Patent Publication Number: US-7917080-B2

Title: Method and apparatus for mitigating interference from terrestrial broadcasts sharing the same channel with satellite broadcasts using an antenna with posterior sidelobes

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 10/872,810, filed Jun. 21, 2004 now U.S. Pat. No. 7,257,370 by Paul R. Anderson, and entitled “METHOD AND APPARATUS FOR MITIGATING INTERFERENCE FROM TERRESTRIAL BROADCASTS SHARING THE SAME CHANNEL WITH SATELLITE BROADCASTS USING AN ANTENNA WITH POSTERIOR SIDELOBES,” which application is a continuation of U.S. patent application Ser. No. 09/480,089, filed Jan. 10, 2000, now issued as U.S. Pat. No. 6,778,810, which application claims benefit of U.S. Provisional Application No. 60/169,005, filed Dec. 3, 1999 by Paul R. Anderson, and entitled “METHOD AND APPARATUS FOR MITIGATING INTERFERENCE FROM TERRESTRIAL BROADCASTS SHARING THE SAME CHANNEL WITH SATELLITE BROADCASTS USING AN ANTENNA WITH POSTERIOR SIDELOBES,” all of which application is hereby incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to systems and methods receiving broadcast signals, and in particular to a system and method for receiving simultaneous terrestrial and satellite broadcasts sharing the same channel using an antenna with posterior sidelobes and anterior nulls. 
     2. Description of the Related Art 
     It has been proposed to cooperatively share the current Broadcast Satellite Service (BSS) frequency bands to allow additional programming material to be transmitted to BSS users or subscribers using the same frequency bands as currently used by BSS satellites. This is implemented through the use of terrestrial based transmitters to transmit the additional programming. 
     One such system is disclosed in U.S. Pat. No. 5,761,605, issued to Tawil et al. on Jun. 2, 1998, which patent is hereby incorporated by reference herein. This patent describes a system in which transmissions from the terrestrial transmitter are received by a second antenna at the user&#39;s premises that faces in the opposite azimuthal direction from the BSS satellite receive antenna. 
     Fundamental to any such system, including the system disclosed in the &#39;605 patent, is that the isolation between the signals from the terrestrial source and the satellite source must be sufficient to prevent interference. While the foregoing system applies to simultaneous channel sharing between satellite and terrestrial transmitters, it requires a BSS satellite ground antenna having highly directional, monocular sensitivity characteristics in order to realize low interference levels. Such antennae can be difficult and expensive to design and produce. Further, it is impractical to replace or modify the several million BSS satellite receive antennae now in use which do not have the desired highly-directional monocular sensitivity. Thus, any simultaneous use of the BSS bandwidth for terrestrial transmissions according to the &#39;605 patent would cause large areas of unacceptably high interference with existing systems. 
     What is needed is a system and method that provides significantly improved interference isolation to provide simultaneous channel sharing without requiring a satellite ground antenna (e.g. a BSS antenna) with highly directional sensitivity characteristics. Preferably, such a system and method would permit terrestrial re-use of the subject BSS broadcast frequencies, with reduced and more acceptable potential interference levels, particularly when used with standard BSS antennae already in widespread use. The present invention satisfies that need. 
     SUMMARY OF THE INVENTION 
     To address the requirements described above, the present invention discloses a method and apparatus for simultaneously receiving a first signal from a non-terrestrial source and a second signal from a terrestrial source on the same or overlapping channels. 
     In one embodiment, the apparatus comprises at least one terrestrial transmitter transmitting information on at least one frequency simultaneously usable by at least one satellite transmitting to a satellite receive antenna having a sensitivity characterizable by a primary sensitive axis directed substantially at the satellite. The terrestrial transmitter includes an azimuthal gain characteristic directed substantially away from the Earth&#39;s Equator. In another embodiment, the terrestrial transmitter is disposed at a location defining a vector angularly displaced from the primary sensitive axis by an angle of less than 90 degrees. 
     In still another embodiment, the apparatus comprises a terrestrial receive antenna for receiving a first signal from a terrestrial transmitter on a frequency usable by at least one satellite transmitting to a satellite receive antenna having a sensitivity characterizable by a primary sensitive axis directed substantially at the satellite. The terrestrial receive antenna includes a primary sensitive axis directed substantially in the direction of the Earth&#39;s Equator. In another embodiment, the terrestrial receive antenna sensitive axis is displaced away from the satellite receive antenna primary sensitive axis by an offset angle of less than 90 degrees. 
     In yet another embodiment, the present invention includes a first antenna having a sensitivity characteristic with a plurality of lobes and nulls therebetween and a second antenna having a sensitivity characteristic with a plurality of lobes and nulls therebetween. The first antenna includes an anteriorly arranged main lobe and one or more posteriorly arranged sidelobes. At least one of the lobes of the first antenna is directed substantially at the non-terrestrial source and at least one of the nulls of the first antenna is directed substantially at the terrestrial source so that the first antenna receives the first signal from the non-terrestrial source, while substantially rejecting the second signal from the terrestrial source. At least one of the second antenna nodes is directed substantially at the terrestrial source so that the second antenna may receive the second signal. In one embodiment, to further enhance isolation between the first signal and the second signal, the second antenna is directed so that in addition to the foregoing, one of the second antenna nulls is directed substantially at the non-terrestrial source. 
     The present invention is also embodied in a method of transmitting information on at least one frequency simultaneously usable by at least one satellite transmitting to a satellite receive antenna having a sensitivity characterizable by a primary sensitive axis directed substantially at the satellite and a posterior secondary sensitive axis. The method is performed by transmitting the information from a terrestrially-based transmitter to a terrestrial receive antenna in a direction substantially away from the Equator. In another embodiment, the invention is embodied by a method for receiving information transmitted on at least one frequency simultaneously usable by at least one satellite transmitting to a satellite receive antenna having a sensitivity characterizable by a primary sensitive axis directed substantially at the satellite, and a posterior secondary sensitive axis. In this embodiment, the method is performed by receiving the information with a terrestrial receive antenna from a terrestrially-based transmitter transmitting the information in a direction substantially away from the Equator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring now to the drawings in which like reference numbers represent corresponding parts throughout: 
         FIG. 1  is a diagram illustrating the general relationship between a terrestrial transmitter, a satellite or other non-terrestrial transmitter, and the orientation of a subscriber&#39;s satellite receive antenna to receive the satellite signal transmitted by the satellite and a second antenna to receive the terrestrial signal transmitted by the terrestrial transmitter; 
         FIG. 2  is a diagram of one embodiment of the satellite receive antenna; 
         FIGS. 3A and 3B  are diagrams showing a sensitivity characteristic of a representative satellite receive antenna  106 , illustrating a posterior sidelobe sensitivity; 
         FIGS. 4A and 4B  are diagrams depicting the sensitivity characteristic of a representative satellite receive antenna; 
         FIG. 5  is a diagram illustrating the application of a foregoing satellite receive antenna in a channel sharing broadcast system; 
         FIG. 6  is an illustration showing a coordinate system; 
         FIG. 7A  is a diagram showing a geometrical relationship between a satellite receive antenna and a plurality of satellite transmitters in a geosynchronous orbit around the Earth; 
         FIG. 7B  is a plot of the azimuth and elevation view angles toward the geosynchronous arc at different satellite receive antenna location latitudes; 
         FIG. 8  is a diagram of the generalized maximum gain of a typical satellite receive antenna at the horizon as the azimuth angle of the satellite receive antenna is swept across a geosynchronous arc; 
         FIGS. 9A-9D  are plots of the worst case satellite receive antenna gain at the horizon for different satellite receive antenna latitudes; 
         FIG. 10  is a plot of a horizontal sensitivity characteristic for an exemplary Southerly-transmitting terrestrial transmitter; 
         FIG. 11  is a diagram illustrating an interference zone for the exemplary Southerly-transmitting terrestrial transmitter; 
         FIG. 12  is a plot illustrating the terrestrial transmitter horizontal gain characteristic for an exemplary Northerly-transmitting terrestrial transmitter; 
         FIG. 13A  is a plot illustrating an interference zone for the exemplary Northerly-transmitting terrestrial transmitter; 
         FIGS. 13B-13G  are plots illustrating the relative size of the interference zone and the service area for varying terrestrial transmitter transmitting directions; 
         FIG. 13H  is a polar plot summarizing the results shown in  FIGS. 13B-13G ; 
         FIG. 14  is a diagram showing the satellite receive antenna spillover lobe geometry; 
         FIGS. 15A and 15B  are diagrams showing the locus of generally preferred directions for terrestrial transmissions; 
         FIGS. 16A and 16B  are diagrams showing the geometry for the elevation angle ε s  and azimuth angle α s  of the satellite receive antenna with its primary sensitive axis directed at the satellite transmitter; 
         FIG. 17  is a plot showing the preferred angular extent of Northerly-directed transmissions from the terrestrial transmitter; 
         FIGS. 18A-18D  are diagrams illustrating relevant geometrical relationships in one embodiment of the present invention, from the perspective of the satellite receive antenna viewing the satellite transmitter and the terrestrial transmitter; and 
