Abstract:
A phase correction apparatus comprising a storing means configured to store a phase correction value associated with each of a plurality of transmission antennas in which the phase correction value is calculated according to an electrical length of a signal path extending from a signal generator generating a transmission signal to the transmission antenna, and correction means configured to correct a phase of the transmission signal to be supplied from the signal generator to each transmission antenna according to the phase correction value for the transmission antenna stored in the storing means.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a phase correction apparatus, a DVOR (Doppler VHF omnidirectional radio range) apparatus, and a phase correction method for radio navigation. 
         [0003]    2. Description of the Related Art 
         [0004]    An example of a DVOR apparatus for providing aircraft with directional information is disclosed in U.S. Pat. No. 4,484,196. The DVOR apparatus of the related art arranges, as shown in  FIG. 5 , a central carrier antenna A and forty-eight sideband antennas B 1  to B 48  along a circle having a diameter of about 13.5 m (a radius R of about 6.7 m in  FIG. 5 ) around the carrier antenna A. 
         [0005]    The carrier antenna A radiates a reference phase signal in all directions. The reference phase signal is an AM wave formed by amplitude-modulating at 30 Hz a carrier of 108 to 118 MHz. The sideband antennas B 1  to B 48  arranged along the circle are sequentially activated at regular intervals of, for example, 30 times a second, so that the sideband antennas B 1  to B 48  successively emit subcarriers whose frequency is higher than that of the carrier by, for example, 9960 Hz. A distance between the sideband antenna emitting a subcarrier and an optional spatial point periodically changes with time, and therefore, the subcarriers received at the optional spatial point periodically change the frequency thereof due to Doppler effect, to form an FM wave of 30 Hz. The phase of this FM wave is dependent on an orientation with respect to a DVOR station where the DVOR apparatus is present. Namely, the sideband antennas B 1  to B 48  radiate the FM wave superposed by a variable phase signal. 
         [0006]    The reference phase signal and variable phase signal are adjusted so that their phases agree with each other on magnetic north, i.e., at zero degrees. An aircraft receives these two signals, detects a phase difference between the AM wave and the FM wave both modulated at the same frequency of 30 Hz, and finds a present orientation of the aircraft. 
         [0007]      FIG. 1  shows a sideband transmission system of the DVOR apparatus according to the related art. In  FIG. 1 , a sideband transmitter  101  generates a half-sine wave signal, supplies the signal to a distributor  3 , and controls the switching of the distributor  3  so that the half-sine wave signal is successively supplied to odd-numbered sideband antennas B 1 , B 3 , . . . , and B 47 . A sideband transmitter  102  generates a half-cosine wave signal, supplies the signal to the distributor  3 , and controls the switching of the distributor  3  so that the half-cosine wave signal is successively supplied to even-numbered sideband antennas B 2 , B 4 , . . . , and B 48 . The sideband antennas B 1  to B 48  are connected through antenna cables C 1  to C 48 , respectively, to the distributor  3 . 
         [0008]      FIG. 2  is a timing chart showing the timing of switching the sideband antennas. As indicated with waveforms (a) to (e) in  FIG. 2 , each of the 48 sideband antennas receives a signal for 1/720 seconds, i.e., the signals are supplied 30 times a second. 
         [0009]    Waveforms (e.g. (a), (c), (e)) provided by the odd-numbered sideband antennas must be continuous between two adjacent ones. 
         [0010]    Also, waveforms (e.g. (b), (d)) provided by the even-numbered sideband antennas must be continuous between two adjacent ones. If waveforms are discontinuous between adjacent odd- or even-numbered antennas, an aircraft is unable to correctly detect a phase difference between the AM wave from the carrier antenna A and the FM wave from the sideband antennas B 1  to B 48 , and therefore, is unable to find a correct orientation. 
         [0011]    To avoid the problem, the DVOR apparatus according to the related art precisely equalizes the lengths of the antenna cables C 1  to C 48  with one another, to align the phases of radio waves provided by the sideband antennas B 1  to B 48 . 
