HF Communication Research Articles

HiCIRF: A high‐fidelity HF channel simulation

A high‐fidelity HF channel simulation has been developed that is suitable for Over‐the‐Horizon Radar (OTHR) and HF communication system design studies and test planning. The simulation capability is called HiCIRF, for High‐frequency Channel Impulse Response Function. HiCIRF provides simulated HF signals corresponding to transmissions from individual transmitter array elements to individual receiver array elements for propagation through the naturally disturbed or undisturbed ionospheric channel. Both one‐way link geometries and two‐way radar geometries can be simulated. HiCIRF incorporates numerical ray tracing and stochastic signal structure computations to realistically simulate signal scatter by small‐scale ionization structure. Stochastic signal generation is employed to generate signal realizations that can be used for OTHR array design and advanced signal processing studies.

Studying the Impact of the Solar Activity on the Maximum Usable Frequency Parameter over Iraq Territory

In this study, the influence of the solar activity on the Maximum Usable Frequency (MUF) parameter has been achieved by adopting a monthly observed sunspots number for the years 2000 & 2010. The values of the MUF parameter has been calculated using the VOACAP international model between Baghdad city and many receiving stations which are distributed on different locations over Iraqi territory. The results of this study showed that the impact of the solar activity on the behavior of MUF parameter at year 2000 is more intense than that at the year of 2010. In the years 1920s and 1930s amateurs and professionals discovered HF communications. In the late '30s, many organizations were involved in the study of HF communications. A considerable effort was made in the U.S.A. and other countries. The Radio Propagation Unit of the U.S. Army Signal Corps provided a great deal of information and guidance on the phenomena of HF propagation in 1945. By 1948, a treatise of ionospheric radio propagation was published by the Central Radio Propagation Laboratory (CRPL) of the national Bureau of Standards. This document (NBS, Circular 462, 1948) outlined the state of the art in HF propagation. Techniques were included for: predicting the maximum usable frequencies (MUF); determining the MUF for any path at any time taking into account the various possible modes of propagation by combining theory and operational experiences; and estimating skywave field strength (1). The Maximum usable frequency (MUF) is the highest frequency at which radio waves are returned to Earth by ionospheric refraction and which can be used to transmit over a particular path under given ionospheric conditions at a specific time, the median value of MUF working 50% of the time specific time. The MUF is used to determine the frequency that provides the optimum performance for the radio communications system. The basic of the MUF represents the highest frequency by which a radio wave can propagate between given terminals by ionospheric propagation alone. The MUF in particular varies greatly throughout the day, seasonally, location of terminals, orientation, and the solar activity. The MUF is reflected from the maximum electron density within a given layer of the ionosphere, therefore the MUF must be statistically based on the electron density of ionosphere, so the frequencies higher than the MUF penetrate the ionosphere and continue into space, while frequencies lower than the MUF tend to refract in the ionosphere and return back to Earth (2).

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.

A Novel Frequency Domain Differential Demodulator in Application to HF Communication

A Novel Frequency Domain Differential Demodulator in Application to HF Communication

A Kind of Optimal Routing Selection Algorithm of HF Ad Hoc Network

The characteristic of not easy to be destroyed makes HF communication be widely used in emergency or special circumstance as earthquake relief, military communication and etc. Traditional routing protocols commonly use least hop number as routing selection criteria. In the HF communication network, for its special channel characteristics, in case of poor link quality, the transmission performance of routing whose hop number is less may be poorer than that have more hops. Taking routing quality as selection criteria has become the best choice. Based on analysis on routing quality calculation method of the third-generation HF communication system, the paper brought out a kind of optimal routing selection algorithm of HF Ad Hoc network based on link quality.