         FIGS. 19 and 20  are flow charts presenting illustrative process steps used to practice embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In the following description, reference is made to the accompanying drawings which form a part hereof, and which is shown, by way of illustration, several embodiments of the present invention. It is understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention. 
       FIG. 1  is a diagram illustrating the general relationship between a terrestrial transmitter  102 , a satellite or other non-terrestrial transmitter  104 , and the orientation of a subscriber&#39;s satellite receive antenna  106  to receive the satellite signal  108  transmitted by the satellite transmitter  104  and a second antenna  110  to receive the terrestrial signal  112  transmitted by the terrestrial transmitter  102 . Typically, the satellite transmitter  104  is disposed in a geosynchronous or geostationary orbit, but the present invention can be practiced with satellite transmitters in other orbits as well. For example, in principle, the present invention can be practiced with two potentially interfering terrestrial transmission networks, so long as those networks comply with the geometrical relationships between transmitters described herein. 
     Depending on the sensitivity and rejection characteristics of the satellite receive antenna  106  and the sensitivity characteristic  116  used to transmit the terrestrial signal  112 , the terrestrial signal  112  may impinge on the satellite receive antenna  106 , whether or not the terrestrial receive antenna  110  is present. This terrestrial signal  112  will be detected by satellite receive antenna  106  as determined by the sensitivity or gain characteristic of the satellite receive antenna  106  in the direction of the terrestrial transmitter  102 , and the received power flux density level of the interfering terrestrial signal  112  at the subscriber premises. 
     Some portion of the satellite signal  108  also impinges on the terrestrial receive antenna  110 , whether or not the satellite receive antenna  106  is present. This satellite signal  108  is detected by the terrestrial receive antenna  110  as determined by the sensitivity or gain characteristic of the terrestrial receive antenna  110  in the direction of the satellite transmitter  104 , and the received power flux density level of the interfering satellite signal at the subscriber premises  114 . 
     One approach to reduce the above-described signal interference is to use a satellite receive and terrestrial receive antennae  106  and  110  with highly directional gain characteristics. For example, U.S. Pat. No. 5,761,605 relies on the use of a satellite receive antenna  106  with a “maximum directional reception range” d max  as measured from the satellite receive antenna center line. In this approach, the satellite receive antenna  106  is designed so that only signals inside the directional reception range can be received, and signals outside of this reception range cannot be received by the satellite receive antenna  106 . The difficulty with this approach is that antennae with such characteristics are difficult to design and produce inexpensively. 
       FIG. 2  is a diagram of one embodiment of the satellite receive antenna  106 . The satellite receive antenna  106  includes a parabolic reflector  202 , which reflects and focuses the energy from the satellite transmitter  104  on a low noise block converter (LNB)  204  disposed at a 22.5 degree angle  206  from the centerline  208  of the reflector  202 . This angle positions the LNB  204  out of the way to minimize attenuation of the incoming signal along the antenna centerline or boresight. The shape of the parabolic reflector  202  includes a slightly ovoid shape to account for the offset. 
     The polar sensitivity characteristic of the satellite receive antenna  106  is a function of a number of interrelated physical and electrical antenna characteristics. These characteristics include, among other things, the sensitivity characteristics and physical location of the LNB  204  relative to the reflector  202 , and the shape of the surface of the reflector  202 . 
     For example, the LNB  204  may be disposed closer to the surface of the reflector  202 , but the focus of the parabolic reflector  202  (and hence its external surface contour) must be changed to account for this modified LNB location. Further, the beamwidth of the sensitive axis of the LNB  204  must be modified to achieve the desired antenna sensitivity. Similarly, the LNB  204  may be placed farther away from the reflector  202 , and other antenna  106  parameters must be modified to reflect this difference. 
     To maximize the antenna sensitivity along its centerline  208 , it is desirable that the beamwidth of the sensitive axis of the LNB  204  be wide enough to accept signals from as much of the reflector  202  surface as possible, including the outer periphery. At the same time, if the beamwidth of the LNB  204  is too wide (exceeding the periphery of the reflector  202 ), spillover from behind the reflector  202  can be received by the LNB  204 . In such cases, the sensitivity characteristic of the antenna  106  will include sidelobes in the posterior (rear) side of the antenna  106  having a significant sensitivity. 
       FIGS. 3A and 3B  are diagrams showing a sensitivity characteristic of a representative satellite receive antenna  106 , illustrating the aforementioned posterior sidelobe sensitivity.  FIG. 3B  is a polar plot. Each curve represents the antenna gain of the satellite receive antenna  106  at the horizon.  FIGS. 3A and 3B  are important when determining the amount of interference that will be received from terrestrial transmitters. Each curve shows the gain for different satellite receiver antenna  106  elevation angles. The elevation angle will change as a function of ground receive antenna location and the location of the desired satellite. 
     The sensitivity characteristic shown in  FIGS. 3A and 3B  show that for all elevation angles, the antenna gain response is relatively high for sidelobes at  302  and  304  angles of 50 degrees on either side of the back antenna centerline (at locations corresponding to approximately 50 degrees  302  and 310 degrees  304 ). At these angles, the antenna gain is approximately 0 dBi, due at least on part, to spillover from the LNB  204 . Thus, interference sources located behind the antenna will be received by the antenna at the indicated significant gain levels. 
       FIGS. 4A and 4B  are diagrams depicting the sensitivity characteristic of a representative satellite receive antenna  106 .  FIG. 4A  depicts an azimuthal slice of the antenna characteristic, while  FIG. 4B  shows a slice along the elevation direction at a zero azimuth angle. 
       FIG. 4A  discloses an azimuthal sensitivity characteristic including an anteriorly-disposed main lobe  402  substantially aligned along a primary sensitive axis  404 , and a plurality of sidelobes  410 A,  410 B,  406 A, and  406 B. Nulls such as null  412 A and null  412 B are disposed between the sidelobes  410 A,  410 B,  406 A, and  406 B. Nulls  412 A and  412 B are disposed substantially along null axes  414 A and  414 B. Posterior sidelobes  406 A and  406 B are substantially along secondary sensitive axes  408 A and  408 B, respectively. As described above, the posterior sidelobes  406 A and  406 B are the result of satellite receive antenna design compromises, resulting, among other things, in spillover from the rear of the reflector  202  to the LNB  204 . 
       FIG. 4B  discloses an elevation sensitivity characteristic including the main lobe  402 , sidelobes  416 A and  416 B substantially along sidelobe axes  418 A and  418 B. Nulls  422 A and  422 B are disposed along null axes  422 A and  422 B, respectively, between the main lobe  402  and the sidelobes  416 A and  416 B, as well as between other sidelobes not illustrated. The depictions of the main  402  and sidelobes in  FIGS. 4A and 4B  above are intended to be representative depictions of the polar sensitivity characteristic of a satellite receive antenna  106  by which the present invention may be practiced. The present invention could be practiced with antennae having sensitivity characteristics with different lobes and null patters with suitable modification. 
       FIG. 5  is a diagram illustrating the application of a satellite receive antenna  106  with the foregoing characteristics in a channel sharing broadcast system. As can be seen in  FIG. 5 , antennae with the foregoing sensitivity characteristics are unusable with the system disclosed in the Tawil disclosure, because the terrestrial transmitters  102  are placed behind the satellite receive antennae  106  where they will impinge on the posteriorly-directed satellite receive antenna sidelobes  408 A and  408 B (i.e. interfering signals broadcast from terrestrial transmitter along path  502 ). Consequently, the Tawil system requires a satellite receive antenna that has no significant posterior sidelobes  406 . This can be accomplished by designing the satellite receive antenna with a narrower beamwidth LNB feed portion (and hence, reduced sensitivity), a resized or reshaped antenna reflector  202 , or other costly modification. Such modification will likely result in a loss of satellite receive antenna  106  performance (for example, reduced sensitivity or greater main lobe beamwidth). The present invention avoids this difficulty by advantageous disposition of the terrestrial transmitter  102 , as described herein. 
     Returning to  FIGS. 3A and 3B , for an interference source located in front of the satellite receive antenna  106 , the worst case or highest relative gain at the horizon (where a terrestrial transmitter  102  would be located) occurs at angles very close to the front antenna centerline  208  (reference angles depicted in  FIGS. 3A and 3B  as near 180 degrees). Note, however, that the gain values near reference angles of 180 degrees are only troublesome for an elevation angle of about 10 degrees (or, equivalently, 10 degrees below the front antenna centerline  208 ). That is, at an elevation angle of 10°, the gain of the antenna at a 180° reference angle (the front of the antenna) is greater than the gain at reference angles of about 50° (at  312 ) and at about 310° (at  304 ). At higher elevation angles, the gain of the satellite receive antenna at the 180° reference angle is in fact significantly lower that the gain seen off the back of the antenna at the 50° and 310° reference angles. This is because at sufficiently high elevation angles, the interfering source is being received at an angle sufficiently far away from the main beam of the antenna that there is sufficient isolation. At elevation angles of 10 degrees and below, the interfering source is within the main lobe of the antenna and isolation becomes poor. 
