Background Art In order to properly receive a satellite broadcasting in a moving body such as a ship, train or automobile, a satellite tracking system which can receive a satellite signal by detecting a position of a satellite in a real-time basis and continuously tracking the detected position of the satellite is required.

In such a satellite tracking system, a hybrid tracking method employing both an open-loop tracking method and a closed-loop. tracking method has been typically used. In some cases, the open-loop tracking method or the closed-loop tracking may be singly used.

To acquire an initial satellite position, the open-loop tracking method generally uses a constant-speed rotation tracking mode in which a satellite is

tracked while rotating in a direction at a constant velocity, and an iterative tracking mode in which, when a signal from a satellite is detected and a subsequent signal is then missed, the satellite is tracked by driving an antenna zigzag around a current position.

In addition, the initial position of a satellite can be tracked using a gyro sensor that is an azimuth angle sensor. Although the gyro sensor provides high performance, it is too expensive.

In order to continuously track positions of a satellite by acquiring a position of a satellite using the open-loop tracking method to receive satellite signals, the closed-loop tracking method employs a step tracking method, a monopulse tracking method, a sequential amplitude sensing method, a conical sensing method or an electronic beam squinting method.

In the step tracking method, a satellite is tracked by detecting a point at which the amplitude of a satellite signal is maximized while moving the direction of an antenna beam left and right in units of steps. Although the system based on the step tracking method is simplified, the accuracy thereof is not ensured so that a gyro sensor that is an acceleration sensor must be employed as an auxiliary device.

On the other hand, in the monopulse tracking method, the same phased direction of a satellite is tracked using the sum of or difference between satellite signals disposed up and down or left and right with respect to an antenna. Since signal processing is continuously performed, the tracking accuracy is high, but a complicated phase tracking system is required.

In the sequential amplitude sensing method, an antenna beam is

mechanically tilted sequentially step by step, up and down, left and right, to measure amplitudes of signals, thereby acquiring the position of a satellite.

In the conical sensing method, an antenna beam is continuously rotated in the shape of a conical circle, thereby acquiring the position of a satellite. In the EBS method, an antenna beam is electronically switched to measure signal amplitudes up and down, left and right, thereby acquiring the position of a satellite. In other words, the monopulse tracking method, the sequential amplitude sensing method, the conical sensing method and the EBS method are characterized as acquiring the position of a satellite by measuring satellite signal amplitude while tilting an antenna beam.

In these existing tracking methods, if an increase in tracking performance is pursued, a complicated, costly satellite tracking system is required. Conversely, construction of a simplified, low-cost satellite tracking system unavoidably results in poor tracking performance. Thus, the above- mentioned conventional tracking methods have been selectively adopted singly or in combination according to the need for satellite tracking.

To overcome the above-mentioned problems, a mobile-mounted satellite antenna apparatus and method thereof has been published in Korean Patent Publication No. 2000-16081 by the present inventor on March 5,2001.

The proposed apparatus is configured to acquire the position of a satellite by measuring and comparing amplitudes of satellite signals by phase shift through movement of the orientation of an antenna beam for receiving satellite signals vertically and horizontally. This system is a simplified, cheap system compared to the conventional system, while exhibiting good tracking

performance. However, according to this system, the phase of a satellite signal should be adjusted inside the system to tilt the orientation of an antenna beam, that is, phase control is difficult to achieve.

Disclosure of the Invention To solve the above-described problems, the present invention provides a simplified, low-cost, highly efficient satellite tracking system that is mounted on a mobile for acquiring and tracking a position of a satellite to receive satellite signals, and a method thereof.

In an embodiment of the present invention, there is provided a satellite tracking system including an antenna unit including a first antenna and a second antenna having feed lines configured to have different lengths for transmitting satellite signals received from the satellite so that directions of an antenna beam vary according to the respective frequencies of satellite channels received from the satellite by a frequency scanning effect, a low noise block down converter (LNB) for receiving the satellite signals of the respective frequencies received through the antenna unit via feed lines and converting the received satellite signals into intermediate frequency (IF) satellite signals, a control board for detecting at least two different signals among the IF satellite signals converted by the LNB and measuring and comparing amplitudes of the signals to acquire a position of the satellite, a motor for rotating the antenna unit to be directed to the position of the satellite acquired by the control board, and a transmission means for supplying power to the respective elements and transmitting the IF satellite signals converted

by the LNB to a TV monitor and a satellite broadcasting receiver.