Probability density function estimation for characterizing hourly variability of ionospheric total electron content

[1] Ionospheric channel characterization is an important task for both HF and satellite communications. The inherent space-time variability of the ionosphere can be observed through total electron content (TEC) that can be obtained using GPS receivers. In this study, within-the-hour variability of the ionosphere over high-latitude, midlatitude, and equatorial regions is investigated by estimating a parametric model for the probability density function (PDF) of GPS-TEC. PDF is a useful tool in defining the statistical structure of communication channels. For this study, a half solar cycle data is collected for 18 GPS stations. Histograms of TEC, corresponding to experimental probability distributions, are used to estimate the parameters of five different PDFs. The best fitting distribution to the TEC data is obtained using the maximum likelihood ratio of the estimated parametric distributions. It is observed that all of the midlatitude stations and most of the high-latitude and equatorial stations are distributed as lognormal. A representative distribution can easily be obtained for stations that are located in midlatitude using solar zenith normalization. The stations located in very high latitudes or in equatorial regions cannot be described using only one PDF distribution. Due to significant seasonal variability, different distributions are required for summer and winter.

Midlatitude ionospheric HF channel reciprocity: Evidence from the ionospheric oblique incidence sounding experiments

[1] Ionospheric HF channel reciprocity is investigated at middle latitudes on the basis of ionospheric oblique incidence sounding experiments. Two identical Wuhan Ionospheric Oblique Incidence Sounding Systems (WIOISS), located at Wuhan (30°32′N, 114°21′E) and Wanning (18°58′N, 110°31′E), are used to carry out the campaign. Comparisons of group distance and Doppler shift between Wuhan-Wanning and Wanning-Wuhan HF ionospheric propagation paths indicate that the reciprocity of the ionospheric HF channel is satisfied at midlatitude region. A 3-D ray tracing simulation is also implemented to evaluate the group distances of the two paths. Midlatitude ionospheric HF channel reciprocity, as verified both experimentally and theoretically in the present study, can be useful for HF communication systems and sky wave over-the-horizon radars.

A technique for calculating ionospheric Doppler shifts from standard ionograms suitable for scientific, HF communication, and OTH radar applications

High‐resolution Doppler ionograms taken at 5 min intervals were obtained from a KEL IPS 71 ionosonde operating over a full ionosonde sweep range. The ionograms were converted into true height profiles using the program POLAN. POLAN also produced an equivalent parabolic layer model of best fit to the true height profile. A parabolic layer model of the ionosphere is defined by three parameters, namely, peak height, maximum electron density/critical frequency, and parabolic thickness. Equations for calculating Doppler shift from a time‐varying parabolic layer model have long been known but have seen remarkably little use in the absence of suitable input data and a means of verifying the results. This paper shows that these equations can provide an accurate means of calculating ionospheric Doppler shift based on standard ionospheric ionograms taken at a 5 min rate over a 24 h period when compared with actual Doppler measurements. Apart from its scientific interest, the demonstrated technique will prove particularly valuable in deriving Doppler shift and associated signal fading for the real‐time control of over‐the‐horizon radar and HF communication links.

Effects of a “day-time” substorm on the ionosphere and radio propagation

Propagation mechanisms of lateral (non-great-circle) signals on a high-latitude HF radio path during magnetospheric substorms that occurred in the day-time have been considered. The path is equipped with oblique ionospheric sounding (OIS) from Murmansk to St. Petersburg. The OIS method gives the possibility to determine propagation modes, MOF (maximum observed frequency) values, signal delays, etc. Data of the CUTLASS radar, the IMAGE magnetometer system, the Finnish riometer chain, and the Tromso ionosonde were also used for the analysis. The main results are the following: (1) the lateral signal propagation takes place, as a rule, if the path midpoint is located near the irregularity region that moves sharply from high to low latitudes. The lateral signal propagation appearing during day-time is a new effect. (2) Formation of dense field-aligned irregularities during a substorm leads to decreasing F2MOF values on radio paths. These results can be useful for problems of radiolocation, HF communications and navigation.

