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maritime DC & PPE Information Center
personnel protection

Man Overboard Indicator (MOBI) Transmitter (For submarine use)


The Man Overboard Indicator (MOBI) system is a personal saltwater-activated man

overboard (MOB) alarm system. The alarm system includes a transmitter, receiver and direction finder. The MOBI transmitter is worn on the MK-1 float coat and the inherently buoyant life preserver.  When activated it emits an RF signal. A visual and audible alarm on the ship-mounted receiver sounds upon receipt of this signal, indicating the identity of the MOB. The transmitter also emits a signal which is processed by the direction finder, providing a continuous bearing to the MOB.  The transmitter is either installed in the pocket on the life preserver next to the sea dye marker or attached on the waist strap on those life preservers that do not have a pocket for the MOBI.  Point of contact for the MOBI system is NAVSEA 05Z.


The three versions of the MOBI in use are:


  1. TX-103 Transmitter model




Note: Switch shall be facing the opening of pouch except for uses on the flight deck or well deck where it should be turned facing away from opening to prevent FOD hazards.


A—Antenna with Strain Relief

B—Manual Activation/Deactivation Switch

C—Saltwater Sensors

D—Distress Marker Light (strobe)


  1. TX-105  Transmitter model


The TX-105 model is  identical to the TX-103 in form, fit, and function, but has ruggedized, encapsulated component packaging internally, and slightly greater transmitted power. Operation of theTX-105 is identical to the TX-103. The TX-105 uses the same alkaline battery as the TX-103.


The TX-103 and TX-105 MOBI transmitter models are now obsolete for new purchase and are to be replaced with TX-104 transmitters when no longer serviceable.


  1. TX-104 Transmitter model

Click to enlarge

The TX-104 is an improved version of the TX-103 in a smaller package. It features all the functionality of the TX-103 and is powered by a Duracell Ultra 123 lithium/manganese dioxide (Li/MnO2) battery for longer life. See the Man Overboard Indicator (MOBI) TX-104 User’s Guide for more information on the installation and operation of the TX-104.