  In-spiraling supermassive black holes should emit gravitational waves, which
would produce characteristic distortions in the time of arrival residuals from
millisecond pulsars. Multiple national and regional consortia have constructed
pulsar timing arrays by precise timing of different sets of millisecond
pulsars. An essential aspect of precision timing is the transfer of the times
of arrival to a (quasi-)inertial frame, conventionally the solar system
barycenter. The barycenter is determined from the knowledge of the planetary
masses and orbits, which has been refined over the past 50 years by multiple
spacecraft. Within the North American Nanohertz Observatory for Gravitational
Waves (NANOGrav), uncertainties on the solar system barycenter are emerging as
an important element of the NANOGrav noise budget. We describe what is known
about the solar system barycenter, touch upon how uncertainties in it affect
gravitational wave studies with pulsar timing arrays, and consider future
trends in spacecraft navigation.
