Ice Depolarization Research Articles

Rain Attenuation Effects on 2.6 GHz WiMAX Networks Deployment in Ghana

WiMAX communication systems operating at 2.6 G frequencies are used for broadband multimedia and internet based services. At these frequencies, the signal will be affected by various propagation impairments such as rain attenuation, cloud attenuation, tropospheric scintillation, ionospheric scintillation, water vapour attenuation, and rain and ice depolarization. Among all the pro-pagation impairments, rain attenuation is the most important and critical parameter. In this research, rain attenuation is calculated at KNUST, Kumasi using ITU-R rain attenuation model. The preliminary results of the work will be used to calculate the attenuation experimentally and comparison can be made, which helps to develop a new rain attenuation model at 2.6 G bands. Rain attenuation is an important aspect of signal propagation above 2.6 GHz frequency. The attenuation time series generation from point rain rate measurement is crucial due to unavailability of actual signal measurements. In this research, a simple and realistic approach has been demonstrated for better estimation of rain attenuation using WiMAX-band signal propagation data and ground rain rate measurements in Ghana. The ITU-R model of rain attenuation has been modified by incorporating an effective slant path model. The effective slant path has been estimated and modeled in terms of a power-law relationship of rain rate data of 2007-2008. The methodology has been validated with the measured data of 2014. Comparison with ITU-R and GMET clearly demonstrates the improved predictability of the proposed model at the present tropical location.

Ice formation in Saharan dust over central Europe observed with temperature/humidity/aerosol Raman lidar

Three gravity‐wave‐induced clouds and a glaciating altocumulus layer were continuously monitored with lidar at Leipzig, Germany, on 21 November 2003. The midtropospheric clouds formed in Saharan dust at heights from 3.5 km (−9°C) to 6.5 km (−27°C). Distinct ice formation in the altocumulus was triggered by the gravity wave. For the first time, an aerosol/cloud study presents height profiles of temperature, water vapor mixing ratio, relative humidity, dust, and cloud optical properties (volume extinction and backscatter coefficients, lidar ratio, depolarization ratio) within the same air column, solely derived from lidar data. The three gravity‐wave‐induced clouds did not show any sign of ice formation. The aged dust particles below 4.5‐km height were probably partly coated and mixed with hygroscopic material and thus deactivated concerning ice nucleation. Ice crystals were generated in isolated air parcels at the cloud edges of a young, optically thin altocumulus layer between 5‐ and 6.5‐km height. An aged altocumulus deck composed of a geometrically thin liquid water layer at cloud top and an extended ice crystal layer (ice virga) was observed 2 hours later in the same height range. Strong ice formation occurred in the altocumulus during the downdraft induced by the gravity wave. Contact freezing was probably the main reason for the observed ice formation on dust particles. Ice depolarization ratios were relatively low with values from 10 to 15% in the altocumulus and indicate plate‐like crystals.

Correction to "Separation of Simultaneous Rain and Ice Depolarization"

Correction to "Separation of Simultaneous Rain and Ice Depolarization"

Separation of Simultaneous Rain and Ice Depolarization

In the case of depolarization due to rain or to ice crystals, the differential attenuation, the differential phase shift, and the canting angle of the depolarizing medium can all be calculated independently from dual polarized measurement results. However, this is not possible when rain and ice are causing depolarization at the same time. This paper presents a calculation method which makes it possible to derive during simultaneous rain and ice depolarization some of the characterising parameters of each part separately. The method makes use of an a priori known relation between the differential attenuation and the differential phase shift of rain and of ice. The calculation method is tested on measured depolarization events, and yields good results.

Slant-path attenuation and depolarisation statisticsat the Ka-band in Belgium

Experimental results of long term attenuation and depolarisation statistics at 20 and 30 GHz using ESA's Olympus satellite are discussed and compared with current prediction methods. While copolar attenuation (CPA) is accurately estimated, crosspolar attenuation (XPD) is poorly predicted, mainly owing to an inadequate account of the ice depolarisation effect.

