ECE 5233 Satellite Communications Prepared by: Dr. Ivica Kostanic Lecture 14: Rain attenuation (Section 8.5-8.7) Spring 2014

Outline Specific attenuation ITU Method for prediction of rain attenuation Example Important note: Slides present summary of the results. Detailed derivations are given in notes.

Prediction o rain attenuation Two components that need to be predicted Specific attenuation – attenuation due to the rain per unit length of signal path Effective path length Approximate approximation for loss in dB Note: rain rate (and losses) vary along the path gR – specific attenuation Leff – effective path Simplifications: Rainfall measured at the earth surface is correlated to the rainfall along the path The actual path length can be adjusted to the effective path lengths

Specific attenuation Specified in ITU-R P838 Calculated at the rain rate encountered at 0.01% threshold Adjustment developed for other reliability thresholds General form of the equation Table of coefficients F (GHz) kH kV aH aV 4 0.00065 0.00059 1.121 1.075 6 0.00175 0.00155 1.308 1.265 8 0.00454 0.00395 1.327 1.310 10 0.0101 0.00887 1.276 1.264 12 0.0188 0.0168 1.217 1.200 20 0.0751 0.0691 1.099 1.065 30 0.187 0.167 1.021 1.000 40 0.350 0.310 0.939 0.929 50 0.536 0.479 0.873 0.868 Note 1: subscripts H and V stand for different polarizations Note 2: for non-tabulated frequencies, the values should be interpolated. Use log scale for k and linear scale for a Example. Calculate specific attenuation @10GHz and rainfall of 40mm/h, when vertical polarization is used A: gR = 0.94dB/km

Procedure for rain rate calculation (ITU-R 618) Semi-empirical approach Statistical summary of many years of measurements Prediction done for 0.01% reliability Adjusted for other reliability thresholds Algorithm inputs R0.01 – point rainfall rate exceeded in 0.01% of time mm/h) hs – height above mean sea level of the earth station (km) q - elevation angle (degrees) f – altitude of the earth station Re – effective radius of the earth (K = 4/3) f – frequency in GHz A: frozen precipitation B: rain height C: liquid precipitation D: earth-space path Reference geometry

ITU-R P.618 (Steps 1-2) Step 1: Calculate the height of the rain. In figure this is quantity h’R- same as h0 in ITU-R P.839 (posted on the web) Step 2: Compute the slant path length below the rain height as

ITU-R P.618 (Steps 3-6) Step 3. Calculate horizontal projection, LG of the slant path length from Step 4. Obtain rainfall rate R0.01. This rain rate is obtained either from local data, from standard exceedance curves, or from climate maps. Step 5. Calculate specific attenuation using coefficients for appropriate polarization and frequency (Recommendation ITU-R P.838) Step 6. Calculate horizontal reduction factor, r0.01 for 0.01% of time

ITU-R P.618 (Step 7) Step 7. Calculate vertical adjustment factor, v0.01, for 0.01% of time Where:

ITU-R P.618 (Steps 8-10) Step 8. The effective path length is Step 9. Predicted attenuation for 0.01% of time Step 10. Adjustment for other percentages of time (p)