rain attenuation prediction in equatorial zones ...

Pre-Print. Article presented at the AIAA ICSSC Conference in 2002. DOI: 10.2514/MICSSC02

RAIN ATTENUATION PREDICTION IN EQUATORIAL ZONES: THEORETICAL ANALYSIS, MEASUREMENT CAMPAIGNS AND MODEL COMPARISON. Joaquín Restrepo Universidad Pontificia Bolivariana (U.P.B), Colombia, Luis D. Emiliani INTERNEXA S.A. E.S.P., Colombia. César Fradique

Abstract: Rain attenuation is the dominant impairment when designing satellite systems in Ku and Ka Band. Thus, the ability to accurately predict the magnitude of this impairment is paramount when the task of designing a radio link at these frequencies is assessed. This article presents a summary of various attenuation measurement campaigns and their results and recommendations, as well as a theoretical comparison of the results of six attenuation prediction models applied to the geographical and climatic characteristics of Colombia, showing the zones where the highest divergence is to be found, aimed at the selection of possible sites for a measurement campaign similar to those described. Furthermore, the results of a measurement campaign conducted in Medellín, Colombia are presented and the first estimation of an EDF for a tropical zone is built. Results are scaled to show behavior of a link in the Ka band. Key words: radiowave propagation, rain, attenuation, Colombia, Satellite communications.

3. 4. 5.

the use of local measurements (radar data) or through the ways discussed in each of the rain attenuation prediction methods available. Estimate the attenuation per unit length (specific attenuation). Estimate the length of the slant path (earth stationsatellite) affected by the rain cell. Estimate the total attenuation on the path. This can be roughly put as the product of the effective slant path length and the specific attenuation.

Several methods exist for the prediction of this impairment and their validity can only be stated through measurement campaigns. Two campaigns, led by space agencies (NASA & ESA), have published their results and conclusions (for Ka band) for stations limited to continental US and Europe, with the exception of two terminals located in Bogotá and Quito, in South America.

1. INTRODUCTION Nowadays, Ku band frequencies are widely used in telecommunication networks via satellite all over the world. The ITU has approved additional bands in higher frequencies (Ka and V) in which data, voice and broadband multimedia services will be developed. All these systems are going to be designed to achieve a high availability figure (>99.6%). The ability to predict the attenuation caused by rain at these frequencies is fundamental when it comes to proper system sizing and design. The procedure for predicting rain attenuation consists basically of the following steps: 1.

2.

Obtain the point rainfall rate for the geographical site under analysis. This can be done through the use of local rain data (1-5 minutes integration time is recommended [CRA96]) or through global precipitation maps and climatic models. Estimate the rain height (height over which no liquid rain is expected). Once again, this can be done through

Furthermore, the theoretical and experimental results for a measurement campaign conducted in Medellín at Universidad Pontificia Bolivariana, with the support of INTERNEXA S.A are presented. This measurement campaign is aimed at determining the applicability of mainstream rain attenuation prediction models to the climatic conditions of a tropical zone. The theoretical results presented show a comparison of the performance of six prediction models using as inputs data from 4500 points of a satellite network in service in Colombia and the rain rates obtained through the use of the Global Climatic Model. Sections of this article are as follows: In Section 2 the methodology and results for three attenuation measurement and model comparison campaigns, conducted in Europe and America are summarized, demonstrating the need for local experiments in tropical regions. In Section 3 a theoretical comparison between six rain attenuation prediction models is made, pointing out the divergence between them, which justifies this project. In Section 4,

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results obtained up to now in this campaign are depicted, showing attenuation distributions and preliminary comparisons. Finally, in Section 5, main conclusions are presented and further works are described.

2. MEASUREMENT CAMPAIGNS AND RESULTS PRESENTED.

1. 2. 3. 4.

DHA [DIS97] Global, R.Crane [CRA96] Two-Components, R.Crane [CRA96] ITU-R P618-4.[CCI90]

Comparison between predictions and measurements was implemented through the RMS deviation for a given probability value. Results are shown in table 1, taken from [CRA97].

2.1 ACTS Campaign [CRA98] The ACTS, Advanced Communication Technology Satellite, was launched by NASA on 1993 mainly to demonstrate advanced telecommunication technologies employing the Ka band. Considering that the characterization of the propagation environment was recognized as a critical issue for the reliable exploitation of the frequencies in this band, NASA developed a propagation experiment within ACTS’s framework to gather the necessary information to accomplish this objective. NASA sponsored two types of experiment: longterm experiments, designed to collect statistics for the development of propagation models, and short term experiments designed to explore countermeasures and special topics.

