Abstract:
A geostationary communication satellite system that uses a hub and spoke configuration, where the hub includes at least one relatively large diameter satellite antenna. The hub of the communication system is capable of receiving satellite communication signals even when the sun transits within the beamwidth of its primary antenna by either redirecting its primary antenna toward a secondary satellite, or switching to a secondary antenna directed toward a secondary satellite.

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part of application Ser. No. 08/988,989, filed Dec. 11, 1997, which is hereby incorporated in its entirety by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to geostationary satellite communication antenna. In particular, it relates to small diameter C-band geostationary satellite antenna. 
     BACKGROUND OF THE INVENTION 
       FIG. 1  shows a geostationary communication satellite system  100  comprising a plurality of satellites  102   1  to  102   i  orbiting the earth  104 . Satellite  102   1  is separated from adjacent satellites  102   2  and  102   3  by approximately a 2° arc (the arc is shown by the separation between the dashed lines on each of  FIGS. 1 ,  2 ,  5 , and  6 , and is typical for geostationary satellites in the United States). Earth  104  has a plurality of earth stations  106   1  to  106   n . Each earth station  106  includes a satellite transmitting and receiving antenna  108 . Communication system  100  operates when antenna  108  generates a communication signal  110  that is received by, for example, satellite  102   1 , and visa versa. 
     As communication signal  110  travels from, for example, earth station  106   1  to its intended destination at satellite  102   1  it spreads over an area  112 . If communication signal  110  spreads beyond the 2° arc between satellite  102   1  and the adjacent satellites  102   2  and  102   3 , then all three satellites  102   1 ,  102   2 , and  102   3  would process communication signal  110  as if it was intended for them. One reason this occurs is that communication signal  110  does not experience significant signal attenuation at the edge of area  112 . In order to prevent satellites  102   2  and  102   3  from processing communication signal  110 , antenna  108  generates a narrow beam communication signal, instead of a wide beam communication signal. 
     The most widely used radio frequency bands for satellite communication are the Ku- and C-bands. In both of these bands, a conventional parabolic reflector antenna generates a narrow communication signal to prevent adjacent satellites from processing communication signals not intended for them. The parabolic reflector antenna for the Ku-band may have a relatively small diameter. The small parabolic reflector antenna provides an efficient, cost-effective mechanism for allowing an earth station to communicate with an individual satellite. Unfortunately, Ku-band radio signals attenuate in atmospheric conditions consistent with periods of moderate-to-heavy precipitation, i.e., rain, sleet, or snow. In most cases, providing facilities with sufficient power to compensate for severe signal attenuation is uneconomical. As a result, satellite communications systems operating in the Ku-band experience periodic system outages that are unacceptable for time critical applications. 
     To avoid periodic system outages due to atmospheric conditions, earth stations typically transmit and receive data using C-band radio frequencies. These frequencies are much less susceptible to attenuation due to precipitation. Therefore, C-band transmitters can economically provide sufficient signal margin to overcome any signal attenuation due to atmospheric conditions. Unfortunately, to generate narrow communication signal beams, C-band parabolic antennas need to be larger than Ku-band antennas. In fact, the minimum C-band parabolic antenna diameter that prevents communication signal  110  from interfering with satellites  102   2  or  102   3  (See  FIG. 1 ) is approximately 3.7 meters. For many applications, however, the installation of a 3.7 meter diameter antenna is too unwieldy, aesthetically unseemly, and/or not structurally prudent. Therefore, it would be desirable to use smaller diameter parabolic reflective antenna to transmit C-band radio frequencies while avoiding unnecessary interference with adjacent satellites. 
     Further, during short periods of each day for several days immediately before and after the vernal and autumnal equinoxes, the sun transits behind geostationary satellites as seen from an earth station&#39;s receiving antenna (i.e., from the perspective of the earth station, the sun passes behind the geostationary satellite). The sun emits a great deal of energy in the form of electromagnetic radiation in the bandwidth occupied by radio wave communications. Therefore, when the sun is located within the beamwidth of the receiving antenna, its energy causes interference in the form of radio frequency noise. This noise causes a decrease in the signal-to-noise ratio of the earth station&#39;s receiver, and can render the earth station inoperative until the sun completes its transit of the antenna&#39;s beamwidth. 
     Because the relative movement of the earth with respect to the sun is known to a high degree of precision, satellite communication system operators are forewarned of the time when the sun will transit the beamwidth of a receiving antenna. Knowledge of a pending problem, however, is only useful if the system operators can keep the system operational during these periods. 
