Abstract:
An antenna alignment apparatus and alignment method is used to effectively position an antenna used in a satellite communication system on the satellite orbital arc, and to ensure that a proper antenna polarization is achieved during installation or alignment of a remote station antenna. The apparatus includes a secondary receiving device mounted on the feed arm of the antenna at a specified distance from the primary receiving device located on the primary plane of the antenna. An antenna using this system and method may be properly aligned on the orbital arc using only simple receiving equipment.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to an antenna pointing apparatus and method used in connection with aligning radio-frequency (RF) emissions from an antenna, and further relates to improving antenna performance in a satellite communication system using only relatively simple equipment. 
     2. Description of the Related Art 
     Typically, an installer desires to maximize or optimize, to the extent possible, a satellite earth station&#39;s performance, including the performance of the antenna which propagates/receives radio-frequency energy to/from one or more satellites. Determining whether the earth station&#39;s antenna&#39;s performance is optimized as much as possible under the particular circumstances is not always a simple matter. Several steps must be accomplished in making that determination. These include, but are not limited to, for example, aligning the earth station antenna in azimuth and elevation so that the antenna “boresight” is pointing directly at a particular satellite of interest, and also adjusting the polarization orientation of the earth station&#39;s antenna so that a cross-polarization isolation parameter measured at a hub station, for example, is acceptable for discriminating between signals having a different polarization than the polarization of a desired signal. 
     Other antenna parameters which may be of interest to installers and operators include gain, noise temperature, voltage standing-wave ratio (VSWR), power rating, receive-transmit group delay, radiation pattern, isolation, and so-called “G/T”, or antenna gain divided by system noise temperature. Many of these parameters are inherent in the fundamental antenna design, while others, such as cross-polarization isolation, may be adjusted to a specified value using an alignment procedure during antenna installation 
     The case of alignment of an earth station&#39;s antenna to a geostationary satellite is a simplified version of the general case of tracking a satellite in a non-geo-synchronous orbit. There are typically two angles of interest between the earth station and the satellite: the elevation angle, and the azimuth angle. The elevation angle is the angle of the satellite above the horizon, as measured from the earth station, and the azimuth is the angle between the line of longitude through the earth station, and the direction of a sub-satellite point, i.e. the point at which a line between the satellite and the center of the earth intersects the earth&#39;s surface. The angle of elevation depends on the latitude of the earth station, and will be relatively large towards the equator, and smaller toward the poles, while the azimuth angle is related to the latitude and longitude of the earth station. Thus, the elevation and azimuth angles define the “boresight” of the receiving antenna for a given receiver location, with respect to a particular geostationary satellite&#39;s orbital position. 
     Conventionally, in order to measure and/or adjust the cross-polarization isolation of an antenna at an earth station, or “remote station”, an installer must first align the antenna boresight to the specific satellite being used. This may be accomplished, for example, by merely roughly setting the azimuth and elevation angles based on a tabulation of angles corresponding to the satellite of interest, as a function of the latitude and longitude of the remote station. For more precise alignment, however, these settings could be used as an initial setting, and then a signal transmitted from the satellite could be monitored, and the azimuth and elevation angles could be more precisely adjusted based on “peaking” or maximizing the received signal, or simply by ensuring that the received signal is greater than a desired threshold signal value. Typically, the maximum signal strength achievable is used in finally adjusting the azimuth and elevation of the remote station antenna to ensure that the boresight of the remote earth station is accurately aligned with the satellite. 
     After the azimuth and elevation angles of the remote station antenna are set, the person installing the antenna would activate the transmitter at the remote station to uplink to the satellite, and the satellite would then, in turn, downlink to the hub station or network operations center. Typical communication satellites may have multiple transponders that receive signals at one frequency from one or more remote stations, and then retransmit these signals at a different offset frequency to the hub. Further, each of these transponders may be configured to receive a signal having a particular type of polarization, e.g. a polarization type that is one of the general cases of linear, circular, or elliptical polarization. More specifically, the satellite transponders may receive horizontal or vertical linear polarization, or right-hand or left-hand circular polarization. Further, the satellite transponders may retransmit to the hub using a different type of polarization than the type originally received from the remote station. For example, in the case of linear polarization, a transponder may receive a horizontally polarized signal from a remote earth station, and may retransmit a vertically polarized signal to the hub station. This same type of transponder approach may be used in the reverse direction, i.e., in transmitting from the hub to one or more remote stations through the satellite and appropriate transponders. 
