Does “fall detection” require GPS + cellular, or does it work with GPS alone?
Does fall detection on Apple Watch 5 require cellular, or can it work with just GPS?
Apple Watch
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If your iPhone is nearby, or if you Apple Watch is connected to WiFi and your iPhone's cell service includes WiFi Calling, then a GPS only model can call emergency services.
If not on WiFi, do not have WiFi calling, and your iPhone is not nearby, then no a GPS only does not have a way to call emergency services.
By nearby, if you are within 40-50 feet (maybe more) of your iPhone, Bluetooth will work. If both the iPhone and the Apple Watch are on the same WiFi network, the Apple Watch can get to the iPhone that way if Bluetooth is out of range.