Array processing of SSR signals in the multilateration context, a decade survey

Secondary surveillance radar (SSR) was originated in the 40´s as an Identification Friend or Foe (IFF) system and, starting from the psila50 and psila60, provided the air traffic control (ATC) with identity and altitude data for the aircraft under coverage, implementing a rough form of cooperative independent surveillance, complementing the non-cooperative surveillance of the primary radar and very often working in a joint manner with it (co-location, co-rotation of the antennae). In the psila70 and psila80 a deep system-level update of the SSR was adopted at international level, namely the selective mode, or Mode S, compatible with previous Modes A and C and providing a data link between the aircraft and the ground-station, i.e. the secondary radar. Following this update and its new signal formats, two main research directions were investigated from half psila90: distributed ground systems enabling multilateration, and array source separation enabling operation in high traffic on the downlink RF channel. This paper provides a survey of this last decade, as seen from the perspective of its authors.