From what I've read as soon as the PLB is activated it begins sending out a signal that can be heard by both the GEO and LEO satellite. Since the GEO satellites are always in view, they receive the signal almost immediately without any location information. Until a GPS fix or Doppler location is available, that signal is being sent without any location information.

SARSAT's web site http://www.sarsat.noaa.gov/ says that even without a location, the initial alert gives them time to verify the alert by calling the telephone numbers given when the PLB was registered.

If the PLB has an onboard GPS it will take 1 to 16 minutes for the GPS to be able to provide a GPS location (assuming it can get a fix). Some PLB's let them obtain a GPS location from an external GPS. Once that location is available it is sent along with the PLB signal.

I always assumed that the Doppler location is something calculated by the 'system' using data from the LEO satellites. I wouldn't think that the PLB does those calculations, since it is not a receiver. At some point after the initial PLB signal is initiated that Doppler location information is provided to the Mission Control Center.

BUT I'm not an expert in all of this. I'm sure Doug can tell you exactly how it works.