Abstract:
Tracking system for flat mobile antenna, which includes: sensors for angular velocity ( 1 ), which sense the rotation of the antenna around their axes; sensors, sensing the orientation of the antenna according to vertical axis ( 2 ); control block ( 3 ), which calculates necessary corrections of the direction of antenna beam and which is connected to outputs of sensors ( 1, 2 ) and with inputs of driving block ( 4 ) and beam control block ( 5 ); at least one motor ( 7 ), which changes the orientation of the antenna and which is connected to the output of driving block ( 4 ) and which drives the antenna panel ( 8 ); block for electronic beam steering ( 5 ), which is connected to antenna panel ( 8 ); power supply block, which converts the voltage from the electrical network of the vehicle into suitable values for providing of power supply of all blocks of the system.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a tracking system for flat antenna with combined mechanical pointing for one axis and electronic beam steering for the other axis, which can be mounted on moving vehicles and platforms and which provides television, Internet and other communication signals, transmitted by satellites.  
       PRIOR ART  
       [0002]     In order to receive signals from satellite, the antenna mounted on a moving vehicle should keep its orientation, so that its beam to be always directed toward the satellite. For this purpose antennas with mechanical or/and electronic beam steering are required, which work under control of a tracking system, which reacts to the changes of orientation of the vehicle (or directly of the antenna) and issues commands to motors or to block for electronic beam control to provide necessary corrections in beam direction. In most cases such tracking systems include sensors for angular velocity (rate gyroscopes), based on quartz, piezo-electric or micro electro-mechanical (MEMS) technology. They feature a low price, but suffer by significant error, produced by temperature change, linear acceleration and other factors on their offset and scale factor. That&#39;s why a key element of different tracking systems, which use such sensors, are different methods for correction of antenna orientation and gyroscope errors, using the measured strength of received signal. Most widely used methods are those for mechanical scanning in a small area around the estimated direction towards the satellite and mono-pulse method.  
         [0003]     U.S. Pat. No. 6,191,734 discloses a system, which uses a combined mechanical control of azimuth of the beam and electronic beam steering on elevation using phase shifters. By using additional phase shifters, beam tilting is obtained at small angles on both axes in order to achieve higher scanning rate. The disadvantage of the mentioned system is the requirement for additional phase shifters, which makes it more complex and expensive. Moreover, the tracking system does not perform an estimation of gyroscope errors, hence, the antenna may lose its direction shortly after the signal path is hidden by an obstacle.  
         [0004]     U.S. Pat. No. 5,900,836 describes an antenna with mechanical drive. Its tracking system commands rotating in a given direction while a decreasing of signal strength is registered, after which the direction of movement is reversed. At particular intervals the movements in both directions are averaged and they are used later for correction of gyroscope offset. Due to big latencies of mechanical driving system the averaging intervals of such system are relatively large, which means that gyroscope offset corrections follow with big delays. Moreover, such a system can be affected by signal fluctuations due to small obstacles which don&#39;t completely interrupt the signal, but only decrease its strength for a short time.  
         [0005]     U.S. Pat. No. 5,309,162 is provided for antenna system, where the control of the motor for every axis is performed using the phase difference between the signals, received by two antenna panels. This system does not use orientation sensors, that&#39;s why it cannot keep its orientation when the signal reception is interrupted by an obstacle.  
       SUMMARY OF THE INVENTION  
       [0006]     An objective of the present invention is to provide a tracking system with low price, which keeps the antenna beam in the direction towards the selected satellite, regardless of the movement of the vehicle, where it is mounted. It is also desirable that the antenna orientation is kept even during temporary interruptions of the reception due to obstacles non-transparent to radio waves like buildings, trees, tunnels, bridges, hills, etc.  
