The 406 beacon links to a dedicated (actually 2 dedicated systems that have combined resources) Search and Rescue system, COPASS/SARSAT so no matter where in the world you are, when it's activated, the international SAR system goes into action quickly. If you get a PLB with a GPS, they know you're in trouble and where you are in minutes. With no GPS it takes a bit longer to get on your position and it's not quite as accurate...
The original COSPAS/SARSAT satellites are out of fuel and in odd-ball orbits that don't work well for SAR anymore.
Before the 121.5 decommissioning we were getting requests from Langley AFB to "search the Front Range area".
"Lieutenant, could you please put the duty Sargeant on the phone? Sarge? Could you explain how big the Front Range is to the new Lieutenant and call us back after you've had a few more satellite passes and have a better location? Thanks." Click.
Is the new system actually called COSPAS/SARSAT? You made me Google. Yup.
http://www.cospas-sarsat.org/
It was my understanding that the 406 receivers are hitchhikers on some NRO birds and thus information about orbital timeframes, etc... is a little more FOUO than it used to be.
Without GPS data, the system has to take the Aquistion of Signal (AOS) and Loss of Signal (LOS) from more than one bird passing overhead (or the same bird on the next orbit) to get an area of interest and narrow it down. They usually also like to see a couple of extra confirmation passes to tighten it up.
Back in the 121.5 days, when those receivers were still active, the time that took was typically hours and the search area miles, just by the physics of VHF.
We might get a heads up call that they were working a signal in the area and to expect refinement of the area in about an hour or two.
If the sun was setting, it was rare to launch any Air SAR assets until daylight. Ground might get rolling but typically the radar tap data from FAA was consulted first to see if ATC had anything go missing. Takes a little while to get that.
406 gives better resolution in these "solutions" without GPS, due to the higher frequency but it still takes a while.
There's also a big problem in that a lot of people aren't *registering* their 406 beacons. Someone here in Colorado gave a PLB to a mountaineer as a present saying it was an avalanche beacon. He'd turn it on every time he went winter hiking and off when he was out of avalanche danger. It wasn't registered. If it was they could have had the Sheriff knock on his door the first night for a welfare check. The search for the on-again, off-again beacon went on for months, as I recall.
The GPS data nails you down to a spot (pun intended) if the satellite can hear the data bursts from the ELT.
Multiple ELTs in an area can really screw with the results, but usually the computers can figure it out. Digital filtering algorithms for $500, Alex. Since most are false alarms by a huge majority, even after the switch to 406, finding the false one's and turning them off is a pretty high priority.
When the false one is cruising at FL350, that's a very widespread problem that can affect finding the real one for -- a couple more hours, especially the contiuous sweep 121.5 ELTs. Thus, why the 406 devices don't transmit continuously. Harder to ground DF without hysteresis on your DF gear, but easier for the satellites.
In all, it's a pretty interesting group of systems. But it's not without some minor technical warts.
Very few ground SAR groups have upgraded to 406 DF gear that can read the data beacons. Too expensive, everyone's broke. Most are homing on the weak 121.5 signal that a 406 beacon transmits. It works but it's tedious with the old gear.
The other disadvantage to 406 is not many pilots know what the short 121.5 sweeps that come and go are. They're used to listening to the continuous sweep from an old-style 121.5 ELT. They wonder if they really heard it, or report it as an "intermittent ELT" which gets a lot less attention. There's some education going on in the SAR world that the "sound" of an active ELT on 121.5 has changed significantly.
A rare few expensive models transmit the tail number of the aircraft and the last good GPS lat/long in *voice* on 121.5. Those kick butt for ground teams, especially looking for a false alarm on an airport. The airport authority usually knows what hangar a particular N-number is in.
One of the TV News choppers here was the first time I heard that. Drove into the airport, first ground searcher on-scene and the ELT said, "November Four Tango Victor". I knew that was the old Channel 4 KCNC chopper and right where they parked it. Walked into the hangar and said, "I think the Channel 4 bird's ELT is running."
Mechanic walked over and deactivated it and found a faulty switch. Thanked me and I left. Fastest ELT search ever.