Patent Publication Number: US-2004051676-A1

Title: Signal cross polarization system and method

Description:
1. RELATED U.S. APPLICATIONS  
     [0001] This application claims the benefit of U.S. Provisional Patent Application Serial No. 60/407,164, filed Aug. 30, 2002 and entitled PROCESS OF CROSS POLARIZING A LINEAR POLARIZED SATELLITE SIGNAL USING AN ADJACENT SATELLITE SIGNAL, which is incorporated herein by reference. 
    
    
     
       BACKGROUND OF THE INVENTION  
       [0002] 1. Field of the Invention  
       [0003] The present invention relates to wireless communication. More specifically, the present invention relates to a system and method for cross polarizing a linear polarized satellite signal to facilitate communication between a satellite and a ground-based antenna.  
       [0004] 2. Description of Related Art  
       [0005] Wireless communication is a continuously expanding field that removes many barriers to communication. Most notably, the communicating parties need not be physically connected together via wires or the like; rather, one or both communicating parties may move relatively freely. Satellites have been especially important for providing information and services such as global position data, television programs, and Internet access.  
       [0006] Many such satellites are in geostationary orbit at an elevation of about 22, 500 miles above the Equator. Satellites in geostationary orbit travel around the Earth at a rate of one cycle per day, and thus remain substantially stationary with respect to their longitudinal positions over the Equator. The orbit followed by geostationary satellites is often called the “Clarke Belt.” The satellites generally have antennas in the form of dishes physically oriented along lines generally tangent to the Clarke Belt so that the dishes transmit signals directly toward the Earth. Generally, ground-based antennas are disposed parallel to their satellite-mounted counterparts in order to permit the antennas to communicate with each other via microwave signals. A ground-based antenna may be rotated about an elevation axis and an azimuth axis to bring the ground-based antenna to an orientation parallel to that of the satellite antenna.  
       [0007] Many satellites transmit and/or receive a linear polarized signal. A polarized signal is generally transmitted along two orthogonal planes, so that the satellite is able to transmit at a bandwidth twice as large as would otherwise be available. In order to properly and efficiently receive such signals, a ground-based antenna must not only be oriented parallel to the satellite antenna via the azimuth and elevation axes, but the ground-based antenna must also be rotated about a skew axis orthogonal to the elevation and azimuth axes to rotationally align the ground-based antenna with the satellite antenna. The ground-based antenna is thus able to properly receive each part of the polarized signal. The process of aligning the ground-based antenna with the satellite antenna via rotation about the skew axis is termed “cross polarization.” Proper cross polarization is required by the FCC.  
       [0008] Unfortunately, determining the exact skew orientation of the satellite antenna can be rather difficult. Due to the gravitational pulls of the sun and the moon, as well as solar weather, satellite positioning must be periodically adjusted to maintain geostationary orbit. In order to conserve fuel, geostationary satellites typically make such adjustments in a manner that keeps them within a specified window, such as a square region seventy miles long and seventy miles wide. Thus, the exact position of the satellite may not be known when the earth-based antenna is set up. In addition to such positional variation, geostationary satellites are known to wobble in orbit by as much as three to four degrees.  
       [0009] Furthermore, a variety of other effects can distort or interfere with signals transmitted between the satellite and the ground-based antenna. For example, solar flares pass through the atmosphere and, in doing so, create magnetic fluctuations of the magnetosphere so intense that the magnetosphere becomes an elongated oval with a length-to-width ratio larger than three-to-one for over an hour. Such magnetic distortion can bend microwave signals. Furthermore, the ionosphere and troposphere have refractive properties that can cause temporary localized effects that are also capable of interfering with microwave signals.  
       [0010] The above-described factors make the skew axis orientation of a satellite antenna somewhat unpredictable. Hence, known cross polarization methods often involve trial and error. According to one known method, a ground-based antenna is first aligned parallel to the satellite antenna, and communication is attempted. A Satellite Operation Center in communication with the satellite provides feedback to the ground-based antenna to suggest adjustments to the skew orientation of the antenna based on known satellite, atmosphere, or magnetosphere conditions or based on analysis of the quality of the signal from the ground-based antenna. Further transmissions may be attempted and additional adjustments may be made accordingly.  
       [0011] The above-described procedure is disadvantageous in a number of respects. First, it is time consuming. Several hours may be required to cross polarize the ground-based antenna. This is particularly problematic for vehicle-mounted systems because each time the vehicle moves, the additional set-up time is required, during which the vehicle is unable to communicate. Furthermore, communication with the Satellite Operation Center is required. Such communication adds an additional point of failure to the satellite network and requires some of the network bandwidth to maintain cross polarization operations.  
       SUMMARY OF THE INVENTION  
       [0012] The apparatus of the present invention has been developed in response to the present state of the art, and in particular, in response to the problems and needs in the art that have not yet been fully solved by currently available cross polarization systems and methods. Thus, it is an overall objective of the present invention to provide a signal cross polarization system and method that remedy the shortcomings of the prior art.  
