Abstract:
A motorized antenna positioning mechanism in a portable microwave communication unit for use as a ground station in a satellite communication system. The antenna positioner has compact and low profile azimuth wire drive mechanics, azimuth and polarization angle sensors that are not affected by slippage and backlash, and an elevation drive mechanism which neutralizes the pressure on the motor axis due to the weight of the parabolic antenna.

Description:
RELATED APPLICATIONS  
       [0001]     The present application claims the benefit of U.S. patent application Ser. No. 11/220,549, filed Sep. 8, 2005. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to antenna positioning systems in certain types of portable terminals for satellite communications where automatic, rather than manual, alignment to the desired satellite is preferred.  
       BACKGROUND OF THE INVENTION  
       [0003]     Typically, such terminals are found in Satellite News Gathering (SNG) and in some military communication systems. Because of the performance required, terminals of this kind are not intrinsically very small, as parabolic antennas around  1 m in diameter are needed. This, in conjunction with the automatic acquisition feature, results in certain mechanical and electrical requirements on the positioner that this invention solves in a novel way.  
         [0004]     In antenna positioners, or positioning systems, the mechanisms for adjusting the azimuth, elevation and polarization angles are typically conceptually different although the azimuth and elevation mechanisms usually share a common physical platform. The polarization angle may be adjusted by rotating the whole antenna but it is more common to rotate only the feed in which case the polarisation mechanism is completely separate from the other two. In addition to the purely mechanical aspects, there is also the issue of angular sensors for azimuth, elevation and polarization. Thus in discussing prior art, it is best to comment on these items separately.  
         [0005]     Re azimuth:  
         [0006]     Many motorized azimuth drives are based on the simple concept of the antenna being mounted on a rotational platform with the motor driving the platform via gears or cables with pulleys. Conceptually, the simplest arrangement involves a driving pulley on the axle of the motor, driving a circular plate via a flexible cable or “wire”. The axles of the motor and the driven plate are normally parallel. This can be modified depending on the specific application. The applicant has identified several US patents, using various embodiments of this basic principle such as e.g. U.S. Pat. No. 2,516,092, U.S. Pat. No. 2,787,169, U.S. Pat. No. 3,194,080, U.S. Pat. No. 4,210,094 etc. Of these, U.S. Pat. No. 2,787,169 is one that specifically applies to antenna control, using a combination of pulleys and long cables to control a TV antenna.  
         [0007]     When the rotational platform carries a relatively heavy item, such as e.g. a 1-meter parabolic antenna with its boom and RF equipment, concerns arise as to the ability of the azimuth control mechanism to drive such a load without slippage. This gave rise to several patents that use what is essentially a screw as the driving element on which the cable is wound in multiple turns, with multiple turns on the periphery of the driven circular plate, or drum, as well. The drum periphery can have grooves, or it can be smooth. An example of the former is U.S. Pat. No. 4,351,197. U.S. Pat. No. 4,757,727 deals with a related subject, namely the termination of the cable on the drum. U.S. Pat. No. 4,787,259 goes further in that it uses several driving elements (motors with pulleys) to drive one rotating platform. U.S. Pat. No. 5,105,672 is similar to U.S. Pat. No. 4,351,197 with the difference of using a drum with smooth outer peripheral surface on which the cable is helically wound.  
         [0008]     The multi-turn structures described in the prior art mentioned above enable turns greater than 360 degrees. This is not necessary for the azimuth control of a satellite terminal. Further, the disadvantage of the above solutions is that the multi-turn devices used result in relatively big (deep) structures compared to the simple one-turn pulley and drum combination.  
         [0009]     Re elevation:  
         [0010]     Many different mechanisms for motorized elevation adjustment are used in the prior art. Most of them use gears in various configurations, some use belts. A good example of a geared system for a parabolic antenna is shown in U.S. Pat. No. 4,725,843. A gear on the motor axle engages a sector gear mounted on a horizontal rotating shaft whose ends are fixedly attached to the antenna. As the motor turns, the shaft turns and thus the antenna turns, changing the elevation angle. U.S. Pat. No. 6,049,306 shows another example of a geared system, designed for a flat antenna, with more conventional gears. U.S. Pat. No. 6,937,299 uses a belt attached at one end to the boom and at the other to the back of the reflector, while the bottom part of the reflector is mounted on a pivot point. U.S. Pat. No. 6,188,367 uses a similar concept.  