     For satellite receive antennae  106  with the foregoing characteristics, the interference regions for all but the lowest elevation angles is reduced when the terrestrial transmitter  102  or other interference source is located in front of the satellite receive antenna  106 , not behind the satellite receive antenna  106  as is the case with the high-directivity antennae used in the Tawil patent. 
       FIG. 5  also depicts a channel sharing system which is usable with satellite receive antennae  106  without a highly directive monocular antenna gain characteristic. Here, terrestrial transmitter  102 C having azimuthal gain characteristics  116 C broadcasts the second signal along path  506  to the terrestrial receive antenna  110 , as desired. The terrestrial receive antenna  110  includes a primary sensitive axis  518  defined by the terrestrial receive antenna main lobe  520 , which is directed substantially at the terrestrial transmitter  102 C. The terrestrial receive antenna  110  also includes one or more sidelobes  522  with null regions  524  between the main lobe  520  and the sidelobes  522 . Typically, there are a plurality of sidelobes  522  on either side of the main lobe  520 , each having additional nulls or null regions therebetween. 
     Unfortunately, the terrestrial transmitter  102 C also transmits the second signal to the satellite receive antenna  106  along path  508 . However, in this channel sharing system, the terrestrial transmitters  102 A- 102 D (hereinafter alternatively referred to as terrestrial transmitter(s)  102 ) are disposed so that the offset angle  540  of their transmissions from the axis defined by the path  512  are greater than a particular offset angle  542  from the axis defined by path  512 . This offset angle (which can be defined in terms of an elevation component and an azimuth component) depends on the beamwidth of the main lobe of the satellite receive antenna  106 . So long as the terrestrial transmitters are disposed so that the offset angle in azimuth and elevation is greater than the effective beamwidth of the satellite receive antenna  106 , sufficient isolation can be realized. It is also noted that, for the continental United States (CONUS), these low elevation angles are only associated with two narrow azimuth ranges, one near 105° (almost due East), and one near 225° (almost due West). Conversely, the significant posterior side lobes are a significant factor over a much wider azimuth range. 
     In one embodiment of the invention, the terrestrial transmitter can be disposed anywhere in front of the satellite receive antenna  106  (including directly in front of it), so long as a minimum elevation angle is maintained. When the sensitivity characteristic of the satellite receive antenna  106  in the direction of terrestrial transmitter  102 C is either in a null or in one of the low sensitivity lobes, the second signal is greatly attenuated when compared to the signal received from the satellite transmitter  104 , thus providing improved isolation between the first signal from the satellite transmitter  104  and the second signal from the terrestrial transmitter  102 C. Similarly, terrestrial transmitter  102 D, also undesirably transmits the second signal to the satellite receive antenna  106 . However, since the satellite receive antenna  106  is elevated by elevation angle  514 , terrestrial transmitter  102 D is substantially aligned with null axis  422 B, and thus, within null  420 B. Hence, the signal from terrestrial transmitter  102 D transmitted along path  510  is sufficiently attenuated so that the second signal from the terrestrial transmitter  102 D does not interfere with the first signal transmitted along path  512 . The main lobe  402  (and hence the primary axis  404 ), however, is substantially aligned with the path  512  to the satellite transmitter  104 . Hence, the satellite receive antenna  106  will receive a strong signal from this source. 
     Hence, unlike the system disclosed in the Tawil patent, which requires the use of a satellite receive antenna  106  with a highly directive monocular antennae gain characteristic, the present system is usable with a satellite receive antenna  106  described by a sensitivity characteristic which may include significant posteriorly-disposed sidelobes. Since transmission paths ( 508  and  510 ) from all of the potentially interfering transmitters ( 102 C- 102 D respectively) are at sufficient offset angle (determined by the combination of the azimuth offset component and the elevation offset component of the angle between the satellite receive antenna  106  and the terrestrial transmitter  102 ), interference is minimized. 
     As can be seen by the foregoing, all but the lowest elevation angles create smaller interference zones when the interference source is located in front of, and not behind, the satellite receive antenna  106 . For the Northern Hemisphere, this means that the preferred direction for a potentially interfering terrestrial transmission is from a southerly direction so that the transmissions are received on the anterior side (but below the main beam  402 ) of the satellite receive antenna  106 . 
     This technique can be applied in either hemisphere. For example, for use in the Southern Hemisphere, the terrestrial transmission system is designed to transmit in a southerly direction so that the interference source is in front of the satellite receive antennas which face north. This concept is also extendable to use with other overlapping transmission channels where there is a desire to minimize interference between terrestrial transmissions and satellite transmissions. 
     In one embodiment of the present invention, elevation angle  514  is selected to be at least approximately 10 degrees. This value provides a reasonable angular distance from the main lobe, while making the most of the transmitting satellite coverage. For 45 cm diameter satellite receive antennae  106 , elevation angles below 10 degrees are at the edge of coverage for a satellite transmitter  104  because the satellite receive antennae  106  appear (from the satellite transmitter&#39;s perspective) to be very close to the limb of the Earth. 
     In another embodiment of the present invention, the terrestrial receive antenna  110  can also be described by a polar sensitivity characteristic having lobes and nulls. The terrestrial receive antenna  110  can be aligned so that one of the nulls of the terrestrial receive antenna is substantially aligned with an axis  516  between the terrestrial receive antenna  110  and the satellite transmitter  104 , while a lobe (preferably the main lobe) of the terrestrial receive antenna is substantially aligned with an axis  506  between the terrestrial receive antenna  110  and the terrestrial transmitter  102 C. 
     Satellite transmitters  104  are typically designed such that the transmitted power levels fall off from higher to significantly lower values at the edge of the primary coverage area, allowing transmitted power to be concentrated on this primary coverage area. Because of this reduced transmitted power at edge of coverage, larger (and typically more expensive) ground receive antennas are required to receive these signals. These larger antennas have greater gain and smaller beamwidths. Thus, they provide increased isolation to in-line terrestrial transmitters relative to the isolation that could be achieved with 45 cm antennas. This improved isolation compensates for the reduced vertical separation angle between any in-line terrestrial transmitters and the beam peak, and should tend to maintain the advantage of northerly directed transmissions over southerly-directed transmissions for some range of elevation angles below 10 degrees. 
     As described above, the present invention describes a system which can cooperatively share the current Broadcast Satellite Service (BSS) frequency bands to allow additional programming material to be transmitted to BSS users or subscribers using the same frequency bands as currently used by BSS satellites. Further, the present invention achieves this goal without requiring expensive specially designed satellite receive antennae  106 . 
     Although not necessary to practice the present invention the terrestrial receive antenna  110  may be disposed proximate the satellite receive antenna  106 . For example, the terrestrial receive antenna  110  and the satellite receive antenna  106  may be attached to the same support structure, or installed on different portions of the customer&#39;s residence. However, it is noted that in the general case, the transmit receive antenna  110  will be used by different customers than the satellite receive antenna  106 , and generally, will not be located proximate the satellite receive antenna. 
     Interference Comparison Between Southerly-Directed Terrestrial Transmissions and Northerly-Directed Terrestrial Transmissions 
       FIG. 5  presents two terrestrial transmission systems. The first (southerly transmitting) system includes transmitters  102 A and  102 B on a posterior side of the satellite receive antenna  106  to transmit the second signal to the terrestrial receive antenna  110 , whereas the second (northerly transmitting) system includes transmitters  102 C and  102 D on an anterior side of the satellite receive antenna  106  to transmit the second signal to the terrestrial receive antenna  110 ). 
     A useful yardstick to measure the performance of the two systems depicted in  FIG. 5  is to determine the size of the interference zones for each system. In this context, the interference zone is defined as the minimum separation distance that must be maintained between the terrestrial transmitter  102  and the satellite receive antenna  106  in order to maintain a pre-defined level of interference between the signals. While the present invention reduces the size of these “interference zones,” they are not entirely eliminated. Hence, even with the substantially improved performance offered by the present invention, satellite communications will be negatively affected by the use of the channel-sharing terrestrial transmission system. An interference zone analysis is presented below. 
       FIG. 6  is an illustration showing a coordinate system used in the calculations for the example that follows. Here,
         Az ES : Azimuth angle from the terrestrial transmitter  102  to the satellite receive antenna  106 . This is shown in column B of Tables 1, 4, and 7, and column O of Tables 5 and 7.   Az TT : Az TT =AZ ES −180°   Az BSS : Azimuth angle of the satellite transmitter  104  from the satellite receive antenna  106 . This is shown in “Sat Az Angle” (Row  110 ) of Table 1.   d ES : Satellite receive antenna  106  to terrestrial transmitter  102  separation distance. This is shown in Table 2, rows 2, 3, and 6, and Table 4, row 11.       

