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 below another one.

Said differently, I have two Arduino boards and I want to know when one board (placed on an object) is below/above the other board (placed on another object) by about 50cm to 1 meter.

Update: @Andyaka and @OliGlaser : no ... I only need it to work when the two boards are 50cm to 1 meter away (bellow/above) from each other.

Here is the application I want to develop: each of the arduinos will be on a goat. And the idea is to know exactly when my male goat is on my female goat for mating.