\begin{table}%t7
\caption {\label{tab:noise_example}Receiver noise properties.  Long duration test antenna \mbox{temperature} pairs for the {\tt RCA28}. }          
%\centering
                          % used for centering table
\begin{tabular}{ c c c c c}        % centered columns (4 columns)
\hline\hline                 % inserts double horizontal lines
\noalign{\smallskip}
 $T_{\rm sky}$[K] &  \multicolumn{2}{c}{$T_{\rm ref}$[K]} &  \multicolumn{2}{c}{$\left ( T_{\rm ref} -T_{\rm sky}\right )$} \\
                   & M  & S  & M & S \\    % table heading 
                   \hline
 8.48 	&  10.21 & 8.85  	& 1.73  & 0.37  \\ 
 9.07 		&  15.16 & 14.66 	&  6.10 &  5.59\\
 9.74 		&  19.45 & 19.36 	&  9.70  &  9.62\\
 19.63 	&  19.45 & 19.36 	&  --0.18 &  --0.27\\
\hline                        % inserts single horizontal line
\end{tabular}
\end{table}