In this paper, a formal language called the aircraft intent description language (AIDL) is proposed as a standard, interoperable means of describing and exchanging predicted aircraft trajectories in trajectory-based operations (TBO). The AIDL provides the necessary elements to unambiguously formulate aircraft intent, which, in the context of trajectory prediction, refers to the information that describes how the aircraft is to be operated within a certain time interval. By expressing aircraft intent according to the AIDL, it is ensured that each instance of aircraft intent defines a unique trajectory. It is anticipated that sharing aircraft intent information expressed in a structured and formal manner, e.g. according to the AIDL, can facilitate the synchronization of the predicted aircraft trajectories held by different automation systems in the context of TBO. The AIDL is characterized by an alphabet and a grammar. The definition of the alphabet and the grammar rules are based on a rigorous mathematical analysis of the trajectory computation process at the core of a TP. This analysis relies on a novel approach to modeling the trajectory computation process based on the theory of differential algebraic equations (DAEs). The paper presents the alphabet of the AIDL, which contains a set of instructions that capture the individual commands and guidance modes available to direct the motion of an aircraft in the ATM context. Then, the AIDL grammar rules, which define the possible combinations of the instructions in the alphabet, are defined and mathematically justified. The AIDL seeks to exploit the physical and mathematical foundations underlying the trajectory computation process to allow describing a priori (i.e. before a trajectory is actually computed) any possible motion behavior that can reasonably be elicited from an aircraft in the ATM context. The objective is that the AIDL encompasses all other methods and formats that may be used to describe aircraft intent, i.e. they would be subsets of the AIDL. The AIDL could thus be seen as a metalanguage for aircraft intent description, containing any other language that may be used to describe aircraft intent in the context of TBO.

A more predictable Air Traffic Management (ATM) system based on automation requires a precise Trajectory Computation Infrastructure (TCI). The accuracy of the computed trajectories does not only depend on the TCI trajectory integration capabilities, but also on the input received from the associated Aircraft Performance Model (APM), the operational instructions defining each trajectory, and the atmospheric data. EUROCONTROL Experimental Centre (EEC) conducts a number of activities in the domain of APMs, which are performed within the scope of Base of Aircraft Data (BADA). EEC has been supported by Boeing Research & Technology Europe (BR&TE) in the definition, development, implementation, and evaluation of an advanced APM suitable for the stringent requirements of future trajectory prediction tools. This new APM will become version 4.0 of BADA, released and maintained by EEC. BR&TE has also supported EEC in the design and development of an advanced bidimensional (vertical plane) TCI intended to exploit all the BADA 4.0 APM capabilities. This TCI accepts either a BADA 4.0 or a BADA 3.x APM, together with all kinds of winds, as well as temperature and pressure deviations over the International Standard Atmosphere (ISA). The TCI requires five simultaneous operational instructions to compute any trajectory segment. These instructions model the commands issued by the flight deck, and comprise the "aircraft intent" (AI). Three of them set the position of the landing gear, high lift devices, and speed brakes, while the other two determine the aircraft motion in the vertical plane. Accepted instructions include different speed, energy, altitude, vertical speed, path angle, and throttle laws, and even optimum speed laws intended to maximize certain flight characteristics, such as range or cost. This paper presents the TCI capabilities and interfaces. It describes the different elements involved in the trajectory computation process, placing special emphasis in the instructions comprising the AI and how they can be combined, as well as the equations describing the aircraft motion and their integration. It also includes an example of the trajectory computation process.

A more predictable air traffic management (ATM) based on automation requires a precise trajectory computation infrastructure (TCI). The predicted trajectories are not only the result of its trajectory integration capabilities, but also depend on the input obtained from the associated aircraft performance model (APM), the operational instructions defining each trajectory, and the weather data. The APM is the core of the trajectory computation process and as such plays a critical role in the resulting capabilities of any future ATM system. EUROCONTROL Experimental Centre (EEC) conducts a number of activities in the domain of APMs, which are performed within the scope of base of aircraft data (BADA). EEC is supported by Boeing Research & Technology Europe (BR&TE) in the definition, development, implementation, and evaluation of an advanced three degrees of freedom APM suitable for the stringent requirements of future trajectory prediction tools. This new APM becomes version 4.0 of BADA, released and maintained by EEC. BADA 4.0 provides expressions for aircraft drag, thrust, and fuel consumption. It represents a significant improvement over current APMs for its high accuracy, its applicability to all types of power plants (jets, turboprops, and piston engines), its validity in the entire flight envelope, and the inclusion of non clean aerodynamic configurations. Additional features include the accurate modeling of the different power plant ratings, the existence of a limitations model to guarantee a proper usage within the flight envelope, the use of a precise atmospheric model that includes possible temperature and pressure differentials over the international standard atmosphere (ISA), and its modern architecture that results in reasonable complexity, maintainability, and computing requirements. This paper describes the different elements involved in the trajectory computation process, with emphasis in the APM as the essential component. It contains an overview of the BADA 4.0 APM, describing its different modules, and provides examples of the achieved results, both in terms of the error with respect to the reference data, as well as the shape of the identified models. A new trajectory engine (TE) capable of exploiting all the BADA 4.0 features is currently being developed. Preliminary results showing some of its capabilities are presented

This paper summarizes the key characteristics of several different methods applied to encode aircraft intent in TP implementations. The differences appear to be significant. We conclude that a standardized formal method to express aircraft intent information (understood as input to the trajectory computation process) is a key enabler to achieve the required consistency. This paper identifies the need for a formal language for the common description of aircraft intent in the context of trajectory computation. Such a language, denoted as aircraft intent description language (AIDL), would allow expressing the aircraft intent with different levels of detail within a single standard unifying framework. By using the AIDL as a common standard for intent sharing, different TP clients could interoperate on the basis of a consistent input to the TP, regardless of their individual requirements. The proposed approach is considered as a possible solution to ensure interoperability among future ATM automation components.