Abstract:
A hybrid satellite antenna comprises an ESA with two steerable dimensions connected to a motor. The motor rotates the antenna about an axis to position the antenna such that a satellite signal can be sufficiently resolved using the two steerable dimensions of the ESA.

Description:
FIELD OF THE INVENTION 
     The present invention is directed generally toward satellite antennas and more particularly to satellite antennas configured for a dynamic environment. 
     BACKGROUND OF THE INVENTION 
     Satellite communication requires precise antenna positioning. When attempting geosynchronous satellite communication from a stationary or nearly stationary location, a satellite antenna, once properly positioned, may require little or no adjustment. When adjustments are required, they are predictable and easily accomplished. 
     However, when attempting satellite communication on the move, the satellite antenna must be constantly and precisely adjusted and repositioned. For example, a satellite antenna affixed to a vehicle must be able to point the beam to within less than 0.5° of a desired orientation while the vehicle is moving; vehicle movement could create a dynamically shifting environment requiring angular acceleration of 120°/s 2 . Satellite communication on the move (SOTM) requires full hemispherical coverage. In addition, Low Earth Orbiting (LEO) satellites are not geosynchronous and therefore require continuous tracking. 
     Electronically steerable antennas (ESAs) can achieve a pointing accuracy of less than 0.5° but any individual planar ESA has only a limited steering range. Planar arrays are the least complex and most commonly used ESA; therefore, multiple planar, expensive ESAs are required to achieve full hemispherical coverage. Spherical ESA are capable of full hemispherical coverage but they are large, complex, expensive and aerodynamically unattractive for airborne applications. 
     Mechanically steerable antennas with two dimensions of movement can achieve full hemispherical coverage with a single antenna. However, the motion control system for military sitcom on the move (SOTM) is extremely complex and costly. It is very challenging to hold a lock on a satellite system while traversing over rough terrain in a ground vehicle when the SOTM antenna has a very narrow beam width, which can be a on the order of 1 degree for Q band systems. The inertial mass, moment arm and center of gravity of the antenna group (antenna positioner, RF front end, modem, etc.) of a typical SOTM antenna group makes motion control with high rates of acceleration with pointing accuracies within 0.5° very challenging. The required motion control systems are expensive, heavy and subject to mechanical failure. Furthermore, mechanically steerable systems are inherently slower than electronically steerable systems. 
     Consequently, it would be advantageous if a lightweight, cost-effective apparatus existed that is suitable for accurately positioning a satellite antenna in a dynamic environment. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a novel method and lightweight, cost-effective apparatus for accurately positioning a satellite antenna in a dynamic environment. 
     One embodiment of the present invention is hybrid antenna with a planar ESA, steerable in two dimensions, mounted to an azimuthal motor. The ESA is mounted to the motor such that the motor can rotate the ESA about an axis to provide 360° of gross movement while the ESA itself provides fine tuning in the azimuth. The ESA is also mounted to the motor at an angle to a horizontal plane so that the range of one of the steerable dimensions in the ESA provides adequate coverage of elevation for satellite systems of interest. 
     Another embodiment of the present invention is a method for steering a hybrid antenna. The method includes monitoring signal strength in an ESA while performing gross position adjustments with an azimuthal motor, then electronically performing fine adjustments in a first steerable dimension of the ESA and electronically performing fine adjustments in a second steerable dimension of the ESA. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention claimed. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention and together with the general description, serve to explain the principles. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The numerous objects and advantages of the present invention may be better understood by those skilled in the art by reference to the accompanying figures in which: 
         FIG. 1  shows a perspective view of a mechanically steerable satellite antenna with two dimensions of mobility; 
         FIG. 2  shows a perspective view of an electronically steerable satellite antenna; 
         FIG. 3  shows a block diagram of a hybrid satellite antenna according to the present invention; 
         FIG. 4  shows a block diagram of a combined phased array for a hybrid satellite antenna such as shown in  FIG. 3 ; 
         FIG. 5  shows a top view diagram of a hybrid satellite antenna; 
         FIG. 6  shows a side view diagram of a hybrid satellite antenna; 
         FIG. 7  shows a perspective view of a hybrid satellite antenna in a radome; and 
         FIG. 8  shows a flowchart of a method for orienting a hybrid satellite antenna. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to the subject matter disclosed, which is illustrated in the accompanying drawings. The scope of the invention is limited only by the claims; numerous alternatives, modifications and equivalents are encompassed. For the purpose of clarity, technical material that is known in the technical fields related to the embodiments has not been described in detail to avoid unnecessarily obscuring the description. 
