25 Feb Does snow damage your Twoobii signal?
The coverage of Twoobii is based on the footprint of IS33e (EPIC) and covers the whole of Africa.
The type of climate areas of the coverage ranges from sub-tropical to desert like environments. There are however, certain areas that will experience snow, especially the Lesotho Highlands and the Drakensberg.
The report will evaluate the effect of snow on the RF signal of a Twoobii remote site.
Attenuation properties of Snow
Very little information on the attenuation coefficient of snow at General Spectrum Frequency (4GHz to 32GHz to cover C-Band to Ka-Band frequencies) is available to quantitatively define the attenuation effect of snowfall as has been done with Rain Fade calculations.
However, research has found several references to the negligible effects of snowfall on the signal levels. Eg. according to Satellite Communications Fourth Edition by Dennis Roddy,
“Hail, Ice and snow have little effect on attenuation because of the low water content”
He refers to the density of water per m³ or g/m³, whereby the general density of a rain drop at 2.5g/m³ produces around 0.2dB/km, a snow flake has a general density of 0.25g/m³ producing attenuation of 0.05dB/km. Some studies shows that the average density of a snow flake is around 0.18g/m³ resulting in much less attenuation per km.
Another study was performed in Ottawa, Canada, whereby the report assesses the attenuation induced by snowfall on a 20.2GHz signal over a 2 year period,
- Experimental assessment of snow-induced attenuation on an Earth-space link operating at Ka-band – by Cesar Amaya, Jose-Miguel Garcia-Rubia, Pierre Bouchard and Tu Nguyen
The conclusion of Research Article concluded that during the 2 year experiment, attenuation levels rarely exceeded 1.5dB to 2dB. This was on a 20.2GHz signal. The effects on 14GHz signal will be much less.
Extensive info on the attenuation effects on the Free-Space Optical Satellite links (FSO links) operating on 96GHz to 200GHz spectrum range is available, which is far from the RF frequency spectrum that the standard VSAT platform is using, indicating the effect of snowfall on 12GHz carriers is too small to consider.
However, the only time snow will become a considerable factor is when snow accumulates in the antenna or on the feed, in which case it will not attenuate the signal, but cause depolarization of the signal, causing CRC errors and packet loss. The offset angle of the 1.2m parabolic antenna and the relatively high look angle to IS33e will ensure the probability for snow to accumulate in the antenna is very low. If packet loss or intermittent service is experienced on site, then it would be a good idea to the check for the accumulation of snow on the antenna or feedhorn.