Abstract:
A method for determining a pre-selected point-to-point separation between a first craft C 1  and a second craft C 2  is provided. The method includes the step of providing two laser devices, L 1 , and L 2 , onboard each craft, C 1  and C 2 , respectively. Then determining a desired skin location vector S 2  of craft C 2  in L 2  coordinates and translating the skin location vector S 2  to L 1  coordinates; and then determining skin separation between crafts C 1  and C 2  in accordance with the translated skin location vector.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to station keeping by platforms in motion and, more particularly, to a laser based method and system for measuring separation distance between maritime vessels.  
           [0003]    2. Prior Art  
           [0004]    In general, maintaining a fixed or controlled separation between sea-going vessels in relative motion is often very difficult and dangerous. An exemplary example is replenishment of fuel and other supplies for ships at sea. In replenishment at sea two or more ships must remain close enough, often from 60 to 300 feet, to accomplish the transfer of supplies from one ship to the other. This is a dangerous situation when considering that the ships involved in such replenishments are usually a supply ship, such as a fuel supply ship, and a ship of the line, such as an aircraft carrier; and that the ships are under power while replenishment takes place. In addition, factors such as rough seas, high winds, and the relative disproportionate displacements of the ships can lead to disasters and tragedy if the ships are not kept a certain distance apart.  
           [0005]    A manual method for maintaining distance between specific points of the ships, also called skin-to-skin separation, involves a manned rope, much like a very long tape measure, which is stretched between the massive ships. Then, separation is measured and maintained by a radio operator watching the rope and signaling a helmsman when something is amiss. But, when considering that the ships are under power, are very close, and are very large, it will be appreciated that if one of the ships is off course, even momentarily, collision and disaster may be imminent. It will be further appreciated that in rough seas the manual method may be prone to errors or that the manned ropes may not be visible to the radio operators. But, even if the ropes are visible, security may require radio silence and therefore slower manual communication, e.g., signal flags or messengers.  
           [0006]    Other methods might include use of the ship&#39;s inertial navigation system (SINS) or the satellite based global positioning system (GPS). However, both these systems may not provide the necessary real time accuracy. Indeed, the margin of error for both systems may well exceed the separation distance between the vessels.  
           [0007]    Therefore, it is desirable to provide a local method and system to dynamically measure and accurately report the rapidly changeable separation between moving platforms. It is also desirable that the method and system be radio silent, which satisfies security requirements, and be capable of operating in an adverse environment.  
         SUMMARY OF THE INVENTION  
         [0008]    The foregoing and other problems are overcome, and other advantages are realized, in accordance with the presently preferred embodiments of these teachings.  
           [0009]    In accordance with one embodiment of the invention a method for determining a pre-selected point-to-point separation between a first craft C 1  and a second craft C 2  is provided. The method includes the steps of providing two laser devices, L 1 , and L 2 , onboard each craft, C 1  and C 2 , respectively; then determining a desired skin location vector S 2  of craft C 2  in L 2  coordinates and translating the skin location vector S 2  to L 1  coordinates; and finally determining skin separation between crafts C 1  and C 2  in accordance with the translated skin location vector.  
           [0010]    In accordance with another embodiment of the invention a laser based system for measuring separation distance between a first platform and a second platform is provided. The system includes a lasing interrogator mechanically attached to the first platform via a first gimbal device adapted to provide interrogator azimuthal and elevation coordinates. The system also includes an interrogatee mechanically attached to the second platform via a second gimbal device, the second gimbal device adapted to provide interrogatee coordinates.  
