoidal as is ahown in FIG. 5, or else be in the form of periodic square signals as is shown in FIG. 6, or else having any other form, particularly triangular, compatible with the demodulation possibilities of the receiver. In all cases, the frequency deviation signal Δf is in phase with the speed signal v. 
     The pairs of antenna strands are switched as illustrated in FIG. 2. The switching control circuit 3a delivers respectively, at eight outputs, the eight antenna switching control signals ae, bf . . . hd. These antenna switching signals are applied, inside the antenna strand switching circuit 1, to pairs of normally open swwitches 9, 10, of any known type, for example PIN diodes, and which are respectively connected to the opposite antenna strands of the same pair. For example, all the left hand switches 9 in FIG. 2 are connected respectively to the antenna strands in the order A, B . . . H whereas all the switches 10 situated on the right side of FIG. 2 are connected respectively to the antenna strands in the order E, F, G, H, A, B, C, D. Thus, the first square wave signal ae controls the closing of the two switched 9, 10 connected respectively to the two antenna strands A and E, and so on for the following square wave signals bf. . . hd. All the left hand switches 9 are connected in common to an input of an amplifier 11 whereas all the right hand switches 10 are connected in common to an input of an amplifier 12, these two amplifiers 11, 12 forming part of a stage providing diametrical switching of the antenna strands by the sub-carrier frequency signal FSP. For the purposes of illustration the outputs of the two amplifiers 11 and 12 are shown as being connected to the two ends of a potentiometer 13 whose slider 13a is moved alternately between the two ends of this potentiometer 13, at the frequency of the sub-carrier FSP. However, it is obvious that in practice switching by the sub-carrier FSP may be provided by any electronic circuit known per se (for example PIN diodes). The slider 13a is connected to an amplifier 14 whose output is connected to a terminal 15 where the modulated frequency signal s(t) appears and which is a combination of the two signals received respectively by the two antenna strands energized at a given time, for example strands A and E. If k is a coefficient defining the instantaneous position of the slider 13a, we have s(t)=ks A(t)+(1-k) sE(t), sA (t) and sE(t) being the two signals respectively received by the two antenna strands A and E as a function of the time t. 
     The modulated frequency signal s(t) is applied to the frequency modulation receiver 2 which delivers at its output a low frequency amplitude modulated signal s1. This signal is applied to the processing module 7 which includes (FIG. 7) an amplifier 16 to which the signal s1 is applied, then a band pass filter 17, then a variable gain amplifier 18 which delivers at its output an amplitude modulation signal s2 (FIG. 8). The signal s2 is shown as corresponding to a high frequency signal source aligned with the antenna strands C-G, that is situated in the direction GC, the antenna strands C being the closest to source S. Thus, when the first pair of antenna strands A-E is energized, there is no frequency modulation of the signal received because the direction AE is perpendicular to the direction GC along which source S is located. The amplitude of the first signal on each plateau during rotation is proportional to the absolute value of the cosine of the angle between the direction of the transmitter of the high frequency signal and the direction of the selected antenna strands. The signal is therefore maximum if the angle formed is zero and on the contrary this signal is zero if the selected antenna strands are situated along the direction perpendicular to the direction of the transmitter. The phase shift of the sub-carrier is zero if the transmitter is in the front sector and 180° if the transmitter is in the rear sector. 
     Signal s2 is applied to a synchronous filter 19, jointly with the sub-carrier frequency signal FSP and the signal FR, 2FR and 4FR. The synchronous filter 19 delivers at its output a staircase signal s3 representing the amplitude plateau of the signal received for the different pairs of antenna strands, this signal being applied to a low pass filter 21 which delivers at its output a sinusoidal signal s4 as can be seen in FIG. 8. This sinusoidal signal s4 is applied to a comparator 22 which produces at its output a measurement signal s5 formed by a square wave which is phase shifted by an angle θ with respect to the rotation reference signal FR. This measurement signal s5 is applied to a stage 23 which calculates, from the phase shift θ, the value of the bearing g with respect to the reference direction AE (in the present case g=90°). 
     The processing module also includes a loop controlling the gain of the amplifier 18. This loop includes an amplifier 24 receiving at its input a signal 8FR from the synchronous filter 19, and the output of which is connected to an input of a comparator 25 receiving, at a second input, a reference voltage Vref corresponding to the desired output voltage level for the amplifier 18. The output of comparator 25 is connected on the one hand to a control input of the variable gain amplifier 18, through a low pass filter 26, and on the other hand to an input of a detector 27 detecting the level of the control signal, which also receives at a second input a reference voltage Vref 1. The output of the level detector 27 is connected dirently to an indicator lamp 28 indicating validity of the bearing and, through an inverter 29, to an input of an AND gate having two inputs 31. The other input of this AND gate receives a signal z indicating the reception of the high frequency signal by the receiver. The output of this AND gate is connected to an indicator lamp 32 forming a vertical indicator. 
     Consequently, when the aircraft carrying the apparatus of the invention passes vertically over the source S of signal HF, this indication of the vertical pasage is shown by the indicator lamp 32 lighting up. In effect, the stage 27 of the level detector delivers a zero output signal, so that the bearing validity lamp 28 is extinguished. On the other hand, this signal is applied, after inversion by the inverter 29, to the first input of the AND gate 31 which receives at its other input the signal z indicating the presence of the signal HF emitted by source S. The gate 31 is then enabled and it causes the indicator lamp 32 to light up indicating the passage of the aircraft vertically over the source S. 
     The bearing validity indicator lamp 28 lights up as long as the aircraft is in the reception zone of the signal HF and it is extinguished as soon as the aircraft leaves the zone, which is signalled by the disappearance of the audio signal s1.