Satellite-tracking millimeter-wave reflector antenna system for mobile satellite-tracking

Abstract

A miniature dual-band two-way mobile satellite-tracking antenna system mounted on a movable vehicle includes a miniature parabolic reflector dish having an elliptical aperture with major and minor elliptical axes aligned horizontally and vertically, respectively, to maximize azimuthal directionality and minimize elevational directionality to an extent corresponding to expected pitch excursions of the movable ground vehicle. A feed-horn has a back end and an open front end facing the reflector dish and has vertical side walls opening out from the back end to the front end at a lesser horn angle and horizontal top and bottom walls opening out from the back end to the front end at a greater horn angle. An RF circuit couples two different signal bands between the feed-horn and the user. An antenna attitude controller maintains an antenna azimuth direction relative to the satellite by rotating it in azimuth in response to sensed yaw motions of the movable ground vehicle so as to compensate for the yaw motions to within a pointing error angle. The controller sinusoidally dithers the antenna through a small azimuth dither angle greater than the pointing error angle while sensing a signal from the satellite received at the reflector dish, and deduces the pointing angle error from dither-induced fluctuations in the received signal.

What is claimed is:  
     
       1. A compact dual-band mobile satellite-tracking antenna system for mounting  mountable on a movable body for communicating with a satellite in earth orbit while said body is at rest or in motion, said antenna system comprising: 
       a parabolic reflector dish having an elliptical aperture with major and minor elliptical axes, said major elliptical axis being aligned in a generally horizontal direction and said minor elliptical axis being aligned in a generally vertical direction;  
       a feed-horn having a back end and an open front end facing said reflector dish at a focal point thereof and comprising vertical side walls opening out from said back end to said front end at a first horn angle and horizontal top and bottom walls opening out from said back end to said front end at a second horn angle, said first horn angle being less than said second horn angle;  
       means for transmitting to said feed-horn signals of a first frequency band for transmission via said reflector dish to said satellite and receiving from said feed-horn signals of a second frequency band reflected by said reflector dish from said satellite; and  
       antenna attitude  control means for maintaining an antenna azimuth direction relative to said satellite, wherein said reflector dish has a fixed elevational angle relative to said movable body corresponding to an elevation of said satellite; and wherein  
       said major elliptical axis is aligned in a generally horizontal direction and said first horn angle is chosen whereby  to maximize azimuthal directionality of said reflector dish, and said minor elliptical axis is aligned in a generally vertical direction and said second horn angle is chosen whereby  to minimize elevational directionality of said reflector dish to an extent corresponding to expected pitch and roll excursions of said movable body.  
     
     
       2. The antenna system of claim  1  wherein said reflectors  reflector dish and feed-horn are mounted on a generally horizontal platform rotatable in azimuth, said antenna attitude  control means comprising: 
       means for rotating said rotatable platform through an azimuth angle in response to sensed yaw motions of said movable body so as to compensate for said yaw motions to within a pointing error angle;  
       means for sinusoidally dithering said rotatable platform through a small azimuth dither angle greater than said pointing error angle while sensing a signal from said satellite received at said reflector dish, thereby sinusoidally modulating in amplitude said signal from said satellite while said signal is being received; and  
       means for extracting amplitude variations in said signal sensed from said satellite corresponding to sinusoidal dithering of the  said rotatable platform by the  said means of  for sinusoidally dithering said rotatable platform and for deducing therefrom said  a deduced pointing error angle, and for transmitting a correction signal corresponding thereof  to said deduced pointing error angle to said means for rotating said rotatable platform.  
     
     
       3. The antenna system of claim  1  wherein said first and second bands comprise K  Ka and Ka  K communication bands, respectively, and wherein said reflector dish extends on the order of ten wavelengths along said major elliptical axis and on the order of several wavelengths along said minor elliptical axis. 
     
     
       4. The antenna system of claim  3  wherein said feed-horn is on the order of 1.5 inches long, 0.75 inches high and 0.5 inches wide at said front end thereof. 
     
     
       5. The antenna system of claim  3  wherein said elliptical aperture is projected onto a paraboloid of said reflector dish along an ellipse projection axis lying at a projection angle with respect to a parabolic axis of said paraboloid of said reflector dish. 
     
     
       6. The antenna system of claim  5  wherein said projection angle is on the order of 25.2 degrees, said paraboloid has a parabolic focal length of 1.673 in. and said elliptical aperture extends on the order of 5.9 in. along said major axis and 2.3 in. along said minor axis. 
     
