Emergency Position Indicating Radio Beacon (EPIRB) is an equipment which helps to determine the location of survivors during a distress situation to enable search and rescue operation(SAR). It is a secondary means of alerting in distress. It transmits on the frequency of 406 MHz.
EPIRB signals indicate that one or more persons are in grave or imminent danger and require assistance fastly. The persons may have abandoned the ship and does not have signal recieving facilities available. 
Whenever an EPIRB transmits a signals, it is not directly recieved by the ships in vicinity. EPIRB uses COSPAS/SARSAT satellite for transmission.COSPAS/SARSAT system have four low polar satellites with orbit 800-1000 kilometres above the surface of earth. Of the four satellites, two are stand by and the system completes a full orbit around the earth in 60 minutes.
EPIRB when activated transmits a 15-digit hexadecimal code, it reaches the satellite and the satellite by the principle of Doppler shift will estimate the position of the EPIRB. Satellite then relays it to the Local user terminal (LUT) ,either in a real-time mode or global mode. LUT then decodes the EPIRB ID and will find the position of the EPIRB. It is then sent to the MCC(Mission control centre) where the EPIRB is registered. The information from MCC is sent to the RCC( Rescue coordination centre), and RCC sents it to the ships in vicinity and the rescue operations are started.

Salient features of EPIRB:

. It works on an 18-volt lithium battery, which works for 48 hours after activation. Lithium battery is used because of its long shelf life of 5 years.                                                
. EPIRB when activated will give a strobe light with 23 flashes per minute with an intensity of 0.75 candela(Cd). This light will help in locating the EPIRB in adverse weather conditions.                                           
. EPIRB can be activated in two ways
(a). Manually by removing the safety pin
(b). Automatic activation by hydrostatic release unit (HRU), when the ship goes below 1.5 to 4 metres.                                      
. EPIRB sends the 15-digit hexadecimal code on 406 MHz, which is repeated every 45-50 seconds.                                                  
. EPIRB also transmits in the 121.5 MHz aeronautical frequency, which is used by SAR aircrafts to home in the position of survivors.

Indian Facilities of EPIRB:
India has two LUT one at banglore and one at lucknow. One Mission Control centre (MCC) is located in the ISRO Complex banglore, on which all the Indian epirbs are registered.
When a ship is transferred to another flag, the EPIRB should be deactivated. When changing the flag the owner should inform the MCC, regarding the deactivation of the equipment. 

Testing of EPIRB:
EPIRB Should be tested by switching it to test mode as per the manufacturer instructions once in a month. Procedure of testing is detailed on the body of EPIRB in the form of a label.

Carriage requirements:
One float-free EPIRB with valid HRU is to be carried. When abandoning the ship, EPIRB is to be carried to the survival craft.

Maintenance of EPIRB:
. Make sure that EPIRB is mounted in such a way that it has no overhead obstruction.

. Mounting bracket of the EPIRB should not have any physical damage.

. A firmly attached lanyard is to be present and it should be cleanly stowed and should be tied to the vessel or the mounting bracket.

. Visual inspections are carried out to detect damage or defects.

. Expiry date of the battery should be checked.

. HRU should be checked for it's expiry.

. Near the EPIRB mounted position IMO symbol should be displayed.