Attenuation model DAH ITU-R Crane 2C Crane Global

Attenuation model DAH ITU-R Crane 2C Crane Global

Attenuation data for 20.2 Ghz. Rain model Crane Global ITU-R R-H 0.28 0.45 0.32 0.42 0.51 0.50 0.60 0.63 0.37 0.47 0.61 0.52

Attenuation data for 27.5 Ghz. Rain model Crane Global ITU-R R-H 0.21 0.51 0.35 0.31 0.53 0.45 0.53 0.73 0.40 0.48 0.71 0.64

The main objectives of the program were: 1. 2. 3.

Provide the necessary knowledge to understand propagation impairments affecting the Ka band. Develop models to predict propagation anomalies on the Ka Band. Develop tools to mitigate the effects caused by this impairment.

Results published in 1997 employed two years of measurements taken from seven stations in the US and results published in 1999 employed data from two southamerican stations. The locations used spanned all the climatic zones proposed by R.Crane [CRA96] and the ITU [ITU94]. To generate the attenuation EDFs, three rain distribution models and four attenuation prediction models were used. The rain statistic prediction models used were: 1. 2. 3.

Global Climatic model, by R. Crane Global Climatic model, by ITU-R. Rice – Holmberg model, by P.Rice and N.Holmberg. [RIC73]

The rain attenuation prediction models compared were

source [CRA97] Table 1. RMS deviation for several models.

Details concerning receiver availability, discarded data and additional effects affecting the measurements can be found in [CRA97]. The ACTS propagation studies extended until 1999. That year the Antenna and Propagation Symposium took place, and the results of a model comparison in tropical zones were presented. Three year results for two stations, located in Bogotá, Colombia and Quito, Ecuador a were shown to partially agree with predictions made employing Crane’s Global and Two Component models. These results are shown in the following graph, taken from [JOH99]

a

Nevertheless, it is important to point out that although Bogotá and Quito are located in tropical latitudes, these cities are located in high altitudes over the andean mountains, hence not being representative of a tropical climatic behavior.



In June, 1996, the ITU-R reported the results of a comparison campaign using predictions from 10 models and 186 stations. The evaluated models were: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

Source [JOH99] Figure 1. Model comparison for a tropical zone.

Johnson’s work indicated that the model which showed the most accurate adjustment to measured data was Crane’s Two-Component. These results differ from those obtained in the US measurements, where the best fit was obtained using rain statistics obtained from Crane’s Global model combined with attenuation predictions from the DHA model [CRA97]. Main conclusions drawn from the ACTS project are [CRA97], [PIN99]: 1) None of the models used for the comparison had good performance (largely underpredicting data). 2) The best approximation to the measured data came from the application of climatic data from the Global model to the DHA regression-based model. 3) When applying the climatic maps developed for each model, the Global model presented a better adjustment when predictions were made for less than 0.01% of the time, whilst DHA gave better predictions for outages above 0.01% of the time . 4) Predictions obtained using physical models (Global, 2C), were inferior to those obtained by empirical models (regression-based models). The former did not include wet antenna or wet feed effects. Furthermore, analysis of figure 1 shows that, for Colombia, the best approximation is obtained when the 2C model is employed.

ITU-R model DHA model. 2C model. ExCell. Leitao-Watson. Misme-Waltdeufel. CCIR – 1986. Karasawa, 1988. García-Lopez, 1988. ITU-Brazil, M.S. Pontes, 1992.

The database included beacon measurements, radiometer measurements and combined beacon radiometer measurements, on frequencies ranging from 11.6 Gz up to 34.5 Ghz. Comparisons were made grouping the data according to specific link conditions. The groups were: ≥ 10 Ghz and < 15 Ghz. ≥ 15 Ghz and ≤ 35 Ghz Rain rate: > 0 mm/hr and ≤ 10 mm/hr >10 and ≤ 20 mm/hr >20 mm/hr Latitude: ≥ -22.5° and ≤ 22.5° ≥ -30 and ≤ 30° 30° Elevation angle: ≤10° >10 and ≤ 20° >20 and ≤ 20° >20 and ≤ 40° >40 and ≤ 60° >60° Results indicated that the enforced ITU model and the DHA model were consistently in the top four performing models in each group, using mean-error and RMS criteria. For frequencies above 15 Ghz, the ExCell model appeared in the top 5. Typical RMS errors were found to be between 30-40% for the top performing models. The mean error was in the 20-25% range. Frequency:

