The SIM-Lite mission has finished its technology programme and continues with an
engineering risk reduction activity while waiting for the results of the 2010 decadal
review. One of the last remaining activities is work on the SCDU testbed, which is
designed to calibrate the spectral bias in interferometric astrometry to 0.1 microarcsec.
This talk will provide a summary of engineering activities on building flight-like
prototypes of the key interferometer subassemblies, as well as an overview of the types of
problems one encounters when attempting to do astrometry below 1 microarcsec, as well as
their solution.