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This paper presents a review on the PECASUS service, which provides advisories on enhanced space weather activity for civil aviation. The advisories are tailored according to the Standards and Recommended Practices of the International Civil Aviation Organization (ICAO). Advisories are disseminated in three impact areas: radiation levels at flight altitudes, GNSS-based navigation and positioning, and HF communication. The review, which is based on the experiences of the authors from two years of running pilot ICAO services, describes empirical models behind PECASUS products and lists ground- and space-based sensors, providing inputs for the models and 24/7 manual monitoring activities. As a concrete example of PECASUS performance, its products for a post-storm ionospheric F2-layer depression event are analyzed in more detail. As PECASUS models are particularly tailored to describe F2-layer thinning, they reproduce observations more accurately than the International Reference Ionosphere model (IRI(STORM)), but, on the other hand, it is recognized that the service performance is much affected by the coverage of its input data. Therefore, more efforts will be directed toward systematic measuring of the availability, timeliness and quality of the data provision in the next steps of the service development.

In this paper, we explored how to more accurately predict the quality of high-frequency links and how to better research and improve the capabilities of high-frequency communication, reconnaissance, and positioning systems. Based on the background electron density generated by the ionospheric assimilation model and 3D ray-tracing technology, more realistic and accurate high-frequency channel parameters with physical meanings were obtained. On this basis, a complete high-frequency channel model that can be used for simulation and prediction was constructed. First, the ionospheric assimilation model, the high-frequency channel model, and the method used for calculating the parameters of the high-frequency channel model based on the background electron density generated by the multi-source ionospheric assimilation model are introduced. Then, the HF oblique sounding experiment and experimental data processing are introduced. Finally, the modeling and simulation of the high-frequency channel are compared with the HF oblique sounding experimental results. The simulation results showed that the modeling results of the high-frequency channel based on the multi-source ionospheric assimilation model proposed in this paper were similar to the HF oblique sounding experimental results. The average deviation of the difference between the simulation results and the experimental ones of the group path, the group path broadening, and the Doppler frequency shift are 29.2200 km, 17.3456 km, and 0.2121 Hz, respectively. The group delay, Doppler frequency shift, and delay broadening results calculated by the high-frequency channel model simulation were relatively accurate and could be used in high-frequency channel quality reporting and prediction, high-frequency reconnaissance and geolocation, and high-frequency radar frequency selection and positioning, etc.

Ionospheric oblique backscatter sounding is an effective means of monitoring the ionosphere which can be used as a frequency selection system to serve HF communication and ensure its quality and stability. But how to obtain effective information from the oblique backscatter ionogram is still a hot issue. Due to this situation, a frequency selecting method for HF communication based on ionospheric oblique backscatter sounding is proposed in this study. After obtaining the ionograms, pattern recognition is used to separate the vertical echoes and the oblique backscatter echoes. Next, the leading edge of the oblique backscatter echoes are extracted, and then a two-dimensional electron density profile can be reconstructed. Then, with the help of ray tracing, the usable frequency range can be estimated. Finally, according to the signal-to-noise ratio reflected by the ionograms, several optimal communication frequencies can be selected. In order to verify this method, oblique ionograms are obtained through oblique sounding experiments to evaluate its accuracy. The result indicates that the usable frequency range and the selected frequencies are in accordance with the echo of the oblique ionogram, so the practicability and accuracy of the method are validated. Eventually, the maximum usable frequencies (MUFs) obtained from oblique backscatter sounding are compared with the MUFs from the oblique sounding ionogram; its Mean Absolute Percentage Error (MAPE) is 7.8% and its root mean squared error (RMSE) is 1.34 MHz.

Aviation communication is significant for the safe, efficient, and orderly operation of air traffic. The aviation industry relies on a sophisticated network to maintain air‐ground communications. However, space weather events can disrupt the ionosphere conditions and damage satellites, leading to High‐Frequency (HF) communication blackouts and satellite communication failures. These disruptions can jeopardize flight safety, especially for flights over polar regions. In response, strategies such as cancellations, rescheduling, or rerouting to lower latitudes may be necessary, despite the low flight efficiency and substantial financial losses. With the background of the anticipated solar maximum in 2025 and a growing number of polar flights, it is indispensable to have a comprehensive understanding of the space weather effects on aviation communication and develop constructive strategies from an Air Traffic Management (ATM) perspective. Hence, we simulate scenarios with different durations of communication failures and assess the corresponding economic losses. Based on the data derived from historical polar flights in 2019, there are daily 18 polar flights with trajectories crossing the north polar region higher than 82°N. Simulation results show that the economic losses associated with these polar flights can range from €0.03 million to €1.32 million, depending on both the duration of communication failures and the adopted air traffic management strategies. We believe that this study can shed light on the effects of space weather‐induced communication failures on polar flight operations and provide guidance for mitigating these effects in the aviation industry.

