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• A detail study of rain fade at Ka band over tropical region has been done. • Performance comparison of experimental link at 20/30 GHz with ITU-R model done. • "s" parameter relating fade slope to filter characteristics is calculated. Dynamic characteristics of Ka band rain fade (or rain attenuation) over tropical region are relatively less studied, though they are essential for implementation of suitable fade mitigation techniques. In this paper, dynamic characteristics of rain fade at 20 GHz and 30 GHz from GSAT-14 satellite are reported with experimental measurements at Ahmedabad, India. ITU-R model for fade slope probability distribution has been analyzed in this paper and Ka band rain fade has been investigated based on this fade-slope analysis. The ITU-R model is found to closely match 20 GHz measurements, though differences increase at 30 GHz. Fade slopes are found to be within ±0.5 dB/s at both 20 and 30 GHz. Both positive and negative slopes of the fade slope distributions are quasi-similar. ITU-R model's s parameter, relating fade slope to filter characteristics, is found to be very close to the measured value for this region. Distributions of fade slopes are also found to be dependent on the standard deviation values similar to ITU-R recommendation at 20 GHz, but significant deviation is observed at 30 GHz. This observed frequency dependency might be due to increase in cloud attenuation at high frequencies or may be due to limited data set. Results indicate needs of further experimental data at high frequencies from other tropical regions to gain further insight into the fade slope characteristics. This will ultimately help in designing a better prediction model in future. [ABSTRACT FROM AUTHOR]

This paper investigates the variability of raindrop size distribution (DSD) and rain integral parameters at Ahmedabad, a tropical location, in relation to the radar estimation of rainfall. Rain DSDs for the years 2006–2007 at Ahmedabad (23°04′N, 72°38′E) have been measured using a disdrometer. Variability of DSD is evaluated for different seasons and its effect on the integral rain parameters like radar reflectivity, rainfall intensity and attenuation are examined. A percentage contribution of different drop diameters on rain integral parameters is studied to understand the seasonal behaviour of rain attenuation and radar reflectivity. It is observed that drops with diameter around 3 mm contribute maximum to the radar reflectivity while drops having a diameter around 2 mm contribute the maximum to the rainfall intensity for the present location. The critical diameter range responsible for the maximum contribution in rain attenuation found to shift towards large drops with an increase in rain rate for a fixed frequency. Linear and non-linear regression analysis between radar reflectivity and rainfall intensity show significant variations in different seasons but does not differ much for different regression techniques. Results point to the necessity of considering the seasonal variability of rain DSD in radar remote sensing and will be helpful for better characterizing of rain parameters from radar measurements. [ABSTRACT FROM AUTHOR]

Abstract: The microstructure of rain has been studied with observations using a vertical looking Micro Rain Radar (MRR) at Ahmedabad (23.06°N, 72.62°E), a tropical location in the Indian region. The rain height, derived from the bright band signature of melting layer of radar reflectivity profile, is found to be variable between the heights 4600m and 5200m. The change in the nature of rain, classified on the basis of radar reflectivity, is also observed through the MRR. It has been found that there are three types of rain, namely, convective, mixed and stratiform rain, prevailing with different vertical rain microstructures, such as, Drop Size Distribution (DSD), mean drop size, rain rate, liquid water content and average fall speed of the drops at different heights. It is observed that the vertical DSD profile is more inhomogeneous for mixed and stratiform type rain than for convective type rain. It is also found that the large number of drops of size <0.5mm is present in convective rain whereas in stratiform rain, drops concentration is appreciable up to 1mm. A comparison of measurements taken by ground based Disdrometer and that from the 200m level obtained from MRR shows good agreement for rain rate and DSD at smaller rain rate values. The results may be useful for understanding rain structures over this region. [Copyright &y& Elsevier]

• Studies on fade duration and fade slope are done at 20.2 GHz over tropical region. • Fade duration statistics are compared with ITU-R and Cheffena-Amaya Model. • Exponent of power law model of fade duration at Ka band is investigated. • A new model on parameters of fade duration statistics is proposed. Some second order rain attenuation statistics such as fade duration and fade slope are investigated on the basis of experimental measurements of received signals using the GSAT-14 satellite beacon signal at 20.2 GHz for three years (2014–2016) over the tropical location Ahmedabad (23.02 0E, 72.510N), India with an Elevation angle of 630. Existing models of fade duration are compared with experimental data in this study and exponent of power law model of fade duration at Ka band is further explored. A new model for fade duration for Ka band for tropical locations is proposed where the constant of exponent of attenuation in the power law is found to be 0.143 instead of 0.055 used in ITU-R. Other relevant parameters for implementation of fade mitigation technique to prevent the link outage like cumulative distribution of signal fade rate, maximum and minimum fade rise and fade fall are also studied. Fade slope asymmetry over tropical region is also investigated. Keeping in view of exploiting the commercial launch of Ka band in Indian region there is an urgent need for validation of the existing models of fade slope (specially looking into fade symmetry) and fade duration. It will help the SATCOM (Satellite Communication) link designer to improve closed loop fade mitigation technique to minimize the possible link failure/link outage over the tropical region. [ABSTRACT FROM AUTHOR]