Patent Publication Number: US-11658396-B2

Title: Methods and systems for mitigating interference with a nearby satellite

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application for patent is a continuation of U.S. patent application Ser. No. 16/846,780 by Diamond et al., entitled “Methods and Systems For Mitigating Interference With A Nearby Satellite” filed Apr. 13, 2020, which is a continuation of U.S. patent application Ser. No. 16/163,808 titled “Methods and Systems for Mitigating Interference with a Nearby Satellite” filed Oct. 18, 2018, which is a continuation of U.S. patent application Ser. No. 15/165,539, titled “Methods and Systems for Mitigating Interference with a Nearby Satellite”, filed May 26, 2016, which claims priority to U.S. Patent Application No. 62/171,418, titled “Methods and Systems for Mitigating Interference with a Nearby Satellite”, filed Jun. 5, 2015, each of which is assigned to the assignee hereof and expressly incorporated by reference herein for any and all purposes. 
    
    
     BACKGROUND 
     The present disclosure relates generally to satellite communications, and more specifically to airborne systems and methods for using such systems to avoid excessive interference with one or more non-target satellites during communication with a target satellite. 
     A geostationary satellite is a satellite that is in geostationary Earth orbit (GEO) about 35,800 km above Earth&#39;s equator, and has a revolution around the Earth synchronized with Earth&#39;s rotation. As a result, the geostationary satellite appears stationary to an observer on the Earth&#39;s surface. 
     Geostationary satellites occupy orbital slots separated in longitude along the geostationary arc above the Earth&#39;s equator. These geostationary satellites, which operate using various frequencies and polarizations, provide a variety of broadcast and communication services. Other types of satellites include low Earth orbit (LEO) satellites set between about 160 km and 2,000 km above Earth&#39;s surface, and medium Earth orbit (MEO) satellites set in orbit with an altitude greater than about 2,000 km and less than about 35,800 km above Earth&#39;s surface. 
     An Earth-based antenna terminal for communication with a satellite typically has high antenna gain and a narrow main beam pointed at the satellite, because of the large distance to the satellite and to avoid interference with other satellites. In order to satisfy interference requirements with the other satellites, a mobile antenna terminal may only be permitted to communicate with the target satellite when at certain geographic locations. In such a case, services provided by the satellite are unavailable to users of the mobile antenna terminal while at these locations, even though they are within the coverage area of the satellite. 
     SUMMARY 
     In one embodiment, an antenna system for mounting on an aircraft is described. The antenna system includes a primary antenna on the aircraft. The primary antenna is mechanically steerable and has an asymmetric antenna beam pattern with a narrow beamwidth axis and a wide beamwidth axis at boresight. The antenna system also includes a secondary antenna on the aircraft. The secondary antenna includes an array of antenna elements. The antenna system also includes an antenna selection system to control communication of a signal between the aircraft and a target satellite via the primary antenna and the secondary antenna. The antenna selection system switches communication of the signal from the primary antenna to the secondary antenna when an amount of interference with a non-target satellite reaches a threshold due to the wide beamwidth axis of the asymmetric antenna beam pattern. 
     In another embodiment, a method is described that includes communicating a signal between a target satellite and an aircraft via a primary antenna on the aircraft. The primary antenna is mechanically steerable and has an asymmetric antenna beam pattern with a narrow beamwidth axis and a wide beamwidth axis at boresight. The method also includes determining that an amount of interference with a non-target satellite reaches a threshold due to the wide beamwidth axis of the asymmetric antenna beam pattern. The method also includes, in response to the determination, switching communication of the signal from the primary antenna to a secondary antenna on the aircraft to reduce interference with the non-target satellite. The secondary antenna includes an array of antenna elements. 
     In yet another embodiment, an antenna system for mounting on an aircraft for communication with a target satellite is described. The antenna system includes a primary antenna comprising a first array of antenna elements and a positioner. The first array of antenna elements has a first main beam with a horizontal half-power beamwidth along a horizontal axis of the first array and has a vertical half-power beamwidth along a vertical axis of the first array. The vertical half-power beamwidth is greater than the horizontal half power beamwidth. The positioner is rotatably coupled with the first array about at least a first axis and a second axis to point the first main beam at the target satellite. The first main beam has a composite half power beamwidth that is less than or equal to a particular value over a first range of skew angles. The first main beam has a composite half power beamwidth that is greater than the particular value over a second range of skew angles. The antenna system also includes a secondary antenna oriented relative to the primary antenna. The secondary antenna includes a second array of antenna elements having a second main beam and a steering mechanism to point the second main beam at the target satellite. The second main beam has a composite half power beamwidth that is less than or equal to the particular value over the second range of skew angles. The antenna system also includes an antenna selection system to select between the primary antenna and the secondary antenna for communication of a signal with the target satellite based on the skew angle. 
     In yet another embodiment, an antenna system for mounting on an aircraft is described. The antenna system includes a primary antenna on the aircraft. The primary antenna has a first acceptable service area for communication of a signal between the aircraft and a target satellite while satisfying an interference requirement with a non-target satellite. The antenna system also includes a secondary antenna on the aircraft. The secondary antenna has a second acceptable service area for communication of the signal between the aircraft and the target satellite while satisfying the interference requirement with the non-target satellite. The second acceptable service area is different than the first acceptable service area. The antenna system also includes an antenna selection system to control communication of the signal between the aircraft and the target satellite via the primary antenna and the secondary antenna. The antenna selection system switches communication of the signal between the primary antenna and the secondary antenna based on a geographic location of the aircraft and the first and second acceptable service areas. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    illustrates an example satellite communications system in which an antenna system as described herein can be used to avoid excessive interference with one more satellites. 
         FIG.  2    is a block diagram illustrating an example antenna system on the aircraft of  FIG.  1   . 
         FIG.  3    illustrates a perspective view of an example primary antenna and an example secondary antenna of an example antenna system. 
         FIG.  4 A  illustrates a perspective view of the main beam of an example asymmetric antenna pattern of an example primary antenna. 
         FIG.  4 B  illustrates an example half-power contour of the asymmetric antenna pattern of main beam  FIG.  4 A . 
         FIG.  4 C  illustrates an example contour of the main beam of the secondary antenna at a particular scan angle to the target satellite, overlayed with the contour of main beam of  FIG.  4 B . 
         FIG.  5 A  illustrates an example acceptable service area of the primary antenna. 
         FIG.  5 B  illustrates the contour of the main beam of the primary antenna for an example geographic location within the acceptable service area. 
         FIG.  5 C  illustrates the contour of the main beam of the primary antenna for an example geographic location outside the acceptable service area. 
         FIG.  5 D  illustrates an example acceptable service area of the secondary antenna. 
         FIG.  5 E  illustrates an example composite acceptable service area for the antenna system. 
         FIG.  6    is an example graph of maximum power spectral density (PSD) curves for the primary antenna and the secondary antenna that satisfy interference requirements with the non-target satellite. 
         FIG.  7    is an example plot of the maximum value of the gain of the primary antenna at 2 degrees from boresight of the main beam versus skew angle. 
         FIG.  8    illustrates an example process for switching between the primary antenna and the secondary antenna. 
