  Context: Occultations of stars by asteroids are an efficient method to study
the properties of minor bodies, and can be exploited as tools to derive very
precise asteroid astrometry relative to the target star. With the availability
of stellar astrometry thanks to the ESA mission Gaia, the frequency of good
predictions and the quality of the astrometry have been strongly enhanced.
  Aims: Our goal is to evaluate the astrometric performance of a systematic
exploitation of stellar occultations, with a homogeneous data set and a given
instrument setup. As a reference instrument, we adopt the example of a robotic
50 cm telescope, which is under construction at the Observatoire de la C\^ote d
Azur. We focus in particular on single chord occultations.
  Methods: We created a data set of simulated light curves, that are modelled
by a Bayesian approach. To build the final statistics, we considered a list of
predicted events over a long time span, and stellar astrometry from Gaia data
release 2.
  Results: We derive an acceptable range of observability of the events, with
clear indications of the expected errors in terms of timing uncertainties. By
converting the distribution of such errors to astrometric uncertainties, we
show that the precision on a single chord can reach levels equivalent to the
performance of Gaia (sub milli arcseconds). The errors on the asteroid position
are dominated by the uncertainty on the position of the occultation chord with
respect to the barycentre of the object.
  Conclusions: The limiting factor in the use of occultation astrometry is not
the light curve uncertainty, but our knowledge of the shape and size of the
asteroid. This conclusion is valid in a wide range of flux drops and magnitudes
of the occulted star. The currently increasing knowledge of the shape, spin
properties, and size, must be used to mitigate this source of error.
