Detect incidence angle with an antenna array
The basic problem is that a directional antenna (one which emits in apparently one direction) will have side-lobes: -



Alternatively, shown as a 360 degree view: -



In other words, how do you determine that you are not receiving a large transmission in the direction of a side lobe compared to a lower power transmission incident on the main lobe? See this wiki article.

Other than the above, it is highly likely that the formula when transmitting will be identical for angle when receiving.