     Therefore, it is not enough to merely align the boresight of the remote station antenna to the satellite of interest. The remote station must also account for and align to the polarization type that the satellite transponder is configured to receive and/or retransmit. If the polarization of the remote station antenna is not properly aligned to the satellite transponder&#39;s polarization (e.g. horizontal or vertical linear polarization), or is of the incorrect type, a reduction in signal strength most likely will occur at both the remote station and at the hub. In addition, cross-polarization interference may also occur between adjacent satellite transponders configured, in an interleaved fashion, to receive signals from other remote stations in the same frequency band, but with different polarizations, if the remote station is transmitting a signal with an undesired polarization. This interference will also then be received at the hub. 
     To preclude such cross-polarization induced interference problems, cross-polarization isolation may be determined at the hub by measuring, for example in the case of linear polarization, the signal strength of the desired linearly polarized signal being transmitted from one transponder on the satellite, and the signal strength of the undesired linearly polarized signal which is transmitted from an adjacent transponder on the satellite. The undesired linearly polarized signal is retransmitted by the transponder primarily due to misalignment of the remote station antenna polarization. If the remote antenna polarization is properly aligned, the undesired linearly polarized signal should theoretically be eliminated. Practically speaking, however, the undesired linearly polarized signal should be at least reduced by a typical value of at least−30 dB (1/1000th) from the signal strength of the desired linearly polarized signal, as measured at the hub. To be able to actually meet this typical value of isolation, the polarization at the remote station is usually iteratively adjusted until the cross-polarization isolation is within the specified value. 
     Unfortunately, this procedure requires that fine adjustments in the remote station antenna alignment must continue to be made until cross-polarization isolation, is at the desired or best-achievable value under the circumstances. Such a procedure necessarily requires dedicated communication (e.g. by telephone) between the installer at the remote station and hub station personnel. This process, as represented by the flow-chart of FIG. 6, takes time, satellite resources, and hub site resources, possibly resulting in interruption of service, and, most likely, increased installation and operating costs. 
     A more reliable approach from a time and satellite resource viewpoint would be to greatly simplify this process. However, the known approaches to solving the problem of antenna alignment does not make use of simplified devices and procedures, and instead merely relies upon the “brute force” method of aligning the antenna which requires use of satellite and technical personnel resources to accommodate a downlink from the satellite to the hub to measure the desired parameter, e.g. cross-polarization, as discussed above. 
     What is needed, therefore, is a relatively low-cost, inexpensive, and reliable apparatus and method using simple equipment for simplifying antenna alignment, and improving system performance. 
     SUMMARY OF THE INVENTION 
     The present invention solves at least one of the aforementioned problems of establishing acceptable antenna alignment during system installation at a remote station, including antenna polarization alignment, which thereby achieves a desired cross-polarization isolation at a hub site, and which reduces the costs associated with installation time and use of satellite and hub station resources required for the installation at the remote station. 
     A first aspect of the present invention embodies an apparatus for aligning an antenna which includes a secondary receiving device arranged on the antenna and separated from a primary receiving device of the antenna by a particular offset distance, wherein the primary receiving device receives a first signal and the secondary receiving device receives a second signal, and wherein alignment of the antenna is made by pointing the secondary receiving device in a direction of the second signal. 
