Leonardi and Maisano Figure 6. Spectrum of received enhanced 1090ES signal to IF at 21.5 MHz. Ten thousand signals are generated with the SDR and injected in the TDR receiver. The recorded signals are than used to evaluate the reported spectrum. are shown. It is possible to note that both the spectrum and the pulse envelope are contained in the frequency and time masks, proving that it is possible to develop an enhanced transponder, compliant with the ICAO recommendations, with a simple hardware (in this case, a 10 Msps digital signal processor for the waveform generation plus a homodyne upconverter). DISCUSSION AND CONCLUSION The starting point of this article was the idea of making room in the Mode S protocol for the insertion of an authentication scheme without degrading the channel performance in terms of total throughput and, at the same time, being compatible with the present Mode S recommendations. It was shown that by introducing a phase modulation on the PPM modulated reply, it is possible to increase the channel capacity and be backward compatible with the current protocol. Moreover, the enhanced protocol is interoperable with the present one and a mixed scenario with old and new transmitter/receiver is possible. It was also shown that the simple BPSK modulation can double the number of bits carried by a single reply maintaining the same performance in term of BER for PPM and BPSK demodulation, obtaining the same overall coverage of the present system without the need to add any receiving stations. Figure 7. Example of a recorder Mode S signal. Envelope of an enhanced 1090ES signal. MAY 2020 IEEE A&E SYSTEMS MAGAZINE 23