This invention relates to displays of traffic in the cockpit of an aircraft and, more particularly, relates to displaying an indication of closure between an Own Ship aircraft and a selected target aircraft in response to the flight characteristics of the selected target aircraft satisfying certain specified criteria. 
A primary task for pilots is to be aware of nearby air traffic by maintaining a constant visual scan.  If traffic is sighted, the pilot must first assess the threat posed by the intruder aircraft, then, if necessary, maneuver to avoid the aircraft.  This strategy is termed xe2x80x9csee-and-avoid.xe2x80x9d The effectiveness of see-and-avoid depends on the ability of a pilot to visually acquire an intruder aircraft early enough in the encounter to enable threat assessment and avoidance.  A Cockpit Display of Traffic Information (CDTI) assists the pilot with see-and-avoid by providing a display of nearby traffic. 
A CDTI display typically displays nearby target aircraft and information about the target aircraft, such as altitude, track, and identification information.  This information may be received from one or more sources, including Traffic Information Service (TIS) from a ground-based air traffic control center, and Automatic Dependent Surveillance-Broadcast (ADS-B) from transponders on other aircraft, or vehicles
The ability to easily determine from the display the movement of nearby aircraft relative to the aircraft containing the display is especially desirable for effective threat assessment and avoidance.  Additionally, aircraft operations can benefit from the timely and effective presentation of traffic information that can assist in decision making for the flight crew.  Thus, there is a need for enhancements to CDTI units to provide easily-readable graphic indications of the relative movement and status of nearby aircraft. 
The present invention is an improvement in displaying traffic information on an aircraft display, such as a CDTI.  More specifically, the invention provides a method and system for graphically displaying on a cockpit display a graphical indication of the rate at which an Own Ship aircraft is closing in on, or receding from, a selected target aircraft.  This rate, which is generally referred to as a xe2x80x9cclosure ratexe2x80x9d, has a positive value if the Own Ship aircraft is closing in on the selected target aircraft, and a negative value if the Own Ship aircraft is receding from the selected target aircraft.  The graphical indication of the rate at which the Own Ship aircraft is closing in on, or receding from, the selected target aircraft is generally referred to as a xe2x80x9cclosure indicator.xe2x80x9d
In a preferred embodiment of the invention, the system displays a closure indicator in response, at least in part, to both of the following requirements being satisfied: (1) the selected target aircraft is within a pre-determined monitoring zone; and (2) the track of the selected target aircraft is within a pre-determined variation angle of the track of the Own Ship aircraft.  In an alternative embodiment of the invention, the system also displays the closure indicator, at least in part, in response to (3) the rate of closure between the selected target aircraft and the Own Ship aircraft being within a predetermined range of values.  In a preferred embodiment of the invention, the system is configured to display no closure indicator, at least partially in response to the system failing to meet one or more of the above requirements. 
As noted above, a system according to a preferred embodiment of the invention only displays a closure indicator if the first two of the above requirements are satisfied.  However, in various alternative embodiments of the invention, the system is configured to display a closure indicator only if a different pre-determined two of the above requirements are satisfied.  For example, in one embodiment of the invention, the system only displays a closure indicator if requirements 1 and 3, above, are satisfied.  Such a system only displays a closure indicator if: (1) the selected target aircraft is within a pre-determined monitoring zone; and (2) the rate of closure between the selected target aircraft and the Own Ship aircraft is within a predetermined range of values.  In another embodiment of the invention, the system only displays a closure indicator if requirements 2 and 3, above, are satisfied.  Such a system only displays a closure indicator if: (1) the track of the selected target aircraft is within a predetermined variation angle of the track of the Own Ship aircraft, and (2) the rate of closure between the selected target aircraft and the Own Ship aircraft is within a predetermined range of values. 
In another alternative embodiment of the invention, the system only displays a closure indicator if each of requirements 1-3 is satisfied.  Such a system only displays a closure indicator if: (1) the selected target aircraft is within a pre-determined monitoring zone; (2) the track of the selected target aircraft is within a pre-determined variation angle of the track of the Own Ship aircraft; and (3) the rate of closure between the selected target aircraft and the Own Ship aircraft is within a predetermined range of values. 
In a further alternative embodiment of the invention, the system displays a closure indicator if a predetermined one of the above requirements is satisfied.  In an additional alternative embodiment of the invention, the system constantly displays a closure indicator on the display screen while the aircraft is in flight. 
The system is preferably configured to remove a closure indicator from display on the display screen in response to either the Own Ship aircraft landing or the selected target aircraft landing. 