           [0011]    The invention is also directed towards a method for determining skin-to-skin distance using multi-purpose laser transceiver devices Laser_ 1  and Laser_ 2 . The method includes the steps of determining a location vector S 1  in Laser_ 1  coordinates and determining a second location vector S 2  in Laser_ 2  coordinates. The next step determines the second location vector S 2  in Laser_ 1  coordinates and gauges at least one skin-to-skin distance vector. Alternatively, location vector S 1  could be determined in Laser_ 2  coordinates for determining the skin-to-skin distance. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    The foregoing aspects and other features of the present invention are explained in the following description, taken in connection with the accompanying drawings, wherein:  
         [0013]    [0013]FIG. 1 is a pictorial diagram of two ships incorporating features of the present invention and shows laser terminal coordinates showing two skin-to-skin distances normal to the centerline of ship  1 ;  
         [0014]    [0014]FIG. 2 is a pictorial diagram showing a ship&#39;s coordinates and laser coordinates parallel to the ship&#39;s coordinates;  
         [0015]    [0015]FIG. 3 is a representation of a laser pan-tilt pedestal coordinate system;  
         [0016]    [0016]FIG. 4 is a representation of a first Euler rotation clockwise Az 2  about Z axis     —     P2  axis which is vertical out of the figure;  
         [0017]    [0017]FIG. 4A is a matrix representation of the first Euler rotation shown in FIG. 4;  
         [0018]    [0018]FIG. 5 is a representation of a second rotation, El 1 +El 2  which is clockwise about second rotated Y P2 =Y′ and is vertical out of the figure;  
         [0019]    [0019]FIG. 5A is a matrix representation of the second Euler rotation shown in FIG. 5;  
         [0020]    [0020]FIG. 6 is a representation of a third rotation about the twice rotated Z P2 =Z″=Z P1  axis;  
         [0021]    [0021]FIG. 6A is a matrix representation of the third Euler rotation shown in FIG. 6; and  
         [0022]    [0022]FIG. 7 is a flow chart of one method for determining platform separation. 
     
    
     DETAILED DESCRIPTION  
       [0023]    Referring to FIG. 1, there is shown a pictorial diagram of two ships incorporating features of the present invention. As shown, gimbaled lasers L 1 , L 2  are pedestal (not shown) mounted aboard each ship C 1 , C 2 , respectively. Preferably, each laser pedestal is mounted facing towards ship&#39;s port or starboard depending on expected relative location of other ships. In alternate embodiments more than one gimbaled laser could be mounted aboard each ship. In the preferred embodiment the gimbaled laser devices are diode lasing transceivers operating at a wavelength of 1550 nm. Although the present invention will be described with reference to the embodiment shown in the drawings, it should be understood that the present invention could be embodied in many alternate forms of embodiments. For example, an alternate embodiment may include blue-green wavelength lasers, which are suited to undersea conditions.  
         [0024]    One feature of the invention advantageously measures ship separation in laser terminal coordinates and not in the ship&#39;s local level tangent plane. Thus, ship&#39;s roll, pitch, and yaw from the Ships Inertial Navigator System (SINS) (FIG. 2, item  22 ) are not required. Of the laser terminal coordinates that are employed, only the azimuthal angle and elevation angle of each laser terminal is necessary.  
         [0025]    Referring also to FIG. 2 there is shown the shipboard laser L 1  and it&#39;s coordinate system with respect to the ship&#39;s coordinate system. The laser L 1  supported by gimbaled laser pedestal LG 1 , is P 1  at (0,0,0) 1 , where the subscript denotes laser L 1 &#39;s coordinate frame. The laser axis, X axis     —     P1 , is parallel to the ship&#39;s longitudinal axis and is positive towards the ship&#39;s bow. The axis Y axis     —     P1  is parallel to the deck and positive towards the ship&#39;s port. The axis Z axis     —     P1  is normal to the ship&#39;s deck or X axis     —     P1 , Y axis     —     P1  plane. The axes form a right-handed set.  
         [0026]    Positive ship&#39;s yaw or heading in the local level or earth&#39;s tangent plane is clockwise about the local vertical Z-axis, a left-handed rotation. Heading is measured from true north to the longitudinal axis of the ship. Positive pitch is bow down, a right-handed rotation about Y. And positive roll is port side down, a left-handed rotation about X. See FIG. 2. It should be appreciated that the ship&#39;s attitude definition is included for the purpose of complete coordinate definition and is not used or necessary in the present invention.  
