Assessment of Radiometeorological Parameters Used in the Development of Maps of Propagation Impairments and Path Availability for Canada at 40/50 GHz

Abstract

Estimates of rain and gaseous attenuation, cloud loss, scintillation fading and total attenuation exceeded for several time percentages of an average year are produced for Canada at both 40 and 50 GHz, based on the in-force ITU-R Recommendations P.618, P.676 and P.840. These predictions assume links to a geostationnary satellite located at 111.1 °W longitude. Total attenuation results are then used to estimate path availability. A map based on long-term rain intensity measurements at various locations across Canada is used for the computation of rain attenuation in lieu of the in-force ITU-R database of rain rate, since this map reflects the precipitation intensity more accurately for our country, especially along the West Coast. The columnar water vapor content V and the columnar cloud liquid water content L are two key radiometeorological input parameters used extensively during the production of these maps. Annual cumulative distribution functions of V and L from the ITU-R database (derived from the ECMWF 40-year Reanalysis ERA-40) are compared to radiosonde or radiometric measurements at selected North American sites and over the tropical Pacific Ocean. I. Introduction Attenuation induced by rain, atmospheric gases (mainly oxygen and water vapor) and clouds on satellite links operating at Q/V-band generally increases compared to that observed at C, Ku, and Ka bands. Although, depending on the link geometry, the magnitude of rain attenuation will be in general the most important, it could be assumed that through the use of fade mitigation techniques, Q/V-band links may operate in the presence of at least lowintensity rain. No propagation campaign in this band has been conducted in Canada yet. However, attenuation levels across the Canadian territory can be estimated using existing propagation models, like those recommended by the ITU-R for the prediction of statistics of attenuation due to rain, gases and clouds, and refractive fading due to scintillation. Fundamental physical input parameters used for the estimation of rain and cloud attenuation and gaseous absorption are the rain rate exceeded during 0.01% of an average year RR0.01%, the columnar cloud liquid water content L, and the columnar water vapor content V, respectively. As part of this investigation, we compare the value of these parameters provided in ITU-R databases with measurements conducted mainly in Canada but also elsewhere, in order to assess their level of agreement. The wet term of the surface refractivity, used by the model recommended by the ITU-R to predict scintillation fade, has not been evaluated in this study. Previous analysis of rain rate and rain-attenuation data collected at Ottawa and Vancouver during propagation measurements using the Ka-band beacon signals of NASA’s ACTS satellite had shown that the rain rate model in Rec. ITU-R P.837 substantially overestimated values collected in Vancouver. A more extensive test of the model using rain rate distributions derived from measurements at 47 locations across Canada led to the development of a new empirical model for RR0.01%, presented in Section II. The next two sections (III and IV) focus at some length on the two integrated quantities, the columnar water vapor content (V) and the columnar cloud liquid content (L), respectively. Section V briefly presents the method recommended by the ITU-R to statistically combine the contributions to total attenuation of rain, gases, clouds and scintillation. The method is applied to develop attenuation maps for Canada at 40 and 50 GHz. A map developed at 50 GHz is presented and discussed. Finally, total attenuation results are used to estimate path availability in the down- and uplink assuming fade margins of 12 and 14 dB, respectively.