         [0011]     EP1465288, which discloses means for manual elevation adjustment, involves a long threaded rod that passes through what is essentially a large nut in a horizontal rotating shaft, appropriately affixed to the antenna structure. As the rod is turned, the nut moves up or down and the antenna inclines thus changing the elevation angle. This approach can be modified for use in a motorized assembly by adding a motor driving the elevation rod either directly or through intermediate gears. As in other systems, direct coupling is preferable as it avoids potential problems due to backlash in a geared system.  
         [0012]     One of the problems in motorized elevation adjustment structures is the pressure exerted on the motor axle due to the weight of the antenna, pushing downwards, or pulling upwards, depending on the elevation angle. Specifically in small portable terminals, the compact elevation mechanism, including the motor, must deal with an antenna that for performance reasons is relatively large. This can have a negative effect on the performance and reliability of the motor. In some of the systems with intermediate gears, as e.g. in the case of U.S. Pat. No. 4,725,843, the pressure acts sideways on the motor axle. In a direct-coupled system this pressure will act in the axial direction of the driving motor.  
         [0013]     Re polarization:  
         [0014]     An example of the conventional practice is U.S. Pat. No. 4,907,003. A servomotor is used to turn the entire feed assembly and a regular potentiometer is employed for polarization angle indication.  
         [0015]     The mechanics of the current invention differ from the above mentioned patent in that the feed is cross-polarization compensated and must stay fixed. Thus the OMT is rotated with respect to the feed which requires the use of a rotary joint.  
         [0016]     Re angular sensors:  
         [0017]     An important aspect of antenna positioners is an accurate indication of the current azimuth, elevation and polarization angle. Those data serve as feedback for the initial pointing in the auto-acquire process.  
         [0018]     Such feedback should preferably be in the form of an electrical quantity, such as voltage. The patents mentioned above, concerned primarily with mechanics, do not address this. However, there are several patents dealing with motorized antenna alignment using such an approach, namely by employing potentiometers. Examples of these are U.S. Pat. No. 4,665,401, U.S. Pat. No. 4,907,003, U.S. Pat. No. 6,049,306, U.S. Pat. No. 6,937,119, U.S. Pat. No. 5,594,460 to cite a few. The approaches appear to use “regular” potentiometers as angular sensors, mostly driven indirectly by gears or cables from the drive motor.  
         [0019]     The disadvantage of the above solutions is the relatively low accuracy and resolution of regular potentiometers and the potential slippage or backlash depending on the method of mechanical coupling to the potentiometers.  
       SUMMARY OF THE INVENTION  
       [0020]     The invention, as described above in detail, contains improvements over the prior art in the azimuth, elevation and polarization mechanisms for motorized antenna positioners, as follows: 
        An azimuth drive, using a crossed cable and a driving pulley with an undersized groove to clamp the wire with greater force and provide more traction to the driven rotational platform with the antenna. A single, rather than multi-turn, wrap around the drum of the platform is used in two horizontal guiding channels. This results in a very flat unit that can be mounted as lid on the top of a typical baseband unit in a portable satellite terminal.     An azimuth angle sensor using a circular potentiometer with large circumference thus providing better accuracy, resolution and freedom from slippage or backlash compared to prior art.     An elevation adjustment mechanism with a de-coupling feature that isolates the elevation motor from the force exerted by the antenna on the elevation assembly.     A polarization adjustment mechanism incorporating a polarization angle sensor using a circular potentiometer. As in the azimuth part of the positioner, better accuracy, resolution and freedom from slippage or backlash is obtained compared to prior art        
 
         [0025]     This invention includes an azimuth drive and azimuth angle sensor for use in a motorized antenna positioner for a small portable satellite terminal. The drive mechanism is sufficiently robust to reliably turn a 1 m antenna while having a very low profile. That enables the azimuth drive to be incorporated in the box housing the terminal&#39;s electronics without significantly affecting its dimensions. The novel angular sensor has improved accuracy and resolution over prior art.  
         [0026]     The invention also includes an elevation drive for use in a motorized antenna positioner for a small portable satellite terminal. The elevation drive has a novel feature where the pressure due to the antenna weight applied to the elevation rod is transferred to the motor housing instead of acting on its axle. This is achieved by means of a bearing imbedded in the top cover of the motor housing.  
         [0027]     The elevation drive has an elevation motor assembly having a housing, a motor and a motor axle. The motor assembly is mounted to a hinge on the rotatable platform.  
