Abstract:
A method for determining the drift vector in a watercraft and then graphically projecting an anticipated drift line for a specified target point. The inventive method is preferably carried out by modifying existing GPS plotters. The user initiates a drift vector determination. A memory device receiving positional data then records the starting position of the watercraft and an ending position of the watercraft a suitable amount of time later. The starting and ending points are then used to calculate a drift vector, which includes at least the orientation of the drift and preferably also information abut the speed of the drift. This information is stored in a memory device. A graphical display is used. At some point the user defines a target point, which is shown within the graphical display. The drift vector information is retrieved from memory and a drift line is projected from the target point in a direction which is 180 degrees away from the direction of the drift vector. The user can then maneuver onto the projected drift line, with the knowledge that if the watercraft is allowed to drift at that point, it will drift over the target point.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    Not Applicable 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not Applicable 
       MICROFICHE APPENDIX 
       [0003]    Not Applicable 
       BACKGROUND OF THE INVENTION 
       [0004]    1. Field of the Invention 
         [0005]    This invention relates to the field of navigation. More specifically, the invention comprises a drift determination and compensation method primarily intended for use in drifting watercraft. 
         [0006]    2. Description of the Related Art 
         [0007]      FIG. 1  shows a plan view of vessel  10  adrift in open water. Two forces primarily determine how the vessel will move. These are wind acting upon the portion of the vessel lying above the waterline and current acting on the portion of the vessel lying below the waterline. These two forces combine to move the vessel along drift vector  14 . 
         [0008]    Those skilled in the art of navigation are able to discern the likely effects of wind by watching the orientation and motion of waves  12 . However, the current is difficult to discern. It is often a function of wind in other areas, tides, and established ocean currents. Even if the current is known, it is difficult for most persons operating a vessel to accurately predict the orientation of drift vector  14 . 
         [0009]    However, the use of GPS receivers now allows the position of the vessel itself to be accurately determined. Many vessels now use an integrated “GPS plotter,” which combines GPS positional data with chart data and in some instances depth readings.  FIG. 2  shows an example of such a device. GPS plotter  16  comprises case  20  attached to mount  18 . A graphical display  22  is provided, which at present is typically a backlit LCD display. Power button  26  turns the device on and off. Input buttons  24  allow a user to select a variety of functions, such as zooming in or out on the chart display, inputting waypoints, etc. 
         [0010]    Those skilled in the art will know that the presence of a device such as shown in  FIG. 2  allows a helmsman to know (1) the vessel&#39;s position; (2) the vessel&#39;s speed; and (3) the vessel&#39;s current heading. However, this knowledge does not directly solve the problem of determining the drift vector shown in  FIG. 1 . 
         [0011]      FIG. 3  provides a situation where it would be desirable to know the drift vector. Target point  28  is a known position where the user wishes to place the vessel. In this example the user wishes to maintain the vessel over target point  28 . One way to do this is to drop an anchor and allow the boat to drift over the target as the anchor line pays out. 
         [0012]    A skilled boat operator knows that the anchor must be dropped upwind. The wind direction can be discerned from wave action. Thus, the operator maneuvers the vessel to anchor point  30  and drops the anchor to the bottom. The anchor line is then extended to allow the boat to drift. The presence of a current—in the direction indicated by the arrow—causes the vessel to drift along actual drift vector  14  and ultimately come to rest at stabilized location  32 . 
         [0013]    The reader will observe that stabilized location  32  is significantly removed from target point  28 . An experienced user will note the error. He or she will then pull up the anchor and maneuver the boat to a new anchor-dropping point that compensates for the orientation of the drift vector. This often becomes an iterative process, with two or more attempts required to actually place the vessel over the target point. 
         [0014]    The iterative anchoring process produces noise and generally disturbs the area around the target point. As the operation described is often performed incident to fishing, noise is undesirable. It would be preferable to provide a system which allows the target point to be reached while dropping the anchor only once. 
       BRIEF SUMMARY OF THE PRESENT INVENTION 
       [0015]    The present invention comprises a method for determining the drift vector of a watercraft and then graphically projecting an anticipated drift line for a specified target point. The inventive method is preferably carried out by modifying existing GPS plotters. The user initiates a drift vector determination. A memory device receiving positional data then records the starting position of the watercraft and an ending position of the watercraft a suitable amount of time later. The starting and ending points are then used to calculate a drift vector, which includes at least the orientation of the drift and preferably also information abut the speed of the drift. This information is stored in a memory device. 
         [0016]    A graphical display is used. At some point the user defines a target point, which is shown within the graphical display. The drift vector information is retrieved from memory and a drift line is projected from the target point in a direction which is 180 degrees away from the direction of the drift vector. The user can then maneuver onto the projected drift line, with the knowledge that if the watercraft is allowed to drift at that point, it will drift over the target point. 
         [0017]    Additional features may be added as well. Visual distance cues may be added along the projected drift line. Information regarding the depth and other factors can be used to calculate a suitable anchoring point along the drift line. This anchoring point can be graphically depicted so that the user knows where to drop anchor in order to drift over the target point. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0018]      FIG. 1  is a plan view, showing the components of drift. 
           [0019]      FIG. 2  is a perspective view, showing a prior art GPS plotter. 
           [0020]      FIG. 3  is a plan view, showing an anchoring operation. 
           [0021]      FIG. 3B  is a schematic view, showing the calculation of a projected drift vector. 
           [0022]      FIG. 4  is a plan view, showing the calculation of a projected drift vector. 
           [0023]      FIG. 5  is a split view, showing a GPS plotter display and a plan view of a drifting vessel. 
           [0024]      FIG. 6  is a plan view, showing the operation of a GPS plotter configured to carry out the present invention. 
           [0025]      FIG. 7  is a plan view, showing the projection of a drift line. 
           [0026]      FIG. 8  is a plan view, showing the use of a drift line to maneuver a vessel. 
           [0027]      FIG. 9  is a split view, showing an anchoring operation and the projection of an anchoring band on a GPS plotter. 
           [0028]      FIG. 10  is a plan view, showing the addition of a steering prompt. 
       