Table number 2, taken from [IPP00] details the results obtained. Model

2.2 Studies conducted by the 3M group, ITU [IPP00].

DHA


Position Full range 15-35 Ghz. 1 2


ITU-R 618-5 ExCell Japan Brazil CCIR Leitao-Watson Misme-Waldteufel Dos-Componentes Spain

2 3 4 5 6 7 8 9 10

3 8 5 6 7 4 10 9 1 Source [IPP00] Table 2. Performance comparison for 10 prediction models, according to the ITU.

1) 2) 3) 4) 5) 6) 7)

LEITAO/WATSON (COST 205, 1985) MI/WA (Misme - Waldteufel, 1980) JAPAN (Karasawa - Matsudo, 1990) EXCELL (Fedi - Paraboni, 1986) CCIR91 (ITU-R) (Rec. 618-2, 1992) CRANE (Crane, 1985) SVIATOGOR (Sviatagor, 1985)

The ITU recommendation was followed comparisons, defining an error variable as:

for

the

2.3 OPEX-OLYMPUS comparison [POI92]. The OLYMPUS program -formerly known as L-Sat- started in 1981, eight years before its launch. The mission objectives were:

0.2   Ap ( p )   Am ( p )     100 Ln    Am ( p )   10  error ( p ) =   Ap ( p )    100 Ln    Am ( p )  

1.

Table 3, taken from [POI92], gathers the results for two exceedance probabilities. Results for other values are found in [POI92].

2.

Develop, launch and operate a multipurpose GEO spacecraft. Development of a series of telecommunication payloads and their in-orbit operation to advance technological capabilities of the payload industry, stimulate users, and promote new market applications through a comprehensive test, demonstration and utilization program.

The mission had a time span of 5 years. The satellite was successfully launched and carried the following platforms: 1) Multi-beam, 12/14 Ghz payload, for experiments with special services. 2) 11 Ghz broadcast payload 3) 20/30 Ghz payload for experiments with communication services such as distance learning, video conferencing and business services. 4) Propagation payload. 2.3.1. Propagation payload. This payload was designed to offer the possibility of long-term propagation experiments in 20 and 30 Ghz, to offer data to compare with the Ku band.

Method LEI/WA MI/WA Japan ExCell CCIR91 Crane Method LEI/WA MI/WA Japan ExCell CCIR91 Crane

Models considered were:

Eq. (1)

for A m ≥ 10 dB

Errors for 0.001% RMS ST. Dev. 47.482 46.756 53.142 46.910 53.966 53.787 50.439 43.440 54.597 53.967 48.187 45.498 Errors for 0.01% Mean RMS ST. Dev. -6.204 36.271 35.736 13.698 37.129 34.510 -4.021 35.474 35-246 16.745 37.973 34.081 -3.333 35.380 35.223 -2.875 41.263 41.163 Source [POI92] Table 3. Results of the OPEX comparison. Mean -10.135 24.970 4.391 25.633 8.272 15.870

Table 4, also taken from [POI92], summarizes the results for all probabilities: Method

2.3.2. Results of the comparison campaign. Data from 33 of the 66 sites equipped with measurement devices were used in the comparison. The minimum operation threshold established in order for the data to be considered was 200 days. Only one year of measurements was used because the number of sites with 2 and 3 year records was not enough for a statistical analysis.

for A m < 10 dB

Mean error RMS 36.885 37.429 36.682 37.047 36.028 40.132

St. Dev 35.072 34.219 36.023 33.512 35.923 40.108 Source [POI92] Table 4. Summary of results obtained in the OLYMPUS program. LEI/WA MI/WA Japón ExCell CCIR91 Crane

Mean -11.423 12.670 -6.923 15.794 -2.743 -1.373

Main conclusions from this campaign are [POI92]:



1) Precision over 30% is difficult to obtain with today’s methods.

Model

Mean error according to ITU Rec. CCIR -14.9 Crane Global -2.38 Leitao-Watson -2.72 Misme-Waldteufel 16.08 Rue -11.36 Excell 0.73 Source [GAL89] Table 5. Error values according to ITU recommendation, for the predictions and measurements used in the Coventry University comparison.

