\begin{table}%t1
\caption{\label{tab_obs}Observed hyperfine frequencies and residuals of the $J_K = 1_0  \leftarrow 0_0$ transition of NH$_3$.}
%\centerline
$$\scriptsize
 { \begin{tabular}{D{-}{-}{3}D{,}{\leftarrow}{1}D{,}{,~}{3}D{.}{.}{8}D{.}{.}{1}D{.}{.}{1}}
\hline\hline \noalign{\smallskip}
\mcl{1}{c}{Order number} & F'_{\rm N},F_{\rm N} & F' & \mcl{1}{c}{Frequency$^{\rm a}$} & \mcl{2}{c}{Residual} \\
& & & \mcl{1}{c}{(MHz)} & \mcl{2}{c}{(kHz)} \\
\mcl{4}{c}{} & \mcl{1}{c}{Fit I} & \mcl{1}{c}{Fit II} \\
\hline  
1  &  1,1           &  2.5,1.5  &  572~497.1247(5)  &  0.1 & -0.1\\
2  &  \mcl{1}{c}{}  &  0.5      &  572~497.1565(5)  & -0.7 & -0.1\\
3  &  2,1           &  3.5      &  572~498.3391(5)  &  0.4 &  0.0\\
4  &  \mcl{1}{c}{}  &  2.5      &  572~498.3885(10) &  1.6 & -0.1\\
5  &  \mcl{1}{c}{}  &  0.5,1.5  &  572~498.3979(10) & -1.2 &  0.1\\
6  &  0,1           &  1.5      &  572~500.1913(5)  & -0.1 &  0.1\\[1ex]
\hline\noalign{\smallskip}
\mcl{5}{l}{crossover transitions$^{\rm b}$}\\
1-3   & \mcl{2}{c}{} &  572~497.7314(10)  &  -0.6 & -0.6\\
   1-4,5 & \mcl{2}{c}{} &  572~497.7588(10)  &  -0.3 & -0.3\\
    2-4,5 & \mcl{2}{c}{} &  572~497.7766(10)  &   0.5 &  1.0\\
    3-4   & \mcl{2}{c}{} &  572~498.3643(10)  &   1.5 &  0.4\\
    1-6   & \mcl{2}{c}{} &  572~498.6595(10)  &   1.2 &  1.3\\
    3-6   & \mcl{2}{c}{} &  572~499.2650(10)  &  -0.1 & -0.2\\
    4,5-6 & \mcl{2}{c}{} &  572~499.2920(10)  &  -1.2 & -0.9\\[1ex]
    \mcl{4}{l}{rms$_{\rm res}$ $^{\rm c}$ / kHz} & 0.9 & 0.6\\
    \mcl{4}{l}{$\sigma^{\rm d}$} & 1.02 & 0.59\\ 
\hline
\end{tabular}}
$$
\medskip
$^{\rm a}$ Estimated uncertainties are reported in parentheses in units of the last quoted digit; $^{\rm b}$ in the first column the order numbers of the dips corresponding to the crossover resonance are reported; $^{\rm c}$ rms error of residuals: $\sqrt{\frac{\sum {\rm residual}^2}{{\rm N\;observations}}}$; $^{\rm d}$ fit standard deviation: $\sqrt{\frac{\sum {\rm (residual/uncert.)}^2}{{\rm degrees\;of\;freedom}}}\cdot$
\end{table}