  The Astrodynamical Space Test of Relativity using Optical Devices (ASTROD) is
a multi-purpose relativity mission concept. ASTROD's scientific goals are the
measurement of relativistic and solar system parameters to unprecedented
precision, and the detection and observation of low-frequency gravitational
waves to frequencies down to $5\times10^{-6}$ Hz. To accomplish its goals,
ASTROD will employ a constellation of drag-free satellites, aiming for a
residual acceleration noise of (0.3-1)$\times$ 10$^{-15}$ m s$^{-2}$
Hz$^{-1/2}$ at 0.1 mHz. Noise sources and strategies for improving present
acceleration noise levels are reported.