Results of 20/12.5 and 30/20-GHz XPD-frequency scaling measurements with the Olympus satellite

Reports on three years of crosspolar discrimination (XPD) frequency-scaling measurements in Germany performed by the Research Center of Deutsche Telekom. Concurrent measurements were carried out at 12.5, 20 and 30 GHz using the OLYMPUS satellite. Two different types of 20/12.5 and 30/20 GHz statistics were analyzed: (1) equiprobability and (2) instantaneous XPD-frequency scaling statistics. The results of equiprobability crosspolar scaling studies for total and ice depolarization are in reasonable agreement with the scaling laws proposed by ITU-R with some deviations, however, depending on the depolarization mechanism. In addition, a variability analysis of instantaneous XPD scaling factors was carried out from their joint conditional distributions, showing the spread which can be expected in long-term scaled SPD statistics. Although total depolarization is strongly affected by ice, the variation of the scaled data is relatively small since the scaling behavior of rain and ice depolarization is very similar in the considered frequency range.

Fraction of ice depolarisation onsatellite links in Ka band

The contribution of ice depolarisation and hence implicitly that of rain depolarisation is estimated from two conditional cumulative XPD statistics, derived from three Earth-satellite links at 20 and/or 30 GHz, using ESA's OLYMPUS satellite.

Ice depolarizations on Ka band (20 GHz) satellite‐to‐ground path and correlation with radar observations

The ice depolarization events without appreciable copolarized attenuation are investigated using the CS‐2 beacon signal (19.45 GHz) and the simultaneous radar observations. In these events, increase of instantaneous depolarizations of the beacon signal closely corresponds to appearance of intense radar echoes at the cloud top. Correlation of the time variations between the depolarizations and the radar echoes is, in general, found in the upper half of the ice region. In addition, the large depolarization events are primarily related to the radar echo intensities at 2.0–2.5 km above the bright band. In these heights the ice crystals may have the fastest growth rate around the temperature of −15°C. The impact of ice depolarizations on the satellite‐to‐ground path is shown to depend strongly upon the height and the background temperature at which ice crystals are first produced near the cloud top. This characteristic feature of ice depolarization is also confirmed in the stratus type events with considerable rain attenuation by theoretically subtracting the effects of raindrops from the total observed depolarization.

Distinction between rain and ice depolarisation by calculation of differential attenuation and phase shift

System-induced depolarisation of single-polarised satellite beacon signals can be cancelled with vector cancellation. It is demonstrated that the residual error after application of this bias removal technique depends only on the polarisation tilt angle and the XPD of the satellite antenna. It appears that for the calculation of the atmospheric-induced differential attenuation A and phase shift β, correction of the system-induced differential phase shift for the copolar and crosspolar signals is necessary. An experiment is described to measure this system-induced differential phase shift. It is demonstrated that it is possible to clearly distinguish between rain and ice depolarisation by looking at the scatterplots of the calculated A and β against measured copolar attenuation.

Effects of ice depolarisation on Ka-band satellite-Earth path in stratus rainfall events

In stratus rainfall events, the effects of ice depolarisation are evaluated using the CS-2 beacon signal and X-band radar observations. Although the ice effects described in terms of the deviation from theoretical rain depolarisation show a large variation according to the rain height, net ice depolarisations deduced above this height are relatively constant.

Prediction of depolarisation distributions on earth-space paths

By using the theoretical relationship between attenuation and depolarisation, a cumulative distribution of depolarisation can be predicted from a distribution of attenuation at the same frequency. In the case of earth-space paths, however, it was found that there is poor correlation between measured attenuation and XPD because of so called ice depolarisation. This implies that the predicted XPD value for an earthspace path includes an error if the theoretical relationship between rain attenuation and XPD is used.In this paper, a new method of predicting cumulative distributions of XPD is proposed in which the effects of the ice depolarisation are taken into consideration by using the factor of ‘ice depolarisation ratio’. The applicability of this depolarisation prediction method is verified by using attenuation and depolarisation statistics which have been measured in some wave propagation experiments along earth-space paths.

