Troposcatter Propagation Research Articles

Study of the Deterministic Transmission Loss Prediction Model for Troposcatter

Troposcatter transmission loss prediction models are based mainly on ITU-R’s empirical models. However, the available troposcatter propagation models do not enable the real-time calculation of the transmission loss for a specific propagation link. Therefore, the primary purpose of this study is to develop a deterministic troposcatter transmission loss prediction model to realize the real-time calculation or prediction of the transmission loss. In this study, the turbulent scattering cross-section, the refractive turbulence structure constant and the radar equation were introduced to our approach, and the troposcatter transmission loss prediction model was established based on the geometry of the troposcatter circuit. In addition, a weighted method for taking the value of the refractive turbulence structure constant, generated from the WRF schema, was proposed, based on the ray tracing method. Furthermore, the proposed model was validated with the experimental measurement of troposcatter, which verifies the proposed model’s availability and reliability.

A Survey and analysis on a troposcatter propagation model based on ITU-R recommendations

Signal scattering in the troposphere is a phenomenon referred to as “troposcatter”, and it is promising method for wireless communication beyond the line of sight (b-LOS). For a troposcatter communication system, it is important to develop a channel model for the tropospheric scatter. Recently, b-LOS communication using troposcatter received much attention for its application to military communication. Thus, a comprehensive review and analysis of conventional ITU-R troposcatter propagation models and recent results would be a timely study. Accordingly, in this study, we analyze and simulate the tropospheric scatter propagation model proposed by the ITU-R. Troposcatter propagation models in ITU-R recommendations consist of three parts: basic transmission loss, gaseous absorption, and a precipitation fading model. The ITU-R P.2001 simulation model shows basic transmission losses of 210 dB and 240 dB at 0.2 GHz and 2 GHz in 300 km distances, respectively. The ITU-R P.452 model results in 20 dB less loss, but this is dependent on measurement conditions. Gaseous absorption and precipitation fading have less than 1 dB loss for frequencies less than 2 GHz, but can increase to be over 10 dB in loss for frequencies of 2 GHz and above. We also review recent studies on tropospheric scatter channel models and communication systems. In addition to these reviews, we examine recent studies on ray tracing methods for tropospheric scatter channel models.

A High-Precision Calculation Method for Troposcatter Propagation Delay by Fusing Atmospheric Reanalysis Data

Troposcatter propagation delay is one of the most significant sources of errors in troposcatter time comparison. The existing methods of calculating troposcatter propagation delay face problems in terms of effectively reflecting the influence of the meteorological environment on troposcatter propagation delay, due to the insufficient spatial and temporal resolution of the meteorological data. This article proposes a high-precision calculation method for troposcatter propagation delay based on atmospheric reanalysis data. The troposcatter propagation path and the refractive index along the troposcatter data were obtained by combining 3D ray tracing with the European Centre for Medium-Range Weather Forecasts’ Reanalysis 5 (ERA5), with high spatial and temporal resolution. This found the hour-level time delay in troposcatter. The geometric delay, path delay, and total propagation delay of the troposcatter were calculated and analyzed via 12 scattering links in 6 typical geographical regions. It was found that the path delay was the main cause of the propagation delay of troposcatter, and that the proportion of geometric delay in the total propagation delay increased along with an increase in link distance. The propagation delay changed noticeably in different seasons. The path delay was higher in summer and lower in winter, the geometric delay was lower in summer and higher in winter, and the total propagation delay was mainly high in summer and low in winter. There were differences in delay date, seasonal fluctuation amplitude, and in hourly fluctuation standard deviation among different geographic areas.

Turbulence: A Significant Role in Clear-Air Echoes of CINRAD/SA at Night

It is commonly believed that clear-air echoes detected by weather radars are caused by atmobios migration. However, clear-air echoes are sometimes inconsistently related to the activity of living creatures. In some cases, the characteristics of radar products seem to conform to biological scattering, but the movement of echoes cannot be observed. For these reasons, we sought to expand the cause of clear-air echoes from a Chinese Doppler S-band Weather Radar (CINRAD/SA) in Beijing. Some contradictions were discovered in a case which diverged from previous conclusions. It was found that the progression and movement of clear-air echoes do not conform to the rules of biological activities. The frequency distribution of dual-wavelength ratio peaks is 21.5 dB, which is in accordance with Villars–Weisskopf’s turbulence theory. From 1 May to 20 May, the 58% dual-wavelength ratio between the S-band and the X-band was distributed between 18 dB and 24 dB. These results show that more than half of the clear-air echoes of CINRAD/SA at night were caused by turbulence in Beijing. A new model of troposcatter propagation, the reflecting-layers model, was then introduced to explain the radar observations. According to the reflecting-layers model, the echoes’ diurnal variation and reflectivity characteristics are attributed to the effects of turbulent mixing. Excessive turbulent mixing affects the generation of the reflective layer, thereby weakening the echo signal. It is necessary to re-examine the position of turbulence in clear-air echoes.