    
    
     REFERENCE NUMERALS IN THE DRAWINGS 
       [0029]      
         [0000]    
       
         
               
               
               
               
             
           
               
                   
               
             
             
               
                 10 
                 vessel 
                 12 
                 wave 
               
               
                 14 
                 drift vector 
                 16 
                 GPS plotter 
               
               
                 18 
                 mount 
                 20 
                 case 
               
               
                 22 
                 display 
                 24 
                 input buttons 
               
               
                 26 
                 power button 
                 28 
                 target point 
               
               
                 30 
                 anchor point 
                 32 
                 stabilized location 
               
               
                 34 
                 projected drift vector 
                 36 
                 establish drift button 
               
               
                 38 
                 vessel display 
                 40 
                 initial position 
               
               
                 42 
                 drifted position 
                 44 
                 cursor control 
               
               
                 46 
                 cursor 
                 48 
                 target 
               
               
                 50 
                 apply drift button 
                 52 
                 drift line 
               
               
                 54 
                 hash mark 
                 56 
                 vessel track 
               
               
                 57 
                 surface 
                 58 
                 bottom 
               
               
                 60 
                 anchor 
                 62 
                 anchor line 
               
               
                 64 
                 50 m mark 
                 66 
                 100 m mark 
               
               
                 68 
                 anchoring band 
                 70 
                 steering prompt 
               
               
                 72 
                 start point 
                 74 
                 end point 
               
               
                   
               
             
          
         
       
     