     In order to calculate the size of interference zones for both a northerly transmitting system and a southerly transmitting system, it is important to determine for each possible azimuth angle to the terrestrial source the worst case gain of the satellite receive antenna  106 . The satellite receive antenna  106  must be protected from interference when it is pointing to any visible portion of the geosynchronous arc down to some minimum elevation angle. 
       FIG. 7A  is a diagram showing a geometrical relationship between a satellite receive antenna  106  and a plurality of satellite transmitters  104  in a geosynchronous orbit around the Earth. To enable line-of-sight communications, the satellite transmitter  104  must be above the horizon  702 . Additionally, to account for atmospheric attenuation, terrestrial obstructions and other effects, there is a minimum elevation angle  704  below which communications between the satellite transmitter  104  and the satellite receive antenna  106  are effectively precluded. The maximum elevation of the arc of the geosynchronous orbit of the satellites  104  depends upon the latitude of the satellite receive antenna  106  location. At lower latitudes (e.g. those near the Tropic of Cancer), the satellite transmitter  104  may be disposed at a high elevation angle, while at higher latitudes (those near the Arctic Circle), the satellite transmitter  104  may never be far from the horizon  702 . As can be seen, each satellite receive antenna can be expected to be directed at any of the satellite transmitters  104 . 
       FIG. 7B  is a plot of the azimuth and elevation view angles toward the geosynchronous arc at different satellite receive antenna  106  location latitudes. These view angles were computed using equations (4) and (5) above. In  FIG. 7B , the horizon is represented by the outside semicircle  702 , and the center point  708  represents the sky directly overhead the satellite receive antenna  106 . Semicircle  710  and  712  represent elevation angles of 60° and 30°, respectively. Plots  714 ,  716 , and  718  indicate views of the geosynchronous arc for a satellite receive antenna  106  located at 60°, 40°, and 20° North latitude, respectively. 
     For each possible geometric alignment of the terrestrial transmitter  102 , satellite receive antenna  106  and satellite transmitter  104 , the worst case (highest satellite receive antenna  106  gain sensitivity at the horizon in the direction of the terrestrial transmitter  102 ) is calculated. This is equivalent to the maximum gain of the satellite receive antenna  106  at the horizon  702  at any given azimuth angle  706  from an easternmost elevation angle (e.g. with the satellite receive antenna disposed as in  106 A) to a westernmost minimum elevation angle (e.g. with the satellite receive antenna disposed as in  106 B), thus providing the worst case potential for interference for all possible orientations of the satellite receive antenna  106 . This result can be used to compute a minimum separation distance to determine the interference zones, and also to quantify the required azimuthal transmission characteristic of the terrestrial transmitters. 
       FIG. 8  is a diagram of the generalized maximum gain of a typical satellite receive antenna  106  at the horizon as the azimuth angle of the satellite receive antenna  106  is swept from position  106 A to  106 B. AZ E    808  represents the easternmost satellite receive antenna  106  pointing angle in the Northern Hemisphere for a given minimum elevation. AZ W    804  represents the westernmost antenna pointing in the Northern Hemisphere for the same minimum elevation. AZ SW    806  represents the azimuthal direction of the highest gain for a posterior spillover lobe  406  when the main lobe  402  is aligned along AZ W    804 . AZ SE    810  is the azimuthal direction of the highest gain for a posterior spillover lobe  406  when the main lobe  402  is aligned along AZ E    808 . Note that the angular location of the gain characteristic  802  of the posterior satellite receive antenna lobes  406  causes a unique characteristic in which the greatest isolation from terrestrial transmissions is in the southerly direction, at an angles from AZ SE    810  to AZ SW    806 . 
       FIG. 9A  is a plot of the worst case satellite receive antenna  106  gain at the horizon  702  for a satellite receive antenna  106  at a location at 40°N latitude, and for a minimum elevation angle of 10° above the horizon  702 . The approximate values for AZ E    808 , AZ SW    806 , AZ SE ,  810  and AZ W    804  are as follows:
         AZ E =106.4°   AZ SW =121.6°   AZ SE =238.4°   AZ W =253.6°       

