System Noise and Accuracy of Primary Flux Density Calibrators and Scales on RadioAstron Space Telescope Data

—We investigate the physical reason of the found difference in the flux density calibration for the space radio telescope on the “primary” calibrators Cassiopeia A and Crab Nebula. Twenty internal noise sources (or noise generators) of space radio telescope are analyzed as “secondary” standards measured relative to the “primary” ones in the units of the Noise Source spectral Equivalent Flux Density (in Jy). This is performed within three accurate flux density scales using monitoring data of space radio telescope calibrations at the wavelengths of 6.2, 18 and 92 cm in 2015–2018. The aims are: (1) to find and eliminate the cause of this discrepancy; (2) to propose a method for verification of flux densities of the calibrators and their scales based on the analysis of Noise Source spectral Equivalent Flux Density; (3) to analyze the System spectral Equivalent Flux Density of the space radio telescope. We have found out that the difference is a result of a variability of “primary” calibrators which is accurately quantified by the new scales proposed in 2014 and 2017. The Noise Source spectral Equivalent Flux Density measured within the new scales turned out to be more accurate than results obtained in the 1977 scale. Averaging these Noise Source spectral Equivalent Flux Density on Crab Nebula and Cassiopeia A eliminates the difference between the scales. The space radio telescope noise sources can be used to verify the quality of calibrators. An artificial standard noise source of a radio telescope can be used not only as an ordinary “secondary” calibrator but also as an indicator of relative accuracy for verifying spectral flux density calibrators and scales under certain conditions.