An MSDL is a system consisting of a minimum of one of each of the following two components:    1. Alerting Unit.    2. Base Unit. 1. ALERTING UNIT A device worn by a person which communicates with a Base Unit to indicate when a Man Over Board Incident has occurred.  A FEW THINGS TO REMEMBER WHEN CHOOSING YOUR ALERTING UNIT: To give you full safety cover your alerting unit must be fully automatic…BE AWARE that all 406 MHz PLBs are not fully automatic…useless if you are knocked unconscious!  406 MHz PLBs must be registered to each person and cannot be exchanged between crew.  A 406 MHz Alert may take 90 minutes to be detected and is fully dependant on you being able to switch it on manually.  ONLY 121.5 MHz ALERTING UNITS ARE FULLY AUTOMATIC AND HAVE THE FULL SUPPORT OF THE SEARCH AND RESCUE AUTHORITIES WORLDWIDE SHOULD YOU NEED IT…SOME AUs TRANSMIT SIGNALS ON OBSCURE FREQUENCIES WHICH CANNOT BE TRACKED BY THE SEARCH AND RESCUE AUTHORITIES. 2. BASE UNIT A device that receives radio signals from one or more Alerting Units to indicate that a person using an Alerting Unit has fallen over board. This can be one of two options:      1. Man Over Board Alarm Only    …Sea Marshall® Crewguard CG-121     2. Man Over Board Alarm with Locating Function …Sea Marshall® SARfinder® 1003 What FUNCTIONS do the Alerting Units (AU) and Base Units (BU) provide?   ALERT Alerting Unit (AU) sends out an R.F. (radio) signal to alert the Base Unit (BU). ALARM Base Unit (BU) constantly listens for a signal from an Alerting Unit (AU). If a signal is received by the Base Unit it will provide an on-board alarm to notify the crew of an MOB situation. LOCATE The Man Over Board Alarm with Locating Function gives crew the ability to locate the Man Over Board. LOCATING FUNCTION By using the Man Over Board Alarm with Locating Function, the SARfinder® 1003, the Sea Marshall® Alerting Unit Alerting Unit provides the means for crew to determine a course to recover the MOB.