         [0007]     To achieve these and other advantages and in accordance with the purpose of the present invention the tracking system for flat mobile antenna comprises:  
         [0008]     sensors for angular velocity (gyroscopes);  
         [0009]     sensors, which measure the orientation of the antenna towards the vertical axis (inclinometers);  
         [0010]     control block, which calculates needed corrections of the direction of antenna beam, depending on measurement from above-mentioned sensors;  
         [0011]     at least one motor, which changes the antenna orientation;  
         [0012]     driving electronics, which drive the motor/s in order to move the antenna in desired direction;  
         [0013]     block for electronic beam steering;  
         [0014]     power supply block, which converts the voltage from the electrical network of the vehicle into suitable values for providing power supply to all blocks of the system.  
         [0015]     It is advisable to use three sensors for angular velocity, each one mounted in parallel to one of the axes of Cartesian coordinate system, fixed with the antenna panel.  
         [0016]     It is advisable in this case to perform a forward coordinate transformation to obtain the necessary corrections of azimuth and elevation angle of antenna beam and reverse coordinate transformation for obtaining corrections of gyroscope offset.  
         [0017]     In one particular variant of the tracking system the axes of two gyroscopes lay on the elevation plane while the axis of the third one is orthogonal to it.  
         [0018]     It is advisable that the antenna panel is moved mechanically in a small angle at one axis of deflecting of antenna beam, while the antenna beam is positioned by electronic control at fixed positions on the other axis and the signal strength, measured in two or more positions in a close proximity to the direction toward the satellite is used for calculation of corrections of gyroscope offsets and for adjusting the orientation of the antenna beam.  
         [0019]     It is advisable in this case that the antenna beam is controlled in such a way, that it keeps staying for a longer period in the fixed position, which is closest to the direction towards the satellite, while it is switched to neighboring fixed positions for much shorter intervals, thus keeping the average signal strength as high as possible.  
         [0020]     It is advisable to perform an additional compensation of offsets for gyroscopes, which axes are horizontal or close to horizontal plane.  
         [0021]     One possible variant of implementing such compensation is to integrate for a given time interval the output value of any one of gyroscope sensors, which axes are horizontal or close to horizontal plane, and if the result is positive, the offset of the corresponding gyroscope is corrected in positive direction, but if the result is negative, the offset is corrected in negative direction.  
         [0022]     Another possible variant of such compensation is to transform the output signals of both gyroscopes into angular velocity vectors, which lie on the horizontal plane, and which sectors are integrated to obtain the inclination angles of the axes of both gyroscopes, which angles are compared with output signals of two inclination sensors (inclinometers), measuring the inclination angles of axes of mentioned gyroscopes, and the result from this comparison is used to adjust the estimated offsets of mentioned gyroscopes.  
         [0023]     The advantages of the tracking system according to the present invention are: using low-cost sensors, decreased impact of gyroscope errors on tracking error, improved speed of correction of gyroscope errors using measured strength of received signal in comparison with antennas with mechanical-only beam movement, improved average signal-to-noise ratio (SNR) due to longer periods when the antenna beam is kept close to direction toward the satellite.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]      FIG. 1  is a block diagram of the tracking system according to the current invention;  
         [0025]      FIG. 2  is a drawing, showing the orientation of gyroscopes in Cartesian coordinate system, fixed with the antenna panel;  
         [0026]      FIG. 3  is an illustration of the orientation of inclination sensors toward axes of Cartesian coordinate system, fixed with the antenna panel and their projections on the horizontal plane;  
         [0027]      FIG. 4  is a diagram, which illustrates angular velocity vectors, corresponding to axes of Cartesian coordinate system, fixed with the antenna panel;  
         [0028]      FIG. 5  represents a variant of implementation of the control block of tracking system, according to present invention;  
         [0029]      FIG. 6  is a block diagram of one variant of implementing the compensation of offset value of one of gyroscopes, which axis is close to horizontal plane;  
         [0030]      FIG. 7  is a block diagram of another variant of implementing the compensation of offset value of gyroscopes, which axes are close to horizontal plane. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0031]     One possible variant of implementation of tracking system according to the present invention is shown at  FIG. 1 . It comprises sensors for angular velocity (gyroscopes)  1 , inclination sensors  2 , control block  3 , driving block  4 , motor  7 , block for electronic beam control  5 , antenna panel with electronic beam steering  8 , down-converter  9 , directional coupler  10 , decoding block  11 , and RF detector  6 .  
