Abstract:
Disclosed is a satellite tracking antenna applied to a satellite tracking antenna system mounted on a vehicle and method using rotation of a subreflector. The antenna includes a reflector controlled to be oriented toward a target satellite, a subreflector for reflecting a signal reflected from the reflector to an entrance end and for identifying relative signals of upper, lower, left, and right sides of the satellite, a subreflector rotating part for rotating the subreflector at a high RPM, a driving device for driving the reflector in at least one of elevation and azimuth directions, and a fixing member for fixing the antenna system on the vehicle. Thus, since the tracking mechanism is realized by operating the elevation and azimuth motors only using the subreflector, the structure of the antenna can be simplified and the satellite tracking is accurately performed.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a satellite tracking antenna mounted on a vehicle, and more particularly, to a satellite tracking antenna and method that can track a satellite using the rotation of a subreflector without using sensors.  
         [0003]     2. Description of the Related Art  
         [0004]     Generally, since a satellite communication is realized by a radio wave with high frequency of a micro-frequency band, a high directional antenna such as a parabolic antenna having a reflector is required to meet an intensive straight-advancing property of the radio wave. Particularly, for a directional antenna mounted on a vehicle such as a motorcar, a ship, and an airplane, there is a need for a function for tracking the satellite in response to a movement of the vehicle.  
         [0005]     Tracking algorithms for the satellite communication can be classified into a closed loop method and an open loop method. The closed loop methods can be further classified into a lobbing method and a mono-pulse method. The closed loop method is designed to control the antenna in a predicted orbit direction by processing satellite orbit forecasting data, standard time data, and antenna digital angle data using a computer. Therefore, the tracking performance of the antenna depends on the accuracy of the data. The lobbing method is designed to control an orientation of the antenna by detecting a coming direction of a bicorn wave by moving a beam of the antenna using a predetermined method. The mono-pulse method is designed to detect an azimuth error on occasion in accordance with a radio wave with a single pulse in a state where the beam of the antenna is fixed.  
         [0006]     The lobbing methods are further classified into a conical scanning method, a beam switching method, and a step tracking method. The conical scanning method is designed to rotate a beam of the antenna in a conical-shape having a minute angle to perform a closed tracking. The beam switching method is designed to determine a relative receiving signal level while discretely moving the beam to more than four predetermined locations disposed around an axis of the antenna. The step tracking method is designed to move the beam in a direction where the receiving level is increased by checking the variation of the receiving level while moving the antenna by a minute angle in a step manner at a predetermined time interval.  
         [0007]      FIG. 1  shows a schematic diagram illustrating a conventional satellite tracking antenna mounted on a vehicle such as a ship.  
         [0008]     Referring to  FIG. 1 , the conventional satellite tracking antenna includes a reflector  100 , a subreflector  101 , a first angle velocity detecting sensor  102  for detecting a movement of the vehicle in an elevation direction, an elevation motor  104  for generating rotational force, an elevation rotating pulley  103  for vertically moving the reflector  100  using the rotational force of the elevation motor  104 , an antenna support  112 , a second angle velocity detecting sensor  105  for detecting a movement of the vehicle in an azimuth direction, an azimuth motor  106  for generating rotational force, an azimuth rotating pulley  108  for horizontally moving the reflector  100  using the rotational force of the azimuth motor  106 , and a base  109 .  
         [0009]     The base  109  of the antenna is fixed on a vehicle body, and the reflector  100  is oriented to face the satellite. The azimuth motor  106  for tracking the azimuth of the antenna and the antenna support  112  for supporting the antenna are disposed on the base  109 . The azimuth rotating pulley  108  for horizontally moving the reflector  100  in accordance with the rotation of the azimuth motor  106  is installed on a lower end of the support  112 , while the elevation rotating pulley  103  for vertically moving the reflector  100  in accordance with the rotation of the elevation motor  104  is installed on an upper end of the support  112 . Accordingly, the reflector  100  is designed to move in the elevation and azimuth directions in accordance with the rotations of the elevation and azimuth rotating motors  104  and  106 , respectively.  
         [0010]     A radio signal from the satellite is concentrated toward the subreflector  101  by the reflector  100  and is then reflected on the subreflector  101 . The reflected radio signal is transmitted to a satellite signal receiver  120  through a feed horn  113 . When the vehicle moves, the movement in the elevation direction is detected by the first angle velocity detecting sensor  102 , and the movement in the azimuth direction is detected by the second angle velocity detecting sensor  105 . When it is detected by the sensors  102  and  105  that the orientation of the reflector  100  deviates from the target satellite, a controller  130  calculates correction values and controls the elevation and azimuth motors  104  and  106  in response to the correction values to rotate the elevation and azimuth rotating pulleys  103  and  108  by the correction values, thereby controlling the reflector  100  to be directed toward the satellite.  
