could be used at baseband - would take frequencies 0 to 96kHz with a cheap USB soundcard and maybe amplify to 5W? Not quite within licenced spectrum: Offcom map
What modem works best, what range will it achieve?
Cool if Pi Zero powered, could leave near WiFi and move out to camp.
Also cool to decode the “Rugby” time signal - https://www.npl.co.uk/msf-signal at 60kHz - should be doable if sample at 192 kHz.