     
    
    
     DETAILED DESCRIPTION 
     An airborne antenna system described herein can provide efficient communication with a target satellite over a large geographical area, while also satisfying interference requirements with other satellites. In some embodiments, the airborne antenna system can provide non-interfering communication with a target satellite, over the entire or substantially the entire coverage area (or footprint) of the target satellite. In doing so, services such as Internet, telephone and/or television services provided by the target satellite can be delivered to airborne users throughout most or all of the satellite&#39;s coverage area, while also satisfying interference requirements with other satellites. 
     The antenna system can include a primary antenna and a secondary antenna on an aircraft such as an airplane. The antenna system can also include an antenna selection system to control communication of one or more signals between the aircraft and the target satellite via the primary antenna and the secondary antenna. 
     The primary antenna can be mechanically steerable about at least one axis to point a main beam of the primary antenna at the target satellite. As used herein, a main beam of an antenna that is “pointed” at a satellite has sufficient antenna gain in the direction of the target satellite to permit communication of one or more signals. The communication can be bidirectional (i.e., the antenna transmits a signal to the satellite and also receives a signal from the satellite) or unidirectional (i.e., the antenna either transmits a signal to the satellite or receives a signal from the satellite, but not both). The direction of the target satellite may be boresight of the antenna. As used herein, “boresight” of an antenna refers to the direction of maximum gain of the antenna. Alternatively, the gain in the direction of the target satellite may be less than the maximum gain of the antenna. In other words, the direction of the satellite may not be in the exact center of the main beam of the antenna. This may for example be due to motion induced pointing accuracy limitations of the antenna. 
     In embodiments described herein, the primary antenna has a non-circular antenna aperture that results in an asymmetric antenna beam pattern at boresight. The non-circular shape of the antenna aperture can be due to the combination of electrical performance requirements and size constraints. Specifically, the non-circular antenna aperture of the primary antenna is designed to have a large enough effective area to provide sufficient antenna gain to satisfy link requirements between the aircraft and the target satellite under various operational conditions, while also having a swept volume small enough that it can be housed under an aerodynamic radome on the aircraft. The primary antenna can vary from embodiment to embodiment. In one embodiment, the primary antenna is an array of antenna elements arranged in a rectangular panel. 
     The asymmetric antenna beam pattern of the primary antenna has a narrow beamwidth axis and a wide beamwidth axis at boresight. As described in more detail below, when the antenna system is at certain geographic locations, the wide beamwidth axis can give rise to excessive interference with one or more other (non-target) satellites, if the primary antenna were used to communicate with the target satellite. 
     The antenna system described herein can avoid the excessive interference that could result due to the wide beamwidth axis of the primary antenna, thereby allowing non-interfering communication with the target satellite over a large geographic area. As described in more detail below, the antenna system includes a secondary antenna, which can be located underneath the same radome as the primary antenna, and an antenna selection system. The secondary antenna can be a different type of antenna than the primary antenna, and/or have a different beam steering mechanism than the primary antenna. 
     The antenna selection system controls whether the primary antenna or the secondary antenna is used to communicate each of the one or more signals communicated between the aircraft and the target satellite. Using the techniques described herein, the antenna selection system can determine when the amount of interference with one or more non-target satellites using the primary antenna, due to the wide beamwidth axis, reaches a threshold. In response to the determination, the antenna selection system can switch to communicating with the target satellite using the secondary antenna. In doing so, the antenna system described herein can provide communication with the target satellite at locations where use of the primary antenna is precluded due to interference requirements. As a result, the service area over which services provided by the target satellite can be delivered to airborne users can be larger as compared to only using the primary antenna. 
       FIG.  1    illustrates an example satellite communications system  100  in which an antenna system  150  as described herein can be used to avoid excessive interference with one more satellites. Many other configurations are possible having more or fewer components than the satellite communication system  100  of  FIG.  1   . 
     As can be seen in  FIG.  1   , the antenna system  150  is mounted on aircraft  102 . In the illustrated embodiment, the aircraft  102  is an airplane. Alternatively, the antenna system  150  can be mounted to other types of aircraft, such as a helicopter, drone, etc. 
     As described in more detail below, the antenna system  150  facilitates communication between the aircraft  102  and satellite  110  (hereinafter referred to as the “target satellite  110 ”), while also satisfying interference requirements with one or more other (non-target) satellites. The antenna system  150  includes an antenna selection system (not shown) to control communication of one or more signals with the target satellite  110  via a primary antenna  152  and a secondary antenna  154 , using the techniques described herein. In the illustrated embodiment, the primary antenna  152  and the secondary antenna  154  are located under the same radome  156 . Alternatively, the primary antenna  152  and the secondary antenna  154  can be located under separate radomes on the aircraft. 
     In some embodiments in which the primary antenna  152  and the secondary antenna  154  are located under the same radome  156 , the shape of the radome  156  may designed to house the primary antenna  152  and satisfy aerodynamic requirements, and the secondary antenna  154  may be selected or designed to fit within remaining room under the radome  156 . 
     The antenna system  150  can also include memory for storage of data and applications, a processor for accessing data and executing applications, and components that facilitate communication over the satellite communication system  100 . Although only one aircraft  102  is illustrated in  FIG.  1    to avoid over complication of the drawing, the satellite communications system  100  can include many more aircraft  102  having respective antenna systems  150  mounted thereon. 
     In the illustrated embodiment, the target satellite  110  provides bidirectional communication between the aircraft  102  and a gateway terminal  130 . The gateway terminal  130  is sometimes referred to as a hub or ground station. The gateway terminal  130  includes an antenna to transmit a forward uplink signal  140  to the target satellite  110  and receive a return downlink signal  142  from the target satellite  110 . The gateway terminal can also schedule traffic to the antenna system  150 . Alternatively, the scheduling can be performed in other parts of the satellite communications system  100  (e.g. a core node, satellite access node, or other components, not shown). Signals  140 ,  142  communicated between the gateway terminal  130  and target satellite  110  can use the same, overlapping, or different frequencies as signals  112 ,  114  communicated between the target satellite  110  and the antenna system  150 . 
     Network  135  is interfaced with the gateway terminal  130 . The network  135  can be any type of network and can include for example, the Internet, an IP network, an intranet, a wide area network (WAN), a local area network (LAN), a virtual private network (VPN), a virtual LAN (VLAN), a fiber optic network, a cable network, a public switched telephone network (PSTN), a public switched data network (PSDN), a public land mobile network, and/or any other type of network supporting communication between devices as described herein. The network  135  can include both wired and wireless connections as well as optical links. The network  135  can include both wired and wireless connections as well as optical links. The network  135  can connect multiple gateway terminals  130  that can be in communication with target satellite  110  and/or with other satellites. 
     The gateway terminal  130  can be provided as an interface between the network  135  and the target satellite  110 . The gateway terminal  130  can be configured to receive data and information directed to the antenna system  150  from a source accessible via the network  135 . The gateway terminal  130  can format the data and information and transmit forward uplink signal  140  to the target satellite  110  for delivery to the antenna system  150 . Similarly, the gateway terminal  130  can be configured to receive return downlink signal  142  from the target satellite  110  (e.g. containing data and information originating from the antenna system  150 ) that is directed to a destination accessible via the network  135 . The gateway terminal  130  can also format the received return downlink signal  142  for transmission on the network  135 . 