       DETAILED DESCRIPTION OF THE INVENTION 
       [0030]      FIG. 3B  shows a graphical depiction of the invention&#39;s basic function. A position determining device—typically a GPS receiver—can be used to monitor a vessel&#39;s position and thereby determine an actual drift vector. The process is started by taking the vessel&#39;s position at start point  72 . The position is then determined at a second, later time. This is denoted as end point  74  in the view. The time interval between the start and end points should be sufficient to allow an accurate drift measurement. This will depend largely on the magnitude and consistency of the vessel&#39;s drift (which will be explained in more detail subsequently). 
         [0031]    A simple way of establishing a drift vector is simply comparing the position of end point  74  to the position of start point  72 . The difference in these positions will establish actual drift vector  14 . Actual drift vector  14  has a magnitude L 1  and a direction θ 1  (the direction is shown using the navigational convention of north being 0 degrees). Projected drift vector  34  may then be calculated. The projected drift vector has the same starting point as the actual drift vector and the magnitude of the projected drift vector is equal to the magnitude of the actual drift vector. However, the direction (or heading) of the projected drift vector is determined by subtracting 180 degrees from the heading of the actual drift vector. In other words, θ 2 =θ 1 −180°. 
         [0032]      FIG. 4  shows how the projected drift vector can be used. If the user defines an arbitrary target point  28 , the start point of the projected drift vector can be mathematically translated to lie upon the target point. The heading and magnitude of the vector remain the same. The result is the location shown for projected drift vector  34  in  FIG. 4 . 
         [0033]    Those skilled in the art will realize that projected drift vector  34  is quite useful in maneuvering the vessel. If the vessel&#39;s operator places the vessel anywhere along the projected drift vector and kills the vessel&#39;s momentum, then the vessel will drift along the projected drift vector and eventually pass over target point  28 . 
         [0034]    Of course, the inventive process preferably includes user interface features allowing its convenient operation.  FIG. 5  illustrates an exemplary embodiment of a user interface. Those skilled in the art will realize that a virtually infinite variety of user interfaces could be provided. Thus, the examples illustrated should properly be viewed as only a few of the many possibilities. 
         [0035]      FIG. 5  is a split view. The upper view shows a GPS plotter modified to carry out the invention. The lower view shows a drifting vessel. The GPS plotter is provided with an establish drift button  36 . Once an operator kills the vessel&#39;s forward momentum, he or she depresses this button, which initiates the drift measurement process. The lower view shows the vessel drifting from initial position  40  to drifted position  42 . Actual drift vector  42  is then determined using any suitable method. 
         [0036]      FIGS. 6-8  show how the projected drift vector can be used in navigation. In  FIGS. 6(A)  and (B) the user defines a target point. In  FIG. 6(A)  the user employs cursor control  44  to move cursor  46  to a desired location. The user then selects this location, which appears as target  48  in  FIG. 6(B)  (target points can be entered in many different ways an are often entered as latitude and longitude coordinates without using a cursor). Many GPS plotters are configured to show a vessel display  38 , and to have the other objects in display appear relative to the vessel (as opposed to some other frame of reference, such as making them appear relative to true north). 
         [0037]      FIG. 7  shows how the user employs the projected drift vector. In  FIG. 7(A) , the user applies the projected drift vector to an active target point by selected the apply drift button  50 . This button causes drift line  52  to appear, with its starting point located on target  48 . Drift line  52  has the same heading as the projected drift vector. The reader will note that the heading of drift line  52  in  FIG. 7(A)  is different from the example of  FIGS. 3(A) and 4 , which represents a difference in the current and wind forces present when the drift measurement of  FIG. 5  was taken. 
         [0038]      FIG. 7(B)  shows the display of the target and drift line in greater detail. The length of drift line  52  is possibly related to the magnitude of the projected drift vector, but it may also be completely unrelated. It may be preferable to extend the drift line to an arbitrary length. Hash marks  54  may be provided along the drift line to show the distance to target  48 . As an example, the hash mark nearest to target  48  could display a distance of 50 meters. The next hash marks could then show 100 m, 150 m, 200 m, and so on. The drift line can be extended off the edge of the display if desired. 
         [0039]      FIG. 8  illustrates two of the many possibilities for using drift line  52 .  FIG. 8(A)  represents an anchoring scenario. The user wishes to anchor the vessel in a position where—once the anchor is set and the anchor line is extended—the vessel will lie over target  48 . The user therefore steers vessel  10  along vessel track  56  as shown. When the vessel reaches a suitable distance along drift line  52 , the anchor is lowered and the vessel is allowed to drift over target  48  as the anchor line is let out. 
         [0040]      FIG. 8(B)  represents a different scenario, in which a fisherman wishes to pass baited lines over target  48 . In this scenario the user does not wish to create noise and wave disturbances in the vicinity of the target. The user therefore pilots the vessel in a wide circle around the target and intersects drift line  52  well away from target  48 . The user then deploys the fishing tack and allows the vessel to drift over the target. 
         [0041]    Some users may wish to have an extension of the drift line appear downstream of target  48 . This can easily be done using the graphical display. The user can then continue to observe the vessel&#39;s actual drift in comparison to the drift line even after the vessel has passed over the target. Other graphical enhancements can be provided. As an example, arrows indicating the direction of drift could be placed on the drift line or elsewhere in the display. 
         [0042]    As the anchoring situation is a common one, it warrants further discussion.  FIG. 9  illustrates an anchoring operation.  FIG. 9(A)  shows an elevation view of vessel  10  anchored in position. Vessel  10  rides along surface  57 , while anchor  60  rests on bottom  58 . If the depth of the water is known, an optimum angle α for anchor line  62  can be calculated to a reasonable degree of certainty. The optimum angle allows a good setting force to be applied to the anchor without using an unduly long anchor line. This then allows the calculation of the anchoring distance, which represents the amount of drift the vessel will experience away from the anchor (which is sometimes referred to as a downwind drift but which, of course, is actually a function of wind and current). 
