A Personal Locator Beacon (PLB) is a personal electronic transmitter used to alert rescuers that there has been a life threatening man over board (MOB) situation with a need to be rescued. When activated, the PLB sends out a signal on either a 406MHz frequency or Local Area System using 121.5MHz, VHF DSC and/or AIS.
A leap second is simply an additional second that is periodically added to Coordinated Universal Time (UTC) to ensure that the Atomic Clock is synchronised with the Earth's rotation. Since introduced in 1972, there have been 26 leap seconds added with the last addition at midnight on 30 June 2015. The prediction is that these will become more frequent in the future as periodic variations, caused by atmospheric winds and the tidal interaction of the Earth and the Moon, cause a slowing down of the Earth’s rotation.