     The target satellite  110  can receive the forward uplink signal  140  from the gateway terminal  130  and transmit corresponding forward downlink signal  114  to the antenna system  150 . Similarly, the target satellite  110  can receive return uplink signal  116  from the antenna system  150  and transmit corresponding return downlink signal  142  to the gateway terminal  130 . The target satellite  110  can operate in a multiple spot beam mode, transmitting and receiving a number of narrow beams directed to different regions on Earth. Alternatively, the target satellite  110  can operate in wide area coverage beam mode, transmitting one or more wide area coverage beams. 
     The target satellite  110  can be configured as a “bent pipe” satellite that performs frequency and polarization conversion of the received signals before retransmission of the signals to their destination. As another example, the target satellite  110  can be configured as a regenerative satellite that demodulates and remodulates the received signals before retransmission. 
     As shown in  FIG.  1   , the satellite communications system  100  also includes another satellite  120  (hereinafter referred to as “non-target satellite  120 ”). Communication of one or more signals between the non-target satellite  120  and the antenna system  150  is undesired or unintended. Although only one non-target satellite  120  is illustrated in  FIG.  1    to avoid over complication of the drawing, the satellite communications system  100  can include many more non-target satellites  120  and the techniques described herein can be used to avoid excessive interference with each of the non-target satellites  120 . 
     The non-target satellite  120  can, for example, be configured as a bent pipe or regenerative satellite. The non-target satellite  120  can communicate one or more signals with one or more ground stations (not shown) and/or other terminals (not shown). 
     As mentioned above, the antenna system  150  includes an antenna selection system to control communication with the target satellite  110  via the primary antenna  152  and the secondary antenna  156 , while also avoiding excessive interference with the non-target satellite  120 . The antenna system  150  is described in more detail below with respect to  FIGS.  2 - 3    and others. 
     As used herein, interference “with” the non-target satellite  120  can refer to uplink interference and/or downlink interference. Uplink interference is interference to the non-target satellite  120  caused by a portion of the return uplink signal  116  transmitted by the antenna system  150  that is received by the non-target satellite  120 . Downlink interference is interference to the antenna system  150  caused by a portion of a signal transmitted by the non-target satellite  120  that is received by the antenna system  150 . 
     In the illustrated embodiment, the target satellite  110  and the non-target satellite  120  are each geostationary satellites. The geostationary orbit slots, and thus the angular separation along the geostationary arc between the target satellite  110  and the non-target satellite  120 , can vary from embodiment to embodiment. In some embodiments the angular separation along the geostationary arc is at least two degrees. In alternative embodiments, one or both of the target satellite  110  and the non-target satellite  120  can be a non-geostationary satellite, such as a LEO or MEO satellite. The non-target satellite  120  can for example be adjacent to the target satellite  110 . As used herein, the target satellite  110  and the non-target satellite  120  are “adjacent” if the effective angular separation between them as viewed at antenna system  150  is less than or equal to 10 degrees. 
       FIG.  2    is a block diagram illustrating an example antenna system  150  on the aircraft  102  of  FIG.  1   . The antenna system  150  can include primary antenna  152 , secondary antenna  154 , antenna selection system  200 , transceiver  210 , modem  230 , network access unit (NAU)  240 , and wireless access point (WAP)  250 . Many other configurations are possible having more or fewer components than the antenna system  150  shown in  FIG.  2   . Moreover, the functionalities described herein can be distributed among the components in a different manner than described herein. 
     In the illustrated embodiment, the primary antenna  152  and the secondary antenna  154  are each housed under the same radome  156  disposed on the top of the fuselage or other location (e.g., on the tail, etc.) of the aircraft  102 . Alternatively, the primary antenna  152  and the secondary antenna  154  can be housed under separate radomes which can be located in different locations on the aircraft  102 . 
     The antenna system  150  can provide for transmission of the forward downlink signal  114  and reception of the return uplink signal  116  to support two-way data communications between data devices  260  within the aircraft  102  and the network  135  via target satellite  110  and gateway terminal  130 . The data devices  260  can include mobile devices (e.g., smartphones, laptops, tablets, netbooks, and the like) such as personal electronic devices (PEDs) brought onto the aircraft  102  by passengers. As further examples, the data devices  260  can include passenger seat back systems or other devices on the aircraft  102 . The data devices  260  can communicate with the network access unit  240  via a communication link that can be wired and/or wireless. The communication link can be, for example, part of a local area network such as a wireless local area network (WLAN) supported by WAP 250. One or more WAPs can be distributed about the aircraft  102 , and can, in conjunction with network access unit  240 , provide traffic switching or routing functionality; for example, as part of a WLAN extended service set (ESS), etc. The network access unit  240  can also allow passengers to access one or more servers (not shown) local to the aircraft  102 , such as a server that provides in-flight entertainment. 
     In operation, the network access unit  240  can provide uplink data received from the data devices  260  to the modem  230  to generate modulated uplink data (e.g. a transmit IF signal) for delivery to the transceiver  210 . The transceiver  210  can upconvert and then amplify the modulated uplink data to generate the return uplink signal  116  ( FIG.  1   ) for transmission to the target satellite  110  ( FIG.  1   ) via the primary antenna  152  or the secondary antenna  154 . Similarly, the transceiver  210  can receive the forward downlink signal  114  ( FIG.  1   ) from the target satellite  110  ( FIG.  1   ) via the primary antenna  152  or the secondary antenna  154 . The transceiver  210  can amplify and then downconvert the forward downlink signal  114  to generate modulated downlink data (e.g., a receive IF signal) for demodulation by the modem  230 . The demodulated downlink data from the modem  230  can then be provided to the network access unit  240  for routing to the data devices  260 . The modem  230  can be integrated with the network access unit  240 , or can be a separate component, in some examples. 
     In the illustrated embodiment, the transceiver  210  is located outside the fuselage of the aircraft  102  and under the radome  156 . Alternatively, the transceiver  210  can be located in a different location, such as within the aircraft interior. In the illustrated embodiment, the transceiver  210  is shared between the primary antenna  152  and the secondary antenna  154 . Alternatively, the antenna system  150  may include a first transceiver coupled to the primary antenna  152 , and a second transceiver coupled to the secondary antenna  154 . In such a case, the modem  230  may be shared by the first transceiver and the second transceiver, or may use separate modems. 
     As described in more detail below, the antenna selection system  200  can control whether the primary antenna  152  or the secondary antenna  154  is used to receive the forward downlink signal  114  from the target satellite  110 , and also whether the primary antenna  152  or the secondary antenna  154  is used to transmit the return uplink signal  116  to the target satellite  110 . The functions of the antenna selection system  200  can be implemented in hardware, instructions embodied in a memory and formatted to be executed by one or more general or application-specific processors, firmware, or any combination thereof. In the illustrated embodiment, the antenna selection system  200  is shown as a separate device. Alternatively, some or all of the components or features of the antenna selection system  200  can be implemented within one or more other components of the antenna system  150 . In the illustrated embodiment, the antenna selection system  200  is located under the radome  156 . Alternatively, some or all of the antenna selection system  200  can be located in a different location, such as within the aircraft interior. As another example, some or all of the antenna selection system  200  may be located in other parts of the satellite communications system  100 , such as the gate terminal  130 , a core node, satellite access node, or other components not shown. 
