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distress frequency[də′stres ‚frē·kwən·sē]
A frequency allotted to distress calls, generally by international agreement; for ships at sea and aircraft over the sea, it is 500 kilohertz.
The frequency used to indicate that there is an emergency or distress situation. Internationally, there are two aeronautical frequencies for this purpose: 121.5 MHz and 243.0 MHz.