Frequency scaling of slant-path attenuation in tropical regions

Measured slant-path attenuation data currently available do not cover every eventuality, thus requiring the use of scaling algorithms to produce estimates of the transmission parameters at a particular site. Several multiple frequency propagation experiments have been conducted over the same path, and the resultant data have been used to derive frequency-scaling models of path attenuation. Most of these experiments have been conducted in temperate latitudes, and there is some doubt that the scaling models will fit other latitudes, particularly those subject to severe rain climates. James Cook University of North Queensland, Australia has been operating a number of radiometers as part of an experiment conducted for INTELSAT. The radiometers used in this experiment operate at 7·5, 11·6, 19·5 and 28·5 GHz with a common azimuth and an elevation angle of 63°. This paper compares the results at the various frequencies with an existing frequency-scaling model and comments on the comparison.     

The intelsat slant-path radiowave propagation experiments in tropical regions

The requirement for INTELSAT to operate world-wide in a diverse range of climates has motivated a number of experiments within the INTELSAT R&D programme to identify and model propagation impairments. The effects of some propagation phenomena, such as depolarization, were anticipated. Others (e.g. ionospheric scintillation at gigahertz frequencies) were unexpected or exhibited unusual features (rain attenuation in tropical climates). This paper presents the background of the INTELSAT R&D propagation measurements, with special emphasis on those in tropical regions, and provides the principal results. Several ongoing experiments are also discussed.     

Modelling attenuation by rain in tropical regions

The data in the CCIR data banks were employed for a statistical study of the relative performance of several rain attenuation prediction procedures in temperate and tropical regions. The results show that the models worked well, in general, when used for prediction at latitudes more than 30° from the equator, but, in the equatorial region, significant prediction errors were observed for all the models. Three sources of error were discovered. The most important is the use of too few rain climate zones to span the wide range of rain conditions present in the equatorial region. The second is an inadequate procedure for taking the naturally occurring vertical variations of specific attenuation into account. Finally, for the CCIR attenuation prediction model, the use of a universal shape for the cumulative distribution of path attenuation must be called into question.     

CCIR data for slant-path propagation in tropical regions: A critical review

The development of methods for predicting the effects of the troposphere and ionosphere on Earth-space paths is a topic of major interest in the work of CCIR. A principal objective is that the methods have world-wide applicability and, being essentially empirical in nature, their development relies on the availability and voluntary contribution of world-wide data, as well as on a knowledge of the global behaviour of the propagation mechanisms involved. Despite some advances in recent years, CCIR propagation prediction methods continue to lack universal applicability, in particular in low latitude tropical areas. This is largely due to a shortage of measurement data for the regions concerned; hopefully, this situation is changing in view of recent efforts to establish appropriate propagation experiments. The paper reviews the current status of CCIR propagation data and information relating to the design of space telecommunication systems in tropical regions, making close reference to the studies and texts of CCIR Study Groups 5 and 6 (Propagation in non-ionized and in ionized media, respectively). Shortcomings in the available information and the lack of general world-wide applicability of the models, highlight the propagation effects for which further data are still required.     

11·6 GHz rain attenuation measurements in Peru

This paper presents the results of a K_u-band propagation experiment conducted in a high-rainfall-rate region in Peru. Sky noise measurements were made with an 11·6 GHz radiometer over a period of 18 months. Simultaneous rainfall was also recorded with a tipping bucket rain-gauge. The paper describes the experimental equipment, the characteristics of the measurement site, and the results obtained from the experiment in the form of long-term distributions of rain rate and path attenuation. Duration statistics of attenuation are also presented. In the absence of an absolute calibration of the radiometer in terms of the path attenuation, selection of the best medium temperature to be used poses a problem. For comparison, sky noise to path attenuation conversion was done with two medium temperatures, 280 and 290 K. When compared with model predictions, 290 K seems to be more appropriate for the climate concerned. Both the attenuation and fade duration statistics are found to follow a lognormal distribution fairly closely.     

