[digs engineering hat from the closet and puts it on]

um, no...relativistic effects have nothing to do with it with regards to the speed or altitude of the plane. If you were able to calculate both an altitude and a speed, the system knew exactly where you were. Accurate altitude determination requires a solid 4-sat fix minimum (4 unknowns require 4 equations/satellites) though due to geometric issues altitude determination will have a larger error bar. By accurate, I mean within the limitations of the CA code and use case. With a 3-sat fix you can get a good approximation of altitude (especially if stationary where you can average out errors to increase accuracy) depending on how clever the programmer was.

And speed requires that such a fix is maintained over time to get the deltas in position over time.The software you were using might not have properly displayed that information for some reason (intentional or otherwise), but that's a display issue and not a calculation one.

However, all that said....thanks for the observation It's always interesting to see people doing novel things with these tablets