Modelling of Rainfall Drop Size Distributions for Application in Fade Prediction on Earth-Space Paths

Abstract

Future earth observation systems that continuously capture sub-metre resolution Earth imagery data spanning a wide swath must have the capability to transfer large volumes of data from the low earth orbiting satellite to a ground terminal during the short window of visibility between the two. This will require operation at Ka-band where there is sufficient allocated radio bandwidth (e.g. 25.5 GHz – 27 GHz) to support the required multi-gigabit per second downlink bit rate capacity. However, at Ka-band the downlink becomes highly susceptible to disruption by rain during each satellite pass when path elevation angles will range anywhere between 90 degrees and 20 degrees or less. The resulting rain attenuation will sometimes exceed what can be feasibly combated through link dimensioning to include a sufficient fixed fade margin. Adaptive fade mitigation will, therefore, be essential to ensure a reliable and robust downlink. The required knowledge of the instantaneous rain attenuation on the downlink could be gained by using ground-based measurement of rain rate and rainfall drop size distribution (DSD) at the earth station location.

This thesis fits the standard lognormal and gamma distributions to one-minute slices of rainfall DSDs captured between 2003 and 2013 at the Chilbolton Observatory in southern England. Unlike previous studies, goodness-of-fit of the models is tested by calculating chi-square goodness-of-fit for distributions fitted. Results show that failure to fit is greater than would normally be expected. This failure to fit is explored and broken down and examined against seasonal variations, different rain rates, atmospheric temperature and wind speed. This thesis investigates the occurrence of multimodality in the rainfall data observed by various researchers, as a possible explanation for the failure to fit. It proposes a Gaussian Mixture Model in this case, predicting the number of modes from rain rates and wind speeds.

Specific attenuation at corresponding instants is computed using the attenuation cross-section of rain drops and their size distributions at ground level. Both disdrometer-measured and standard-modelled DSDs are employed and the results are validated by comparison to the ITU specific attenuation for the beacon frequency. Results show that specific attenuation tends to increase with the drop sizes, and the smaller drops contribute little to the overall attenuation experienced by signals.

Effective rainy slant path length is defined as the ratio of the total instantaneous slant path rain attenuation and the specific attenuation. Assuming a latitude-dependent fixed rain height, the variability of instantaneous effective rainy slant path length with rain rate is modelled. From this length and the specific rain attenuation, the instantaneous total rain attenuation can be estimated. This in principle enables telecommunications service providers to provide better quality of service at reduced cost.

Date of Award	May 2017
Original language	English
Supervisor	Ifiok Otung (Supervisor) & Francis Hunt (Supervisor)