Hopefully everyone airborne transmits on 1090, but equipment fails, and wears out, so that many transponders are off frequency or not running perfectly. Thus, the specification is rather loose. The transmitter can be off +/- 1 MHz and still be in spec.
The ground and airplanes also transmit on 1030 MHz to interrogate other transponders, for example TCAS. The ground not only interrogates Mode-S, but also the older modes.
The transponder pulses are very narrow, and therefore require a high bandwidth receiver. The spectrum can be 14 MHz wide at 20 dB down, which would cover as much as 88 to 102 MHz on the FM dial in comparison. So between the uplink and downlink it would be like 88 to 116 or the whole FM dial plus some... Did I say the signal was wideband :-)
