Seasonal Variations of Rain Attenuation on Radio Propagation ... - URSI

Seasonal Variations of Rain Attenuation on Radio Propagation Paths in South Africa 1

1

M.O. Odedina, 2T.J. Afullo

School of Electrical, Electronic and Computer Engineering, University of KwaZulu-Natal, Durban, 4000, South Africa. Email: [email protected] 2

As (1) above, but E-mail: [email protected]

Abstract Seasonal statistics of rain attenuation are analyzed to investigate the characteristics and variations associated with the different seasons in various climatic zones in South Africa. A 5-year locally observed rain rate data in these climatic zones has been utilized to estimate the cumulative distributions of rain attenuation for the different seasons in each zones. From these, appropriate figures of fade margin were derived which gives the percentage of time for which a certain attenuation level is exceeded for different seasons in each climatic zone. A 1-year signal attenuation level recorded in Durban (coastal region) at 19.5 GHz on a 6.73 km path length was also used to determine the average attenuation for the year and these measurements show reasonable agreement with the results obtained with ITU-R models.

1. Introduction Attenuation caused by rain can be obtained directly through measurement or predicted from the knowledge of rain rate, drop-size distribution and other relevant parameters of the radio path [1]. Several attenuation prediction models proposed by various authors have been used to estimate rain attenuation in cases where adequate direct measurements are not available [1]. Rainfall being a natural and time-varying phenomenon is highly seasonal over most of southern Africa. South Africa which is located on the latitude 29o00'S and longitude 24o00'E in the most southern tip of the Africa continent [2] is climatically moderated by its surrounding oceans. The cold Benguela current that pushes northwards from the Antarctic along the Atlantic coastline runs up to the west coast, and a much warmer stream, the Agulhas current also moves southwards from the Indian Ocean which is the chief source of rain over most of the country. The Eastern seas' steady evaporation provides generous rainfall while the Benguela current retains its moisture to cause desert conditions in the west [3, 4]. South Africa has an average annual rainfall of 502 mm as against the world mean of 857 mm [3]. The climatic conditions in South Africa generally range from Mediterranean in the southwestern corner of the country to temperate in the interior plateau, and subtropical in the northeast. A small area in the northwest has a desert climate [5]. In a previous presentation, the cumulative distributions of rainfall rate have been estimated for different locations in South Africa using the rain rate statistics measured by the South Africa Weather Services for 5 years (2000-2004) [6]. The rain data was processed and converted to the ITU-R recommended 1-min integration time rain rate at 0.01% exceedance value for South Africa [6, 7]. This paper presents the seasonal cumulative distributions of rain attenuation for 4 major climatic zones in South Africa and these zones are: Coastal, Temperate, Mediterranean and Desert in which Durban, Pretoria, Cape Town and Brandvlei are located respectively. At this point, appropriate figures for fade margin are derived at different percentages of time for the different seasons in each climatic zone. Also, the signal attenuation measurement recorded in Durban at 19.5 GHz in 2004 was utilized to determine the average attenuation for the year

2. Seasonal Distributions of Rain Attenuation for Different Climatic Zones The seasonal cumulative distributions of rain attenuation in each climatic zone is estimated using the ITU-R global attenuation prediction model [8] and the 1-min integration time rain rate observed for a period of 5-years in each climatic zone in South Africa. South Africa can be classified into 4 seasons and they are; summer (midOctober to mid-February), autumn (mid-February to April), winter (May to July) and spring (August to midOctober) [4]. Most of the summer months are characterized by hot, sunny weather and the rainfall region are

