Take A, B and C respectively, so that SC is perpendicular to CA and SB is perpendicular to AB.

Projection from a to BSC a'

Let the length of SC be 2,

AA'= radical number 2,

The length of CA is the root of 2.

SBA'C is a rectangle, so SA'=BC.

Therefore, the sine of ASA' is 1/2, so the angle ASA'=30 degrees.