"On" or "at' should be put in the following sentences:
- They are working on (at) distress frequency now.
- He heard it at (on)distress frequency.
P.S.: distress frequency (term taken from marine radio engineering) is intended for distress alerts at sea.
Of the choices you present, Google Ngram Viewer finds only on a distress frequency . CJ
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Of the choices you present, Google Ngram Viewer finds only on a distress frequency.
CJ