         [0027]    Referring also to FIG. 3 there is shown a laser pan-tilt pedestal coordinate system for either ship shown in FIG. 1. Pan angle is azimuth (Az) and tilt angle is elevation (E 1 ). Line-Of-Sight (LOS) is the direction of the laser beam. The R vector is range measured at angles Az and E 1  with magnitude |R|. The coordinate components R=(R x , R y , R z ).  
         [0028]    The laser pan-tilt gimbaled pedestal for ship C 1  is set to coordinates (0,0,0) 1  with its pan-azimuth rotations in the laser X axis     —     P1 ,Y axis     —     P1  plane, a left-handed rotation about Z axis     —     P1 . The zero azimuth (Az 1 ) reference is aligned parallel to the ship&#39;s longitudinal axis towards the bow; azimuth is positive clockwise, a left-handed rotation, and the tilt-elevation angle (El 1 ) is positive upward measured from the X axis     —     P1 ,Y axis     —     P1  plane of the ship C 1 , a left-handed rotation about once rotated Y axis     —     P1 . In the preferred embodiment the pan-tilt pedestal does not have slip rings and the pan-tilt pedestal rotation capability is limited to substantially +/−159° in pan-azimuth and up 31° and down 47° in tilt-elevation. In alternate embodiments with slip rings the pan-tilt pedestal rotation capability may be unlimited. The reported pan-tilt azimuth is preferably referenced to the laser X-axis and reads zero when the laser is aimed along the X-axis, which is towards the ship&#39;s bow. In the preferred embodiment a software bias is required to boresight the laser to reference laser&#39;s X-axis. The pan-tilt pedestal is preferably mounted to the ship such that the laser X-axis parallels the ship&#39;s X-axis and the laser&#39;s Y-axis parallels the ship&#39;s Y-axis to substantially within +/−1°. Laser L 2  is similarly mounted on ship C 2 . In alternate embodiments any suitable orientation of the laser&#39;s pedestal may be used.  
         [0029]    Still referring to FIG. 1, and also to FIG. 7, an example illustrating features of the present invention shows a desirable point (skin) location S 1  (step  72 ) of craft C 1  is at S 1 =(X S1 , Y S1 , Z S1 ) 1  in L 1  coordinates and a desirable skin location S 2  (step  71 ) on ship C 2  at S 2 =(X S2 , Y S2 , Z S2 ) 2  in L 2  coordinates, where the S 2  coordinates are referenced from P 2 =(0,0,0) 2 . The S 2  coordinates may be determined from the known geometry of the ship C 2  and stored in look up tables or calculated as necessary from the ship&#39;s known shape.  
         [0030]    The laser terminal L 2  on ship C 2  is P 2  at (X P2 , Y P2 , Z P2 ) 1  in L 1  coordinates measured at a range |R 1 |:  
         ( X   P2 ) 1   =|R   1 | Cos ( Az   1 ) Cos (El 1 )  
         ( Y   P2 ) 1   =−|R   1 | Sin ( Az   1 ) Cos (El 1 )  
         ( Z   P2 ) 1   =|R   1 | Sin ( El   1 )  Eq. set (1)  
         [0031]    The pan-azimuth (Az 2 ) and tilt-elevation (El 2 ) of laser L 2  along with Az 1 , El 1 , and range vector R 1  determine the location of S 2  in laser L 1  coordinates.  
         [0032]    Both laser terminals measure the same R vector when in track, except in reverse, i.e. R 1 =−R 2 . Furthermore, the R vector is oriented to each ship through the pan-tilt gimbals of each laser terminal except for a rotation about the R vector. With the lasers aimed at each other and parallel to the ship&#39;s Y pitch axis, relative rotations about the R vector occurs if each ship has a different pitch angle; and difference in pitch angles has negligible effect on skin-to-skin distance. Consequently, and advantageously, neither ship&#39;s inertial navigation is required to determine skin-to-skin distance.  