Effect of Ice-Induced Cross-Polarization on Digital Earth- Space Links

Among the techniques proposed to increase the capacity of the allocated frequency bands, reuse of the same channel twice with orthogonal polarizations seems to be very promising. However, imperfect isolation between the two orthogonal channels, arising, for example, from nonspherical scatterers encountered by the transmitted waves during propagation, is likely to limit the possibilities of this frequency reuse technique. While rain-induced cross-polarization is relatively well understood, the impact of cross-polarization coupling due to ice crystals on communication systems has to be thoroughly investigated. In this paper a statistical analysis of depolarization events observed at 20 GHz from the linearly polarized beacon of ATS-6 is carried out. It shows that ice depolarization events occur as frequently and are as significant as rain depolarization events, and also that ice crystals are typically canted within <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\pm10\deg</tex> of the vertical. From the results obtained, the effect of ice cross-polarization on a dual-polarized microwave link utilizing <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</tex> -ary PSK transmission is investigated. These results point out the necessity of implementing cross-polarization cancellation devices.

The system implications of 6/4 ghz satellite‐to‐ground signal depolarization results from the intelsat propagation measurement programme

AbstractThe results of six experiments to measure 6/4 GHz signal depolarization are presented. The measurement sites varied from the tropics to 57°N latitude and elevation angles between 12° and 38° were employed. In two of the experiments, simultaneous measurements at 6 GHz and 4 GHz were made along the same path to measure the correlation between the depolarizations occurring at the two frequencies.It was found that, in all cases, rain was the dominant depolarizing mechanism. Where the occurrence of ice depolarization was significant the magnitude of the ice depolarization occurring was not system limiting on a statistical basis. The correlation between up‐link and down‐link depolarization occurring along the same path was found to be very good both on an instantaneous basis and a statistical basis. The use of open‐loop up‐link depolarization compensation mechanisms should therefore be feasible. An examination of the depolarization fluctuation spectra obtained in some severe depolarizing events during one of the experiments would indicate that the settling time for such a mechanism should be on the order of 1 s. The use of simple, ‘non‐perfect’, depolarization compensation mechanisms allied to a low‐cost, spectrally efficient FEC system may prove to be the best compromise in some locations that experience severe depolarization.

Data analysis for dual-polarized communication link experiments

Performance of dual-polarized radio links operating in the microwave and millimeter wave regions is affected by the propagation medium. The attenuation ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A</tex> ) and cross-polarization discrimination (XPD) are used to quantify the medium effects. Of particular importance is the XPD versus <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A</tex> relationship. Three ways of displaying XPD versus <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A</tex> data are examined: mean, median, and equiprobable. It is shown that mean and medium are essentially identical measures, whereas equiprobable is sensitive to several error sources and is especially sensitive to ice depolarization. It is concluded that XPD versus <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A</tex> data sets using mean and median can be safely compared, but equiprobable results must be treated with caution.

Calculation of ice depolarization on satellite radio paths

In this paper, theoretical methods are presented for calculating the depolarization of ice clouds. Rayleigh scattering from single particles is used. A general theory is introduced that includes distributions of ice particle shapes and orientation angles for both single and multiple scattering. It is shown that the more accurate multiple scattering formulation yields cross‐polar discrimination values significantly worse than those for single scattering over the 10‐ to 30‐GHz band for high ice content. Comparisons are made to measurements using the SIRIO satellite.

The impact of ice along satellite‐to‐earth paths on 11‐GHz depolarization statistics

This paper summarizes measurements of ice depolarization made using a unique facility consisting of a dual‐site (7.3‐km site spacing), dual‐circularly polarized, low‐elevation angle (10.7°) receiving system for the SIRIO 11.6‐GHz beacon. Significant depolarization (cross‐polar discrimination as low as 10 dB) in the absence of attenuation occurred relatively frequently. However, the impact of the ice on the cross‐polar discrimination statistics for a 6‐month time period was small.