The complementary M -ary orthogonal spreading with orthogonal frequency division multiplexing (CMOS-OFDM) is an efficient scheme for providing high bit error rate (BER) performance and low peak to mean envelop power ratio (PMEPR) in high frequency (HF) communications system. In the CMOS-OFDM, orthogonal spreading OFDM is implemented based on orthogonal complementary sequence pairs where there is an issue that one data symbol is transmitted through two OFDM symbols. In this paper, we present the M -ary orthogonal spreading OFDM scheme using complex complementary sequences which satisfy real domain orthogonality. The complementary real orthogonal spreading OFDM (CROS-OFDM) is restrict the PMEPR to 3dB and provides two times the bit rate of CMOS-OFDM. The CROS-OFDM scheme uses zero-center complementary sequences like the CMOS-OFDM and hence it is suitable to the HF OFDM and Wi-Fi system where the direct current subcarrier is not used. The simulation results show that the CROS-OFDM provides higher bit rate and BER performance.

This paper investigates 7 MHz ionospheric radio wave propagation between pairs of distant countries that simultaneously lie on the terminator. This is known as greyline propagation. Observations of amateur radio beacon transmitters recorded in the Weak Signal Propagation Reporter (WSPR) database are used to investigate the times of day that beacon signals were observed during the year 2017. The WSPR beacon network consists of thousands of automated beacon transmitters and observers distributed over the globe. The WSPR database is a very useful resource for radio science as it offers the date and time at which a propagation path was available between two radio stations, as well as their precise locations. This paper provides the first systematic study of grey-line propagation between New Zealand/Eastern Australia and UK/Europe. The study shows that communications were predominantly made from the United Kingdom (UK) to New Zealand at around both sunset and sunrise times, whereas from New Zealand to the UK, communication links occurred mainly during UK sunrise hours. The lack of observations at the UK sunset time was particularly evident during the UK summer. The same pattern was found in the observations of propagation from Eastern Australia to UK, and from New Zealand and Eastern Australia to Italy and the surrounding regions in Europe. The observed asymmetry in reception pattern could possibly be due to the increase in electromagnetic noise across Europe in the summer afternoon/evening from thunderstorms.

Designing an automatic modulation classifier (AMC) for high frequency (HF) band is a research challenge. This is due to the recent observation that noise distribution in HF band is changing over time. Existing AMCs are often designed for one type of noise distribution, e.g., additive white Gaussian noise. This means their performance is severely compromised in the presence of HF noise. Therefore, an AMC capable of mitigating the time-varying nature of HF noise is required. This article presents a robust AMC method for the classification of FSK, PSK, OQPSK, QAM, and amplitude-phase shift keying modulations in presence of HF noise using feature-based methods. Here, extracted features are insensitive to symbol synchronization and carrier frequency and phase offsets. The proposed AMC method is simple to implement as it uses decision-tree approach with pre-computed thresholds for signal classification. In addition, it is capable to classify type and order of modulation in both Gaussian and non-Gaussian environments.

This study investigates the effect of solar flares on absorption of high frequency (HF) radio signals over Turkey. For this purpose, the highest affected frequency (HAF) values by 1 dB absorption due to solar X‑ray flux over Turkey were analyzed for different phases of solar flare, different local times (LT), different solar flare classes and different days. The HAF and ΔHAF values were calculated from an empirical model using X-ray flux data with 1-min resolution measured by the Geostationary Operational Environmental Satellite-15 (GOES-15) and solar zenith angle data. The increase in X-ray flux density during the ascending phase of the solar flare causes a sudden and large increase in HAF values. During this phase of flare, the HAF has a logarithmic relationship with X-ray flux values. The HAF reaches its maximum values at the solar flare peak. During the descending phase of solar flare, the HAF values gradually decrease as X-ray flux density decrease. The local time has a significant effect on HF absorption. The greatest increase in ΔHAF values occurs around noon. Comparisons between solar flare classes show that the ΔHAF values increases significantly as the solar flare density increases. For different days of year, the value of ΔHAF increases with decreasing solar zenith angles and the mean ΔHAF has a linear relationship with the values of mean solar zenith angle. The results of this study are important because it is the first attempt to examine the effect of solar flares on HF absorption over Turkey.

In an interview, Dejan Filipovic from the Antenna Research Group at the University of Colorado Boulder, talks about the applied electromagnetics, including Schiffman phase shifters and their applications. The research group focuses on applied electromagnetics research where, for example, they demonstrated an antenna-based interconnect system for multi-core communications over 170-220 THz. They have shown that Schiffman phase shifters can now be designed and fabricated to work over a 15-45 GHz range with very low loss (<0.6 dB) and imbalances (<0.25 dB and <2.5°). The device is designed and built for the emerging surface micromachining technology PolyStrata(TM).

U.S. put forward the protocol standard of 3G generation HF communication, which emphasizes HF network automation degree and interconnection capacity between HF and other network to support a larger network application. HF in the form of access network and the integration of existing IP network should be able to get rid of the island model of traditional HF communication of point to point and self-managed,self-used. give full play to the HF over-the-horizon transmission and strong anti-damage ability of their own advantages and efficient multiple IP network data transmission service ability, provide a wider range of communication for cable users, provide efficient management for wireless subscribers.Compared with the traditional HF communication mode. HF network achieve efficient planning, management and control through the integrated network management system based on IP. Enhanced the network management and control ability as well as anti-damage and anti-disturbance ability.