Decoding techniques for SSR mode S signals in high traffic environment

SSR (secondary surveillance radar) exploitation in high traffic environment calls for enhancing surveillance and communication capacity. In the multilateration applications, the use of omni directional (or, anyway, wide beam) antennas and the presence of both SSR mode S replies and squitter make the superimposition of 1090 MHz signals probable, without mentioning the presence of modes A/C signals on the same band. These limitations in SSR signal analysis and decoding, can be mitigated by using both classical (i.e. linear) algorithms and by exploiting the super resolution algorithms capability to discriminate signals in the frequency domain. This work summarises the results obtained by the application of both classical and super resolution (i.e. music, esprit) algorithms to decode overlapped mode S replies and squitters acquired in a live environment. Moreover, it is recognized that the main limit for super resolution is represented by the "too high" quality of the on board transponder local oscillator which causes a too small spectral separation between two overlapped signals, and a possible solution is presented for vehicles.