  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.