       [0013] To achieve the foregoing objective, and in accordance with the invention as embodied and broadly described herein in the preferred embodiment, a network may include first and second satellites in geostationary orbit around the Earth, and a communication station. The first and second satellites are displaced from the center of the Earth by a first vector and a second vector, respectively.  
       [0014] The first and second vectors may not be initially available to the communication station; however, each of the first and second satellites has a drift area within which the satellite must be disposed, and these drift areas are available to the communication station. Each of the first and second satellites thus has a window, comprising a space extending from the Earth&#39;s center to the drift area, within which the corresponding vector must be disposed. Each of the first and second satellites has a tangent to the Clark Belt, which is the direction along which the corresponding satellite antenna (e.g., dish), is oriented.  
       [0015] The antenna of the communication station is to be disposed parallel to the antenna of the satellite with which it communicates. Hence, orientation of the antenna perpendicular to the first or second vectors orients the antenna for communication with the first or second satellites, respectively. Orientation of the antenna structure parallel to the corresponding first or second tangent provides the proper skew angle for cross polarization of the antenna of the communication station. Hence, if the communication station is to communicate with the first satellite, the antenna of the communication station should be oriented parallel to the first tangent for proper cross polarization. This refers to orientation of the antenna structure itself; not the direction along which signals are received by the antenna.  
       [0016] If the antenna is coupled to two LNB&#39;s (low noise, block down conversion devices), the antenna may be oriented parallel to the first tangent automatically by obtaining the first and second vectors. The LNB&#39;s are disposed such that the antenna can be oriented to simultaneously receive first and second signals from the first and second satellites, respectively. Thus, the antenna is first pointed at the first window via rotation of the antenna about the elevation and azimuth axes. The antenna is then moved until a peak signal is found. The orientation of the antenna that provides the peak signal within the first window is the first vector.  
       [0017] The antenna is then rotated about the skew axis until the antenna points at the second window. The antenna is further rotated about the skew axis until a peak signal is found. The peak signal within the second window is the second vector. The antenna is then aligned at the proper skew angle for cross polarization of the first signal from the first satellite.  
       [0018] Alternatively, if the antenna is only coupled to a single LNB, vector mathematics can be used to obtain the proper skew angle. The first tangent can be obtained by first obtaining the first and second vectors. The first and second vectors are obtained by pointing the antenna along the first and second windows, and moving the antenna until a peak signal is found. The vector along which the antenna points when the signal peaks within the first window is the first vector and the vector along which the antenna points when the signal peaks within the second window is the second vector. The first and second vectors are then processed, i.e., via vector subtraction or the like, to obtain a third vector extending between the first and second satellites.  
       [0019] The first vector is within the plane of the Clark Belt but is offset from the tangent to the first satellite by an angle. The angle is half the angle between the first and second vectors. Thus, the third vector can be offset by half the angle between the first and second vectors to obtain the first tangent. The skew angle is then provided by the third tangent.  
       [0020] The above-described methods may be carried out through the use of computer code stored within a control unit of the communication station. The control unit may be coupled to an LNB (and a second LNB, if one is present), a computer, a sensor array attached to the antenna, and a motor array disposed to rotate the antenna about the elevation, azimuth, and skew axes. Thus, the control unit can be initiated and/or controlled via the computer, assess signal strength from one or both LNB&#39;s, receive position and orientation data, and provide motor control signals. The control unit may accordingly have components such as an RF receiver/ADC (analog-to-digital converter), NIC (network interface card), sensor signal receiver/ADC, processor, memory, and motor controller/DAC (digital-to-analog converter). The components may be digitally linked via a bus.  
       [0021] The computer code may be stored within the memory of the control unit. The computer code may include modules such as a window acquisition module that acquires the first and second windows based on sensor data, and a tuning module that determines the first and second vectors within the first and second windows, respectively. If only a single LNB is used, the computer code may include the above plus a vector manipulation module that mathematically uses the first and second vectors to obtain the third vector, and an arc adjustment module that adjusts the third vector to obtain the skew angle.  
       [0022] Through the use of the apparatus and method of the invention, satellite signal cross polarization may be more rapidly and/or accurately accomplished, without involvement from a satellite operation center. These and other features and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0023] In order that the manner in which the above-recited and other features and advantages of the invention are obtained will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:  
     [0024]FIG. 1 is a perspective view of a network including a plurality of satellites in geosynchronous orbit and an Earth-based communication station;  
     [0025]FIG. 2 is a schematic block diagram of the communication station of FIG. 1;  
     [0026]FIG. 3 is a schematic block diagram illustrating various hardware components of the control unit of the communication station of FIG. 1;  
     [0027]FIG. 4 is a logical block diagram depicting cross polarization of the antenna of FIG. 1;  
     [0028]FIG. 5 is a flowchart diagram illustrating a cross polarization method that may be carried out in the logical block diagram of FIG. 4;  
     [0029]FIG. 6 is a logical block diagram depicting cross polarization of an antenna of a communication station according to one alternative embodiment of the invention; and  
     [0030]FIG. 7 is a flowchart diagram illustrating a cross polarization method that may be carried out in the logical block diagram of FIG. 6. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0031] The presently preferred embodiments of the present invention will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout. It will be readily understood that the components of the present invention, as generally described and illustrated in the figures herein, could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of the embodiments of the apparatus, system, and method of the present invention, as represented in FIGS. 1 through 7, is not intended to limit the scope of the invention, as claimed, but is merely representative of presently preferred embodiments of the invention.  