Troposcatter array signal detection based on frequency and spatial fading correlation

Troposcatter propagation mechanism is valuable for beyond-line-of-sight surveillance. However, channel fading caused by multipath scattering limits the sensing performance of the existing equipment. Based on the troposcatter multipath channel fading model on frequency and spatial domain, a modified covariance-based detector is proposed. The captured array signals are first maximal ratio combined in frequency domain and then weighted in spatial domain before accumulation. The performance of the proposed detector is derived theoretically and verified through simulations, showing significant improvement in the troposcatter scenario.

Receiving antenna mode of troposcatter passive ranging based on the signal group delay

The beyond-line-of-sight (BLOS) ranging of radiation sources can be realised through the troposcatter propagation of electromagnetic signals. Present troposcatter ranging systems are based on multi-station cross localisation theory wherein synchronisation and data fusion of all stations are not tractable. The troposcatter signal group delay properties provide a new ranging mechanism that can be realised through a single station conveniently and inexpensively. However, for radiation sources with multibeams, e.g. frequency-scanned radar, which forms several sub-beams with different elevation angles in the vertical direction, the scattering signal of sub-beams with high elevation angles has overall hysteresis compared to lower ones. This effect is called group delay between sub-beams. In this study, a one-to-one correspondence between group delay and target distance is derived to a closed-form expression with target and receiver antenna parameters. Furthermore, a group-delay-based passive ranging mechanism is proposed for exploiting this correspondence to increase the range of the BLOS target. Then two indicators for evaluation, ranging resolution and delay-measure requirement, are proposed for investigating ranging performance under the receiving antenna mode with different elevation angles, vertical beamwidths and erection heights. Results show the ranging resolution is ∼0.1–0.3 μs/300 km and delay-measure requirement is ∼0.002–0.033 μs for the given parameters.

Study on the maximum calculation height and the maximum propagation angle of the troposcatter wide‐angle parabolic equation method

In this study, a method to choose the maximum calculation height and the maximum propagation angle of the split-step troposcatter wide-angle parabolic equation (TWAPE) method is presented. The method is implemented with the height and the scatter angle of the troposcatter common volume. The performance of this method is then demonstrated with a few examples which rely heavily on the choosing of the maximum calculation height and the maximum propagation angle. These examples include the comparison of calculated results of TWAPE for choosing different maximum calculated height and maximum propagation angle at frequencies of 3 and 1 GHz, path lengths of 300 and 500 km, respectively. Finally, the seasonal variation mechanism of the troposcatter propagation loss is discussed based on the seasonal variation of the refractive-index structure parameter.

Statistics of duration and intensity of path loss in a microwave transhorizon sea-path

A preliminary analysis of basic transmission loss and duration of events is presented from one and a half years of data obtained in a 152 km transhorizon path over a sea environment at 11.65 GHz. The distributions for troposcatter and anomalous propagation conditions indicate a path loss well modelled by two distinct normal distributions. The statistics of event durations D conditioned to a decrease in a given path loss L, P(D/L), are accurately modelled by the log-normal distribution. Illustrative values are presented in graphical and tabular form.

Observations of troposcatter and anomalous propagation signal levels at 11.6 GHz on 155 km path over the sea

Microwave CW transmissions from the French coast near Le Havre are being monitored at Portsmouth along a 155 km path. Preliminary observations indicate that the troposcatter levels of about −117 dBm agree with conventional calculations. However ducting has been seen to persist for long periods, with signal levels exceeding the free space value of −48 dBm by several decibels. On other occasions enhancements of several tens of decibels have been associated with warm and cold fronts and with heavy rain.

A Note on the Ambiguity of Troposcatter Aircraft Interference Patterns

Earlier investigators of troposcatter propagation phenomena have suggested that aircraft-created interference patterns observed at the receiving site can be used to measure aircraft motion within and near the common volume region. A simplified signal and scattering model is used to show that these interference signatures are ambiguous unless prior knowledge of the aircraft trajectory is available.

L-Band Low Noise Transistor Amplifier

This paper describes an L-Band low noise transistor preamplifier fabricated in micro-stripline configuration for microwave receiver for troposcatter propagation measurements. The amplifier provides a gain of 15.5±0.5 dB and noise figure of 4±0.5 dB in the frequency range of 1 to 2 GHz. The output power of 1 dB gain compression is zero dBm.

A troposcatter propagation experiment at 15.7 GHz over a 500 km path

An experiment to study tropospheric structure through the use of high resolution antennas having a very small common volume is described. Measurements at 15.7 GHz over a 500 km path between Boston and Ottawa have been conducted, and signals as large as 25 dB above the receiver noise level have been obtained.