Abstract:
An airborne radio frequency (RF) antenna terminal system includes a two-axis gimbals control system and a phased array antenna. The phased array antenna electronically steers the receive and transmit beams using phase shifters. The electronically steered beams provide a virtual third-axis for the two-axis gimbals control system. The combination of the electronically steered beams and the two-axis gimbaled system provides accurate beam steering for the keyhole region of the two-axis gimbals control system so that the RF communication link is prevented from being lost in the keyhole region.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention generally relates to accurate beam pointing in the keyhole region of an airborne radio frequency (RF) antenna and, more particularly, to using phased array beam steering for third-axis motion in a two-axis gimbaled antenna control system.  
         [0002]     Airborne radio frequency (RF) antenna terminal systems have been developed for the FAB-T (Family of Advanced Beyond line-of-sight Terminal) program for military EHF (Extremely High Frequency) satellite communication systems. Such RF antenna terminal systems may, for example, be mounted on a moving platform—such as a B-52 aircraft—and are designed to acquire and track a geostationary satellite payload or a polar satellite payload to establish a two-way digital beyond line-of-sight communication service that is secure, jam-resistant, scintillation-resistant (scintillation loss results from rapid variations in a communication signal&#39;s amplitude and phase due to changes in the refractive index of the Earth&#39;s atmosphere), and has a low probability of intercept and detection.  
         [0003]     In order to meet the required communication link performance for such a communication service, the antenna pointing for tracking the satellite payload is required to be precisely controlled in the presence of platform motion. For example, the total signal loss due to antenna pointing error is typically required to be less than 1 decibel (dB), at the 3 sigma (standard deviation) level specified over a field-of-regard (FOR) given by 0 to 360 degrees in azimuth and 5 to 90 degrees in elevation.  
         [0004]     One prior art RF antenna designed for existing EHF communication terminals used a two-axis gimbaled control system, which could not maintain the required pointing accuracy in the vicinity of the keyhole region—the region where the antenna pointing elevation angle is close to 90 degrees. Thus, in the keyhole region, the communication link could be temporarily lost due to pointing error using the two-axis gimbaled control system. A three-axis gimbaled control system was proposed and designed during the early phase of the FAB-T program to eliminate this keyhole problem. Because of the available antenna dome volume, however, the three-axis gimbaled control system could not accommodate the required antenna aperture to meet the desired antenna gain performance.  
         [0005]     As can be seen, there is a need for accurate antenna pointing in the keyhole region from a moving platform. Moreover, there is a need for accurately pointing an antenna in the keyhole region of a moving platform that does not require a larger antenna dome, or a smaller antenna aperture.  
       SUMMARY OF THE INVENTION  
       [0006]     In one aspect of the present invention, a communication system includes a two-axis gimbals control system having a gimbals azimuth axis and a gimbals elevation axis; and an antenna mounted to the two-axis gimbals control system along the elevation axis. The antenna generates an electronically steered beam that adjusts the antenna pointing direction relative to a cross-elevation axis that is perpendicular to the gimbals elevation axis.  
         [0007]     In another aspect of the present invention, a method for antenna pointing includes steps of: controlling antenna pointing using a two-axis gimbals control system when an antenna LOS pointing vector is outside a keyhole region; and controlling antenna pointing using the two-axis gimbals control system with additional electronic beam steering using electronically steered angles when the antenna LOS pointing vector is inside the keyhole region.  
         [0008]     In a further aspect of the present invention, a method for communication system antenna pointing from a moving platform includes steps of: commanding an azimuth angle and an elevation angle to a two-axis gimbals control system having a gimbals azimuth axis and a gimbals elevation axis. The two-axis gimbals control system is located on the moving platform. The method also includes steps of: computing a cross-azimuth angle and cross-elevation angle for an antenna mounted to the two-axis gimbals control system along the elevation axis; and adjusting the antenna pointing direction electronically relative to a cross-elevation axis that is perpendicular to the gimbals elevation axis, using the cross-azimuth angle and cross-elevation angle.  
         [0009]     These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a geometrical diagram for a satellite communication system in accordance with an embodiment of the present invention;  