         [0011]      FIG. 2  shows a block diagram illustrating a satellite tracking algorithm of the conventional satellite tracking antenna.  
         [0012]     Referring to  FIG. 2 , a satellite position correcting signal is generated by receiving a satellite signal through a dithering process  201  in a Ts1 cycle ( 202 ). An angle velocity correcting signal is generated by receiving an angle velocity signal from a gyro sensor  211  in a Ts2 cycle ( 212 ). A level correcting signal is generated  214  by receiving a sensor signal from a level sensor  213  in a Ts3 cycle ( 214 ). The correcting signals are put together and transmitted to a position controller  203 . The motor  204  is driven by the position controller  203  in response to the correcting signals to control the orientation of the antenna. As described above, in the conventional satellite algorithm, the satellite position correction and the posture control are essentially required.  
         [0013]     However, the conventional satellite tracking antenna requires two angle velocity sensors to detect the movement of the vehicle as well as two motors to rotate the antenna in response to the detected value. Therefore, the structure of the antenna and the control mechanism are complicated. Furthermore, additional hardware and software are required to initiate the sensors and to compensate for the reference value.  
       SUMMARY OF THE INVENTION  
       [0014]     Accordingly, the present invention is directed to a satellite tracking antenna and method using rotation of a subreflector that substantially obviate one or more problems due to limitations and disadvantages of the related art.  
         [0015]     A first object of the present invention is to provide a satellite tracking antenna that can be designed in a simple structure and can provide an accurate satellite tracking performance by rotating a subreflector using only a satellite signal without using sensors.  
         [0016]     A second object of the present invention is to provide a satellite tracking method using such an antenna.  
         [0017]     Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the drawings.  
         [0018]     To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, there is provided a satellite tracking antenna applied to a satellite tracking antenna system mounted on a vehicle, comprising: a reflector controlled to be oriented toward a target satellite; a subreflector for reflecting a signal reflected from the reflector to an entrance end and for identifying relative signals of upper, lower, left, and right sides of the satellite; a subreflector rotating part for rotating the subreflector at a high RPM; driving means for driving the reflector in at least one of elevation and azimuth directions; and fixing means for fixing the antenna on the vehicle.  
         [0019]     In another aspect of the present invention, there is provided a method for tracking a target satellite using an antenna mounted on a vehicle, the method comprising: the steps of searching a target satellite in a state where a tracking function of the antenna is turned off; receiving position signals from a subreflector and satellite signals corresponding to the position signals in a state where the tracking function of the antenna is turned on when the target satellite is searched; generating a position correcting signal by comparing the satellite signals transmitted to corresponding positions and calculating a difference between the satellite signals; and tracking the target satellite by correcting an orientation of the antenna in response to the position correcting signal.  
         [0020]     It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principle of the invention. In the drawings:  
         [0022]      FIG. 1  is a schematic view of a conventional satellite tracking antenna mounted on a vehicle;  
         [0023]      FIG. 2  is a block diagram illustrating a conventional satellite tracking algorithm;  
         [0024]      FIG. 3  is a schematic view of a satellite tracking antenna according to an embodiment of the present invention;  
         [0025]      FIGS. 4   a  and  4   b  are schematic views illustrating a mounting concept of a subreflector depicted in  FIG. 3 ;  
         [0026]      FIGS. 5   a  and  5   b  are schematic views illustrating a concept of deflection caused by the rotation of a subreflector;  
         [0027]      FIG. 6  is a schematic view illustrating a satellite tracking algorithm according to the present invention;  
         [0028]      FIG. 7  is a flowchart illustrating a satellite tracking process according to the present invention; and  
         [0029]      FIGS. 8   a ,  8   b  and  8   c  are schematic views illustrating a variety of modified examples of a subreflector depicted in  FIG. 3 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings.  
         [0031]      FIG. 3  shows a satellite tracking antenna according to the present invention. The satellite tracking antenna includes an antenna system mounted on an outer body of vehicle body and a satellite signal receiving/transmitting device  320  and an antenna position controller (tracker)  330  that are installed in a control room (communication room) of the vehicle.  
         [0032]     Referring to  FIG. 3 , the antenna system is coupled on the outer body of vehicle by a base  313 . A reflector  300  is supported by a support  314  fixed on a rotational plate  309 . The reflector  300  is designed to vertically move in response to rotation of an elevation motor  307  and to horizontally move in response to rotation of an azimuth motor  308 . That is, when the elevation motor  307  rotates by a control signal, an elevation driving pulley  306  rotates to rotate a driven pulley  304  by a belt  305 , thereby vertically moving the reflector  300 . When the azimuth motor  308  rotates by a control signal, an azimuth driving pulley  312  rotates to rotate a driven pulley  310  by a belt  311 , thereby horizontally moving the reflector  300 . By this operation, the rotational plate  309  rotates to vary the orientation of the antenna vertically and horizontally.  