In another embodiment of the present invention, there is provided a method tracking a satellite by measuring and comparing satellite signals received from a satellite, including a) setting a first tuner of a control board to a reference frequency fo among a plurality of channel frequencies transmitted from the satellite, and setting a second tuner to a frequency fn corresponding to the reference frequency, b) first and second antennas of an antenna unit receiving satellite signals to transmit the same to a low noise block down converter (LNB) through feed lines having different lengths, c) converting the satellite signals transmitted to the LNB into intermediate frequency (IF) satellite signals to transmit the same to the first and second tuners of the control board, d) detecting satellite signals of the corresponding frequencies set in the first and second tuners among the IF satellite signals transmitted to the first and second tuners and outputting the detected signals, e) comparing amplitudes of the satellite signals output in step d) to acquire a position of the satellite and rotating the antenna unit to the acquired position of the satellite.

Brief Description of the Drawings FIG. 1 is a conceptual diagram of a frequency scanning effect used when a satellite tracking system according to the present invention tracks the position of a satellite ; FIG. 2 is a diagram illustrating the design concept of an antenna for tracking the position of a satellite using the frequency scanning effect according to the present invention;

FIG. 3 is a plan view of an antenna realized according to the design concept shown in FIG. 2; FIG. 4 is an overall block diagram of a satellite tracking system using a frequency scanning effect according to the present invention; FIG. 5 is a conceptual diagram illustrating the orientation of an antenna in the satellite tracking system according to the present invention; FIG. 6 is a schematic diagram showing a state in which the satellite tracking system according to the present invention; FIG. 7 is a flow diagram illustrating the procedure of tracking the position of a satellite by the satellite tracking system according to the present invention; FIG. 8 is an overall block diagram of a satellite tracking system for tracking the azimuth and elevation angles of a satellite according to another embodiment of the present invention ; and FIG. 9 is a flow diagram illustrating the procedure of a satellite tracking system rotating an antenna portion by tracking a satellite of an elevation angle according to another embodiment of the present invention.

Best mode for carrying out the Invention The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown.

FIG. 1 is a conceptual diagram of a frequency scanning effect used when a satellite tracking system according to the present invention tracks the

position of a satellite.

A"frequency scanning effect"termed in the present invention is used to refer to directivity of a phased array antenna beam for receiving satellite signals varying with frequencies. Differently directed beams are formed at different frequencies by designing a phased array antenna such that the orientation of a directed beam thereof is made to vary with frequencies.

As shown in FIG. 1, satellite signals fo, fi, f2,..., fn transmitted from a satellite have different frequencies. That is, orientations of an antenna beam are different with frequency. When channel frequencies of a satellite signal transmitted from a satellite are fo, fi, f2,..., fn, orientations of an antenna beam for receiving the satellite signal are different depending on the frequencies, thereby tracking the position of the satellite.

FIG. 2 is a diagram illustrating the design concept of an antenna for tracking the position of a satellite using the frequency scanning effect according to the present invention.

As shown in FIG. 2, an antenna composed of a first antenna A1 and a second antenna A2 can be designed as the simplest structure of using the frequency scanning effect. A satellite signal received by the antenna is transmitted to a low noise block down converter (LNB) to be converted into an intermediate frequency (IF) signal. When a feed line between the first antenna A1 and the LNB is set to a, a feed line between the second antenna A2 and the LNB is set to b, and a distance between the first and second antennas A1 and A2 is set to d, the relationship between the frequency and wavelength of an electromagnetic wave is represented by equation (1):

wherein c represents the velocity of light, that is, 3#108 m/s, and £eff represents the effective dielectric constant. According to the equation (1), the wavelength varies with frequency. Assuming that channel frequencies of a satellite signal are fo, fi, f2, f3,..., fn, antenna design is made such that a length difference between the feed lines becomes nXo, that is, b-a=nAo-- (2) wherein Xo represents a wavelength corresponding to the frequency fo, and n is an integer.

Since the phases of frequencies passing through the feed lines and b in the antenna designed as described above are the same at the frequency fo, an antenna beam is vertically disposed.

However, when the frequency is fn, the corresponding wavelength becomes Xn, the phases of the feed lines a and b are not the same. If fo < fn, Thus, when a frequency of a satellite signal is fn, the phase of the second antenna A2 is delayed by AL Thus, an antenna beam is tilted toward the second antenna A2 by Q@.

