Patent Publication Number: US-6911949-B2

Title: Antenna stabilization system for two antennas

Description:
This application claims priority from U.S. Provisional Application No. 60/419,543 filed on 21 st  Oct. 2002. 

   FIELD AND BACKGROUND OF THE INVENTION 
   The present invention relates to antennas of geo-stationary satellites and, in particular, it concerns stabilizing two antennas mounted on a single pedestal. 
   By way of introduction, various geo-stationary satellites are located at approximately 36,000 Km from the surface of the earth around the equator in a belt known as the “Clark Belt”. These satellites serve satellite TV channels and two way communication such as internet, data video conferencing and voice communications. However, not all the TV channels are available from the communication satellites. For example, in the U.S.A. the communication satellites (FSS) which are located at 91 degrees West, 99 Degrees West and 116.8 degrees West do not include the Broadcast TV channels which are provided by the BSS satellites at 101 degrees West, 110 degrees West and 119 degrees West. Typically, on a mobile platform, for example, but not limited to a marine, airborne or ground mobile platform, there is a need to provide both two way communication and to receive broadcast TV channels. Therefore, there is a need to mount two antennas on the mobile platform in order to provide simultaneous links with two satellites, one for TV Receive Only communications (TVRO) and the other for two way (Tx/Rx) communication. 
   The simple and common solution is to use two separate pedestal/tracking antenna systems. This solution requires a large amount of space, is not cost effective and there may be interference between the two antennas if they are placed to close together. In addition, two radomes or one large radome are required which takes up additional space and is very expensive. 
   It is known in the field of antenna alignment to use a single antenna with multiple feeds, such that the antenna receives signals from a plurality of satellites. However, the Regulatory authorities, such as the FCC and ETSI require that the end-user terminal be aligned very accurately with a satellite in order for the end-user to transmit to the satellite. The alignment accuracy required by the Regulatory authorities cannot be achieved using a multiple feed system. 
   It is also known in the field of antenna alignment systems to mount two antennas on a single pedestal for tracking low earth orbit (LEO) satellites. An example of such a system is taught by U.S. Pat. No. 6,310,582 to Uetake, et al. The aforementioned system is suitable for LEO satellites, but is not suitable for tracking two geo-stationary satellites. 
   There is therefore a need for a cost and space effective stabilization system for two antennas associated with geo-stationary satellites where at least one of the antennas is linearly polarized. 
   SUMMARY OF THE INVENTION 
   The present invention is an antenna stabilization system construction and method of operation thereof. 
   According to the teachings of the present invention there is provided, a system for stabilizing at least two antennas on a mobile platform, the antennas including a first antenna associated with a first geo-stationary satellite and a second antenna associated with a second geo-stationary satellite, the system comprising: (a) an upper alignment system configured for being a common support for the antennas, the upper alignment system having at least one degree of freedom, the upper alignment system including an intermediate element, the upper alignment system being configured for pointing the antennas relative to the intermediate element, such that the angular displacement between the first antenna and the second antenna is substantially matched with the angular displacement between the first geo-stationary satellite and the second geo-stationary satellite; and (b) a lower alignment system mechanically connected to the upper alignment system and the mobile platform, the lower alignment system having three degrees of freedom, the lower alignment system being configured for maintaining the orientation of the intermediate element in order to compensate for rotation of the mobile platform, such that the first antenna and the second antenna are maintained pointing toward the first geo-stationary satellite and the second geo-stationary satellite, respectively. 
   According to a further feature of the present invention, the three degrees of freedom are rotational degrees of freedom, the three degrees of freedom including roll, pitch and yaw, the lower alignment system being configured for maintaining the orientation of the intermediate element in order to compensate for movements of yaw, pitch and roll of the mobile platform. 
   According to a further feature of the present invention, the upper alignment system and the lower alignment system are configured, such that the lower alignment system maintains the orientation of the intermediate element in order that movement of the first antenna and the second antenna is substantially restricted to pointing to satellite of the Clark belt. 
   According to a further feature of the present invention, the upper alignment system is configured, such that the polarization of the first antenna is adjustable. 
