Influence of fluctuations in atmospheric radio thermal emission on the sensitivity of a radio telescope

Abstract

We describe the experimental procedure for measuring fluctuations in atmospheric radio emission. We present the results of measurements, beth with and without clouds, at a wavelength of 1.35 cm. The structure function for the radio-brightness temperature is observed to increase monotonically with increasing time interval (from 6 to 240 sec) and increasing angular interval (from 1 to 18~ the increase is close to a power law. Onthe basis of experimental data at a wavelength of 1.35 cm, we estimate the intensity of fluctuations in atmospheric radio emission at wavelengths between 0.6 and 10 cm on account of fluctuations in water vapor. We discuss the results which we have obtained. w 1o With incr easing r ec eiver s ens itivity, the pos s ibility of observing faint s our c es of radio emis s ion will be determined to a considerable extent by fluctuations in the antenna temperature of the radio telescope. Radio thermal emission from the atmosphere enters both the main lobe and the side lobes of the directional diagram of the antenna. The effect of this emission on the antenna temperature depends on meteorological conditions, and it becomes more serious at wavelengths shorter than 3 cm. Fluctuations in atmospheric radio emission are due to nonuniformities in meteorological parameters, such as humidity, amount of water, temperature, phase composition, etc. These nonuniformities arise from turbulent motions in the troposphere. Between 0~ and 2.0 cm, under a cloudless sky, theoretical and experimental data indicate that the major contribution to the intensity of fluctuations in atmospheric radio emission comes from pulses of humidity [1,2]. Experimental data on fluctuations in atmospheric radio emission have been given in [3-6]. These data were obtained with a receiver having a sensitivity considerably lower than that which can be achieved at the present time, and they refer mainly to measurements in cloudy weather. There are no experimental data in the literature concerning variations of fluctuations in radio-brightness temperature of a cloudless atmosphere as functions of time and angle. In the present work, we describe a procedure for measuring the variations of the fluctuations in radiobrightness temperature of the atmosphere as functions of time and angle. We present results for various weather conditions in wintertime and in summertime at a wavelength of 1.35 cm. From the experimental values of the fluctuations in radio emission from a cloudless atmosphere we estimate the effect of the fluctuations on the sensitivity of a radio telescope at wavelengths between 0.6 and 10 cm. w We used a superheterodyne modulation radiometer for the observations, with a fluctuation sensitivity of 0.16~ for a time constant of 1 sec in the output device. The temporal instability of the gain coefficient of the radiometer was 1o5-2.0% in a time of 30 min. In the summertime, the measurements were made on the RT-22 telescope of the P. N. Lebedev Physics Institute of the Academy of Sciences of the USSR. For the winter observations, we used a horn antenna to pick up the emission: the antenna had a beamwidth of 6.5 ~ at the half-power level. The antenna and the VHF unit of the radiometer were mounted on a movable platform: this permitted us to vary the zenith angle of the antenna in steps.