         [0033]    The determination of ship C 2  (in laser L 2  coordinates) in laser L 1  coordinates is uniquely achieved with three sequential Euler rotations (step  73 ), which rotate the laser L 2  coordinates into laser L 1  coordinates. For example, starting with laser L 2  coordinates, let (X 2 , Y 2 , Z 2 ) be a point in L 2  coordinates.  
         [0034]    Referring to FIG. 4, the first Euler rotation rotates Az 2  clockwise about Z axis     —     P2 , where Z axis     —     P2  is vertical out of the paper.  
         [0035]    The second Euler rotation, referring also to FIG. 5, is in elevation, El 2 +El 1 , about the once rotated Y′=Y axis     —     P2 . Essentially there are two rotations: one in laser L 2  and one in laser L 1  coordinates which both happen to be about the same Y axis. The first elevation rotation with laser L 2  is clockwise as shown in FIG. 4, whereas the second rotation with laser Li is counter clockwise and since the R vector reverses for laser Li relative to laser L 2 , El 1  is therefore added to El 2 .  
         [0036]    [0036]FIG. 6 shows the third Euler rotation where Az 1  is rotated about the twice rotated Z″=Z axis     —     P2 =Z axis     —     P1 . The rotation is counterclockwise but 180 degrees must be added to Az 1  to account for the reversal of the R vector. The Euler rotation matrices shown in FIGS. 4, 5, and  6  are sequentially rotated and substituted to represent the original vector (X 2 , Y 2 , Z 2 ) 2  in laser L 2  coordinates in laser L 1  coordinates as (X 2 , Y 2 , Z 2 ) 1 R. The substitutions and matrix multiplies yields the following Eq. Set (3), where S=sine and C=cosine:  
                 (     X   2     )       1      R       =            [         -     S        (     Az   2     )              S        (     Az   1     )         -       C        (     Az   2     )            C        (       El   2     +     El   1       )                                        C        (     Az   1     )       ]            X   2       +     [         -     C        (     Az   2     )              S        (     Az   1     )         +       S        (     Az   2     )            C        (       El   2     +     El   1       )                                      C        (     Az   1     )       ]          Y   2       -       S        (       El   2     +     El   1       )            C        (     Az   1     )            Z   2                       (     Y   2     )       1      R       =                -     [         S        (     Az   2     )            C        (     Az   1     )         -       C        (     Az   2     )            C        (       El   2     +     El   1       )            S        (     Az   1     )           ]            X   2       +                              [         -     C        (     Az   2     )              C        (     Az   1     )         -       S        (     Az   2     )            C        (       El   2     +     El   1       )            S        (     Az   1     )           ]          Y   2       +                            S        (       El   2     +     El   1       )            S        (     Az   1     )            Z   2                       (     Z   2     )       1      R       =                [       -     C        (     Az   2     )              S        (       El   2     +     El   2       )         ]          X   2       +       [       S        (     Az   2     )            S        (       El   2     +     El   1       )         ]          Y   2       +                            C        (       El   2     +     El   1       )            Z   2                                   
 
         [0037]    The vector S 2  (X S2 , Y S2 , Z S2 ) 2  in laser L 2  coordinates is transformed into laser L 1  using Eq (3) by substituting (X S2 , Y S2 , Z S2 ) 2  for (X 2 , Y 2 , Z 2 ) 2  to produce (X S2 , Y S2 , Z S2 ) 1 R and adding to Eq. set (1), namely (X P2 , Y P2 , Z P2 ) 1  to translate the rotated vector i.e.  
