\documentstyle{article}
\title{
        Effects of precession uncertainties
          on planetary ephemerides.}
                  \author{H.G. Walter \\
(Astronomisches Rechen-Institut Monchhofstr. 12-14\\
 D-69120 Heidelberg, Germany}
\date{}

\begin{document}
\maketitle
   Recent estimates of the luni-solar precession suggest a correction of
$-0.31 \pm  0.02$ arcsec/cy to the adopted 1976 IAU value. If angular
measurements of bodies of the solar system are used for the
determination of the semimajor axis(a), its relative variation  $\Delta a/a$ as
a function of the precession uncertainty is estimated at 0.2 * $10^{-8}$
multiplied by the time difference of observations measured in years.
Assuming a time difference corresponding to one half of an orbital
period,  $\Delta a$ is of the order of 6 km for objects of 5 AU barycentric
distance. On the other hand, the theoretical precision of planetary
radar ranging is as small as 0.1 km which, in practice, increases to
1-3 km for planets and some 10 km for asteroids depending on the
quality of the surface models. A corrected value of precession would
render equivalent the determination of semimajor axes of orbits from
ranging and angular measurements.


\end{document}