A phase difference between the first and second antennas A1 and A2 is calculated as follows : #I = n(#0-#n) -------- (3) Wherein Aq) is represented by: and wherein a frequency-dependent tilt angle of the antenna beam Decan be adjusted by varying the integers n and d.

FIG. 3 is a plan view of an antenna realized according to the design concept shown in FIG. 2.

As shown in FIG. 3, an antenna unit 100 includes a first flat antenna 110 and a second flat antenna 120 for receiving satellite signals, the first and second antennas 110 and 120 spaced apart from each other by a distance d.

A feed line for transmitting a satellite signal received at the first antenna 110 to the LNB via a wave guide is denoted by a, a feed line for transmitting a satellite signal received at the second antenna 120 to the LNB via a wave guide is denoted by b, and a length difference between the feed lines, that is, b-a, equals nXo.

Assuming that frequencies of a satellite signal depending on channel are fo, fi, f2, f3,..., fn, the following antenna beams are formed for the respective frequencies: For fo, an antenna beam is vertically disposed with respect to the antenna because the phases of satellite signals respectively received by the first and second antennas 110 and 120 become the same when the wavelength is Xo and the length difference between the feed lines a and b is n xi- For fn, an antenna beam is tilted due to a phase difference between

satellite signals respectively received by the first and second antennas 110 and 120 when the wavelength is Xn and the length difference between the feed lines a and b is nXo. Since b > a, based on the equation (5), the antenna beam is tilted toward the second antenna 120 by In an embodiment of the present invention, the antenna unit 100 is designed to receive a direct broadcasting satellite (DBS) from KoreaSat 3, which will now be described. The DBS channel of KoreaSat 3 utilizes frequencies of 11.747 Gh and 11.824 GHz.

That is to say, fo = 11.747 GHz, and fun = 11.824 GHz. Based on the equation (1), #0 # 25. 54 mm and Xn 25. 37 mm assumingthat ceff=1.

When b - a = 10#0 (n=10) and d = 164.8mm, <BR> <BR> <BR> <BR> <BR> 10(25.54-25.37)<BR> ##=arcsin #arcsin0.01032#0.6a 164.8 In other words, if the frequency of a satellite signal transmitted from a satellite is 11.747 GHz, a vertical antenna beam is formed to receive the satellite signal. If the frequency of a satellite signal transmitted from a satellite is 11.824 GHz, the antenna beam is tilted toward the second flat antenna 120 by approximately 0.6° to receive the satellite signal.

A position of a satellite can be tracked by comparing amplitudes of the satellite signal received by the vertically disposed antenna beam at frequency 11.747 GHz and the satellite signal received by the antenna beam tilted by 0.6° at frequency 11.824 GHz, using a high-speed central processing unit (CPU) to

be described later.

A coverage angle (high angle) of frequency fn with respect to frequency to can be further increased or decreased by adjusting the length difference b-a and the distance d.

FIG. 4 is an overall block diagram of a satellite tracking system using a frequency scanning effect according to the present invention. As shown in FIG. 4, the satellite tracking system according to the present invention includes an antenna unit 100 for receiving a satellite signal from a satellite, a low noise block down converter (LNB) 200, for converting the satellite signal received by the antenna unit 100 into an intermediate frequency (IF) signal, a control board 300 for analyzing the IF signal converted by the LNB 200 to acquire a position of the satellite, a motor 400 for rotating the antenna unit 100 to the acquired position of the satellite, and a rotary joint 500 for transmitting the IF signal to a satellite broadcasting receiver 600.

The antenna unit 100, as shown in FIG. 3, including the first flat antenna 110 and the second flat antenna 120 having different feed line lengths to reach the LNB 200, receives satellite signals of various frequencies corresponding to satellite channels and transmits the received satellite signals to the LNB 200 through the feed lines a and b. The LNB 200 converts and amplifies the satellite signals transmitted from the antenna unit 100 to transmit the same to the control board 300.