     
       7. The antenna system of claim  2  wherein said means for transmitting to said feed-horn signals of a first frequency band and receiving from said feed-horn signals of a second frequency band comprises a microwave rotary joint concentric with said rotatable platform for coupling signals of said first and second bands between said feed-horn and a microwave transmitter and a microwave receiver. 
     
     
       8. The antenna system of claim  7  wherein said means for transmitting to said feed-horn signals of a first frequency band and receiving from said feed-horn signals of a second frequency band further comprises: 
       an orthomode transducer coupled to said back end of said feed-horn and having first and second ports corresponding to said first and second frequency bands; and  
       a diplexer having a common port for conducting signals of both said first and second bands, said common port being connected to said rotary joint, and separate ports corresponding to  for said first and second bands respectively , said separate ports of said diplexer being connected to corresponding ones of said first and second ports of said orthomode transducer.  
     
     
       9. The antenna system of claim  1  wherein said lesser horn angle is on the order of 2 degrees and said greater horn angle is on the order of 13 degrees. 
     
     
       10. The antenna system of claim  2  wherein said dither angle is on the order of 1 degree to the right and  1  degree to the left of a current antenna azimuth angle commanded by said means for rotating and wherein said sinusoidal dithering has a rate on the order of 2 Hz. 
     
     
       11. The antenna system of claim  10  further comprising a yaw rate sensor for sensing a rate of change of said yaw angle with a bandwidth on the order of 300 Hz. 
     
     
       12. The antenna system of claim  1  further comprising a radome covering said reflector dish and said feed-horn and having a diameter on the order of 23 cm. 
     
     
       13. The antenna system of claim  1  wherein a beam of said reflector dish is aligned with an elevational angle of on the order of 46 degrees  corresponding to an elevation angle of any particular satellite selected for communications from a given geographical region and said feed-horn points at said reflector dish at an angle of  on the order of 4 degrees with respect to horizontal. 
     
     
       14. A compact dual-band mobile satellite-tracking antenna system for mounting  mountable on a movable body for communicating with a satellite in earth orbit, said antenna system comprising: 
       a reflector dish having a non-symmetrical aperture with major and minor axes, said major axis being aligned in a generally horizontal direction and said minor axis being aligned in a generally vertical direction;  
       a feed-horn having a back end and an open front end facing said reflector dish at a focal point thereof and comprising vertical side walls opening out from said back end to said front end at a first horn angle and horizontal top and bottom walls opening out from said back end to said front end at a second horn angle, said first horn angle being less than said second horn angle;  
       means for transmitting to said feed-horn signals of a first frequency band for transmission via said reflector dish to said satellite and receiving from said feed-horn signals of a second frequency band reflected by said reflector dish from said satellite; and  
       antenna attitude  control means for maintaining an antenna azimuth direction relative to said satellite, wherein said reflector dish has a fixed elevational angle relative to said movable body corresponding to an elevation of said satellite; and wherein,  said major axis is aligned in a generally horizontal direction and said first horn angle is chosen whereby  to maximize azimuthal directionally of said reflector dish, and said minor axis is aligned in a generally vertical direction and said second horn angle is chosen whereby  to minimize elevational directionally of said reflector dish to an extent corresponding to expected pitch and roll excursions of said movable body.  
     
     
       15. The antenna system of claim  14  wherein said reflector dish and said feed-horn are mounted on a generally horizontal platform rotatable in azimuth, said antenna control means comprising: 
       means for rotating said rotatable platform through an azimuth angle in response to sensed yaw motions of said movable body so as to compensate for said yaw motions to within a pointing error angle;  
       means for sinusoidally dithering said rotatable platform through a small azimuth dither angle greater than said pointing error angle while sensing a signal from said satellite received at said reflector dish, thereby sinusoidally modulating in amplitude said signal from said satellite while said signal is being received; and  
       means for extracting amplitude variations in said signal sensed from said satellite corresponding to sinusoidal dithering of the  said rotatable platform by said means for sinusoidally dithering said rotatable platform and for deducing therefrom said  a deduced pointing error angle, and for transmitting a correction signal corresponding thereto to said means for rotating said rotatable platform.  
     
     
       16. The antenna system of claim  15  wherein said first and second frequency bands comprise K  Ka and Ka  K communication bands, respectively, and wherein said reflector dish extends on the order of ten wavelengths along said major axis and on the order of several wavelengths along said minor elliptical axis. 
     
     
       17. The antenna system of claim  16  wherein said feed-horn is on the order of 1.5 inches long and said feed- horn at a front end thereof is  0 . 75  inches high and  0 . 5  inches wide.    
     
