Abstract:
A scan acquisition technique for acquiring terminals ( 62, 64 ) that does not rely on precise alignment between a sensor ( 66, 70 ) and a transmitter ( 68, 72 ) associated with the terminals ( 62, 64 ). The terminals ( 62, 64 ) separate uncertainty regions ( 76, 78 ) into a plurality of sections ( 88, 90 ). Scan beams ( 82, 84 ) include encoded information of what section ( 88, 90 ) the scan beam ( 82, 84 ) is currently scanning. Each terminal ( 62, 64 ) will eventually receive the scan beam ( 82, 84 ) of the other terminal ( 62, 64 ). When it does, it will encode its scan beam ( 82, 84 ) with both the outgoing code and the return code for that section ( 88, 90 ), so that when it&#39;s scan beam ( 82, 84 ) is received by the other terminal ( 62, 64 ), that terminal ( 62, 64 ) will know what scan section ( 88, 90 ) the other terminal ( 62, 64 ) is located.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates generally to a system and method for acquiring a distant terminal for transmitting an optical data beam thereto and, more particularly, to a system and method for mutually acquiring two distant terminals, where each terminal simultaneously scans an uncertainty region in which the other terminal is located, and where the scan beams include both encoded information about which section of the uncertainty region the scan beam is currently scanning and the position in the uncertainty region decoded from the other scan beam at the time the terminal of origin was last illuminated.  
           [0003]    2. Discussion of the Related Art  
           [0004]    Optical beams are sometimes employed to transmit digital data between two distant sources or terminals, such as space-to-space and space-to-ground optical communications, at very low power levels. The optical data beams have low divergence, and thus have extremely narrow beamwidths (for example 1-20 microradians) when they reach the target terminal to operate at the desired low power level. The optical data beam impinges on a collection aperture on the terminal. Conventional optics are used to focus the optical beam onto a detector to extract the data.  
           [0005]    When the terminals wish to exchange data, and currently aren&#39;t tracking each other, an acquisition technique is employed to align the terminals so that the optical beam is transmitted in the proper direction with high accuracy. First, both terminals are informed of the general vicinity of the other terminal, defined herein as an uncertainty region. The uncertainty region is much larger than the angular beamwidth of the communications data beam. The terminals then initiate the acquisition process so that at the conclusion, their beam is pointed directly at the other terminal&#39;s telescope to exchange the data. During the acquisition process, information is exchanged between the two terminals until the uncertainty region is reduced to less than half the angular beamwidth of the data beam. If the incoming and outgoing beams are perfectly co-aligned, tracking can then commence to maintain the alignment. Because the information extracted by a local sensor from the arriving beam only tells the direction of the incoming beam, a misalignment of greater than one-half of the beamwidth between the incoming and outgoing beam will result in a failure to achieve track even if the incoming beam knowledge is perfect.  
           [0006]    [0006]FIG. 1 is an illustration of a communications system  10  for transmitting optical data between a first terminal  12  and a second terminal  20  some distance apart. The terminal  12  includes a telescope having a sensor  14  for receiving optical beams from the terminal  20  and a transmitter  16  for transmitting optical beams to the terminal  20 . Likewise, the terminal  20  includes a telescope having a sensor  22  for receiving optical beams from the terminal  12  and a transmitter  24  for transmitting optical beams to the terminal  12 .  
           [0007]    The terminal  12  gives its best estimate of its location to the terminal  20 , and the terminal  20  gives its best estimate of its location to the terminal  12  for subsequent data transmissions. However, neither of the terminals  12  and  20  will give their location to the other terminal  12  or  20  with a high enough accuracy. Therefore, the actual location of the terminals  12  and  20  is unknown to the other terminal  12  or  20  before a signal beam is acquired. The given position of the terminals  12  and  20  is shown here as terminals  12 ′ and  20 ′, which is some unknown distance from the actual location of the terminals  12  and  20 . Thus, an uncertainty region  28  is defined for the terminal  12  in which the terminal  12  is located, and an uncertainty region  30  is defined for the terminal  20  in which the terminal  20  is located. For example, the uncertainty region  28  or  30  may be 100 times the diameter of the data beamwidth, which provides a factor of 10,000 times of total area. The expected location of the terminals  12  and  20  is set at the center of the uncertainty regions  28  and  30 , respectively, in two dimensions.  