         [0011]      FIG. 2  is a schematic diagram for antenna pointing axes on an antenna platform for a satellite communication system in accordance with an embodiment of the present invention;  
         [0012]      FIG. 3  is a geometrical diagram for a satellite communication system in accordance with one embodiment of the present invention;  
         [0013]      FIG. 4  is a set of four graphs comparing prior art antenna pointing performance with that of one embodiment of the present invention; and  
         [0014]      FIG. 5  is a flow chart of a method for communication system antenna pointing according to one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]     The following detailed description is of the best currently contemplated modes of carrying out the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims.  
         [0016]     Broadly, the present invention uses the electronically steered beams generated by a phased array antenna to add a third-axis motion for a two-axis gimbaled control system for antenna beam pointing from a moving platform for radio-frequency (RF) communication systems. For example, one embodiment is especially useful for antenna beam pointing in a beyond line-of-sight communications link between an aircraft and a satellite and provides reliable antenna pointing and signal strength in the keyhole region of the aircraft. One embodiment thus differs from prior art two-axis gimbals control systems—which do not provide reliable antenna pointing in the keyhole region—by effectively providing a three-axis gimbals control that provides reliable antenna pointing in the keyhole region. One embodiment differs from prior art three-axis gimbals control systems, which rely on a third mechanical gimbal to provide three-axis gimbals control, by using electronic steering of the beam to achieve the third axis control and providing an antenna having a larger aperture than can be provided in a mechanical three-axis gimbals system having the same volume. One embodiment thus maximizes the antenna gain performance while solving the keyhole problem.  
         [0017]     For example, because the FAB-T (Family of Advanced Beyond line-of-sight Terminal) antenna is a phased array antenna, which has the capability to electronically steer the received and transmitted beams using phase shifters, one embodiment can make use of electronically steered beams to accommodate the third-axis gimbaled motion. Using the two-axis gimbaled system with the aid of electronically steered beams, one embodiment can annihilate the keyhole region while optimizing RF performance. As pointed out in the case of a prior art three-axis gimbals system, the size of the antenna aperture needs to be reduced to satisfy the same volume constraints because of additional volume needed for the cross-elevation (third) gimbals axis. The three-axis gimbals approach not only degrades the antenna gain, it also increases the system weight and power. Since the FAB-T antenna is a phased array antenna, it can steer its received and transmitted beams away from its boresight using the available phase shifters (5-bit phase shifters). Hence, one embodiment can use a two-axis gimbaled system and electronically steer the beams off to compensate for the pointing error when the line of sight (LOS) enters the keyhole region.  
         [0018]     Referring now to the figures,  FIG. 1  shows a communication system  100  in accordance with an embodiment of the present invention. Communication system  100  may include a beyond line-of-sight communications link (not shown) between a moving platform  102 —e.g., an aircraft—and a satellite  104 . Communication system  100  may refer to an Earth-centered Earth-fixed (ECEF) reference frame  106 . For example, ECEF reference frame  106  may have coordinate axes  108  originating at the planet Earth&#39;s center of mass and rotating with the Earth. ECEF reference frame  106  may be contrasted, for example, to an Earth-centered inertial (ECI) reference frame (not shown) having coordinate axes originating at the planet&#39;s center of mass and pointing toward fixed stars. A platform ECEF coordinate vector R P    110  may represent the position of platform  102  relative to ECEF reference frame  106 . Likewise, a satellite ECEF coordinate vector R S    112  may represent the position of satellite  104  relative to ECEF reference frame  106 .  
         [0019]     A range pointing vector R R    114  may represent the position of satellite  104  relative to platform  102  and may also be described as a vector from the platform  102  to the satellite  104  (e.g., a vector in the direction of the line-of-sight (LOS) from the platform  102  to the satellite  104 ). Range pointing vector R R    114  may be computed in the ECEF coordinate frame  106  by vector subtraction of vector R P    110  from vector R S    112 , i.e., R R =R S −R P . As well known, a unit vector (vector having a length of one) in the direction of vector R R    114  may be computed by scalar division of vector R R    114  by its length |R R | to provide a normalized (i.e., unit length) range pointing vector {right arrow over (r)} LOS   ECEF    116  with respect to the ECEF reference frame  106 , i.e.,  
                 r   -&gt;     LOS   ECEF     =         R   R            R   R            ..             (   1   )             
 