     Referring to  FIG. 1 , a perspective view of a mechanically steerable satellite antenna is shown. A mechanically steerable satellite antenna may include an azimuth positioning mechanism  100  connected to an azimuth positioning motor  106 . The azimuth positioning mechanism  100  may support an elevation positioning mechanism  102  and elevation positioning motor  108 . The elevation positioning mechanism  102  may rotate an antenna  104  about an axis substantially parallel to the horizon to orient the antenna  104  to point toward a desired elevation. The entire mechanically steerable satellite antenna may be housed inside of a radome  110 . 
     Because satellite communication requires accurate positioning and orientation of the antenna to within 0.5°, a control system must be able to rotate the azimuth positioning unit to within 0.5° of a desired orientation and maintain such orientation even under stress due to external motion and acceleration of the host vehicle. Furthermore, the elevation positioning mechanism  102  adds additional weight to the azimuth positioning mechanism  100 , which therefore adds additional momentum during positioning which must be compensated for by the control system and by stiff bearings and a powerful motor. The elevation positioning mechanism  102  also requires stiff bearings to achieve elevation orientation within 0.5°. 
     Stiff bearings and correspondingly powerful yet precise motors and precision control systems are expensive. Mechanically steerable satellite antennas are also large, necessitating a large radome  110  that decreases the aerodynamic efficiency of the vehicle housing the antenna for airborne applications. 
     Referring to  FIG. 2 , a perspective view of a multi-panel ESA is shown. The receive array is abutted to the transmit array for each of the panels shown. The ESA includes one or more planar receiving arrays  200 , and a plurality of planar transmitting arrays  202 . An ESA panel may also be configured as one or more arrays in a common aperture such that the transmitting array  202  and receiving array  200  potentially share at least one common radiating element. Another ESA panel configuration is a “nested” transmit array superimposed within the receive array. The transmit and receive arrays are then effectively “interlaced”. The configuration of  FIG. 2  offers optimal performance and the applicability of the other configurations described depend on the harshness of ESA systems requirements. ESAs are also called phased array antennas; the beam from a phased array antenna may be steered by electronically adjusting the individual phase shifter of each radiating element in the phased array to create constructive and destructive interference that nullifies the beam in undesirable directions and enhances the beam in desirable direction; i.e. the beam is effectively “steered” to the desired elevation and azimuth position. Two-dimensional planar phased array antennas are operable to steer a beam within a conical volume as referenced to the axis normal to the surface of the phased array antenna panel. The structure and design of a phased array antenna may determine the scan volume in which the phased array antenna can steer a beam. Steering a beam with a phased array antenna is very fast as compared to a mechanically steerable antenna since phase shifter adjustments can typically be made on the order of tens of microseconds. A single, spherical phased array antenna may realize full hemispherical scan volume, but may be more expensive and significantly higher profile. 
     An ESA such as in  FIG. 2  may be statically mounted to a vehicle. Where transmitting arrays  202  are oriented substantially perpendicular to each other in a plane defined by the azimuth, four transmitting arrays  202  may technically cover substantially the entire hemisphere. However, each transmitting array  202  may not provide the same signal integrity as a beam is steered away from the direction normal to the surface of the transmitting array  202 . The scan volume of a planar phased array panel, in a single dimension, can be predicted by the equation: Gain=G o *cos n (θ), where θ is the angle the beam scans off array normal and G o  is the gain at array normal. For an ideal array, n=1.0 and n is greater than one for actual phased arrays. This equation readily shows that the gain progressively becomes less as the array is scanned off array normal. 
     Where a satellite is positioned at the periphery of the effective view (i.e. off perpendicular) of any one transmitting array  202 , performance of the ESA may be compromised. Also, where the receiving array  200  is fixed in a certain position, the receiving array  200  cannot be oriented to improve signal integrity on the receiving end. Furthermore, phased array antennas are expensive; a hemispherical coverage ESA necessarily requires enough phased array antennas to cover the entire hemisphere at all times. 