     [0032] For this application, the phrases “connected to,” “coupled to,” and “in communication with” refer to any form of interaction between two or more entities, including mechanical, electrical, magnetic, electromagnetic, and thermal interaction. The phrase “attached to” refers to a form of mechanical coupling that restricts relative translation or rotation between the attached objects. The terms “rotate” and “pivot” are used interchangeably to refer generally to turning about an axis; neither term implies any limitation of the angle through which rotation is able to occur.  
     [0033] Referring to FIG. 1, a perspective view illustrates a network  10  in which the cross polarization system and method of the present invention may be employed. FIG. 1 depicts the Earth  12 , which has a center  14  and an Equator  16 . The Clark Belt  18  is also shown encircling the Equator  16 . The Earth  12  and the Clark Belt  18  are shown by way of illustration, and may not be to scale in FIG. 1.  
     [0034] As shown, the network  10  includes a first transmitter  30 , a second transmitter  32 , and a communication station  34 . The invention is usable with a wide variety of wireless transmission systems, including satellites and ground-based antennas. In the embodiment of FIG. 1, the first transmitter  30  is a first satellite and the second transmitter is a second satellite  32 . The first and second satellites  30 ,  32  are disposed in geosynchronous orbit around the Earth  12 , and are thus positioned in the Clark Belt  18 , as shown. The communication station  34  is disposed at some arbitrary point on the surface of the Earth  12 .  
     [0035] The present invention provides a system and method whereby the communication station  34  may be rapidly and easily configured to communicate with a transmitter such as the first satellite  30  or the second satellite  32 . The communication station  34  may, according to one example, be mounted on a vehicle. The communication station  34  must therefore be reconfigured for communication with the first or second satellite  30 ,  32  each time the vehicle stops moving and communication is desired. In this application, “communication” involving an antenna refers to transmission of a wireless signal to and/or from the antenna.  
     [0036] The first satellite  30  is displaced from the center  14  of the Earth  12  by a first vector  40 , and the second satellite  32  is displaced from the center  14  of the Earth  12  by a second vector  42 . The first and second vectors  40 ,  42  are separated from each other by an angle  43 . When setup of the communication station  34  commences, the first and second vectors  40 ,  42  may not be directly available to the communication station, but may be obtained to provide cross polarization, as will be described hereafter. In this application, a “vector” comprises a geometric displacement of at least two dimensions. A “vector” may be expressed in a variety of coordinate systems including Cartesian, spherical, and cylindrical coordinates.  
     [0037] The first satellite  30  has a first satellite drift area  44  that surrounds its nominal position on the Clark Belt  18 . According to one example, the first satellite drift area  44  may be generally square in shape, and may be on the order of seventy-by-seventy miles in size. The first satellite  30  may be permitted to drift within the first satellite drift area  44  until the first satellite  30  approaches the edge of the first satellite drift area  44 , at which point thrusters may be engaged to return the first satellite  30  to its nominal position at the center of the first satellite drift area. The second satellite  32  similarly has a second satellite drift area  46  that surrounds its nominal position on the Clark Belt  18 .  
     [0038] Due to satellite drift, the first and second vectors  40 ,  42  are not initially known to the communication station. However, the first and second satellite drift areas  44 ,  46  are stationary, and their locations can thus be obtained with reference to the communication station  34  once the position and orientation of the communication station  34  are known. As will be described subsequently, the communication station has sensors that provide position and orientation data to enable the first and second satellite drift areas  44 ,  46  to be located with respect to the communication station  34 .  
     [0039] Location of the first and second satellite drift areas  44 ,  46  provides first and second windows  48 ,  50 . The first window  48  is the space within which the first vector  40  may be disposed, and comprises the volume between the center  14  of the Earth  12  and the first satellite drift area  44 . The first window  48  may comprise a generally inverted pyramidal shape. Similarly, the second window  50  is the space within which the second vector  42  may be disposed, and comprises the volume between the center  14  of the Earth  12  and the second satellite drift area  46 .  
     [0040] As shown, the first satellite  30  has a first tangent  52  to the Clark Belt  18 . The antenna (for example, dish) of the first satellite  30  is oriented generally parallel to the first tangent  52 . Thus, the dish (not shown) faces the Earth such that one of the polarized signals is transmitted within the plane of the Clark Belt  18 , while the other is transmitted substantially perpendicular to the plane of the Clark Belt  18 . The communication station  34  will receive a first signal from the first satellite  30  at maximum strength when the antenna (not shown) of the communication station  34  is disposed parallel to the dish of the first satellite  30 .  
     [0041] Similarly, the second satellite  32  has a second tangent  54  to the Clark Belt  18 , and the antenna of the second satellite  32  is oriented generally parallel to the second tangent  54 . The communication station  34  will receive a second signal from the second satellite  32  at maximum strength when the antenna of the communication station  34  is disposed parallel to the dish of the second satellite  32 .  