     The primary antenna  152  can include an array of antenna elements that are operable over the frequency ranges of both the forward downlink signal  114  and the return uplink signal  116 . In such a case, the same antenna elements of the array can transmit the return uplink signal  116  and receive forward downlink signal  114 . Alternatively, the primary antenna  152  can include a first group of one or more antenna elements to transmit the return uplink signal  116 , and a second group of one or more antenna elements to receive forward downlink signal  114 . 
     The primary antenna  152  can include a positioner rotatably coupled to the array of the primary antenna  152  to mechanically steerable the array about at least one axis to point the main beam of the array of the primary antenna  210  at the target satellite  110  as the aircraft  102  moves. In some embodiment, the primary antenna  152  is fully mechanically steered using an elevation-over-azimuth (EL/AZ), two-axis positioner. Alternatively, the positioner may include other mechanisms for providing adjustment in azimuth and elevation. For example, in some alternative embodiments, the primary antenna  152  can include a combination of mechanical and electrical scanning mechanisms. As another example, the primary antenna  152  includes a fully mechanically steered using three-axis positioner to provide adjustment in azimuth, elevation and skew. The primary antenna  152  can also include an antenna control unit to provide control signals to the positioner. 
     The primary antenna  152  has a non-circular antenna aperture that results in an asymmetric antenna beam pattern of the main beam at boresight. The non-circular shape of the antenna aperture can be due to the combination of electrical performance requirements and size constraints. Specifically, the non-circular antenna aperture of the primary antenna  152  can be designed to have a large enough effective area to provide sufficient antenna gain to satisfy link requirements between the aircraft  102  and the target satellite  110  under various operational conditions, while also having a swept volume small enough that it can be housed under an aerodynamic radome  156  on the aircraft  102 . 
     The primary antenna  152  can be any type of antenna that fits under an aerodynamic radome and provides an asymmetric antenna beam pattern, and can vary from embodiment to embodiment. In some embodiments, the primary antenna  152  is an array of waveguide antenna elements arranged in a rectangular panel. Each of the one or more antenna elements can include a waveguide-type feed structure including a horn antenna. Alternatively, other types of structures and antenna elements can be used for the primary antenna  210 . For example, in another embodiment, the primary antenna  210  can include one or more feeds illuminating a reflector having an asymmetric reflector surface. As another example, the primary antenna  152  can include multiple, separately moveable panels that together provides an asymmetric antenna aperture. 
     The asymmetric antenna beam pattern of the primary antenna  152  has a wide beamwidth axis and a narrow beamwidth axis. As described in more detail below, when the antenna system  150  (and thus the aircraft  102 ) is at certain geographic locations, the wide beamwidth axis can give rise to excessive interference with the non-target satellite  120 , if the primary antenna  152  were used to communicate with the target satellite  110 . 
     When using the primary antenna  152  to communicate with the target satellite  110 , the antenna selection system  200  can switch to communicating with the target satellite  110  using the secondary antenna  154  when the amount of interference with the non-target satellite  120 , due to the wide beamwidth axis, reaches a threshold. In doing so, the antenna system  150  can provide communication with target satellite  110  at geographic locations where use of the primary antenna  152  is precluded due to interference requirements. As a result, the techniques described herein can ensure that the interference generated is within acceptable limits to other satellite system operators, while at the same time satisfying link requirements between the aircraft  102  and the target satellite  110 . 
     The secondary antenna  154  can include an array of antenna elements and a steering mechanism for pointing a main beam of the array at the target satellite  110  as the aircraft  102  moves. The secondary antenna  154  can be a different type of antenna than the primary antenna  152 , and/or have a different beam steering mechanism than the primary antenna  152 . As described in more detail below, the secondary antenna  152  is arranged relative to the primary antenna, and has different composite beamwidth characteristics versus skew angle than the primary antenna  152  at various geographic locations, such that the secondary antenna  154  can provide an acceptable service area for communication with target satellite  110  that is different than the acceptable service area provided by the primary antenna  152 . 
     Thus, at a given geographic location that is within the acceptable service area of the secondary antenna  154  and also outside the acceptable service area of the primary antenna  152 , the secondary antenna  154  can satisfy interference requirements with the non-target satellite  110 . In other words, switching to the secondary antenna  154  can reduce interference with the non-target satellite  120  as compared to the primary antenna  152 , while still permitting communication between the aircraft  102  and the target satellite  110 . In doing so, the secondary antenna  152  can provide for communication with the target satellite  110  at geographic locations where use of the primary antenna  152  is precluded due to interference requirements. 
     At some or all of geographic locations for the aircraft  102 , the primary antenna  152  may be designed to provide better performance characteristics than the secondary antenna  154  for communicating at least one of the return uplink signal  116  and the forward downlink signal  114  with the target satellite  110 . For example, the primary antenna  152  can have one or more of higher gain, lower sidelobes, cross-polarization, etc. 
     As used herein, the interference “with” non-target satellite  120  can be uplink interference and/or downlink interference. Uplink interference is interference to the non-target satellite  120  caused by electromagnetic energy from a portion of the return uplink signal  116  that is received by the non-target satellite  120 . Downlink interference is interference to the antenna system  150  caused by radiated electromagnetic energy from the non-target satellite  120  that is received by the antenna system  150 . The downlink interference can increase the equivalent noise temperature at a receiver of the antenna system  150 , which in turn reduces the signal-to-noise ratio of the forward downlink signal  114  received by the antenna system  150 . 
     The antenna selection system  200  can switch between the primary antenna  152  and the secondary antenna  154  based one or more thresholds for the amount of interference with the non-target satellite  120 . The one or more thresholds can be based on uplink interference and/or downlink interference and can vary from embodiment to embodiment. 
     In some embodiments, the same threshold can be used for switching from the primary antenna  152  to the secondary antenna  154 , and for switching from the secondary antenna  152  to the primary antenna  152 . In other words, the antenna selection system  200  can switch from the primary antenna  152  to the secondary antenna  154  when the amount of interference reaches the threshold, and switch back to the primary antenna  152  when the amount of interference using the primary antenna  152  will be below the threshold. In some other embodiments, the threshold for switching from the primary antenna  152  to the secondary antenna  154  can be different than the threshold for switching from the secondary antenna  154  to the primary antenna  152 . In such a case, the antenna selection system  200  can avoid rapidly switching between the antennas  152 ,  154  when the aircraft  102  is near the boundary of the acceptable service area of the primary antenna  152 . 
     In some embodiments, the value of the threshold for switching transmission of the return uplink signal  116  from the primary antenna  152  to the secondary antenna  154  can for example be based on regulatory requirements imposed by regulatory agencies (e.g. FCC, ITU, etc.) on the maximum power spectral density (or other metric) that can be radiated to the non-target satellite  120 , or coordination agreements with the operator of the non-target satellite  120 . Additionally, the threshold for switching transmission of the return uplink signal  116  from the primary antenna  152  to the secondary antenna  154  can account for one or more of motion induced pointing accuracy limitations of the primary antenna  152 , etc. 
     The antenna selection system  200  can determine when to switch based on a comparison(s) of the threshold(s) to the amount of interference with the non-target satellite  120  at the current geographic location and attitude of the aircraft  102 . The current geographic location may for example be provided via a global positioning system (GPS) or other equipment on the aircraft  102 . The attitude (including yaw, roll and pitch) of the aircraft  102  may for example be provided via an internal reference unit (IRU) on the aircraft  102 . 
