Transhorizon Propagation Research Articles

Analysis of the Atmospheric Duct Existence Factors in Tropical Cyclones Based on the SHAP Interpretation of Extreme Gradient Boosting Predictions

The atmospheric duct (AD) is an anomalous structure in which electromagnetic waves can make transhorizon propagation. ADs often occur in the formation, development and disappearance of tropical cyclones (TCs). In this work, the eXtreme Gradient Boosting (XGBoost) model is used to predict TC ducts and a relatively high accuracy of 81.3% is obtained. Shapely additional explanations (SHAP) values of the features including TC parameters and local meteorological parameters are employed to interpret XGBoost model predictions of the TC ducts existence. Furthermore, the importance ranking of the features is revealed, among which the distance between dropsondes and TC eyes is the most important. In addition, the detailed relationships between the AD existence and the features are presented. Hence, this work can not only improve the knowledge of the relationship between TC ducts and the features, but also be of great value to the ducts prediction.

A New Model for Estimating Troposcatter Loss and Delays Based on Ray-Tracing and Beam Splitting With ERA5

The atmospheric environment is one of the critical factors affecting troposcatter transmission loss and propagation delay. This article proposes a new estimation model that can accurately calculate troposcatter transmission loss and propagation delay with a numerical weather model (NWM). The ERA5 reanalysis data as the NWM are applied to construct the new model. The 3-D ray-tracing and beam splitting are used to calculate propagation paths and delays. Compared with the existing methods, the new model thoroughly considered the bending and delaying effects of the atmospheric environment on electromagnetic waves, resulting in more accurate estimates. The transmission loss calculation capability of the new model is compared with the International Telecommunication Union (ITU) model and the terrestrial trans-horizon propagation loss data banks. The propagation delay calculation capability is compared with the Bello model. These comparison results show that the new model sufficiently reflects the meteorological environment’s influence on transmission loss and propagation delay. Finally, the daily variation characteristics of losses and delays are analyzed using the new model and further validate the model performance.

C band transhorizon signal characterisations in evaporation duct propagation environment over Bohai Sea of China

Evaporation duct is an anomalous mechanism of transhorizon propagation in marine and coastal regions. In order to better understand the characteristics of signal fading, a C band signal transhorizon propagation experiment was carried out with a 107km path length over the Bohai Sea of China. Using an iron tower with meteorological and oceanographic (METOC) sensors at multiple altitudes, a direct diagnostic method of evaporation duct is presented. The dependence of signal strength on evaporation duct height, antenna height, and METOC parameters is analysed. The experimental results indicate that the signal strength tends to be linearly dependent on the evaporation duct height when the transmitter and receiver antennas are submerged within the duct layer. Normally, the signal strength is stronger when the relative humidity is lower. Stronger signal strength tends to occur under near-neutral or stable atmospheric conditions or when the air-sea temperature difference is higher. The results indicate that the signal strength distribution of fast fading is close to Rayleigh distribution, and the probability density of fade slope is symmetric at 0dB/s. These results have important implications for the design of ducting communications system.

Study on the Prediction of Troposcatter Transmission Loss

In this article, a novel semiempirical average annual troposcatter transmission loss prediction model is presented. The model is generated by optimizing the correlation coefficients of propagation path conditions utilizing the modern optimization algorithm. The statistical relativities of troposcatter transmission loss as captured by changing frequency, path length, scatter angle, and meteorological condition are each analyzed using the terrestrial trans-horizon propagation loss data banks released by the International Telecommunication Union (ITU). For the percentages of time transmission loss not falling below 50%, the model is combined with the model of anomalous propagation mechanism introduced in the Recommendation ITU-R P.2001. The prediction results of this new troposcatter model are compared with other troposcatter models and with the trans-horizon propagation loss data banks. The comparisons show that the new model has a better estimated accuracy.

ESTIMATING SHADOW-ZONE PARAMETERS OF TROPOSPHERIC REFRACTION FROM THE RADIATION OF REMOTE SOURCES. PART II: EXPERIMENT

Obtaining quasi-real time estimates of troposphere refraction is a task of crucial importance for improving the performance of radio systems. This paper presents experimental results on troposphere refraction over the sea, recovered from radio signals of GPS navigation satellites. The work features measurements in the shadow zone where signal parameters are subject to great variations and happen to be particularly sensitive to changes in refraction. The shadow-zone level of the GPS signals has been increased by the use of a parabolic antenna. The observations of radio occultations of GPS satellites over the sea involved cases of standard and enhanced refraction, and of tropospheric ducting. The measured data have been used to retrieve height gradients of the refractive index profiles n(h) in the near-surface layer of the above-sea troposphere. The appropriate calculations are done within the model developed in Part I of the paper, using the data on the occultation angle and slope of the diffraction-controlled portion of the basic propagation loss curve. The magnitudes so obtained are confronted with the 'effective' gradients that are recovered from the signal levels measured along a near-surface trans-horizon propagation path. The correlation factors estimated for elements of the gradient arrays, compiled for the fixed near-surface and the satellite radio links, were about 0.53 to 0.78 (samples taken from neighboring measurements). The figures can increase up to 0.85 or 0.9 upon averaging. The results may prove useful for developing satellite-based systems of diagnosing the troposphere refraction.

