Basically if anyone receives it that wasn't expecting to it's illegal. That is at the core of the FCC rules in the US, you can't interfere with the lawful use o

Basically if anyone receives it that wasn't expecting to it's illegal. That is at the core of the FCC rules in the US, you can't interfere with the lawful use of the band, nor can you cause "interference" on unused bands. That said, the process for prosecuting you is long and somewhat convoluted as they have to file a report, then the FCC will confirm the report, Etc. But this kind of project (and a similar I saw that broadcasts on every FRS[1] channel at the same time) is a great example of something you can do with SDR that you cannot (easily) do with transistors and boring old analog radio.[2] It is one of the reasons I enjoy it (playing around with SDRs and DSP) so much as I keep learning new things. [1] Family Radio Service [2] I know, I know, building 20 modulators isn't all that tough yadda yadda.