         [0012]    However, to align the phases of radio waves provided by the sideband antennas, the 48 antenna cables C 1  to C 48  of the DVOR apparatus must precisely be processed into identical electrical lengths. This process needs a long time and skill. Even with the identical electrical lengths, the related art is still vulnerable to phase shifts that may occur due to the aging of the antenna cables after installing the DVOR apparatus at a site. 
       SUMMARY OF THE INVENTION 
       [0013]    According to a first aspect of the present invention, a phase correction apparatus includes storing means and correction means. The storing means stores a phase correction value associated with each a of plurality of transmission antennas. The phase correction value is calculated according to an electrical length of a signal path extending from a signal generator generating a transmission signal to the transmission antenna. The correction means corrects a phase of the transmission signal supplied from the signal generator to each transmission antenna according to the phase correction value for the transmission antenna stored in the storing means. 
         [0014]    According to a second aspect of the present invention, provided is a DVOR apparatus having a carrier antenna radiating a carrier signal and a plurality of sideband antennas arranged along a circle around the carrier antenna and sequentially emitting a sideband signal. The DVOR apparatus includes a signal generator, storing means, correction means, switching means, and control means. The signal generator generates a sideband signal. The storing means stores a phase correction value associated with each of the sideband antennas. The phase correction value is calculated according to an electrical length of a signal path extending from the signal generator to the side band antenna. The correction means corrects a phase of the sideband signal supplied from the signal generator to each sideband antenna according to the phase correction value for the sideband antenna stored in the storing means. The switching means switches the sideband antennas from one to another so that the phase-corrected sideband signal is supplied to the sideband antenna. The control means controls the phase correction means, so that the correction means corrects the phase of the sideband signal in synchronization with the switching of the switching means. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a view showing a sideband transmission system of a DVOR apparatus according to a related art; 
           [0016]      FIG. 2  is a timing chart showing the timing of switching sideband antennas from one to another according to the related art; 
           [0017]      FIG. 3  is a view showing a sideband transmission system of a DVOR apparatus according to an embodiment; 
           [0018]      FIG. 4  is a view showing the timing of switching sideband antennas from one to another carried out by a distributor in the DVOR apparatus of  FIG. 3  and waveforms of half-sine wave signals at input ends of the distributor and each sideband antenna; 
           [0019]      FIG. 5  is a top view showing an arrangement of a carrier antenna and sideband antennas in a DVOR apparatus; 
           [0020]      FIG. 6  is a view showing a DVOR apparatus according to a second embodiment; 
           [0021]      FIG. 7  is a view showing a modification of a sideband transmitter of the first embodiment; and 
           [0022]      FIG. 8  is a view showing a modification of a sideband transmitter of the second embodiment. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0023]    Embodiments of the present invention will be explained with reference to the accompanying drawings. 
       First Embodiment 
       [0024]      FIG. 3  is a view showing a sideband transmission system of a DVOR apparatus according to an embodiment of the present invention. In  FIG. 3 , the same parts as those shown in  FIG. 1  are represented with the same reference numerals and the detailed explanations thereof are omitted. The DVOR apparatus according to the embodiment of  FIG. 3  arranges a carrier antenna A and forty-eight sideband antennas B 1  to B 48  in the same manner as that shown in  FIG. 5 .  FIG. 3  shows only the sideband transmission system of the DVOR apparatus, and a carrier transmission system of the DVOR apparatus is not shown therein. 
         [0025]    In  FIG. 3 , a sideband transmitter  1  has a signal generator (SG)  12 , a power amplifier (AMP)  13 , a phase corrector  4 , a phase correction controller  5 , and a phase correction table  6 . In particular, a half-sine wave signal as a sideband signal is generated by the signal generator  12 , amplified by the power amplifier  13 , and supplied to the phase corrector  4 . The switching of a distributor  3  is controlled by a switching control signal s 1  outputted from the signal generator  12  so that the amplified half-sine wave signal is sequentially supplied to odd-numbered sideband antennas B 1 , B 3 , . . . , and B 47 . The signal generator  12  outputs a synchronization signal s 2  to a phase correction controller  5  so that phase correction is carried out by the phase corrector  4  in synchronization with the switching of the distributor  3 . 
