What is VSAT Satellite Link Rain Fade?

VSAT rain fade is the deterioration of the microwave RF signal levels caused by rain precipitation on either end of the satellite VSAT link. Typically, Ku and Ka band links are affected most (frequencies above 11GHz) and can occur at the VSAT remote or the Teleport end of the link. RF energy is absorbed and scattered by the rain droplets and affects the higher frequencies more because of the wavelength of the signal and the size and shape of the rain droplet. Cross polarization isolation will also be reduced.

Line of Sight

Rain storms many km away from the antenna could still have a detrimental effect, especially if the elevation angle is quite low and the storm is in the line of sight of the dish. The rain fade can last for prolonged periods of time.

Satellite Dish

Water, snow and Ice on the surface of the satellite dish and RF feed assembly will also cause a decrease in signal levels by approx. 5-15%. A Radome will protect the VSAT electronics from the weather and corrosion but when it rains the water on the surface of the Radome will attenuate the signal.

Clouds

Clouds are made from water vapour, right? Clouds can cause significant loss on high frequency satellite links, the typical attenuation at Zenith (0° Elevation) it is only a few dB below 100 GHz.

Satellite Rain Fade Solutions

Possible ways to overcome the effects of rain fade are;

1. Satellite Diversity – have multiple satellites available that the remote can switch to in the event of rain fade.
2. Uplink Power Control (UPC) – The hub can control the transmit power of a remote site to compensate and to keep the remote VSAT site in the network.
3. Adaptive Coding and Modulation (ACM) – This is a technology that can automatically changes the modulation  type and error correction (MODCOD) used on the link to make it more robust.
4. Hydrophobic Coatings – The dish and Radome can have a coating that actively repels water droplets.
5. C-Band VSAT Communications – This frequency range is less effected by rain fade (Uplink 5.925-6.425 GHz and Downlink 3.7-4.2 GHz).
6. Link budgets – Rain fade margins should be calculated when designing the system and power levels.
7. Antenna Heated Blankets – Heating the satellite dish will prevent the build up of snow and ice.