47 CFR 80.322: Form of acknowledgement.

(a) The acknowledgement of receipt of a radiotelegraph distress message is transmitted in the following form:

(1) The distress signal SOS;

(2) The call sign of the station sending the distress message, sent three times;

(3) The word DE;

(4) The call sign of the station acknowledging receipt, sent three times;

(5) The group RRR;

(6) The message signal SOS.

(b) The acknowledgement of receipt of a radiotelephone distress message is transmitted in the following form:

(1) The distress signal MAYDAY;

(2) The call sign or other identification of the station sending the distress message, spoken three times;

(3) The words THIS IS;

(4) The call sign or other identification of the station acknowledging receipt, spoken three times;

(5) The word RECEIVED;

(6) The distress signal MAYDAY.