The Earth orientation sensor (ERS) device installed on board the spacecraft (SC) is discussed. In this device related to the local vertical reference, the so-called circular scanning principle is implemented. The circular scanning principle implies the angular field being scanned in a circumferential direction, with the angular radius coinciding with the angular radius of the Earth, and the signal of the receiving device being detected by two phase-sensitive detectors, which generate signals that determine the deviation of the instrument's optical reference axis from the vertical in two mutually perpendicular planes. Provided a mathematical description of the kinematic model of the Earth orientation sensor (EOS). Performed the validation of the analytical description of the device by using the Matlab system and with a model-based approach. As a result, defined that for reliable recognition of four points of the Earth-Space border intersection, it is desirable that the time between transitions approaches to 125ms. Thus, to receive a substantial measurement base on Z axis the angle at the apex of the scanning cone should be reduced. Modeling has shown that the optimal angle at the apex of the scanning cone for an orbital altitude range of 200-2000 km is 55 degrees.
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