When activated manually - or automatically by immersion in water or as a result of high 'g' forces on impact - ELTs transmit a distress signal which can be detected by non-geostationary satellites and then located precisely by either or both of GPS trilateration and doppler triangulation.
Herein, what is an ELT in aviation?
ELTs are emergency transmitters that are carried aboard most general aviation aircraft in the U.S. In the event of an aircraft accident, these devices are designed to transmit a distress signal on 121.5, 243.0-megahertz frequencies (and for newer ELTs, on 406 MHz).
How long is the aircraft's airworthiness certificate valid?
A standard airworthiness certificate remains valid as long as the aircraft meets its approved type design, is in a condition for safe operation and maintenance, preventative maintenance, and alterations are performed in accordance with 14 CFR parts 21, 43, and 91.
How long does an ELT transmit for?
Per COSPAS/SARSAT specifications new 406MHz ELTs are required to be able to transmit continuously for a minimum of either 24 or 48 hours (depending on the type of beacon). I believe the older 121.5 MHz ELTs also have a 24-hour continuous transmission requirement. I presume you are referring to this FAR.