1. Turn the GPS on. Usually I leave the GPS turned off, in order to
maximise battery life.
2. If the GPS can't see the sky, the telephone network is used for location.
3. Regardless of the current volume settings, the volume is turned up
to the maximum, and an alarm sound (or the standard phone ring) is
4. The app can be activated by SMS or online.
5. The location can be shown on Google Maps (or equivalent).
These are must-haves.
Some security features may be worthwhile, but are not necessary.
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