     [0042] Consequently, proper cross polarity for receiving the first signal from the first satellite  30  can be obtained by disposing the antenna of the communication station  34  parallel to the first tangent  52 . Similarly, proper cross polarity for receiving the second signal from the second satellite  32  can be obtained by disposing the antenna of the communication station  34  parallel to the second tangent  54 . The first tangent  52  or the second tangent  54  may be obtained via the intermediate step of obtaining a third vector  56  that extends between the first and second satellites  30 ,  32 .  
     [0043] The third vector is offset from each of the first and second tangents  52 ,  54  by an angle  57  equal to half the angle  43  between the first and second vectors  40 ,  42 . Thus, when the first and second vectors  40 ,  42  have been obtained, the third vector  56  may be obtained by processing the first and second vectors  40 ,  42 . The first or second tangent  52 ,  54 , including the offset angle, may then be derived from the third vector  56 . Alternatively, if a desired signal is to be received from a third satellite (not shown) midway between the first and second satellites  30 ,  32 , the third vector  56  will be parallel to the tangent to the third satellite, so the third vector  56  may be used without adjustment to provide the skew angle. The third satellite must simply be angularly halfway between the first and second satellites  30 ,  32 , i.e., the third satellite must bisect the angle  43  between the first and second vectors  40 ,  42 .  
     [0044] As illustrated, the communication station  34  is displaced from the center  14  of the Earth  12  by a communication station location vector  58 . The antenna of the communication station  34  is to be disposed parallel to the antenna of the satellite with which it communicates, regardless of the location of the communication station  34  on the Earth  12 . Thus, the first and second vectors  40 ,  42  may be repositioned for purposes of illustration. This is shown in FIG. 1 in the form of first and second vectors  60 ,  62 , separated by an angle  63 , first and second satellite drift areas  64 ,  66 , first and second windows  68 ,  70 , first and second tangents  72 ,  74 , a third vector  76 , and an angle  77  that are the same as those discussed above, but have the communication station  34  as their origin.  
     [0045] These repositioned vectors and angles may be analyzed to determine the skew angle in the same manner described previously. Hence, the antenna of the communication station  34  is positioned for optimal communication with the first satellite  30  when the first vector  60  is normal to the antenna. Similarly, the antenna of the communication station  34  is positioned for optimal communication with the second satellite  32  when the second vector  62  is normal to the antenna.  
     [0046] Referring to FIG. 2, a schematic block diagram illustrates various components of the communication station  34 . As mentioned previously, the communication station  34  may be mounted on a vehicle (not shown). The term “communication station” is not limited to the combination of elements illustrated in FIG. 2, but may include any component or combination of components that provides wireless communication with at least one polarized signal transmitter. In the embodiment of FIG. 2, the communication station  34  has an antenna  80 , which may be generally dish-like in shape. If desired, the antenna  80  may have a generally rectangular or elliptical, rather than circular, profile.  
     [0047] A first LNB (low noise block down conversion device)  82  is coupled to the antenna  80  so that electromagnetic signals such as microwave signals can bounce from the antenna  80  and be received by the first LNB  82 . The first LNB  82  converts the received electromagnetic signals into an electrical RF signal. A second LNB  84  may operate in a similar manner and may also be coupled to the antenna  80 . In this application, an “antenna” need not necessarily convert wireless signals to electrical signals, but may simply reflect the wireless signals for receipt by a separate device, such as an LNB.  
     [0048] The second LNB  84  may be angled from the first LNB  82  so that the second LNB  84  receives signals from a different angle than the first LNB  82 . For example, the first LNB  82  may receive signals from a source perpendicular to the antenna  80 , while the second LNB  84  receives signals from a source offset from perpendicularity to the antenna  80 . The first and second LNB&#39;s  82 ,  84  may thus be used simultaneously to communicate with two different satellites. If desired, the first LNB  82  may provide two-way communication for Internet access and the second LNB  84  may receive television signals.  
     [0049] The electrical RF signal from the first LNB  82  may be conveyed to an RF splitter  86  that further conveys the RF signal to a modem  88  and to a control unit  90 . The modem  88  may include components such as a mixer/oscillator (downconverter) designed to convert the RF signal to an IF frequency for broadband demodulation, an ADC (analog-to-digital converter), and/or any other components needed to convert the RF signal to digital, computer readable form.  
     [0050] The modem  88  transmits the computer-readable signals to a personal computer  92 . As mentioned previously, the first LNB  82  may be designed to provide Internet access. The personal computer  92  may be connected to the control unit  90  in such a manner that the personal computer  92  can be used to initiate satellite acquisition and/or cross polarization via operation of the control unit  90 , or to modify the operation of the control unit  90 .  
     [0051] The electrical RF signal from the second LNB  84  may be conveyed to an RF splitter  96  that further conveys the RF signal to a television display screen  102  and to the control unit  90 . As mentioned previously, the second LNB  84  may be designed to receive television signals.  