Storm sudden commencement and its effect on high‐latitude HF communication links

Following particle precipitation at high latitudes that occurs as a result of geomagnetic disturbances, the electron density of the E region of the ionosphere undergoes substantial variation. These variations modify the E layer propagation characteristics which adversely affect HF propagation paths both unexpectedly and in many cases for long periods of time. This paper presents two case studies of how the enhancement of E region electron density immediately following storm sudden commencement affects high‐latitude HF communications links in and through the auroral zone. Using data gathered from ionosondes, incoherent scatter radar and an oblique sounder, the Advanced Composition Explorer Satellite, and riometers and magnetometers, the physical effects of particle precipitation on the ionospheric E layer and the resulting effect on HF links is revealed. Both case studies show that besides the expected occurrence of severe short‐term absorption and the reduction in the critical frequency of the F layer caused by a redistribution of the electron density in the upper layers of the ionosphere, there is the likelihood a sporadic E layer will form and/or be enhanced because of significant and sustained particle precipitation. When the critical frequency of the sporadic E layer increases above the norm and in cases exceeds the critical frequency of the F layer, there are severe implications for many HF radio communication systems used at high latitudes unless corrective action is taken.

Observations on the variability and screening effect of Sporadic-E

Sporadic-E is a problem in high latitude HF communications because of its irregular and as yet unpredictable behaviour. This paper characterises the change in E-layer critical frequency ( f o E) that occurs in the four hours following a storm sudden commencement (SSC) event which is a precursor to magnetospheric storms which adversely affect ionospheric communications. Following checks for any seasonal or solar activity dependency in the data, further analysis determines the occurrence of full blanketing (screening) Sporadic-E layer formation at high latitudes as seen on vertical ionosondes in Northern Finland (67° Latitude). An appropriate threshold value of f o E is proposed that could be used following the commencement of ionospheric storms, indicating that there is a high probability that F-layer screening will occur. As far as the authors are aware this is the first statistical analysis of the onset of Sporadic-E following SSC.

Vertical and oblique ionospheric soundings over a very long multihop HF radio link from polar to midlatitudes: Results and relationships

A vertical incidence sounder (VIS) was installed at the Spanish Antarctic Base (62.7°S, 299.6°E) during the Antarctic summer survey 2004–2005. In addition, an oblique sounding (OS) system for HF communications between Antarctica and Spain (12,700 km) has been operated during the Antarctic summers since December 2003. OS results are compared to Rec533 model and to VIS data obtained from stations located along the radio path, including VIS stations at the emitter and receiver sites. The results presented in this paper show the potential of this infrastructure for ionospheric monitoring and research purposes at and from polar regions. The results also show that relevant information (e.g., maximum received frequency) of very long range radio links can be obtained from appropriate VIS stations along the path.

Real‐time specification of HF propagation support based on a global assimilative model of the ionosphere

An HF propagation program, HFNowcast, has been developed that takes advantage of the availability of a real‐time global model of the ionosphere to specify in real time the range of frequencies that would be supported on a given HF communications circuit. HFNowcast determines the range of frequency support for a specified circuit, from the lowest usable F layer frequency (FLUF) to the highest (FMUF), together with the E layer ELUF and EMUF. The lack of real‐time observations of HF absorption and HF noise at the receiver means that it is not possible to provide real‐time specifications of the signal‐to‐noise ratio (SNR), the key parameter for HF communications. The global model (developed by Utah State University (USU)) includes particle E precipitation at high dip latitudes during the night, which often precludes the determination of foF2 and M(3000)F2 and thus the simple determination of propagation support in those regions. The FMUF is therefore derived by first calculating the vertical incidence ionogram at the control point(s) and then transforming that ionogram to the oblique incidence (OI) ionogram for the given circuit/hop length. HFNowcast has been validated favorably against several months of oblique propagation observations on midlatitude circuits, although its day‐to‐day variability does not match the observations very well, and it tends to underestimate the FMUFs. The systematic differences between the predictions and observations can be attributed directly to limitations of the current version of the USU model of the ionosphere.

