ece 5233 satellite communications prepared by: dr. ivica kostanic lecture 14: rain attenuation...

ECE 5233 Satellite Communications

Prepared by:

Dr. Ivica Kostanic

Lecture 14: Rain attenuation

(Section 8.5-8.7)

Spring 2014

Florida Institute of technologies

Specific attenuation

ITU Method for prediction of rain attenuation

Example

Outline

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

o Specific attenuation – attenuation due to the rain per unit length of signal path

o Effective path length

Approximate approximation for loss in dB

effLA R

gR – specific attenuation

Leff – effective path

Simplifications:

1. Rainfall measured at the earth surface is correlated to the rainfall along the path

2. The actual path length can be adjusted to the effective path lengths

L

R LdllRfA0

eff

Note: rain rate (and losses) vary along the path


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

dB/km01.0 RkR

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

Table of coefficients

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

Reference geometryA: frozen precipitationB: rain heightC: liquid precipitationD: earth-space path

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

Algorithm inputs


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

5,

sin2

sin

2

5,

sin

'2

'

'

e

SR

sR

SR

s

R

hh

hh

hh

L



Step 3. Calculate horizontal projection, LG of the slant path length from

cosSG LL

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

GRG L

f

Lr

2exp138.078.01

101.0


ITU-R P.618 (Step 7)

Step 7. Calculate vertical adjustment factor, v0.01, for 0.01% of time

degtan01.0

'1

rL

hh

G

sR

,sin

,cos

'

01.0

SR

G

R

hh

rL

L

45.0131sin1

1

21/

01.0

f

Le

vRR

Where:

36,36

36,0



Step 8. The effective path length is

01.0vLL RE

Step 9. Predicted attenuation for 0.01% of time

ER LA 01.0

Step 10. Adjustment for other percentages of time (p)

otherwisesin25.48.1360.005-

25 and36 and%136005.0

36or%1,0

p

p

dB01.0

sin1ln045.0ln033.0655.0

01.0

01.0 pAp

p

pAA

ece 5233 satellite communications prepared by: dr. ivica kostanic lecture 14: rain attenuation...

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