Legacy systems are quite common in critical infrastructures. The Air Traffic Management (ATM) domain is no exception. The combination of their legacy technology and use in critical infrastructures make them attractive and sometimes easy targets for cyber-attacks. Like many critical infrastructures, ATM is heavily constrained and strongly regulated. This limits the possibilities of short term changes. The development and operational life-cycles of these systems are usually quite long. This increases the need for transitional security mechanisms that can be retrofitted in a minimal invasive way without compromising safety and performance requirements while taking into account the ATM constraints. In this paper a Proof of Concept study is described that investigates a flexible and lean implementation for the ATM surveillance chain, based on standardized protocols and open source software.