Global Positional Service (GPS) is the worse of all the time sources mostly because you can't receive it in doors. The Very Low Frequency (VLF) radio signals can penetrate most buildings. WWVB (Bolder Co, USA) power output was increased by a factor of three about two years ago which can now be received in north and South America. Most of the other VLF transmitter will also be increased in power in the next few years. Japan just installed a 79 kHz transmitter which covers most of Asia. Australia also has a VLF transmitter. It is not as regional as you might think.
The VLF receivers are the most cost efective also. $17 retail. $7 wholesale. GPS will never be this cheap.
Regards,
Charles Bannister