     
       18. The antenna system of claim  15  wherein said means for transmitting to said feed-horn signals of a first frequency band and receiving from said feed-horn signals of a second frequency band comprises a microwave rotary joint concentric with said rotatable platform for coupling signals of said first and second bands between said feed-horn and a microwave transmitter and a microwave receiver. 
     
     
       19. The antenna system of claim  18  wherein said means for transmitting to said feed-horn signals of a first frequency band and receiving from said feed-horn signals of a second frequency band further comprises: 
       an orthomode transducer coupled to said back end of said feed-horn and having first and second ports corresponding to  for said first and second frequency bands, respectively; and  
       a diplexer having a common port for conducting signals of both said first and second bands, said common port being connected to said rotary joint, and separate ports corresponding to  for said first and second frequency bands respectively , said separate ports of said diplexer being connected to corresponding ones of said first and second ports of said orthomode transducer.  
     
     
       20. The antenna system of claim  15  wherein said dither angle is on the order of 1 degree to the right and  1  degree to the left of a current antenna azimuth angle commanded by said means for rotating and wherein said sinusoidal dithering has a rate on the order of 2 Hz. 
     
     
       21. The antenna system of claim  20  further comprising a yaw rate sensor for sensing a rate of change of said yaw angle with a bandwidth on the order of 300 Hz. 
     
     
       22. The antenna system of claim  14  further comprising a radome covering said reflector dish and said feed-horn and having a diameter on the order of 23 cm. 
     
     
       23. The antenna system of claim  14  wherein a beam of said reflector dish is aligned with an elevational angle of on the order of 46 degrees  corresponding to an elevation angle of any particular satellite selected for communications from a given geographical region and said feed-horn points at said reflector dish at an angle of  on the order of 4 degrees with respect to horizontal. 
     
     
       24. A compact dual-band  mobile satellite-tracking antenna system for mounting  mountable on a movable body for communicating with  receiving communications from a satellite in earth orbit while said body is at rest or in motion, said antenna system comprising: 
       a reflector dish having a non-symmetrical aperture with orthogonal major and minor axes connected to said body, said major axis being aligned relative to said body in a generally horizontal direction whereby  to maximize azimuthal directionality of said reflector dish and said minor axis being aligned in a generally vertical direction whereby  to minimize elevational directionality of said reflector dish to an extent corresponding to expected pitch excursions of said movable body;  
       a feed-horn having a back end and an open front end facing said reflector dish at a focal point thereof and comprising vertical side walls opening out from said back end to said front end at a first horn angle and horizontal top and bottom walls opening out from said back end to said front end at a second horn angle, wherein said first horn angle is less than said second horn angle, and wherein said reflector dish and said feed-horn are mounted on a generally horizontal platform rotatable in azimuth;  
       means for rotating said rotatable platform through an azimuth angle in response to sensed yaw motions of said movable body so as to compensate for said yaw motions to within a pointing error angle;  
       means for sinusoidally dithering said rotatable platform  reflector dish and said feed- horn as a unit  through a small azimuth dither angle greater than said pointing error angle while sensing a signal from said satellite received at said reflector dish, thereby producing an amplitude- modulated signal from said signal being sensed;    
       means for extracting from amplitude variations in said amplitude- modulated  signal sensed from said satellitean error signal corresponding to sinusoidal dithering of thesaid rotatable platform by the  means for sinusoidally dithering and for  deducing therefrom saida deduced pointing error angle, and  
       means for transmitting a correction signal corresponding thereto to said means for rotating said rotatable platform corresponding to said deduced pointing error angle.  
     
     
       25. The antenna system of claim  24 for transmitting communications to said satellite as well as receiving communications therefrom using first and second frequency bands, wherein said first and second frequency bands comprise KKa and KaK communication bands, respectively, and wherein said reflector dish aperture extends on the order of ten wavelengths along said major elliptical  axis and on the order of several wavelengths along said minor elliptical  axis. 
     
     
       26. The antenna system of claim  25  wherein said feed-horn is on the order of 1.5 inches long and said feed- horn at a front end thereof is  0 . 75  inches high and  0 . 5  inches wide.    
     
     
       27. The antenna system of claim  24  further comprising means for transmitting to said feed-horn signals of a first frequency band and receiving from said feed-horn signals of a second frequency band. 
     
     
       28. The antenna system of claim  27  wherein said means for transmitting and receiving comprises a microwave rotary joint concentric with said rotatable platform for coupling signals of said first and second bands between said feed-horn and a microwave transmitter and a microwave receiver. 
     