       FIG. 9B  is a plot of the worst case satellite receive antenna  106  gain at the horizon  702  for a satellite receive antenna  106  at a location at 20°N latitude, and for a minimum elevation angle of 10° above the horizon  702 . The approximate values for AZ E    808 , AZ SW    806 , AZ SE ,  810  and AZ W    804  are as follows:
         AZ E =97°   AZ SW =131°   AZ SE =229°   AZ W =263°       

       FIG. 9C  is a plot of the worst case satellite receive antenna  106  gain at the horizon  702  for a satellite receive antenna  106  at a location at 50°N latitude, and for a minimum elevation angle of 10° above the horizon  702 . The approximate values for AZ E    808 , AZ SW    806 , AZ SE ,  810  and AZ W    804  are as follows:
         AZ E =113.6°   AZ SW =114.4°   AZ SE =245.6°   AZ W =246.4°       

       FIG. 9D  is a plot of the worst case satellite receive antenna  106  gain at the horizon  702  for a satellite receive antenna  106  at a location at 60°N latitude, and for a minimum elevation angle of 10° above the horizon  702 . The approximate values for AZ E    808 , AZ SW    806 , AZ SE ,  810  and AZ W    804  are as follows:
         AZ E =125.5°   AZ SW =102.5°   AZ SE =257.5°   AZ W =234.5°       

     These results were generated using the sensitivity characteristic for a typical offset feed parabolic reflector 45 cm satellite receive antenna  106 , having the characteristics shown in  FIGS. 3A and 3B . Table 1, presented in the appendix, illustrates the computations used to generate the results shown in  FIGS. 9A-9D . The last three columns of Table 1 (labeled AL, AM, and AN), describe the maximum horizon gain of the satellite receive antenna  106  for minimum elevation angles restricted to 15, 10, and 5 degrees, respectively. This calculation is performed as a function of both the relative difference in longitude between the location of the satellite receive antenna  106  and the location of the satellite transmitter  104 , and the relative azimuth angle from the terrestrial transmitter  102  to the satellite receive antenna  106  (AZ ES ). 
     Each interior column in Table 1 represents a specific longitudinal difference between the location of the satellite receive antenna  106  and satellite transmitter  104 . The elevation angle from the satellite receive antenna  106  to the satellite transmitter  104  is different for each column, and the value is shown in degrees at the bottom of the Table. Each interior row in Table 1 represents a different azimuth angle from the terrestrial transmitter  102  towards the satellite receive antenna  106 . 
     The entry at the intersection of each interior row and column represents the horizon gain of the satellite receive antenna  106  as calculated for a particular geometry and for a particular elevation angle to the satellite transmitter  104 . Rows  110  and  111  in Table 1 provide the particular azimuth and elevation angles in degrees. For the purposes of example, the horizon gain is chosen from the horizon gain antenna pattern closest in elevation angle to the true elevation angle. 
     The worst case (maximum) horizon gain for each row (i.e., each selected azimuth angle from the terrestrial transmitter  102 ) is then noted and recorded in the rightmost columns of the Table (columns AL-AN). This is the gain value that is used in the separation distance calculation to protect reception of a satellite receive antenna  106  to all considered locations of satellite transmitters located along that particular radial line from the terrestrial transmitter  102 . 
     From  FIG. 9A , it is apparent that the maximum satellite receive antenna  106  gain is minimized for northerly directed terrestrial transmitter  102  transmissions between the angles AZ SE    810  and AZ SW    806 . Note that over a significant angular range, the maximum gain values for northerly directed terrestrial transmissions are significantly lower than are seen for southerly directed transmissions, providing increased isolation to interference sources. 
     Table 2 illustrates the terrestrial transmitter  102  vertical (elevation) sensitivity characteristic, which is used in the calculations that follow. 
     Interference Analysis for a Southerly-Transmitting Terrestrial Transmitter 
       FIG. 10  is a plot of the horizontal (azimuthal) sensitivity characteristic  1002  for a southerly-transmitting terrestrial transmitter  102 . This sensitivity characteristic is analogous to the sensitivity characteristics  116 A and  116 B of  FIG. 5 . 
     Table 3 presents a path loss calculation as a function of the distance from the terrestrial transmitter  102 . The equation used to derive path loss values, which can be expressed in decibels (dB) is: 
                   PL   =         -   10     ⁢     log   ⁡     [     4   ⁢           ⁢   π   ⁢     {         (     d   s     )     2     +       (     h   TT     )     2       }       ]         +     10   ⁢     log   ⁡     [       λ   2       4   ⁢           ⁢   π       ]       ⁢           ⁢   in   ⁢           ⁢   dB               (   1   )               
wherein λ=the wavelength of the transmissions, d s  is the separation distance, and h TT  is the height of the terrestrial transmitter  102  above the location of the satellite receive antenna  106 .
 