         [0028]     The housing supports a bearing, which is engaged with said elevation rod such that said elevation rod may rotate freely about its axis relative to said housing. The elevation rod is coupled to the motor axle such that rotation of the motor causes rotation of the elevation rod. A threaded nut mounted to the antenna is threadably engaged with said the elevation rod such that rotation of the elevation rod causes the threaded nut to move longitudinally along the elevation rod causing a change in the angle of elevation of the antenna. Any force applied longitudinally along the elevation rod, for example due to gravity or wind acting on the antenna, is transferred through the bearing to the housing.  
         [0029]     The invention further includes a polarization adjustment drive and angle sensor for use in a motorized antenna positioner for a small portable satellite terminal. The novel aspect of the polarization adjustment assembly is its angular sensor that, similar to the one in the azimuth unit, has improved accuracy and resolution over prior art.  
         [0030]     The polarization adjustment drive and angle sensor includes a fixed part to which is mounted the feed. There is a rotatable part connected to the fixed part, the rotatable part rotatable about an axis relative to the fixed part. There is an OMT and LNB mounted to the rotatable part such that the OMT and LNB rotate about the axis with the rotatable part. A rotary potentiometer is attached to the fixed part having a circular conductive trace and a circular resistive trace concentric about the axis and fixed relative to rotation of the rotatable part. A plunger attached to the rotatable part traces a circular path as the rotatable part rotates about the axis, and is positioned such that it contacts the rotary potentiometer at a point of contact and connects the circular traces together. A circuit is connected to the potentiometer and applies a constant current to one end of the circular resistive trace and one end the circular conductive trace and outputs a voltage, indicative of the position of the plunger. The feed is aligned so as to receive a signal along the axis, and the OMT is aligned so as to receive a signal from the feed.  
         [0031]     In its preferred embodiments, the current invention is applied to small portable satellite terminal antenna applications and it addresses the azimuth drive problems while keeping the structure flat. This is desirable for mounting considerations and very important for overall size and weight of the terminal.  
         [0032]     The current invention provides a solution that is free of backlash or slippage. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]     Further features and advantages will be apparent from the following detailed description, given by way of example, of a preferred embodiment taken in conjunction with the accompanying drawings, wherein:  
         [0034]      FIG. 1  depicts a portable satellite terminal;  
         [0035]      FIG. 2  shows a front view of the antenna with its boom;  
         [0036]      FIG. 3  depicts the components of the boom and feed assembly;  
         [0037]      FIG. 4  shows the details of the boom attachment to the reflector;  
         [0038]      FIG. 5  is a close-up view of the azimuth and elevation adjustment mechanism, showing the antenna mounted on the azimuth rotational platform and the attachment of the elevation adjustment mechanism;  
         [0039]      FIG. 6  depicts the baseband unit with the embedded azimuth drive mechanism, with the antenna removed;  
         [0040]      FIG. 7  shows a side view of the compact azimuth drive mechanism;  
         [0041]      FIG. 8  depicts the pulley, cable and drum of the azimuth drive;  
         [0042]      FIG. 9  shows the details of how the driving cable is terminated on the drum of the azimuth wire drive;  
         [0043]      FIG. 10  is a side view of the pulley and drum assembly, showing the guiding channels (grooves) on the drum;  
         [0044]      FIG. 11  depicts the cable with its terminations  
         [0045]      FIG. 12  is a detailed drawing of the whole azimuth positioner assembly;  
         [0046]      FIG. 13  shows the construction of the rotary sensing potentiometer;  
         [0047]      FIG. 14  depicts the elevation adjustment mechanism in its deployed state;  
         [0048]      FIG. 15  shows the internal details of the drive unit with rotational coupling, and linear de-coupling, between the motor axle and the elevation rod;  
         [0049]      FIG. 16  is an outside view of the motor housing on hinge to be attached to the azimuth rotational platform;  
         [0050]      FIG. 17  shows a cross-section of the upper boom arm assembly with the feed, OMT and polarization motor; and  
         [0051]      FIG. 18  is an enlarged view of  FIG. 17 , focusing on the components of the subsystem for polarization adjustment. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0052]     The antenna positioner and sensing mechanisms of this invention are preferably part of a portable communication unit capable of transmitting/receiving high-speed data and broadcast quality video via satellite. However, they may be used in a wide variety of settings and applications. To achieve good performance while preventing undue interference to or from other systems, a 1-meter parabolic antenna is preferably employed, together with a powerful RF amplifier. For ease of setup, the unit preferably contains all the necessary hardware and software for automatic acquisition of the desired satellite.  