     
       29. The antenna system of claim  28  wherein said means for transmitting to said feed-horn signals of a first frequency band and receiving from said feed-horn signals of a second frequency band further comprises: 
       an orthomode transducer coupled to said back end of said feed-horn and having first and second ports corresponding to  for said first and second frequency bands, respectively; and  
       a diplexer having a common port for conducting signals of both said first and second bands, said common port being connected to said rotary joint, and separate ports corresponding to  for said first and second frequency bands respectively , said separate ports of said diplexer being connected to corresponding ones of said first and second ports of said orthomode transducer.  
     
     
       30. The antenna system of claim  24  wherein said dither angle is on the order of 1 degree to the right and  1  degree to the left of a current antenna azimuth angle commanded by said means for rotating said platform and wherein said sinusoidal dithering has a rate on the order of 2 Hz. 
     
     
       31. The antenna system of claim  30  further comprising a yaw rate sensor for sensing a rate of change of said yaw angle with a bandwidth on the order of 300 Hz. 
     
     
       32. The antenna system of claim  24  further comprising a radome covering said reflector dish and said feed-horn and having a diameter on the order of 23 cm. 
     
     
       33. The antenna system of claim  24  wherein a beam of said reflector dish is aligned with an elevational angle of on the the order of 46 degrees  corresponding to an elevation angle of any particular satellite selected for communications from a given geographical region and said feed-horn points at said reflector dish at an angle of  on the order of 4 degrees with respect to horizontal. 
     
     
       34. The antenna system of claim  24  wherein said reflector dish is adjustable to any fixed elevational orientation corresponding to an elevational angle of any particular satellite within a predetermined range of elevational orientations corresponding to a geographical region of mobility of said antenna system  movable body. 
     
     
       35. A satellite- tracking antenna system comprising an antenna having a fixed rotation axis with respect to a mobile body, said fixed rotation axis being approximately vertical while said mobile body is at rest or moving on a horizontal surface and not vertical while moving on a nonhorizontal surface, wherein said antenna system is rotatable in azimuth about said fixed rotation axis for receiving at least a pilot signal from a satellite while said body is at rest or in motion, and wherein said antenna has an aperture with orthogonal major and minor axes, said major axis being aligned in a generally perpendicular orientation with respect to said rotation axis to maximize azimuthal directionality of said antenna and said minor axis being aligned in a plane at an angle with respect to said rotation axis generally equal to the elevation angle of said satellite, thereby providing less elevational directionality of said antenna to an extent corresponding to expected pitch and roll of said body while providing pronounced azimuthal directionality of said antenna; and further comprising:    
       
         antenna azimuth control means for rotating said antenna about said rotation axis while said body is in motion, said antenna azimuth control means operating automatically to maintain azimuthal pointing of said antenna toward said satellite to within a pointing error angle while said body is in motion and orientation of said antenna is maintained by optimally tracking in azimuth the signal strength of said pilot signal with said antenna maintaining said minor axis at a fixed angle relative to said rotation axis.  
       
     
     
       36. The antenna system of claim  35  including means for receiving said pilot signal from said satellite through said aperture of said antenna whereby said antenna azimuth control means includes: 
       
         means for sensing yaw motions of said body;  
       
       
         means for rotating said antenna through an azimuth angle in response to sensed yaw motions of said body so as to compensate for said yaw motions to within said pointing error angle;  
       
       
         means for sinusoidally dithering said antenna about said rotation axis through a small azimuth dither angle greater than said pointing error angle while sensing a signal received from said satellite by said receiving means through said aperture of said antenna, thereby sinusoidally amplitude modulating said signal received from said satellite, and  
       
       
         means for extracting amplitude variations in said signal sensed from said satellite corresponding to sinusoidal dithering of said antenna and for deducing therefrom said pointing error angle, and further means for transmitting a correction signal corresponding to said pointing error angle to said means for rotating said antenna about said rotation axis to null and pointing error angle.  
       
     
     
       37. The antenna system of claim  36  wherein said means for receiving said pilot signal from said satellite is adapted to receive said pilot signal and other signals in a particular frequency band, and wherein said antenna aperture extends on the order of ten wavelengths of said particular frequency band along said major axis and on the order of several wavelengths of said particular frequency band along said minor axis.  
     
     
       38. The antenna system of claim  37  including means for transmitting signals to said satellite in a frequency band distinct from said particular frequency band of received signals.  
     
     
       39. The antenna system of claim  38  wherein said distinct frequency band and said particular frequency band are Ka and K communication bands, respectively.