The function spa has no outputs. It instead prints the results ot the screen. I would modify the function declaration to return the structure created by the function.
Also note that the clear command is unnecessary in a function.
function [spa] = spa()
spa_const
..
end
You could then access the fields in the structure using dot notation. So a simple main.m might be
my_spa = spa;
my_spa.azimuth_astro
