RAIN FADE CALCULATIONS Introduction Rain fade is the dominant factor in path loss variation above 10GHz, and can have an effectbelowthatfrequencyatlowelevations.Thisshortnotedescribesonofthe ITU-Rmethodsforcalculatingrainfade.Itcanalsobeusetorelatetherequired availabilityofsatellitecircuitstothefademarginthatmustbeincludedinthelink budget. Fain fades vary with frequency, location, polarisation and rainfall rate.The depth of fade in dB can be calculated from: LRAIN = R DRAIN Where: LRAIN is the rain loss in dB R is the specific attenuation (dB/Km) DRAIN is the path length through the troposphere in Km, To calculate the rain attenuation we need to know: Latitude and longitude of the earth station to within a degree. Altitude of the station in Km. The frequency of operation The polarisation of the signal. The required availability of the satellite circuit. Determining DRAIN DRAINiseffectivelytheslantrangeoftheportionofthesignalthatliesbelowthe freezingpoint(zerodegreeisotherm)intheatmosphere.Theassumptionisthatall rain originates at this level. hANTENNAhRAINRain height(zero degree isotherm)elevationangle e AntennaheightDatumDRAIN DRAIN can be calculated from simple trigonometry from the above diagram. ) () (e Sinh hDANTENNA RAINRAIN= ThisimpliesknowledgeoftherainheighthRAIN.ITU-RRecommendationP.839 relates rain height to location.It is reproduced at table 1. Latitude hRAINRegion > 23 N5 - 0.075 (- 23)Nhemisphere(exceptNAmerica& Europe) 0 23N5Nhemisphere(exceptNAmerica& Europe) 3.2-0.075 ( - 35)NhemisphereNAmerica&Europe west of 60E Longitude 0 21S5S hemisphere 21S > 71S5 + 0.1( +21)S hemisphere 71S > S hemisphere Table 1 From this we can calculate hRAIN, and through that DRAIN. Determining R InordertodetermineRitisfirstnecessarytoidentifytherainfallregionfromthe attachedmaps(Appendix1).Table2linkstherainfallratetothepercentageofthe time it is exceeded in any year by rainfall region. ZonePercentage of time R exceeded ABCDEFGHJKLMNPQ 1.0