     The amount of interference at a given geographic location can be determined using various techniques, and can be characterized or represented in different ways. For example, in some embodiments the amount of interference is represented in terms of power spectral density (PSD). 
     The amount of uplink interference can for example be determined based on one or more of the known antenna pattern characteristics of the primary antenna  152  and the secondary antenna  154 , the transmission parameters (e.g. transmit power, frequency range, etc.) of the return uplink signal  116 , the geographic location of the aircraft  102 , the attitude of the aircraft  102 , the locations of the target satellite  110  and non-target satellite  120 , the operating frequency, system gain-to noise temperature (G/T) and/or polarization of operation of the non-target satellite  120 , etc. Alternatively, other and/or additional information can be used to calculate the amount of interference. The amount of downlink interference can be calculated in a similar manner based on the parameters of a signal from the non-target satellite  120  that is received by the antenna system  150 . 
     In some embodiments, the comparison of the threshold(s) to the amount of interference at the various geographic locations has been previously calculated for each of the primary antenna  152  and the secondary antenna  154 . In such a case, the antenna selection system  200  can store a look-up table indicating which of the primary antenna  152  and secondary antenna  154  to use based on the current geographic location and attitude of the aircraft  102 . 
     The manner in which the antenna selection system  200  controls the switching between the primary antenna  152  and the secondary antenna  154  can vary from embodiment to embodiment. In some embodiments, the antenna selection system  200  provides control signals to the transceiver  210  (or transceivers) to enable/disable operation of electronics associated with the primary antenna  152  and the secondary antenna  154 . In other embodiments, the antenna selection system  200  controls switches that route the signals between the modem  230  and the antennas  152 ,  154  through the transceiver  210 . Alternatively, other techniques can be used. 
     In some embodiments, the antenna selection system  200  switches both the transmission of return uplink signal  116  and the reception of forward downlink signal  114  when switching between the primary antenna  152  and the secondary antenna  154 . In such a case, the same antenna (either primary antenna  152  or the secondary antenna  154 ) is used for transmitting the return uplink signal  116  and for receiving the forward downlink signal  114  at a particular time during operation. 
     In some other embodiments, the antenna selection system  200  switches only one of the transmission of return uplink signal  116  and the reception of forward downlink signal  114  when switching between the primary antenna  152  and the secondary antenna  154 . For example, in embodiments in which the switching is done to avoid excessive uplink interference with the non-target satellite  120 , the primary antenna  152  can be used to receive forward downlink signal  114  regardless of whether the return uplink signal  116  is transmitted via the primary antenna  152  or the secondary antenna  154 . In doing so, overall system performance can be improved in embodiments in which downlink interference is not excessive and the primary antenna  152  can provide better performance characteristics (e.g. higher gain, etc.) than the secondary antenna  154  for reception of the forward downlink signal  114 . Using the secondary antenna  154  only for transmission of the return uplink signal  116  may simplify the secondary antenna  154  and the associated electronics. For example, the secondary antenna  154  may be operable over the frequency range of the return uplink signal  116 , but not the frequency range of the forward downlink signal  114 . In embodiments in which the primary antenna  152  is coupled to a dedicated transceiver, the secondary antenna  154  can be coupled to a transmitter rather than another transceiver. As another example, the primary antenna  152  may be coupled to a receiver, and a shared transmitter may be selectively switched between the primary antenna  152  and the secondary antenna  154 . 
     In addition to switching between the primary antenna  152  and the secondary antenna  154 , the antenna selection system  200  can also change the transmission parameters of the return uplink signal  116  to avoid excessive interference when needed. For example, the antenna selection system  200  can change one or more of transmitted power level of the return uplink signal  116 , spreading the return uplink signal  116  over a larger bandwidth, or any other technique for reducing the power spectral density in the direction of the non-target satellite  120 . 
     In some embodiments, the primary antenna  152  and the secondary antenna  154  each remain pointed at the target satellite  110  regardless of which antenna  152 ,  154  is currently being used. In other words, the antenna system  150  maintains pointing of the primary antenna  152  and the secondary antenna  154  at the target satellite  110  following switching of the communication with the target satellite  110 . In such a case, the handover time between the primary antenna  152  and the secondary antenna  154  can be minimized. 
     In some embodiments, the antenna system  150  maintains the return link operating point (e.g., energy per symbol to noise power spectral density E s /N 0 ) regardless of whether the primary antenna  152  and the secondary antenna  154  is used to transmit the return uplink signal  116 . For example, in embodiments in which the gain of the primary antenna  152  is greater than the gain and the secondary antenna  154 , the antenna system  150  may increase the transmit power of the return uplink signal  116  upon switching from the primary antenna  152  to the secondary antenna  154 . The antenna system  150  may then reduce the transmit power upon switching back to the primary antenna  152 . In some alternative embodiments, the antenna system  105  can have different return link operating points for the primary antenna  152  and the secondary antenna  154 . The different operating points can be due to differences in the gains of the primary antenna  152  and the secondary antenna  154 , and/or different transmit powers of the return uplink signal  116  when using the primary antenna  152  and the secondary antenna  154 . 
       FIG.  3    illustrates a perspective view of an example primary antenna  152  and an example secondary antenna  154  of an example antenna system  150 . 
     The primary antenna  152  can include a positioner  300  and an array  310  of antenna elements. The array  310  of antenna elements has a non-circular aperture that includes a major axis  312  (referred to hereinafter as “horizontal axis  312 ”), which is the longest line through the center of array  310  of antenna elements. The array  310  of antenna elements that also includes a minor axis (referred to hereinafter as “vertical axis  314 ”), which is the shortest line through the center of the array  310  of antenna elements. The non-circular aperture of the array  310  of antenna elements defines an antenna beam having an asymmetric antenna beam pattern at boresight. 
     In the illustrated embodiment, the array  310  of antenna elements is a direct radiating two-dimensional array which results in boresight being normal to a plane containing the antenna elements of the array  310 . As a result, in the illustrated embodiment the asymmetric antenna beam pattern has a narrow beamwidth axis aligned with the horizontal axis  312  and a wide beamwidth axis aligned with the vertical axis  314 . Alternatively, the array  310  of antenna elements can be arranged and/or fed in a different manner such that boresight is not normal to the plane containing the antenna elements of the array  310 . 
     The positioner  300  is responsive to commands from an antenna control unit (not shown) of the antenna system  150  to mechanically steer the primary antenna  152  to point the main beam of the array  310  in the direction of the target satellite  110 . In the illustrated embodiment the positioner  300  is an elevation-over-azimuth (EL/AZ) two-axis positioner that provides full two-axis mechanical steering. The positioner  300  includes a mechanical azimuth adjustment mechanism to move the primary antenna  152  in azimuth  320 , and a mechanical elevation adjustment mechanism to move the primary antenna  152  in elevation  320 . Each of the mechanical adjustment mechanisms can for example include a motor with gears and other elements to provide for movement of the primary antenna  152  around a corresponding axis. In some alternative embodiments, the steering mechanism for the primary antenna  152  may include a combination of mechanical and electrical steering of the main beam. 
