Rain fade
Encyclopedia
Rain fade refers primarily to the absorption of a microwave
radio frequency
(RF) signal by atmospheric rain, snow or ice, and losses are especially prevalent at frequencies above 11 GHz
. It also refers to the degradation of a signal caused by the electromagnetic interference
of the leading edge of a storm front. Rain fade can be caused by precipitation at the uplink or downlink location. However, it does not need to be raining at a location for it to be affected by rain fade, as the signal may pass through precipitation many miles away, especially if the satellite dish
has a low look angle. From 5 to 20 percent of rain fade or satellite signal attenuation may also be caused by rain, snow or ice on the uplink or downlink antenna reflector, radome or feed horn.
Possible ways to overcome the effects of rain fade are site diversity
, uplink power control, variable rate encoding, receiving antennas larger than the requested size for normal weather conditions, and hydrophobic coatings. Only superhydrophobic, Lotus effect
surfaces repel snow and ice.
the CCIR interpolation formula:
where Ap is the attenuation in dB exceeded for a p percentage of the time and A001 is the attenuation exceeded for 0.01% of the time.
where
and f is the frequency in GHz.
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
radio frequency
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
(RF) signal by atmospheric rain, snow or ice, and losses are especially prevalent at frequencies above 11 GHz
GHZ
GHZ or GHz may refer to:# Gigahertz .# Greenberger-Horne-Zeilinger state — a quantum entanglement of three particles.# Galactic Habitable Zone — the region of a galaxy that is favorable to the formation of life....
. It also refers to the degradation of a signal caused by the electromagnetic interference
Electromagnetic interference
Electromagnetic interference is disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit...
of the leading edge of a storm front. Rain fade can be caused by precipitation at the uplink or downlink location. However, it does not need to be raining at a location for it to be affected by rain fade, as the signal may pass through precipitation many miles away, especially if the satellite dish
Satellite dish
A satellite dish is a dish-shaped type of parabolic antenna designed to receive microwaves from communications satellites, which transmit data transmissions or broadcasts, such as satellite television.-Principle of operation:...
has a low look angle. From 5 to 20 percent of rain fade or satellite signal attenuation may also be caused by rain, snow or ice on the uplink or downlink antenna reflector, radome or feed horn.
Possible ways to overcome the effects of rain fade are site diversity
Site diversity
Site diversity is a technique to limit the effect of rain fade in satellite communications, mainly in the Ka band. The downlink transmissions of satellites cover very large areas, that will have different weather...
, uplink power control, variable rate encoding, receiving antennas larger than the requested size for normal weather conditions, and hydrophobic coatings. Only superhydrophobic, Lotus effect
Lotus effect
The lotus effect refers to the very high water repellence exhibited by the leaves of the lotus flower ....
surfaces repel snow and ice.
Uplink power control
The simplest way to compensate the rain fade effect in satellite communications is to increase the transmission power: this dynamic fade countermeasure is called uplink power control (UPC). Until more recently, uplink power control had a limited use since it could not provide very large margins without compressing the transmitting amplifier. Modern amplifiers coupled with advanced uplink power control systems that offer automatic controls to prevent transponder saturation make uplink power control systems an effective, affordable and easy solution to rain fade in satellite signals.Parallel fail-over links
In terrestrial point to point microwave systems ranging from 11 GHz to 80 GHz, a parallel backup link can be installed alongside a rain fade prone higher bandwidth connection. In this arrangement, a primary link such as an 80GHz 1 Gbit/s full duplex microwave bridge may be calculated to have a 99.9% availability rate over the period of one year. The calculated 99.9% availability rate means that the link may be down for a cumulative total of ten or more hours per year as the peaks of rain storms pass over the area. A secondary lower bandwidth link such as a 5.8 GHz based 100 Mbit/s bridge may be installed parallel to the primary link, with routers on both ends controlling automatic failover to the 100 Mbit/s bridge when the primary 1 Gbit/s link is down due to rain fade. Using this arrangement, high frequency point to point links (23GHz+) may be installed to service locations many kilometers farther than could be served with a single link requiring 99.99% uptime over the course of one year.CCIR interpolation formula
It is possible to extrapolate the cumulative attenuation distribution at a given location by usingthe CCIR interpolation formula:
- Ap = A001 0.12 p-(0.546 - 0.0043 log10 p).
where Ap is the attenuation in dB exceeded for a p percentage of the time and A001 is the attenuation exceeded for 0.01% of the time.
ITU-R frequency scaling formula
According to the ITU-R, rain attenuation statistics can be scaled in frequency in the range 7 to 55 GHz by the formulawhere
and f is the frequency in GHz.