     Table 4, shows a computation of the isotropic signal received at the satellite receive antenna  106  as a function of distance from the terrestrial transmitter  102  and the azimuth angle from that transmitter. This was computed using the terrestrial transmitter antenna characteristics shown in  FIG. 10  and the vertical sensitivity characteristic shown in Table 2. The isotropic signal strength is the power that would be seen at the output port of an omnidirectional receive antenna with exactly 0 dB of gain. 
     Note that since the transmitter antenna provides the greatest sensitivity at 180 degrees, the results presented in Table 4 refers to southerly directed transmissions. The equation used to derive the RSS i  value is as follows:
 
 RSS   i   =P+L+A+G   TP   +G   TH   +G   TV   +PL  in dB  (2)
 
wherein P represents the terrestrial transmitter  102  power in dBW, L represents line losses from the terrestrial transmitter  102  to the satellite receive antenna  106  in dB, A represents atmospheric losses in dB, G TP  represents the terrestrial transmitter antenna peak gain in dB, G TH  represents the terrestrial transmitter antenna relative horizontal gain in dB, and G TV  represents the terrestrial transmitter antenna relative vertical gain in dB.
 
     Table 5 provides a calculation of the threshold or trigger value of RSS i  that exceeds a given interference criteria. For purposes of this example and for an isotropic receive antenna with a gain of 0 dBi, the threshold interfering signal level is set at −152.1 dBW. Where the horizon gain of the satellite receive antenna  106  is different from 0 dBi, this gain difference must be taken into account. The “BSS Ant Gain” column (column P) of Table 5 is the maximum horizon gain for a satellite receive antenna  106  at the azimuth angles indicated in column O. This column comes directly from column AM of Table 1. The value in this column is subtracted from the RSS i  threshold or trigger value to arrive at the RSS i  value needed to protect the satellite receive antenna  106 . These results are shown in the rightmost column of Table 5. 
     Given the threshold RSS i  value calculated in Table 5, each row in Table 5 can be examined to identify the minimum satellite receive antenna  106  separation distance at which the RSS i  level falls below the corresponding threshold value. This is repeated for each row in the Table, representing a range of azimuth angles from the terrestrial transmitter  102  to the satellite receive antenna. 
       FIG. 11  illustrates the results of this above analysis. A terrestrial transmitter  102  is located at the point indicated. The outer curve  1102  represents the service area of the terrestrial transmissions. For this example, the service area is assumed to require a minimum terrestrial signal strength of −158.7 dBW. 
     The inner curve  1104  in  FIG. 11  represents the required minimum separation distance for satellite receive antenna  106 . Inside of this curve  1104  the interfering signal level to a satellite receive antenna  106  is above the calculated RSS i  threshold level, and unacceptable levels of interference result. 
     Interference Analysis for a Northerly Transmitting Terrestrial Transmitter 
     The same calculation can be performed for northerly-directed terrestrial transmissions. This calculation uses the same parameters for satellite receive antenna  106  gain sensitivity patterns, terrestrial transmitter antenna  102  gain patterns and power, required service area signal strength, and interference threshold criteria. The only difference is that the terrestrial transmissions are now directed substantially away from the Equator (in the Northern Hemisphere, in a Northerly direction instead of a Southerly direction, and in the Southern Hemisphere, in a Southerly direction instead of a Northerly direction). 
       FIG. 12  is a plot illustrating the terrestrial transmitter horizontal gain characteristic for a northerly transmitting terrestrial transmitter such as transmitters  102 C and  102 D in  FIG. 5 . The illustrated sensitivity characteristic is analogous to the sensitivity characteristic  116 C and  116 D of  FIG. 5 . A comparison with  FIG. 10  shows that this pattern has the same shape but the azimuth angle for peak gain (0 dB relative gain) is at zero degrees, or at 0° North. The terrestrial transmitter antenna  102  vertical gain pattern is the same as that shown in Table 2. 
     Table 6 shows a path loss calculation for northerly directed transmissions. This table is the same as Table 3 except that the terrestrial transmitter  102  to satellite receive antenna  106  azimuth angle has been adjusted for the northerly directed transmissions. 
     Table 7 provides a calculation of received isotropic signal strength. The Table is similar to Table 4 but presents the result for northerly-directed transmissions. The equation used to derive the RSS i  value is the same as described above for Table 4. 
     Table 8 provides the calculation of the threshold or trigger value of RSS i  that exceeds the same interference criteria used in the derivation of  FIG. 11 . The “BSS Ant Gain” column (column P) is the maximum horizon gain for a satellite receive antenna  106  at each of the indicated azimuth angles. The data in this column comes directly from the column AM of Table 1. The value in this column is subtracted from the RSS i  threshold or trigger value to arrive at the RSS i  value needed to protect the satellite receive antenna  106 . These results are shown in the rightmost column of Table 8. 
     Note that the values in the rightmost column of Table 8 are significantly higher overall than the values in the rightmost column of Table 5. This means that the separation distances can be shorter for northerly-directed transmissions, resulting in a smaller interference zone. 
       FIG. 13A  illustrates the result of applying the RSS i  threshold values calculated in Table 8 to the calculated RSS i  values as a function of distance from the terrestrial transmitter (Table 7). Note that the shape of the service area is inverted from that in  FIG. 11 , and this is a result of the northerly directed terrestrial transmissions. 
     The outer curve  1302  of  FIG. 13A , as it did in  FIG. 11 , represents the service area of the terrestrial transmissions. Similarly, the inner curve  1304  in  FIG. 13A  represents the required minimum separation distance for a satellite receive antenna  106 . The interference zone enclosed by the line  1304  defined by the minimum separation distance is significantly smaller than the corresponding zone in  FIG. 11 . For this example, the area of the interference zone for southerly-directed transmissions is approximately 36 square kilometers. The area of the interference zone for northerly-directed transmissions ( FIG. 10 ) is approximately 16 square kilometers, less than half the area of that required for southerly directed transmissions. 
     As the foregoing discussion illustrates, the use of northerly-directed terrestrial transmissions significantly minimizes interference from terrestrial transmitters  102  in the satellite receive antenna  106 . The foregoing analysis can be extended to further refine these results to determine the optimal horizontal sensitivity characteristic of the terrestrial transmitter  102 . 
       FIGS. 13B-13H  are diagrams illustrating the relationship between the direction of the transmissions from the terrestrial transmitter  102 , and the size and shape of the resulting interference zone.  FIG. 13B  shows the relationship between the service area and the interference zone for southerly-directed transmissions (180°).  FIG. 13C  shows the relationship between the service area and the interference zone for transmissions directed southeasterly at a 140° angle.  FIG. 13D  shows the relationship between the service area and the interference zone for transmissions directed east-by-southeasterly at a 100° angle.  FIG. 13E  shows the relationship between the service area and the interference zone for transmissions directed east-by-north-easterly at a 60° angle.  FIG. 13F  shows the relationship between the service area and the interference zone for transmissions directed in a northeasterly direction at a 30° angle.  FIG. 13G  shows the relationship between the service area and the interference zone for transmissions directed in a northerly 0° angle. Note that the interference zone for northerly directed transmissions is about one-half as large in area as the interference zone for southerly directed transmissions.  FIG. 13H  is a polar plot which summarizes the results of  FIGS. 13B-13G  by showing the relative area of the interference zone according to the transmission angle of the terrestrial transmitter  102 . 
     Note that the interference zone can be further reduced in size by selecting the horizontal sensitivity characteristic of the terrestrial transmitter  102  to limit the angle of transmission so that the small “wings” of the interference zone are substantially excluded. As can be seen by comparing  FIGS. 13B and 13G , this technique is especially well suited to a system with terrestrial transmissions in a generally northerly direction. This is discussed more fully below. 
     Optimal Terrestrial Transmitter Horizontal Sensitivity Characteristic 
     The foregoing results can be improved even more by limiting the transmissions from the terrestrial transmitter to be within specific azimuthal angular limits. 
       FIG. 14  is a diagram showing the satellite receive antenna spillover lobe  406  geometry. As described earlier, the source of the satellite receive antenna spillover lobe  406  is the relationship between the beamwidth  1404  of the LNB  204 , the diameter of the reflector  202 , and the distance of the LNB  204  from the reflector  202 . When the beamwidth of the LNB  204  exceeds the diameter of the reflector  202 , RF energy from behind the reflector  202  can be sensed by the LNB  204 . This allows the satellite receive antenna  106  to have a gain characteristic with significant posterior lobes  406 . As shown in  FIG. 14 , the peak of the posterior side lobe (or spillover lobe  406 ) is at an angle 180°−S degrees from the satellite receive antenna  106  boresight  208 , where S represents the angle (in degrees) between the rear-facing portion of the antenna centerline  206  and the peak of the posterior side lobe  406  in direction  2402 . 
     Given that AZ E , and AZ W  the following relationships can be established
 
