This invention relates generally to distress signal detection systems and more particularly to systems of such type which are adapted to provide unattended monitoring of the 500 KHz international radio-telegraph distress frequency.
As is known, the International Convention For Safety of Life At Sea (SOLAS) requires that certain classes of vessels carry radio-telegraph auto alarm equipment for enabling the unattended monitoring of a distress signal transmitted by a distant ship. Such auto alarm equipment is designed to monitor the 500 KHz international radio-telegraph distress frequency and provide an audible warning signal at a location on the vessel where a watch-stander is generally present when a properly coded distress signal has been detected. Ideally, the distress signal consists of four-second dashes with 1-second spaced intervals. The auto alarm equipment should respond to detection of four of such dashes. After transmission of the coded distress signal the distant ship transmits the international distress SOS and then its position.
In order to allow for human inaccuracies in transmitting a properly coded signal and because atmospheric noise having a frequency component of 500 KHz will also be detected by the auto alarm equipment, such equipment must be capable of responding to dashes which are somewhat less or somewhat greater than four seconds in duration and to spaces which may be correspondingly greater than or less than one second. In particular, SOLAS requires that the auto alarm equipment be activated when dashes vary in length from 3.5 to 6 seconds and spaces vary in duration from 10 milliseconds to 1.5 seconds.