  SIM-Lite is an astrometric interferometer being designed for
sub-microarcsecond astrometry, with a wide range of applications from searches
for Earth-analogs to determining the distribution of dark matter. SIM-Lite
measurements can be limited by random and systematic errors, as well as
astrophysical noise. In this paper we focus on instrument systematic errors and
report results from SIM-Lite's interferometer testbed. We find that, for
narrow-angle astrometry such as used for planet finding, the end-of-mission
noise floor for SIM-Lite is below 0.035 uas.