         [0026]    Based on an output from the phase correction controller  5  to be explained later, the phase corrector  4  corrects a phase of the half-sine wave signal generated by the signal generator  12  and amplified by the power amplifier  13  and outputs the phase-corrected half-sine wave signal to the distributor  3 . In response to a switching control signal from the signal generator  12 , the distributor  3  switches the sideband antennas from one to another so that the phase-corrected half-sine wave signal from the phase corrector  4  is supplied to a proper one of the sideband antennas. 
         [0027]    In response to a synchronization signal s 2  from the signal generator  12 , the phase correction controller  5  refers to a phase correction table  6  and provides the phase corrector  4  with a phase correction control signal together with a phase correction value retrieved from the phase correction table  6 . 
         [0028]    The phase correction table  6  stores a phase correction value for each of the odd-numbered sideband antennas B 1 , B 3 , . . . , and B 47 , the phase correction values being necessary to secure consecutiveness of radio waveforms emitted from these odd-numbered sideband antennas. The phase correction values are calculated by measuring electrical lengths of signal paths (antenna cables C 1 , C 3 , . . . , and C 47 ) from the signal generator  12  to the odd-numbered sideband antennas B 1 , B 3 , . . . , and B 47  with the use of, for example, a network analyzer and by finding differences among the measured electrical lengths. To cope with the aging of the antenna cables C 1 , C 3 , . . . , and C 47 , the phase correction values may be updated by periodically measuring the electrical lengths of the signal paths. 
         [0029]    The odd-numbered sideband antennas B 1 , B 3 , . . . , and B 47  are connected through the antenna cables C 1 , C 3 , . . . , and C 47 , respectively, to the distributor  3  and sequentially emit radio waves based on the half-sine wave signal generated by the signal generator  12  and amplified by the power amplifier  13 . 
         [0030]    In the DVOR apparatus of  FIG. 3 , a sideband transmitter  2  has a signal generator (SG)  15 , a power amplifier (AMP)  16 , a phase corrector  7 , a phase correction controller  8 , and a phase correction table  9 . A half-cosine wave signal is generated by the signal generator  15 , amplified by the power amplifier  16 , and supplied to the phase corrector  7 . The signal generator  15  controls the switching of the distributor  3  so that the half-cosine wave signal is sequentially supplied to even-numbered sideband antennas B 2 , B 4 , . . . , and B 48 . The signal generator  15  outputs a synchronization signal s 4  to a phase correction controller  8  so that phase correction is carried out by the phase corrector  7  in synchronization with the switching of the distributor  3 . 
         [0031]    The phase corrector  7  corrects a phase of the half-cosine wave signal generated by the signal generator  15  and amplified by the power amplifier  16  and sends the phase-corrected half-cosine wave signal to the distributor  3 . In response to a switching control signal s 3  from the signal generator  15 , the distributor  3  switches the sideband antennas from one to another so that the phase-corrected half-cosine wave signal from the phase corrector  7  is supplied to a proper one of the sideband antennas. 
         [0032]    In response to a synchronization signal s 4  from the signal generator  15 , the phase correction controller  8  refers to a phase correction table  9  and provides the phase corrector  7  with a phase correction control signal together with a phase correction value retrieved from the phase correction table  9 . 
         [0033]    The phase correction table  9  stores a phase correction value for each of the even-numbered sideband antennas B 2 , B 4 , . . . , and B 48 , the phase correction values being necessary to secure consecutiveness of radio waveforms emitted from these even-numbered sideband antennas. The phase correction values are calculated by measuring electrical lengths of signal paths (antenna cables C 2 , C 4 , . . . , and C 48 ) from the signal generator  15  to the even-numbered sideband antennas B 2 , B 4 , . . . , and B 48  and by finding differences among the measured electrical lengths. To cope with the aging of the antenna cables C 2 , C 4 , . . . , and C 48 , the phase correction values may be updated by periodically measuring the electrical lengths of the signal paths. 
