Originally Posted by Snowflake
One other thing to keep in mind: If you're relying on the crash to set off the ELT, you're throwing away your best chance of being found *at all*. Your ELT (or SPOT, InReach, whatever) needs to be calling out the cavalry *before* you hit the ground. The GPS in a fixed 406 ELT won't even start trying to find it's position until it's activated... if it's upside down, it won't ever find it.
Rob - not disagreeing here, just hoping to refine the general understanding of ELT operation...
An ELT which is connected to an external position source is getting position data all the time, as long as that position source is active and can see the sky. With this in mind we should understand that such an ELT will broadcast GPS position coordinates in the first transmission burst, 50 seconds after crash activation or manual switch activation.
Other ELTs such as the Kannad Integra which have a built-in GPS receiver behave as you have mentioned, needing a view of the sky in order to get a position fix, and that fix may not be broadcast on the first burst, depending on how quickly the GPS can get a position. It may never get a position fix, and if that's the case the ELT still broadcasts but without the position data.
Interestingly, the search radius associated with 406MHz ELTs is considerably smaller than that associated with 121.5MHz ELTs, so even without GPS data the 406MHz ELT has already reduced the search area considerably. That's when the built-in 121.5MHz homing signal, which broadcasts continuously except when the 406MHz burst is being transmitted, comes into play, allowing searchers to home on the ELT.
Another interesting point is that the 406MHz signal is very robust, about 5 watts, compared to a typical 100 milliwatts of 121.5MHz signal. That whackin' big 406MHz signal can get through a surprising amount of "stuff". More than one ELT owner has discovered the hard way that 406 signals travel right through hangars!