     The secondary antenna  154  can include an array  350  of antenna elements having a main beam. The secondary antenna  154  includes a steering mechanism to point the main beam of the array  350  at the target satellite  110 . The type of antenna elements, orientation of the antenna elements, and the steering mechanism of the secondary antenna  154  can vary from embodiment to embodiment. In some embodiments, the array  350  includes antenna elements that are operable over the frequency ranges of both the forward downlink signal  114  and the return uplink signal  116 . In such a case, the same antenna elements of the array  350  can transmit the return uplink signal  116  and receive the forward downlink signal  114 . In some alternative embodiments, the array  350  includes a first group of one or more antenna elements to transmit the return uplink signal  116 , and a second group of one or more antenna elements to receive the forward downlink signal  114 . In embodiments in which the secondary antenna  154  is only used for transmission of the return uplink signal  116 , the antenna of the array  350  may be operable over the frequency range of the return uplink signal  116 , but not the frequency range of the forward downlink signal  114 . 
     In the illustrated embodiment, the antenna elements of the array  350  are arranged in a circular two-dimensional array arranged in a plane  352 . Alternatively, the antenna elements of the array  350  may be arranged in a different fashion. For example, the array  350  may have a non-circular antenna aperture that results in an asymmetric antenna beam pattern at boresight. In such a case, the asymmetric antenna beam pattern of the secondary antenna  154  has a narrow beamwidth axis and a wide beamwidth axis at boresight. In some embodiments, the steering mechanism of the secondary antenna  154  includes a mechanical azimuth adjustment mechanism responsive to commands (e.g., from an antenna control unit, the antenna selection system, etc.) to rotate the secondary antenna  154  in azimuth, and an azimuth/elevation adjustment mechanism to steer the main beam of the secondary antenna  154  in the direction of the target satellite. As the aircraft  102  moves, the mechanical azimuth adjustment mechanism can be used to maintain alignment of the narrow beamwidth axis with a line defined by the target satellite  110  and the non-target satellite  120 . By aligning the narrow beamwidth axis with that line, the amount of interference with the non-target satellite  120  can be minimized while the secondary antenna  154  is being used. 
     In the illustrated embodiment, the array  350  is a non-movable, fully electronic scanned phased array antenna. The array  350  can include feed networks and phase controlling devices to properly phase signals communicated with some or all the antenna elements of the array  350  to scan the beam in azimuth and elevation from the normal to the plane  352 . 
     Alternatively, the secondary antenna  154  can include a different steering mechanism, which can vary based on the antenna type of the secondary antenna  154 . For example, in some alternative embodiments, the secondary antenna  154  can be an electro-mechanically steered array that includes one mechanical scan axis and one electrical scan axis, such as a variably inclined continuous transverse stub (VICTS) antenna. As another example, the secondary antenna  154  can be an offset fed, parabolic cylinder reflector antenna, such as an antenna of the type of DBS-2130 antenna available from L-3 Communications. As yet another example, the secondary antenna  154  can be an EXPLORER 9092H or 9092M antenna available from Cobham, plc. 
     The combination of the primary antenna  152  and the secondary antenna  154  can vary from embodiment to embodiment. In some embodiments in which the target satellite  110  operates at Ka-band, the primary antenna  152  is Aero Mobile Terminal Model 2540 available from ViaSat Inc., and the secondary antenna  154  is a ThinAir Falcon-Ka2517 VICTS antenna available from ThinKom. In embodiments in which the secondary antenna  154  is only used for transmission of the return uplink signal  116 , the secondary antenna  154  may only include the transmit antenna aperture of the ThinAir Falcon-Ka2517. 
       FIG.  4 A  illustrates a perspective view of the main beam  422  of an example asymmetric antenna pattern of an example primary antenna  152 . The main beam  422  has a 3-dB (half power) contour with an elliptical shape about boresight  430 . The positioner  300  ( FIG.  3   ) can move the primary antenna  152  to point the boresight  430  of the main beam  422  is the direction of the target satellite  110 . The direction can be described in terms of azimuth  424  and elevation  434 . Azimuth  424  refers to the angle between boresight  430  and reference  402 , and elevation  434  refers to the angle between boresight  430  and horizon  401 . 
       FIG.  4 B  illustrates an example half-power contour of the asymmetric antenna pattern of main beam  422   FIG.  4 A . The main beam  422  has a first half-power beamwidth (hereinafter referred to as “horizontal half-power beamwidth”) along the narrow beamwidth axis  440  that corresponds the horizontal axis  312  of the primary antenna  152 , and a second half-power beamwidth (hereinafter referred to as “vertical half-power beamwidth) along the wide beamwidth axis  450  corresponding to the vertical axis  314  of the primary antenna  152 . The horizontal half-power beamwidth and the vertical half-power beamwidth can vary from embodiment to embodiments. In some embodiments, the vertical half-power beamwidth is at least three times greater than the horizontal half-power beamwidth, such as being at least four times greater. For example, in some embodiments the vertical half-power beamwidth can be less than three degrees, and the horizontal half-power beamwidth can be less than one degree. Alternatively, the vertical half-power beamwidth and the horizontal half-power beamwidth may be different than the examples above. 
     As shown in  FIG.  4 B , the main beam  422  has a skew angle  460 . As used herein, “skew angle” refers to the angle between the narrow beamwidth axis of the main beam of an antenna (e.g. narrow beamwidth axis  440  of the main beam  422 ), and a line defined by the target satellite  110  and the non-target satellite  120 . The half-power beamwidth of the main beam  422  along the line defined by the target satellite  110  and non-target satellite  120  is referred to herein as a “composite half-power beamwidth”  470 . The composite half-power beamwidth  470  is a mixture of the half-power beamwidths along the narrow beamwidth axis and wide beamwidth axis respectively, and depends on the skew angle  460 . For example, in embodiments in which the target satellite  110  and the non-target satellite  120  are geostationary satellites along the geostationary arc, the skew angle  460  is the angle between the narrow beamwidth axis  440  and the geostationary arc, and the composite half-power beamwidth  470  is the beamwidth along the geostationary arc. 
     The skew angle  460 , and thus the composite half-power beamwidth  470 , varies depending upon the geographic location of the aircraft  102  that includes the antenna system  150 . For example, if the antenna system  150  is located at the same longitude as the target satellite  110 , the skew angle  460  is zero degrees and the composite half-power beamwidth  470  is the horizontal half-power beamwidth along the narrow beamwidth axis  440 . In such a case, the composite half-power beamwidth  470  can be narrow enough to satisfy interference requirements with the non-target satellite  120 . However, if the antenna system is located at a different longitude than the target satellite  110 , the skew angle  460  is non-zero and the composite half-power beamwidth  470  is a mixture of the vertical half-power beamwidth and the horizontal half-power beamwidth. As a result, at certain geographic locations, the composite half-power beamwidth  470  can be wide enough to cause excessive interference with the non-target satellite  120 , if the primary antenna  152  were used to communicate with target satellite  110 . In other words, due to the vertical half-power beamwidth along the wide beamwidth axis  450 , at certain geographic locations within the service area of the target satellite  110 , the interference level could exceed the threshold amount of interference with the non-target satellite  120  if the primary antenna  152  were used. 