         [0034]    The even-numbered sideband antennas B 2 , B 4 , . . . , and B 48  are connected through the antenna cables C 2 , C 4 , . . . , and C 48 , respectively, to the distributor  3  and sequentially emit radio waves based on the half-cosine wave signal generated by the signal generator  15  and amplified by the power amplifier  16 . 
         [0035]    Operation of the DVOR apparatus shown in  FIG. 3  will be explained. Although the following explanation relates to the sideband transmitter  1  supplying a half-sine wave signal to the odd-numbered sideband antennas B 1 , B 3 , . . . , and B 47 , the explanation is similarly applicable to the sideband transmitter  2  supplying a half-cosine wave signal to the even-numbered sideband antennas B 2 , B 4 , . . . , and B 48 . 
         [0036]    The sideband transmitter  1  controls the distributor  3  so that the sideband antennas are switched from one to another every 1/720 seconds to receive a half-sine wave signal from the signal generator  12 . Supplying a half-sine wave signal generated by the signal generator  12  to the sideband antenna B 1  will be explained. The signal generator  12  sends the generated half-sine wave signal to the phase corrector  4  through the power amplifier  13 . Also, the signal generator  12  provides the distributor  3  with a switching control signal s 1  so that the distributor  3  may supply the half-sine wave signal to the sideband antenna B 1 . At the same time, the signal generator  12  provides the phase correction controller  5  with a synchronization signal s 2 . 
         [0037]    In response to the synchronization signal s 2  from the signal generator  12 , the phase correction controller  5  refers to the phase correction table  6 , retrieves a phase correction value corresponding to the sideband antenna B 1  from the phase correction table  6 , and provides the phase corrector  4  with a phase correction control signal together with the stored phase correction value. The phase corrector  4  uses the phase correction value corresponding to the sideband antenna B 1  supplied from the phase correction controller  5 , to correct the half-sine wave signal provided by the signal generator  12  and amplified by the power amplifier  13  and supplies the phase-corrected half-sine wave signal to the distributor  3 . 
         [0038]    The distributor  3  supplies the phase-corrected half-sine wave signal from the phase corrector  4  to the sideband antenna B 1  through the antenna cable C 1 . 
         [0039]    Thereafter, the signal generator  12  provides the distributor  3  with a switching control signal so that the half-sine wave signal is supplied to the next sideband antenna B 3  and the other sideband antennas. The signal generator  12  provides a phase correction controller  5  with a synchronization signal s 2  so that the phase correction of the selected sideband antenna is conducted in synchronization with the switching of the distributor  3 . In this way, the odd-numbered sideband antennas B 1 , B 3 , . . . , and B 47  sequentially receive the half-sine wave signal whose phase is corrected with the use of phase correction values corresponding to the odd-numbered sideband antennas, respectively. 
         [0040]      FIG. 4  is a view showing the switching timing ((a)-(d)) of sideband antennas carried out by the distributor  3 , and waveforms ((e)-(h)) of a half-sine wave signal at input end (a 0  in  FIG. 3 ) of the phase corrector  4 , input ends of the distributor (a 1  in  FIG. 3 ) and sideband antennas (b 1 , b 3 , . . . , and b 47 ). Waveforms of the half-sine wave signal at the input ends (b 1 , b 3 , . . . , and b 47 ) of the sideband antennas B 1 , B 3 , . . . , and B 47  are deformed as shown in (f) of  FIG. 4  with respect to that at the input end (a 0 ) of the phase corrector as shown in (e) of  FIG. 4  due to variations in the electrical lengths of the antenna cables C 1 , C 3 , . . . , and C 47  if no phase correction is conducted by the phase corrector  4 . Thus, radio waveforms to be emitted from the sideband antennas B 1 , B 3 , . . . and B 47  will be discontinuous. 
         [0041]    On the contrary, in a case where phase correction is conducted by the phase corrector  4 , waveforms of the half-sine wave signal at the input end (a 1 ) of the distributor are corrected as shown in (g) of  FIG. 4 . Therefore, waveforms of the half-sine wave signal at the input end (b 1 , b 3 , . . . , and b 47 ) are as shown in (h) of  FIG. 4 , and thereby, the continuity of the radio waveforms radiated from the sideband antennas B 1 , B 3 , . . . , and B 47  can be maintained. In other words, the phase correction is performed so that the phase of the output waveform and the switching timing of the distributor  3  are synchronized with each other and thereby the continuity of radiated waveforms can be maintained. 
