HF Communication Research Articles

Calculation of optimal operating frequencies of a communication radio line according to oblique ionosphere sounding

A hardware-software complex for real-time automatic determination of the optimal operating frequencies of a communication radio line according to oblique chirp ionosphere sounding is created. Path tests of the chirp complex on midlatitude radio lines are performed. Bit error probability and the reliability of HF communication for narrow-band and broadband communication systems are estimated from the results of oblique chirp sounding. It is shown that the quality of a communication channel greatly depends on the ratio of the regular and fluctuation components of a signal, as well as on the magnetic activity level. The created chirp complex can be used as a part of the ionospheric-wave and frequency-control service for dynamic management of the radio-line frequency resource in the interests of efficient operation of different-purpose radioelectronic systems.

Numerical simulation of the structure of the high-latitude ionospheric F region during meridional HF propagation

A previously developed model of the high-latitude ionosphere is used to calculate the distribution of the ionospheric parameters in the polar region. A specific method for specifying input parameters of the mathematical model, using the experimental data obtained by the method of satellite radio tomography, is used in this case. The spatial distributions of the ionospheric parameters characterized by a complex inhomogeneous structure in the high-latitude region, calculated with the help of the mathematical model, are used to simulate the HF propagation along the meridionally oriented radio paths extending from middle to high latitudes. The method for improving the HF communication between a midlatitude transmitter and a polar-cap receiver is proposed.

Validation of the Utah State University Global Assimilation of Ionospheric Measurements (GAIM) model predictions of the maximum usable frequency for a 3000 km circuit

The accuracy of the Utah State University Global Assimilation of Ionospheric Measurements (GAIM) monthly median values of the maximum usable frequency for an HF communications circuit has been determined for 3000 km circuits centered on Australian ionosonde locations for March and April 2004. The accuracy is discussed in terms of foF2, M(3000)F2, and the product MUF(3000)F2 = foF2 * M(3000)F2. Ground truth is provided by hourly values of foF2 and M(3000)F2 that have been hand‐scaled from ionograms. In general, the accuracy of the GAIM predictions of foF2 exceeds that for M(3000)F2. Given that M(3000)F2 is a function of the height and shape of the F2 subpeak, it follows that these parameters of the GAIM F2 peak need to be improved. The GAIM errors in foF2 are quite large at Learmonth during the day, even though Learmonth is a digital ionospheric sounding systems (DISS) station that provides real‐time data to GAIM. We interpret these errors in terms of limitations of the current GAIM model. Comparisons of the GAIM errors in the monthly median MUF(3000)F2 with those given by the propagation prediction program Advanced Stand Alone Prediction System (ASAPS) show that the GAIM errors are generally somewhat greater than the ASAPS errors, but we consider GAIM's performance to be creditable given that most of the data available for assimilation is GPS total electron content observations. Overall, the daytime GAIM errors are 3 MHz or 10%, while the nighttime errors are 3 MHz or 16%. On average, the GAIM errors are about 3/2 the ASAPS errors during the day and twice the ASAPS errors at night.

Near-Field Coupling Between Broadband Over Power Line (BPL) and High-Frequency Communication Systems

Electromagnetic coupling between a broadband over power line (BPL) system and an HF communication system is discussed in this paper. Numerical analyses using the method of moments were performed to determine the near-field coupling between the BPL and communication systems at 3.5, 7, 14, and 28 MHz. Half-wave dipole and quarter-wave vertical antennas were positioned at 1-m intervals along the power line to find the maximum and minimum signal levels caused by the BPL system. Signal levels induced into the BPL system by a 100-W HF transmitter are also presented.

Practical measures for combating communication system impairments caused by large magnetic storms

HF communication circuits, on an individual basis, may perform poorly during ionospheric disturbances associated with geomagnetic storms. This is especially true at middle and high latitudes. To achieve a greater degree of reliability, it is necessary to exploit various forms of diversity. The most powerful forms of diversity at HF must exploit multiple communication paths and frequencies. The most reliable communication with an individual entity (e.g., aircraft) will involve a star network involving several independent paths. Moreover, these (redundant) communication paths must have a wide range of communication bands at their disposal. Much of the paper deals with the process by which such diversity systems may provide acceptable communication reliability, even under adverse propagation conditions. A significant portion of the paper will deal with a particular system during a very realistic scenario. The specific system of interest is the GLOBALink™/HF system, a unique data communication network.