       FIG.  4 C  illustrates an example contour of the main beam  480  of the secondary antenna  154  at a particular scan angle to the target satellite  110 , overlayed with the contour of main beam  422  of  FIG.  4 B . In embodiments in which the secondary antenna  154  is electronically scanned in at least one axis, the contour of the main beam  480  can vary with pointing direction (scan angle) to the target satellite  110 . In other words, at least one of the vertical half-power beamwidth and the horizontal half-power beamwidth of the main beam  480  of the secondary antenna  154  can vary based on the geographic location of the aircraft  102 . In embodiments in which the primary antenna  152  is fully mechanically steered, the main beam  422  does not vary with pointing direction. 
     The vertical half-power beamwidth and the horizontal half-power beamwidth of the main beam  480  of the secondary antenna  154  can vary from embodiment to embodiment. In some embodiments, the vertical half-power beamwidth is less than three times than the horizontal half-power beamwidth. 
     Line  490  represents the maximum acceptable skew angle for the main beam  422  of the primary antenna  152  that satisfies interference requirements with the non-target satellite  120 . That is, for a range  492  of skew angles, the composite half-power beamwidth of the main beam  422  is less than or equal to a particular value, such that the amount of interference with the non-target satellite  120  when using the primary antenna  152  is at or below the threshold. Accordingly, for a range  494  of skew angles, the composite half-power beamwidth of the main beam  422  is greater than the particular value, such that the amount of interference with the non-target satellite  120  would exceed the threshold if the primary antenna  152  were used. 
     As can be seen in  FIG.  4 B , for the range  494  of skew angles, the composite half-power beamwidth of the main beam  480  of the secondary antenna  154  is less than the particular value of the composite half-power beamwidth of the main beam  422  along the line  490 . Thus, for a group of geographic locations corresponding to the range  494  of skew angles at which the amount of interference with the non-target satellite  120  using the primary antenna  152  exceeds the threshold, the interference level when using secondary antenna  154  can be less than or equal to the threshold, such that the secondary antenna  154  can be used to communicate with the target satellite  110 . The antenna selection system  200  can thus switch from the primary antenna  152  to the secondary antenna  154  when the skew angle reaches the maximum acceptable skew angle. Similarly, when the skew angle returns to a value below the maximum acceptable skew angle, the antenna selection system  200  can switch back to the primary antenna  152 . 
     In the illustrated embodiment, range  494  of skew angles extends from the line  490  to the wide beamdwidth axis  450  ( FIG.  4 B ) corresponding to the skew angle of ninety degrees. In such a case, the secondary antenna  154  can avoid excessive interference with the non-target satellite  120  at all the geographic locations at which the main beam  480  of the secondary antenna  154  has the contour illustrated in  FIG.  4 C . Alternatively, the range  494  of skew angles may not extend to the skew angle of 90 degrees. 
     The range  492  of skew angles and the range  494  of skew angles can vary from embodiment to embodiment. In some embodiments, range  492  of skew angles is at least 40 degrees, and the range  494  of skew angles is at least 30 degrees. For example, range  492  of skew angles may be from zero to sixty degrees, and range  494  of skew angles may be from sixty to ninety degrees. 
       FIG.  5 A  illustrates an example acceptable service area  510   a ,  510   b  of the primary antenna  152 . In the illustrated embodiment, the target satellite  110  and the non-target satellite  120  are both geostationary satellites. 
     The acceptable service area  510   a ,  510   b  are geographic locations of the antenna system  150  where the amount of interference with the non-target satellite  120  when using the primary antenna  152  is at or below the threshold, and the signal communication with the target satellite  110  has acceptable or desired performance characteristics. In other words, within the acceptable service area  510   a ,  510   b , the skew angle of main beam of the primary antenna  152  is less than the maximum acceptable skew angle. The boundary  512  corresponds to the line  490  of  FIG.  4 B . In the illustrated example, the acceptable service area  510   a ,  510   b  account for the attitude of the aircraft  102 . 
     The maximum acceptable skew angle, and thus the acceptable service area  510   a ,  510   b  of the primary antenna  152 , can vary from embodiment to embodiment. The maximum acceptable skew angle can depend on the radiation pattern of the primary antenna  152 , the locations of the target satellite  110  and non-target satellite  120 , the threshold amount of interference with the non-target satellite  120 , the transmission parameters of the return uplink signal  116 , etc. 
     As described above, the skew angle of the main beam  422  of the primary antenna  512 , and thus the composite half-power beamwidth along the geo arc, varies depending upon the geographic location of the antenna system  150 .  FIG.  5 B  illustrates the contour of the main beam  422  of the primary antenna  152  for an example geographic location  520  within the acceptable service area  510   a ,  510   b . In this example, the composite half-power beamwidth along the geo arc  540  is small enough that the amount of interference with the non-target satellite  120  is less than or equal to the threshold. 
       FIG.  5 C  illustrates the contour of the main beam  422  of the primary antenna  152  for an example geographic location  530  outside the acceptable service area  510   a ,  510   b . In this example, the composite half-power beamwidth along the geo arc  540  is large enough to cause excessive interference with the non-target satellite  120 , if the primary antenna  152  were used to communication with the target satellite  110 . 
       FIG.  5 D  illustrates an example acceptable service area  510   c  of the secondary antenna  154 . The acceptable service area  510   c  are geographic locations where the amount of interference with the non-target satellite  120  when using the secondary antenna  154  is at or below the threshold, and signal communication with the target satellite  110  has acceptable or desired performance characteristics. In the illustrated embodiment, the acceptable service area  510   c  is for a secondary antenna  154  that includes a non-movable, fully electronic scanning phased array antenna. At higher latitudes around the same longitude as the target satellite  110 , the boundary  514  of the acceptable service area  510   c  can be due to scan loss of the array which precludes signal communication having acceptable performance characteristics with the target satellite  110 . At lower latitudes near the equator  500 , the boundary  514  can be due to an increase in the composite half-power beamwidth of the main beam  480  along the geo arc at larger scan angles to the target satellite  110 . 
     As can be seen upon comparison of  FIGS.  5 A and  5 D , in the illustrated embodiment a portion of the acceptable service area  510   c  of the secondary antenna  514  overlaps with the acceptable service area  510   a ,  510   b  of the primary antenna  512 . The determination of whether to use the primary antenna  512  or the secondary antenna  514  when the aircraft  102  is at a geographic location within this overlap can vary from embodiment to embodiment. For example, at a given geographic location with this overlap, the antenna selection system  200  can select the primary antenna  152  or secondary antenna  154  based on which antenna  152 ,  154  provides performance characteristics at the given geographic location for communicating with the target satellite  110 . In embodiments in which the primary antenna  152  can provide better performance characteristics than the secondary antenna  154  for communicating with the target satellite  110  when the aircraft  102  is throughout the overlap, the antenna selection system  200  can select the primary antenna  152  for use. 
       FIG.  5 E  illustrates an example composite acceptable service area  510   d  for the antenna system  150 . The composite acceptable service area  510   d  are geographic locations where the amount of interference with the non-target satellite  120 , when using the primary antenna  152  or the secondary antenna  154 , is at or below the threshold, and signal communication with the target satellite  110  has acceptable or desired performance characteristics. The composite acceptable service area  510   d  is a union of the acceptable service area  510   a ,  510   b  of the primary antenna  512  and the acceptable service area  510   c  of the secondary antenna  514 . As can be seen in  FIG.  5 E , the primary antenna  512  and the secondary antenna  514  provides a larger acceptable service area than a system that includes only one of the antennas  152 ,  154 . 
