Abstract:
A display system for command and control system for a vessel, the system including a visual display unit (VDU), and means for providing to the VDU signals for providing a display having coordinates of vessel heading and time, with areas of the display indicating certain heading constraints which areas should be avoided by the commander when selecting his course.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a course recommendation display for a command and control system on board ship. This will be described here in the context of a Naval surface ship, but is also applicable to any vessel such as merchant surface ships, submarines, or hovercraft. For the purposes of this specification, &#34;vessel&#34; is intended to means any structure which can float in or to adjacent water. 
     BACKGROUND ART 
     FIG. 1 shows schematically a typical command and control system for a small naval vessel. One essential function of such a system is to organise and present to the Commanding Officer the information he requires in order to make tactical decisions concerning the allocation of weapons to targets and for manoeuvring the ship. This information is derived from sensors on board the ship such as the surveillance radar and sonar, and also from encyclopaedic data and from external sources including sensors on other ships or aircraft, transmitted via a data link. Information is presented on an electronic graphical display console in the form of a plan view display of the world around the ship. 
     Factors which have to be considered by the commander to manoeuvre his ship in a practical battle situation include weapon and sensor blind arcs, the direction, range and speed of approach of threats and the positions of other vessels which may pose a risk of collision. For example a missile system mounted at the back of a ship cannot be used to engage targets within a forward facing arc because it is obstructed by the superstructure of the ship as illustrated by FIG. 2. A radar may be similarly obstructed. These areas in which a weapon or sensor system is ineffective are termed `blind arcs` and impose constraints on the ships heading to ensure that all threats can be engaged successfully without the targets allocated to each weapon entering the blind arc of the tracking radar or the weapon. 
     A problem arises in that a plan view display is not always the most convenient form of presentation, and it becomes difficult for the commander to recognise manoeuvre options from this kind of display when there are several potential problems to consider simultaneously. This situation may arise for example if there are two or more targets to be engaged and also possibly a collision risk to be considered. A simple plan view display can be made to present all of the necessary information, including the relative positions and velocities of targets and other objects, and the weapon and sensor system coverage, but in the plan-view form it can be difficult to recognise the options available for the best course to steer. 
     Other forms of display are in common use. For example it is common practice in submarine command and control systems to present active sonar information in cartesian coordinates of range and bearing and to present sensor information representing the position and possibly the velocity of objects such as other ships and aircraft. 
     SUMMARY OF THE INVENTION 
     The purpose of this invention is to present information required for manoeuvring decisions in a form which makes the situation clear and easy to assimilate. 
     The present invention provides a display system for a command and control system for a vessel, the system including a visual display unit (VDU), and means for providing to the display unit signals for generating a display having coordinates of vessel heading and time, with areas of the display representing certain heading constraints, which areas should be avoided by the commander when selecting his course. 
     The display in accordance with the invention represents the manoeuvring constraints imposed by other objects, rather than the physical positions of the objects. In the case of a naval vessel, each constraint is imposed by a particular weapon allocation or collision risk. These constraints are presented graphically in a two dimensional picture by regions which represent heading limits and the time over which the limits apply. Any convenient form of presentation could be employed, but the preferred method has cartesian coordinates of heading and time, and indicates headings which should be avoided by regions whose width indicates the heading constraint and height indicates the time for which that constraint applies. In this form it is possible to recognise at a glance how to steer the ship, when to steer and how much time is available within which to make manoeuvres. 
     This form of display is also applicable to merchant ships for collision avoidance, particularly for large vessels with slow steering response where early assessment of collision risks is essential. 
    
    
     Brief Description of the Drawings 
     The invention will now be further explained with reference to the accompanying drawings, wherein: 
     FIG. 1 is a schematic view of the command and control system for a naval vessel; 
     FIG. 2 is a schematic view of a naval vessel illustrating the problem of a blind arc of a weapon system; 
     FIG. 3 is a view of a VDU in a control system in accordance with the invention having a typical display thereon; 
     FIG. 4 is a flow chart showing the method of operation of the system according to this invention; and, 
     FIGS. 5 and 6 are diagrams of typical situations which arise in computing the heading constraints to be displayed as in FIG. 3. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 3, the horizontal axis of the display represents true heading in terms of compass bearing, and the vertical axis represents time in the future with `now` at the bottom of the display. The present heading of the ship is represented by a cursor 2 on the horizontal scale and the proposed future course of the ship by a line 4 beginning at the current heading. Both axes may be calibrated in any convenient scale. As shown, the time scale is calibrated in minutes which is a sensible choice for a surface ship where the turning performance dictates an interest in future headings over a period of several minutes. 
     Three regions marked on this display 6, 8 and 10 indicate three heading constraints, that is headings which should be avoided for given periods of time in order to ensure that two hostile air tracks can be engaged and to avoid the risk of collision with another ship. 
     Each of these regions in this example has been annotated by a label 12, 14, 16 summarising the reason for the constraint. The form of presentation of these labels is chosen to match that which is already familiar to the user. In this example the top line of the label indicates the hostility (H=hostile, F=friendly) and classification (A=aircraft, S=ship, M=mines, O=oil rig, etc.) of the other vehicle (e.g. HA=Hostile Aircraft). The second line indicates the system track number of the object (that is an identity by which it is known by the computing system) and the third line identifies a weapon system of the ship (e.g. FM=forward missile, AM=aft missile) when this is associated with the constraint. Other information such as height of an aircraft also could be included if required. 