           [0008]    The uncertainty region  28  is shown relative to the sensor  22  of the terminal  20 ′, and the uncertainty region  30  is shown relative to the sensor  14  of the terminal  12 ′. The uncertainty defined by the uncertainty regions  28  and  30  includes both positional uncertainty and angular uncertainty. It is the angular uncertainty that causes the uncertainty regions  28  and  30  to be shown relative to the sensors  14  and  22  for the expected locations of the terminals  12 ′ and  20 ′.  
           [0009]    In one known acquisition technique, the uncertainty regions  28  and  30  are flooded with a beacon of light from the terminals  12  and  20 , respectively, to determine the position of the other terminal  12  or  20  by looking for the direction of the other terminals flood beam. This technique requires a separate beam than the data beam, and is typically relatively slow at providing acquisition if the power levels are low. Particularly, because of the distance between the terminals, the sensors  14  and  22  see the other terminals flood beam as a point source on its detector, such as a charge coupled device (CCD) array. It may take a significant amount of time for the CCD array receiving the low level flood beam to integrate enough charge to provide an indication of the direction of the other terminals flood beam.  
           [0010]    Some of the problems with beacon type acquisition have been alleviated by employing scan beams that scan the uncertainty regions  28  and  30 , where the beams are detected by the sensors  14  and  22 , respectively. FIG. 2 shows the system  10  where only the actual locations of the terminals  12  and  20  are provided for illustrating a scan acquisition technique. The terminal  12  transmits a scan beam  34  from the transmitter  16 , having the same beamwidth as the data beam, that is scanned across the uncertainty region  30  to illuminate the terminal  20 . At the same time, the terminal  20  transmits a scan beam  36  from the transmitter  24  that scans across the uncertainty region  28  to illuminate the terminal  12 . Each time the scan beam  34  or  36  is received by the sensor  22  or  24  of the terminal  12  or  20 , that terminal  12  or  20  knows the approximate direction of the other terminal  12  or  20  because of where the beam impinges the sensors field-of-view. Thus, the terminals  12  and  20  can home in on each other by receiving the other terminals scan beam  34  or  36  until acquisition is completed. This occurs when the uncertainty regions  28  and  30  are reduced to less than half the beamwidth of the scan beams  34  and  36 .  
           [0011]    [0011]FIG. 3 depicts the terminals  12  and  20  after being acquired by the scan beams  34  and  36 , after which the terminals  12  and  20  can track each other to maintain the pointing in the event that one or both of the terminals  12  or  20  is moving. Because the terminals  12  and  20  are simultaneously scanning for the other terminal  12  or  20 , the average acquisition time can be reduced because once one terminal  12  or  20  is illuminated by the other terminal  12  or  20  its uncertainty area is reduced allowing it to acquire faster.  
           [0012]    Recent improvements have been made in the known scan acquisition techniques to more quickly acquire the terminal of interest. Particularly, U.S. patent application Ser. No. 09/481,924 titled “Satellite Optical Communication Beam Acquisition Techniques,” filed Jan. 13, 2000, assigned to the Assignee of this application and herein incorporated by reference, discloses one improvement. In this scan acquisition technique, the position of one terminal is determined by subdividing a sensor of the other terminal into sensor quads, and then continually subdividing each sensor quad after the terminal receives the scan beam until acquisition.  