     [0052] The control unit  90  may also be connected to a sensor array  104  attached to the antenna  80 . The sensor array  104  includes sensors such as a GPS (global positioning satellite) receiver, a compass, a level, and a tilt indicator (not shown). The sensor array  104  may thus provide three dimensional location data and three dimensional orientation data so that the disposition of the antenna  80  is fully obtained.  
     [0053] The control unit  90  is also connected to a motor array  106  coupled to the antenna  80  to rotate the antenna  80  about three axes: an azimuth axis  107 , an elevation axis  108 , and a skew axis  109 . The motor array  106  may thus have a plurality of motors, such as rotary electrical motors, linear actuators, or any other known motors and/or actuators. The axes  107 ,  108 ,  109  are shown by arrows in FIG. 2. The azimuth axis  107  extends between the vertical extents of the antenna  80 , the elevation axis  108  extends between the sides of the antenna  80 , and the skew axis  109  is perpendicular to the antenna  80 . Rotation along the azimuth axis  107  and the elevation axis  108  may be used to bring the antenna  80  parallel to the corresponding antenna of the first or second satellites  30 ,  32 , while rotation along the skew axis  109  may be used to cross polarize the first or second signals with the antenna  80 . This concept will be described in greater detail subsequently.  
     [0054] Referring to FIG. 3, a schematic block diagram illustrates the control unit  90  in greater detail. As shown, the control unit  90  may have various components designed to permit the control unit  90  to substantially automatically set up the antenna  80  for communication with the first satellite  30  or the second satellite  32 . The components may include a bus  110 , an RF signal receiver/ADC (analog-to-digital converter)  112 , a NIC (network interface card)  114 , a sensor signal receiver/ADC  116 , a processor  118 , a memory  120 , and a motor controller/DAC (digital-to-analog converter)  122 . The bus  110  may serve to digitally connect the other components of the control unit  90  together.  
     [0055] The RF signal receiver/ADC may receive the RF signals from the first and second LNB&#39;s  82 ,  84  via the splitters  86 ,  96 , and may convert them into digital form for processing. The NIC  114  may be designed to transmit data to and from the personal computer  92 , and may include any of a variety of digital connection types including Ethernet, parallel, serial, USB, USB2, and firewire (IEEE 1394) connections. The NIC  114  may receive commands from the personal computer  92 , such as commands to set up the antenna  80  for communication, to adjust the antenna  80  to enhance communication quality, or to fold the antenna  80  for storage or travel. The NIC  114  may also be used to provide feedback to the personal computer  92 , such as the current status of the antenna  80  and/or the quality and strength of the signals received.  
     [0056] The sensor signal receiver/ADC  116  is coupled to the sensor array  104  to receive position and orientation data from the sensor array  104 . As mentioned previously, the sensor array  104  may include a GPS receiver, a compass, a level, and a tilt indicator that cooperate to provide three dimensional position data and three dimensional orientation data. The position and orientation data are converted to digital form for use in the antenna alignment/cross polarization process.  
     [0057] The processor  118  may comprise any of a number of structures designed to process digital signals. For example, the processor  118  may be a microprocessor, a RISC (reduced instruction set) processor, an ASIC (application specific integrated circuit), or an FPGA (field programmable gate array). The processor  118  generally carries out simple instructions like signal strength logging and comparison, vector mathematics, and the like.  
     [0058] The memory  120  may include RAM (random access memory)  124  and ROM (read only memory)  126 . If desired, the ROM  126  may be true read-only memory such as a PROM (programmable read only memory). Alternatively, EEPROMs (electrically erasable and programmable read only memory), a hard drive, or the like may be used. The ROM  126  may contain executable instructions or other data. The ROM  126  may contain the instructions to perform the methods outlined in connection with the discussion of FIG. 1.  
     [0059] The RAM  124  may contain data such as the position and orientation data, signal strength data for comparison, vector data such as the first and second vectors  60 ,  62 , and the like. The RAM  124  may use any type of rewritable memory, including EEPROMs, DIMM or SIMM modules, or the like. Alternatively, the RAM  124  and the ROM  126  may be integrated, with executable code and operating data stored in the same type of memory.  
     [0060] The motor controller/DAC  122  may include circuitry to receive digital signals from the bus  110  and to convert them into control signals suitable for receipt by the motor array  106 . The control signals may provide position commands, displacement commands, or the like to any given motor of the motor array  106 . If desired, the motor array  106  may provide feedback to the motor controller/DAC  122  to indicate the positions of the motors, thereby enabling further tuning of the motor positions.  
     [0061] In the communication station  34  of FIG. 2, the control unit  90  is configured to operate substantially independently to configure the antenna  80  for communication. In alternative embodiments, some of the functions of the control unit  90  may be moved to the personal computer  92  to simplify the configuration and operation of the control unit  90 . Some of the structures described above may thus be omitted or moved to the personal computer  92 . In certain embodiments, the control unit  90  may be omitted entirely, and all of its functions may be carried out by a personal computer with hardware such as motor control cards and sensor signal receipt cards. In the alternative or in addition to the above, the control unit  90  may be minimized, mounted on the antenna  80 , and/or integrated with the sensor array  104  and/or the motor array  106 .  