     In this aspect of the invention, the antenna is preferably aligned in azimuth and elevation to the desired satellite using a signal received by the primary receiving device along the antenna boresight. The proper antenna polarization with respect to the first signal may first be adjusted by aligning the secondary receiving device mounted on the antenna using a second signal transmitted by a second satellite adjacent to the desired satellite. The secondary receiving device then preferably receives the second signal at a scan angle offset from the antenna boresight, wherein the offset scan angle is determined, at least in part, by the particular offset distance of the secondary receiving device from the primary receiving device. In addition, the particular offset distance may be a variable offset distance which is adjustable by the installing technician, so that a variety of satellites adjacent to the primary or desired satellite may function as the second or adjacent satellite, depending on the variable offset distance chosen. 
     A second aspect of this invention is directed to a method of aligning an antenna, which preferably includes the steps of monitoring a received signal strength of a primary signal; adjusting a position of the antenna based on the received signal strength of the primary signal; monitoring a received signal strength of a secondary signal; and further adjusting the antenna based on the received signal strength of the secondary signal. 
     In this aspect of the invention, the polarization angle of the antenna may be brought into the proper alignment with respect to the desired satellite transmission by further aligning the antenna to “peak” or substantially maximize the signal strength of the secondary signal received from a satellite adjacent to the desired satellite. In this method, a secondary receiving device is arranged in a position separated from a primary receiving device in such a manner that a rotational position of the antenna about its boresight axis aligns the primary receiving device along the boresight axis with the desired satellite, and also aligns the secondary receiving device with an adjacent satellite offset from the boresight by an offset angle. By using such an alignment method, the use of dedicated hub resources (personnel, equipment, and bandwidth) is not necessary, and the antenna polarization with respect to the desired satellite may easily be set using only simple test equipment and procedures. 
     The present invention has a number of features that distinguish it over conventional antenna alignment approaches that attempt to determine the proper polarization alignment by monitoring the signal levels of cross-polarized signals transmitted from the remote station to the hub through satellite transponders. For example, the method and system of the present invention uses simple and relatively inexpensive approach to remote station satellite antenna alignment, whereas conventional approaches to solving this problem have relied only upon the “brute force” method which requires extensive communication and coordination between the remote station and the hub, and which unnecessarily requires the use of channel bandwidth and technical personnel resources to align the remote station antenna after installation. 
     These and other objects of the present application will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent from this detailed description to those skilled in the art. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings in which: 
     FIG. 1A depicts a plan view of an embodiment of the satellite antenna pointing apparatus of the present invention, FIG. 1B provides an elevation view in cross-section along section line IB-IB′ of FIG. 1A; FIG. 1C provides a detailed view of the leg attachment to the primary and secondary receiving devices; and FIG. 1D provides a view in cross-section along section line ID-ID′ of FIG.  1 C. 
     FIG. 2 provides a representative receiving device block diagram. 
     FIG. 3 provides a representation of a satellite communication system in which the apparatus and method of the present invention may be applied. 
     FIG. 4 provides a representative graph of offset-feed gain versus boresight angle of the apparatus of the present invention. 
     FIG. 5 provides a representative flow-chart depicting the improved satellite antenna alignment process using the apparatus and method of the present invention. 
     FIG. 6 provides a representative flow-chart depicting a conventional satellite antenna alignment process. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A preferred embodiment of the apparatus of the present invention is described below. Turning to FIGS. 1A and 1B, satellite antenna pointing apparatus  100  is depicted. Satellite antenna pointing apparatus  100  preferably includes secondary receiving device  150  and secondary feedhorn  155  separated by leg  160  from primary receiving device  120  at an offset distance “D”. This offset distance preferably corresponds to a secondary focal point, which is off main axis  140 , the boresight of the antenna. Secondary receiving device  150 , secondary feedhorn  155 , and leg  160  may be separate “add-on” components temporarily configured to align the antenna to a desired satellite, or may be a permanent part of the installation, so that different signals or services from two different satellites separated in azimuth may simultaneously be received. 
     Primary receiving device  120  is preferably arranged at a primary focal point of antenna reflector  110 , and also preferably includes primary feedhorn  125  supported by one end of feedhorn support  130 , which is attached at an opposite end to antenna reflector  110  on main axis  140  of antenna reflector  110 . Antenna reflector  110  preferably is parabolic in shape, to collect RF energy from a far-away source, for example, a satellite. Primary feedhorn  125  also may direct the collected RF energy into primary receiving device  120  for processing. FIG. 1B provides a cross-sectional view of the antenna, including the satellite antenna pointing apparatus, taken through section line IB-IB′ shown in FIG.  1 A. 