2) It is not possible to make positive conclusions regarding the best performing model, due to the small differences between model performances. 3) Models employing rain cells statistics give the closest approximation to the measured data. 4) ITU-R models can be employed in the range 20-30 Ghz. Improvements to account for clouds attenuation and gas absorption should be made.

2.4 Studies conducted at Coventry University. [GAL89]. In 1989, Coventry University conducted an investigation directed at determining possible sites for the evaluation of rain attenuation. A comparison between measured and predicted attenuation was also conducted for 9 locations in continental Europe, for 0.01% time exceedance. Locations were: 1. 2. 3. 4. 5. 6. 7. 8. 9.

Fucino. 31° elevation, 5 years data in 11 Ghz, 4 years at 17.8 Ghz. Kirkkonum. 20.6° elevation. 2 years at 11.5 Ghz. Lario. 32° elevation. 5 years at 11.6Ghz, 4 years at 17.8 Ghz. Leeheim. 32.9° elevation. One year data 11.6 Ghz. Lustbuehel. 35.2° elevation, 4 years data at 11.6 Ghz. Nederhorst. 30° elevation. 3 years data at 11.6 Ghz. Sodankyla. Elevation: 13.2°. 5 years data at 11.6 Ghz. Spino D’Adda. Elevation: 32°. 3 years worth of data at 11.6 Ghz. Stockholm. Elevation: 22.4°. One year of data at 11.6 Ghz, one year at 14.5 Ghz.

Models employed in the comparison were 1) 2) 3) 4) 5) 6) 7)

CCIR, 1986. [CCI86] Global - Crane. [CRA96] 2-Components, R. Crane. [CRA96] Misme-Waldteufel [MIS80] Rue. [RUE85] Leitao-Watson . [LEI86] Excell. [CAP87]

Results are summarized in table 5:

The table shows the average of the errors calculated for each site and frequency of operation. According to these results, the ExCell model yields the smallest value for the error variable.

3.

THEORETICAL COMPARISONS DEVELOPED FOR COLOMBIA.

As shown in the campaigns summarized above, a large uncertainty exists between model predictions when applied to tropical zones. This divergence is manifested through a high deviation between different predictions. In a previous article presented in the Andean Telecommunications Congress (Congreso Andino de Telecomunicaciones, ANDICOM2000 [FRA00]), the authors pointed out that for the rain rates expected in Colombia (over 100 mm/hr, 0.01% of the time), results diverged by 10 dB.

1. 2. 3.

Models evaluated at the time were DHA. [DIS97] Global - R. Crane. [CRA96] ITU-R P618-5. [ITU97]

Parameters used in the comparison were the geographical data for a station in Medellín, and a satellite located at 304.5°E, with a downlink at 12.6 Ghz. Since then, reach of the study was extended to obtain, according to the predictions of six models, which zones in the Colombian geography have the largest divergence between predictions. Comparisons were made using a database with over 4500 sites (Compartel 1 – [COM02]), part of a rural telecommunications network in Colombia. A plot of the data contained in the database is shown in figure 2.



3.2 Results. For each point in the database, predictions for the above mentioned models were obtained. The standard deviation of the prediction set was calculated and color-coded, and a map using this deviation data was built.

Figure 2 . A geographical plot of the station database.

Models compared are: 1. 2. 3. 4. 5. 6.

ITU P618-6 [ITU99-2] Global – R. Crane [CRA96] Two Component model. R. Crane [CRA96] Simple - García-López [GAR88] Pontes-DaSilva Mello Leitao-Watson [LEI86]

The Leitao-Watson model was implemented considering showery rain as worst case. 3.1 Comparison parameters. The station database includes geographical information (latitude, longitude and height). Additional parameters required for the comparison are: 1. 2. 3. 4.

Figure 3 . Results of the divergence analysis performed.

A graph of the average slant path length versus the standard deviation of the predictions (figure 4) illustrates that this representation of the deviation among model predictions constitutes the utmost evidence of the need for awareness of the accuracy of the output values.

Frequency of operation: 14089 Ghz. Required availability: 99.99% of the average year. Rain rate for the availability required: Average rate according to ITU-R: 110 mm/hr. Satellite orbital location: 304.5°E.