Transhorizon propagation of decameter waves

Solutions to the problem of the point source field in a spherically layered medium are analyzed. For a three-layer waveguide model, a solution in the form of the Watson integral was used. A consideration of the singularities in the plane of the integration variable made it possible to represent the integral as a superposition of three waves. Two of them are connected with the interaction of the primary spherical wave with the lower convex and upper concave interfaces. The third wave is connected with the alternate action with both interfaces. The fourth wave is caused by the interaction between the primary wave and random inhomogeneities of the external medium (the ionosphere). Here, simulation was carried out based on Green equations. The considered unique data of flight measurements of the point source field strength indicate the efficiency of simulating the transhorizon propagation of decameter waves based on the superposition of all four aforesaid wave packets.

Path loss statistics and mechanisms of transhorizon propagation over a sea path

Carrier amplitude data at 11.6 GHz captured on four parallel transhorizon links over the sea have been divided into four groups according to the identified dominating propagation mechanisms. The statistics of path loss are each found to be lognormal. The rate of occurrence of the mechanisms is investigated relative to transmitter/receiver heights, diffraction and evaporation ducting.

Anomalous transhorizon propagation and meteorological processes of a multilink path

An event‐based approach is developed and applied to 3 years of data of a four link transhorizon path over the English Channel (La Manche). The signal enhancements (events) observed on the chart recordings during anomalous propagation are selected and, according to their dynamical signatures, categorized into three main patterns of ducting, layer reflection, and high k factor diffraction. The statistical analysis carried out on this event basis gives the correlation between the signal levels of different links and the cross correlation between signal levels and local N index derived from surface meteorological monitoring. The results reveal the fundamental properties of the anomalous propagation regimes. Using case studies of several interesting events, the meteorological processes and resultant impact on transhorizon transmission are also explained. This extensive work contributes to a better understanding of anomalous propagation mechanisms present in transhorizon links.

Dynamic behaviour of radio channels due to trans-horizon propagation mechanisms

To study the propagation mechanisms of trans-horizon radio links, the Letter proposes an event-based approach. Data from one year captured on a trans-horizon link of 156 km are divided into different groups according to the signal amplitude signature of the chart recordings. The propagation mechanisms for each group are investigated using the cross-correlation between received signal amplitudes along the same path. These amplitudes are obtained using two different transmitter antenna heights.

Worst month prediction model for transhorizon microwave interference

A model is developed to calculate the (average) worst month clear-air microwave interference statistics for land, sea and mixed land/sea paths. The model predicts the parameter Q, which relates the worst month statistics to the average annual statistics, derived either by measurementsor by prediction. By providing a reliable means of computing worst month statistics, the model addresses a major deficiency in the existing CCIR interference prediction procedures. The validity of the model is established by comparison with measured data from 1.3GHz transhorizon propagation experiments in northwest Europe.

The effects of atmospheric convective circulation on a narrow radio beam

In considering the basic mechanism of heat exchange by convective circulation in the atmosphere it is apparent that the resultant thermal cell structure, as proposed by Bénard, may possess refractive index properties which will influence the cross-sectional shape of a narrow conical radio beam. A simplified classical toroidal cell is considered for the purpose of analysis to show that the core of such a cell in the atmosphere may act as a cylindrical concave lens. The resultant distortion of a narrow beam is discussed, and the concept is then developed further to show that trans-horizon propagation may result from an in-line array of thermal cells.

Propagation study for a tropospheric transhorizon radar

Abstract : Computation methods for tropospheric propagation losses in transhorizon propagation are used to provide estimates for the detectability of over-the-horizon targets. Losses depend on frequency, distance, climate, refractivity, and on an empirically derived attenuation function which also takes into account the effects of antenna heights. Equivalent antenna temperatures are assumed to be due to galactic noise. Receiver noise is characterized by noise figure F. Signal-to-noise ratios are illustrated for several target cross-sections as a function of frequency and distance, parametric in antenna heights, distance or frequency, for the median loss 50%, as well as 1, 10, 90, and 99% levels of variability.