   According to a further feature of the present invention, the upper alignment system is configured, such that the polarization of the second antenna is adjustable. 
   According to a further feature of the present invention, the one degree of freedom of the upper alignment system is a rotational degree of freedom configured for setting the cross-elevation of the first antenna and the second antenna. 
   According to a further feature of the present invention, the upper alignment system, the lower alignment system, the first antenna and the second antenna fit under a single radome. 
   According to a further feature of the present invention, the upper alignment system and the lower alignment system are configured to provide full hemispherical coverage for the first antenna and the second antenna. 
   According to the teachings of the present invention there is also provided a method for stabilizing at least two antennas on a mobile platform, the antennas including a first antenna associated with a first geo stationery satellite and a second antenna associated with a second geo stationery satellite, the method comprising the steps of: (a) mechanically connecting the antennas to an element; (b) pointing the antennas relative to each other such that the angular displacement between the first antenna and the second antenna is matched with the angular displacement between the first geo-stationary satellite and the geo-stationary second satellite; and (c) maintaining the orientation of the element in order to compensate for rotation of the mobile platform, such that the first antenna and the second antenna are maintained pointing toward the first geo-stationary satellite and the second geo-stationary satellite, respectively. 
   According to a further feature of the present invention, the step of maintaining includes at least one of a roll adjustment, a pitch adjustment and a yaw adjustment in order to compensate for movements of roll, pitch and yaw of the mobile platform, respectively. 
   According to a further feature of the present invention, the step of maintaining is performed, such that movement of the first antenna and the second antenna is restricted to pointing to satellite of the Clark belt. 
   According to a further feature of the present invention, there is also provided the step of adjusting the polarization of the first antenna. 
   According to a further feature of the present invention, there is also provided the step of adjusting the polarization of the second antenna. 
   According to a further feature of the present invention, there is also provided the step of disposing the antennas in a single radome. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: 
       FIG. 1  is a schematic isometric view of an antenna stabilization system that is constructed and operable in accordance with a preferred embodiment of the present invention; 
       FIG. 2  is a schematic view of the system of  FIG. 1  mounted on a mobile platform; and 
       FIG. 3  is an isometric view of an antenna stabilization system that is constructed and operable in accordance with a most preferred embodiment of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention is an antenna stabilization system construction and method of operation thereof. 
   The principles and operation of an antenna stabilization system according to the present invention may be better understood with reference to the drawings and the accompanying description. 
   Reference is now made to  FIGS. 1 and 2 .  FIG. 1  is a schematic isometric view of an antenna stabilization system  10  that is constructed and operable in accordance with a preferred embodiment of the present invention.  FIG. 2  is a schematic view of antenna stabilization system  10  mounted on a mobile platform  16 . Antenna stabilization system  10  is a system for stabilizing two antennas  12 ,  14  on a mobile platform  16 . Antenna  12  is associated with a geo-stationary satellite  18 . Antenna  14  is associated with a geo-stationary satellite  20 . Antenna stabilization system  10  includes a lower alignment system  22  and an upper alignment system  24 . Lower alignment system  22  is mechanically connected to mobile platform  16 . Lower alignment system  22  includes an intermediate element  26 . Intermediate element  26  is generally an elongated element. Lower alignment system  22  is mechanically connected to upper alignment system  24  via intermediate element  26 . Intermediate element  26  of upper alignment system  24  is a common support for antenna  12  and antenna  14 . 
   Lower alignment system  22  has three rotational degrees of freedom including a roll adjustment  34 , a pitch adjustment  36  and a yaw adjustment  38  for adjusting the orientation of intermediate element  26 , as described in more detail below. 