         [0042]    In this way, according to the present embodiment, the electrical lengths of signal paths from the sideband transmitter  1  to the sideband antennas B 1 , B 3 , . . . , and B 47  are measured, phase correction values for the sideband antennas are calculated, respectively, according to the measured electrical lengths, and the phase of a half-sine wave signal supplied from the sideband transmitter  1  to each sideband antenna is corrected according to the phase correction value for the sideband antenna. Consequently, without precisely equalizing the lengths of the antenna cables C 1 , C 3 , . . . , and C 47 , the consecutiveness of radio waveforms emitted from the sideband antennas can be secured according to the present embodiment. Furthermore, the continuity of output waveforms of radiation can be maintained even if all of the electrical lengths between each of input end a 1  of the distributor  3  associated with each corresponding sideband antenna and the corresponding terminals p 1 , p 3 , . . . , and p 47 . 
         [0043]    The electrical lengths of the signal paths may periodically be measured to update the phase correction values in the phase correction table  6  accordingly. This technique can easily handle, without hardware readjustment, phase shifts that may occur due to the aging of the antenna cables C 1 , C 3 , . . . , and C 47 . 
       Second Embodiment 
       [0044]    Two signal generators  12  and  15  of the first embodiment may be integrally constituted. According to a second embodiment, as shown in  FIG. 6 , the half-sine wave signal outputted from the power amplifier  13  is divided at a dividing point d 1 , phase shifted by 90 degrees through the phase shifter  18 , and inputted to the power amplifier  16  as the half-cosine wave signal. According to the second embodiment of the DVOR apparatus, two sideband transmitters are realized using only one signal generator  12 . 
       Modifications 
       [0045]    As shown in  FIG. 3 , the power amplifier (AMP)  13  of the DVOR apparatus of the first embodiment is connected to the output side of the signal generator  12 . The power amplifier (AMP) can be also connected between the output end of the phase corrector  4  and the input end of the distributor  3 , and therefore, a sideband transmitter  1 ′ can be implemented as shown in  FIG. 7 . 
         [0046]    In a similar manner, with regard to the DVOR apparatus of the second embodiment, the power amplifier (AMP)  13  ( 16 ) can be also connected between the output end of the phase corrector  4 ( 7 ) and the input end of the distributor  3 , and therefore, a sideband transmitter  21 ′ ( 22 ′) can be implemented as shown in  FIG. 8 . 
         [0047]    As to the DVOR apparatus of the first embodiment, the signal generator  12 ( 15 ), the phase corrector  4 ( 7 ), the phase correction controller  5 ( 8 ), and the phase correction table  6 ( 9 ) can be integrated into one integration circuit as the signal generation part  30  shown in  FIG. 7 . That is, the phase correctors  4 ,  7  and the phase correction tables  6 ,  9  can be realized as inner functions of the signal generation part  30 . 
         [0048]    In a similar manner, with regard to the DVOR apparatus of the second embodiment, a signal generation part including the signal generator  12 , the phase corrector  4 , the phase correction controller  5 , and the phase correction table  6 , the signal generator  12 , the phase shifter  18 , the phase corrector  7 , the phase correction controller  8 , and the phase correction table  9  can be integrated into one integration circuit  31  as shown in  FIG. 8 . 
       INDUSTRIAL APPLICABILITY 
       [0049]    The present invention is applicable to radar signal processors of radar systems. 
         [0050]    This application claims benefit of priority under 35 U.S.C. 119 to Japanese Patent Application No. 2006-163355 filed on Jun. 13, 2006, the entire contents of which are incorporated by reference herein. Although the present invention has been described above by reference to certain embodiments of the present invention, the present invention is not limited to the embodiments described above. Modifications and variations of the embodiments described above will occur to those skilled in the art in light of the teachings. The scope of the present invention is defined with reference to the appended claims.