Skip Zone Research Articles

Review on HF Band Vehicular Antenna with NVIS Communication

Antennas play a fundamental role in modern communication systems by facilitating signal transmission and reception across various frequency bands. Each antenna is designed for specific applications based on its operating frequency range. Whether deployed for Car-to-Car Communication or military operations, antennas serve critical functions, with information security being paramount in military contexts. Vehicular antennas typically operate within the L Band, approximately 1 to 2 GHz, relying on satellites for signal communication. However, traditional transmission through the ionosphere faces limitations, notably the skip zone, where signal reception is hindered, leading to communication failures. To address this limitation, Near Vertical Incident Skywave (NVIS) communication has been developed. NVIS utilizes antennas with very high (> 75°) radiation angles and low HF frequencies to overcome skip zone challenges. Despite existing research efforts, achieving high radiation efficiency remains a challenge, often due to improper antenna angle settings. Additionally, previous studies have overlooked the importance of narrow beam focusing, crucial for long-range communication and direction finding. This paper aims to enhance antenna performance parameters such as efficiency and gain through the adoption of specific techniques. By optimizing antenna design and configuration, we aim to improve performance for ionospheric communication, particularly in military applications involving data and voice transmission.

Performance analysis of a RIS-assisted RoFSO communication system over Malaga distribution for smart city applications.

Radio over free space optics (RoFSO) is one of the potential technologies that can satisfy the requirements of 5G services in a smart city. However, as RoFSO is line-of-sight (LOS) communication, one of its limitations is the occurrence of a skip zone in the targeted areas. In this work, a reconfigurable intelligent surface (RIS) is proposed as the solution to overcome this connection difficulty, which prevents signal blocking by generating LOS connections. These RIS modules extend the communication channel coverage, making it more intelligent and controllable. The performance analysis based on outage probability, ergodic channel capacity, and bit error rate has been performed using heterodyne detection. Malaga distribution has been used to model atmospheric turbulence. The exact closed-form expressions of the probability density function and cumulative distribution function of the end-to-end signal-to-noise ratio are derived. Exploiting these derived statistics, system performance is investigated through the ergodic channel capacity, outage probability, and average bit error rate for M-ary quadrature amplitude modulation and two binary modulation schemes: non-coherent binary frequency-shift keying and coherent binary phase-shift keying. Numerical results are compared among different turbulence conditions, link lengths, and scattering errors. The results show that the proposed RIS-assisted RoFSO technology has the potential to be effective for 5G smart city applications.

Deep Electron Density Depletion Near Sunset Terminator on St. Patrick's Day Storm and Its Impacts on Skywave Propagation

AbstractThe plasma density distribution in the equatorial and low latitude ionosphere can often be severely disturbed during active space weather events that can have paramount impacts on the long‐distance high frequency (HF) radio wave (Skywave) communications through the ionosphere. On the St. Patrick's Day storm of 17 March 2015, a deep depletion of plasma with more than two orders of magnitude was observed over a narrow longitudinal sector near the sunset terminator. The controlled SAMI2 (Sami2 is Another Model of the Ionosphere) simulations indicate that a large equatorial vertical drift around sunset terminator can produce such a deep electron density depletion and strong reinforcement of equatorial ionization anomaly. The impacts of these ionospheric density disturbances on the Skywave communication systems have been investigated using an HF propagation simulator that solves the propagation path of radio waves under given background ionospheric conditions. The results clearly demonstrate that the usable HF spectrum for Skywave communications is reduced by more than 50% over the region of depletion. Further, large areas of skip zones, where the Skywave signals are not receivable, are produced due to low ionospheric densities over this region. This study can have important applications in the planning and operation of Skywave systems during the active space weather periods.

Seasonal and Diurnal Dynamics of Radio Noise for 8–20 MHz Poleward‐Oriented Mid‐Latitude Radars

AbstractBased on ray tracing in a smooth ionosphere described by the IRI‐2012 model we have inferred the seasonal‐diurnal dynamics of radio noise observed by four mid‐latitude high‐frequency (HF) radars. In the calculations, noise is assumed to be homogeneous and stationary, but the main contribution comes from the radar skip zone boundary due to focusing radiowaves effect. Noise absorption along the ray path is simulated from the IRI‐2012 electron density, and from the molecular nitrogen density and electron temperatures obtained from the NRLMSISE‐00 model. Earth magnetic field is not taken into account both in the absorption and ray‐tracing calculations due to insufficient accuracy of the ionospheric model. The model results are compared with experimental radar data, and good agreement between the two is demonstrated. It is shown that experimentally observed seasonal and diurnal dynamics of the noise correlates well with model predictions. We demonstrated saturation effect at low noise levels. The model makes it possible to estimate the amount of absorption in D‐ and E‐layers using noise observations at SuperDARN and SuperDARN‐like poleward‐oriented radars, especially at mid‐latitudes. This is important for the retrieval of long term variations in the electron density in the lower ionosphere, by using wide coverage provided by these radars' network. The model also makes it feasible to interpret vertical absorption by experimental noise observations, thereby significantly expanding the capability of HF radars to monitor the lower ionosphere, and to provide data for joint analysis with other data, obtained by these radars at E‐ and F‐layer heights.