           [0013]    The operation of this acquisition technique will be discussed herein with reference to the terminal  12 , the sensor  14  and the scan beam  36  from the terminal  20 . FIG. 4 shows a sensor  40 , representing the sensor  14  that is applicable for this purpose. The &#39;924 application used a sensor divided into sensor quads for two-dimensional acquisition. However, for illustration purposes, this technique can be shown in only one-dimension with the sensor  40 . The sensor  40  is separated into a first sensor half  42  and a second sensor half  44  separated by a line  46 . The sensor halves  42  and  44  only determine if the scan beam  36  arrives through the portion of the uncertainty region  28  being watched by that sensor half. Because there are only two cells, high performance materials and electronics can be used to maximize sensitivity and minimize noise at reasonable cost. A CCD with the same level of performance would be extremely expensive. One example of a sensor suitable for this purpose is an InGaAs cell, well known to those skilled in the art.  
           [0014]    As the scan beam  36  scans the uncertainty region  28 , the scan beam  36  will eventually impinge the sensor  14 . The sensor  40  includes suitable circuitry to determine which sensor half  42  or  44  is illuminated by the scan beam  36  (referred to as a “hit”). The sensor  40  will then adjust its field-of-view so that the line  46  falls half-way through the portion of the uncertainty region  28  that was previously covered by the sensor half  42  or  44  that was “hit” by the scan beam  36 . For example, if the field-of-view of the sensor  40  is 16°, and the scan beam  36  is detected by one of the sensor halves  42  or  44 , the sensor  40  will then move the center of its field-of-view to bisect the portion of the original field-of-view covered by the particular sensor half  42  or  44  that detected the beam  36 .  
           [0015]    By positioning the sensor  40  at the field-of-view for the sensor half  42  or  44 , the uncertainty region  28  is cut in half, and now this half is covered by both of the sensor halves  42  and  44  around the line  46 . When the sensor  40  is illuminated by the scan beam  36  again, it will again hit one of the two sensor halves  42  or  44 , and thus the field-of-view can be divided in half again. This process is continued until the scan beam  36  is simultaneously detected by both sensor halves  42  or  44  at the line  46 . For a two-dimensional scan, a quad cell sensor would be employed in this manner.  
           [0016]    Once the uncertainty region  28  is reduced to half of the beamwidth of the scan beam  36 , the transmitter  16  of the terminal  12  should be aligned with the sensor  22  of the terminal  20 . However, the process described above requires that the sensor  14  be accurately aligned with the transmitter  16  because the transmitter  16  transmits the scan beam  34  in the direction just determined by the sensor  14 . Misalignment between the sensor  14  and the transmitter  16  will induce an undetectable bias in uncertainty region  28  which may be greater than one-half of the beamwidth. Therefore, it is necessary to precisely align the transmitted beam to the center of the sensors field-of-view using precise alignment devices or applying an extremely stable structure.  
         SUMMARY OF THE INVENTION  
         [0017]    In accordance with the teachings of the present invention, a scan acquisition technique is disclosed for acquiring two communications terminals that does not rely on precise alignment between a sensor and a transmitter associated with each terminal. The terminals are located within known uncertainty regions. The uncertainty regions are separated into a plurality of scan sections where each section is designated by a code. The scan beams include “forward” encoded information about what scan section the scan beam is currently scanning within the uncertainty region. As one terminal scans the uncertainty region trying to illuminate the other terminal, it will eventually receive the scan beam from the other terminal. When it does receive the other terminals scan beam, it will then decode the “forward” encoded information. The scan beam will contain two pieces of information. The first is the forward encoded information and the second is “return” information containing the decoded data from the terminal of origin. Thus, when the terminal of origin receives the scan beam, that terminal will know what scan section of the uncertainty region it was in when it successfully illuminated the other terminal.  