     FIG. 1C provides an exemplary embodiment of the leg attachment arrangement, using bolts  180  to attach leg  160  to primary receiving device  120  and secondary receiving device  150 . In a further preferred embodiment as shown in FIG. 1D, slots  190  are used in conjunction with bolt  180  to provide a means to adjust the position of secondary receiving device  150  relative to primary receiving device  120 , i.e., offset-distance (D) can be varied, preferably to a position which corresponds to an offset-scan angle of the antenna which aligns with a satellite adjacent to the primary satellite. Although bolts and slots are depicted, any other means for attaching receiving devices  120  and  150  to leg  160 , and for allowing adjustment of the offset-distance (D) may be used, e.g. a clamping arrangement (not shown). 
     Primary receiving device  120  may also include components and associated circuitry for transmitting an RF signal through feedhorn  125 , which is then collimated and reflected by reflector  110  in a relatively narrow beam to a satellite, for example, or to another desired location. 
     A receiving device representative of a type that may be used both for primary and secondary receiving devices  120  and  150 , is depicted in FIG.  2 . Primary and secondary receiving devices may each include, for example, low-noise amplifier (LNA)  210  receiving RF energy from feedhorn  125  or  155  and providing an amplified signal to bandpass filter  220 . The band-limited signal from bandpass filter  220  is provided as one input to multiplier  230 , which has local oscillator  240  as a second input. A down-converted signal at an intermediate frequency (IF) is output from multiplier  230 , and may be further band-limited by IF filter  250 , and amplified by IF amplifier  260 . This amplified IF signal may be provided through connector  140  or  170  to other circuitry (not shown), for further processing of the received signals. Such further processing after initial processing in what may be described above as a “low-noise block” (LNB), may include, for example, demodulation or other baseband signal analysis. 
     As a further example, the primary receiving device may include a switch such as a waveguide switch (not shown), to allow the received signal to be routed to one of two receivers which preferably include what is known in the industry as a “low-noise block” (LNB), depending on which of two polarizations are desired to be received, e.g. or vertical or horizontal linear polarization. A LNB essentially includes the abovementioned circuitry, including a LNA, and the down conversion/band-limiting circuitry discussed above. 
     Intermediate frequency signals are preferably provided through connectors  140  and  170  by coaxial cables (not shown) to the other receiver circuitry, for example. Further, connectors  140  and  170  may also be used to provide test point access for evaluating the received signal strengths of primary receiving device  120  or secondary receiving device  150  using, for example, a hand-held or portable meter to measure the average power of the IF signal output of either of the receiving devices  120  or  150 , as an indication of the received signal strength. Such a signal access point may be useful to the installing technician in determining the proper antenna alignment in azimuth and elevation, for example. 
     Referring to FIG. 3, typical satellite communication system  300 , which may employ the apparatus and method of the present invention, is shown. Hub station  310  (“hub”), which may also be called a network operations center, transmits an uplink portion of uplink/downlink  320  to primary satellite  330 . Through a transponder arrangement previously discussed (not shown), primary satellite  330  retransmits a downlink portion of downlinks  340   a  and  340   b  to remote stations  350   a  and  350   b . Each of remote stations  350   a  and  350   b  may include primary and secondary receiving devices  120  and  150  to receive the signals from primary satellite  330  and adjacent satellite  335 , respectively. Remote stations  350   a  and  350   b  may also transmit to primary satellite via an uplink portion of uplink/downlink  340   a  and  340   b , which may then be retransmitted by a transponder on satellite  330  as a downlink portion of uplink/downlink  320  to hub  310 . 
     Adjacent satellite  335  may transmit a set of signals on broadcast  325 , for example. Secondary receiving devices  150  in each of remote stations  350   a  and  350   b  are preferably arranged to receive broadcast  325 . Finally, Hub  310  may also provide access to/from the internet, an intranet, and/or a terrestrial link, e.g. a telephone line, via gateway  360 . 
