@grizzled: for the edification of the casual reader/lurker/ we could point them to the Tech Log forum,
where there's a thread with a discussion of ELT's with
at least one good post. Summary from that (PPruNer avspook)
- ELTs transmit on 121.5/243/406 MHz. (406 MHz contacts a satellite monitoring system. It uplinks messages to a Satellite that includes aircraft identifying information. Thanks to David Reid for the specifics in his post a few down from this one)
- Some ELT's are tied into the Navigation system to uplink last position.
- Many rely on the Satellite system to direction find the signal
- An ELT (due to the frequencies involved) is not able to contact the satellite from underwater. It is also not a waterproofed box.
- The aircraft hull mounted ELTs are typically set up to transmit when a G-load of a particular magnitude is detected (crash)
Sample product sheet (Honeywell; Thales and some other vendors also make products in this class).
FAA Spec TSO-C126A (Update: pages 9-16
of this link are TSO-C126b.
DO-204A, DO-160F are specs cited on some product sheets (DO-204b is pending based on some ICAO docs posted on line).