Indeed, the 406 ELT location data, which is derived from polar orbiting satellites takes a while to develop, although with one pass of a satellite, RCC will have a GENERAL location at least. The polar orbiting satellites require between 90 and 110 minutes to orbit. Therefore, there is a PERIOD of time during which they will be overhead, not just a snapshot.
AND, there is a geo synchronous satellite down there over the equator that is continuously monitoring the 406 frequency as well. So, if your 406 alerts, the geosynchronous satellite will at the very least, notify RCC that your beacon has alerted. If you've done your job, as in filed a flight plan, or left an itinerary with a trusted loved one, the search can begin.
The advantage of the GPS enabled ELT is that it sends out a GPS coordinate set with the first burst, and that will be picked up either by a polar orbiting satellite or the geosynchronous satellite. Either way, RCC will both know who you are, AND where you are. Even if the plane burns.
As to the plane on its top preventing an alert, consider that my 406 ELT had a little switch issue last summer, enroute to OSH. The thing started alerting, enroute. I got on the ground, called RCC (who had already called my cell phone) and informed them that I was fine. The ELT alerted again that afternoon, so I removed it from the mount, turned the switch OFF, and disconnected it from the antenna and the airplane. It was sitting on the floor of my airplane at that point, where the back seat would be. Next morning, I got a call from RCC. It was alerting again. I informed them that the switch was OFF, and the thing was not connected to the antenna. No matter, and in fact, they were able to download a position and tell me where the thing was. And, it was within a few yards.
The switch has now been fixed, but remember that the 406 ELT, unlike the old 121.5 ELT, transmits at a full 5 watts nominal power. The 121.5 beacons only transmitted in the VERY low power range, somewhere around 200 to 500 miliwatts. So, these things really hammer out a signal, as compared to the old beacons. Secondly, they transmit in bursts, every 50 seconds, not continuously, like the 121.5 units, which permit these beacons to transmit as this very high power for the same length of time that the 121.5 beacons could.
And, yes, there are beacons now with an onboard GPS unit. The Emergency Lifesaving technology unit http://www.elt406.net/
does. Bring money. This unit, by the way, does not include a 121.5 transmitter....