Aviation radar systems (Jan 2002): the practical allocation of II codes

Within Europe, air traffic service providers are currently implementing plans to upgrade SSR (secondary surveillance radar) radar systems to Mode S radar systems. Even in the short term, the demand for interrogator identifier (II) codes by air traffic service providers has already exceeded the number of II codes available. In this presentation, we describe why II codes are needed for Mode S systems and the problems that arise if radars with the same II code overlap in coverage. We offer a summary of strategies that can be used to mitigate against the shortage of II codes and offer some thoughts on how, in practice, II codes might be allocated.