Mitigation and exploitation of the ionosphere: A military perspective

The ionosphere is a critical element in the design and operation of many military RF systems. While judicious engineering choices, such as transmission frequency, can mitigate many ionospheric effects, the research scientist and engineer are being asked to meet ever more demanding operational requirements. These, in turn, require ever more sophisticated situational awareness, mitigation, and exploitation techniques, which are being variously realized against a backdrop of evolving ground‐ and space‐based instrumentation. The latter, in combination with improved modeling and assimilative techniques, promises a new generation of more capable real‐time ionospheric models. Through examples this paper describes the benefits and problems associated with the ionospheric medium. HF communications and space radar systems are discussed as exemplars, and an emphasis is placed on the need to understand the engineering details of those systems if ionospheric research and development is to provide its full benefit.

On the applications of the space time premise when exploiting location diversity of multiple antenna sites

This paper presents novel techniques to exploit the spatial dimension and the space time (ST) premise in a Large Scale Communication Networks LSCN(s). These techniques are based on an encapsulation framework aimed at enforcing an artificial none line of sight (NLOS) condition. The framework integrates the spatially diverse multiple antenna sites (MAS); composing parts of an LSCN into joint transmission/reception systems. The location diversity of these MAS provides for exploiting the spatial dimension whether for multiplexing gain, diversity gain and/or array gain. Whereas multiplexing gain is best exploited in multi input multi output (MIMO) architectures to boost spectrum efficiency; diversity and array gains lend themselves to equally useful applications e.g., in VSAT technology; array/diversity gain can improve the uplink efficiency to satellite, diversity gain may resolve uncertainty issues in HF link, etc. This paper develops a framework for exploiting MIMO in a network to network communications. It also outlines possible applications of location diversity of MAS in LSCNs. Selected scenarios include terrestrial satellite networks, VSAT network topologies, HF communications and broadband global area networks (BGAN).

Protection of HF Transmitters from Reflection Failure by Help of Semiconductor Isolators

The fundamental problem of the transmitters protection against microwave energy reflections from aircraft antenna is discussed. The paper presents some applications of active and passive isolators and circulators as a tool for reflections protection. It has been stated that electrical properties of ferrite isolators and circulators grow worse at lower operational frequencies (below 50 MHz) and temperatures (cryogenic). Semiconductor magnetoplasma exhibits gyrotropic effects similar to those of magnetized ferrites and can solve the problem non-reciprocity in the HF range from 3 to 30 MHz, which is important for HF communication. The semiconductor device realization, optimal parameters, and operational temperature range, based on minimum forward loss α and maximum reverse attenuation β, were chosen. Major electromagnetic parameters of semiconductor isolators were determined.

Atmospheric impact of the Carrington event solar protons

The Carrington event of August/September 1859 was the most significant solar proton event (SPE) of the last 450 years, about four times larger than the solar proton fluence of the largest event from the “spacecraft era” (August 1972). Recently, much attention has focused upon increasing our understanding of the Carrington event, in order to better quantify the impact of extreme space weather events. In this study the Sodankylä Ion and Neutral Chemistry (SIC) model is used to estimate the impact of the Carrington event to the neutral atmosphere and the ionosphere, and the disruption to HF communication. We adopt a reported intensity‐time profile for the solar proton flux and examine the relative atmospheric response to different SPE‐energy spectra, and in particular, the comparatively soft energy spectrum of the August 1972 or March 1991 SPE which is believed to provide the best representation of the Carrington event. Our calculations indicate that large changes in electron density and atmospheric constituents occur during the period of SPE forcing, depending upon the nature of the spectrum and also on the hemisphere considered. However, the most important SPE‐driven atmospheric response is an unusually strong and long‐lived Ox decrease in the upper stratosphere (Ox levels drop by ∼40%) primarily caused by the very large fluxes of >30 MeV protons. This depletion is an indication of the extreme changes possible for the largest SPE. We find that there are comparatively small long‐term differences in the atmospheric and ionospheric response between the 3 suggested SPE spectra.