The control board 300 includes a first power divider 310, a second

power divider 320, first and second tuners 330 and 340, an analog-to-digital converter (ADC) 350, a central processing unit (CPU) 360, and a motor driver 370. The first power divider 310 divides each intermediate frequency satellite signal transmitted from the LNB 200 into two equal intermediate frequency satellite signals. The second power divider 320 subdivides each of the two intermediate frequency satellite signals into two intermediate frequency satellite signals. The first and second tuners 330 and 340 receive the intermediate frequency satellite signals from the second power divider 320 to detect satellite signals each having a set frequency. The ADC 350 performs an analog-to-digital conversion on the intermediate frequency satellite signals transmitted from the first and second tuners 330 and 340. The CPU 360 measures amplitudes of the satellite signals transmitted from the ADC 350 to acquire the position of a satellite. The motor driver 370 drives a motor 400 to rotate the antenna unit 100 to the position of a satellite acquired by the CPU 360.

The first and second tuners 330 and 340 of the control board 300 are set so as to detect satellite signals having different frequencies.

For example, if frequencies corresponding to channels of a satellite are fo, f, f2, f3,..., fn, the first tuner 330 is set to a frequency fo and the second tuner 340 is set to a frequency fn. In other words, the first tuner 330 is tuned to a reference frequency fo and the second tuner 340 is tuned to a different frequency fn. The first and second tuners 330 and 340 output automatic gain control (AGC) signals of set frequencies. The ADC 350 performs an ADC conversion on the AGC signals output from the first and second tuners 330

and 340 to transmit the converted signals to the CPU 360.

The CPU 360 compares amplitudes of the AGC signals of the frequencies fo and fn input via the ADC 350, thereby acquiring the position of a satellite. This is possible because the channel frequency of a satellite to be tracked is known and the orientation and tilt angle of an antenna beam at the frequency fn with respect to the frequency fo are also known. Since the orientation of the antenna beam tilted at the frequency fn is previously input to the CPU 360, the CPU 360 acquires the position of the satellite by comparing the amplitude of the AGC signal at frequency fo with that of the AGC signal at frequency fn. After acquiring the position of the satellite, the CPU 360 controls the motor driver 370 to drive the motor 400, thereby controlling the antenna unit 100 to be directed toward the satellite.

Since the CPU 360 performs comparison of signal amplitudes depending on frequency and motor driving control very rapidly, a satellite mounted on a vehicle moving very fast can be sufficiently tracked. In an embodiment of the present invention, the CPU 360 compares/analyzes amplitudes of AGC signals at a processing speed of approximately 1000 cycles/sec. The motor rotates at a rotation speed of 90°/sec. Thus, in the antenna unit 100 for receiving satellite signals from the KoreaSAT 3, as illustrated above, even if there is a slight difference in the beam direction between two satellite signals, that is, 0.6°, the satellite signals are compared and analyzed very fast to then drive the motor, thereby smoothly tracking the position of the satellite.

The rotary joint 500 transmits the IF signals divided by the first power

divider 310 of the control board 300 to the satellite broadcasting receiver 600 so that the signals transmitted to the satellite broadcasting receiver 600 are displayed through a TV monitor. Also, the rotary joint 500 supplies externally applied power to the respective elements.

FIG. 5 is a conceptual diagram illustrating the orientation of an antenna in the satellite tracking system according to the present invention. In FIG. 5, the abscissa indicates azimuth angle and the ordinate indicates amplitude of a satellite signal according to frequency. Also, angles of an antenna beam formed with frequencies are denoted by A, B and C, and amplitudes of AGC signals of frequencies fo and fn are denoted by So and Sn, respectively. The CPU 360 acquires a position of a satellite by comparing So with Sn. If So>Sn, which means that the satellite is positioned at"A", the motor 400 is driven leftward to make the antenna unit 100 toward"B". On the contrary, if So<Sn, which means that the satellite is positioned at"C", the motor 400 is driven rightward to make the antenna unit 100 toward"B". If So=Sn, which means that the antenna unit 100 is oriented at the same direction in which the satellite is positioned, the motor 400 is not driven. Since this procedure can be performed within a very short time, it is possible to keep track of the position of a satellite even when a moving body such as a vehicle or train moves.

FIG. 6 is a schematic diagram showing a state in which the satellite tracking system according to the present invention. As shown in FIG. 6, the satellite tracking system according to the present invention is installed inside a hemispherical radome 10 to be protected from natural environments such as

rain, snow or wind, and is then mounted on a moving body such as a vehicle.