Preliminary plans for a propagation experiment in Mali

Measurements of propagation effects on satellite down-links to tropical locations are rare. This paper describes two experiments that will provide new and useful data for radiowave propagation in tropical Africa. A discussion of the African tropical climate is included that shows the wide range of rainfall patterns throughout the continent. Also, the broad distribution of climate in Mali, the West African country where the experiments will be located, is described. A rain-rate measurement programme involving autonomous terminals based on tipping bucket rain-gauges is presented. An experiment measuring the slant-path attenuation of the 12·5 GHz beacon of the ESA satellite OLYMPUS is described. The paper also discusses how the measured data in both experiments will be relayed daily from Mali to Virginia Tech by the low earth orbit satellite VaSTAR.     

Some aspects of tropical rainfall and their effect on microwave propagation

Some characteristics of tropical rain rate and raindrop size distribution are briefly reported, making use of the data obtained at locations in Nigeria. Models and empirical relations have been obtained for the computation of the effects of tropical rain on radiowave propagation at centimetre and millimetre waves. Results are presented for the effect of integration time on rain rate as well as the rain-rate duration characteristics, rain-induced phase shift, attenuation and depolarization, in addition to the relationship between the rain rate and other rainfall parameters. Comparisons have also been made in many cases with the CCIR reports and results from other locations.     

Ku-band slant-path radiometric measurements at three locations in Brazil

This paper presents results of the 12 GHz propagation experiment that is being conducted in Brazil. The data obtained are useful to investigate the impairment due to rain attenuation in satellite links operating in tropical and equatorial climates. Radiometric sky noise temperature and rainfall rate are being measured at three locations. At one location, site diversity measurements are also being conducted. Cumulative distributions of sky-noise temperature, equivalent attenuation and rainfall rate at each site are presented together with worst-month statistics. Also, the joint-probability distribution of equivalent attenuation at the diversity sites is shown. Finally, the measured probability distributions of attenuation are compared to those obtained from the measured rainfall rate distributions, using prediction models.     

Prediction of attenuation due to rain: Characteristics of the 0°C isotherm in temperate and tropical climates

A detailed investigation of the vertical structure of rain and of the characteristics of the 0°C isotherm height in both rainy and non-rainy conditions has been carried out in connection with the prediction of rain attenuation on Earth-satellite links. Radiosonde data have been analysed for eight and nine locations in the southern and northern hemispheres, respectively. The temporal, latitudinal variations and the relationship between the 0°C isotherm height and ground meteorological parameters such as rainfall, temperature and relative humidity, are reported. The equivalent rain height measured along the Earth-satellite link is compared with the heights obtained from meteorological data.     

Measurements of 11 GHz attenuation using the intelsat V beacon in Indonesia

A campaign of measurements in Indonesia has been carried out according to a co-operation agreement concluded between the Indonesian and French Administrations with the aim of estimating rain attenuation on slant paths. The experiment was planned using two earth-stations, at Cibinong and Padang, both receiving a K_u-band beacon from an INTELSAT V satellite, coupled with rain-rate measurements at a number of sites. Rain-rate statistics are given for several locations in the Indonesian Archipelago. Cumulative distributions of attenuation and simultaneous rain rate, obtained at the earth-stations, are presented. The analysis of the results has led to the proposal of an adaptation of the CCIR attenuation prediction method for low latitudes.     

The joint African radiometric propagation measurement programme

This paper summarizes the principal aspects of a major co-operative radiowave propagation experiment that was designed to collect data for improving rain attenuation prediction models for tropical Africa. A pressing need for such data had previously been identified by Resolution 79 of the CCIR. In a unique joint arrangement with three African governments, INTELSAT, COMSAT, the U.S. Agency for International Development, the U.S. National Telecommunications and Information Administration and the U.S. Telecommunications Training Institute (USTTI) collaborated in setting up a K_u-band radiometric measurement campaign in Cameroon, Kenya and Nigeria. A brief historical overview is given, together with the major technical parameters of the sites and the equipment installed there. The anticipated characteristics of the three locations are outlined with regard to meteorological and propagation conditions, and some preliminary indications of the results are presented based on an inspection of the early event data.     

