\subsection{GNSS}
Need ultra-precise time sync ($c \approx 0.3\; m/ns$). \bi{Errors}
\begin{itemize}
    \item Multipath problem (signal bounce) (0.5 - 100m)
    \item Ionosphere delays (10m)
    \item Satellite pos. err, trop. delay (1m)
\end{itemize}