           [0018]    Additional advantages and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    [0019]FIG. 1 is an illustration of a communications system employing communications terminals positioned in uncertainty regions;  
         [0020]    [0020]FIG. 2 is an illustration of the communications system shown in FIG. 1 depicting the terminals scanning the uncertainty regions with a scan beam for acquisition purposes;  
         [0021]    [0021]FIG. 3 is an illustration of the communications system shown in FIG. 1 where the two terminals have acquired each other so that the uncertainty region is reduced to half of the scan beamwidth;  
         [0022]    [0022]FIG. 4 is a depiction of a sensor having two sensor halves for use in a scan acquisition technique;  
         [0023]    [0023]FIG. 5 is an illustration of a sensor including a plurality of sensor sections for use in a scan acquisition technique, according to an embodiment of the present invention; and  
         [0024]    [0024]FIG. 6 is an illustration of a communications system employing a scan acquisition technique, according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0025]    The following discussion of the embodiments of the invention directed to a scan acquisition technique for acquiring optical terminals is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses.  
         [0026]    According to one embodiment of the present invention, the sensors  14  and  22  are separated into a plurality of sensor sections to decrease the scanning acquisition time. For example, FIG. 5 shows a sensor  50  including eight separate sensor sections  52  separated by lines  54 . As above with the sensor  40 , when the sensor  14  or  22  receives the scan beam  36  or  34 , respectively, the particular sensor section  52  that receives the beam  36  or  34  provides an indication of which direction the scan beam came from. Therefore, the uncertainty region  28  or  30  for the particular terminal  12  or  20  can be reduced to the size of that section  52 , here ⅛ of the original size of the uncertainty region  28  or  30 . For example, if the original uncertainty region  28  or  30  is 16°, the first hit allows the uncertainty region  28  or  30  to be reduced to 2°. Thus, the first time that the sensor  50  receives the scan beam  34  or  36 , the uncertainty region  28  or  30  can be considerably reduced beyond that for the sensor  40  including the two sensor halves  42  and  44 .  
         [0027]    The sensor field-of-view of the sensor  50  is then moved so that one of the lines  54  falls at the center of that section  52  that previously received the scan beam  34  or  36 . Therefore, only two of the sensor sections  52  are employed and the remaining sensor sections  52  are not used. Also, once the first section  52  is determined, then that section  52  can only be divided in half for each subsequent hit. If too many sensor sections are employed, then the same problem of using a sensor that is relatively unsensitive occurs. Also, the several circuits required to monitor more sensor sections becomes more complex. After the first hit, the many circuits that are required for the several sensor sections would not be used. This process continues until the uncertainty region  28  or  30  is reduced to one-half of the beamwidth of the scan beam  34  and  36 . The improvement comes as a result of the reduction in the size of the uncertainty region  28  or  30  after the first initial detection of the scan beam  34  or  36 .  
         [0028]    According to the invention, a scan acquisition technique is disclosed that eliminates the requirement that the sensor and the transmitter be precisely aligned. FIG. 6 depicts a communications system  60  employing a first terminal  62  and a second terminal  64  that wish to communicate with each other through optical data beams. The terminal  62  includes a sensor  66  and a transmitter  68 , and the terminal  64  includes a sensor  70  and a transmitter  72 . At the initiation of the acquisition, it is known that the terminal  62  is positioned within an uncertainty region  76 , and the terminal  64  is positioned within an uncertainty region  78 . The transmitter  68  of the terminal  62  transmits a scan beam  82  that scans the uncertainty region  78  to be received by the sensor  70  of the terminal  64 . Likewise, the transmitter  72  of the terminal  64  transmits a scan beam  84  that scans the uncertainty region  76  to be received by the sensor  66  of the terminal  62 .  
         [0029]    According to the invention, the uncertainty region  76  is separated into a plurality of contiguous scan sections  88 , here fifteen. Likewise, the uncertainty region  78  is separated into a plurality of contiguous scan sections  90 , also fifteen sections. Each scan section  88  and  90  is identified by a particular code, here a number from one to fifteen for simplicity purposes. Each section  88  and  90  represents the steering angle of the respective scan beam  82  and  84 . The scan beams  82  and  84  are encoded with the specific scan section that they are currently scanning through. The scan beam  84  sequentially scans through all of the scan sections  88  of the uncertainty region  76 , and the scan beam  82  sequentially scans through all of the sections  90  of the uncertainty region  78 .  
