lat lon to ECEF conversion derivation...

cpu_pg

Most texts on geodesy seem to jump right into the formulas for converting LLA to ECEF as follows:

X = [N + H] cos ¢. cos £

Y = [N + H] cos ¢. sin £

Z = [(b 2 /a 2) N + h] sin ¢

My question is, how does one arrive at this? I can't seem to find any derivations for this. Any pointers to some material?