PO is different from PO' and O and O' lay on ground level which truly makes the shaded triangle right angled. also the height does not varry !
The signal is projected vertically down at first than at an angle when roll motion takes place.
thank you...
so imagine you have an airplane traveling at an axis projecting a signal vertically down along AF axis and you have some point O on AF axis and a refference angle Sun-O-Plane . the plane gets affected by some rolling motion at a certain known angle... now the axis shifts to A'F' and O become O'...