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