         [0032]     The outputs of gyroscope sensors  1  and outputs of inclination sensors  2  are fed to control block  3 , as well as the outputs of RF detector  6  and block for decoding of received signal  11  are. One of the outputs of the control block  3  is fed to the input of the block for electronic beam control  5 , while the other output is fed to the input of the driving block  4 , which output is fed to the motor  7 , which moves the antenna panel  8 . The antenna panel  8  output is fed to the input of the down-converter  9 , which output is connected to inputs of RF detector  6  and the decoding block  11  through directional coupler  10 .  
         [0033]     The angular velocity sensors could be solid-state gyroscopes, for example based on quartz, piezo-crystal, MEMS or another technology. They are fixed to antenna panel  8  and provide signals, which are proportional to its speed of rotation around particular axes. In the present embodiment are used three gyroscopes  1   a ,  1   b  and  1   c , ( FIG. 2 ), which are co-linear with the axes of Cartesian coordinate system Oxyz, fixed to antenna panel  8 .  
         [0034]     The inclination sensors  2  could be solid-state, liquid-based or devices, based on another principle, which allow electronic measuring of the inclination of antenna panel toward the horizontal plane. In the present variant two inclination sensors,  2   a  and  2   b , are used, which are fixed to antenna panel  8 . It is also possible to use a two-axis sensor, which measures the inclination of two of antenna panel axes towards the horizontal plane ( FIG. 3 ). It is desirable that these axes are the same as the axes of two of the gyroscopes  1   a  and  1   b , as this simplifies the data processing by the control block.  
         [0035]      FIG. 5  is a block diagram of the data processing, performed by the control block during satellite tracking. The output signal from gyroscope  1   a  is fed to the first input of subtracting block  12 , to which second input the output signal from offset memory ω x0    13  is fed. The output signal from gyroscope  1   b  is fed to the first input of subtracting block  14 , to which second input the output signal from offset memory ω y0    15  is fed. The output signal from gyroscope  1   c  is fed to the first input of subtracting block  16 , to which second input the output signal from offset memory ω z0    17  is fed. The output signals from subtracting blocks  12 ,  14  and  16  are fed to the inputs of the block for forward coordinate transformation  18 . The first output of the block for forward coordinate transformation  18  is fed to the first input of adder  19 , which second input is fed to the output of scaling block  20 . The second output of the block for forward coordinate transformation  18  is fed to the first input of adder  21 , which second input is fed to the output of scaling block  22 . The output of adder  19  is fed to the input of integrator  23 , which output is fed to the first input of subtracting block  24 . The second input of subtracting block  24  is fed to the output of azimuth scanning block  25 . The output of subtracting block  24  is fed to the input of motor control block  26 , which output is fed to the input of driving block  4 . The output of adder  21  is fed to the input of integrator  27 , which output is fed to the input of beam selection block  28 . The output of beam selection block  28  is fed to the input of the block for electronic beam control  5 .  
         [0036]     The signal from RF detector  6  is fed to the input of switch  29 , input of signal drop detector  30  and input of sign inverter block  31 . The first output of switch  29  is fed to the input of first beam memory  32 , while its second output is fed to the input of second beam memory  33 . The output of the first beam memory  32  is fed to the first input of computation block  34 , which second input is fed to the output of second beam memory  33 . The output of the computation block  34  is fed to the first output of subtracting block  35 , which second input is fed to the output of integrator  27 . The output of subtracting block  35  is fed to the first input of block for reverse coordinate transformation  36 .  
         [0037]     The output of sign inverter block  31  is fed to the input of integrator  37 , which output is fed to the input of memory block  38 . The output of memory block  38  is fed to the second input of block for reverse coordinate transformation  36 . The first output of block for reverse coordinate transformation  36  is fed to the input of sign detection block  39 , which output is fed to the input of scaling block  40 . The second output of block for reverse coordinate transformation  36  is fed to the input of sign detection block  41 , which output is fed to the input of scaling block  42 . The third output of block for reverse coordinate transformation  36  is fed to the input of sign detection block  43 , which output is fed to the input of scaling block  44 .  