   Upper alignment system  24  has three rotational degree of freedom  28   30   32 . Antenna  12  is mechanically connected to one end of intermediate element  26  via degree of freedom  28 . Antenna  14  is mechanically connected to one end of intermediate element  26  via degree of freedom  30  and degree of freedom  32 . The axis of rotation of degree of freedom  28  and degree of freedom  30  are perpendicular to the direction of elongation of intermediate element  26 . The axis of rotation of degree of freedom  32  is parallel to the direction of elongation of intermediate element  26 . Degree of freedom  28  and degree of freedom  30  are configured for adjusting the polarization of antenna  12  and antenna  14 , respectively. If antenna  12  and/or antenna  14  are not linearly polarized, then degree of freedom  28  and degree of freedom  30  are not needed, respectively, for example, but not limited to when antenna satellite  20  is a TVRO satellite, degree of freedom  30  is generally not needed. 
   Lower alignment system  22  and upper alignment system  24  include motors (not shown) for adjusting the degrees of freedom of antenna stabilization system  10 . The motors are driven by a servo driver unit  40  (SDU) motor driver. 
   The operation of antenna stabilization system  10  is best described by first assuming that mobile platform  16  is completely stationary without tilting, rocking, or turning. In this scenario, lower alignment system  22  is configured by adjusting roll adjustment  34 , pitch adjustment  36  and yaw adjustment  38 , such that the direction of elongation of intermediate element  26  is perpendicular to a plane which includes all the satellites in the Clark Belt and antenna  12  is pointing toward satellite  18 . Therefore, as degree of freedom  32  is parallel to the direction of elongation of intermediate element  26 , the movement of antenna  14  is restricted, such that antenna  14  is only able to point to satellites in the Clark belt. Degree of freedom  32  is adjusted, such that antenna  14  points toward satellite  20 . In other words, degree of freedom  32  substantially matches the angular displacement between antenna  12  and antenna  14  with the angular displacement between the satellite  18  and satellite  20 . The term “substantially matches” is defined herein such that the angular displacement is matched well enough, such that antenna  12  can communicate with satellite  18  and antenna  14  can communicate with satellite  20 . The angular displacement between satellite  18  and satellite  20  is defined as the angle between two lines, the first line connecting satellite  18  and a point on antenna stabilization system  10 , the second line connecting satellite  20  and the same point of antenna stabilization system  10 . The angular displacement between antenna  12  and antenna  14  is defined as the angle between a “line of sight” of antenna  12  and a “line of sight” of antenna  14 . The term “line of sight” is defined herein as a line joining the communication center of an antenna and the communication center of a satellite, the antenna and the satellite being aligned for peak communication. In other words, degree of freedom  32  is configured for setting the cross-elevation of antenna  12  and antenna  14 . 
   The operation of antenna stabilization system  10  is now described by assuming that mobile platform  16  is rotating. Rotating is defined herein as to include tilting, rocking, or turning of mobile platform  16 . Antenna stabilization system  10  also includes an inertial measurement unit  42  (IMU) for measuring movement of mobile platform  16 . Antenna stabilization system  10  also includes a controller  44 . Controller  44  is configured for processing the measurements of inertial measurement unit  42  as well as running algorithms for continuous peak signal-strength detection. Therefore, measurements from inertial measurement unit  42  provide data for coarse adjustment of lower alignment system  22  and upper alignment system  24 , while signal-strength algorithms provide data for fine adjustment of lower alignment system  22  and upper alignment system  24 . Therefore, the signal strength algorithms enable the accuracy and therefore the cost of inertial measurement unit  42 , lower alignment system  22  and upper alignment system  24  to be reduced. U.S. Pat. No. 6,608,950 to Naym, et al. describes a novel system for adjusting for polarization using auto-correlation. It will be appreciated by those ordinarily skilled in the art that the auto-correlation method can be used for aligning roll of antenna stabilization system  10 . Methods for adjusting yaw and pitch using signal strength techniques are known by those skilled in the art. Controller  44  is configured for instructing servo driver unit  40  to adjust the motors of lower alignment system  22  and upper alignment system  24  in order to adjust for movements of mobile platform  16 . Therefore, lower alignment system  22  is configured for maintaining the orientation of intermediate element  26  in order to compensate for rotation of mobile platform  16  relative to satellite  18  and satellite  20 , such that the direction of elongation of intermediate element  26  is perpendicular to a plane which includes all the satellites in the Clark Belt and antenna  12  is pointing toward satellite  18 . In other words, lower alignment system  22  is configured for maintaining intermediate element  26  in a constant angular and rotational position. The angular displacement between antenna  12  and antenna  14  does not need to be adjusted by adjusting degree of freedom  32 . This is because the angular displacement between satellite  18  and satellite  20  does not alter significantly enough to effect communication between antennas  12 ,  14  and satellites  18 ,  20 , respectively. The angular displacement between antenna  12  and antenna  14  only needs to be adjusted when there is a significant change in longitude or latitude of mobile platform  16 , which effects communication. 