The vehicle-mounted satellite tracking system tracks the position of a satellite to smoothly receive satellite signals irrespective of the moving direction of the vehicle. The satellite tracking system according to the present invention receives satellite signals by acquiring a position of a satellite and at the same time tracking the position. The received satellite signals are transmitted to the TV monitor 700 via the satellite broadcasting receiver 600 to then be displayed.

The operation and procedure of the aforementioned satellite tracking system according to the present invention acquiring a position of a satellite and keeping track of the acquired position will now be described.

FIG. 7 is a flow diagram illustrating the procedure of tracking the position of a satellite by the satellite tracking system according to the present invention. As shown in FIG. 7, the satellite tracking system according to the present invention tracks a position of a satellite as follows.

In step S110, the system is initialized to tune the first and second tuners 330 and 340 of the control board 300 to channel frequencies fo and fn.

Based on the frequencies fo and fn set to the first and second tuners 330 and 340 and parameters known by design of the antenna unit 110, the direction of an antenna at frequency fn is obtained to then be stored in a memory of the CPU 360. In other words, the azimuth direction and angle of the antenna beam with respect to the perpendicular direction of the antenna are determined at frequency fn.

In step S120, the first and second flat antennas 110 and 120 of the

antenna unit 100 receive satellite signals from a satellite. The satellite signals received by the antenna unit 100 are satellite signals of all frequencies corresponding to satellite channels.

In step S130, the satellite signals received in step S120 are input to the LNB 200 via the respective feed lines a and b having different lengths, and the LNB 200 converts the input satellite signals into IF signals and amplifies the IF signals.

The IF satellite signals converted by the LNB 200 are applied to the first power divider 310 of the control board 300, and the first power divider 310 divides the IF satellite signals to then be transmitted to the rotary joint 500 and the second power divider 320, respectively.

In step S131, the IF satellite signal transmitted to the rotary joint 500 is displayed on the TV monitor via the satellite broadcasting receiver 600.

In step S132, if a viewer of satellite broadcasting intends to stop watching, power supply of the system is interrupted to terminate the operation of the system.

In step S140, the respective IF satellite signals applied to the second power divider 320 in step S130 are sub-divided into two equal IF satellite signals to then be input to the first and second tuners 330 and 340. The first tuner 330 detects only the IF satellite signal corresponding to frequency fo to then output an AGC signal. The AGC signal is subjected to analog-to-digital conversion by the ADC 350 to then be applied to the CPU 360. The CPU 360 measures an amplitude So of the AGC signal corresponding to frequency fo.

In step S150, the CPU 360 compares So with a predetermined set value. The set value represents an amplitude of a satellite signal with which a satellite broadcasting can be smoothly viewed, inclusive of a certain margin, and the CPU 360 is previously tuned to the set value.

In step S151, if So is less than the set value, which means that the initial position of a satellite is not properly acquired, the motor 400 is initially driven in a predetermined direction so as to allow the antenna unit 100 to receive a satellite signal at a new position. While the motor 400 is driven, So is continuously measured for being compared with the set value.

In step S160, if it is determined in step S150 that So is less than the set value, which means that the initial position of a satellite is acquired, an amplitude Sn of the AGC signal corresponding to frequency fn set in the second tuner 340 is measured for acquiring a more accurate position of the satellite.

In step S170, So corresponding to fo is measured again.

In step S180, So measured in step S170 is compared with the set value. This is for providing against cases of instantaneously missing a position of a satellite in step S160 or losing an acquired position of the satellite in subsequent steps.

In step S181, if So is less than the set value, it is determined that the acquired position is lost, and a position of the satellite is acquired again, which is achieved by tracking a point at which So corresponding to fo exceeds the set value by means of the motor 400 driven zigzag while increasing the tracking range about the current position. In this case, the drive range of the motor

400 is limited to prevent the motor 400 from driving zigzag infinitely.

In step S182, if So exceeds the set value within a predetermined zigzag range, it is determined that the position of a satellite is acquired again and then the process goes back to step S160 to operate the subsequent routine. If So does not exceed the set value within the predetermined zigzag range, it is determined that the position of a satellite is missed and the process goes back to step S151 for tracking a position of the satellite again from the beginning.

In step S190, if it is determined in step S180 that So is greater than the set value, which means that the position of the satellite is kept track of, it is determined whether So-Sn > a for acquisition of more accurate position.

Here, denotes a margin value for preventing the antenna unit 100 from keeping moving due to a difference between So and Sn even if a satellite broadcasting can be sufficiently received.