     For conventional satellite systems, each individual receive antenna might be effected by the sun&#39;s positioning during this period. Some conventional systems use costly terrestrial communications facilities to provide continuing operations as the sun transits behind a satellite with respect to its earth station&#39;s receiving antenna. Other systems remain off-the-air for the duration of these periods. The inherent inconvenience of this option, however, renders it particularly unattractive. Finally, some conventional satellite systems continue operation by switching each earth station&#39;s antenna to a secondary satellite during the period that the sun is within the beamwidth of the antenna. This process requires manual intervention and/or complex automated mechanical mechanisms to perform the daily repositioning of the antenna during its sun transit outage. The cost of the daily repositioning of each antenna so effected renders this option uneconomical. 
     Therefore, a need exists for a satellite communication system to efficiently provide communication during sun transit outages. 
     SUMMARY OF THE INVENTION 
     Systems and methods consistent with the present invention address this need by providing a mechanism for repositioning an earth station&#39;s antenna during a sun transit outage. Alternatively systems and methods consistent with the present invention provide a second antenna at the earth station directed toward a second satellite. 
     In accordance with the purpose of the invention, as embodied and broadly described herein, a point-to-multipoint satellite communication system, comprises a first satellite antenna for generating a wide beam communication signal to illuminate a plurality of satellite, means for generating a return communication signal from each of the plurality of satellites, a second satellite antenna for receiving the return communication signal from only one of the plurality of satellites, and a satellite antenna repositioning system for repositioning said second antenna when the sun transits within the beamwidth of said second antenna. 
     Both the foregoing general description and the following detailed description are exemplary and explanatory, and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate preferred embodiments of the invention and, together with the description, explain the goals and principles of the invention. In the drawings, 
         FIG. 1  is an illustration of a geostationary satellite communication system; 
         FIG. 2  is an illustration of a geostationary satellite communication system consistent with the present invention; 
         FIG. 3  is a flow chart illustrating the reception operation of the communication system of  FIG. 2 ; 
         FIG. 4  is a flow chart illustrating the transmission operation of the communication system of  FIG. 2 ; 
         FIG. 5  is an illustration of a second geostationary satellite communication system consistent with the present invention; 
         FIG. 6  is an illustration of a third geostationary satellite communication system consistent with the present invention; 
         FIG. 7  is an illustration of a fourth geostationary satellite communication system consistent with the present invention; and 
         FIG. 8  is an illustration of a fifth geostationary satellite communication system consistent with the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The following detailed description of the invention refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. 
     Systems and methods consistent with the present invention provide efficient and continuous communications during sun transit outages by providing a secondary channel for communications to continue during the outages. 
     Communication systems consistent with the present invention comprise a “hub and spoke” configuration. In this configuration, a central earth station acts as the hub and a plurality of earth stations act as the spokes. Communication from the central earth station to any one of the plurality of earth stations is direct in that it involves a single transmission to the satellite and a single transmission from the satellite. Communication between spokes, however, is not direct. A transmitting earth station communicates with the central earth station, which retransmits the signal to a receiving earth station. In this case, there are two transmissions to a satellite and two transmissions from a satellite. 
       FIG. 2  is a diagram of satellite communication system  200  that uses a relatively small diameter C-band antenna (also called a very small aperture terminal (VSAT) antenna) for the transmission and reception of communication signals. System  200  includes a plurality of satellites  202   1  to  202   i , a central earth station  204 , and a plurality of earth stations  206   1  to  206   n . Central earth station  204  transmits to the plurality of satellites  202  via a communication signal  208 . Each of the earth stations  206  transmits to the plurality of satellites  202  via a communication signal  210 . Each of the satellites  202  communicates with central earth station  204  and the plurality of earth stations  206  with a return communication signal (not shown). 
     Central earth station  204  includes a relatively large C-band antenna  214  having a relatively narrow beamwidth. Conversely, each of the plurality of earth stations  206  includes a relatively small C-band antenna  216  having a relatively wide beamwidth. 
       FIG. 3  is a flow chart  300  of a return communication from one of the satellites  202  to antenna  216 . First, central earth station  204  aligns its narrow beam antenna  214  to illuminate a single satellite  202 , for example satellite  202   1  (step  302 ). Next, antenna  214  generates a narrow communication signal  208  (step  304 ), which is received solely by satellite  202   1  (step  306 ). Based on communication signal  208 , satellite  202   1  broadcasts a return communication signal (step  308 ). Antenna  216  receives the return communication signal (step  310 ). 
       FIG. 4  is a flow chart  400  illustrating the transmission of a communication signal  210  from antenna  216  to the plurality of satellites  202 . First, one of the earth stations  206  aligns its antenna  216  to illuminate satellite  202   1  (step  402 ). Next, antenna  216  generates a relatively wide communication signal  210  (step  404 ), which is received by satellite  202   1 , along with the other satellites within the gain pattern of signal  210 , such as satellites  202   2  and  202   3  (step  406 ). In response to communication signal  210 , each of the satellites broadcasts return communication signals (step  408 ). Due to its narrow beamwidth, however, antenna  214  receives the return communication signal from the single satellite at which it is pointed (i.e., satellite  202   1 ). 