СИНТЕЗ ДОПЛЕРОГРАМ КВАЗІВЕРТИКАЛЬНОГО ЗОНДУВАННЯ ІОНОСФЕРИ ДОПЛЕРІВСЬКИМ ВЧ РАДАРОМ ІЗ РОЗНЕСЕНИМ ПРИЙМАННЯМ

Purpose: The ionospheric channel is widely used for the communication, radio navigation, radar, direction finding, radio astronomy, and remote radio probing systems. The radio channel parameters are characterized by nonstationarity due to the dynamic processes in the ionosphere, and therefore their study is one of the topical problems of space radio physics and earth-space radio physics of geospace. This work aims at presenting the results of synthesis of temporal variations in the Doppler spectra obtained by the Doppler probing of the ionosphere at vertical and quasi-vertical incidence. Design/methotology/approach: One of the most effective methods of ionosphere research is the Doppler sounding technique. It has a high time resolution (about 10 s), a Doppler shift resolution (0.01–0.1 Hz), and the accuracy of Doppler shift measurements (~0.01 Hz) that permits monitoring the variations in the ionospheric electron density (10–4–10–3) or the study of the ionospheric plasma motion with the speed of 0.1-1 m/s and greater. The solution of the inverse radio physical problem, consisting in determination of the ionosphere parameters, often means solving the direct radio physical problem. In the Doppler sounding technique, it belongs with the construction of variations in Doppler spectra and comparing them with the Doppler spectra measurements. Findings: For the radio wave ordinary component, three echoes being produced by three rays are observed. Influence of the geomagnetic fi eld and large horizontal gradients in the electron density of δ≥10 % give rise to complex ray structures with caustic surfaces. The ionospheric disturbances traveling along the magnetic meridian form the skip zones. The longitudinal and transverse displacement of the ray reflection point attains a few tens of kilometers along the vil. Haidary to vil. Hrakove quasi-vertical radiowave propagation path, for which the great circle range is 50 km. For the vertical incidence, the signal azimuth at the receiver coincides with the traveling ionospheric disturbance azimuth. The synthesis of temporal variations in the HF Doppler spectra has been made and compared with the temporal variations in the Doppler spectra recorded with the V. N. Karazin Kharkiv National University radar. The estimate of δ=15 % obtained confirms the existence of large horizontal gradients in electron density. Conclusions: Temporal variations in Doppler spectra and in azimuth have been calculated for the vertical and quasi-vertical incidence with allowance for large horizontal gradients of the electron density caused by traveling ionospheric disturbances. Key words: ionosphere, Doppler sounding at oblique incidence, synthesis of temporal variations in HF Doppler spectra, traveling ionospheric disturbances, electron density

INVERSION OF BACKSCATTER IONOGRAMS INTO QUASI-PARABOLIC IONOSPHERIC LAYER PARAMETERS

We present an inversion scheme of the backscatter signal leading edge into parameters of the quasi-parabolic electron density profile, which is based on the comparison of experimental and calculated minimum delays of scattered signals with corresponding distance to the skip zone border. Input parameters are frequency dependences of minimum group path of signal propagation, derived from processing and interpreting backscatter ionograms. For a fixed sounding frequency, the ionospheric parameter pair — the critical frequency and height of the F2-layer maximum — is defined as the intersection point of two curves representing solutions of minimization problems for discrepancy functionals of the minimum group path and the range to the skip zone border. Determining the ionospheric parameters by this inversion scheme on the sounding frequency grid allows us to construct a two-dimensional distribution of electron density in the direction of backscatter sounding.

Инверсия ионограмм возвратно-наклонного зондирования в параметры квази-параболического ионосферного слоя

We present an inversion scheme of the backscatter signal leading edge into parameters of the quasi-parabolic electron density profile, which is based on the comparison of experimental and calculated minimum delays of scattered signals with corresponding distance to the skip zone border. Input parameters are frequency dependences of minimum group path of signal propagation, derived from processing and interpreting backscatter ionograms. For a fixed sounding frequency, the ionospheric parameter pair — the critical frequency and height of the F2-layer maximum — is defined as the intersection point of two curves representing solutions of minimization problems for discrepancy functionals of the minimum group path and the range to the skip zone border. Determining the ionospheric parameters by this inversion scheme on the sounding frequency grid allows us to construct a two-dimensional distribution of electron density in the direction of backscatter sounding.

