Random thoughts: If the mobile phone and mobile phone with a wireless distance measurement function

For example, in a tunnel, how do I know where I am currently in the tunnel?

If the tunnel is 3 km long, after entering the tunnel there is no GPS signal, the mobile phone will not be able to use satellite positioning, and then navigation failure.

If the phone and the phone can be interconnected (perhaps through the 2G, 3G, 4G signal interconnection), coupled with a mobile phone has a measure of the distance between the phone function (such as radio ranging, measuring azimuth), then can be from a phone without a GPS signal connected to a mobile phone that receives a GPS signal, And with its GPS signal as the reference point, you can calculate the location of the phone in the middle of the tunnel. This function is very useful, and the GPS satellite positioning network just complementary, the formation of ground, space and vertical positioning system, greatly improving the positioning ability of mobile devices.

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Random thoughts: If the mobile phone and mobile phone with a wireless distance measurement function