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R&D N 35/2005

Terje Tjelta, Lars Erling Brten and Tor Ove Breivik

Predicting the Attenuation

Distribution on Line-of-Sight Radio

Links Due to Melting Snow

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Predicting the Attenuation Distribution on Line-of-Sight...

Telenor R&D N 35/2005

R&D ScientificDoc.

N 35/2005

Title Predicting the Attenuation Distribution on Line-of-

Sight Radio Links Due to Melting Snow

ISBN

ISSN 0809-1021

Project No TFRN36

Program Access Networks

Security Gr. OPEN

No. of pages

Date 2006.01.19

Author(s)Terje Tjelta, Lars Erling Brten and Tor Ove Breivik

Subject headingsAttenuation due to melting snow and sleet

AbstractThere are a large number of radio links working under climate conditions where meltingsnow should be taken into account, as well as other propagation effects. This is, inparticular, important since it is established that melting ice and snow particles attenuate

electromagnetic waves more than water particles with the same amount of liquid water.This paper reviews some basics of propagation through sleet. It summarises the recentlyproposed Bacon and Eden prediction method, applies it, and compare with measurements.Experimental data provide evidence for critical parts of the prediction method and fullmonthly distributions compares well with the predicted ones in the shallow regions likelycaused by melting snow.

The Norwegian Defence Logistic Organisation, Telenor Nordic Fixed and Norkring arejointly financing the project.

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Telenor ASA 2005.11.23

All rights reserved. No part of this publication may be reproduced or utilized in any form orby any means, electronic or mechanical, including photocopying, recording, or by anyinformation storage and retrieval system, without permission in writing from the publisher.

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Contents

1 Introduction ......................................................................................2

2. Line-of-sight radio links in regions with wet snowfall...............3

3. Melting layer attenuation...............................................................4

4. Basics of the combined rain and wet snow attenuationprediction method............................................................................6

4.1. Zero degree isotherm and rain height ................................................................ 64.2. Distribution of the zero degree isotherm height ................................................6

5. Experimental evidence....................................................................85.1. Observations in Japan and Canada .................................................................... 8

6. Conclusions .....................................................................................12

8. References......................................................................................13

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2 - Telenor R&D N 35/2005

1 IntroductionIt has been known for over 50 years, that melting ice or snow particles attenuateelectromagnetic waves more than water particles with the same amount of liquid water.Also the first attempts to estimate the excess attenuation compared to what droplets withthe same amount of water causes are old. It is now well established that sleet, or meltingsnow, can cause attenuation several times more than that caused by rain of the same rainfallrate intensity.

Correct dimensioning of radio systems is important from the point of view of providingsatisfactory service and making best possible use of the frequency spectrum. Rather thanusing an arbitrary additional margin to account for sleet attenuation it is of great interest tomake use of an accurate prediction method. This also minimise the misuse of spectrumimproving the overall economy of radio systems. This has been an un-solved problem for

many years.However, recently a method has been proposed to predict the attenuation distribution due toboth rain and sleet on any line-of-sight radio link. There are several reasons for theslowness of developing a prediction method for wet snow compared to predicting theattenuation due to rain only. The problem is complicated both from a theoretical as well aspractical measurements points of view. Several theoretical results give confidence in theestimated extinction coefficients for sleet particles, but there is a great variability due to theparticle shape and the mixture of liquid water with snow and ice. The measurements aredifficult because it is important to only estimate the effect of sleet in the atmosphere andnot antenna wetting or wet snow effects on the antenna radomes.

Until recently, there have not been available global maps describing meteorological

features to help describing the amount of wet snow in the total rainfall at any point of thesurface of the Earth. However, recently Bacon and Eden suggested a method that makes useof global maps and indirect information to make an estimate of the amount of sleet and anaverage attenuation profile to obtain excess attenuation in the melting layer. It furtherassumes a certain distribution of the height of rainfall, or zero-degree isotherm, as well as afixed shape of the melting layer excess attenuation as a function of the position in the layer.With this information it is possible to predict the attenuation distribution on any link takingboth rain and sleet into considerations.

This report reviews some basics of propagation through sleet and is based on [1]. Itsummarises the Bacon and Eden prediction method and applies it and compare withobserved results published. In particular data from Japan obtained in the fifties to seventies

have been found very interesting, in addition to data from Canada. These data provideattenuation and precipitation during sleet events. There have been a number of reports fromseveral parts of the world indicating excess attenuation due to the melting layer. Analysis ofnew data from recent measurements in Norway is compared with the predictions. In spite oflimited amount of measured data available, the proposed prediction method is promising. Adetailed description of the measurement set-up is given in [2].

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Telenor R&D N 35/2005 - 3

2. Line-of-sight radio links in regions with wet

snowfallRadio links are extensively used in all parts of the world, also in regions with a significantamount of frozen precipitation. These links are designed to operate satisfactory according tolocal climate conditions. Wet snow, or sleet, is a condition difficult to account for, sincethere has been no available prediction method. However, recently, Bacon and Edenproposed a prediction method generally applicable everywhere [3].

Heavy radiowave attenuation in the melting layer, consisting largely of wet snow, may besignificant for radio links in climate regions such as coastal Norway. This may be aproblem for telecommunication operators guaranteeing reliable service with short outagetimes independent of weather. Rain attenuation can generally be ignored for frequencies

below 5 - 10 GHz. The importance of rain attenuation with respect to propagationimpairment increases rapidly with frequency. Even for lower frequencies wet snow cancause significant attenuation events compared to pure rain due to the larger size of the sleetparticles compared to raindrops. Dry snow, which is a mixture of ice and air, is known tocause insignificant attenuation on radio links.

The current ITU-R recommended method for predicting attenuation caused byhydrometeors does not take into account sleet precipitation. The link attenuation iscalculated based on the annual rain rate statistics. One of the challenges when performingexperimental work on sleet attenuation is to characterise the form of precipitation.Avoidance of sleet accumulating on the surface of the antennas is important to avoidanother source of anomalous propagation degradation, when the propagation impairmentsof the transmission medium are of interest.

Prediction of melting layer attenuation should take into account the link geometry relativeto the melting layer. If the link is elevated, contributions from different particle types withvarying degree of relative water content, such as rain and sleet, must be accounted for. Thisis slightly different from estimating rain attenuation, where an effective path length is usedto account for both the path length through the rain and the inhomogeneous rain intensity.

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Telenor R&D N 35/2005 - 5

Figure 2 Zero-degree isotherm

The proposed multiplication factor is given by g(h), Equation (1)

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N_35_2005

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