 Thus, normalized range pointing vector {right arrow over (r)} LOS   ECEF    116  may be described as a unit vector in the direction of the line-of-sight from the platform  102  to the satellite  104  relative to the ECEF reference frame  106 . 
 
         [0020]      FIGS. 2 and 3  show a body reference frame  200  and the relationship of its various axes to an antenna  202  for communication system  100  and to the body (e.g., platform  102 ) in relation to which body reference frame  200  is fixed. For example, the body may be platform  102 , and platform  102  may be assumed to be an aircraft for purposes of the terminology used in  FIG. 2 .  FIG. 2  also shows the relationship of the axes of body reference frame  200  to a set of gimbals axes.  
         [0021]     Antenna  202  may have an antenna pointing vector  204  which generally represents the direction of maximum beam energy of RF radiation of antenna  202  and may also be considered as the RF line-of-sight of antenna  202 . Antenna  202  may have a long a-b axis  206  and a short axis  207  perpendicular to long axis  206 . The direction of antenna LOS pointing vector  204  may be controlled relative to axis  206  by electronic beam steering, e.g., shifting the relative phase of antenna elements of antenna  202 . Operating the link of communication system  100  between platform  102  and satellite  104  requires aiming antenna pointing vector  204  in the direction of satellite  104 , e.g., aligning pointing vector  204  with range pointing vector {right arrow over (r)} LOS   ECEF    116 .  
         [0022]     Although  FIG. 2  schematically represents a gimbals having 3 axes, it is to be understood that  FIG. 2  is a schematic diagram only and that antenna pointing function of at least one of the gimbals axes may be achieved, according to one embodiment, by electronically steering the beam of antenna  202  to change the direction of antenna pointing vector  204 , while antenna pointing function of other gimbals axes may be achieved through the mechanical mounting of the antenna  202  to mechanical gimbals which change the direction of antenna pointing vector  204  by mechanically moving the antenna  202 .  
         [0023]     Body reference frame  200  may include an X-axis  208 , having a positive direction in the direction of the nose of the aircraft, e.g., platform  102 , and may be considered as an aircraft roll axis with a positive roll angle  209  moving the right wing down. The X-axis  208  may be used to measure the r 1  coordinate of {right arrow over (r)} LOS   Body    316  (see  FIG. 3 ), the representation of normalized range pointing vector {right arrow over (r)} LOS   ECEF    116  with respect to body reference frame  200 . Body reference frame  200  may include a Y-axis  210 , having a positive direction in the direction of the left wing of the aircraft body and may be considered as an aircraft pitch axis with a positive pitch angle  211  moving the nose up. The Y-axis  210  may be used to measure the r 2  coordinate of range pointing vector {right arrow over (r)} LOS   Body    316  with respect to body reference frame  200 . Body reference frame  200  may include a Z-axis  212 , having a positive direction in the direction of the top of the aircraft body and may be considered as an aircraft yaw or heading axis with a positive yaw angle  213  turning the aircraft clockwise as viewed from the top. The Z-axis  212  may be used to measure the r 3  coordinate of range pointing vector {right arrow over (r)} LOS   Body    316  with respect to body reference frame  200 .  
         [0024]     A two-axis gimbals control system  201  may include a gimbals azimuth axis  222  and a gimbals elevation axis  220 . The gimbals azimuth axis  222  may coincide with Z-axis  212 , as shown in  FIG. 2 . In the example used to illustrate one embodiment, gimbals azimuth axis  220  may be a mechanical axis. An azimuth angle AZ  223  may have positive direction corresponding to that of positive yaw angle  213 . The gimbals elevation axis  220  may be held perpendicular to gimbals azimuth axis  222  and may lie in the plane of X-axis  208  and Y-axis  210 . For example,  FIG. 2  shows gimbals elevation axis  220  in a position that coincides with Y-axis  210 . In the example used to illustrate one embodiment, gimbals elevation axis  220  may be a mechanical axis. An elevation angle EL  221  may have positive direction corresponding to that of positive pitch angle  211 . Antenna  202  may be mounted to gimbals elevation axis  220  so that the long axis  206  of antenna  202  is along gimbals elevation axis  220 .  
         [0025]     A cross-elevation axis  218  may be perpendicular to gimbals elevation axis  220  and may lie in the plane of X-axis  208  and Y-axis  210 . For example,  FIG. 2  shows cross-elevation axis  218  in a position that coincides with X-axis  208 . In the example used to illustrate one embodiment, cross-elevation axis  218  may be a virtual axis provided by electronic steering of antenna pointing vector  204  rather than a mechanical gimbals axis. A cross-elevation angle XEL  219  may have positive direction corresponding to that of positive roll angle  209 .  
         [0026]     When range pointing vector {right arrow over (r)} LOS   ECEF    116  ({right arrow over (r)} LOS   Body    316 ) is not in the keyhole region  302  (see  FIG. 3 ), the two-axis gimbals system using azimuth axis  222  and elevation axis  220  may be used to point RF antenna  202  from platform  102  in the direction of satellite  104 , i.e., to command pointing vector  204  to align with range pointing vector {right arrow over (r)} LOS   ECEF    316 , which is the representation of normalized range pointing vector {right arrow over (r)} LOS   ECEF    116  with respect to body reference frame  200 . The commanded azimuth angle AZ  223  and elevation angle EL  221  may be computed by:  
               AZ   =     -       tan     -   1       ⁡     (       r   2       r   1       )           ;     EL   =       tan     -   1       (       r   3           r   1   2     +     r   2   2           )               (   2   )             
 
