Millisecond pulsars and solar-system ephemerides

J.F. Chandler
Harvard/Smithsonian Center for Astrophysics

The analysis of pulsar time-of-arrival data is intimately bound up
with planetary ephemerides.  Highly accurate ephemerides are required
for the Earth and Moon and, to a lesser degree, for the other planets,
in order to make full use of the timing data for millisecond-class
pulsars.  These data, in turn, present an opportunity for improving
planetary ephemerides in a variety of ways.  Fitting the Earth and
Moon orbital parameters to the timing data is the obvious first step,
though it is less valuable in the short term for many applications
than using the current accumulation of spacecraft-tracking and Lunar
laser ranging data.  By themselves, the pulsar timing data convey no
information on the orientation of the Earth's orbit, since each
pulsar's position in the sky must be determined from those same data.
However, independent pulsar position measurements by techniques such
as VLBI, in combination with the timing-derived positions, can serve
to fix the orientation of the Earth's orbit with respect to the radio
reference frame and thereby link the planetary and radio frames.  In
the long run, the acquisition of timing data over increasing time
spans and with improving precision should prove to be an important
factor in determining the shape, as well as the orientation, of the
Earth's orbit.  In addition, pulsar timing over a sufficiently long
span can directly measure a planet mass through the reaction of the
rest of the solar system.  The effect must be observed for a major
fraction of the orbital period of the planet in question so that the
signature can be separated from that of the ordinary spin-down of each
pulsar.  Finally, pulsar timing can be used to probe gravitational
physics, a basic part of the framework for constructing the
ephemerides.