     [0062] Referring to FIG. 4, a logical block diagram  140  provides greater detail regarding how the communication station  34  may be configured to communicate with a satellite, such as the first satellite  30  of FIG. 1. Various components of the communication satellite  34 , including the control unit  90 , the sensor array  104 , and the motor array  106 , are illustrated in logical block form.  
     [0063] As shown, the sensor array  104  has a GPS receiver  142  that receives signals broadcast by GPS (global positioning system) satellites (not shown). The sensor array  104  also has a level, tilt indicator, and compass  144 . The level, tilt indicator, and compass  144  provide measurements of the angular displacement of the antenna  80 , which are somewhat analogous to the pitch, roll, and yaw, respectively, of a plane. As shown, the GPS receiver  142  provides an antenna position  146 , and the level, tilt indicator, and compass  144  provides an antenna orientation  148 , to the control unit  90 . As mentioned previously, the antenna position  146  and the antenna orientation  148  may each include three dimensional data.  
     [0064] The antenna position  146  and the antenna orientation  148  are received by a window acquisition module  150 , which may reside within the memory  120  of the control unit  90 , such as within the ROM  126 . The window acquisition module uses the antenna position  146  and antenna orientation  148 , in combination with the known location of the first satellite drift area  44 , to determine the first window  152 , which corresponds to either of the first windows  48 ,  68  of FIG. 1.  
     [0065] The window acquisition module  150  transmits instructions to the motor controller/DAC  122  to initiate motion of the antenna  80  to point to the first window  152 . “Pointing to the first window” comprises orienting the antenna  80  generally normal to some vector, originating from the communication station  34 , within the first window  152 . Orienting the antenna  80  in such a manner comprises rotating the antenna  80  about the azimuth and elevation axes  107 ,  108 . Thus, the instructions are transmitted to an azimuth controller  160  and an elevation controller  162  of the motor controller/DAC  122 . The azimuth controller  160  and the elevation controller  162  send control signals  164 ,  166 , respectively, to an antenna azimuth motor  168  and an antenna elevation motor  170  of the motor controller  10 .  
     [0066] The first window  152  also gets passed to a tuning module  180 , which also resides within the memory  120 . The tuning module  180  receives the first window  152  and initiates motion of the antenna  80  along a pattern to generally point along vectors throughout the first window  152 . The tuning module  180  continuously receives data indicating the strength of the signal received, which may be obtained via the RF signal receiver/ADC  112 . When the signal strength reaches a maximum value within the first window  152 , the tuning module  180  records the vector at which the antenna  80  is pointing. This vector is the first vector  182 , which corresponds to the first vectors  40 ,  60  of FIG. 1. This process may be termed “peaking” the antenna  80  on the first satellite  30 . Although this process may involve trial and error, the peaking process is not the same as the trial and error process traditionally used to obtain the proper skew angle through the use of a satellite operation center.  
     [0067] The tuning module  180  transmits instructions to the motor controller/DAC  122  to trigger motion of the antenna  80  to point at the first vector  182 , or to stop motion of the antenna  80  if the antenna  80  is already pointing at the first vector  182 . Again, the instructions are transmitted to the azimuth controller  160  and the elevation controller  162 . The azimuth controller  160  and the elevation controller  162  again send control signals  164 ,  166 , respectively, to the antenna azimuth motor  168  and the antenna elevation motor  170  to obtain the desired position of the antenna  80 .  
     [0068] The first LNB  82  is used to acquire the first window  152  and the first vector  182 . When the antenna  80  has been oriented to point along the first vector  182 , the first LNB  82  receives the first signal from the first satellite  30  at maximum strength. The window acquisition module  150  then determines the second window  184  via processing of the antenna position  146 , the antenna orientation  148 , and the known location of the second satellite drift area  46 . Instructions are sent to a skew controller  186  of the motor controller/DAC  122  to trigger orientation of the antenna  80 . The skew controller  186  transmits a control signal  188  to an antenna skew motor  190  of the motor array  106  so that the antenna  80  remains pointed along the first vector  182  via the first LNB  82 , and the antenna  80  simultaneously points toward the second window  184  via the second LNB  84 .  
     [0069] The second window  184  is also transmitted to the tuning module  180 , which moves the antenna  80  via rotation only about the skew axis  109  to point along various vectors within the second window  184 . When the signal strength reaches a peak within the second window  184 , the tuning module  180  records the vector at which the antenna  80  is pointing via the second LNB  84 . This is a second vector  194 , which corresponds to the second vectors  42 ,  62  of FIG. 1. The tuning module  180  transmits instructions to the skew controller  186 , and the skew controller  186  transmits a control signal  188  to the antenna skew motor  190  to move the antenna  80  such that the antenna  80  points along the second vector  194  via the second LNB  84 .  
