Degarbling Mode S replies received in single channel stations with a digital incremental improvement

Multilateration and automatic dependent surveillance – broadcast systems are used in air traffic control to detect, locate and identify cooperating aircraft using signals emitted by airborne transponders and received by dedicated ground stations. In areas with a high traffic density, these stations may receive simultaneously several superimposed signals. Present operational systems use only one receiving channel connected to a non-directional antenna. When the received replies are superimposed, that is, ‘garbled’, their detection and/or decoding are severely affected in nowadays equipment. The aim of this study is to transform the single channel problem into a multiple channels problem in order to solve it using specific knowledge about the Mode S signals. In fact, the multiple channels problem is a typical signals separation problem applied to Mode S mixture for which several algorithms already exist. The authors’ algorithm, named projection algorithm (PA) single antenna, is based on the existing PA and can be easily implemented on existing receiving stations. The effectiveness of their method is demonstrated using real data collected from their experimental receiver.