     Referring to  FIG. 3 , a block diagram of a hybrid satellite antenna is shown. The hybrid satellite antenna may comprise a combined phased array  300 . The combined phased array may comprise a receive ESA panel  302  and a transmit ESA panel  304 . Both the receive ESA panel  302  and transmit ESA panel  304  may have substantially the same orientation such that each of the receive ESA panel  302  and transmit ESA panel  304  may communicate with the same satellite at the same time. The combined phased array  300  may be connected to a motor  306 . The motor  306  may rotate the combined phased array  300  about an axis. One skilled in the art may appreciate that although a combined phased array  300  having separate receive ESA panel  302  and transmit ESA panel  304  is shown, an ESA may be configured as one or more arrays interlaced such that a transmitting array and receiving array potentially share at least one common radiating element (cell). 
     The motor  306  may be connected to a processor  308  and the processor  308  may be connected to memory  310  for storing computer executable program code. The processor  308  may actuate the motor  306  to rotate the combined phased array  300  about the axis to an azimuth with sufficient precision that the combined phased array may electronically adjust a beam to achieve optimal signal integrity. The processor  306  may be connected to a transceiver  312  that is further connected to the receive ESA panel  302  and to the transmit ESA panel  304 . The transceiver  312  may relay signals to the transmit ESA panel  304  from the processor  308  and relay signals from the receive ESA panel  302  to the processor  308 . The processor  308  may monitor signal strength through the receive ESA panel  302  to determine when to actuate the motor  306  and when to electronically adjust the combined phased array  300 . 
     Recall that the scan volume of a planar phased array panel, in a single dimension, can be predicted by the equation: Gain=G o *cos n (θ), where θ is the angle the beam scans off array normal and G o  is the gain at array normal. The hybrid configuration proposed herein minimizes azimuthal scan loss by the use of the azimuthal motor. Furthermore, because the phased array panel  300  is light weight, and offers final azimuthal beam adjustment via electronic beam scanning, the motion control system ( 306 / 308 / 310 ) may be much simpler and less expensive as compared to those used in traditional 2-axis mechanically steered SOTM systems. 
     Referring to  FIG. 4 , a detailed view of a combined phased array  300  is shown. The combined phased array  300  may include a receive ESA panel  302  and a transmit ESA panel  304 . The receive ESA panel  302  may comprise a plurality of array cells  400  and the transmit ESA panel  304  may comprise a plurality of array cells  402 . Each array cell  400  may be a component of a receive phased array, configured to interact with other of the plurality of array cells  400  to produce a directional beam. Array cells  400  contain phase shifter modules to electronically steer the receive beam. In addition, an array cell  400  may contain receive modules which include T/R switches, phase shifters, attenuators, low noise amplifiers (LNA) and limiter functions. The relative phase shift between each of the array cells  400  determines the beam pointing position relative to the array normal. 
     Each array cell  402  may be a component of a transmit phased array, configured to interact with other of the plurality of array cells  402  to produce a directional beam. Array cells  402  contain phase shifter modules to electronically steer the transmit beam. In addition, an array cell  402  may contain receive modules which include T/R switches, phase shifters, attenuators, and power amplifier functions. The relative phase shift between each of the array cells  402  determines the beam pointing position relative to the array normal. 
     Referring to  FIG. 5 , a top view of a hybrid satellite antenna is shown. When a hybrid satellite antenna is mounted in a vehicle, the hybrid satellite antenna may be oriented such that the motor  306  (obscured by the combined phased array) may rotate the combined phased array in the azimuth plane. The motor  306  may make gross adjustments to the position of the combined phased array in the azimuth as the vehicle is moving. The motor  306  may adjust the position of the combined phased array to a minimum precision such that the processor may electronically adjust array cells in phased array columns and phased array rows to steer a beam to within 0.5° of a desired orientation. The processor may continue to make electronic adjustments as necessary to maintain desired signal strength. 
