Radiocommunication Study Groups
Received: 8 June 2013
Source:
Document 3J/47-E
Document 3M/98-E
11 June 2013
English only
Recommendation ITU-R P.618-10
China (People’s Republic of)
SUPPORTING DOCUMENT FOR IMPROVEMENT OF RAIN ATTENUATION
PREDICTION METHOD IN RECOMMENDATION ITU-R P.618-10
More analysis on anomalies observed in the Earth-Space
rain attenuation prediction models
1
Introduction
Document 3J/50 submitted by ESA and Italy in 2008 described an “anomalous” behavior of the rain
attenuation model in Recommendation ITU-R P.618-10 for low latitudes (between 36° S and 36° N)
and low elevation angles (below 25°). As mentioned in Annex 8 to Working Party 3M Chairman’s
Report in 2012, the source of the behaviour has been identified as the  parameter of equation (8)
in Recommendation ITU-R P.618-10. This behaviour may affect the prediction for Earth-space
telecommunication systems or Earth-observation systems using non-GEO satellites.
Document 3M/17 submitted by Brazil in 2003 described the dependence of rain attenuation with
elevation angle in Recommendation ITU-R P.618-10. The rain attenuation decreases up to an
elevation angle around 60, however, for higher angles the attenuation increases showing a possible
physical inconsistence of the prediction model. It is another “anomalism” behavior of the rain
attenuation model in Recommendation ITU-R P.618-10. This may cause overestimate of rain
attenuation for links with high elevation angles, particularly for low latitudes countries, where high
elevation angles are used quite often.
In recent years, several rain attenuation prediction models have been presented to ITU-R SG 3 for
modification of rain attenuation prediction model in Recommendation ITU-R P.618. These models
have been tested with ITU-R SG 3 databank. They are the model in Document 3M/28 submitted by
United Kingdom in 2003 (UK model). Three prediction models in Documents 3M/107, 3M/25 and
3M/31 submitted by China in 2005 (China_2005 model), 2008 (China_2008 model) and 2012
(China_2012 model), respectively. The former one is based on the cylindrical rain cell and the latter
two are based on the EXCELL rain cell. The prediction model in Document 3M/27 submitted by
Brazil in 2008 based on the concept of an effective rain cell (Brazil model). A new effective pathlength model for rain-attenuation prediction in Document 3M/45 has proposed by Korea in 2012
(Korea model) based on rain cell measurements by radar.
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In this document, the dependences of rain attenuation with elevation angle for both these models
and data in the ITU-R DBSG 3 databank are analysed. The main results are as follows:
1)
The measured rain attenuation data in the ITU-R DBSG 3 databank have decrease trend
as the elevation angle increase monotonously.
2)
Most of the models have the similar behaviour as the model in Recommendation ITU-R
P.618 except the China_2012 model and Korea model (for R ≥ 20 mm/h),wherethe rain
attenuation decreases up to an elevation angle, then increases for higher elevation angle,
this elevation angle change from about 25 to 75 for different models.
3)
4)
The rain attenuation predicted by Korea model (for R≥20mm/h)increase as the
elevation angle increase monotonously, which have an opposite variation trend as the
measured rain attenuation data in the ITU-R DBSG 3 databank.
The rain attenuation predicted by China_2012 model decrease as the elevation angle
increase monotonously, and has the same variation trend as the measured rain
attenuation data in the ITU-R DBSG 3 databank.
The “anomalous” behaviours of Recommendation ITU-R P.618-10 described in both input
Document 3M/17 of Study Period 2002-2007 and Document 3J/50 of Study Period 2007-2012
results in overestimate of rain attenuation for low latitudes (between 36° S and 36° N) and low
elevation angles (below 25°), and also for high elevation angles (above 60°), The behaviours may
affect the planning of Earth-space systems for using GEO and non-GEO satellites. The rain
attenuation prediction model in Recommendation ITU-R P.618-10 should be improved as soon as
possible.
2
Dependence of predicted rain attenuation with elevation angle for
different models
Input Document 3M/17 of Study Period 2002-2007 shows that the rain attenuation predicted by
Recommendation ITU-R P.618-10 do not decreases with the elevation growing up monotonously.
The rain attenuation predicted in Recommendation ITU-R P.618-10 decreases up to an elevation
angle around 60, however, for higher angles the attenuation increases. In recent years, several rain
attenuation prediction models have been presented to ITU-R SG 3 for modification of rain
attenuation model in Recommendation ITU-R P.618, the variations of the rain attenuation with the
elevation predicted by these models are analysed. The results can be seen in Figure 1-Figure 7.