         [0053]     As shown in  FIG. 1 , the communication unit  100  is a portable satellite terminal consisting of a 1 m diameter parabolic segmented antenna  101  with a boom assembly  102  with a feed horn and receiver assembly  103  mounted on the end. The boom assembly  102  breaks into two parts for disassembly and transport. On the lower back part of the antenna  101 , the RF transmit (Tx) electronics assembly  104  is mounted to a U-shaped carrier  502  (see  FIG. 5 ). When the communications unit  100  is deployed, as shown in  FIG. 1 , the antenna  101  and RF transmit electronics assembly  104  are mounted on the baseband housing  105  (i.e. the housing for the “non-RF”, or “baseband” (BB) electronics). The baseband housing  105  has a main body  108  and foldable legs  106 , which together with the main body  108  act as a tripod, providing a stable platform for the communication unit  100 . The top of the housing contains the positioner elements for azimuth and elevation adjustment to which the antenna is attached. The polarization adjustment element is physically separated from the azimuth/elevation elements and is part of the feedhorn/receiver assembly  103  at the end of boom assembly  102 .  
         [0054]     As shown in  FIG. 2 , antenna  101  breaks into six segments  110  specially shaped for rigidity and compact stowage. The Transmit RF electronics assembly  104  remains attached to the back of the “main” segment  112 . The boom assembly  102  connects to a U-shaped carrier  502  (see  FIG. 5 ) behind the main segment  112 .  
         [0055]      FIG. 3  shows the two parts of the boom assembly  102 . The upper boom assembly consists of boom arm  301  with feed  302 , Transmit/Receive separator (OMT)  303  and receiver (LNB)  304 . The feed, OMT and LNB are rotated for polarization alignment by motor and gear  305 , with manual override  306 . The Transmit port of the OMT is connected, via flexible waveguide  307 , to solid waveguide  308  running inside the boom arm  301 . The boom arm is terminated with a quick connect device that will attach the above-described assembly to the waveguide flange  310  of the lower boom arm  311 . The lower boom arm is terminated with another quick-connect device  312  (e.g. screw-on), which connects to the U-shaped carrier  502  (see  FIGS. 4 and 5 ).  
         [0056]     FIGS.  4 ( a ) and  4 ( b ) show that the quick-connect device  312  on the lower boom arm  311  attaches to the flange,  402 , mounted on the U-shaped carrier  502 , which in turn is mounted on the main reflector segment  112 .  
         [0057]      FIG. 5  shows the antenna  101  mounted to the baseband housing  105 .  
         [0058]     The main reflector segment  112  is attached to the U-shaped carrier  502  on which is mounted the RF transmit (Tx) electronics assembly  104 . The U-shaped carrier  502  also has connected to it the elevation assembly of the antenna positioner, namely threaded nut  504 , with elevation rod  505  and elevation motor  506 . The whole antenna assembly (antenna  101 , RF transmit (Tx) electronics assembly  104 , U-shaped carrier  502 , and the elevation assembly of the positioner) is pivotally mounted, via hinges  507  and  508 , on the rotational platform  509  for azimuth alignment driven by the drive unit  510 . This platform and the motor are parts of the azimuth assembly of the positioner that in turn is part of the baseband housing  105 .  
         [0059]      FIG. 6  shows the baseband housing  105  with the legs  106  folded, after removal of the antenna assembly (not shown) from rotational platform  509  of the positioner. The baseband housing  105  contains the components needed to process data to and from a laptop computer or similar device into a form suitable for the Transmit RF electronics assembly  104  on the back of the antenna and the feed horn and receiver assembly  103 . Attachment points  602  are for the attachment of the hinges  507  (see  FIG. 5 ). Attachment point  603  is for attaching the elevation hinge  508  (see  FIG. 5 ).  
         [0060]     The azimuth, elevation and polarization elements of the positioner are now described in detail as follows:  
         [0061]     The azimuth positioning mechanism (see  FIGS. 7-12 ) employs a wire drive and comprises:  
         [0062]     a) Drive unit  510  consisting of the step motor  701  that propels driving pulley  703  via gear reduction box  702 . Motion is translated to drum  704  by the use of flexible wire  705 . Drum  704  and plate  707  form part of previously mentioned rotational platform  509  that carries the antenna assembly and thus provides antenna azimuth angle adjustment.  
         [0063]     b) Driving pulley  703  with groove  1001 , undersized relative to the wire size, to capture the wire  705 , thus clamping the wire with greater force as the wire is tightened creating a substantially higher rotating moment transfer. The crossed wire results in a 300 degree winding angle around the drive pulley. These two factors make it possible to drive the antenna load with a single wrap around the pulley and drum, compared to multiple wraps of greater than 360 degrees around a solid or helical drive shaft that are otherwise needed to drive said antenna load according to the prior art.  