                 (       X   S2     ,     Y   S2     ,     Z   S2       )     1     =         (       X   P2     ,     Y   P2     ,     Z   P2       )     1     +       (       X   S2     ,     Y   S2     ,     Z   S2       )       1      R                 Eq                   (   4   )                                 
 
         [0038]    Finally, the skin-to-skin distance SK YS2  calculated in laser L 1  coordinates is determined (step  74 ) from the magnitude of the Y component:  
           SK   YS2 =|( Y   S2   −Y   S1 ) 1 |  Eq (5)  
         [0039]    Another skin-to-skin distance associated with R 1  is the Y component of P 2  in laser #1 coordinates:  
           SK   YP2 =|( Y   P2 ) 1 |,calculated in Eq (1).  
         [0040]    In the preferred embodiment the shape of the ships C 1  and C 2  and their relative positions are displayed on a visual device. In alternate embodiments various alarms may be used to signal alerts.  
         [0041]    The following numeric examples further illustrate features of the present invention.  
       Case #1  
       [0042]    Let  
                                                       R 1  = 200 feet               Az 1  = 90 0     Az 2  = 270 0             El 1  = 0 0     El 2  = 0 0             S 1  = (−300, −50, 0)   S 2  = (−300, +50, 0)                      
 
         [0043]    Substituting these values into Eq. set (3),  
         ( X   2 ) 1R =[−(−1)×1−0×1×0 ]X   2 +[−0×1+(−1)×1×0 ]Y   2 −0×0 Z   2    
         ( X   2 ) 1R   =X   2    
         ( Y   2 ) 1R =−[−1×0 −0×1×1 ]X   2 +[−0×0−(−1)×1×1 ]Y   2 +0×1 Z   2    
         ( Y   2 ) 1R   =Y   2    
         ( Z   2 ) 1R =[−0×0 ]X   2 +[−1×0 ]Y   2 +1 Z 2    
         ( Z   2 ) 1R   =Z   2    
         [0044]    From Eq (1);  
         ( X   P2 )=200 Cos (90°) Cos (0°)=0  
         ( Y   P2  ) 1 =−200 Sin (90°) Cos (0°)=−200  
         ( Z   P2 ) 1 =200 Sin (0°)=0  
         [0045]    From Eq set (4)  
         ( X   S2   , Y   S2   , Z   S2 ) 1 =( X   P2   , Y   P2   , Z   P2 ) 1 +( X   S2   , Y   S2   , ZS   2 ) 1R                    (       X   S2     ,     Y   S2     ,     Z   S2       )     1     =                (     0   ,     -   200     ,   0     )     1     +       (       -   300     ,     +   50     ,   0     )       1      R                     =              (       -   300     ,     -   150     ,   0     )     1                                   
         [0046]    From Eq (5) the skin-to-skin distance  
           SK   YS2 =|( Y   S2   −Y   S1 ) 1 |=|(−150+50) 1 |=100 feet  
       Case #2  
       [0047]    Let  
                                                       R 1  = 200 feet               Az 1  = 90 0     Az 2  = 270 0             El 1  = 0   El 2  = 45 0             S 1  = (−300, −50, 0)   S 2  = (−300, +50, 0)                      
 
         [0048]    Substituting these values into Eq (3):  
                 (     X   2     )       1      R       =                [         -     (     -   1     )       ×   1     -     0   ×   0.707   ×   0       ]          X   2       +     [         -   0     ×   1     +                                        (     -   1     )     ×   0.707   ×   0     ]            Y   2       -     0.707   ×   0                   Z   2         =     X   2                     (     Y   2     )       1      R       =                -     [         -   1     ×   0     -     0   ×   0.707   ×   1       ]            X   2       +     [         -   0     ×   0     -       (     -   1     )     ×                                      0.707   ×   1     ]          Y   2       +     0.707   ×   1        Z   2         =       0.707        Y   2       +     0.707                   Z   2                         (     Z   2     )       1      R       =                  [       -   0     ×   0.707     ]          X   2       +       [       -   1     ×   0.707     ]          Y   2       +     0.707                   Z   2         =   -                            0.707                   Y   2       +     0.707                   Z   2                                     
 
