Abstract:
A support for a deployable reflector for use on a modular satellite antenna assembly is constructed of an elongated boom supported at both ends by a pair of two axis actuators. The boom is attached at its inboard end to the satellite structure in close proximity to the point of attachment of the associated signal feed assembly to minimize the differential thermal stress throughout the antenna assembly.

Description:
FIELD OF THE INVENTION 
     The present invention is directed to a mounting structure for a reflector which is deployed from a stowed position during launch to an extended position when the satellite obtains orbit. The deployed reflector is aligned with its associated feed horn and sub-reflector in the deployed position. 
     BACKGROUND OF THE INVENTION 
     Space satellites require antennas for signal reception and/or transmission. Such satellites and antennas must be relatively lightweight, strong, capable of being stowed into compact condition, and capable of being activated remotely into deployed condition in which they are operational for their intended purposes. The antenna systems generally consist of a reflector, feed horn, and a sub-reflector. It is generally desirable to use antenna reflectors which are attached to the supporting spacecraft platform by hinges so that they can be pivoted up against the sides of the spacecraft in a streamlined stowed position during the launching of the spacecraft. Once the satellite is launched into orbit, the reflector may be deployed by pivoting the reflector away from the body of the satellite into its operational position. 
     As shown in FIG. 1, a single axis mounting structure is used to connect the reflector to the spacecraft body. The mounting structure consists of a hinge secured to the bottom of the spacecraft which allows actuators associated with the hinge to swing the reflector outward for operational deployment. A mounting structure of this type is described in commonly owned U.S. Pat. No. 5,673,459. Deployment in the system of the &#39;459 patent is actuated by a bias spring which pivots the reflector outward upon release of holddowns. 
     Reflectors must be maintained in alignment with its signal source or target after deployment. This is particularly critical in communication applications where the reflector needs to be accurately aligned with its associated signal feed horn. Therefore in some applications it is necessary to adjust the position of the reflector further to obtain full operational deployment. Deployment in such applications, may involve rotating the antenna supports on a hinge axis to unfold the reflectors to a position in which they extend perpendicular to the sides of the spacecraft, and also rotating the reflectors about a second axis, perpendicular to the first axis, to aim the reflectors in the direction of the signal source or target. Actuators which provide such two axis movement have been devised as illustrated in U.S. Pat. No. 5,864,320. 
     It has been found that the alignment between reflector and feed can be significantly distorted by differential thermal stress between the two elements. This distortion is compounded in the configurations of the prior art by mounting the reflector at the bottom of the spacecraft body and mounting the feed horn at the top. This distance is mandated by the aligned physical relation between reflector and feed and the limited amount of movement available for deployment. Generally the feed remains fixed and the reflector moves into the deployed position. 
     It is a purpose of this invention to minimize the thermal differential between the reflector and feed and thereby maintain the aligned relation in the deployed position. Another purpose of this invention is to mount the reflector support structure in close proximity to the feed apparatus. It is a purpose of this invention to accomplish the deployment using multiple two axis actuators. In addition it is a purpose of this invention to provide a antenna sub-module incorporating these features which will facilitate the testing and installation of the antenna system. 
     SUMMARY OF THE INVENTION 
     A satellite antenna sub-module is constructed in which the signal feed and sub reflector are secured in a fixed mutual relation on a frame which is to be, in turn, assembled within a spacecraft/satellite. The associated primary reflector is mounted on the frame by means of a support boom at a location on or in close proximity to the feed attachment point. The attachment points of the primary reflector boom and the associated feed horn and sub-reflector are positioned as close as possible in order to minimize thermal distortion throughout the reflector system. The boom is connected at one end to the frame by means of a two axis actuator which provides powered rotary motion about two orthogonal axis&#39;. The reflector is mounted at the other end of the boom by a second similar two axis actuator. 
     By sequentially rotating the boom and reflector through a series of movements, the reflector is deployed from its stowed position, where it is secured for launch, to its fully deployed position, in which it extends outward from the side of the space craft for operation in alignment with its feed horn and sub-reflector. 
     The reflector system described above is constructed for use in satellites having multiple antenna which must be stowed in a nested relation to present a streamlined contour for the exterior of the spacecraft while the craft is being launched into orbit. To properly nest the multiple antenna they are mounted in pairs on independent booms as described above. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described by way of example with reference to the accompanying drawings, wherein like reference numerals refer to like elements, and in which: 
     FIG. 1 is a perspective view of a satellite having a reflector mounted on a single axis hinge according to the prior art; 
     FIG. 2 is a perspective view of a satellite antenna sub-module constructed according to the subject invention; 
     FIG. 3 is a perspective view of a satellite showing one side of an antenna sub-module with the reflectors nested in the stowed position; 
     FIG. 4 is a perspective view of the antenna sub-module of FIG. 3 with one of the reflectors deployed; 
     FIG. 5 is a perspective view of an reflector support boom constructed according to this invention; and 
     FIGS. 6 a  through  6   e  are perspective views of the satellite with the reflector at sequential position of deployment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A typical mounting system of the prior art is shown in FIG. 1 in which a satellite  1  is shown. A reflector  3  is mounted through a frame  4  to a hinge  5  for pivotal movement about axis  6 . The hinge  5  is secured to the body  2  of the satellite  1  at the bottom of the satellite  1 . To provide alignment between the reflector  3  and a signal feed (not shown), the signal feed is mounted at the top of the satellite. This is necessitated, at least in part by the limited movement allowed by the reflector on hinge  5 . It has been found that a significant thermal differential can occur between the top and the bottom of the satellite  1  as it is launched and positioned in orbit. This thermal differential can cause distortion in the reflector mounting structure which may result in misalignment of the reflector with its associated signal feed after deployment. 