     In the illustrated embodiment of  FIG.  5 E , the primary antenna  152  is selected by the antenna selection system  200  for use within the overlap between the acceptable service area  510   a ,  510   b  of the primary antenna  152  and the acceptable service area  510   c  of the secondary antenna  154 . In some alternative embodiments, the antenna selection system  200  may select the secondary antenna  154  for use within some or all of the overlap. 
     Line  550  represents an example flight path for the aircraft  102  between source  552  and destination  554 . At geographic locations along a first segment  570  of the flight path, the aircraft  102  is within the acceptable service area  510   b  of the primary antenna  152 . Thus, along the first segment  570  the antenna selection system  200  selects the primary antenna  152  for communication with the target satellite  110 . At geographic location  556  the aircraft  102  leaves the acceptable service area  510   b  of the primary antenna  152  and enters the acceptable service area  510   c  of the secondary antenna  154 . Thus, at geographic location  556  the antenna selection system  200  switches communication with the target satellite  110  from the primary antenna  152  to the secondary antenna  154 , and continues to use the secondary antenna  154  along the segment  572 . At geographic location  558  the aircraft  102  enters the acceptable service area  510   a  of the primary antenna  152 . Thus, at geographic location  558  the antenna selection system  200  switches communication with the target satellite  110  from the secondary antenna  154  to the primary antenna  152 , and continues to use the primary antenna  152  along the segment  574  to the destination  554 . 
     In the illustrated embodiment the antenna selection system  200  switches communication between the primary antenna  152  and the secondary antenna  514  at the boundaries between the various acceptable service areas. In other embodiments, the switching along the flight path may occur at geographic locations different than these boundaries. For example, if at least a portion of the segment  574  adjacent geographic location  558  is within the overlap of the acceptable service areas  510   a ,  510   c , the antenna selection system  200  may continue to use the secondary antenna  154  for some or all of that portion. In contrast, if the flight path were in the other direction, the antenna selection system  200  switches from the primary antenna  152  to the secondary antenna  154  at geographic location  558 , since a portion of the segment  572  adjacent the geographic location  558  is not within the overlap of the acceptable service areas of the primary antenna  152  and the secondary antenna  154 . In other words, the geographic locations at which the antenna selection system  200  switches between the primary antenna  152  and the secondary antenna  154  may depend on whether the aircraft  102  is moving from the acceptable service area  510   a ,  510   b  of the primary antenna  152  to the acceptable service area  510   c  of the secondary antenna  154 , or is moving from the acceptable service area  510   c  of the secondary antenna  154  to the acceptable service area  510   a ,  510   b  of the primary antenna  152 . 
       FIG.  6    is an example graph of maximum power spectral density (PSD) curves for the primary antenna  512  and the secondary antenna  514  that satisfy interference requirements with the non-target satellite  120 . As can be seen in the graph, the curve  600  of the maximum PSD for the primary antenna  512  decreases with increasing skew angle. This is due to the increase in the composite beamwidth of the main beam  422  of the primary antenna  512  as the skew angle increases. In contrast, the curve  610  of the maximum PSD for the secondary antenna  514  increases as the skew angle of the main beam  422  of the primary antenna  512  approaches 90 degrees. This is due to the increasing projected aperture of the secondary antenna  154  along the line defined by the target satellite  110  and the non-target satellite  120 . In other words, the composite half-power beamwidth of the secondary antenna decreases as the skew angle of the primary antenna increases. 
     In the illustrated embodiment of  FIG.  6   , the switching by the antenna selection system  200  between the primary antenna  152  and the secondary antenna  154  occurs over a non-zero switching range  620  between skew angle S 1  and skew angle S 2 . The switching range  620  corresponds to at least a portion of the overlap between the acceptable service areas of the primary antenna  152  and the secondary antenna  154 . The switching by the antenna selection system  200  from the primary antenna  152  to the secondary antenna  154  occurs at skew angle S 2 , whereas the switching from the secondary antenna  154  to the primary antenna  152  occurs at skew angle S 1 . The skew angle S 2  may for example correspond to geographic locations (e.g. geographic location  556 ) along the boundary of the acceptable service area  510   a ,  510   b  of the primary antenna  152 . The skew angle S 1  can correspond to geographic locations within the overlap and inside the boundary of the acceptable service area  510   a ,  510   b  of the primary antenna  152 . By having separate skew angle values S 1 , S 2 , rapid switching can be avoided when the aircraft  102  flies near the boundaries of the acceptable service area  510   a ,  510   b  of the primary antenna  152  and the acceptable service area  510   c  of the secondary antenna  154 . In alternative embodiments, the skew angle S 1  and skew angle S 2  may be the same. 
     As can be seen in  FIG.  6   , the minimum PSD P min  over the range of skew angles from 0 to 90 degrees that the antenna system  150  can provide by switching between the primary antenna  152  and the secondary antenna  154  is significantly greater than can be provided by either antenna  152 ,  154  separately. 
       FIG.  7    is an example plot  700  of the maximum value of the gain of the primary antenna  152  at 2 degrees from boresight of the main beam versus skew angle. Line  710  represents the maximum value of the gain that satisfies interference requirements with the non-target satellite  120 . As can be seen in  FIG.  7   , the plot  700  crosses the line  710  at a skew angle value of about 65 degrees in this example. Thus, in this example the maximum acceptable skew angle for the primary antenna  152  is about 65 degrees. 
       FIG.  8    illustrates an example process  800  for switching between the primary antenna  512  and the secondary antenna  514 . Other embodiments can combine some of the steps, can perform the steps in different orders and/or perform different or additional steps to the ones illustrated in  FIG.  8   . In the illustrated embodiment, the process  800  includes steps performed by the antenna selection system  200  discussed above. 
     At step  802 , a signal is communicated between a target satellite and an aircraft via a primary antenna on the aircraft. In the illustrated embodiment, the primary antenna is mechanically steerable and has an asymmetric antenna beam pattern with a narrow beamwidth axis and a wide beamwidth axis at boresight. The primary antenna can for example be the primary antenna  152  discussed above. 
     At step  804 , the determination of whether an amount of interference with a non-target satellite reaches a threshold due to the wide beamwidth axis of the asymmetric antenna beam pattern. If not, the process  500  returns to step  802 . 
     If the determination is made at step  804  that the amount of interference with the non-target satellite reaches the threshold, the process continues to step  806 . At step  806 , communication of the signal is switched from the primary antenna to a secondary antenna on the aircraft to reduce interference. The secondary antenna can for example be the secondary antenna  154  discussed above. 
     At step  808 , the signal is communicated between the target satellite and the aircraft via the secondary antenna. 
     At step  810 , the determination of whether an amount of interference with the non-target satellite using primary antenna will be below the threshold. The step  810  can for example be performed as the aircraft  102  moves. If not, the process returns to step  808 . 
     If the determination is made at step  810  that the amount of interference with the non-target satellite using the primary antenna will be below the threshold, the process returns to step  802 . 
     While the present disclosure is described by reference to the examples detailed above, it is to be understood that these examples are intended in an illustrative rather than in a limiting sense. It is contemplated that modifications and combinations will readily occur to those skilled in the art, which modifications and combinations will be within the spirit of the disclosure and the scope of the following claims.