     During transmission from antenna  216 , both satellites  202   2  and  202   3  receive communication signal  210 . Due to its wide beamwidth, antenna  216  receives return communication signals from all three satellites  202   1 ,  202   2  and  202   3 , though it is pointed only towards satellite  202   1 . In the above example, when antenna  216  is aligned with satellite  202   1 , it can receive return communication signals from each of satellites  202   1 ,  202   2 , and  202   3 . 
     If an antenna outside communication system  200  mistakenly illuminates a satellite within system  200 , the received signal is seen by system  200  as an interference signal (“interference signal” is defined as a communication signal generated by an antenna outside a communication system that operates on the same frequency band). The illuminated satellite retransmits the signal to antenna  216 , because the satellite does not distinguish the source of the signal. 
     Similarly, when antenna  216  illuminates satellites  202   1 ,  202   2 , and  202   3  with communication signal  210 , each of satellites  202   1 ,  202   2 , and  202   3  transmits a return communication signal. An antenna outside of communication system  200  that is aligned with one of the satellites would receive the return communication signal. In order to avoid these types of interference, it is preferable to obtain exclusive use, on satellites  202   1 ,  202   2 , and  202   3 , of the particular frequencies that communication system  200  will use. 
     Although the disclosure is directed to a communication system with a central and two adjacent satellites, virtually any number of satellite configurations are possible. For example,  FIG. 5  shows a communication system  500  that uses two satellites  502   1 , and  502   2 .  FIG. 6  shows a communication system  600  that uses five satellites  602   1 ,  602   2 ,  602   3 ,  602   4 , and  602   5 . Communication systems  500  and  600  both operate in a manner similar to system  200  described above. 
     As noted above, it is preferable to exclude other satellite communication systems from using the bandwidth employed by communication system  200 . However, it is not possible to control the frequencies emitted by the sun as it transits behind satellites  202  with respect to the earth. Large C-band antennas, such as antenna  214 , are particularly sensitive to the noise signal emitted by the sun. This sensitivity is caused by the amplification of the sun signal received within the narrow beamwidth of the large antenna. Smaller VSAT antennas  216  do not receive as large a noise signal due to the lower level of amplification of the signal received within their wide beamwidth. 
     Sun transit outage is of particular concern to operators of large point-to-multipoint (hub and spoke) satellite systems as described herein. In these hub and spoke type networks, such as system  200 , all communications necessarily pass through hub antenna  214  of central earth station  204 . During the transit of the sun through the beamwidth of antenna  214 , the entire system becomes inoperative. 
       FIG. 7  is a diagram of a satellite communication system  700  that includes a satellite antenna repositioning system  720  to overcome the problem of sun transit outages. Because relatively small C-band antenna  216 , or VSAT antenna, has a relatively wide beamwidth, antenna  216  communicates with several satellites, including, for example, satellites  202   1 ,  202   2 , and  202   3 . Upon receiving a signal  210  from antenna  216 , each of satellites  202   1 ,  202   2 , and  202   3  broadcasts a return communication signal. During the period that the sun passes through the beamwidth of antenna  214  (i.e., behind satellite  202   1 ), satellite antenna repositioning system  720  repositions antenna  214  to point to one of the proximate secondary satellites  202   2  or  202   3 . As noted above, due to the relatively wide beamwidth of antennas  216 , they remain in operation while the sun transits their beamwidths. Following the repositioning, therefore, antenna  214  can both transmit signals to and receive signals from antennas  216 . 
       FIG. 8  is a diagram of a satellite communication system  800 , which includes a second relatively large C-band antenna  814  installed at the central earth station  204 . Station  204  directs antenna  214  at satellite  202   1 , and antenna  814  at one of the proximate secondary satellites  202   2  or  202   3 . During the period of transit of the sun behind satellite  202   1  with respect to antenna  214 , central earth station  204  discontinues use of antenna  214  and switches to antenna  814 . The operation of switching from one antenna to another is performed by an antenna switch selector (not shown). Once again, because of the relatively wide beamwidth of antenna  216 , the sun does not have as large an effect on the signal-to-noise ratio of the received signal as the sun transits within the beamwidths of antennas  216 . The relatively wide beamwidths of antennas  216  also results in the illumination of satellites  202   1 , and proximate secondary satellites  202   2 , and  202   3 . The communication link between antenna  216  and central earth station  204  is thereby maintained during the sun transit of the beamwidth of satellite  202   1  by receiving the signal from a proximate secondary satellite using antenna  814 . 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the methods and apparatus consistent with the present invention without departing from the scope or spirit of the invention. Other modification will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. The specification and examples should be considered as exemplary only, with the true scope and spirit of the invention being indicated by the following claims.