In step S191, if So-Sn > a, it is determined that the satellite is positioned leftward and the motor 400 is driven leftward. This is because the antenna beam at frequency fn is directed rightward compared to that at frequency fo if fn is greater than to. In this case, an appropriate driving range of the motor 400 may be previously set to be stored in the CPU 360, or may be determined by calculation based on a difference between So and Sn and an azimuth angle of the antenna beam for more accurate driving. Then, the motor 400 is driven leftward to make the antenna unit 100 rotate leftward, and then the process goes back to step S160 to proceed the subsequent routine.

In step S200, if it is determined in step S190 that the difference value,

i. e., So-Sn is not greater than a, it is further determined whether So-Sn <-a.

In step S201, if So-Sn <-a, which means that the satellite is positioned rightward, the motor 400 is driven rightward. Thereafter, the process goes back to step S160 and the subsequent routine is repeated.

In step S210, if the difference value, i. e., So-Sn, is not greater than a and is not smaller than-a, there is a slight difference between So and Sn within the margin value a, which means that the satellite tracking system according to the present invention performs accurate tracking of a satellite. In this case, the motor 400 is not driven and the process goes back to step S160 to repeat the above-described procedure for accurately tracking the satellite.

As described above, the satellite tracking system according to the present invention operates such that the antenna unit 100 detects an AGC signal of a basic frequency set at the first tuner 330 during initial rotation at a constant speed and measures an amplitude of the AGC signal to acquire an initial position of a satellite, and then accurate positions of the satellite are constantly tracked by measuring amplitudes of AGC signals depending on various frequencies of satellite signals received by the antenna unit 100, the antenna unit 100 designed such that the orientation of an antenna beam is tilted according to frequencies corresponding to satellite channels.

In the satellite tracking antenna system according the present invention, an elevation angle of the antenna unit 100 is fixed at an initial elevation angle position of a satellite and azimuth angles of a satellite varying with movement of a moving body are acquired and tracked using a frequency scanning effect, thereby controlling the antenna unit 100 to direct to the

position of the satellite. In the case where the moving body is a vehicle, a variation in elevation angle is not severe. Thus, the satellite tracking antenna system can smoothly receive a satellite signal just by controlling the azimuth angles.

However, in the case where the moving body is a special vehicle or ship, since pitch frequently occurs, it is necessary to control both azimuth and elevation angles of an antenna unit.

FIG. 8 is an overall block diagram of a satellite tracking system for tracking the azimuth and elevation angles of a satellite according to another embodiment of the present invention.

As shown in FIG. 8, the satellite tracking system for tracking the azimuth and elevation angles of a satellite according to another embodiment of the present invention includes an antenna unit 100, an LNB 200, a control board 300, a motor 400 and a rotary joint 500, which are the same as those in FIG. 4, and further includes a gyro sensor 800 for sensing movement of a moving body. Also, a motor driver 370 of the control board 300 further includes an azimuth angle motor driver 371 for controlling an azimuth angle of the antenna unit 100 and an elevation angle motor driver 372 for controlling an elevation angle of the antenna unit 100. The motor 400 further includes an azimuth angle motor 410 for rotating an azimuth angle of the antenna unit 100 and an elevation angle motor 420 for rotating an elevation angle of the antenna unit 100.

The gyro sensor 800 senses a variation in elevation angle according to movement of a moving body. Elevation angle variation information sensed

by the gyro sensor 800 is transmitted to the control board 300. A CPU 360 of the control board 300 controls an elevation angle of the antenna unit 100 according to the variation in elevation angle by means of the elevation angle motor driver 372 and the elevation angle motor 420. When the CPU 360 controls the elevation angle of the antenna unit 100, the elevation angle variation information transmitted from the gyro sensor 800, a rotation speed of the elevation angle motor 420 and amplitudes of satellite signals received through the antenna unit 100 are taken into consideration.

An operation of the above-described satellite tracking antenna system according to another embodiment of the present invention in controlling an elevation angle of an antenna unit by tracking the elevation angle of a satellite will now be described in detail.

FIG. 9 is a flow diagram illustrating the procedure of a satellite tracking system rotating an antenna portion by tracking a satellite of an elevation angle according to another embodiment of the present invention. The elevation angle control is performed at the same time with the azimuth angle control shown in FIG. 7. The azimuth angle control of the antenna unit 100 is the same as that in FIG. 7, and a detailed explanation will not be given.

