Within the Surveillance function in the A-SMGCS frame, Multilateration (MLAT) systems are being more and more used for location and identification of aircraft and of cooperating vehicles [1] on the airport surface. This situation, with increasing traffic in the downlink SSR channel, may lead to a too high number of superimposed, i.e. garbled, signals. The aim of this work is the need for reliable, simple effective algorithms to separate overlapped SSR signals. We proposed earlier some solutions [2,3], as well others can be found in the literature [5,6]. All of them require an array antenna at the receiver stations, while today's receiver stations are equipped with only one omni-directional antenna. In this paper, the sources separation algorithm based on array processing (PA: Projection Algorithm) is adapted to a single-antenna configuration, where the variation of carrier frequencies of different vehicular transponders [8] is exploited. Simulations with signals synthesized from real measurements give results that demonstrate the effectiveness of the algorithm in a frequency agility environment.