     [0070] Once the antenna  80  has been rotated along the azimuth, elevation, and skew axes  107 ,  108 ,  109  to simultaneously point along the first and second vectors  182 ,  194 , via the first and second LNB&#39;s  82 ,  84 , respectively, the antenna  80  is disposed at the proper skew angle for receiving the first and second signals from the first and second satellites  30 ,  32 . The first and second LNB&#39;s  82 ,  84  are angled in such a manner that proper cross polarization is obtained with the first and second satellites  30 ,  32  at the same skew angle of the antenna  80 .  
     [0071] This is accomplished without necessarily processing the first and second vectors  182 ,  194 . Hence, recording the first and second vectors  182 ,  194  is optional because as long as the antenna  80  is pointed toward the first and second vectors  182 ,  194 , proper cross polarization is achieved. Hence, “obtaining” or “determining” the first and second vectors  182 ,  194  need not include recording or processing the first and second vectors  182 ,  194  mathematically. Rather, the first and second vectors  182 ,  194  may be obtained implicitly by pointing the antenna  80  along the first and second vectors  182 ,  194 .  
     [0072] Referring to FIG. 5, a flowchart diagram illustrates the cross polarization method  200 , or method  200 , followed in the logical block diagram  140  of FIG. 4. The method  200  starts  210  with adjusting  212  the azimuth and elevation of the antenna  80  to point parallel to the first window  152 . Then, the azimuth and elevation of the antenna  80  are tuned  214  via motion of the antenna  80 . The strength of the first signal from the first satellite  30  is measured  216 , for example periodically.  
     [0073] If the first signal from within the first window  152  has not peaked  218 , i.e., reached a maximum strength, the method  200  continues with the tuning operation  214  until a peak has been reached. If the first signal from within the first window  152  has peaked  218 , the skew angle of the antenna  80  is adjusted  222  such that the antenna  80  points to the second window  184 . As mentioned in connection with the previous embodiment, when two LNB&#39;s are used, communication may be maintained with the first satellite  30  while the antenna is being oriented about the skew axis  109  to communicate with the second satellite  32 .  
     [0074] The skew angle of the antenna  80  is then tuned  224  by rotating the antenna  80  about the skew axis  109  such that the antenna  80  points to a plurality of vectors within the second window  184 . The strength of the second signal from the second satellite  32  is measured  226 , for example, periodically. If the second signal from within the second window  184  has not peaked  228 , the method  200  continues with the tuning operation  224  until a peak has been reached. If the second signal from within the second window  184  has peaked  228 , the method  200  ends  230  because the antenna  80  has been properly cross polarized to receive the first and second signals from the first and second satellites  30 ,  32 , respectively.  
     [0075] The accuracy of the skew angle is depends upon how far apart the first and second satellites  30 ,  32  are. Greater angular displacement between the first and second satellites  30 ,  32  provides a greater accuracy. An angular displacement of fifteen degrees, for example, results in a skew angle with an error of less than about plus or minus 0.6 degrees. Total cross polarization error, including satellite wobble and drift, should then be less than about plus or minus two degrees. The angular displacement of the first and satellites  30 ,  32 , with respect to the antenna  80 , is determined by the positioning of the first and second LNB&#39;s  82 ,  84 . However, in the following method, only the first LNB  82  is used, and enables determination of the skew angle based on satellites with a larger or smaller angular displacement.  
     [0076] Referring to FIG. 6, a control unit  290  according to one alternative embodiment of the invention is illustrated. The control unit  290  has a memory  320  analogous to the memory  120  of the control unit  90  described previously. The control unit  290  may be incorporated into a communication station (not shown) that is like the communication station  34  of FIG. 2, except for the differences in the control unit  290 , which will be set forth in greater detail below, and the fact that only a single LNB (such as the first LNB  82  of FIG. 2) is included.  
     [0077]FIG. 6 illustrates a logical block diagram  340  in which the control unit  290  is incorporated. As shown, a sensor array  104  like that of FIG. 4 is coupled to the control unit  290 . The sensor array  104  has a GPS receiver  142  and a level, tilt indicator, and compass  144 , which provide the antenna position  146  and the antenna orientation  148 , respectively, to the control unit  290 . The control unit  290  has a window acquisition module  150  like that of the previous embodiment. The window acquisition module  150  is stored in the memory  320 , and operates in a manner substantially similar to the window acquisition module  150  of FIG. 4. Hence, the window acquisition module  150  processes the antenna position  146  and the antenna orientation  148 , in combination with the known first satellite drift area  44 , to provide the first window  152 .  
     [0078] As in the logical block diagram  140  of FIG. 4, the window acquisition module  150  instructs the azimuth controller  160  and the elevation controller  162  of the motor controller/DAC  122  to orient the antenna  80  to point toward the first window  152 . The azimuth controller  160  and the elevation controller send control signals  164 ,  166  to the antenna azimuth motor  168  and the antenna elevation motor  170  to induce rotation of the antenna  80 .  