 where r 1 , r 2 , and r 3  are the three coordinates, with respect to body frame  200  of  
                 r   -&gt;     LOS   Body     =       [           r   1               r   2               r   3           ]     =         [     C   LL   Body     ]     ⁡     [     C   ECEF   LL     ]       ⁢       r   -&gt;     LOS   ECEF                 (   3   )             
 
 where C LL   Body  is the aircraft body attitude with respect to a local level (LL) frame, and C ECEF   LL  is the LL attitude with respect to the ECEF frame  106 . For example, C LL   Body  may be a three by three coordinate transformation matrix from an LL reference frame (e.g., a reference frame (not shown) centered at reference frame  200  but with the negative Z-axis pointing toward the center of mass of the planet) into the body reference frame  200 , and C ECEF   LL  may be a three by three coordinate transformation matrix from the ECEF reference frame  106  into the LL reference frame. 
 
         [0027]     The following considerations apply, however, when range pointing vector {right arrow over (r)} LOS   ECEF    116  ({right arrow over (r)} LOS   Body    316 ) enters the keyhole region  302 . The azimuth rate, d(AZ)/dt—e.g., the spinning velocity of the gimbals around azimuth axis  222 —and the azimuth acceleration, d 2 (AZ)/dt 2 —e.g., spinning force, or torque, on the gimbals around azimuth axis  222 —can be shown to be approximated as:  
                             ⅆ     (   AZ   )         ⅆ   t       ≈       ⁢       -     (       r   1         r   1   2     +     r   2   2         )       ⁢       r   .     2         =       -     r   1             r   1   2     +     r   2   2             ⁣         sin   ⁡     (   EL   )             r   1   2     +     r   2   2           ⁢     ϕ   .                   ≈       ⁢       cos   ⁡     (   AZ   )       ⁢     tan   ⁡     (   EL   )       ⁢     ϕ   .               ⁢     
     ⁢   and           (   4   )                     ⅆ   2     ⁢     (   AZ   )         ⅆ     t   2         ≈         (       sin   ⁡     (   AZ   )       ⁢     tan   ⁡     (   EL   )         )     ⁢     ϕ   .     ⁢   A   ⁢           ⁢     Z   .       -       (       cos   ⁡     (   AZ   )       ⁢     tan   ⁡     (   EL   )         )     ⁢     ϕ   ¨                 (   5   )             
 
