How to measure the distance covered by iphone during free fall?

Question

During the free fall the iphone is supposed to send acceleration values as 0 on all the three axis. So how to detect the distance covered by the iphone?

Answer 1

One solution that would take into account the issue of wind resistance is you could use the difference between acceleration from gravity and your actual norm of acceleration you're reading,

d²x / dt² = g - |a|

Where g = 9.8 and |a| = sqrt(a₁² + a₂² + a₃²) where a_n are the readings from the accellerometer on each axis.

Then solve the differential equation numerically, with something like Euler's Method.

You could even be clever and lookup local value of g using GPS.