I suppose the most likely scenario is that these are part of some long-running experiment or are used for some fairly mundane purpose like radio navigation or calibration. My first thought was that it might be for a military or intelligence fail-deadly system (e.g. to mount an attack, perform subterfuge, deploy suitcase nukes across U.S. cities, or check in with a handler).
My first thought was some sort of nuclear assurance. In the UK we have radio 4 broadcasting constantly, if there's no communication and radio 4 isn't broadcasting then we assume a London may have been bombed and a nuclear alert is issued.