     In the system of this invention, an antenna sub-module  7  is constructed as shown in FIG.  2 . Module  7  consists of a top mounting plate  8  and side support plates  9  which extend downward. A pair of antenna packs are mounted to the support plates  9  and each includes nested reflectors  10 ,  11 ,  12 , and  13  with their associated signal feeds  14 ,  15 ,  16 , and  17  ( 17  not shown). Mounting plate  8  is secured across the top of the satellite with the antenna packs extending downward on either side prior to deployment, as shown in FIG.  3 . This modular construction allows the complete assembly of the antenna system for testing prior to installation on the satellite and facilitates the installation. 
     Reflectors  10 - 13  are respectively mounted on independent support booms  18 , and  19 - 20  ( 20  not shown). Reflector  10  is shown in the fully deployed position in FIG.  4 . To accomplish this deployment, the boom  18  is connected to the antenna module  7  and its associated reflector  10  by a pair of two axis actuators which may be of the type described in U.S. Pat. No. 5,864,320 the disclosure of which is incorporated herein by reference. 
     The support boom  18  is shown in FIG.  5  and is connected at its outboard end  25  to reflector  10  by actuator assembly  30  and at its inboard end  23  to the satellite sub-module frame portion  9  by actuator assembly  30 . Each of the end connections is made through two axis actuator assemblies  30  and  31 . The actuator assemblies  30  and  31  may comprise spring biased gear mechanisms, as described in the above referenced &#39;320 patent, they may also comprise a pair of stepping motor driven, reduction gear assemblies, as shown in FIG.  5 . The use of stepping motor drives is preferred to provide a more accurate and adjustable deployment of the reflector  10 . It should be noted that the feed assembly, consisting of feed horn  14 , support boom  32  and sub-reflector  21  are fixed to satellite sub-module  7  on frame  9  in close proximity to the attachment point of boom  18 . 
     In the preferred embodiment actuator assemblies  30  and  31  are driven through a series of deployment steps by electrically powered stepping motors  26  through  29 . Actuation of the drive motors, cause the boom  18  and reflector  10  to rotate at each end about a pair of orthogonal axis identified by the reference letters A,B,C, and D in FIG.  5 . The deployment motion may be controlled by digital signals, generated by a microprocessor component of the satellite computer according to preprogrammed instructions or manually by commands uploaded from ground control. 
     The sequence of motions will depend on the axial relationship of the individual actuators. Based on the orientation of the axis A-B shown in FIG. 5, an appropriate sequence of movements are shown in FIGS. 6 a - 6   e  to move the reflector  10  from its stowed position (see FIG. 2) to its deployed position (see FIG.  3 ). 
     For clarity only the reflector  10  is shown in the series of FIGS. 6 a - 6   e . The starting position of FIG. 6 a  has the reflector  10  in its nested position. To begin deployment a digital signal is sent to stepping motor  27  which prompts stepping motor  27  to rotate the boom  18  about axis B through an angle θ 1  as shown in FIG. 6 b . At this point boom  18  is partially deployed, but reflector  10  is not aligned with its sub-reflector  21 . This will take several steps to accomplish. First reflector  10  is rotated about axis D by energizing stepping motor  28  to cause the pivoting of reflector  10  through angle θ 2  as shown in FIG. 6 c . FIG. 6 d  shows the rotation of the reflector  10  through an angle θ 3  about axis C by actuation of stepping motor  29  to place the reflector in a closer position to receive signals from its feed assembly. To complete the alignment process, reflector  10  is pivoted downward about axis A by actuating stepping motor  26  through angle θ 4  and further by triggering stepping motor  28  to pivot reflector  10  about axis D through an angle θ 5 , as shown in FIG. 6 e . At this position, reflector  10  is positioned to receive signals from feed horn  14  via sub-reflector  21  and transmit the signals to a remote target for example another satellite or earth receiving station. The relative values of the angles θ-θ 5  will depend on the dimensions of the reflector and the clearances provided in the antenna envelop of satellite  1 . It is readily observed that the order of motions may be reversed to stow the reflector or otherwise altered to accommodate the configuration of the components. 
     It should be appreciated from the above description that the other reflectors on the satellite antenna sub-module will be operated in a similar manner. The reflector  11 , for example, can be deployed by movements which are the mirror image of the above motions. 
     In this manner an accurately adjustable mechanism is provided to nest an antenna array for launch and to deploy the antenna when the satellite has achieved orbit. The mechanism allows the mounting of the components of the antenna assembly to be mounted closely together on the satellite  1  to avoid distortion of the alignment of the antenna components due to thermal stress.