 AZ   SE   =AZ   E +(180 °−S ), and  (3a)
 
 AZ   SW   =AZ   W −(180 °−S ).  (3b)
 
     Because the shape of the worst case horizon gain characteristic  802  shown in  FIG. 8 , it is preferred to direct transmissions from the terrestrial transmitters  102  in a direction such that they are directed at the satellite receive antenna  106  between AZ SE    810  and AZ SW    806 . 
       FIG. 15A  is a diagram showing the locus of generally preferred directions  1502  for terrestrial transmissions relative to the satellite receive antenna  106  (located at the origin). 
       FIG. 15B  is a diagram showing the locus of generally preferred directions  1504  for terrestrial transmissions relative to the terrestrial transmitter  102 . 
     The foregoing can be generalized to determine AZ SE  and AZ SW  for different satellite receive antenna  106  location latitudes and minimum elevation angles ε min  as follows. 
       FIGS. 16A and 16B  are diagrams showing the geometry for the elevation angle ε s  and azimuth angle α s  of the satellite receive antenna  106  with its primary sensitive axis  404  directed at the satellite transmitter  104 . 
     Assuming satellite transmitters  104  are in orbits with a substantially zero inclination (i=0), the elevation angle ε s  (δ) and azimuth angle α s  (δ) of the satellite receive antenna  106  with its primary sensitive axis  404  directed at the satellite transmitter  104  can be shown to be 
                         ɛ   s     ⁡     (   δ   )       =     arcsin   ⁡     [         K   ⁢           ⁢   cos   ⁢           ⁢       ψ   s     ⁡     (   δ   )         -   1         [     1   +     K   2     -     2   ⁢   K   ⁢           ⁢   cos   ⁢           ⁢       ψ   s     ⁡     (   δ   )           ]       1   /   2         ]         ,           ⁢   and           (   4   )                     α   s     ⁡     (   δ   )       =     arccos   ⁡     [     -       cos   ⁢           ⁢     ψ   s     ⁢   sin   ⁢           ⁢   ζ       sin   ⁢           ⁢     ψ   s     ⁢   cos   ⁢           ⁢   ζ         ]         ,           (   5   )               
where
         ψ s =arccos [cos ζ cos δ] (great circle arc);   ζ=Earth station latitude (latitude of the location of the satellite receive antenna  106 ;   δ=longitude difference (Δ) between the location of the satellite transmitter  104  and the satellite receive antenna  106 ; and   K=radius of the orbit of the satellite transmitter  104 /radius of the Earth≈6.62.       

     Using the above relationships, AZ SE  or AZ SW  can be determined from AZ E  or AZ W , respectively. AZ E  is a function of the latitude of the satellite receive antenna location ζ and the minimum allowed elevation angle ε s,min  of the satellite receive antenna  106 . In other words, AZ E =ƒ(ε s,min ,ζ). To find AZ SE  for a given latitude ζ, different values of the longitude difference δ are tried until ε s,min  reaches the desired value 
     
       
         
           
             
               
                 
                   
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     Table 9 shows the longitude difference δ for satellite receive antenna  106  locations from 0 to 70 degrees North, and for minimum satellite receive antenna  106  minimum elevation angles of 5, 10, and 15 degrees. 
     Once the longitude difference δ is known, one can solve for the easternmost azimuth angle corresponding to the specified minimum elevation angle, using equation (5). The result of the foregoing computation of AZ E , including the interim computation of the great circle arc ψ s  is shown in Tables 10 and 11. Given the easternmost AZ angle of the satellite receive antenna  106 , the preferred easternmost AZ angle for significant terrestrial transmitter  102  transmissions can be computed. AZ SE  can be computed from the equation (3a). Recalling the diagram presented in  FIG. 15A , it can be seen that terrestrial transmissions should approach the satellite receive antenna  106  between the AZ SE  radial  810  and the AZ SW  radial  806 . The azimuth angle of a terrestrial transmitter  102  pointing along AZ TE  and toward the satellite receive antenna  106  is 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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     Table 12 presents values for AZ TE  for a typical 45 cm offset feed parabolic satellite receive antenna  106  corresponding to the Table 10. Similarly, the value for AZ TW  is
 
 AZ   TW =360°− AZ   TE .  (8)
 