In step S110 shown in FIG. 7 to initialize the system, the antenna unit 100 moves to an initial elevation angle position of a satellite to be initialized.

Since the initial elevation angle of a satellite is predetermined by a location of a moving body, the CPU 360 of the control board 300 controls the elevation angle motor driver 372 and the elevation angle motor 420 to allow an elevation angle of the antenna unit 100 to be directed to the satellite. Then,

as shown in FIG. 7, a satellite signal is received and the amplitude thereof is compared with a set value (steps S120 through S170 shown in FIG. 7).

In step S180, if So corresponding to fo is greater than the set value, the procedure moves to step S190 shown in FIG. 7 to track an azimuth angle of a satellite, thereby driving the azimuth angle motor 410 to perform steps following after the step of controlling the azimuth angle of the antenna unit 100.

In step S300, if So corresponding to fo is greater than the set value, the procedure also tracks and controls a variation in elevation angle of the satellite at the same time with step S190. For tracking and controlling the elevation angle of a moving body, it is first checked whether an elevation angle of the moving body has been changed or not. This process is performed by the CPU 360 of the control board 300 analyzing elevation angle variation information of the moving body transmitted from the gyro sensor 800. If there is no variation in elevation angle of the moving body, the process goes back to step S160 shown in FIG. 7 without a change in the elevation angle of the antenna unit 100 and then the procedure of measuring amplitudes of satellite signals is repeated.

In step S310, if the elevation angle variation information is sensed by the gyro sensor 800 in step S300, the CPU 360 of the control board 300 analyzes the elevation angle variation information to calculate an elevation angle motor driver determination value Vsum. The Vsum is determined based on the elevation angle variation information transmitted from the gyro sensor 800, So and a drive speed of the elevation angle motor 420. If the elevation angle is varied upward, the Vsum is greater than 0. If the elevation

angle is varied downward, the Vsum is smaller than 0.

In steps S320 and 321, if the Vsum calculated in step S310 is greater than 0, the CPU 360 controls the elevation angle motor driver 372 and the elevation angle motor 420 to shift the antenna unit 100 upward and then the process goes back to step S160 shown in FIG. 7 to repeat the subsequent routine.

In steps S330 and S331, if the Vsum calculated in step S310 is smaller than 0, the CPU 360 controls the elevation angle motor driver 372 and the elevation angle motor 420 to shift the antenna unit 100 downward and then the process goes back to step S170 shown in FIG. 7 to repeat the subsequent routine. If the Vsum is equal to 0, which means that there is no variation in elevation angle, the process goes back to step S170 shown in FIG. 7 to repeat the subsequent routine.

In step S340, if So corresponding to fo is smaller than the set value in step S180, it is determined that the previously acquired position of a satellite is instantaneously lost and the elevation angle of the antenna unit 10 is first shifted to the initial position to acquire a position of the satellite again. That is to say, the elevation angle of the antenna unit 100 is initialized to the initial elevation angle position of the satellite set during the system initializing step (step S110 shown in FIG. 7).

Thereafter, the process proceeds to step S181 shown in FIG. 7 to drive the azimuth angle motor 410 zigzag, thereby acquiring a position of the satellite again.

As described above, the satellite tracking antenna system performs

control of elevation and azimuth angles of the antenna unit 100, thereby acquiring and tracking a position of a satellite more accurately.

The satellite tracking system according to the present invention may be modified in different types of systems performing the same function. For example, the antenna unit 100 may be a parabola antenna rather than a flat antenna. Also, as the number of antenna elements increases, a position of a satellite can be tracked more accurately. The second power divider 320, first and second tuners 330 and 340 of the control board 300 can be replaced with a single tuner capable of tuning to a plurality of channels. Instead of the expensive rotary joint 500, a coaxial cable can be used. Therefore, the invention is not limited to the above-described embodiments and it will be evident to those skilled in the art that various modifications may be made thereto within the scope of the appended claims and their equivalents.

Industrial Applicabilitv As described above, the satellite tracking system using a frequency scanning effect according to the present invention is simplified and reduces the manufacturing cost while accurately tracking a position of a satellite, by designing its antenna unit such that an antenna beam is tilted according to channel frequencies, rather than by adjusting a phase of an antenna beam through system control.