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The SPHERE project studies primary cosmic rays by detection of the Cherenkov light of extensive air showers reflected from the snow covered surface of the earth. Measurements with the aerial-based detector SPHERE-2 were performed in 2011–2013. The detector was lifted by a balloon to altitudes of up to 900 m above the snow covered 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.

The main purpose of the Sphere-Antarctica project is connected to the fundamental problems of the cosmic ray physics and general astrophysics - the determination of the energy and mass composition of cosmic ray particles of ultra high and extremely high energies 1018 − 1020 eV. In the energy region above 6 · 1019 eV modern experiments (Telescope Array and Pierre Auger Observatory) observed anisotropy and the clustering of arrival directions of cosmic rays in some areas. The scientific importance of this problem stems from the lack of generally accepted acceleration mechanism of the CR particles above 3 · 1018 eV, the unknown nature of the sources of such particles, the inconsistencies of the results of major experiments in the part of the mass of CR composition and the discrepancy of experimental and model data. Scientific novelty of this project is in the methodology registration of the extensive air showers over a large area ∼ 600 km2 from an altitude 30 km, that allows to measure the two optical components of the shower Vavilov-Cherenkov radiation and fluorescence light by the same SiPM sensitive elements of the detector simultaneously.

Preliminary results on the development of a separation method for Cerenkov (CL) and fluorescence (FL) light from EAS are shown. The results are based on the measurement of attenuation coefficients of CL and FL for different filters. A total of six optical filters were investigated: filters from optical glass UFS-1, UFS-5, FS6 (analogue BG3) and interference filters SL 360-50, SL 280-380, FF01-375/110. The measurements were performed using silicon photomultipliers (SiPM). To improve existing fluorescent light detectors, a segment of 7 SiPM was developed, which would be able to separate both components of the light flux from EAS at the level of primary data processing.

Preliminary results on the development of a separation method for Cerenkov (CL) and fluorescence (FL) light from EAS are shown. The results are based on the measurement of attenuation coefficients of CL and FL for different filters. A total of six optical filters were investigated: filters from optical glass UFS-1, UFS-5, FS6 (analogue BG3) and interference filters SL 360-50, SL 280-380, FF01-375/110. The measurements were performed using silicon photomultipliers (SiPM). To improve existing fluorescent light detectors, a segment of 7 SiPM was developed, which would be able to separate both components of the light flux from EAS at the level of primary data processing.