 where φ is the aircraft roll angle, e.g., roll angle  209 . (Dot and double dot above a variable follow the standard mathematical notation for first and second time derivatives of the variable.) Hence, as the elevation angle EL  221  approaches 90 degrees, e.g., the keyhole region  302 , the azimuth rate and azimuth acceleration “become infinite” (due to tan(EL) increasing without bound). Thus, antenna pointing cannot be precisely controlled when the antenna elevation is near 90 degrees, or in the keyhole region  302 . It is noted that depending on the gimbals configuration the keyhole region  302  may occur at different elevation (EL  221 ) or azimuth (AZ  223 ) angles. For a given two-axis gimbaled antenna system, the keyhole region  302  may be defined as being where the corresponding elevation rate, or azimuth rate, approaches infinite at any operating gimbal angle range. The methods described in embodiments of this invention also apply to those cases where keyhole regions, as defined, exist. 
 
         [0028]     To provide a first approach to precise control when the antenna line-of-sight (LOS), e.g., antenna pointing vector  204 , enters the keyhole region  302 , a third gimbals axis, e.g., cross-elevation axis  218 , nested within the elevation axis  220 , as shown in  FIG. 2 , may be considered. In this first approach, the azimuth gimbals axis  222  would be limited to its maximum azimuth acceleration and maximum azimuth rate. Thus, the above formulas for azimuth rate and azimuth acceleration may be used to find a value of EL, based on the physical properties of the particular gimbals system being used, that suggests what the appropriate keyhole region should be for the particular gimbals system and a keyhole region  302  may be defined for the particular gimbals system being used. For example, a keyhole region  302  for a typical gimbals system may include all elevation angles EL between 87 and 90 degrees, with the boundary or threshold  304  of the keyhole region  302  in this example being a locus of points at an elevation angle of 87 degrees as shown in  FIG. 3 . When the LOS pointing vector  204  enters the keyhole region, the elevation angle EL  221 , and the cross-elevation angle XEL  219 , may be computed in the first approach as follows:  
               EL   =       cotan     -   1       ⁡     (       r   1   ′       r   3   ′       )         ⁢     
     ⁢     XEL   =     -       tan     -   1       (       r   2   ′           r   1     ′   ⁢           ⁢   2       +     r   3   ′2           )         ⁢          ⁢   with           (   6   )                 [           r   1   ′               r   2   ′               r   3   ′           ]     =       [           cos   ⁡     (     AZ   m     )             -     sin   ⁡     (     AZ   m     )             0             sin   ⁡     (     AZ   m     )             cos   ⁡     (     AZ   m     )           0           0       0       1         ]     ⁡     [           r   1               r   2               r   3           ]               (   7   )             
 
 where AZ m  is the measured azimuth angle AZ  223  which may be provided, for example, by a gimbal resolver, as known in the art. 
 
         [0029]     Thus, in accordance with one embodiment using electronic beam steering to make cross-elevation XEL adjustments about cross-elevation axis  218 , when the antenna line-of-sight (LOS), e.g., antenna pointing vector  204 , enters the keyhole region  302 , the azimuth angle AZ  223  and the elevation angle EL  221  may be commanded as follows:  
               AZ   =     -       tan     -   1       ⁡     (       r   2       r   1       )           ⁢     
     ⁢     EL   =         cotan     -   1       ⁡     (       r   1   ′       r   3   ′       )       .               (   8   )             
 
         [0030]     A corresponding LOS pointing error vector Δ{right arrow over (r)}  315  (see  FIG. 3 ) between range pointing vector {right arrow over (r)} LOS   Body    316  and keyhole coast-through pointing vector {tilde over (r)} LOS   Body    317  is then given by: 
 
Δ {right arrow over (r)}={tilde over (r)}   LOS   Body   −{right arrow over (r)}   LOS   Body    (9) 
 
 where:  
                 r   ~     LOS   Body     =     [             cos   ⁡     (     EL   m     )       ⁢     cos   ⁡     (     AZ   m     )                     -     cos   ⁡     (     EL   m     )         ⁢     sin   ⁡     (     AZ   m     )                   sin   ⁡     (     EL   m     )             ]             (   10   )             
 
 and where AZ m  and EL m  are measured values for azimuth angle AZ  223  and elevation angle EL  221  and may be measured, for example, by gimbals resolvers, as known in the art. 
 