In a preferred embodiment of the invention, the pre-determined monitoring zone referenced above is positioned adjacent to a front portion of the Own Ship aircraft.  This pre-determined monitoring zone is preferably a substantially cone-shaped segment of airspace.  Additionally, the cone-shaped segment of airspace is preferably positioned so that the apex of the cone-shaped segment of airspace is adjacent a front portion of the Own Ship aircraft.  Furthermore, the cone-shaped segment of airspace is preferably oriented so that the axis of the cone-shaped segment of airspace is substantially co-linear with the track of the Own Ship aircraft.  In one embodiment of the invention, the apex of the cone-shaped segment of airspace is immediately adjacent a front portion of the Own Ship aircraft and the axis of the cone-shaped segment of airspace is oriented in substantially the same direction as the track of the Own Ship aircraft.  The vertex angle of the cone-shaped segment of airspace is preferably about 40 degrees.  In a preferred embodiment of the invention, the system is configured for allowing a user to at least partially modify at least one boundary of the monitoring zone. 
Turning now to the requirement that the track of the selected target aircraft be within a predetermined variation angle of the Own Ship aircraft""s track, the predetermined variation angle is preferably about 20 degrees.  In a preferred embodiment of the invention, the system is configured to allow a pilot or other user to modify the predetermined variation angle to account for current flying conditions. 
The closure indicator displayed by the system preferably comprises an alphanumeric value indicating the current rate at which the Own Ship aircraft is closing in on, or receding from, the selected target aircraft.  The closure indicator also preferably includes a plus or minus sign immediately adjacent to the alphanumeric value to indicate whether the Own Ship aircraft is closing in on, or receding from, the selected target aircraft.  The closure indicator preferably includes a plus sign if the Own Ship aircraft is closing in on the selected target aircraft.  Similarly, the closure indicator preferably includes a minus sign if the Own Ship aircraft is receding from the selected target aircraft. 
In a preferred embodiment of the invention, if the rate of closure between the selected target aircraft and the Own Ship aircraft is within a predetermined range of values, the closure indicator also comprises a closing/receding indicia that further indicates whether the Own Ship aircraft is closing in on, or receding from, the selected target aircraft.  In a preferred embodiment of the invention, the closing/receding indicia comprises an upwardly-directed arrow if the Own Ship aircraft is closing in on the selected target aircraft, and a downwardly-directed arrow if the Own Ship aircraft is receding from the selected target aircraft. 
More particularly, the system is preferably configured to display, at least in part in response to the closure rate being above a predetermined upper threshold value, a closure indicator that includes a closing/receding indicia that indicates that the Own Ship aircraft is closing in on the selected target aircraft.  Similarly, the system is also preferably configured to display, at least in part in response to the closure rate being below a predetermined lower threshold value, a closure indicator that includes a closing/receding indicia that indicates that the Own Ship aircraft is receding from the selected target aircraft.  Furthermore, in this embodiment of the invention, the system is configured to display a closure indicator that includes no closing/receding indicia if the closure rate is both: (1) equal to or above the predetermined lower threshold value; and (2) equal to or below the predetermined upper threshold value. 
In a preferred embodiment of the invention, if the closure rate is above a predetermined upper threshold value, the system displays the closure indicator above a symbol representing the Own Ship aircraft.  Similarly, if the closure rate is below a predetermined lower threshold value, the system displays the closure indicator below a symbol representing the Own Ship aircraft.  In this embodiment of the invention, the system is configured to display the closure indicator to one side of a symbol representing the Own Ship aircraft if the closure rate is both: (1) equal to or above the predetermined lower threshold value; and (2) equal to or below the predetermined upper threshold value. 
In a preferred embodiment of the invention, the predetermined upper threshold value is about 5 knots and the predetermined lower threshold value is about xe2x88x925 knots.  The system is preferably configured to allow a user to modify either the predetermined upper threshold value, the predetermined lower threshold value, or both the predetermined upper threshold value and the predetermined lower threshold value to accommodate current flying conditions. 
In an alternative embodiment of the invention, the system is configured to only display a rate of closure on the display screen if the rate of closure between the selected target aircraft and the Own Ship aircraft is within a predetermined range of values.  In this embodiment of the invention, the system is configured to display a closure indicator in response, at least in part, to the closure rate being above a predetermined upper threshold value.  Furthermore, the system is also preferably configured to display a closure indicator in response, at least in part, to the closure rate being below a predetermined lower threshold value.  In this embodiment of the invention, the system is further configured to display no closure indicator on the system""s display screen if the closure rate is both: (1) equal to or above the predetermined lower threshold value; and (2) equal to or below the predetermined upper threshold value.