The variations of rain attenuation with the elevation are shown in these figures for a slant path
characterized by the following parameters:
–
Percentage time of an average year of rain attenuation: 0.01%
–
Precipitation rate for 0.01% of time: 88mm/h (for Korea model, another rain rate is
added)
–
Latitude of the earth station: 22N
–
–
–
–
Longitude of the earth station: 114W
Rain height: 5.21km
Frequency: 30 GHz
Polarization: Horizontal
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FIGURE 1
Variation of the ITU-R model with the elevation
85
Attenuation（ dB）
80
75
70
65
60
55
20
30
40
50
60
Elevation Degree（ ）
70
80
FIGURE 2
Variation of the China_2005 model with the elevation
64
62
60
Attenuation（ dB）
58
56
54
52
50
48
46
44
20
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40
50
60
Elevation Degree（ ）
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FIGURE 3
Variation of the China_2008 model with the elevation
80
Attenuation（ dB）
75
70
65
60
55
50
45
10
20
30
40
50
60
Elevation Degree（ ）
70
80
FIGURE 4
Variation of the China_2012 model with the elevation
80
75
Attenuation（ dB）
70
65
60
55
50
45
20
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30
40
50
60
Elevation Degree（ ）
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FIGURE 5
Variation of the UK model with the elevation
70
Attenuation（ dB）
65
60
55
50
45
20
30
40
50
60
Elevation Degree（ ）
70
80
FIGURE 6
Variation of the Brazil model with the elevation
85
80
Attenuation（ dB）
75
70
65
60
55
50
45
40
35
20
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30
40
50
60
Elevation Degree（ ）
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FIGURE 7
Variation of effective length in Korea model with elevation
8
R=15mm/h
7.5
Effective Length
7
6.5
6
5.5
5
4.5
10
20
30
40
50
60
Elevation Degree（ ）
(a). R  20mm / h
70
80
14
R=88mm/h
Effective length(km)
12
10
8
6
4
2
20
30
40
50
60
Elevation Degree（ ）
(b). R  20mm / h
70
80
It can be seen from Figure 1, Figure 5, Figure 6 and Figure 7(a) , that the UK model , Brazil model
and Korea model (for R＜20mm/h) have similar behavior as the model in Recommendation ITU-R
P.618. That is, the rain attenuation decrease as the elevation grows up to an elevation angle which
changes from about 25 to 75 for different models, for higher angles the attenuation increases. As
shown in Figure 2 and Figure 3, the variation of China_2005 model and China_2008 model with the
elevation have also similar behaviour as Recommendation ITU-R P.618, and is more complicated
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than other models. Figure 7 (b) presents the variation of effective length in the Korea model (for
R ≥ 200 mm/h) with elevation. It can be seen that the effective length increase monotonously as the
elevation increasing, that means that rain attenuation have same behaviour.
Figure 4 presents the variation of the China_2012 model with the elevation. It can be seen that the
predicted rain attenuation decrease monotonously as the elevation increasing.
3
Rain attenuation data analysis
The conception of path adjustment factor is usually utilized to obtain effective path length. The rain
attenuation of the earth-space path can be calculated by the following equation and the geometry is
illustrated in Figure 8.
A   R LE  kR Ls r
(1)
 R (dB·km-1) is the specific attenuation which can be calculated by Recommendation ITU-R P.
838-3. LE (km) is the effective path length, Ls (km) is the slant-path length, r is the path adjustment
factor.
FIGURE 8
Schematic presentation of an Earth-space path
D
A
B
C
Ls
hR – hS
hR

hS
LG
（A: frozen precipitation，B: rain height，C: liquid precipitation，D: Earth-space path）
To make sure whether the anomalism of these prediction models with elevation is the inherent
characteristic of the rain attenuation, all rain attenuation data in ITU-R DBSG 3 is analysed. That is
to say all the flags for the data are ignored. The actual effective path can be expressed in following
from:
LE 
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A
(2)
R
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The variation of the effective path length with elevation and the variation of rain attenuation with
elevation are the same. The distribution of the effective path length with elevation obtained from all
rain attenuation data in ITU-R DBSG 3 is shown in Figure 9.
FIGURE 9
The relationship between effective path lengths with elevation
70
Effective length（ km）
60
50
40
30
20
10
0
0
20
40
60
Elevation Degree（ ）
80
It can be seen from Figure 9 that the effective path length have decrease trend as the elevation
increasing. That is to say the rain attenuation will decrease when the elevation increases. Then the
rain attenuation estimated by the prediction model should have the same variation as the data shown
in Figure 9 if the model is proper. Unfortunately, most of these models except the China_2012
model have different variations in the high elevation range.
4
Conclusion
The variation of rain attenuation with elevation analysed with all rain attenuation data in ITU-R
DBSG 3 shows that the rain attenuation should decrease monotonously as the elevation increases,
but most of models submitted to SG 3 in recent years and in Recommendation ITU-R P.618 except
the China_2012 model have different behaviours. The rain attenuation predicted by the China_2012
model decrease as the elevation increases, which is agreement with the ITU-R measured data.
5
References
[1]
ITU-R new database on earth-satellite links,
http://saruman.estec.esa.nl/dbsg3/categories.jsp?category=annualRainAttStat
[2]
Document 3M/17, comments on Recommendation ITU-R P.618-7: Propagation data
and prediction methods required for the design of earth-space telecommunication
systems, Brazil, 2003.
[3]
Document 3M/29, Proposed modification to Recommendation ITU-R P.618-8: An
improved model of rain attenuation prediction along Earth-space path, UK, 2003.
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[4]
Document 3M/108, Proposed modification to Recommendation ITU-R P.618-8: An
improved model of rain attenuation prediction along Earth-space path, China, 2005.
[5]
Document 3M/27, Prediction of Rain Attenuation in Slant Paths and Terrestrial Links
Using the Full Rainfall Rate Distribution, Brazil, 2008.
[6]
Guidelines for testing Earth-Space prediction methods, 2009.
[7]
Document 3M/25, Modelling and Prediction Methods of Rain Attenuation
Statistics,China,2008.
[8]
Annex 11 to WP 3M Chairman's Report Recommendation ITU-R P.618: Proposed
Modifications and Future work, 2004.
[9]
Annex 10 to Working Party 3M Chairman's Report Recommendation ITU-R P.618-9 Proposed Revisions and Future work, 2008.
[10]
Annex to WP 3M Chairman's Report Recommendation ITU-R P.618-9 Proposed
Revisions and Future work, 2009.
[11]
Annex 10 to Working Party 3M Chairman's Report Working Document towards a draft
revision of Recommendation ITU-R P.618-10, 2010.
[12]
Document 3M/31, Modification to rainfall rate adjustment factor for Modelling and
Prediction Methods of Rain Attenuation Statistics, China, 2012.
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