Measurements of ice depolarization at 28.56 GHz using the COMSTAR Beacon simultaneously with a 16.5 GHz polarization diversity radar

Measurements on ice depolarization made at the National Research Council in Ottawa, using a 16.5-GHz polarization diversity radar concurrently with the 28.56-GHz COMSTAR beacon, are reported. The radar, which is normally operated in a circular polarization mode, provides data on polarization parameters such as the cancellation ratio and the degree of preferred orientation. Frequently, the differential propagation parameters can be determined. From these data, identification of different kinds of hydrometeors (i.e., rain, melting layer, and ice crystals) along the path is possible. The beacon receiver measures the differential attenuation and the differential phase shift between two linear orthogonal components of the received signal, and the copolar attenuation. Several occurrences of ice depolarization in the presence of rain were observed. From radar data, identification of the hydrometeors was made and the contribution of each type toward the differential propagation parameters was determined. Good agreement between radar and beacon receiver measurements was obtained. Also several events of ice depolarization in the absence of rain are reported. The results presented here provide new data on ice depolarization and demonstrate the usefulness of simultaneous polarization diversity radar and beacon measurements for obtaining a better understanding of the nature of ice depolarization. Because of the limited observation period, emphasis is given to the nature of the phenomenon rather than to the statistics of its occurrence.

Ice depolarization of the COMSTAR beacon at 28.56 GHz during low fades and correlation with radar backscatter

Radar correlation with significant ice depolarization events accompanied by low copolarization fades of the 28.56-GHz COMSTAR beacon signal are described for an experimental program at Wallops Island, VA. Using a Faraday switch at the front end of the receiver, the copolarization and cross-polarization levels of the 28.56-GHz beacon signal are sequentially monitored. A nearby high resolution S -band radar pointing along the Earth-satellite path monitors the simultaneous ice and rain reflectivity. Excellent correlation is noted between the cross-polarization events and relatively large and extended ice reflectivities along a segment of the Earth-satellite path at altitudes near and above the 0°C isotherm. The radar and receiver data strongly suggest the cross-polarization mechanism is due to a hailshaft which intersects the path at altitudes well below the 0° isotherm. Since the intervening ice results in a cross-polarization signal which either adds or subtracts to the cross-polarization antenna residual, a method is described to remove the residual from the resultant measured cross-polarization level, without employing a phase measurement. Cumulative, month-of-year and time-of-day statistics associated with the depolarization signals are established.

Results from the 19- and 28-GHz COMSTAR satellite propagation experiments at Crawford Hill

Strong interactions of radio waves with rain and ice particles in the lower atmosphere significantly affect the performance of earth-space radio links operating at frequencies above 10 GHz. Because of the uncertainties and variabilities in the microstructures of weather phenomena, direct measurements of the effects of rain and ice on radio propagation are needed for the economic design of reliable satellite communications systems. Unique and comprehensive space-earth propagation measurements have been made at Crawford Hill, New Jersey, using the 19- and 28-GHz beacons on the COMSTAR satellites. This paper summarizes these measurements of rain attenuation, rain and ice depolarization, phase and amplitude dispersion, off-path rain-scatter coupling, angle-of-arrival variations, and cloud scintillation. These results provide empirical design data and will serve as checks on theoretical models needed for predicting communication system performance in geographical regions where measurements are not available.

Depolarization of radio waves by atmospheric hydrometeors in earth‐space paths: A review

Many measurements of earth‐space radio wave depolarization from 10 to 30 GHz have been made in recent years. This review relates the results from these measurements within the framework of theoretical relationships derived from rain and ice depolarization models. The review includes measured statistical relationships between depolarization and rain attenuation and contains cumulative statistics showing the relative frequencies of occurrence of rain and ice depolarization. Depolarization is usually strongest for circular polarizations and for fixed linear polarizations oriented 45° from vertical and horizontal. Depolarization is usually minimized for fixed vertical and horizontal polarizations. For frequencies up to 30 GHz and elevation angles ranging from 10° to 50°, amplitude ratios of ice and rain depolarization are approximately proportional to frequency and to cos2 θ/sin θ, where θ is the path elevation angle.