     Referring to  FIG. 6 , a side view of a diagram of a hybrid satellite antenna is shown. A satellite antenna must be able to adjust the orientation of a beam along an elevation as well as an azimuth. The combined phased array  300  may be oriented such that the operational surface of the combined phased array  300  is oriented away from the horizon when the hybrid satellite antenna is mounted in a vehicle. The combined phased array  300  may be oriented such that the phased array rows may steer a beam within an elevation range of between 0° and 90° relative to the horizon. The nominal elevation angle of orientation of combined phased array  300  is designed such that the array normal generally points in the elevation angle of the desired satellite being communicated. This minimizes scan loss in the elevation plane while at the same time maintaining a low profile for the hybrid satellite antenna assembly. The orientation of the combined phased array  300  may remain substantially unchanged relative to the horizon as the motor  306  rotates the combined phased array  300 . The processor may electronically adjust array cells in phased array rows (elevation scanning) and phased array columns (azimuthal scanning) to steer a beam to within 0.5° of a desired elevation. The processor may continue to make electronic adjustments as necessary to maintain a desired signal strength. 
     A hybrid satellite antenna according to the present invention may utilize a motor, bearings and control system conforming to less rigorous standards as compared to satellite antennas known in the art. A hybrid satellite antenna according to the present invention may also utilize a single phase array antenna as opposed to multiple, expensive phased array antennas. A hybrid satellite antenna according to the present invention may track a desired satellite signal while in a moving vehicle, even under conditions requiring tracking velocity of 60°/s and tracking acceleration of 120°/s 2 . 
     Referring to  FIG. 7 , a hybrid satellite antenna  300  in a radome  700  is shown. A hybrid satellite antenna according to the present invention may have the smallest possible footprint of any satellite antenna with any type of mechanical steering, having an antenna of comparable size and capability (hemispherical coverage). A hybrid satellite antenna according to the present invention may be placed inside a radome  700  having a diameter defined by the size of the combined phased array  300  and a height defined by the size of the combined phased array  300  as it is angled relative to the horizon. By comparison, the solely mechanically steerable antenna described in  FIG. 1  may require a larger radome for a similarly sized antenna. 
     Referring to  FIG. 8 , a flowchart is shown for a method of orienting a hybrid satellite antenna. A processor may determine  800  an initial course pointing adjustment of the azimuthal motor and elevation/azimuth scan of the ESA. The initial course pointing adjustment may be determined mathematically based on the known satellite coordinates and the vehicle&#39;s GPS/Inertial Navigation System (INS) based local coordinates. A processor in a hybrid satellite antenna may also monitor  801  signal strength at a desired frequency through a receiving array. The processor may monitor signal strength for some absolute value, or for the strongest possible signal within the capabilities of the hybrid satellite antenna. Sequential lobing techniques may be used with ESA electronically steering to rapidly lock to the satellite&#39;s receive signal. The processor may then adjust  802  the orientation of the hybrid satellite antenna in the azimuth by actuating a motor to rotate the hybrid satellite antenna about an axis substantially perpendicular to the plane of the horizon. The processor may stop the motor based on some determination that no further gross adjustments in the azimuth are necessary or beneficial. The process may make such determination based on continual monitoring  800  of signal strength, or based on other factors known in the art. The processor may then adjust  804  the azimuth orientation of a beam by electronically manipulating array cells in phased array columns in a combined phased array in the hybrid satellite antenna. The processor may continue to electronically adjust the combined phased array until an optimal azimuth orientation is achieved within 0.5°. Optimal azimuth orientation may be defined by signal strength or other factors known in the art. The processor may then adjust  806  the elevation orientation of a beam by electronically manipulating array cells in phased array rows in the combined phased array in the hybrid satellite antenna. The processor may continue to electronically adjust the combined phased array until an optimal elevation orientation is achieved within 0.5°. Optimal elevation orientation may be defined by signal strength or other factors known in the art. One skilled in the art will appreciate that the processor may also utilize information such as known satellite locations and vehicle location based on some global positioning system to make an initial decision as to the orientation of the hybrid satellite antenna. 
     It is believed that the present invention and many of its attendant advantages will be understood by the foregoing description, and it will be apparent that various changes may be made in the form, construction, and arrangement of the components thereof without departing from the scope and spirit of the invention or without sacrificing all of its material advantages. The form herein before described being merely an explanatory embodiment thereof, it is the intention of the following claims to encompass and include such changes.