relative position: above,bellow

Other Hardware Sensors
1

Hi,

I would like to know if you have any idea of what kind of sensor I need to be able to tell the relative position of an arduino board with regard to another one. I only need to know if an arduino is above or bellow another one.

Thanks

2

Barometer?

How far above or below?
In what sort of environment, with what accuracy and what update rate?

3

AWOL:
Barometer?

How far above or below?
In what sort of environment, with what accuracy and what update rate?

The two objects will be about 1 to 1 1/2 meters bellow/above and I only need to know if one is above/bellow the other and do not need by how much.