| || || Prasad, Ashneel Avishek.|
| || || Investigation of the effects of cloud cover on Ku-Band satellite link and earth's potential gradient |
Author:Prasad, Ashneel Avishek.
Institution: University of the South Pacific.
Call No.: Pac QC 924 .F5 P73 2009
Copyright:Under 10% of this thesis may be copied without the authors written permission
Abstract: This thesis investigates the effect of cloud cover on one of the universally present atmospheric parameter, namely the Earth’s vertical electric field or the Potential Gradient (PG), close to the surface of the Earth and the effect of cloud cover on satellite down link. The measurement site was The University of the South Pacific (18.13°S, 178.47°E). As cloud cover directly affects the solar insolation, it was used as one of the parameters to distinguish between fair-weather and cloudy days. Continuous record of the PG for the period 1st June 2005 to 30th June 2006 and simultaneous records of solar insolation during this period were analyzed. Days with peak insolation > 800 W/m2 and having the expected bell shaped variation of solar insolation were considered as fair-weather days, and on this basis, 31 days were classified as fair-weather days. In analyzing the PG values, records which fell in the range 0 – 1000 V/m were considered. The average diurnal variation of PG showed a bimodal oscillation with a dominant peak in the morning, close to 7 am LT. The evening peak coincided with the Carnegie peak and the daily average PG was ~ 125 V/m. The relatively low value of this average value indicates that the measurement site is relatively “clean” during fair-weather days with little effect from atmospheric aerosols. For the period of study, another 31 days showed the bell shaped variation in solar insolation but with decreased peak values and were classified as cloudy days. Assuming the same criteria used for the selection of PG data on fair-weather days, the daily averages of PG for all these days were found to be significantly less than 125 V/m. As the solar insolation and the PG varies with time of the day, their magnitudes compared to the average fair-weather day values are further analyzed to find the correlation between cloud cover and the surface PG. Observations clearly show that the percentage reduction in PG is nearly proportional to the intensity of the cloud cover. A possible reason for this dependence of PG on cloud cover is discussed. To study the effect of cloud cover on satellite down links, the Ku-band signal centered at 12.648 GHz from Intelsat 701 was monitored. Simultaneous measurements of RF signal strength and solar insolation were made for 3 months (29th August to 26th October 2007) continuously. During this period, 11 days were classified as fair weather days. Contrary to the expectation that the RF signal strength should remain constant, it exhibited variation (~ 61 dBm – ~ 68 dBm) in addition to the scintillations. The signal remained relatively high in the nighttime hours compared to the daytime hours. During the period of study, 37 days were classified as cloudy days, records on these days again showed a diurnal variation with a mean strength of ~ 50 dBm. Further analysis of the cloudy day records showed a nearly linear dependence of the percentage reduction in RF signal strength to the intensity of the cloud cover. This reduction is attributed to the increased scattering loss by cloud droplets.