Measurement of the K-band atmospheric opacity using the sky-dips method with a horn antenna

The attenuation of radio signals by the atmosphere is measured through atmospheric opacity. The atmospheric absorption effect must be corrected through an accurate amplitude calibration of the radio signals. Obtaining atmospheric mean temperature, sun brightness, sky dips, and radiosonde data modeling are the most commonly used methods for measuring atmospheric opacity. We develop an independent measuring system of K-band atmospheric opacity with a horn antenna to implement the sky-dips method to further investigate and explore the atmospheric opacity test. After many attempts, we employ the K-band atmospheric opacity measurement system to perform a comparison test with the K-band cryogenic receiver of the Nanshan 25 m telescope at the same time, with a comparison error of approximately 30%. Although the test error is due to the measurement accuracy of power and elevation, ground noise will be added in the side and back lobes if the corrugated horn is directly employed as the receiving antenna. To build the ground noise model of the test system, we employ the conventional hot and cold load method, as well as a load at ambient temperature, to cover the corrugated horn port surface and test the power output of the receiver at various elevation angles. The above opacity fitting data are corrected using the ground noise model. All the corrected data are properly fitted, and the comparison error with the 25 m telescope has been reduced to 10-20%. Although the initial test results of the system met the requirements of the atmospheric opacity test, it still needs to be improved in terms of the beam width, airmass, and side and back lobe.