Performance Limiters of Near-Vertical-Incidence Skywave Propagation: A Scientific Approach

Near-vertical-incidence skywave (NVIS) propagation is defined as providing continuous coverage from nearly 0 km (just beyond the line of sight) to a couple hundred kilometers from the transmitter with no skip or dead zones. NVIS communications are especially effective during disaster-relief operations when infrastructure is severely damaged. The ability to accurately determine the performance limiters of NVIS propagation can help in the planning of high-frequency (HF) (3-30 MHz) emergency communication links. In the literature, widely varying radial distances (from as few as 50 to up to 160 or even 320 km) for the coverage attainable by NVIS propagation have been reported. It is very difficult to plan an NVIS link for homeland security or disaster relief when the published guidelines vary to such a degree. In this study, a scientific approach was utilized to determine the NVIS performance limiters for varying solar conditions, times of day, and geophysical locations.

The Design and Implement of Propagation Path-Loss Model over Sea-Surface under HF

A new algorithm “Propagation Path-Loss Model” (PPLM) is introduced in this paper. As a correction coefficient and equivalent reflection coefficient are introduced to simulate the reflection loss of calm and rough sea-surface, a mathematical prediction model of sea-surface propagation is devised. This model focus on the analysis of propagation path and transmission loss of skywave, including the propagation loss in free space and ionosphere and reflection loss over the sea-surface. PPLM permits calculating the propagation losses and evaluating propagating capacity of a certain environment by predicting the propagation distance and maximum hops of signal at a certain power. It also can be utilized to predict the communication time based on a geometrical model of skip zone. All the implements indicates that the model is effective and functional.

Features of HF Radio Wave Attenuation in the Midlatitude Ionosphere Near the Skip Zone Boundary

We briefly describe the history of studying the decameter radio wave attenuation by different methods in the midlatitude ionosphere. A new method of estimating the attenuation of HF radio waves in the ionospheric F region near the skip zone boundary is presented. This method is based on an analysis of the time structure of the interference field generated by highly stable monochromatic X-mode radio waves at the observation point. The main parameter is the effective electron collision frequency νeff, which allows for all energy losses in the form of equivalent heat loss. The frequency νeff is estimated by matching the assumed (model) and the experimentally observed structures. Model calculations are performed using the geometrical-optics approximation. The spatial attenuation caused by the influence of the medium-scale traveling ionospheric disturbances is taken into account. Spherical shape of the ionosphere and the Earth’s magnetic field are roughly allowed for. The results of recording of the level of signals from the RWM (Moscow) station at a frequency of 9.996 MHz at point Rostov are used.

NVIS 통신을 활용한 HF 도약지대 극복가능성 검증

HF 통신은 별도의 중계 없이 근거리 및 장거리 통신이 가능한 방법으로 해외에서는 주/부 통신수단으로 활용되고 있으나, 국내에서는 도약지대 극복 문제와 전리층 변화에 따라 사용가능한 주파수가 변한다는 이유로 사용자 기피 현상이 심화된 상태였다. NVIS 통신은 일반적인 전리층 반사 통신에 비해 잡음이 적고 도약지대를 포함한 근거리내에서도 통신이 가능한 방법으로써, 이번 연구를 통해 HF 통신의 기존 단점들을 극복하고 HF 통신의 활성화를 위해 외국의 NVIS 통신방법에 대해서 실험을 통해 검증해 보았다. 본 논문에서는 NVIS 통신의 개념을 설명하고, 실제 국내의 HF 통신운용 현황을 확인하였다. 이를 바탕으로 NVIS 통신의 도약지대 극복가능성을 검증하였으며, 효율적인 HF 통신을 위한 예측프로그램의 적용가능성도 판단해보았다. The HF communication method is capable of communicating short and long distances without a separate relaying method and is used as the primary/secondary communication method in other nations. However, the Korean military strongly discouraged the use of the method due to issues regarding the skip zone and the fact that the usable frequency changes according to irregularities in the ionosphere. The NVIS communication is less susceptible to noise than typical communications using ionosphere reflection, and is also able to communicate short distances containing skip zones. In this paper, we inspect the NVIS communication methods of foreign nations in order to facilitate the use of HF communications, as well as provide solutions to the issues mentioned above. This paper explains the concept of NVIS communication, and investigates how the Korean military is implementing HF communications based on actual communications data of military corps. Based on this result, we have verified the possibility of overcoming skip zones through NVIS communications, and have considered the applicability of a prediction program in order to enhance the efficiency of HF communications.

