IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 47, NO. 12, DECEMBER 1999 1749 Evidence of Long-Term Correlation Between Clear-Air Attenuation and Scintillation in Microwave and Millimeter-Wave Satellite Links Frank Silvio Marzano, Member, IEEE, and Carlo Riva, Member, IEEE Abstract— Long-term correlation between microwave scintil- lation and path attenuation in clear-air troposphere is quanti- tatively evaluated carrying out a numerical and experimental analysis on a monthly basis. Amplitude scintillation variance is simulated by means of a weak-fluctuation propagation model, while path attenuation is obtained from ground-based brightness temperature data using a radiative transfer model. Both the scintillation and radiative transfer models are applied to a set of radiosounding observations, performed in Milan, Italy, during 1989. Regression formulas relating clear-air mean radiative tem- perature to meteorological quantities and slant-path attenuation to amplitude scintillation variance are derived from numeri- cal simulations. Their validity should be restricted, in general, to mid-latitude subcontinental climates. Monthly predictions of radiometer-derived path attenuation and correlation between attenuation and scintillation are tested using both multichannel radiometric data and Italsat beacon measurements at 18.7, 39.6, and 49.5 GHz, acquired at Spino d’Adda, Italy, ground station in 1995. A fairly good agreement is found by performing a comparison between estimates and measurements. Index Terms— Clear-air amplitude scintillation, microwave propagation, satellite communications. I. INTRODUCTION N EW fixed and mobile communication services are planned to exploit beacon frequencies at band and above [1]. At these frequencies, scintillation plays an important role as a signal degradation source [2], [3]. Scintillation phenomena are attributed to turbulent refractive index inhomogeneity, which induces time and space random variations of the amplitude, the phase, and the angle of arrival of the received signal. The renewed interest in scintillation studies is mainly due to their relevant impact on digital links that have low fade margins available, but require high performance [4]. Recently, the application of electromagnetic propagation models, describing the interaction between microwave ra- diation and turbulent atmosphere, has been proposed for developing statistical prediction methods of clear-air scintil- Manuscript received January 22, 1999; revised October 5, 1999. This work was supported in part by the Italian Ministry of University and Scientific and Technological Research (MURST) and the Italian Space Agency (ASI). F. S. Marzano is with the Dipartimento di Ingegneria Elettrica, Universit` a dell’Aquila, Monteluco di Roio, L’Aquila, 67040 Italy. C. Riva is with the Dipartimento di Elettronica e Informazione and Centro di Studio per le Telecomunicazioni Spaziali, Politecnico di Milano, Milano, 32-20133 Italy. Publisher Item Identifier S 0018-926X(99)09974-3. lation using meteorological data [5]–[7]. This model-based approach has opened the possibility to include predictors other than surface data, showing that the vertically integrated water- vapor content is significantly correlated with the amplitude scintillation variance [6]. Indeed, this extension of the statisti- cal prediction methods is very appealing since the integrated water-vapor content can be accurately retrieved by both dual- channel microwave radiometers [8], [9] and global positioning system (GPS) receivers [10], [11]. The foundations of these new estimation techniques are ba- sically related to the correlation between clear-air scintillations and brightness temperatures. This correlation has been shown in literature by using both simulated and experimental data on a short- and long-term basis [6], [12]–[16]. Since path attenuation can be derived from radiometric measurements, it is expected that scintillation is also correlated with path attenuation in clear-air conditions, i.e., without clouds and precipitation. An analysis of the correlation between rain path attenuation and scintillation intensity has been already carried out using Olympus measurements on a short-term basis [17]. In rain conditions, zenithal path attenuation at 20 GHz can reach values up to 15 dB (for precipitation intensity of about 50 mm/h). On the contrary, in clear-air conditions, we expect zenithal attenuation no greater than about 1 dB (for very humid air with 50 mm of vertically integrated water vapor) [18], [19]. This means that the dynamic range of path attenuation in clear- air conditions is much smaller than in rain conditions and, correspondingly, when clear-air high scintillation variances are present, the measured path attenuation values are generally small. In this work, numerical and experimental evidence of the long-term correlation between microwave scintillation and path attenuation in clear-air troposphere is argued. Amplitude scintillation variance is derived both from a weak-fluctuation propagation model and from Italsat measurements at the Spino d’Adda ground station (Italy). Path attenuation is obtained from ground-based brightness temperature data, using both a radiative transfer model and a multichannel radiometric system available at the Italsat receiving station. Both the scintillation and radiative transfer models are applied to a set of radiosounding observations (RAOB’s), performed in Milan, Italy, during 1989. Regression models, relating clear-air slant- path attenuation to amplitude scintillation, are derived from numerical simulations and tested against Italsat data. 0018–926X/99$10.00  1999 IEEE