         [0033]     Meanwhile, a subreflector  301  is disposed facing the reflector  300  and rotates at a relatively high RPM by a rotational motor  303 . Accordingly, a signal reflected from the oval-shaped reflector  300  is concentrated on the subreflector  301  and reflected thereon. The reflected signal is transmitted to the feed horn  315  and is then further transmitted to the satellite signal receiving/transmitting device  320  through a coaxial cable. At this point, a dielectric lens  316  may be inserted through an end of the feed horn  315  to further sharpen a beam. A position of the subreflector is detected by a position sensor  302 , and the detected signal is transmitted to the antenna position controller  330 .  
         [0034]     When comparing the inventive antenna with the conventional antenna, gyro sensors such as elevation and azimuth sensors that are used in the conventional antenna are all omitted in the inventive antenna. That is, it is noted that the structure of the inventive antenna is more simplified. In addition, the subreflector  301  of the present invention is inclined at a predetermined angle.  
         [0035]     A position sensor  302  attached on a rotational part of the subreflector  301  is provided to accurately detect an inclined direction of the subreflector  301 . In addition, the position sensor  302  further detects a rotation cycle of the subreflector  301  to create a Ts cycle illustrated in  FIG. 6 , thereby determining a sampling cycle of a controller.  
         [0036]     The position controller  330  receives a satellite signal from the satellite signal receiving/transmitting device  320  as well as a sub-reflection position signal from the position sensor  302  to control the elevation and azimuth motors  307  and  308 , thereby controlling the orientation of the antenna toward a target satellite. At this point, the satellite signal receiving/transmitting device  320  includes an information analyzing part for analyzing a data signal transmitted from the satellite and determining if a satellite toward which the antenna is currently directed is the target satellite.  
         [0037]      FIGS. 4   a  and  4   b  show a subreflector installing concept and  FIGS. 5   a  and  5   b  show a concept of deflection caused by the rotation of the subreflector.  
         [0038]     Specifically,  FIG. 4   a  shows a state where a central axis of the subreflector  301  is deviated from a central axis C of the reflector  300 , and  FIG. 4   b  shows a state where the subreflector  301  is rotated in a state where it is inclined with respect to the central axis C of the reflector  300  at a predetermined angle.  
         [0039]     These two states are all possible in the present invention, realizing an identical performance. At this point, vertical and horizontal positions of the subreflector  301  are determined using the position sensor  302 .  
         [0040]      FIGS. 5   a  and  5   b  show states where the subreflector  301  installed as in  FIG. 4   a  or  4   b  is deflected by rotation. That is,  FIG. 5   a  shows a state where the subreflector  301  is deflected in a horizontal direction, and  FIG. 5   b  shows a state were the subreflector  301  is deflected in a vertical direction.  
         [0041]     When the subreflector  301  is inclined with respect to the central axis C of the reflector  300  to accurately track the target satellite (i.e., when the orientation of the reflector is accurately directed to the target satellite), satellite signals coming to upper, lower, left and right sides have identical signal intensity and are all identical to each other. However, when the subreflector  301  is deflected to a side, the intensity of a signal transmitted to the deflected side is greater than those of others. That is, when the orientation of the reflector  300  is deflected to the right side with respect to the target satellite, the intensity of a receiving signal obtained when the subreflector  301  is deflected to the right side will be greater than that obtained when the subreflector  301  is deflected to the left side. When the orientation of the reflector  300  is inclined to the upper side, a receiving signal obtained when the subreflector  301  is deflected to the upper side will be greater than that obtained when the subreflector  301  is deflected to the lower side.  
         [0042]     Accordingly, it will be identified which direction the orientation of the antenna is deviated with respect to the target satellite by comparing the receiving signals obtained when the subreflector  301  is deviated to the upper, lower, right and left sides. That is, an intensity difference between the receiving signals is scaled and a position correcting signal is generated by a scaled value. The corresponding motor is driven in response to the position correcting signal so as for the orientation of the antenna to be directed to track the target satellite.  
         [0043]     Next, the satellite tracking process according to the present invention will be described hereinafter.  
         [0044]     Again referring to  FIG. 3 , the reflector  300  is a part for receiving a satellite signal. The satellite signal directed to the reflector  300  is transmitted through the subreflector  301 . Since the subreflector rotates in a state where it is inclined with respect to its rotational axis, the maximum signal value is lowered, but it provides information on the satellite direction where the reflector  300  should move and an amount of movement of the reflector.  
