A memo describing the creation of the DE405 generic planet ephemeris is available from NAIF or from the author: Dr. Myles Standish of JPL's Solar System Dynamics Group. Because this memo was produced using the TeX processor and includes numerous equations, tables and plots, only a small extract of it can be included here as plain ASCII text.



       >>> Beginning of extract from Standish's DE405 memo <<<


JET PROPULSION LABORATORY                         INTEROFFICE MEMORANDUM
IOM 312.F - 98 - 048
August 26, 1998

TO:           Distribution
FROM:         E M Standish
SUBJECT:      JPL Planetary and Lunar Ephemerides, DE405/LE405



INTRODUCTION

The latest JPL Planetary and Lunar Ephemerides, "DE405/LE405" or just 
"DE405," have been released and are now available.

DE405 represents an improvement over its predecessor, DE403, described in 
detail by Standish et al (1995), and hereafter referred to as the "DE403 
Memo." Many of the data sets, reduction techniques, etc. are the same as 
those used for DE403; they will not be described at length in this memo.  
Instead, this memo concentrates on the differences: additional data, 
refinements to the reduction techniques, etc.

The memo discusses the improvement in the orientation of the ephemerides 
onto the (J2000) International Celestial Reference Frame (ICRF). It shows 
the increased observational data set used for DE405 and the associated 
residuals. There is a description of the improved method for modeling the 
perturbations of asteroids upon the planetary orbits. The individual 
ephemerides in DE405 are compared with those of DE403. Tables are included 
of the initial conditions and dynamical constants, resulting from the 
least squares fitting process and used for the integration.


II. ORIENTATION OF DE405

As has been discussed often, the ephemerides of the four innermost planets 
along with the moon and the sun are all well-known with respect to each 
other because of the accurate ranging observations to which the 
ephemerides are adjusted. In turn, the orientation of this inner system 
onto the ICRF is also accurately determined, mainly by the VLBI 
observations of the Magellan Spacecraft orbiting Venus, but also somewhat 
by VLBI observations of the Phobos Spacecraft approaching Mars, a frame-
tie linkage using ground surveys and lunar laser ranging (LLR)data, and 
the range observations of the Viking Spacecraft on the surface of Mars.  
Some of the Magellan VLBI observations are new since DE403. These data are 
discussed in the next section. It is believed that the orientation of the 
whole inner planet ephemeris system of DE405 is now accurate to about 
0.001 arcseconds.  A verification of this estimate was provided by the 
arrival of the Pathfinder Spacecraft at Mars in July 1997, where the 
ephemeris error was about 0.001 arcseconds, corresponding to 1 km at that 
distance; it was mostly in the down-track direction.

Ephemerides of the outer planets rely almost entirely upon optical 
observations these were initially referenced to various stellar 
catalogues, then transformed onto the FK4 using the formulae of Schwan 
(1983), then onto the FK5 system by applying the equinox offset and motion 
parameters of Fricke (1982), and finally onto the ICRF using tentative 
transformation tables supplied by Morrison (1996).


{Substantial intervening text that included equations has been left out.}


CONCLUSION

DE405 represents the most accurate planetary positions available.  
Certainly, they are not perfect; extrapolation forward or backward in time 
will always show some amount of deterioration. Subsequent improvements 
will continue with further acquisition of observational measurements.

        >>> End of extract from Standish's DE405 memo <<<