Ionospheric modeling for short‐ and long‐term predictions of F region parameters over Indian zone

The equatorial and low latitudinal F region ionosphere is highly dynamic and unpredictable because of various geophysical mechanisms operating therein. In the present study, two HF prediction models for short and long term are developed for equatorial and low‐latitude F region ionosphere. In the first approach, multiple regression analysis (MRA) for the dependence of F region parameters, namely, foF2 and M(3000)F2, on solar 2800 MHz flux (F10) and geomagnetic index Ap are generated, and in the second one, second‐degree (SD) coefficients are generated, both by fitting monthly median foF2 and M(3000)F2 with corresponding 12 monthly mean sunspot numbers (R12) using data over three solar cycles. For MRA, daily foF2 and M(3000)F2 values for each hour obtained from Delhi (28.6°N, 77.1°E) digital ionosonde for about half a solar cycle are used. MRA coefficients for foF2 and M(3000)F2 are obtained for every month over 2400 UT times using daily F10 and Ap values separately for quiet (Ap < 25) and disturbed (Ap > 25) periods. Similarly, SD coefficients are obtained each month at all local times for all the 14 stations covering a geographic latitude range from about 0°N to 45°N. In this way, once appropriate coefficients for each hour for all the 12 months are obtained, they are used by the computer‐based MRA and SD models to predict ionospheric hourly foF2 and hmF2 values for given inputs such as month, F10, Ap, and R12, as the case may be. Predicted model values of foF2 and hmF2, calculated on short‐ and long‐term basis, are then compared with the observed data over Delhi and also with those obtained using international reference ionosphere (IRI)‐2001 model. From our comparative studies it is observed that MRA and SD models show better agreement with observations compared to the IRI model for both long‐ and short‐term basis and among the two the MRA model provides best agreement with the observed ones, even during the magnetic storm periods. The SD model, on the other hand, which is based on monthly median values, is useful for providing long‐term predictions for HF communication applications.

Accuracy of USU‐GAIM specifications of foF2 and M(3000)F2 for a worldwide distribution of ionosonde locations

The Utah State University (USU) Global Assimilation of Ionospheric Measurements (GAIM) model has replaced the Parameterized Real‐Time Ionospheric Specification Model (PRISM) for operational use at the Air Force Weather Agency (AFWA). Validation of the model by the Air Force Research Laboratory has therefore passed from the sanity checking stage to investigations of the accuracies and limitations of the model when it is used to generate Operational Space Environment Network Display (OpSEND) products. This paper presents the results of an extensive analysis of the errors in the USU‐GAIM specifications of the key ionogram characteristics foF2 and M(3000)F2 for 21 ionosonde locations distributed around the world, 9 in Australia and 12 in other countries, for September 2006. The ground‐truth observations of foF2 are manually scaled hourly values. The ground‐truth observations of M(3000)F2 are manually scaled hourly values for the Australian ionosondes, and filtered autoscaled values for the other 12 ionosondes. The analysis provides representative latitude‐dependent median errors for both characteristics, together with errors in the values of the maximum usable frequency for the standard 3000 km HF communications circuit. In general, GAIM performs well at all locations. The errors seem to be a function of the availability of data from nearby GPS TEC sites, the accuracy of the program's background ionosphere, and the validity of the assimilated TEC data, rather than of any limitations of the assimilation technique.

A Lumped-Circuit Model for a Triband Trapped Dipole Array-Part II: Stacked Arrays

Demands on emergency rf links have shown how important HF (330 MHz) communications can be for medium to long range. Efficient triband antennas have been developed and are quite popular for HF communications. In part 1, we described a lumped circuit model of the KLM 4-element KT-34A antenna. Here we consider its larger companion the 6-element KT-34XA and describe retuning the model for operation in the 10-, 15- and 20-m U.S. Army Military Affiliate Radio System (MARS) bands. We retune the parallel LC-circuits acting as traps and adjust the element lengths to obtain a MARSarray. A numerical model of this type of antenna is of interest to study antenna placement and the use of stacked arrays. We compare the radiation pattern of the 6-element KLM and MARS arrays and show examples for a stacked configuration over an imperfect ground plane. The model results for the individual arrays have the expected front and rear main beam angles but when stacked these angles change in a nonintuitive manner.