  Many atmospheric and climatic criteria have to be taken into account for the
selection of a suitable site for the next generation of imaging air-shower
Cherenkov telescopes, the "Cherenkov Telescope Array" CTA. Such data are not
available with sufficient precision, thus a comparison of the proposed sites
and final decision based on a comprehensive characterization is impossible.
Identical cross-calibrated instruments have been developed which allow for
precise comparison between sites, the cross-validation of existing data, and
the ground-validation of satellite data. The site characterization work package
of the CTA consortium opted to construct and deploy 9 copies of an autonomous
multi-purpose weather sensor, incorporating an infrared cloud sensor, a newly
developed sensor for measuring the light of the night sky, and an
All-Sky-Camera, the whole referred to as Autonomous Tool for Measuring
Observatory Site COnditions PrEcisely (ATMOSCOPE). We present here the hardware
that was combined into the ATMOSCOPE and characterize its performance.