Tropical rainfall drop-size distribution

Based on disdrometer measurements carried out in Brazil, this paper discusses the problem of rainfall drop-size distribution in tropical areas and its effect on radiowave propagation at frequencies above 10 GHz. A comparison between Laws and Parsons and lognormal models is made. In spite of the best fit to the experimental data shown by the lognormal distribution, in the range from 10 to 20 GHz there is no significant difference between the two models. However, as the frequency increases this difference becomes larger and may be an important source of error in the design of both terrestrial and satellite links in low-latitude regions. Similar results from Nigeria (Africa) are also taken into account.     

The measurement of rainfall in Papua New Guinea,and its effect on microwave propagation

A programme of measurement on rainfall characteristics has been commenced at Lae, Papua New Guinea, using pluviometers, tipping bucket rain-gauges and a rapid-response rain-gauge. Pluviographs extending over many years have been analysed by computer to give data on general rain conditions and on the probability of rainfall intensity exceeding particular levels. The resultant distribution compares well with the empirical formula of Moupfouma, and corresponding parameters are given. Simultaneous measurements are also being made on the 12 GHz signal level variation from the Aussat television signal, and on the attenuation on a 10 GHz terrestrial link.     

Elevation angle dependence of fading for satellite PCS in urbanareas

Images of urban Japan taken vertically through a 180° fisheye lens were analysed to derive, as a function of elevation, the fractions of sky that are clear, shadowed by trees, or blocked by buildings. At a 32° elevation, the results match those derived from satellite measurements fit to a three-state fade model. Using the same model, the elevation angle dependence of mobile satellite fading is predicted for the first time     

Elevation angle dependence of fading for satellite PCS in urbanareas

Images of urban Japan taken vertically through a 180° fisheye lens were analysed to derive, as a function of elevation, the fractions of sky that are clear, shadowed by trees, or blocked by buildings. At a 32° elevation, the results match those derived from satellite measurements fitted to a three-state fade model. Using the same model, the elevation angle dependence of mobile satellite fading is predicted for the first time     

Path loss prediction in microcellular environments at 900 MHz

A model is proposed to estimate path loss in urban environments at 900 MHz when the base station antenna is below the average height of the buildings. It shows that the percentage of area occupied by buildings explains more than 20 dB of variation of the mean path loss.     

Rainfall rate characteristics for microwave systems in tropical and equatorial areas

The wave propagation at frequencies from about 10 GHz can suffer rain effects such as attenuation, scattering and depolarization. The magnitude of these effects depends on rain characteristics. This paper presents results of the propagation experiments undertaken in Douala (Cameroon) and in Brazzaville (Congo) under the auspices of the ITU (Geneva) with the collaboration of CNET (France Telecom). Data based on five months measurements of rain rates in Douala with a network of four rain-gauges are presented with reference also to some results of a propagation campaign in Brazzaville. The rain measurement period in Douala includes all of the rainy season. The measurement data are used to investigate the rain effect characteristics of the low latitude tropical regions of Africa and are discussed in relation to the influence of rain on microwave system performances in the region.     

Analysis of the influence of building modelling in the prediction of over-rooftop multiple diffraction losses in urban areas

A study of the influence of building modelling when predicting the signal attenuation due to over-rooftop multiple diffraction in urban environments is presented. A straightforward formulation expressed in terms of Uniform Theory of Diffraction (UTD) coefficients is used for the above-mentioned analysis assuming a spherical-wave incidence over the array of buildings and considering the latter modelled as series of different representations such as knife edges, double knife edges, wedges and rectangular sections. The results clearly indicate the importance of an appropriate building modelling in obtaining realistic path-loss predictions in urban environments when multiple diffraction has to be considered.     

Land mobile communications in Ku-band. Results of a test campaign on eutelsat I-F1

This paper presents and discusses the results of technical tests on land mobile communications conducted in June and July 1989 in Europe using a EUTELSAT satellite. The land mobile communication system under test was that already operated by QUALCOMM in the U.S.A. under the registered name OmniTRACS. The field trial was organized in conjunction with the Signatories who expressed to EUTELSAT an interest in a full-scale technical investigation of the system. The aim of the field trial was to evaluate the technical characteristics of the system and test its performance in different operating conditions. The results establish the relationship between transmission performance (i.e. essentially the required number of transmission attempts in order to get a message through the communication channel), and the environment of the land mobile, the satellite transponder's mode of operations and the interference conditions.