         [0030]    In this example, the terminal  62  is positioned within scan section  1  of the uncertainty region  76  and the terminal  64  is positioned within scan section  15  of the uncertainty region  78 . However, this is for illustration purposes only. The scan beam  84  starts with scan section  1  of the uncertainty region  76 , sequentially scans to scan section  15 , and then returns to scan section  1 . Likewise, the scan beam  82  starts with scan section  1  of the uncertainty region  78 , sequentially scans to scan section  15 , and then returns to scan section  1 . The direction of the scan is also for illustration purposes.  
         [0031]    Each time the scan beam  84  or  86  moves from one scan section  88  or  90  to the next scan section  88  or  90 , it is encoded with the code for that section. For example, when the terminal  64  begins its scan in scan section  1  of the uncertainty region  76 , the scan beam  84  is encoded with the number for that section  88 . Because the terminal  62  is located in scan section  1 , the sensor  66  of the terminal  62  receives the scan beam  84  and decodes the section code therefrom. The terminal  62  then encodes its scan beam  82  with both the outgoing section code and the decoded section code  1  just received from the scan beam  84 . However, the terminal  64  does not receive the scan beam  82  until it reaches scan section  15  of the uncertainty region  78 .  
         [0032]    When the scan beam  82  does reach scan section  15  of the uncertainty region  78 , it is still coded with both the outgoing section code  15  and the return scan section code  1  from the uncertainty region  76 . Because the terminal  64  is located in scan section  15 , it receives and decodes the scan beam  82  and now knows which section  88  the terminal  62  is located. Further, when the sensor  70  of the terminal  64  receives the scan beam  82 , the terminal  64  encodes its scan beam  84  with both the outgoing section code  1  and the return section code  15  of the uncertainty region  78 . Thus, when the scan beam  84  returns to scan section  1  of the uncertainty region  76 , the terminal  62  will now know that the terminal  64  is in scan section  15 .  
         [0033]    Each time the terminals  62  and  64  know what scan section  88  or  90  the other terminal  62  or  64  is located, the uncertainty regions  76  and  78  are reduced to the size of that section  88  or  90 . That section  88  or  90  is then subdivided fifteen times where each scan section  88  or  90  is again designated by the particular code. Thus, each time the data makes a round trip, the uncertainty regions  76  and  78  are reduced by {fraction (1/15)}. This process of decreasing the uncertainty regions  76  and  78  continues until the uncertainty regions  76  and  78  are one-half the size of the beam width of the scan beams  82  and  84 . Thus, the relative orientation between the sensors  66  or  70  and the transmitters  68  or  70  is not important. This is because the information being exchanged is not just the arrival angle of the incoming beam, but is also the departure angle necessary to illuminate the other terminal. Once the uncertainty region  76  or  78  is less than half of the beamwidth, it is certain that a track can be entered since co-alignment will be automatically compensated.  
         [0034]    The percentage reduction of the uncertainty regions  76  or  78  for a single round trip scan piece of data is limited by the amount of data that can be packed into a hit.  
         [0035]    The example given above separates the uncertainty region  76  and  78  into fifteen scan sections. However, this is by way of a non-limiting example for illustration purposes. In a practical example, ten bits can be provided on the scan beams  82  or  84  for coding. This provides 1024 scan sections for each of the uncertainty regions  76  and  78 . Further, each time a scan section is subdivided, it can be subdivided by a different number of times than was previously done. Further, the two uncertainty regions  76  and  78  can be subdivided into different numbers of scan sections. Also, the field-of-view of the sensors  68  and  70  can be the entire field-of-view for the current uncertainty region  76  or  78 , and thus do not need to be subdivided as was done in the embodiment discussed above. The example discussed above is a one-dimensional example. However, those skilled in the art will readily recognized that the scan can actually be a two-dimensional scan, where the invention can be used for that type of scan.  
         [0036]    The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.