A review of transhorizon propagation phenomena

Interference problems underlie the current interest in transhorizon propagation. In particular, statistics of the rare, high‐level fields are of interest. This paper reviews the propagation mechanisms which produce the high‐level fields and summarizes recent work in the modeling of the transhorizon propagation.

Interference on microwave digital radio links due to transhorizon propagation

General periods of meteorological conditions which lead to multipath effects on line of sight links can also be those during which anomalous transhorizon propagation is present. An empirical method for predicting the probability of outage due to the resulting interference is described, which is based on the prediction models for the individual multipath and transhorizon effects.

Atmospheric structure, transhorizon propagation, and their relationship to remote sensing

Previous papers suggest that the characteristics of atmospheric structure cannot be inferred unambiguously from transhorizon scatter measurements. It is shown that the form of the refractivity spectrum can be correctly inferred from appropriate beam-swinging measurements. One analysis includes contributions from both scattered and partially reflected components of the transhorizon signal, and shows how the characteristics of the angular response pattern of the received signal varies with changes in these components. Analytically derived patterns are compared with measured response patterns. The form of the refractivity spectrum inferred from the comparison is consistent with the form suggested from physical arguments and by the results of independent transhorizon experiments. Because the transhorizon signal includes scattered and partially reflected signals, remote-sensing techniques using transhorizon methods should employ both signal components to correctly derive atmospheric parameters. Finally, both of these signal components should be used for channel models that use adaptive algorithms to optimize the design of troposcatter communication networks.

Tropospheric transhorizon propagation characteristics at 2 GHz in Northern India

Based on radiometeorological observations at 2 GHz and 120 MHz over two co-located transhorizon tropospheric propagation paths in Northern India during 1971, this paper describes the propagation characteristics and analyses the mechanisms of propagation at 2 GHz. Correlation of hourly median signal with refractivity gradient was found to be good while correlation with surface refractivity was much poorer. Evidence is presented to show that scattering was responsible for weaker values of observed signal, reflexion for intermediate signal levels which include the long-term median, while the guided mode was responsible for the higher levels of observed signal.

Trans-horizon propagation techniques for examining disturbances in stratified tropospheric layers

This paper discusses the merits of using measurement techniques, in trans-horizon propagation experiments, which obtain various types of information simultaneously - such as, information on angle of arrival, transmission delay, Doppler spectrum, etc. It points out examples where such techniques have been used to a limited degree. It emphasizes the importance of extending this type of approach in the application of trans-horizon measurements to the study of specific formations on layers: waves and instabilities.

Review of Techniques in Transhorizon Propagation Research and their Relationship to Parameters of the Troposphere

Transhorizon propagation techniques and their usefulness in determining atmospheric parameters are discussed. It is concluded that the state or structure of the atmosphere cannot be inferred unambiguously from individual measurement techniques used in experiments reported to date. However, because of the sensitivity of the transhorizon radio signals to changes in the parameters of the troposphere, it is suggested that combinations of available techniques be pursued to determine the parameters uniquely.

Modeling UHF transhorizon scatter propagation

A laboratory model of a transhorizon propagation path has been used in an experiment designed to simulate the transmission of UHF energy beyond the radio horizon via the mechanism of tropospheric scatter. Electromagnetic energy was scattered over the radio horizon by dropping very small glass spheres through the beam of the transmitting antenna.

Observation of Wind‐Produced Doppler Shifts in Tropospheric Scatter Propagation

Persistent average frequency shifts obtained from phase data have been consistently observed on a 230‐km closed‐loop troposcatter circuit when the antennas were synchronously pointed off the great‐circle azimuth. The magnitude of the “Doppler shift” systematically increases with the antenna pointing angle from the great circle. The sign of the Doppler shift reverses across the great circle and depends upon the direction of the crosspath wind simultaneously observed in the lower common volume. Average Doppler shift data are presented for several fixed‐antenna‐offset and antenna‐beam‐swinging experiments conducted under varied meteorological conditions. Observations are interpreted in terms of atmospheric “scatterers” drifting across the path at the average crosspath wind speed. A model based on phase geometry is derived for the average Doppler shift and for Doppler spectra in terms of the crosspath wind velocity, antenna patterns and propagation mechanism.It is concluded that Doppler frequencies observed with a bistatic transhorizon microwave system depend primarily upon crosspath wind speed and the prevailing atmospheric structure. Crosspath motions thus appear to be an effective source of interference fading in tropospheric transhorizon propagation.