   Therefore, adjustment of at least one of roll adjustment  34 , pitch adjustment  36  and yaw adjustment  38  of lower alignment system  22  is enough to compensate for at least one of roll, pitch and yaw movement of mobile platform  16  relative to satellites  18 ,  20 , such that antenna  12  and antenna  14  are maintained pointing toward satellite  18  and satellite  20 , respectively, without needing to adjust upper alignment system  24 . Therefore, one of the important advantages of antenna stabilization system  10  is that only the degrees of freedom of lower alignment system  22  need to be adjusted to realign both antenna  12  and antenna  14  toward satellite  18  and satellite  20 , respectively. Therefore, degree of freedom  28 , degree of freedom  30  and degree of freedom  32  of upper alignment system  24  only need to have a low-dynamic response, for example, for selecting a different pair of satellites or for accurate correction and/or compensation of slight variations of the angular displacement of satellite  18  and satellite  20  due to geographical longitudinal or latitudinal movement of mobile platform  16 . Roll adjustment  34 , pitch adjustment  36  and yaw adjustment  38  of lower alignment system  22  need to have a high dynamic response, typically having a velocity up to 30 degrees per second, and an acceleration of up to 30 degrees per second per second. Antenna stabilization system  10  typically has a pointing accuracy better than 0.3 degrees RMS. Additionally, antenna stabilization system  10  typically has a resolution of less than 0.01 degree, enabling very smooth operation and high quality continuous step-track. 
   The rotational requirement of the degrees of freedom of antenna stabilization system  10  are typically as follows. Yaw adjustment  38  is continuous. Pitch adjustment  36  is from minus 10 degrees to plus 90 degrees. Roll adjustment  34  is from minus 60 degrees to plus 60 degrees. Degree of freedom  28  and degree of freedom  30  are both from minus 90 degrees to plus 90 degrees. Degree of freedom  32  is from minus 90 degrees to plus 90 degrees. 
   The system and method of the present invention also includes the following advantages. First, antenna stabilization system  10  enables selection of any pair of satellites. Second, antenna stabilization system  10  enables antenna  12  and antenna  14  to be pointed toward a single satellite or two very close satellites. Third, antenna stabilization system  10  including antenna  12  and antenna  14  fits under a single radome  52 . Fourth, there is no communication blockage between antenna  12  and antenna  14 . Fifth, the lower alignment system  22  and upper alignment system  24  are configured to provide full hemispherical coverage for the antenna  12  and antenna  14 , typically down to minus 10 degrees elevation (pitch) and continuous azimuth (yaw) rotation. 
   Reference is now made to  FIG. 3 , which is an isometric view of an antenna stabilization system  46  that is constructed and operable in accordance with a most preferred embodiment of the present invention. Antenna stabilization system  46  is the same as antenna stabilization system  10  ( FIG. 1 ) except for the following differences. Pitch adjustment  36  and roll adjustment  34  are both disposed very close to intermediate element  26 . Therefore, lower alignment system  22  has a curved elongated element  48  disposed between pitch adjustment  36  and yaw adjustment  38  in order that movement of antennas  12 ,  14  is not restricted, such that antenna stabilization system  10  provides full hemispherical coverage for antenna  12  and antenna  14 . Additionally, upper alignment system  24  includes a counterweight arrangement  50  disposed on intermediate element  26  in order to reduce the load on the motors (not shown) of antenna stabilization system  46 . 
   It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and sub-combinations of the various features described hereinabove, as well as variations and modifications thereof that are not in the prior art which would occur to persons skilled in the art upon reading the foregoing description.