         [0038]     The output of scaling block  40  is fed to the second input of subtracting block  45 , which first input is fed to the output of offset memory ω x0    13 . The output of subtracting block  45  is fed to the input of offset memory ω x0    13 . The output of scaling block  42  is fed to the second input of subtracting block  46 , which first input is fed to the output of offset memory ω y0    15 . The output of subtracting block  46  is fed to the input of offset memory ω y0    15 . The output of scaling block  44  is fed to the second input of subtracting block  47 , which first input is fed to the output of offset memory ω z0    17 . The output of subtracting block  47  is fed to the input of offset memory ω z0    17 .  
         [0039]      FIG. 6  presents a block diagram of one variant for additional compensation of one of gyroscopes  1   a , which axis is near to horizontal plane. A similar compensation is used for the second gyroscope  1   b . The output signal of gyroscope  1   a  is fed to the first input of subtracting block  48 , which second input is fed to the output of offset memory ω x0    13 . The output of subtracting block  48  is fed to the signal input of integrator  49 , which reset input is fed to the first output of timer block  50 . The output of integrator  49  is fed to the input of sign detector block  51 , which output is fed to the input of scaling block  52 . The output of scaling block  52  is fed to the signal input of switch  53 , which control input is fed to the second output of timer block  50 . The output of switch  53  is fed to the first input of adder  54 , which second input is fed to the output of offset memory ω x0    13 . The output of adder  54  is fed to the input of offset memory ω x0    13 .  
         [0040]      FIG. 7  presents a block diagram of another variant for additional compensation of gyroscopes  1   a  and  1   b , which axes are close to horizontal plane. The output of gyroscope  1   a  ω x  is fed to the first input of subtracting block  55 , which second input is fed to the output of offset memory ω x0    13 . The output of subtracting block  55  is fed to the first input of the first block for coordinate transformation  56 , which second input is fed to the output of integrator  57 . The output of the first block for coordinate transformation  56  is fed to the first input of adder  58 , which second input is fed to the output of scaling block  59 . The output of adder  58  is fed to the input of integrator  60 . The first input of subtracting block  61  is fed to the output of inclinometer  2   b , while its second input is fed to the output of integrator  60 . The output of subtracting block  61  is fed to inputs of scaling blocks  59  and  62 . The first input of subtracting block  63  is fed to the output of offset memory ω x0    13 , while its second input is fed to the output of scaling block  62 . The output of subtracting block  63  is fed to the input of offset memory ω x0    13 .  
         [0041]     The output of gyroscope  1   b  ω y  is fed to the first input of subtracting block  64 , which second input is fed to the output of offset memory ω y0    15 . The output of subtracting block  64  is fed to the first input of the second block for coordinate transformation  65 , which second input is fed to the output of integrator  60 . The output of the second block for coordinate transformation  65  is fed to the first input of adder  66 , which second input is fed to the output of scaling block  67 . The output of adder  66  is fed to the input of integrator  57 . The first input of subtracting block  68  is fed to the output of inclinometer  2   a , while its second input is fed to the output of integrator  57 . The output of subtracting block  68  is fed to inputs of scaling blocks  67  and  69 . The first input of subtracting block  70  is fed to the output of offset memory ω y0    15 , while its second input is fed to the output of scaling block  69 . The output of subtracting block  70  is fed to the input of offset memory ω y0    15 .  
