find the minimum coaltitude which can directly receive a signal from a geostationary satellite.

Explanation

The farthest point on earth, which can receive signals from the parking orbit is the point where a length is drawn on earth surface from satellite as shown in figure. The colatitude `lambda` of point P can be obtained from figure as

`sin lambda = (R_(e))/(R_(e) + h) ~= (1)/(7)`
We known for a parking orbit `h ~= R_(e)`
Thus we have `lambda = sin^(-1)((1)/(7))`