associated with violent convection storms accompanied by thunder, lighting, and often hail [3, 4]. Most rainfall in South Africa normally comes in the summer with the exception of Western Cape with its Mediterranean climate that gets its rain in winter [4].The winter rain is often long lasting and not very intense except along the mountains, where the orographic effect may induce heavy showers. Between the winter and summer rainfall regions lies a transitional area, where rain comes in all seasons-that is, neither in winter nor in summer which is autumn and spring. This transitional area can be divided into two sub-regions: a southern coastal strip with annual total of 375 to 875 mm, and a drier inland corridor behind the coastal ranges with an annual total of 50 to 250 mm [2]. Using the ITU-R attenuation prediction model at 20 GHz [8], figs. 1 (a)-(d) were produced from the 5-year locally observed rain rate data from each climatic zone. The average seasonal cumulative distributions provide a good insight into the behavior of attenuation at different seasons in each zone. It can be seen that attenuation strongly depends on the season and most of the summer and autumn seasons provide the largest amount of attenuation except at the Mediterranean climatic zone which has most of its rain in winter and spring. The attenuation produced in the summer and autumn seasons reflect the high degree of precipitation in that season especially in Durban which lies along the coastal region of South Africa. As for Cape Town (Fig 1(b)), the highest attenuation is in spring and winter .The winter is seen to lie close to spring and overlaps at 3.76dB. The lowest attenuation is recorded during summer season. Pretoria which has a temperate climate, also show similar characteristics as that of Durban but from the plot (Fig. 1(c)), it is observed that, Pretoria just divides the season into two; summer/autumn and the winter/spring are seen to overlap to certain extent. Brandvlei which is found in the desert records its highest attenuation in the autumn season and the lowest in the winter and spring but at 1.28 dB, the springs tends to record lower attenuation than winter for the average year.

3. Fade Margin and Link Availability From the seasonal attenuation distribution for an average of 5-years, fade margin figures were derived for different percentages of time for which certain attenuation is exceeded. Tables 1(a)-(d) gives the required fade margin at 20 GHz for various level of availability in different seasons for various climatic zones in South Africa. A link availability of 99%, 99.9% and 99.99% indicates a signal outage of 88 hrs, 8.8 hrs and 53 mins respectively in a year (This does not necessarily imply total outage, but signal may be present at reduced quality) [10]. The ITU-R [10] recommends 0.01% exceedance attenuation value which gives the 99.99% availability on a radio link. Fig.2 gives the fade margin at 20 GHz for each season at 99.99% in South Africa. It is seen that a relatively small fade margin is required when availability has to be guaranteed during the winter and spring months. However, a large fade margin up to 22 dB and more has to be implemented when a high degree of availability is required during the summer and autumn months in South Africa. Table 1(a) Durban on lat.29o97'S and long. 30o95'E

Table 1(b) Cape Town on lat. 33o97'S and long 18o60' E

Availability

99%

99.9%

99.99%

Availability

99%

99.9%

99.99%

Fade margin in dB (Summer)

2.79

10.18

22.73


0.52

2.43

5.54

Fade margin in dB (Autumn)

1.92

10.12

22.10


0.62

3.00

7.72

Fade margin in dB (Winter)

1.30

6.38

11.04


1.76

5.09

9.52

Fade margin in dB (Spring)

1.77

5.23

8.93

1.83

5.60

o

o

'

Fade margin in dB (Spring) o

Table 1(c) Pretoria on lat.25 73'S and long. 28 18'E

11.39 o

Table 1(d) Brandvlei on lat. 30 47'S and long 20 48'E Availability 99% 99.9% 99.99%

Availability

99%

99.9%

99.99%


2.66

8.94

20.66


0.09

2.43

8.21


2.22

8.82

17.79


0.44

4.67

13.62


0.15

2.06

6.08


0.07

0.97

3.73


0.24

2.51

12.07


0.05

1.05

2.76

4. Average Attenuation along 6.73 km path at 19.5 GHz in Durban A terrestrial radio link was set up in Durban at a frequency of 19.5 GHz and propagation path length of 6.73 km between two points; this records the rain rate at 1-min integration time and the signal attenuation for 1-year.

Characteristics of the link have been published elsewhere [11]. From here, the average path attenuation and the corresponding rain rate is determined for the year. And these measurements were then compared with the results obtained from the ITU-R model [10]. Fig. 3 shows the measured maximum, average, and minimum rain attenuation values per rain rate and the ITU-R results for the entire year along the 6.73 km path at 19.5 GHz. It is seen in this figure that, the ITU-R attenuation curves fall within the bounds up to rain rate of about 70 mm/h. This suggests that the ITU-R model can produce a justifiable result when used with the available rain data in South Africa. Nevertheless, longer measurements may be needed to buttress this point.

5. Conclusion A 5-year rain rate data obtained from the South Africa Weather Service (SAWS) has been utilized to study and identify the seasonal variability of rain attenuation on radio propagation paths at a frequency of 20 GHz for the seasons in different climatic zones in South Africa. The seasonal behavior of attenuation for these seasons in each climatic zone is seen to be linked to their climatic characteristics. That is, high attenuation is expected in a rain environment and season of high rain rate, like the summer and autumn; except for Cape Town that has its rainy season during winter and spring due to Mediterranean type of climate and location. Analysis of fade margin also indicates a seasonal and climatic dependence. This fade margin when utilized by radio engineer for each season gives better performance, and confines the effect of rain attenuation to very heavy and very infrequent rain periods. The measurement recorded in Durban at 19.5 GHZ on 6.73 km path length for a period of 1- year and the resulted prediction by the ITU-R model shows that the ITU-R model can thus be used reasonably to estimate rain attenuation for low to medium rain rates for different seasons in a year.

