The ELTs in general aviation aircraft contain crash activation sensor, or G-switch, which is designed to detect the deceleration characteristics of a crash and automatically activate the transmitter.  However, it is always safest to assume that the automatic activation feature failed.  Place the ELT function switch to "ON" as soon as possible after the crash.  With the SARSAT and COSPAS satellites now in orbit, ELT signals will be detected, normally within 90 minutes, and SAR agencies alerted.   Military aircraft also monitor 121.5 MHz or 243.0 MHz and will notify ATS or SAR agencies of any ELT transmissions they hear.  ELT detection ranges can be improved if the ELT is placed upright, with the antenna vertical, on the highest nearby point. Do not delay ELT activation until flight-planned times expire, as such delays will only delay rescue.  Do not cycle the ELT through "OFF" and "ON" positions to preserve battery life, as irregular operation reduces localization accuracy and will hamper homing efforts.  Once you have turned your ELT "ON", do not switch it "OFF" until you have been positively located and directed to turn it off by the SAR forces.