## 2 Airport Operations Reference Scenario  
In order to define the functionalities of the XRP decision support tool, we introduce in this section a series of general assumptions that contribute in defining a reference scenario for the airport environment in which the XRP is intended to operate. As stated in the introduction, Airport-Collaborative Decision-Making (A-CDM) is the key element of the reference scenario. A-CDM is a concept that aims at improving air traffic flow and capacity management at airports by reducing delays, improving the predictability of events and optimizing the utilization of resources. The decision making by the Airport CDM partners is facilitated by sharing accurate and timely information and by adapted procedures, mechanisms and tools [2]. We will assume that the arrival and departure flows are managed by two of such advanced ATCo tools, which cooperate with the Airport-CDM concept by sharing information and optimizing arrivals and departures, as follows [2]:  
* Arrival Manager (AMAN), which is an arrival flow management tool, optimizes the traffic flow into a Terminal Management Area (TMA) and/or runway(s) by calculating Target Landing Time (TLDT), taking various constraints and preferences into account. * Departure Manager (DMAN), which is a planning system to improve the departure flows at an airport by calculating Target Take-Off Time (TTOT) and Target Start-up Approval Time (TSAT) for each flight, taking multiple constraints and preferences into account. The arrival and departure processes we will use as a reference for deriving the XRP functionalities are developed according to EUROCONTROL's Milestones Approach, part of the Airport-CDM concept. In practice, the milestones approach standardizes, as possible, the sequence of events, denoted as \"milestones\", that shall occur to the typical profile of a flight transiting at an airport. We focus only on the reference arrival and departure processes, enhancing the elements relevant to the taxi route assignment, referring the interested reader to [2]. A major milestone for inbound flights, at least from the routing standpoint, is the Estimated Landing Time (ELDT). Given the guidelines for information sharing in the reference A-CDM concept, relevant information of an inbound flight is available with reasonable accuracy to the routing planning process at ELDT--20 min. The AMAN tool is in charge of determining the optimal sequence of ELDTs for all inbound flights, improving the exploitation of runways' capacity and increasing punctuality. For doing so, an estimate of the time at which the aircraft will arrive at its assigned gate, the Estimated In-Block Time (EIBT), shall be computed by means of the Estimated Taxi in Time (EXIT), which is the estimated taxi time between landing and in-block. The reference inter-operational concept assumed in this paper is that AMAN computes the optimal ELDT sequence. The ELDTsequence is then delivered to XRP, which determines the EIBT for each inbound aircraft, by means of an accurate computation of the EXIT. For an outbound flight, the milestone sequence is more involved, also when focusing only on the aspects relevant to the route assignment problem. A major role is played by the Target Off-Block Time (TOBT), which is the time that the aircraft operator and handling agents estimate that an aircraft will be ready, all doors closed, boarding bridge removed, push back vehicle present, ready to start-up/push back immediately upon reception of clearance from the control tower (TWR). The availability of an accurate TOBT is the pre-requisite for performing a collaborative pre-departure sequence optimization. Given the guidelines for information sharing in the reference A-CDM concept, the relevant information of an inbound flight is available with reasonable accuracy to the routing planning process around 20 min before off-block. After a certain amount of time the TOBT is issued, the ATC issues the TSAT: the time that an aircraft can expect to receive start-up/push back approval.