     When a feedhorn of a parabolic antenna is displaced at offset distance D or at an angle from the main focal point, the main beam radiation pattern moves in the opposite direction, at nearly the same angle. A scanned beam shows a decrease in antenna gain ΔG, as notionally depicted in FIG.  3 . Sidelobes in the direction of the scan are suppressed, and the trailing sidelobes increase. The term “coma lobe” is sometimes used to describe the largest trailing sidelobe. 
     Practically speaking, the scanning process can continue until the gain loss and coma lobe interference produce objectionable results. However, other higher order aberrations, such as astigmatism, filling in of pattern nulls, and beam broadening, are associated with large scan angles, and are preferably avoided. For 2-degree spaced satellites, for example, scan angles can be confined to a narrow ±8-degree or less range. Over this range the decrease is gain may be approximated by: 
     ΔG=−0.045×(2Φ), where “Φ” is the scan angle off the boresight. 
     Generally, speaking, the maximum off-axis scan available is a function of the antenna dish size and the antenna focal length and/or distance of a LNB from the antenna surface. Off-axis scan angles of between 20-30 degrees may be achievable under some circumstances, depending on the antenna configuration. 
     Turning now to FIG. 5, the improved satellite antenna alignment process using the above-described apparatus will now be described. First, primary satellite is preferably “active”, i.e., transmitting a downlink portion in uplink/downlink  340   a , which may be received by primary receiving device  120  in remote station  350   a , for example. In addition, adjacent satellite  335  is also preferably activated to transmit a signal that may be received by secondary receiving device  150 , e.g., broadcast  325 . The person installing or adjusting the antenna of remote station  350   a  preferably aligns the antenna in azimuth and elevation by iteratively adjusting these angles based on a signal strength of a primary signal received by primary receiving device  120 . A hand-held IF power meter connected to primary receiving device  120  through connector  140 , for example may be used to measure this signal strength. 
     Once the primary signal is “peaked”, or essentially maximized, by iteratively adjusting the azimuth and elevation angles of the antenna, a secondary signal, e.g. broadcast  325 , is preferably monitored by the installer using, for example, a hand-held IF power meter connected to secondary receiving device  150  through connector  170  may be used to measure this signal strength. While keeping the azimuth and elevation angles of the main antenna axis, or boresight, fixed in their orientation, the installer preferably rotates the entire antenna reflector  110  about the boresight (as indicated by the dashed line shown around main axis  140  in FIG.  1 A). Secondary receiving device  150  then moves in a prescribed arc about the boresight when antenna reflector  110  is rotated in such a manner. By either pre-setting offset-distance (D), or adjusting offset-distance (D) in the field during installation to a particular distance, the secondary receiving device can be made to align, through the offset-scan angle of the antenna, with a different satellite adjacent to the primary satellite. Because of the allocation of orbital slots in the so-called “Clarke Belt” where geo-synchronous satellites orbit, there is only one offset-distance (D) which will allow the boresight to be aligned with the primary satellite, while aligning the secondary receiving device, through the offset-scan angle of the antenna, with a satellite adjacent to the primary satellite, at a different longitudinal angle. 
     For example, if the primary satellite is geo-synchronously located at 91 degrees west longitude, and the adjacent or secondary satellite is located at 101 degrees west longitude (a 10 degree orbital offset), there is only one offset-distance (D) which will allow both of these satellites to be received by remote station  350 , and there is only one orientation of antenna reflector  110  which will ensure that the proper polarization alignment of the remote station antenna is achieved. 
     Although discussion of a preferred embodiment of the present invention has been directed to a satellite communication system, the method and system of the present invention is not limited to such an implementation. For example, the present invention may also be applicable to other communication links, for example, a terrestrial point-to-point microwave communication could also benefit from the alignment features and techniques provided by the present invention. 
     It will be obvious that the present invention may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims. The breadth and scope of the present invention is therefore limited only by the scope of the appended claims and their equivalents.