         [0064]     c) Drum  704  with two guiding channels  1002  and  1003  and two openings  902  and  903  for wire termination;  
         [0065]     d) Flexible cable  705 , with one end secured to the first drum termination point  902 , running inside the first drum guiding channel  1002  with 200 degree winding angle, traveling to driving pulley  703  and resting inside undersized groove  1001  with 300 degree winding angle, traveling back to drum  704 ; running inside the second drum guiding channel  1003  with a 200 degree winding angle, and second end secured to the second drum termination point  903 .  
         [0066]     In a preferred embodiment, commercial quality “aircraft grade” type cable strand 7×19 is used. It consists of 1 center core bundle of 19 wires, which is straight, and 6 bundles of 19 wires helically stranded around the core. This provides the strongest and most flexible of cables, with greatest stretch. The stretch is compensated by springs ( 1101 , 1102 ) tensioning the cable terminations. The choice of cable is important to provide the friction needed for the drive pulley to drive the drum with the antenna assembly without slipping.  
         [0067]     The whole assembly as described above is mounted on baseplate  706  which in turn is part of baseband housing  105 .  
         [0068]      FIG. 12  shows a more detailed drawing of the entire azimuth positioning unit. Motor  701  and gearbox  702  with pulley  703  are joined by coupler  1201 . The other side of the motor axle is equipped with a hand wheel  1202  for manual override. The drive assembly is covered by cover  1203 . Power to the motor is brought through waterproof strain relief feedthrough  1215 .  
         [0069]     Skirt  1204 , with thrust washer  1205  on top of it, envelopes drum  704  and is attached to baseplate  706 . It contains slots  1206  for the entry of the previously mentioned flexible cable  705  (see  FIGS. 7-11 ). Drum  704  is attached to, and turns with, the upper part of bearing  1207  that fits into circular opening  1208  in baseplate  706 . The lower part of bearing  1207  is attached to bearing plate  1209 . Bearing plate  1209  also carries rotary sensing potentiometer  1210 , and is attached to the underside of baseplate  706 . Plate  707  is attached to drum  704  to form rotational platform  509  carrying the antenna assembly via attachment points  602 ,  603  and hinges  507 ,  508  as shown in  FIGS. 5 and 6 . Plate  707  also carries the waterproof strain relief feedthrough subassembly  1212  with a connector plate and gasket. This allows external wiring to be brought through opening  1208  to the baseband unit  105  on which baseplate  706  is mounted. Baseplate  706  also contains spirit level  1214  for help with setup.  
         [0070]     As can be seen in  FIG. 5 , the above-described azimuth positioner construction results in a very flat unit that adds little additional height to the baseband unit. This facilitates compact stowage of the terminal as a whole.  
         [0071]     The Azimuth angle indicator (see  FIGS. 12 and 13 ) consists of:  
         [0072]     a) Rotary potentiometer  1210  with self-adhesive backing, attached to bearing plate  1209 . As shown in  FIG. 13 , it is made up of two dielectric layers  1301  and  1302 , one of which contains a circular conductive trace  1303  serving as the potentiometer wiper and the other a resistive circular trace  1304 , said traces being on the adjacent sides of the dielectric layers that are separated by a spacer layer  1305 .  
         [0073]     Similar devices are commercially available, for example from Spectra Symbol, of Salt Lake City, Utah.  
         [0074]     b) Plunger subassembly  1213  (see  FIG. 12 ) where a spring-loaded plunger in a tubular carrier is attached to the underside of drum  704 . The plunger connects the said traces of potentiometer  1210  together directly underneath, due to the downward pressure of said plunger, thus enabling the wiper action, as demonstrated in  FIG. 13 .  
         [0075]     c) A circuit is (not shown) connected to the linear end of rotary sensing potentiometer  1210 . The circuit is mounted on the underside of baseplate  706  and protected by cover  1211 . The circuit applies dc voltage to the two ends of the resistive trace in potentiometer  1210  and outputs the voltage between the conductive trace  1303  and one of the said ends of the resistive trace  1304  to an Analog-to-Digital Converter (ADC) connected to the circuit. This voltage is proportional to the angle of rotation of drum  704 . The said ADC converts this voltage value from its analog form to a digital value for further processing by the terminal&#39;s computer. In the preferred embodiment the ADC is a 10-bit device, therefore, theoretically the voltage will be represented by 2 10 =1024 values. Of this, the actual usable range is closer to about 800, so each 1-bit step corresponds to 360/800=approximately a 0.5 degree change in the antenna azimuth direction. To insure accurate correlation with the real antenna position, a calibration process is used with the aid of the communication unit&#39;s software.  
