SPOT use geosynchronous satelites. There's no "waiting" for coverage, there is no "low on the horizon" stuff either. You are either in the coverage area or you aren't.

And as usual, you don't understand the SPOT usage model either. Since a SPOT user has been broadcasting location information often, (perhaps at every rest break or continually if they have TRACKing), when SAR gets the 911 call they'll start from the last broadcast coordinates.

I own a SPOT and I purchased tracking. If I can see a 30* slice of the sky, SPOT gets the message out, everytime. And track-mode is nice because SPOT is sending location messages every 10 minutes automatically.

Ask and I'll email you a kml file for Google Earth that logs my last backpacking trip. It was a perfect mix of canopy and open area. You can see where the track messages didn't get through.