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.