     Hence, the preferred azimuthal angular limits (as illustrated generically in  FIG. 15B ) for transmissions from the terrestrial transmitter  102  are as shown in the Table 13. 
     Table 14 presents the angular extent φ  1504  of these transmissions between the limits AZ TE  and AZ TW . 
       FIG. 17  is a plot showing the preferred angular extent of Northerly-directed transmissions from the terrestrial transmitter. Examination of  FIG. 17  (and of  FIGS. 9A-9D ) reveals that the worst case gain region falls between AZ SE  and AZ SW  for latitudes of 20°N, 40°N, and 50°N, with the angular extent varying from 98° at 20°N to 131° at 50°N. However, as the latitude of the satellite receive antenna  106  increases, AZ E    808  crosses AZ SW    806  and AZ W    804  crosses AZ SE    810 , the main beam gain associated with AZ E    808  and AZ W    804  become the limiting factors, and the size of the least sensitive region begins to decline. 
       FIGS. 18A-18C  are diagrams illustrating relevant geometrical relationships in one embodiment of the present invention, from the perspective of the satellite receive antenna  106  viewing the satellite transmitter  104  and the terrestrial transmitter  102 . In this illustration, the primary sensitive axis  404  of the satellite receive antenna  106  is directed substantially at the satellite transmitter  104 . The satellite transmitter  104  is disposed at an elevation angle ε greater than the minimum elevation angle ε min . The satellite receive antenna  106  can be characterized by an effective beamwidth  1804  having an angular extent β substantially described by the main lobe  404 . The terrestrial transmitter  102  is disposed at a location defining a vector  1808  angularly displaced from the primary sensitive axis  404  of the satellite receive antenna  106  by an offset angle γ, having a first component and a second component, wherein the first component is the azimuth component γ AZ  which extends from −90° to 90° (e.g., γ AZ ε(−90°, 90°), and an elevation component γ EZ  which extends from −90° to 90° (e.g., γ EZ ε(−90°, 90°). In one embodiment of the present invention, γ EZ  is set to a value greater than β, and γ AZ  can be any angle between −90° and 90°. In another embodiment of the invention, γ AZ  is further limited to further minimize interference. As described above, interference can be reduced by limiting the azimuthal gain characteristic  1504  of the terrestrial transmitter  102 . In such cases, to assure that the terrestrial receive antenna  110  can receive signals from the terrestrial transmitter  102 , the terrestrial transmitter is disposed so that γ AZ  is limited to smaller angles. Consistent with the analysis presented above, this angular limitation on γ AZ  is a function of minimum elevation angle ε min  and the latitude ζ of the satellite receive antenna  106 . 
       FIG. 18D  is a diagram illustrating a geometrical relationship between the satellite receive antenna  106 , the satellite transmitter  104 , and the terrestrial transmitter  102 . In this case, the satellite is not located directly due South from the satellite receive antenna  106  (as was the case illustrated in  FIG. 18B ). Instead, the angle between the satellite receive antenna  106  and the satellite transmitter  104  is displaced from due South by an angle δ  1812 . In this case, the angle defined by a vector between the satellite receive antenna  106  and the satellite  104  and a line of constant latitude  1810  is described as 90°−δ. 
       FIG. 19  is a flow chart presenting illustrative process steps used to practice one embodiment of the present invention. Block  1902  illustrates the step of aligning an azimuth and elevation angle of a first antenna such as the satellite receive antenna  106  to direct a first antenna main lobe substantially at a non-terrestrial source such as the satellite transmitter  104 . Then, the azimuth and elevation angle of the first antenna are adjusted to direct a null of the first antenna substantially at the terrestrial source and to direct a first antenna sidelobe substantially away from the terrestrial source. The foregoing azimuth and elevation angles can be adjusted independently, or the antenna can be set to be aligned the main lobe with the non-terrestrial source, then rotated to minimize interference from terrestrial transmitters  102 . 
       FIG. 20  is a flow chart presenting illustrative process steps used to practice one embodiment of the present invention. Block  2002  illustrates the step of receiving a first signal with the first antenna via a first antenna main lobe. Block  2004  presents the step of rejecting (i.e. sufficiently attenuating) the second signal with the first antenna via a first antenna null angularly displaced from the main lobe by less than 90 degrees. In one embodiment, the second signal is attenuated via a first antenna gain characteristic that is close enough to a null to prevent interference, but not necessarily coincident with a null. Block  2006  illustrates the step of receiving the second signal with a second antenna. In one embodiment, the first signal can also be rejected by the second antenna using a second antenna null. The foregoing steps can be performed in any order, but are preferably performed substantially simultaneously, to permit simultaneous reception of different signals on the same channel. 
     CONCLUSION 
     This concludes the description of the preferred embodiments of the present invention. In summary, the present invention discloses a method and apparatus for simultaneously receiving a first signal from a non-terrestrial source and a second signal from a terrestrial source on the same or overlapping channels. 
     The apparatus comprises at least one terrestrial transmitter transmitting information on at least one frequency simultaneously usable by at least one satellite transmitting to a satellite receive antenna having a sensitivity characterizable by a primary sensitive axis directed substantially at the satellite. The terrestrial transmitter includes a azimuthal gain characteristic directed substantially away from the Earth&#39;s Equator. In an alternative embodiment, the terrestrial transmitter is disposed at a location defining a vector angularly displaced from the primary sensitive axis by an angle of less than 90 degrees. 
     A method of transmitting information has also been disclosed. In this method the information is transmitted on at least one frequency simultaneously usable by at least one satellite transmitting to a satellite receive antenna having a sensitivity characterizable by a primary sensitive axis directed substantially at the satellite and a posterior secondary sensitive axis. The method is performed by transmitting the information from a terrestrially-based transmitter to a terrestrial receive antenna in a direction substantially away from the Equator. 
     The foregoing description of the preferred embodiment of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. 
     For example, in the foregoing discussion, mention is made of lobes and nulls in the satellite receive antenna  106  and the terrestrial receive antenna  110 . It should be understood, that the antenna nulls do not necessarily reflect a region of zero antenna sensitivity. In practice, some residual sensitivity exists, even at the antenna null points. Hence, such nulls include regions where the gain of the antenna is such that the relevant signal is essentially rejected. Further, it should be noted that although the foregoing invention has been described in terms of a satellite receive antenna  106  and a terrestrial receive antenna  110  disposed proximate thereto, it is possible and perhaps desirable in some instances to fashion both antennae from a single structure with shared components. Hence, the satellite receive antenna  106  and the terrestrial receive antenna  110  can be broadly described as antennas, which may or may not share structural components. For example it is possible to design a dual feed antenna with a single reflector (perhaps of a more complex shape) to implement the foregoing invention. 
     Further, in the foregoing description, mention is made of frequency sharing between the terrestrial and non-terrestrial transmission networks. However, it is understood that there exist transmission networks using modulation techniques such as code division multiple access (CDMA), in which frequency sharing can be described in terms of channel sharing. 
     It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto. The above specification, examples and data provide a complete description of the manufacture and use of the composition of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended.