152 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 7, 2008

Cloud Attenuation for Satellite Applications OverEquatorial Climate

J. S. Mandeep and S. I. S. Hassan

Abstract—A propagation experiment has been carried out atPenang using the SUPERBIRD-C satellite beacon. Cloud oc-currences were observed during different months and it is seenthat the low cloud occurrences over Penang is very significantfrom October to January. The cloud attenuation results that arepresented, which include the testing of models, have been obtainedfrom the data gathered over five years.

Index Terms—Cloud attenuation, equatorial climate, satellitecommunications.

I. INTRODUCTION

T ROPOSPHERIC propagation impairments that affectsatellite communication signals increase in severity with

the increase of frequency. Precipitation effects are the mainimpairment factor for millimeter-wave signals propagatingthrough the atmosphere. However, many projected Ka-bandand V-band services uses small terminals and, for these, raineffects may only form a relatively small part of the total propa-gation link margin. But cloud attenuation, that may cause deepfades in these band, is one of the components that need to beconsidered for low availability satellite links owing to its higherprobability of occurrence [1], [2]. A number of experimentalstudies of atmospheric attenuation at millimeter wave havebeen conducted. Based on the studies, researchers with someconfidence can estimate the losses due to gaseous absorptionand rain throughout the millimeter wavelength spectrum. How-ever it is difficult to estimate the losses due to clouds since onlylimited experimental data are available [3].

In this letter, five years results for cloud attenuation, includingcomparison of experimental distributions with model predic-tions are presented and discussed. An attempt has been made toprovide the statistics of cloud occurrences for low cloud typesand total (all cloud types included).

II. EXPERIMENTAL SYSTEM CONFIGURATION

The receiving station includes a 2.4-m antenna to collect hor-izontal signal polarization at 12.255 GHz by means of a dig-ital beacon receiver. The diameter of the antenna was set at2.4 m as a compromise to have large fade margin (20 dB), anda not-too-narrow antenna bandwidth (0.5 ), so that a trackingsystem was not necessary. A meteorological station, equippedwith different sensors (humidity, pressure, temperature, wind

Manuscript received October 12, 2007; revised December 21, 2007.The authors are with the School of Electrical and Electronic Engineering,

Universiti Sains Malaysia, Engineering Campus, 14300 Nibong Tebal, SeberangPerai Selatan, Pulau Pinang, Malaysia (e-mail: mandeep_s75@yahoo.com).

Digital Object Identifier 10.1109/LAWP.2008.919324

TABLE IPROBABILITY OF OCCURRENCE (%) FOR LOW CLOUD TYPES AND TOTAL (ALL

CLOUD TYPES INCLUDED) FOR THE AVERAGE YEARS

direction and velocity and rain gauge, is installed at the samesite. Data from the Meteorological Department, correspondingto the Penang station (4.6 km away from the University), arealso available and can be used to detect malfunctions in the sta-tion. Detail of the measurement setup is described in [4]. Tworadiometers were installed, one pointing upwards and the otherat 45 elevation. The radiometer data were processed to providetime series of attenuation, using the relationship

(1)

where is the antenna noise temperature, is the effectivemedium temperature and is the clear sky temperature.

III. RESULTS AND DISCUSSION

Clouds are composed of either water droplets or ice crystals.Ice clouds, by virtue of the low dielectric constant of ice andthe small size of the constituent particles, are not expected tocause appreciable attenuation to radiowaves in the frequencyrange below 50 GHz [2]. It has been reported by researchers [5]that clouds density and concentration varies from 0.15 to1 and 70 per to 450 per , respectively. Slobin [6]has reported that the average diameter of cloud particles variesfrom 9 to 20 while the size of rain drops distribution is100 . The thickness of the cloud is usually 1.5 km to 2.5 kmdepending on the type of cloud. For light, medium and heavyclouds the thickness are 0.2 km, 0.5 km and 1 km, respectively

1536-1225/$25.00 © 2008 IEEE

MANDEEP AND HASSAN: CLOUD ATTENUATION FOR SATELLITE APPLICATIONS OVER EQUATORIAL CLIMATE 153

Fig. 1. Monthly variation of 0 � isotherm height from 2000 to 2004 and average years.

Fig. 2. Monthly cumulative distributions of experimental measurements.

[6]. For equatorial climate, the low clouds that were observedover Penang pertain to the height from 3.5 km to 6 km.

The cloud characteristics have been derived from the lowcloud data. Averaged monthly and yearly cloud cover percent-ages for the measurement period (2000–2004) are presented inTable I.

Four types of low cloud are considered [7], cumulonimbus(Cb), nimbostratus (Ns), cumulus (Cu) and stratus/stratocu-mulus (St/Sc). It is seen that low cloud occurrence over Penangis very significant during October to January. The monthlycumulative distribution of rainfall is influenced by the seasonalmonsoons, namely the Northeast monsoon from October toMarch, and the Southwest monsoon from April to September.The Northeast monsoon is marked by heavy rainfall. The

monthly variation of rain height in relation to 0 isothermheight is presented in Fig. 1. It is seen that 0 isotherm heightvaries from 4.3 km to 4.9 km and has found to be maximumduring the Northeast monsoon season. The cumulative distri-bution shown in Fig. 2 shows the monthly variability of cloudattenuation for all 12 months of the average years.

November presents the highest measured cloud attenuationfor most percentage of time reaching up to 2.9 dB for 0.01%of time. This is due to a high percentage of occurrences (9.5%)of nimbostratus clouds (Table I). January, April, May, October,November and December gave high probability of occurrenceof certain types of clouds, such as cumulus, cumulonimbus andnimbostratus. July presents the lowest measured cloud attenua-tion reaching up to 2 dB for 0.01% of time.

154 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 7, 2008

Fig. 3. Experimental cumulative distributions with different prediction models.

TABLE IIERROR (RMS) BETWEEN EXPERIMENTAL AND ESTIMATED CUMULATIVE

DISTRIBUTIONS

The experimental yearly cumulative distribution is comparedto attenuation predictions obtained from several models. TheSalonen and Uppala model [8] and ITU-R [9] estimate the cloudattenuation from cloud liquid water content. The DAH [7] modelemploys cloud statistics derived from SYNOP reports. The Alt-shuler and Marr and Dintelman and Ortgies [10] modelsuse ground level meteorological data, such as temperature andrelative humidity.

Fig. 3 shows the comparison of several estimated cumulativedistributions of cloud attenuation with experimental data, all ofthem for the five years averaged. The ITU-R, DAH and Salonenand Uppala model predictions show very good agreement withmeasurements. The ITU-R and DAH model deviate from themeasured data for percentages of time lower than 0.1%. TheDintelmann and Ortgies and Asthuler and Marr models under-estimate the measurements in the whole range of time percent-ages.

In Table II, the rms errors between experimental and es-timated cumulative distributions are presented. The bestresult (0.02 dB) corresponds to Salonen and Uppala model.This is because the cloud attenuation component is obtainedfrom the liquid water content, which is calculated for eachradio-sounding profile. The cumulative distribution is thenobtained from the set of individual attenuation values.

IV. CONCLUSION

The type of input data used for the cloud attenuationmodels that have been tested appear to be related to the results

achieved. The best results are obtained with models basedon radio-sounding profiles and poor results are obtained formodels that use ground level meteorological parameters.

ACKNOWLEDGMENT

The authors would like to thank Universiti Sains Malaysia forsupport of equipments.

REFERENCES

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