         [0049]    From Eq (1):  
         ( X   P2 ) 1 =200 Cos (90°) Cos (0°)=0  
         ( Y   P2 ) 1 =−200 Sin (90°) Cos (0°)=−200  
         ( Z   P2 ) 1 =200 Sin (0°)=0  
         [0050]    From Eq (4)  
         ( X   S2   , Y   S2   , Z   S2 ) 1 =( X   P2   , Y   P2   , Z   P2 ) 1 +( X   S2   , Y   S2   , Z   S2 ) 1R                    (       X   S2     ,     Y   S2     ,     Z   S2       )     1     =                (     0   ,     -   200     ,   0     )     1     +       (       -   300     ,     +   35.35     ,   0     )       1      R                     =              (       -   300     ,     -   164.6     ,   0     )     1                                   
         [0051]    From Eq (5), the skin-to-skin distance is:  
           SK   YS2 =|( Y   S2   −Y   S1 ) 1 |=|(−164.6 +50) 1 |=114.6 feet  
       Case 3  
       [0052]    Let  
                                                       R 1  = 200 feet               Az 1  = 90 0     Az 2  = 255 0             El 1  = 0 0     El 2  = 0 0             S 1  = (−300, −50, 0)   S 2  = (−300, +50, 0)                      
 
         [0053]    Substituting these values into Eq (3),  
                 (     X   2     )       1      R       =                [         -     (     -   0.966     )       ×   1     -       (     -   0.259     )     ×   1   ×   0       ]          X   2       +     [         -     (     -   0.259     )       ×   1     +                                      (     -   0.966     )     ×   1   ×   0     ]          Y   2       -     0   ×   0                   Z   2         =                  0.966                   X   2       +     0.259                   Y   2                         (     Y   2     )       1      R       =                -     [         -   0.966     ×   0     -       (     -   0.259     )     ×   1   ×   1       ]            X   2       +     [         -     (     -   0.259     )       ×   0     -                                      (     -   0.966     )     ×   1   ×   1     ]          Y   2       +     0   ×   1                   Z   2         =         -   0.259                     X   2       +     0.966                   Y   2                                     
 ( Z   2 ) 1R =[−(−0.259)×0 ]X   2 +[−0.966×0 ]Y   2 +1 Z   2   =Z   2    
         [0054]    From Eq (1)  
         ( X   P2 ) 1 =200 Cos (90°) Cos (0°)=0  
         ( Y   P2 ) 1 =−200 Sin (90°) Cos (90°)=−200  
         ( Z   P2 ) 1 =200 Sin (0°)=0  
         [0055]    From Eq (4)  
         ( X   S2   , Y   S2   , Z   2 ) 1 =( X   P2   , Y   P2   , Z   P2 )  1 +( X   S2   , Y   S2   , Z   S2 ) 1R                    (       X   S2     ,     Y   S2     ,     Z   S2       )     1     =                (     0   ,     -   200     ,   0     )     1     +       (       -   276.8     ,     +   126.0     ,   0     )       1      R                     =              (       -   276.8     ,     -   74.0     ,   0     )     1                                   
         [0056]    From Eq (5) the skin-to-skin distance:  
           SK   YS2 =|(Y S2   −Y   S1 ) 1 |=|(−74.0+50) 1 |=24 feet  
         [0057]    Finally, the skin-to-skin distance for SK YP2 =|(YP 2 ) 1 =200 feet minus the distance from the laser terminals to the adjacent skin locations which is 50+48.3=98.3 if the laser terminals are each 50 feet inboard. Thus, the skin-to-skin distance is 200−98.3=101.7 feet.  
         [0058]    It should be understood that the foregoing description is only illustrative of the invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the invention. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variances that fall within the scope of the appended claims.