     Thus region 6 is a heading constraint imposed by the allocation of hostile air track `2345` to weapon `FM`. It indicates that headings between 225 degrees and 270 degrees should be avoided between 2 minutes and 6 minutes in the future, because the earliest time of engagement of `2345` by `FM` is 2 minutes ahead, and the last feasible time of engagement is 6 minutes. During the time of engagement (2-6 minutes from now) it is necessary that the ship should not head in the direction 225° to 260° if the weapon system FM can remain trained on the vehicle HA. 
     Region 8 is a heading constraint imposed by the allocation of hostile air track `1423` to weapon `AM`. It indicates that headings between 350 degrees and 045 degrees should be avoided after 3 minutes if the weapon system AM is to remain trained on the aircraft, and that the constraint changes to a range between 010 and 065 degrees between 3 and 7 minutes in the future. In this case the target is not heading directly towards the ship so its bearing is changing with time, therefore the heading constraint changes with time and the displayed constraint region is a rhomboid instead of rectangular shape. 
     Region 10 is a heading constraint imposed by the risk of collision with a friendly ship, surface track `5678`. It indicates that headings between 080 degrees and 140 degrees should be avoided from 7 minutes in the future. In this case the heading constraint becomes more severe as time passes because the vessels approach one another, so the heading constraint becomes wider, until at 10 minutes the constraint extends between 100° and 170°. 
     As time progresses the regions representing heading constraints will move downwards towards the bottom of the display, and new constraints may be added as new threats are perceived and weapons are allocated. The regions shown are all computed based on the assumption that both the ship and the object remain on this same course and speed. If course/speed changes the display will change accordingly. 
     The line 4 representing the proposed course of the ship can be changed by defining a new course. This may be done by any convenient means such a typing commands at a keyboard 18, or by indicating new headings directly on the display picture via a touch screen, graphic tablet or mouse and cursor. Changes to the future course of the ship will affect the shape of collision risk regions such as region 10 and the display will provide immediate feedback of the effectiveness of a proposed course change in avoiding a potential collision. Hence the commander may define a number of proposed courses and judge from the resultant displays which is the preferred course. 
     Thus with the display indicated, it is possible to recognize at a glance how to steer the ship, when to steer and how much time is available within which to make manoeuvres. It will be appreciated that the display is of great assistance where there are a number of different problems such as hostile targets and collision risks for the commander to consider at the same time. 
     The heading constraints for a target engagement are determined by the target position and velocity and the performance of the weapon and associated sensor. The method of computing the heading constraints will now be described in the context of an engagement of an air target by a surface to air missile system with an associated target tracking radar. A missile launcher generally will have at least one blind arc within which a missile cannot be launched without endangering the ship, and the associated tracking radar will have a blind are within which its view of a target will be obstructed by the superstructure of the ship. If these arcs overlap then they can be considered as one combined constraint. If they do not overlap then they can be considered independently as two distinct constraints. The following discussion applied to the former case. 
     Referring to FIG. 5, given a blind arc between bearings of α and β relative to the ship&#39;s heading, and a target at a true bearing of φ (relative to north), the ship must avoid steering on a heading between φ-α and φ-β in order to keep the target outside the blind arc. Thus the heading constraints can be found by subtracting the limiting angles of the blind arc (relative to the ships head) from the true (compass) bearing of the target. 
     The time period over which the heading constraint applies will be determined by the speed of approach of the target and the weapon performance. The earliest time will be the time at which the target must be detectable by a radar in order to achieve a successful engagement at the greatest possible range of the weapon, and the latest time will be the last opportunity for a successful engagement at the shortest effective range of the weapon. 
     If a target is travelling directly towards, or away from the own ship then the constraint will be represented by a simple rectangular shape such as region 6 in FIG. 3. Otherwise the constrant will be represented by a rhomboid such as region 8 because the target bearing will be changing with time. The constraints can be calculated quite simply from a knowledge of the position and velocity of the target, the range of the missile radar, the missile speed, and least and greatest effective range and blind arcs. 
     FIG. 6 illustrates the situation of the risk of collision with another vessel, e.g. an oil tanker which is moving with a velocity vector v at a distance d and bearing φ relative to ship S, which has a velocity vector u. If there is a direction in which the ship can steer to intercept the other vessel, then there is a risk of collision in that direction. Assuming that collision risk is defined in terms of &#34;miss distance&#34; (i.e. distance at closest point of approach) then it is a simple matter to calculate the heading constraints, and the time over which the risk would exist, from a knowledge of the above factors. 
     FIG. 4 is a schematic view of course recommendation display computing system according to the invention, showing the main data and processing elements and the information flow between them. 
     `Maintain track information` is a process which maintains the internal database called the `track table` from information provided by radar and sonar and possibly external sources received via datalinks. Knowledge of the position and motion of the own ship is required for this process, and is obtained from the navigation system. 
     `Compute heading constraints` is a process which computes the heading constraints and the times over which they apply, given the weapon allocations, the positions and velocities of targets and own ship and the weapon and sensor blind arcs in the manner described above. 
     `Generate course recommendation display` is a process which generates a definition of the course recommendation display picture in the form required by the display system. 
     `Interpret user&#39;s commands` is an interfacing process which accepts commands from the user to allocate weapons to targets, and to define the proposed future course, and stores that information in the appropriate data areas. 
     `Track table` is a data area containing the current best estimates of the positions and velocities of all objects such as ships, aircraft or missiles which have been detected by radar or sonar, or possibly from information received via data links. It also contains the position and velocity of the own ship, derived from the navigation system. 
     `Weapon and sensor geometry` is a data area which contains the weapon and sensor blind arc information. 
     `Heading constraints` is a data area which contains a list of all the heading constraints currently imposed by weapon allocations or collision risks, along with the time periods over which the constraints apply. 
     `Proposed course` is a data area which contains the intended future course of the own ship.