The United Kingdom’s Civil Aviation Authority published the national aviation forecast in 2008. The forecast predicts that domestic traffic will increase by 3.5% per year, and that international traffic will grow, on average, by 4.5% during 2010-2020. Based on this prediction, the traffic density will increase dramatically in the future, and airspace will be more and more congested. Usually, there are two potential solutions to deal with this situation: improving the ability of air traffic flow management is one solution; reducing the separation minimum of aircraft is another solution. However, this thesis focuses on the second solution, based on constraints of communication, navigation and surveillance systems (CNS). Cont?d.