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

2. 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.

3. 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

4. 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
1.121
1.075 6
1.308
1.265 8
1.327
1.310 10
0.0101
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 and rainfall of 40mm/h, when vertical polarization is used
A: gR = 0.94dB/km

5. 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

6. 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

7. ITU-R P.618 (Steps 3-6)
Step 3. Calculate horizontal projection, LG of the slant path length from
Step 4. Obtain rainfall rate R 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

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

9. 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)