  The fringe sensor unit (FSU) is the central element of the phase referenced
imaging and micro-arcsecond astrometry (PRIMA) dual-feed facility for the Very
Large Telescope interferometer (VLTI). It has been installed at the Paranal
observatory in August 2008 and is undergoing commissioning and preparation for
science operation. Commissioning observations began shortly after installation
and first results include the demonstration of spatially encoded fringe sensing
and the increase in VLTI limiting magnitude for fringe tracking. However,
difficulties have been encountered because the FSU does not incorporate
real-time photometric correction and its fringe encoding depends on
polarisation. These factors affect the control signals, especially their
linearity, and can disturb the tracking control loop. To account for this,
additional calibration and characterisation efforts are required. We outline
the instrument concept and give an overview of the commissioning results
obtained so far. We describe the effects of photometric variations and
beam-train polarisation on the instrument operation and propose possible
solutions. Finally, we update on the current status in view of the start of
astrometric science operation with PRIMA.
