2.2.2. TargetWindow

Another optical supporting function used for guidance assistance in operational environments for 3D-FMS aircraft is an indicator in the form of a target circle or arrow on the centreline and final approach. This target circle visualises a position for the merging of two arrival streams. These systems also do consider several turns of an aircraft [30,31]. Another approach is using "slot marker" circles to show the aircraft's expected position along its trajectory if it were conforming to the schedule [32]. Similar target position indicators may also be used for certain waypoints in upper airspace, for wake vortices [33] or in lower airspace for aircraft on several arrival routes, which are mapped onto one centreline [28,34]. For the introduction of the European wake turbulence categories and separation minima on approach and departure (RECAT-EU) I, a new categorisation of the mandatory wake vortex separates them into six. While today most countries use four categories, EUROCONTROL implemented the Leading Optimised Runway Delivery (LORD) display aid for approach ATCOs [35,36]. With two additional triangular symbols for each inbound aircraft moving on the final approach, it follows the principle of DLR's TargetWindow without indicating an additional safe area around the optimal position on the final approach.

A TargetWindow on the ATCOs traffic situation display was a marked interval on the centreline where it was safe for individually guided aircraft to be fed into the planned or established arrival stream by the ATCO [37]. Target positions in this window indicated the optimal positions after a turn-to-base manoeuvre to meet the AMAN's calculated trajectory and touchdown time. When aircraft were flying on downwind, they received a turn-to-base command to perform the base and final leg by the feeder ATCO [38]. It did not matter whether an aircraft was turned in from downwind or guided to the final approach by a direct or a fan approach. The decisive factor was that the aircraft manoeuvres were precisely presented within the TargetWindow when reaching the its last phase of approach.

On the one hand, the task of approach ATCOs was to clear the turn not too early to avoid wake vortex separation violations on final approach. On the other hand, the clearance had to be given early enough, so that the aircraft was not too far behind its predecessor, thereby reducing the capacity and effectiveness of the airport after the turn manoeuvre. A

special challenge in this context was the wind, as its influence on the airspeed can change extremely during the 180◦-turn from downwind on the final approach.

In the TargetWindows concept, target positions for turning aircraft were indicated by a dotted semicircle on the final approach with the open side facing the for this position by the AMAN scheduled aircraft (Figure 5). The surrounding dotted lined TargetWindow symbolises a safe area around this optimal target position even if the aircraft does not hit its planned position exactly. Furthermore, there is a buffer of half a nautical mile, shown by a tapering of the TargetWindow at both ends. This helps ensuring that ATCOs do not violate separation minima from predecessors and successors.

**Figure 5.** Schematic illustration of the TargetWindow concept displayed on an ATCO's traffic situation display. The dashed pointed area (grey) moves with the time in the direction to the runways. The ATCO's task is to turn the aircraft at the right time, as they fit in the open areas in the TargetWindow to meet their scheduled landing time perfectly. Additionally, ATCOs have the ability to quickly perceive if an aircraft is too fast or too slow. Thus, they can assess if these deviations will have any impact on the wake-vortex safety distances.

With the passage of the time, the TargetWindows moved with the speed of the expected aircraft in the direction of the runway. In this way, the ATCO was shown the current sequence planning and also planned distances of aircraft from each other. From this point of view, the TargetWindow also represented a "ghost", since it projects the position of the corresponding aircraft from another route onto the centreline depending on the distance still to be flown until touchdown, at least as long as the aircraft was moving along its planned trajectory. Unlike a ghost, however, a TargetWindow did not change its movement on the final approach because it represented the ideal position when the corresponding aircraft had to be turned on base and final approach. The ATCOs therefore did use the TargetWindow as an indicator of whether the aircraft was too early or too late at the LMP, and thus at the threshold. Nevertheless, the ATCO retained both the responsibility for the approach guidance and all freedom to follow the AMAN's suggestions or to establish his own sequence. The TargetWindow reacted just as adaptable to traffic changes as the entire AMAN.