         [0042]     The operation of the tracking system according to present invention is as follows:  
         [0043]     The control block  3  operates in two modes—acquisition and tracking. During acquisition mode the motor  7  is commanded to rotate the antenna panel  8  around vertical axis with particular velocity. At the same time the block for electronic beam control  5  switches consecutively the antenna beams to cover the whole field of view of the antenna, while the RF detector  6  measures the strength of received signal. The described action continues until a local maximum of the signal strength is found. After that the decoding block  11  reads the identification data from the transport stream of received signal, which is compared with a defined value by the control block  3 . If the received data do not match the defined value, the control block  3  continues the acquisition mode. When the identification data match the internal value, the control block  3  enters tracking mode. In this mode the control block  3  uses the signals from gyroscope block  1  and RF detector  6  to calculate the changes in antenna panel orientation towards the satellite direction, and issues the necessary control signals to the driving block  4  and the block for electronic beam control  5  to keep the antenna beam always directed towards the satellite.  
         [0044]     The detailed description of the operation is as follows:  
         [0045]     The contents of the offset memory ω x0    13  is subtracted from the signal value ω x  of the first gyroscope  1   a . The initial value, which is contained in the offset memory ω x0    13  can be obtained by measurement of gyroscope signal during stand-still condition or can be read from a table, prepared in advance, which defines the temperature dependence of the offset of particular gyroscope. The same operation is performed for output signals ω y  and ω z  of gyroscopes  1   b  and  1   c . The resultant compensated signals correspond to angular velocities of antenna panel towards the three axes of Cartesian coordinate system Oxyz ( FIG. 4 ), which are labeled as ω′ x , ω′ y  and ω′ z . These three values are converted by the block for forward coordinate transformation  18  into angular velocities, collinear to axes of the coordinate system Oxsz, where s is a vector, pointing towards the satellite. As a result both angular velocities on elevation axis ω θ  and on azimuth axis ω φ  are obtained. By their integration by integrators  23  and  27  the deflection angles between the direction to the satellite and antenna panel axes by azimuth φ and by elevation θ are obtained. The azimuth scanning block  25  produces a sine signal with small amplitude and low frequency, which is subtracted from azimuth angle φ. The resultant difference signal is processed by the motor control block  26  in such a way, that the azimuth angle φ is kept approximately equal to the output signal of azimuth scanning block  25 . As a result the motor  7  drives the antenna panel to oscillate slowly around the expected satellite azimuth.  
         [0046]     The sign inverter block  31  either passes with no change the signal from RF detector  6  or inverts its polarity, which is synchronized with the sine signal, produced by azimuth scanning block  25  in such a way, that the signal polarity of RF detector  6  is inverted when the sine signal is negative, or it is passed with no change when the sine signal is positive. The resultant signal at the output of sign inverter block  31  is integrated by integrator  37  for one sine period for the signal of azimuth scanning block  25 . At the end of sine period the result of integrating is stored in the memory  38  and the value of integrator  37  is reset. The result, stored in memory  38  is used as an azimuth error in orientation of the antenna panel ε φ . It is scaled by some coefficient in the scaling block  20  and is added to azimuth angular velocity ω φ .  
         [0047]     The beam selection block  28  determines the two beams, which are closest to estimated elevation angle θ. The beam, which has a minimum distance to estimated elevation angle θ is considered as main beam, while the other—as secondary beam. The beam selection block  28  issues commands to the block for electronic beam control  5 , so that the main beam is selected for a long period, while the secondary beam is selected for a short time, sufficient to measure the signal strength in its direction. The beam selection block  28  synchronizes the switch  29  with beam switching, so that the signal strength of the main beam is stored in the first beam memory  32 , while the signal strength of the secondary beam is stored in the second beam memory  33 . The values, stored in both beam memories  32  and  33  are used by the computation block  34  for calculation of the real elevation angle of the satellite toward the antenna panel θ RSSI . The subtracting block  35  produces the difference between θ RSSI  and obtained by gyroscope measurements elevation angle θ, thus giving as result the elevation error co. It is scaled by some coefficient in the scaling block  22  and is added to elevation angular velocity ω θ .  
