ITU-R methods for calculating rain fade. It can also be use to relate the required
availability of satellite circuits to the fade margin that must be included in the link.
RAIN FADE CALCULATIONS Introduction Rain fade is the dominant factor in path loss variation above 10GHz, and can have an effect below that frequency at low elevations. This short note describes on of the ITU-R methods for calculating rain fade. It can also be use to relate the required availability of satellite circuits to the fade margin that must be included in the link budget. Fain fades vary with frequency, location, polarisation and rainfall rate. The depth of fade in dB can be calculated from: LRAIN = γR DRAIN Where: LRAIN is the rain loss in dB γR 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 DRAIN is effectively the slant range of the portion of the signal that lies below the freezing point (zero degree isotherm) in the atmosphere. The assumption is that all rain originates at this level.
Rain height (zero degree isotherm)
DRAIN hRAIN e
elevation angle
hANTENNA
Datum
Antenna height
DRAIN can be calculated from simple trigonometry from the above diagram. DRAIN =
(hRAIN − h ANTENNA ) Sin(e)
This implies knowledge of the rain height hRAIN. ITU-R Recommendation P.839 relates rain height to location. It is reproduced at table 1.
Region N hemisphere (except N America & Europe) 5 N hemisphere (except N America & Europe) N hemisphere N America & Europe 3.2-0.075 (ϕ - 35) west of 60E Longitude 5 S hemisphere S hemisphere 5 + 0.1(ϕ +21) S hemisphere Table 1 hRAIN 5 - 0.075 (ϕ - 23)
Latitude ϕ ϕ > 23 N 0 ≤ ϕ ≤ 23N
0 ≥ ϕ ≥ 21S 21S > ϕ ≥ 71S 71S > ϕ
From this we can calculate hRAIN, and through that DRAIN.
Determining γR In order to determine γR it is first necessary to identify the rainfall region from the attached maps (Appendix 1). Table 2 links the rainfall rate to the percentage of the time it is exceeded in any year by rainfall region. Percentage of time R A exceeded
