Asides from that, there is always the option of using normal VHF/UHF radio to squawk current location. Again, only in emergency, but sure to be recorded by any aircraft and ground-stations in range. It could be turned on for instance if the radio was set to the emergency frequency and would append a 'selcall' or DTMF burst containing GPS coordinates at the end of any message. Many land-mobile radios have this feature already.
Pilots are taught to aviate, navigate
and then communicate (their emergency).
Primary activation has to be automatic (with manual backup).
Even Scully (Hudson River ditching) forgot to activate the ditching switch on his Airbus. (Correction: Scully didn't have time to run through his checklists)
You need a position reporting system which operates even after main electrical bus failures (as experienced in engine flameouts and mid-air breakups) and in situations in which the pilots are fighting for control (hands busy), etc. If you have electrical bus problems, the aircraft position sensors may not be sending the data to the emergency transmission system.
IMHO, constant or
frequent periodic transmission seems to be a much more reliable method.