6. References 1. G.O Ajayi, S.Feng, S.M. Radicella, B.M Reddy (Ed): Handbook on Radiopropagation Related to Satellite communications in Tropical and Subtropical Countries. ICTP, Trieste, Italy 1996, pp.140 2. South Africa Longitude and Latitude: http://www.mapofworld.com/lat_long/south-africa-lat-long.html 3. Republic of South Africa 1989-1990: Official Year Book of the Republic of South Africa, 15th Edition, Bureau for information pp1-22 4. South Africa Weather: http://www.southafrica.info/plan_trip/travel_tips/questions/climate.htm 5. South Africa Climate and Weather, Conditions: http://www.sa-venues.com/no/weather.htm 6. M.O. Fashuyi, P.A. Owolawi, T.J. Afullo, “Rainfall Rate Modelling for LOS Radio Systems in South Africa,” Africa Research Journal of the South Africa Institute of Electrical Engineering, Vol. 97 No.1, March 2006, pp.7481 ISSN No.1991-1696. 7. Recommendation ITU-R 837-1, 2, 3, 4, “Characteristics of Precipitation for Propagation Modelling,” Vol. 19922003, P Series, International Telecommunication Union, Geneva, Switzerland 8. Recommendation ITU-R 838-3 “Specific Attenuation Model for Rain for Use in Prediction Models,”Vol.2005, P Series, International Telecommunication Union, Geneva, Switzerland, 2005 9. M. Marcus and B. Pattan, “Millimeter Wave propagation-Spectrum Management Microwave Magazine, Vol. 6, No2, June 2005, pp. 54- 61.

Implications,”

IEEE

10. Recommendation ITU-R 530-11, “Propagation Prediction Techniques and Data Required for the Design of Terrestrial Line-of-Sight Systems,” Vol. 2001, P Series, International Telecommunication Union, Geneva, Switzerland, 2005, pp.271-295 11. M.O Fashuyi and T.J Afullo, “Rain Attenuation Prediction and Modeling for Line-of-Sight Links on Terrestrial Paths in south Africa,” Radioscience Journal, Vol. 42, RS5006, doi: 1029/2007RS003618, Oct. 2007

28

12 Summer

Summer

Autumn

Autumn

24

10

W i nt e r

winter

Spring

Spring

A t t e n u a t i o n (d B )

20 A t t e n u a t I o n (d B )

8

16

6

12

4

8

4

2

0 0.01

0.1

1

0 0.01

10

% of time ordinate exceeded

0.1 1 % of time ordinate exceeded

10

Fig. 1 Average seasonal cumulative distribution of attenuation at 20 GHz at (a) Durban, and (b) Cape Town 25

16

Summer

Summer

Autumn

Autumn

Winter

20

Winter

Spring

A t t e n u a t I o n (d B )

A t t e n u a t I o n (d B )

12

15

10

Spring

8

4

5

0 0.01

0.1

1

10

0 0.01

0.1

1

10

% of tIme ordInate exceeded

% of tIme ordInate exceeded

Fig. 1 Average seasonal cumulative distribution of attenuation at 20 GHz at (c) Pretoria, and (d) Brandvlei 100 Durban (Coastal) Pretoria (Temperate)

20

Cape Town (Mediterranean) Brandvlei (Desert)

15

10

P a th A tte n u a tIo n (d B )

F a d e m a r g i n (d B ) a t 9 9 .9 9 % a v a i l a b i l i t y

25

10

M i n. M e a s u r e d A t t e n u a t i o n

1

M a x. M e a s u r e d A t t e n u a t i o n

5

A v. M e a s u r e d A t t e n u a t i o n ITU-RModel 0 Summer

Autumn

Winter

Spring

Seasons

Fig. 2 Required fade margin for 99.99% availability as a function of season

0.1 0

10

20

30

40

50

60

70

80

90

R a I n R a t e (m m / h )

Fig. 3 Rain attenuation for Durban along 6.73km link at 19.5 GHz in 2004