P.A. Watson

Analytical models for cross-polarization on earth-space radio paths for frequency range 9–30 GHz

Depolarization and attenuation of radiowaves along earth-space paths due to rain storms are characterised. Frequency-dependent expressions for specific attenuation and phase shift at (PC and 20°C and for Laws-Parsons raindrop size distribution are given. Using small amplitude and phase approximations, a simple relation for cross-polar discrimination due to rain in terms of co-polar attenuation, frequency, angle of elevation and polarization angle is derived. Expressions for depolarization due to ice crystals are given, treating them as Rayleigh scatterers of spheroidal shape. For both rain and ice the relationship between linear and circular crosspolar ratio can be shown to be simply sin 2 θ, where θ is the polarization angle.AnalyseOn caractérise pour les liaisons Terre-satellite la dépolarisation et l’affaiblissement des ondes électromagnétiques causés par les hydrométéores. On présente des expressions pour l’affaiblissement linéique et le déphasage linéique en fonction de la fréquence à 0°C et 20°C pour une distribution de la dimension des gouttes conforme à la loi de Laws-Parsons. En utilisant une approximation pour les petites amplitudes et phases, on déduit une expression simple pour le découplage de polarisation en fonction de l’affaiblissement de la composante copolaire, la fréquence, l’angle de site et l’angle de polarisation. On présente des expressions pour la dépolarisation par les particules de glace. Les particules sont traitées comme des diffuseurs de Rayleigh sphéroïdaux. On montre que pour la pluie et la glace, la relation entre les découplages pour les polarisations rectilignes et circulaires est simplement sin 2 θ si θ est l’angle de polarisation.

Evidence for the presence of turbulent attenuation on low-elevation angle Earth-space paths. I. Comparison of CCIR recommendation and scintillation observations on a 3.3/spl deg/ path

This paper presents the results of and conclusions drawn from two experimental studies of low-elevation slant-path scintillation. The paper describes observations made on an 11.2 GHz, 3.3/spl deg/ path over a two and a half year period. Received signal strength and a variety of concurrent meteorological parameters were recorded. The resulting database has enabled the veracity of the 1990 International Telecommunications Union-Radio [(ITU-R)-formerly International Consultative Radio Committee (CCIR)] model to be determined. An interesting discrepancy between the CCIR prediction procedure and the experimental data suggests an as yet unmodeled scintillation mechanism operating in wet summer weather.

Evidence for the presence of turbulent attenuation on low-elevation angle Earth-space paths. 2. Frequency scaling of scintillation intensity on a 10/spl deg/ path

For pt.1 see ibid., vol.45, no.1, p.73-84 (1997). This paper presents the results of and conclusions drawn from two experimental studies of low-elevation, slant-path scintillation. Part I related to 11.2-GHz measurements at an elevation angle of 3.3/spl deg/. Part II describes 14- and 11-GHz observations made on a (nominal) 10/spl deg/ path over a four year period. The resulting time-series data have enabled frequency scaling to be investigated in some depth. Scintillation occurring during periods unassociated with fading is found to be well modeled in its frequency scaling behavior by classical theories of clear air turbulence (CAT). This is not the case, however, for scintillation that occurs during, just before, or just after rain-induced fading. A rapidly fluctuating attenuation mechanism, not separable from CAT-induced scintillation on the basis of its spectrum alone, is thought to be responsible for this anomalous frequency-scaling result.

Review and comparison of tropospheric scintillation prediction models for satellite communications

An overview of the origin and characteristics of tropospheric scintillation is presented and a measurement database against which scintillation models are to be tested is described. Maximum likelihood log-normal and gamma distributions are compared with the measured distribution of scintillation intensity. Eleven statistical models of monthly mean scintillation intensity are briefly reviewed and their predictions compared with measurements. RMS error, correlation, percentage error bias, RMS percentage error and percentage error skew are used in a comprehensive comparison of these models. In the context of our measurements, the ITU-R model has the best overall performance. Significant difference in the relative performance of the models is apparent when these results are compared with those from a similar study using data measured in Italy.

Rain rate and drop size distribution measurements in Malaysia

Satellites for television, telephone and data communication employ ever-increasing radio frequencies in order to satisfy the growing demand for communications. At frequencies above 10 GHz, the effects on radio propagation of clouds, snow and, in particular, rain have to be considered. Measured data, and models to predict the effect of rain on communication systems, are therefore required before establishing satellite communication systems. Crane (1990) has indicated that out of 317 rain attenuation data bases available from CCIR either from radiometer or beacon receiver measurements, only 18 are in the wet tropical region. In view of the importance of such data, several measurement programmes have recently been made or proposed throughout the world concentrating on tropical regions. Preliminary results of rain behaviour in wet tropical climates such as in Indonesia, Papua New Guinea and Malaysia have already been reported. However, it is too early to derived a definitive attenuation model since long term data collection is required and each location has a different geographical nature which plays a significant role in the rain rate distribution. In this paper the results of one year programme of measurements of rain rate and drop size distribution (DSD) are presented.<<ETX>>

Generation of attenuation time-series for EHF SATCOM simulation

In this paper, a method for deriving time-series of attenuation on fixed satellite or terrestrial links is described. The method uses meteorological model forecast data and radar data and hence permits time coincident derivations for multiple sites that include the spatial correlation properties inherent in weather systems. The success of the technique depends upon insertion of the short interval temporal properties (varying typically over 1 second to 15 minutes) that are statistically independent between stations. The technique has immediate application to simulation of the performance of multi-media satellite to earth links and has longer-term significance in terms of operational real-time performance prediction.

Rain attenuation measurements at 6.75 GHz in Malaysia

Attenuation of radio frequency signal by clouds, snow and in particular, rains have to be considered at frequencies above 10 GHz. Indeed, below 10 GHz, rain may cause severe degradation at certain threshold frequencies especially when operating in tropical regions. At high rain rates, it causes significant absorption, scattering and depolarization. Therefore, data and prediction models are required for establishing any satellite-Earth communication links. The successful development of predicted models calls for the knowledge of propagation characteristics along the signal path. Such knowledge is acquired both from theoretical studies of relevant propagation phenomena and statistical analysis of long-term propagation data bases. In view of the importance of such data the University has started to do measurements to model the slant path attenuation and the rain structure. In this paper, results of a year of measurement of radiometer and rain gauges are presented. The measured sky noise temperatures are analyzed in both event and statistical terms. The values of the sky temperature are converted into attenuation using the radiometer equation with medium temperature of 290 K. The cumulative distribution of attenuation was calculated. The rain rate measurements are made using two tipping bucket rain gauges employing one-minute integration time. Investigation of several events shows that the rain rate is not uniform over a short distance (3 km) especially at high rain rates. Comparison of rain rates and attenuation with the International Radio Consultative Committee (CCIR) proposed model shows discrepancy especially at low percentages of time. More data are required to develop a new model.