Estimated Accuracy of Spectral-Selective Radiometer

A. Ipatov, I. Rahimov, N. Koltsov, T. Andreeva

Transactions of IAA RAS, issue 61, 44–52 (2022)

DOI: 10.32876/ApplAstron.61.44-52

Keywords: broadband radio spectrum, broadband radio spectrums calibration, accuracy of radiometric measurements, estimation of the monitoring accuracy of flux scale, spectral interference rejection, spectral-selective radiometer

Abstract

The regular measurements of broadband radio emission flux densities (FD) are carried out on the RT-32 radio telescopes of the “Svetloe” observatory. The main aim of these measurements is to study the variability of cosmic sources emission and refine the flux scale formed by the “artificial moon” method. The FD studies were carried out in four frequency bands (L, S, C and X), but in 2009 they were suspended in the L and S frequency bands due to a significant increase in radio interference generated by mobile communication systems. The observations were resumed in 2013 after the creation of a digital broadband spectral-selective radiometer with protection against radio interference which replaced the traditional radiometer with an amplitude quadratic detector of broadband noise signals. Radiation variability study and flux scale monitoring require measurements with an accuracy of no worse than 2–3 %. Achieving such accuracy is a rather complicated task that can only be solved with the help of radiometric equipment that measures the power and noise temperature of the received broadband signal with extremely high accuracy. The accuracy of a radiometric system determines the scatter of the FD measurement results and, accordingly, the required amount of repeating measurements. The accuracy is supreme important for radiometric studies performed in the presence of radio interference. Along with random errors in FD measurements which depend on the radiometer accuracy there can be systematic errors that depend on the accuracy of determining the effective antenna area as well as the accuracy of determining the reference source FD used (flux scale standard). The purpose of the article is to analyze the accuracy in measuring noise temperatures and radio emission power by a spectral-selective radiometer. Besides, the article determines observations volume necessary to obtain a given accuracy of radiometric measurements. This will allow optimizing programs for radiometric observations performed by the RT-32 radio telescope in order to study the variability of radio emission from sources. When estimating the random error in measuring noise temperature, broadband signal accuracy and received radio emission FD under the influence of radio interference, methods common for radiometry and measuring technology are used. Data on systematic errors of FD measurements are determined on the basis of published materials on the RT-32 parameters and the flux scale accuracy. The random (noise) error in the FD measurement with an accumulation time of 1 s is (4–15) % depending on the frequency band and signal level and it decreases in a series of repeated measurements. For the RT-32 radio telescope with a spectral-selective radiometer the measurement volumes (accumulation time τ and the number of repeated measurements M) are determined. In the frequency bands C and X in order to obtain a root-mean-square error of about 1 % it is necessary to accumulate a signal with an FD of about 5 Jy for τM ≥ 4÷5 s, and a signal with an FD of 1 Jy for 36 s. In the L and S frequency bands a signal with a level of 5 Jy needs τM ≥ 22 s, and a signal at a level of 2 Jy — at least 144 s. In a spectral-selective radiometer with square-wave modulation of the receiving device the signal and noise spectra are accumulated separately. This allows implementing a mode with signal and noise accumulation at different time intervals (asymmetric accumulation mode). In this case it is possible to increase the accuracy of the radiometer by 29 %. The RT-32 radio telescope with a spectral-selective radiometer is used to monitor the “artificial moon” flux scale and to study the variability of broadband radio emission from space sources.

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