Rain fade

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Rain fade refers primarily to the absorption of a microwave radio frequency (RF) signal by atmospheric rain, snow, or ice, and losses which 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. 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% 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. Rain fade is not limited to satellite uplinks or downlinks, as it can also affect terrestrial point-to-point microwave links (those on the earth's surface).

Microwave form of electromagnetic radiation

Microwaves are a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter; with frequencies between 300 MHz (1 m) and 300 GHz (1 mm). Different sources define different frequency ranges as microwaves; the above broad definition includes both UHF and EHF bands. A more common definition in radio engineering is the range between 1 and 100 GHz. In all cases, microwaves include the entire SHF band at minimum. Frequencies in the microwave range are often referred to by their IEEE radar band designations: S, C, X, Ku, K, or Ka band, or by similar NATO or EU designations.

Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around twenty thousand times per second to around three hundred billion times per second. This is roughly between the upper limit of audio frequencies and the lower limit of infrared frequencies; these are the frequencies at which energy from an oscillating current can radiate off a conductor into space as radio waves. Different sources specify different upper and lower bounds for the frequency range.

Rain liquid water in the form of droplets that have condensed from atmospheric water vapor and then precipitated

Rain is liquid water in the form of droplets that have condensed from atmospheric water vapor and then become heavy enough to fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides suitable conditions for many types of ecosystems, as well as water for hydroelectric power plants and crop irrigation.

Contents

Rain fade is usually estimated experimentally and also can be calculated theoretically using scattering theory of rain drops. Raindrop size distribution (DSD) is an important consideration for studying rain fade characteristics. [1] Various mathematical forms such as Gamma function, lognormal or exponential forms are usually used to model the DSD. Mie or Rayleigh scattering theory with point matching or t-matrix approach is used to calculate the scattering cross section, and specific rain attenuation. Since rain is a non-homogeneous process in both time and space, specific attenuation varies with location, time and rain type.

The raindrop size distribution (DSD), or granulometry of rain, is the distribution of the number of raindrops according to their diameter (D). Three processes account for the formation of drops: the accumulation of small drops on large drops and collisions between sizes. According to the time spent in the cloud, the vertical movement in it and the ambient temperature, the drops that have a very varied history and a distribution of diameters from a few micrometers to a few millimeters.

Total rain attenuation is also dependent upon the spatial structure of rain field. Horizontal, as well as vertical, extension of rain again varies for different rain type and location. Limit of the vertical rain region is usually assumed to coincide with 0˚ isotherm and called rain height. Melting layer height is also used as the limits of rain region and can be estimated from the bright band signature of radar reflectivity. [2] The horizontal rain structure is assumed to have a cellular form, called rain cell. Rain cell sizes can vary from a few hundred meters to several kilometers and dependent upon the rain type and location. Existence of very small size rain cells are recently observed in tropical rain. [3]

Possible ways to overcome the effects of rain fade are site diversity, uplink power control, variable rate encoding, and receiving antennas larger than the requested size for normal weather conditions.

Site diversity is one of six techniques used to improve the reliability of satellite communications by limit atmospheric effects, particularly those caused by rain fade. A diversity scheme is typically required when using frequencies in the Ka, V, or W-band. The downlink transmissions of satellites cover very large areas, that will have different weather. The site diversity technique consists of linking two or more ground stations receiving the same signal: this way, if the signal is heavily attenuated in one area, another ground stations can compensate it. These intense rain areas, for example, supercells, often have a horizontal length of no more than a few kilometers: putting the ground stations at a sufficient distance the possibility of rain fade in the downlink signal will be reduced.

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 limited use, since it required more powerful transmitters - ones that could normally run at lower levels and could be increased in power level on command (i.e. automatically). Also uplink power control could not provide very large signal 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.

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 80 GHz 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 (23 GHz+) 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 using the CCIR interpolation formula: [4]

Ap = A001 0.12 p(0.546 0.0043 log10p).

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, [5] rain attenuation statistics can be scaled in frequency in the range 7 to 55 GHz by the formula

where

and f is the frequency in GHz.

See also

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Communication channel physical transmission medium such as a wire, or logical connection

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Ultra high frequency The range 300-3000 MHz of the electromagnetic spectrum

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Satellite dish antenna for TV and radio reception

A satellite dish is a dish-shaped type of parabolic antenna designed to receive or transmit information by radio waves to or from a communication satellite. The term most commonly means a dish used by consumers to receive direct-broadcast satellite television from a direct broadcast satellite in geostationary orbit.

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Unified S-band

The Unified S-band (USB) system was a tracking and communication system developed for the Apollo program by NASA and the Jet Propulsion Laboratory (JPL). It operated in the S band portion of the microwave spectrum, unifying voice communications, television, telemetry, command, tracking and ranging into a single system to save size and weight and simplify operations. The USB ground network was managed by the Goddard Space Flight Center (GSFC). Commercial contractors included Collins Radio, Blaw-Knox, Motorola and Energy Systems.

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C band (IEEE) 4-8GHz

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References

  1. Das, Saurabh; Maitra, Animesh; Shukla, Ashish K. (2010). "PIER B Online - Rain Attenuation Modeling in the 10-100 GHz Frequency Using Drop Size Distributions for Different Climatic Zones in Tropical India". Progress in Electromagnetics Research B. 25: 211–224. doi:10.2528/PIERB10072707.
  2. Das, Saurabh; Maitra, Animesh; Shukla, Ashish K. (2011-07-01). "Melting layer characteristics at different climatic conditions in the Indian region: Ground based measurements and satellite observations". Atmospheric Research. 101 (1–2): 78–83. doi:10.1016/j.atmosres.2011.01.013.
  3. Shukla, Ashish K.; Roy, Bijoy; Das, Saurabh; Charania, A. R.; Kavaiya, K. S.; Bandyopadhyay, Kalyan; Dasgupta, K. S. (2010-02-01). "Micro rain cell measurements in tropical India for site diversity fade mitigation estimation". Radio Science. 45 (1): RS1002. doi:10.1029/2008RS004093. ISSN   1944-799X.
  4. CCIR [1990] Report 564-4 "Propagation data and prediction methods required for earth-space telecommunication systems"
  5. “Propagation Data and Prediction Methods Required for the Design of Earth-Space Telecommunication Systems,” Recommendations of the ITU-R, Rec. P.618-10, 2009.