\begin{table}%t1
\caption{\label{tab1}Number of observed positions in the database}
\small \begin{tabular}{lll}
\hline\hline \noalign{\smallskip}
 Satellites   & Period of time &Number\\
              &               &of data\\
\hline
 M-1 M-2  &1877-2005 &3438 \\
%\hline
J-1  &1891-2007 &3307 \\
%hline
J-2 &1891-2007 &2758 \\
%\hline
  J-3  &1891-2007 &3086 \\
%\hline
  J-4  &1891-2007 &3104 \\
%\hline
 all J-1-2-3-4 &1891-2007 &12~255 \\
%\hline
%\hline
  J-5  &1954-2001 &677 \\
%\hline
   J-14 &1981-2001 &521 \\
%\hline
  J-15  &1988 &48 \\
%\hline
  J-16  &1988-2000 &178 \\
%\hline
 all J-5-14-15-16 &1954-2001 &1424 \\
%\hline
%\hline
  J-6 J-7 J-8 J-9 J-10 J-11  &1894-2008 &4598 \\
%\hline
   J-12, J-13 &1951-2008 &294 \\
%\hline
  J-17 to J-49 and unnumbered  &1975-2006 &975 \\
%\hline
 all J-6-7-8-9-10-11-12-13-17 &1894-2008 &3020 \\
%\hline
%\hline
 S-1  &1874-2003 &1820 \\
%\hline
 S-2  &1874-2007 &5786 \\
%\hline
 S-3  &1874-2007 &9671 \\
%\hline
  S-4  &1874-2007 &9017 \\
%\hline
  S-5  &1874-2007 &8072 \\
%\hline
  S-6  &1874-2007 &7994 \\
%\hline
  S-7  &1874-2007 &4321 \\
%\hline
  S-8  &1874-2007 &6833 \\
%\hline
 all S-1-2-3-4-5-6-7-8 &1891-2007 &57~355 \\
%\hline
%\hline
S-10, S-11  &1966-2000 &188 \\
%\hline
 S-12  &1980-1996 &286 \\
%\hline
 S-13, S-14  &1981-1996 &90 \\
%\hline
  S-16, S-17  &1994-2002 &410 \\
%\hline
 all S-12-13-14-16-17 &1966-2002 &974 \\
%\hline
%\hline
 S-9  &1904-2008 &1520 \\
%\hline
 from S-18 to S-52 and unnumbered  &2000-2008 &939 \\
%\hline
 all S-9-18-... &1904-2006 &2459 \\
%\hline
%\hline
  U-1  &1983-2006 &1495 \\
%\hline
  U-2  &1983-2006 &1529 \\
%\hline
  U-3  &1983-2006 &1385 \\
%\hline
  U-4  &1983-2006 &1605 \\
%\hline
 U-5  &1983-2006 &1430 \\
%\hline
 all U-1-2-3-4-5 &1983-2006 &7441 \\
%\hline
 U-6 to U-15  &1994-2004 &65 \\
%\hline
U-16 to U-20  &1984-2006 &479 \\
%\hline
%\hline
N-1  &1877-2006 &3095 \\
%\hline
N-2  &1949-2008 &667 \\
%\hline
 N-1 N-2 &1877-2008 &3762 \\
%\hline
%\hline
N-3 to N-8  &1991-2002 &149 \\
%\hline
%\hline
 N-9 to N-13  &1999-2004 &170 \\
%\hline
%\hline
P-1  &1992-2006  &78 \\
%\hline
P-2, P-3  &2002-2006 &35 \\
%\hline
 all P-1-2-3 &1992-2006 &113 \\
\hline
\end{tabular}
\end{table}