Slant path length vs. std. deviation

Station height Vs. Prediction value 25

14 12

Avg. prediction

20

ITU-R

10 15

8 6

10

4 5

2 0

0 0

2000

4000

6000

0

8000

Slant path length (mts)

Figure 4. Average slant path length versus standard deviation of predictions

Provided the differences in nature and approaches between models, there is no definite conclusion as to a set of input values for which there is consistency of one result compared to the others. An additional plot of the station height vs. prediction values (fig. 5) shows that the current ITU model is close in magnitude to the average of the predictions. From this graph follows that the ITU model represents the most suitable approach to predicting attenuation in a slant path, because it is a fairly good representative value of the average prediction of several rain attenuation methodologies. The ITU prediction is the least compromising value that can be used, as it cannot be characterized as either conservative nor optimistic. This feature turns out very handy when local measurements or alternative criteria are not available.

1000

2000

3000

4000

5000

St. height(mts)

Figure 5. Station height vs. prediction values.

3.3 Rain distribution comparison. After processing rain accumulation data for the decade of 1990-2000 and data for 2001 and 2002, a theoretical comparison between rain distributions was performed. The climatic models compared were Crane-Global, Moupfouma [MOU95] and Rice-Holmberg. The only parameter needed to generate Crane’s rain distribution was the climatic zone. Colombia lies entirely in the H zone. For the Rice-Holmberg model, the parameters used were: Parameter name U - Number of thunderstorm days expected in an Average year Mm - Highest monthly Precipitation observed M - average annual accumulation

Value 77 400.1 1614.38

β0

0.38

β

0.4 Table 6. Parameters used in the RH model.

Parameter β was calculated according to [DIS97]. For Moupfouma’s model, the parameters for a tropical zone were considered, as described in [MOU95]. Results are displayed in figure 6.



operation since November 9, 2001 and the first results of the pilot tests are available now.

Theoretical comparison of rain distributions

600

Rain rate exceeded X% of a year (mm/hr)

Rice-Holmberg Crane-Global

500

Moupfouma 400

300

200

100

0 98.85

99.05

99.25

99.45

99.65

99.85

100.05

Availability (% of a year)

Figure 6. Results of the theoretical comparison of rain distributions for Medellín, Colombia.

Data for percentages of outage of common use are shown on table 7.

1.00

3.95

7.74

Crane-Global rain rate 12.47

0.50

7.449

14.62

22.71

0.40

8.76

17.43

26.97

0.30

10.7

21.49

33.25

0.20

14.25

28.14

43.78

0.10

26.93

41.44

66.91

0.01

101.1

98.55

209.70

0.001

165.4

158.25

544.41

Outage (%) RH rain rate

Moupfouma rain rate

Figure 7. Vsat terminal employed in the pilot test of the measurement campaign.

Figure 8 illustrates a typical fading event as registered by the terminal, after processing the data to extract downlink attenuation.

Table 7. Values for two rain rate distributions for percentages of service outage time of common use.

4.

RESULTS OF THE MEASUREMENT CAMPAIGN.

In the third quarter of 2001, a cooperative effort between INTERNEXA S.A. and Universidad Pontificia Bolivariana led to the installation of a Vsat terminal at Universidad Pontificia Bolivariana’s campus in Medellín, with the objective of implementing the pilot test of an attenuation measurement campaign and the verification of its experimentation methodology aimed at determining the model that best fitted the conditions of attenuation due to rain in Colombia. This Vsat terminal (figure 7) has been in

Figure 8. A fading event.

Using four months of measurements (November 2001February 2002) an empirical distribution function for the attenuation in Medellín (6.2 °N, -76.4°W, 1600 mts.) was constructed. This distribution is shown in figure 9.



Cumulative attenuation function

Cumulative attenuation function 100.00%

10.000%

Percentage of time given attenuation is exceeded

Percentage of time attenuation is exceeded

100.000%

12 Ghz.

1.000%

0.100%

0.010%

0.001%

0.000%

1.00%

0.10%

0.01%

1

3

5 7 Attenuation (dB)

9

11

1

Figure 9. Cumulative attenuation distribution function - 12.6 Ghz.

Based on this results, Figures 10 and 11 show the Ku band distribution scaled to Ka band using the method proposed in ITU’s recommendation 618-6.

