  Despite an emerging interest in MIMO radar, the utilization of its
complementary strengths in combination with optical sensors has so far been
limited to far-field applications, due to the challenges that arise from mutual
sensor calibration in the near field. In fact, most related approaches in the
autonomous industry propose target-based calibration methods using corner
reflectors that have proven to be unsuitable for the near field. In contrast,
we propose a novel, joint calibration approach for optical RGB-D sensors and
MIMO radars that is designed to operate in the radar's near-field range, within
decimeters from the sensors. Our pipeline consists of a bespoke calibration
target, allowing for automatic target detection and localization, followed by
the spatial calibration of the two sensor coordinate systems through target
registration. We validate our approach using two different depth sensing
technologies from the optical domain. The experiments show the efficiency and
accuracy of our calibration for various target displacements, as well as its
robustness of our localization in terms of signal ambiguities.
