An emergency telecommunications system wherein the distress signals are transmitted repeatedly in successive sequences each comprised of a detect interval during which an alarm signal is transmitted and of a message transmit interval during which the distress message itself is transmitted indicating particularly the identity of the ship, its location and the nature of distress. During the detect interval, the transmitted signal is comprised of a carrier phase modulated by a sub-carrier, the frequency of the latter being a multiple of the information transmission speed. During the message transmit interval, the information is superposed to the sub-carrier by phase binary modulation. The presence of an alarm signal during each detect interval is detected in an alarm receiver monitoring the receive frequency band simultaneously in several channels of different narrow bandwidths so as to generate a control signal which indicates the presence of a distress signal in anyone of the channels. In response to this control signal, the message signal received during the subsequent message transmit interval is controlled accurately, and demodulated and interpreted coherently in a message receiver so as to restore the distress message.
A method and apparatus for determining the position of a vessel in distress and transmitting an auditory distress message containing the position of the vessel. The apparatus obtains the position of the vessel as determined by a LORAN-C type receiver from either a cassette or directly from the LORAN-C receiver and generates a "Mayday" message including the identification of a vessel, the position of the vessel and the nature of the emergency. The "Mayday" message is synthesized into an auditory human or Morse code message. The language of the auditory message is preferably chosen to correspond with the most likely language used by a possible receiver of the "Mayday" message. Also preferably, the apparatus determines whether the primary distress radio frequency is clear; should the primary distress frequency be in use, the apparatus automatically shifts to a secondary frequency and transmits the "Mayday" message. During transmission on the secondary frequency the apparatus continues to monitor primary frequency; when the primary frequency clears, the "Mayday" message is transmitted thereon.