         [0043]    The determination of the anchoring distance can be calculated using the known depth (which can be provided by an integrated or separate depth finder, as well as simply being entered by the user from a chart or other data source).  FIG. 9(B)  shows one way in which the anchoring distance information could be graphically displayed to the user. This view shows target  48  and drift line  52  extending away from the target. This particular embodiment has hashmarks shown. It also displays an anchoring band  68 , which is preferably set out in a distinctive graphic or color. The center of the anchoring band in this example lies between 50 m mark  64  and 100 m mark  66 . The band actually covers a distance of about 30 m, indicating that a successful anchoring operation can likely be carried out anywhere within this band. If the user then drops anchor with the vessel&#39;s position lying along drift line  52  and lying within anchoring band  68 , the anchor line can be let out and the boat should drift down the drift line until it lies over target  48 . The anchor line can then be secured so that the vessel remains in position over the target. 
         [0044]    Those skilled in the art will realize that many enhancements can be added to the inventive processes thus disclosed.  FIG. 10  shows one such enhancement. The vessel is maneuvering along drift line  52 . The vessel has passed over target  48  and is attempting to travel along the drift line. The reader will note, however, that vessel display  38  has deviated to the left of the drift line. Steering prompt  70  appears indicating that the user should steer to the right. If the user deviates to the left of the drift line, a steering point indicating that the user should steer to the left would appear. Audible signals, or other types of signals, could be substituted for the graphical ones. 
         [0045]    Some users may prefer to know the actual distance the vessel is from the drift line. For example, a GPS plotter can project the vessel&#39;s path based on its current heading. This projected path can be intersected with the drift line and a distance from the vessel to the drift line can then be calculated. The boat operator may wish to know this distance so that he or she can smoothly decelerate as the vessel approaches the drift line. 
         [0046]    Having thereby received an explanation of the invention&#39;s fundamental operative features, the reader may wish to know some more detail about certain operations. A simple two-point method of determining the actual drift vector was explained previously. Many other methods could be used for this. For instance, many GPS receivers provide track monitoring at a rate of between 1 and 10 samples per second. This is used to generate instantaneous velocity vectors at the same rate. These vectors can be averaged over time to create a good approximation of the drift angle and speed. These vectors can also be used to determine how many samples should be gathered before determining the actual drift vector. A slow drift rate may require the averaging of samples over a relatively long period, such as 5 minutes. A fast drift rate may provide sufficient data in 1 minute or less. An algorithm running on a computing device can make this determination and automatically adjust the measurement period. On the other hand, the device can simply allow a user to select the period. The device can even allow the user to manually start and manually stop the measurement period. 
         [0047]    The calculations and displays discussed obviously require the presence of a computing device and an associated memory. Modern GPS plotters already have an internal computing device and associated memory (as well as sophisticated display technology). Thus, the invention can be implemented simply by modifying the software of an existing GPS plotter. 
         [0048]    The drift line needed to accurately predict the vessel&#39;s drift will not—of course—remain constant. Changing wind and current conditions will eventually cause the drift line to become inaccurate. It is therefore preferable to recompute the drift line from time to time. A prompt can be provided to remind the user of the need to create a new drift line after a fixed interval. The software can also predict the likely interval by noting the variability and strength of the wind and current forces. It is even possible to provide corrections to the direction of the drift line by observing continuously taken position samples and noting how they deviate from the projected drift. 
         [0049]    One good approach to evaluating the continued validity of the drift line is to evaluate the linearity of the boat&#39;s actual drift over time. If the drift remains constant, then position samples being taken by a GPS device will all lie approximately along a single line. If the drift varies, however, the sample points will begin to curve away from the line. An error threshold can be defined so that the user is prompted once a significant error is detected. 
         [0050]    The GPS device will typically measure and store position data continuously. Thus, when the error threshold is exceeded, the device can be configured to use the last portion of the data collected to determine a new drift vector and a new drift line. The user can be prompted to initiate such a recomputation, or it can be performed automatically. 
         [0051]    Some users may prefer other variations in the graphical display. As an example, the hashmarks shown along the drift line could be used to display time to the target rather than distance from the target. The expected drift velocity is known from the samples taken. Thus, it is possible to place a hashmark at a distance along the drift line from which it will take the vessel 1 minute to drift over the spot (as well as 5 minutes, 10 minutes, and so on). The actual time-to-target and distance-to-target can be displayed as well. 
         [0052]    Those skilled in the art will know that the cursor functions found in most graphical displays allow the inventive process to be easily used in many ways. As an example, once a drift line is presented, the user can employ the cursor to select two points along the drift line. The time to drift from the first point to the second point can then be computed and displayed to the user. 
         [0053]    Finally, although the invention has been presented in the context of sport fishing and pleasure boating operations, it should not be seen as limited to these types of operations. Many commercial vessels could employ the invention for docking and slow maneuvering. Military applications—particularly in the field of anti-submarine warfare—are also possible. 
         [0054]    Although the preceding description contains significant detail, it should not be construed as limiting the scope of the invention but rather as providing illustrations of the preferred embodiments of the invention. The inventive device could be realized in many different ways. Thus, the scope of the invention should be fixed by the following claims rather than the examples given.