Human spinal arachnoid villi revisited: immunohistological study and review of the literature

Few have described the relationship between arachnoid protrusions (villi) and adjacent spinal radicular veins, and the descriptions that do exist are conflicting. Some authors have even denied the presence of spinal arachnoid villi, suggesting that they play no role in cerebrospinal fluid (CSF) absorption. To further elucidate these structures, laminectomies from C-2 inferiorly to S-2 were performed in 10 fresh human adult cadavers. Following removal of the laminae, the dural nerve sleeves were identified and the spinal nerves excised 1 cm lateral and medial to the intervertebral foramina. Samples were submitted for histological and immunohistological analysis. The authors identified arachnoid villi in all specimens. The length of these structures was approximately 50 to 170 microm. Regionally, these villi were more concentrated in the lumbar region, but they were not present at every vertebral level, with observed skip zones. Occasionally, more than one villus was identified per vertebral level. The majority of villi were intimately related to an adjacent radicular vein. There was a direct relationship between the size of the adjacent radicular vein, and the presence and number of arachnoid villi. Findings in the present study have demonstrated that arachnoid villi exist and are morphologically associated with radicular veins. These data support the theory that CSF absorption occurs not only intracranially but also along the spinal axis. Further animal studies are necessary to prove that CSF traverses these villi and is absorbed into the spinal venous system.

Trends in critical frequencies and layer heights over Tromso and their consequential impact for radio system modelling

A unique resource providing continuous observations of the critical frequencies since 1935 and layer heights since 1952 has beenused to determine the long-term trends above Tromsø in Northern Norway (69°N, 19°E). These, and the recent results of Ulich et al. [Ulich, T., Cliverd, M.A., Rishbeth, H. Determining the long-term change in the ionosphere. EOS Trans. 84(52), 581–585, 2003] have then been used to illustrate the effects of layer height changes on radio system modelling and operation by reference to high frequency ground range and skip zone variations. The analysis ignores the effects of the variations in the underlying E-region and changes in the F2 layer critical frequencies. Consequently, the system impact is expected to be higher still. It is shown that a 16 km drop in h m F2 will introduce ground errors of ∼100 km with higher values under certain circumstances. The Tromso analysis indicate that these ground range errors will not occur until the year 2110, however, the results of Ulich et al. [EOS Trans. 84(52), 581–585, 2003] indicate that this condition occurred in the year 2000. This study demonstrates an urgent need to reach a consensus understanding of long-term ionospheric trends. A reassessment of the CCIR and URSI worldwide maps of ionospheric characteristics is likely necessary.

Evolution of near vertical incidence skywave communications and the Battle of Arnhem

The use of near vertical incidence skywaves (NVIS) in battlefield communications is now commonplace. Though not referred to as such until recently, the propagation of HF radio waves over short distances without the intervention of the skip zone is a natural consequence of the use of appropriate frequencies plus transmitting and receiving antennas that favour high angles of radiation. It has occasionally been suggested that the first dedicated use of NVIS techniques took place during conflicts in the 1960s, whereas evidence exists of its use during the D-Day landings of June 1944. However, wartime documents have recently come to light which show that the British Army Operational Research Group carried out dedicated research into this method of short-range HF communication at least a year earlier and released its reports containing operational recommendations in 1943, prior to the Battle of Arnhem.

Effects of large-scale clouds of ionosphericirregularities on the propagation of high-frequency radiowaves

We have devised a technique for computing the radio wave scattering usingasymptotic formulas and numerical simulations, allowing for a wide variety of electron densityprofiles of a regular ionosphere. For a model ionosphere containing, against a background ofregular layers, large-scale clouds of ionization with small-scale fine structure, a study is made ofthe effects due to the signal scattering into the skip zone produced by obliquely propagatinghigh-frequency radio waves. By means of a numerical simulation it is shown that, for typical finestructure parameters, the upper boundary of the operating frequency range can be increased by1.5–2 MHz through the scattering by irregularities. The study revealed previously unknownregions where the influence of random irregularities upon the radio wave propagation is enhanceddramatically in consequence of the ray field defocusing in a regular ionosphere. For paths with thehop range close to a limiting range, it is shown that there is a fundamental possibility of thereception zone being extended through the scattering of upper rays.