Cumulative attenuation function 100.000%

17 Ghz.

d e d e e c x e is n o it a u n tte a e tim fo e g a tn e c re P

27 Ghz. 10.00%

10.000%

1.000%

0.100%

11

To reduce the number of assumptions as far as rain distributions are concerned, the comparison was made for 0.01% of the time. In this way, the rain rate predictions from ITU p.837-2 can be used directly without the need for scaling to other percentages, which would add uncertainties. Data from the decade of 1990 plus 2001 and January 2002 were processed using the Rice-Holmberg model to obtain a rain rate exceeded 0.01% of the time. Thus, three climatic models were used in the comparison, along with the six prediction methods mentioned in Section 3. The error variable as defined in [ITU01] was employed in this comparison.

Attenuation model 9 Attenuation (dB)

13

17

41

4.1 Model comparison. Results of a model comparison with these measured values are shown in tables 8, 9 and 10.

0.001%

5

31

Figure 11. Attenuation CDF (scaled) 27 Ghz.

0.010%

1

21 Attenuation (dB)

Attenuation data for 12.6 Ghz – 0.01% Rain model Crane Global ITU-R

R-H

21

Figure 10. Cumulative attenuation distribution function (scaled) – 17.7 Ghz.

Garcia Le-Wa ITU-R 618-6 Crane 2C Pontes-DeMello Crane Global

0.5 0.27 0.58 1.35 0.55 1.23

-0.26 -0.19 0.12 0.5 -0.3 0.45

-0.19 -0.15 0.16 0.56 0.22 0.51

Table 8. Results of the model comparison for Ku band.



Attenuation model

Attenuation data for 17.7 Ghz (scaled)– 0.01% Rain model Crane Global ITU-R R-H


0.56 0.31 0.82 1.3 0.62 1.19

-0.11 -0.06 0.38 0.52 -0.15 0.46

-0.04 -0.02 0.42 0.58 -0.08 0.52

Table 9. Results of the model comparison for Ka band.

Attenuation model

Attenuation data for 27 Ghz (scaled)– 0.01% Rain model Crane Global ITU-R


1.43 0.91 1.62 1.65 1.49 1.55

0.89 0.47 1.25 0.92 0.85 0.86

R-H -0.01 -0.19 -0.0007 -0.13 0.08 -0.24

Table 10. Results of the model comparison for Ka band (2).

It should be noted that Garcia’s model was recommended for use in a tropical zone (southeast asia - Thailand) as a result of an evaluation performed under the AI3 initiative [BAH97]. Table 11 ranks the models in performing order: Rank 1 2 3 4 5 6

12 Ghz ITU + ITU837 Le-Wa + RH Garcia + RH Pontes + RH C.Global + ITU 837 C.2C + ITU 837

Model 17.7Ghz Le-Wa + RH Garcia + RH Pontes + RH ITU + ITU 837 C.Global + ITU 837 C. 2C + ITU 837

27 Ghz ITU + RH Garcia + RH Pontes + RH C. 2C + RH Le-Wa + RH C. Global + RH

Table 11. List of models in performing order.

Raw and processed data obtained in this campaign will be made available on the project’s website, located at www.upb.edu.co/gidat/proyectos/radiocomunicaciones. 5. CONCLUSIONS, RECOMMENDATIONS AND FURTHER WORKS. Insofar as the applicability of existing methods is concerned, and according to the conclusions exposed in section 2, the agencies have concurred that the best

performing model today is the current ITU recommendation. For tropical – equatorial zones, ACTS measurements indicated that Crane’s Global and 2Component models had the best adjustment. However, the authors’ opinion is that the Crane Global Climatic Model overestimates rain rates for 0.01% of the time, as a result of applying the Rice-Holmberg model to local rain accumulation data. A measurement campaign is required to validate this assertion. Based upon the previous maps and slant path length vs. std. deviation graphs, it can be concluded that it is not possible to make an a-priori statement referring to the consistency of the outputs of these models considering only the altitude and rain rate at particular earth station. In a previous article, it was stated that the threshold over which the resulting values for the three models diverge was 50 mm/hr for a station at an altitude of 1800m. Therefore the choice of one particular model at a site with no previous experimental verification becomes a critical issue, as the outputs may vary dramatically with this selection. As stated above, the ITU model output represents a very fair mid-point in the range of possible outputs and consequently ought to be the first option for the estimation of exceeded rain attenuation. The first results of the campaign show that the best performing model so far for the four month period under review is the current ITU model, coupled with the current ITU rain attenuation distribution. For the scaled values, the best performing model was found to be Leitao-Watson model coupled with Rice-Holmberg predictions for 17.7 Ghz and again the current ITU model, this time coupled with Rice-Holmberg’s estimates for rain intensity, for 27 Ghz. It should be noted that for the scaled values, the best adjustment is always found when using local climatological data. Based on these results, at this stage of the experiment the authors recommend that the current ITU model, ITU P618-6, be used for attenuation predictions in tropical zones. Nevertheless, these results are not yet conclusive, and further measurements are required to confirm this initial analysis. Along with the pilot test in UPB, an additional site with the same characteristics is operational and located 12 Km



apart. This second site will deliver data to validate site diversity improvements.

