Global Maritime Distress and Safety System: 

A global communications service based upon automated systems, both satellite-based and terrestrial, to provide distress alerting and promulgation of maritime safety information for mariners.

More info :
http://www.imo.org/en/OurWork/Safety/Regulations/Documents/GMDSSandSAR1999.pdf/

https://www.navcen.uscg.gov/?pageName=GMDSS