         [0048]     Both errors are used by the block for reverse coordinate transformation  36 , which converts them into three components, respectively collinear to axes Ox, Oy and Oz of the Cartesian coordinate system Oxyz ( FIG. 4 ). Every one of them is processed by a corresponding sign detector (respectively  39 ,  41  and  43 ), which gives a result of +1 if the corresponding component is positive or −1 in the case of negative component. The output values from three sign detectors  39 ,  41  and  43  are scaled in corresponding scaling blocks  40 ,  42  and  44 , which in result gives the corrections, which need to be applied to offsets of three gyroscopes ε x , ε y  and ε z . Every correction is subtracted by subtracting blocks, respectively  45 ,  46  and  47 , from the contents of offset memories, respectively  13 ,  15  and  17 . The results from the subtraction are stored back into the same offset memories.  
         [0049]     The signal drop detector  30  checks the strength of received signal. When it drops by more than a specified threshold value, it clears ε φ  and ε θ  errors, to prevent changing of ω x0 , ω y0 , ω z0 , φ and θ due to the noise of detected signal during interruption of signal reception by some obstacle between antenna and satellite. In this case the antenna beam orientation is controlled only by gyroscope signals.  
         [0050]     The described algorithm is able to calculate two independent error values ε φ  and ε θ , based on measurements of the signal strength at different points around the estimated direction to the satellite. However, in the reverse coordinate transformation there is no single solution for the corrections ε x , ε y  and ε z  of three particular gyroscopes  1   a ,  1   b  and  1   c . To resolve this ambiguity additional corrections of offsets of gyroscopes  1   a    1   b , which axes are close to the horizontal plane, are applied.  
         [0051]     One variant of such correction, which does not use an additional sensor, is shown at  FIG. 6 . From output signal of one of gyroscopes  1   a  the value ω x0 , stored in the offset memory  13  is subtracted, which gives as a result the corrected signal ω′ x . It is further integrated by an integrator  49  for a period, determined by timer  50 . The sign detector  51  produces +1 as output signal when the result from integration is positive or −1 in the case of negative result. The resultant value is multiplied by a certain coefficient in scaling block  52  and at the end of integration period is fed by switch  53  to adder  54 , where it is added to the value, stored in offset memory  13 . The result of the addition is stored back into offset memory  13 . At the end of integration period the timer  50  resets the value of integrator  49 . As a result of described actions the stored offset value ω x0  is updated in up or down direction until the number of periods with positive value at integrator output becomes equal to the number of periods with negative value. The same actions are applied for gyroscope  1   b.    
         [0052]     Another variant of correction of offsets of gyroscopes  1   a  and  1   b , which axes are close to the horizontal plane, using inclinometers, is shown at  FIG. 7 . From output signal of one of gyroscopes  1   a  the value ω x0  is subtracted, stored in the offset memory  13 , which gives as a result the corrected signal ω′ x . Using coordinate transformation, performed by block  56  the signal is converted in angular velocity, which vector is collinear to the sense axis of inclinometer  2   b , which measures the inclination of axis Oy towards the horizontal plane. Further, the converted angular velocity is integrated by integrator  60 . The result from integration is compared in subtracting block  61  with the signal from inclinometer  2   b . The difference value is multiplied by certain coefficients in scaling blocks  59  and  62 . The result from scaling block  59  is used for correction of the result from the coordinate transformation in block  56  by adder  58 , while the result from scaling block  62  is subtracted from the value, stored in offset memory  13  by subtracting block  63 .  
         [0053]     The same procedure is applied to the signal of gyroscope  1   b . From its output signal the value ω y0  is subtracted, stored in the offset memory  15 , which gives as a result the corrected signal ω′ y . Using coordinate transformation, performed by block  65 , the signal is converted in angular velocity, which vector is collinear to the sense axis of inclinometer  2   a , which measures the inclination of axis Ox toward horizontal plane. Further, the converted angular velocity is integrated by integrator  57 . The result from integration is compared in subtracting block  68  with the signal from inclinometer  2   a . The difference value is multiplied by certain coefficients in scaling blocks  67  and  69 . The result from scaling block  67  is used for correction of the result from the coordinate transformation in block  65  by adder  66 , while the result from scaling block  69  is subtracted from the value, stored in offset memory  15  by subtracting block  70 .