Radar performance during propagation fades in the mid-Atlantic region

Periodically through the year, an Atlantic coast vessel traffic control radar, located near the entrance to the Delaware Bay, observes a reduction in detection range from 37 (20 nmi) to about 17 km (9 nmi). Sometimes ships can be seen visually from the radar tower before they can be observed on the radar screen. The reduction in the radar detection range usually lasts several hours and occurs often when fog is present. An investigation of this phenomenon, referred to as a radar deep fade (RDF), was undertaken to find out what causes the fade and to determine the best method of increasing the radar detection range when one occurs. The investigation indicated that these RDFs arose during the presence of a propagation condition known as subrefraction, which is an increase in the atmospheric index of refraction with altitude above the ocean surface at microwave frequencies. This increase in the index of refraction with altitude results in the effective radius of the Earth being much smaller than the usual 4/3 Earth radius that is observed for radar during standard atmospheric conditions. During subrefractive conditions, the effective Earth's radius can be between its true radius and one-half of its true radius. As a result, the radar horizon is shortened and the radar detection range is reduced. These RDFs will occur during propagation conditions known as sustained deep fades (SDFs), which arise when subrefraction lasts for more than two hours and causes one way losses exceeding 20 dB relative to free-space for line-of-sight (LOS) geometries. Subrefraction is caused by the movement of subtropical moist air over the cold ocean surface. The recommended solution for increasing the radar range in the presence of subrefractive conditions is to place the radar on a higher tower than the present 19-m-high tower, one that is perhaps 60-m high. This would increase the radar range by 50% when the Earth's effective radius is half of normal. This increase in radar height also usually increases the radar detection performance when skip zones associated with surface-based ducts occur or when evaporation ducts (EVDs) are present.

Generation of HF sidebands in the ionosphere during oblique heating: First results

Results from the first high‐power two‐frequency heating experiment to employ obliquely incident modifying waves are presented. Diagnostic instruments measuring the effects at HF and ULF were deployed in the skip zone of the modifying waves to investigate aspects related to HF sidebands. Although occurring far less frequently than with vertically incident heating, HF sidebands were observed on a number of occasions. Two categories of sideband excitation mechanisms are currently recognized (phase modulation and downconversion) and could be responsible for the observed sidebands. Theoretical initiatives are needed to help sort out the contributions of these processes during oblique heating.

HF radio wave field strength on long mid‐latitude paths

Analyses are presented of experimental field strength values on two long mid‐latitude circuits, Fort Collins to Hiraiso and Hawaii to Hiraiso on a frequency of 15 MHz, for the different seasons of the 1975–1986 solar cycle and for different ratios of the wave frequency to the MUF. It is shown that on average the field strength during the night and twilight increases with solar activity and the electron concentration and also with MUF increase or wave frequency decrease. Comparison of experimental field strength values with values calculated by the adiabatic invariant method for equinoctial conditions shows the existence of extra energy losses on the circuits. The losses arise when f < MUF during night and twilight. They are not related to radio wave absorption in the ionosphere. Their average value is 20 to 30 dB, and the values decrease with solar activity and electron concentration increase or with radio wave frequency decrease. Strong leakage of the radio waves into the skip zone for f > MUF is also observed during the night, sunrise, and sunset. The observed phenomena are explained in terms of multiple radio wave scattering by irregularities in the ionosphere. A similar, but smaller, effect also occurs during the day.

On some problems of the theory of radio wave propagation in a randomly inhomogeneous ionosphere

In considering HF propagation in a random inhomogeneous ionosphere it is necessary to take into account regular and random caustics. Regular caustics connected with regular refraction of radio waves in the ionosphere form a skip zone and determine the maximum usable frequency (MUF) and the maximum of the oblique incidence backscatter sounding (OBS) signal. Random fluctuations of ionospheric radio rays “wash out” field enhancement in the vicinity of MUF and the maximum OBS signal. The presence of random caustics results in strong intensity fluctuations of ionospheric radio waves. The above mentioned problems are considered by the interference integral method. A scintillation index formula for strong fluctuations of oblique ionospheric radio waves is obtained. Average intensity and average pulse signal which form in the vicinity of MUF are investigated. The peculiarities of oblique multihop radio wave propagation, taking into account random ionospheric inhomogeneities, terrestrial surface roughness, and caustics focusing in skip distance are discussed.

HF radio in Southwest Asia

The use of an HF propagation mode, called rear-vertical incidence skywave (NVIS), by the US Marine Corps is discussed. In an NVIS system, HF signals are radiated nearly vertically. The signal returned from the ionosphere covers the skip zone with an omnidirectional pattern providing a communications range from the transmitter site out to a radial distance of 0 to 300 miles. The development of HF communication systems by the Marine Corps in the 1980s is outlined. The application of several HF communication systems during Operation Desert Storm is described. >