[GAR88] J.A. García López, J.M Hernando and J.M. Selga "Simple rain attenuation prediction method for satellite Radio links" IEEE Transactions on Antennas and Propagation, Vol 36. No. 3 March 1988. Pp 444-448.

ACKNOWLEDGMENTS The authors wish to extend special thanks to Juan Gonzalo Zuluaga Botero for his help in developing the base maps for the theoretical comparison and in processing the vsat data to generate the attenuation distribution, and to Diana Quiceno for her aid in obtaining thunderstorm information for the Rice-Holmberg model. Additional thanks go to GILAT COLOMBIA and Simón Hernandez, for providing the station database. REFERENCES [BAH97] http://www.cs.ait.ac.th/ai3/research/BBM/main.shtml. Bahadur Mishra, Birat. Link analysis and rain attenuation considerations in AI3-AIT satellite link project. [CAP87] Capsoni, C, Fedi, F. And Paraboni, A., “A Comprehensive Meteorologically Oriented Methodology for the Prediction of Wave Propagation Parameters in Telecommunication Application Beyond 10 Ghz”, Radio Science, Vol 22, No. 3, 1987. [CCI86] CCIR “Propagation data and prediction methods required for earth-space telecommunications systems”. ITU. International Telecommunications Union, Report 564-3, 1986.

[ITU94] ITU-R. P.837-1. “Characteristics Of Precipitation For Propagation Modeling”. ITU. International Telecommunications Union, Geneva, 1994. 19pp. [ITU97] ITU-R. P.618-5. “Propagation data and prediction methods required for the design of earth-space telecommunications systems”. ITU. International Telecommunications Union, Geneva, 1997. 19pp. [ITU99-1] RECOMMENDATION ITU-R P.837-2. “Characteristics Of Precipitation For Propagation Modeling.” ITU-R. International Telecommunications Union. Radiocommunication Sector, Geneva, 1999. 8p. [ITU99-2] ITU-R. P.618-6. “Propagation data and prediction methods required for the design of earth-space telecommunications systems”. ITU. International Telecommunications Union, Geneva, 1999. 21pp. [ITU01] RECOMMENDATION ITU-R P.311-10. “ Acquisition, presentation and analysis of data in studies of tropospheric propagation”. ITU. International Telecommunications Union, Geneva, 2001. 8pp. [IPP89] Ippolito, Louis J. Propagation Effects Handbook for Satellite System Design. A Summary Of Propagation Impairments On 10-100 Ghz. Satellite Links With Techniques For System Design. NASA Scientific and Technical Information Program. 1989. [IPP00] Ippolito, Louis J. Propagation Effects Handbook for Satellite System Design. Fifth Edition, Second revision. ITT Industries. Sept. 2000.

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Joaquín G. Restrepo M: Dr. Restrepo received his B.Sc degree in Electronic Engineering and M.Sc. degree in Technology Management from Universidad Pontificia Bolivariana. He received his M.Sc. and Ph.D. degrees in satellite telecommunications from E.N.S.T. site de Tolouse. He is currently director of GIDAT, the telecommunications R&D group in Universidad Pontificia Bolivariana. He has authored several articles and conferences in Non-Geo systems in America and Europe and co-authored two European patents in non-geo systems. César A. Fradique-Méndez : received his B.Sc in Electrical Engineering, B.Sc in Physics and his MBA from Los Andes University in Bogotá, Colombia. He is with Nortel Networks de Colombia, as a Field Service Engineer in the areas of switching, satellite networks and management systems. Luis David Emiliani A.: received the B.Sc in Electronic Engineering and Specialist Degree in Telecommunications from Universidad Pontificia Bolivariana. Candidate for the Master of Engineering Degree from Universidad Pontificia Bolivariana, he is currently a network engineer in INTERNEXA S.A., where he is in charge of satellite networks engineering.