         [0031]     To derive the required cross-elevation and cross-azimuth electronically steered angles, xEL  330  and xAZ  340  (see  FIG. 2 ), for canceling the LOS pointing error vector Δ{right arrow over (r)}  315 , we first define the following parameters:  
               [           r   1   ″               r   2   ″               r   3   ″           ]     =       [           cos   ⁡     (     EL   m     )           0         sin   ⁡     (     EL   m     )               0       1       0             -     sin   ⁡     (     EL   m     )             0         cos   ⁡     (     EL   m     )             ]     ⁡     [           r   1   ′               r   2   ′               r   3   ′           ]               (   11   )             
 
 and then solve the following equations for xEL  330  and xAZ  340 :  
               [         1           0           0         ]     =         [           cos   ⁡     (   xAZ   )           0         sin   ⁡     (   xAZ   )               0       1       0             -     sin   ⁡     (   xAZ   )             0         cos   ⁡     (   xAZ   )             ]     ⁡     [           cos   ⁡     (   xEL   )             -     sin   ⁡     (   xEL   )             0             sin   ⁡     (   xEL   )             cos   ⁡     (   xEL   )           0           0       0       1         ]       ⁡     [           r   1   ″               r   2   ″               r   3   ″           ]               (   12   )             
 
 which gives:  
               xEL   =     -       tan     -   1       ⁡     (       r   2   ″       r   1   ″       )           ⁢     
     ⁢     xAZ   =         tan     -   1       (       r   3   ″             (     r   1   ″     )     2     +       (     r   2   ″     )     2           )     .               (   13   )             
 
         [0032]     The angles xEL  330  and xAZ  340  may then be used to electronically steer the beam of antenna  202  to correct the antenna pointing, aligning antenna LOS pointing vector  204  with range pointing vector {right arrow over (r)} LOS   Body    316  (range pointing vector {right arrow over (r)} LOS   ECEF    116 ).  
         [0033]      FIG. 4  shows graphs for a set of simulation results for a two-axis gimbaled system with—graphs  401 ,  402 —and without—graphs  411 ,  412 —the electronically steered beams for antenna LOS in the keyhole region. Using one embodiment of the present invention—see graphs  401 ,  402 —the communication link between platform  102  and satellite  104  remains operative even when the LOS pointing vector  204  enters the keyhole region  302 . For example, maximum antenna pointing error loss  403  remains less than 1 decibel (dB) when elevation angle EL  221  is in the keyhole region at point  404  on graph  401 . On the other hand, as shown on graphs  411  and  412 , the communication link between platform  102  and satellite  104  can be temporarily lost (antenna pointing error loss  413  exceeds 1 dB) for a two-axis gimbaled system without the electronically steered beam when its LOS enters the keyhole region at point  414  on graph  411 .  
         [0034]     A method  500  for communication system antenna pointing is illustrated in  FIG. 5 . At step  502 , a keyhole region  302  is defined for a two-axis gimbals control system  201 . At step  504 , antenna pointing is controlled using two-axis gimbals control system  201  when LOS pointing vector  204  is outside keyhole region  302 . At step  506 , when LOS pointing vector  204  is inside keyhole region  302 , antenna pointing is controlled using two-axis gimbals control system  201  with additional electronic beam steering to provide electronically steered angles xEL  330  and xAZ  340 , calculated using Equation (13), for example, for canceling the LOS pointing error vector Δ{right arrow over (r)}  315  and aligning antenna LOS pointing vector  204  with range pointing vector {right arrow over (r)} LOS   Body    316 (=range pointing vector {right arrow over (r)} LOS   ECEF    116 ). The method may alternate between step  504  and step  506  depending on whether the LOS pointing vector  204  is inside keyhole region  302  or outside keyhole region  302 .  
         [0035]     It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.