         [0045]     For example, when the inclined surface of the subreflector  301  faces the upper side of the reflector  300 , the intensity of the signal transmitted to the upper side of the reflector  300  becomes greater than others. When it faces the lower side of the reflector  300 , the intensity of the signal transmitted to the lower side of the reflector  300  becomes greater than others. Therefore, when it is determined that the signals transmitted to the upper and lower sides are identical, it should be noted that the orientation of the antenna is correctly directed toward the satellite with regard to the vertical direction. When the signal transmitted to the upper side is greater than that transmitted to the lower side, the elevation motor  307  is rotated in an upper direction to correct the position of the reflector  300  by rotating the driven pulley  304  through the elevation driving pulley  306 . When the signal transmitted to the lower side is greater than that transmitted to the upper side, the elevation motor  307  is rotated in a lower direction to correct the position of the reflector  300  by rotating the driven pulley  304  through the elevation driving pulley  306 . At this point, the driving amount of the elevation motor  307  is determined in proportion to the intensity difference between the signals.  
         [0046]     Likewise, when the inclined surface of the subreflector  301  faces the left side of the reflector  300 , the intensity of the signal transmitted to the left side of the reflector  300  becomes greater than others. When it faces the right side of the reflector  300 , the intensity of the signal transmitted to the right side of the reflector  300  becomes greater than others. Therefore, when it is determined that the signals transmitted to the left and right lower sides are identical, it should be noted that the orientation of the antenna is correctly directed toward the satellite with regard to a horizontal direction. When the signal transmitted to the left side is greater than that transmitted to the right side, the azimuth motor  308  is rotated in a left direction to correct the position of the reflector  300  by rotating the driven pulley  310  through the azimuth driving pulley  312 . When the signal transmitted to the right side is greater than that transmitted to the left side, the azimuth motor  308  is rotated in a right direction to correct the position of the reflector  300  by rotating the driven pulley  310  through the azimuth driving pulley  312 . At this point, the driving amount of the azimuth motor  308  is determined in proportion to the intensity difference between the signals.  
         [0047]      FIG. 6  shows a satellite tracking algorithm according to the present invention, and  FIG. 7  shows a flowchart illustrating a satellite tracking process according to the present invention.  
         [0048]     Referring first to  FIG. 6 , a satellite position correcting part  604  generates a position correcting signal by (a) receiving a rotational position signal of the subreflector  301  from a subreflector rotating part  603  and satellite signals at each side, (b) comparing the satellite signals, and (c) calculating a signal difference between the satellite signals. At this point, the rotation time Ts of the subreflector  301  becomes a cycle for generating a position command. The position correcting signal is transmitted to a position controller  602  in a Ts cycle, and the position controller  602  controls a corresponding motor  601  in response to the position correcting signal to track the satellite. At this point, different from the conventional dithering method, the position correcting cycle Ts is fast enough to track the satellite in real-time. Therefore, the satellite tracking can be quickly realized even without using an angle velocity sensor and a level sensor.  
         [0049]     Referring to  FIG. 7 , an initialization is performed after the antenna is operated (S 1 ), and the satellite searching is processed (S 2 ). When the satellite is searched (S 3 ), it is identified if the searched satellite is a target satellite by obtaining satellite information and reading the information (S 4  and S 5 ). When it is determined that the searched satellite is not the target satellite, the above steps (S 2 -S 5 ) are repeated until the target satellite is searched. When it is determined that the searched satellite is the target satellite, a tracking operation is started (S 6 ), after which a position signal of the subreflector and satellite signals are inputted and the satellite signals are compared with each other (S 7 ). When it is identified by the comparison that the intensities of the signals are identical to each other, it is determined that the orientation of the antenna is correctly directed toward the target satellite. When it is identified by the comparison that there is an intensity difference between the signals, a correcting signal corresponding to the intensity difference is generated and the motor is driven in response to the correcting signal so as for the orientation of the reflector of the antenna to be directed toward the target satellite (S 8 ).  
         [0050]      FIGS. 8   a ,  8   b , and  8   c  show a variety of modified examples of the subreflector.  
         [0051]     In the above-described embodiment, the subreflector  301  is formed in a flat type. However, the present invention is not limited to this. That is, the subreflector  301  may be formed in a concave type (see  FIG. 8   a ), a convex type (see  FIG. 8   b ), or a V-shape type (see  FIG. 8   c ).  
         [0052]     As described above, when the antenna is deviated by the movement of the vehicle in rolling, pitching, and yawing directions, the satellite signal is varied in vertical and horizontal directions. At this point, the upper and lower signals are compared with each other and the elevation motor is driven in the larger signal direction. In addition, the left and right signals are also compared with each other and the azimuth motor is driven in the larger signal direction. Accordingly, the orientation of the antenna is controlled toward the satellite. In addition, by analyzing the satellite data, it can be identified if a searched satellite is a target satellite. As described above, since the tracking mechanism is realized using the elevation and azimuth motors without using a variety of sensors attached on the vehicle, the structure of the antenna can be simplified and the satellite tracking is accurately performed.  
         [0053]     It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.