         [0076]     The design described above has the advantage over the prior art in that is provides more accurate indication of the antenna azimuth angle, with better resolution and freedom from slippage or backlash.  
         [0077]     The elevation adjustment mechanism of the positioner (see FIGS.  14  to  16 ) consists of:  
         [0078]     a) Elevation motor assembly  506 , pivoting on elevation hinge  508  which is attached to azimuth rotational platform  509 ,  
         [0079]     b) Elevation rod  505  connected to the motor axle inside motor assembly  506 , and with its threaded upper portion connected to gear  
         [0080]     c) Gear  504  that is essentially a nut that pivots about an axle turning between the two right-angled corners of U-shaped carrier  502 .  
         [0081]      FIG. 15  shows the details of the coupling between the elevation rod and the motor. Motor  1501  is centred within main housing  1502 . Elevation rod  505 , with hand wheel  1509  for manual override, is press-fitted into ball bearing  1503  which in turn is attached to lid  1504 . The lower end of elevation rod  505  is connected to axle  1505  of motor  1501  by coupler  1506 , with set screw  1507 . With bearing  1503  holding elevation rod  505 , the push or pull by the antenna on rod  505  is diverted from motor axle  1505  onto housing  1502  through bearing  1503  and lid  1504 .  
         [0082]     The power to the motor is brought through connector  1510 . Housing  1502  is held on elevation hinge  508  by means of axle  1511 , around which the whole elevation assembly pivots. Hinge  508  is attached to the Az/El Plate  707  of the azimuth rotational platform  509  by means of guiding pins  1601  and hand screw  1602  shown in  FIG. 16 .  
         [0083]     The decoupling of the motor from the elevation rod achieved by the above design results in better and more reliable performance of the motor and thus the entire elevation adjustment mechanism.  
         [0084]     The polarization adjustment mechanism of the positioner is built into the feed/OMT subassembly mounted on the upper boom arm assembly as depicted in  FIG. 3 .  FIGS. 17 and 18  provide additional details specifically with respect to the polarization adjustment mechanism itself.  
         [0085]      FIG. 17  is a cross-sectional side-view of the feed/OMT assembly showing feed  302 , OMT  303  and polarization motor  305  with hand wheel  306  for manual override. Also shown is the Rx reject filter  1701  in the Tx waveguide  1702  with the latter terminated by flange  1703 .  
         [0086]     From there, a flexible waveguide (not shown) connects to the lower flange on the OMT.  
         [0087]      FIG. 18  is an enlarged view of the interface of feed  302 , OMT  303  and polarization motor  305 , showing the relevant parts of the polarization adjustment mechanism. Since the feed is a cross-pol compensated type that must not be rotated, a rotary joint is used to connect the feed to the OMT, thus enabling OMT rotation with respect to the feed for polarization adjustment.  
         [0088]     As shown in  FIG. 18 , OMT  303  is attached to rotating part  1801  of the rotary joint and the feed  302  is attached to fixed part  1802 . Rotating part  1801  carries main driven gear  1803 , engaged with interface gear  1804 , which in turn is driven by driver gear  1805 , attached to the axle of polarization motor  305 . Thus when the motor turns, OMT  303  and rotating part  1801  of the rotary joint turn with respect to the stationary feed.  
         [0089]     The flange of the feed has attached to it the circular part of rotary sensing potentiometer  1806 . The potentiometer is of the same type as the one for azimuth adjustment, but of different size. The rotating part  1801  of the rotary joint has mounted on it spring-loaded plunger  1807 , pushing on the potentiometer and enabling the wiper action. The linear part  1808  of the potentiometer contains the input/output traces and connects to a cable that also provides power to motor  305 . The cable terminates in connector  1704  ( FIG. 17 ). Thus at this connector a voltage proportional to the polarization angle is available for feedback to the auto-acquire system of the terminal. Again, as in the case of the azimuth sensor, this analog voltage can be converted to a digital form for further processing. With a 10-bit ADC, 0.5 degree steps in polarization angle are obtained. As in the azimuth sensor case, this method provides better accuracy and resolution and freedom from of slippage or backlash compared to more conventional approaches.