  The optical truss interferometer (OTI) is a contingent subsystem proposed for
the LISA telescopes to aid in the verification of a $1
\frac{\mathrm{pm}}{\sqrt{\mathrm{Hz}}}$ optical path length stability. Each
telescope would be equipped with three pairs of compact fiber-coupled units,
each forming an optical cavity with a baseline proportional to the telescope
length at different points around the aperture. Employing a Pound-Drever-Hall
approach to maintain a modulated laser field on resonance with each cavity, the
dimensional stability of the telescope can be measured and verified. We have
designed and developed prototype OTI units to demonstrate the capability of
measuring stable structures, such as the LISA telescope, with a $1
\frac{\mathrm{pm}}{\sqrt{\mathrm{Hz}}}$ sensitivity using a set of freely
mountable fiber-injected cavities. Aside from its initial motivation for the
telescope, the OTI can also be readily integrated with other systems to aid in
ground testing experiments. In this paper, we outline our experimental setup,
measurement results, and analyses of the noise limitations.
