\begin{table}%t6
      \caption{Comparisons of observed and predicted satellite positions.
         \label{tab6}}
     $$ 
%         \begin{array}{p{0.5\linewidth}l}
         \begin{tabular}{c c c c}
            \hline\hline
            \noalign{\smallskip}
             & & JPL model &  ESOC model \\
            \noalign{\smallskip}
            \hline
            \noalign{\smallskip}
\par
Phobos	& along-track *   &	+12 km	&	 --2 km   \\
	& across-track    &	+/-- 1 km	&	+/-- 8 km \\
\par
Deimos	& along-track *   &	+ 50 km		&        --5 km   \\
	& across-track	  &        0 km	        &      +/-- 18 km \\
\par
            \noalign{\smallskip}
            \hline
         \end{tabular} %{array}
     $$ 
\par
\smallskip
$^{{\rm *}}$ Positive numbers indicate that the satellite is ahead of the predicted position
all numbers with errors +/-- 2~km.
\par
   \end{table}