SPHERE-2 balloon experiment results. Part I: EAS observation conditions

The SPHERE project studies primary cosmic rays by detection of the Cherenkov light of extensive air showers reflected from the snowy surface of the earth. Measurements with the aerial-based detector SPHERE-2 were performed in 2011-2013. The detector was lifted by the balloon at altitudes up to 900 m above snowed surface of Lake Baikal, Russia. The results of the experiment are summarized now in a series of papers that opens with this article. An overview of the SPHERE-2 detector telemetry monitoring systems is presented along with the analysis of the measurements conditions including atmosphere profile. The analysis of the detector state and environment atmosphere conditions monitoring provided various cross-checks of detector calibration, positioning and performance.