     [0079] The first window  152  is also conveyed to a tuning module  350  that receives the first window  152  and instructs the azimuth controller  160  and the elevation controller  162  to move the antenna  80  to point along a plurality of vectors within the first window  152 . The tuning module  350  receives signal strength data and instructs the azimuth controller  160  and the elevation controller  162  to move to a first vector  182  (or stop moving at the first vector  182 ). Again, control signals  164 ,  166  are sent to the antenna azimuth motor  168  and the antenna elevation motor  170  to induce rotation of the antenna  80 . The first vector  182  is the vector within the first window  152  along which the signal strength is maximized.  
     [0080] The window acquisition module  150  then obtains the second window  184  in a manner similar to that of the first window  152 . More precisely, the window acquisition module  150  uses the antenna position  146  and the antenna orientation  148 , in combination with the known second satellite drift area  46 , to provide the second window  184 . Since only the first LNB  82  is present, the antenna  80  cannot communicate with multiple satellites simultaneously. Accordingly, the antenna  80  must be moved between communication with the first satellite  30  and communication with the second satellite  32 . The azimuth controller  160  and the elevation controller  162  are instructed to initiate motion of the antenna to point to the second window. Control signals  164 ,  166  are transmitted to the antenna azimuth motor  168  and the antenna elevation motor  170  to rotate the antenna  80  accordingly.  
     [0081] The second window  184  is also conveyed to the tuning module  350 , which moves the antenna  80  to point along a plurality of vectors within the second window. When the second vector  194 , i.e., the vector within the second window  184  along which the greatest signal strength is received, is determined by the tuning module  350 , the first and second vectors  182 ,  194  are conveyed to a vector manipulation module  360  that mathematically manipulates the first and second vectors  182 ,  194  to obtain a third vector  362 , which is analogous to the third vectors  56 ,  76  illustrated in FIG. 1.  
     [0082] The third vector  362  extends from the first satellite  30  to the second satellite  32 . The third vector  362  may, for example, be obtained by subtracting the first vector  182  from the second vector  194 . The third vector  362  is then conveyed to an arc adjustment module  370  that adds an offset to the third vector  362  to obtain the first tangent  52  (shown in FIG. 1). As mentioned previously, the offset is the angle  57  or  77 , as shown in FIG. 1, which is readily determined because it is half the angle  43  or  63 . The angle  43  or  63  between the first and second vectors  182 ,  194  is easily determined via vector mathematics.  
     [0083] The first tangent  52  is disposed at the skew angle; thus, finding the first tangent  52  results in obtaining a skew angle  372  to which the antenna  80  is to be rotated about the skew axis  109  to provide proper cross polarization. The skew controller  186  of the motor controller/DAC  122  is instructed to dispose the antenna  80  at the skew angle  372 . Hence, the skew controller  186  transmits a control signal  188  to the antenna skew motor  190  of the motor array  106 . The antenna  80  is then rotated to the skew angle  372  for cross polarization.  
     [0084] Referring to FIG. 7, a flowchart diagram illustrates a cross polarization method  400 , or method  400 , that may be followed by the logical block diagram  340  of FIG. 6. As shown, the method  400  starts  210  with adjusting  212  the azimuth and elevation of the antenna  80  to point parallel to the first window  152 . Then, the azimuth and elevation of the antenna  80  are tuned  214  via motion of the antenna  80 . The strength of the first signal from the first satellite  30  is measured  216 , for example periodically.  
     [0085] If the first signal from within the first window  152  has not peaked  218 , i.e., reached a maximum strength, the method  400  continues with the tuning operation  214  until a peak has been reached. If the first signal from within the first window  152  has peaked  218 , the azimuth and elevation along which the peak signal was obtained are recorded  410  to obtain the first vector  182 .  
     [0086] Then, the azimuth and elevation of the antenna  80  are adjusted  412  to point parallel to the second window  184 . The azimuth and elevation of the antenna  80  are tuned  414  via motion of the antenna  80 , and the strength of the second signal from the second satellite  32  is measured  416 , for example periodically.  
     [0087] If the second signal from within the second window  184  has not peaked  418 , i.e., reached a maximum strength, the method  400  continues with the tuning operation  414  until a peak has been reached. If the second signal from within the second window  184  has peaked  418 , the azimuth and elevation along which the peak signal was obtained are recorded  420  to obtain the second vector  194 .  
     [0088] The first and second vectors  182 ,  194  are then used 430 to obtain a third vector  362  extending from the first satellite  30  to the second satellite  32 . An offset is added  440  to the third vector  362  to provide the skew angle  372 . As mentioned before, the offset is equal to half the angle between the first and second vectors  182 ,  194 . The antenna  80  is then aligned  450  with the skew angle  372  to cross polarize the antenna  80  with respect to the first signal. As an alternative, the third vector  362  may be offset by the same angle in the opposite direction to cross polarize the antenna  80  with respect to the second signal. However, since the antenna  80  has only the first LNB  82 , the antenna  80  cannot simultaneously be properly aligned and/or cross polarized with the first and second satellites  30 ,  32 .  
     [0089] The present invention may be embodied in other specific forms without departing from its structures, methods, or other essential characteristics as broadly described herein and claimed hereinafter. The described embodiments are to be considered in all respects only as illustrative, and not restrictive. The scope of the invention is, therefore, indicated by the appended claims, rather than by the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.