Calculating the ECEF coordinates out of LLA coordinates is a slightly easier task than the other way round. The following way refers to \cite{wiki:1}. With the known constants
\begin{align}
a	&= 6,378,137				\\
f	&= \frac{1}{298.257223563}	\\
e^2	&= \sqrt{2f-f^2}			
\end{align}
the value
\begin{equation}
\chi = \sqrt{1-e^2\sin^2 \lat}
\end{equation}
can be precomputed and used in
\begin{align}
x &= \left(\tfrac a \chi + h \right) \cos \lat \cos \lon \\
y &= \left(\tfrac a \chi + h \right) \cos \lat \sin \lon \\
z &= \left(\tfrac a \chi (1-e^2) + h \right) \sin \lat
\end{align}
\inCfile{ecef\_of\_lla\_i(EcefCoor\_i* out, LlaCoor\_i* in)}{pprz\_geodetic\_int}
\inCfile{ecef\_of\_lla\_f(EcefCoor\_f* out, LlaCoor\_f* in)}{pprz\_geodetic\_float}
\inCfile{ecef\_of\_lla\_d(EcefCoor\_d* ecef, LlaCoor\_d* lla)}{pprz\_geodetic\_double}