The Contract of Objectives (CoO), which is based on Target Windows (TWs), constitutes a new concept of operations for Air Traffic Management. TWs are represented by 4-D windows to be respected during the flight execution. They are negotiated and formally agreed by all the different actors involved in the execution of a flight and are located at the transfer of responsibility areas between them. ... [Show full abstract] This paper focuses on the TW modelling process which is at the base of the operational assessment carried on in the framework of the CATS project to investigate the impact of this concept on Air Traffic Controllers and pilots' working methods. In particular in this paper we focus on the TW model which has been developed for the first Human In the Loop (HIL) experiment, a real time simulation carried on to assess the impact of the concept on air traffic controllers working methods. A different work by CATS project elaborates instead on the specific indicators measured during this experiment, regarding both system performances and human performances observed during the HIL.