if we do pulse position modulation with one sample per pulse, the
waveform will not look very 'square like' as it should. By using two
samples per pulse position (4MS/s at 1Msym/s) this improves
significantly.
Using that block you can feed ascii-encoded ADS-B messages via TCP into
gnuradio, which will then take care of encoding/transmitting the
respective bursts.