Naval Structural Antenna Systems for Broadband HF Communications

This paper introduces the concept of multipurpose structural antenna system for shipborne installations. By combining properly shaped and loaded wire radiators together with an existing naval superstructure, such as the funnel or a big mast, it is possible to achieve a broadband high-frequency compact radiating system which permits handling both sea-wave and near vertical incidence sky-wave communications. When provided with a plurality of feeding points, the naval structural antenna may be used in multichannel mode, with a huge efficiency improvement over the conventional multichannel broadband systems requiring a combining network. Additionally it is shown that, when fed in monochannel mode, the structural antenna system exhibits focusing features with the possibility to achieve sectorial coverages. This paper demonstrates the advantage and limitation of this new system by carefully taking into account the interantenna coupling.

Daily ionospheric forecasting service (DIFS) III

Abstract. The daily variability of the ionosphere can greatly affect HF or SATCOM communications. HF skywave operators plan frequency schedules months in advance, however, they also require daily knowledge of the ionospheric conditions in order to modify assignments. SATCOM operators also require daily information about the levels of scintillation, which are variations in phase, amplitude, polarisation and angle of arrival that can cause severe degradation of the received signal. Using a number of ionosonde measurements and geomagnetic and solar values, a Daily Ionospheric Forecasting Service (DIFS) has been developed, which provides HF and SATCOM operators with daily forecasts of predicted ionospheric conditions. The system uses in-house algorithms and an externally developed Global Ionospheric Scintillation Model (GISM) to generate HF and SATCOM forecasts. HF forecasts consist of a past summary and a forecast section, primarily displaying observed values and predicted categories for the Maximum Usable Frequency (MUF), as well as an Ionospheric Correction factor (ICF) that can be fed into the ionospheric propagation prediction tool, WinHF. SATCOM forecasts give predictions of global scintillation levels, for the polar, mid and equatorial latitude regions. Thorough analysis was carried out on DIFS and the results conclude that the service gives good accuracy, with user feedback also confirming this, as well.

An application of DAPSK in HF communications

The 64-DAPSK-OFDM transmission techniques for HF communications are investigated in this paper. The error probability of 64-DAPSK in mobile fading channels is evaluated, the optimum ring ratio is given and detection thresholds are determined. Extensive computer simulation results show that 64-DAPSK-OFDM is superior to the widely adopted 64-QAM and it can be a potential choice for spectrally efficient digital transmission over different HF channels.

2003년 10월의 태양활동과 우주환경의 영향

본 연구에서는 2003년 10월과 11월에 발생한 강력한 태양활동과 우주환경의 변화에 대한 국내외 관측결과를 분석하였다. 이러한 태양활동은 거대한 흑점군, X급 이상의 강력한 플레어, 연이은 코로나물질 방출(Coronal Mass Ejections: CMEs) 및 프로톤 현상 등으로 특징지어 질 수 있다. 특히 이때 발생한 고속의 CME들은 지구 방향으로 진행하여 매우 강력한 지자기 폭풍을 일으켰다. 미국 해양대기청 우주환경예보센터에서 제시한 우주환경기준(Space Weather Scales)에 따라 국내외 관측 자료를 분석하고 위성 및 통신에 미치는 영향을 예측하였다. 또한 같은 기간동안 우리나라에서 관측된 전리층 총전자함유량(Total Electron Contents: TEC), 오로라, 전리층의 F2 임계주파수, 그리고 아리랑 위성 1호의 궤도자료를 분석함으로서 우주환경변화가 우리나라 상층대기, 위성궤도, 무선통신 등에 미치는 영향을 조사하였다. In this paper, we present a good example of extreme solar and geomagnetic activities from October to November, 2003. These activities are characterized by very large sunspot groups, X-class solar flares, strong particle events, and huge geomagnetic storms. We discuss ground-based and space-based data in terms of space weather scales. Especially, we present several solar and geomagnetic disturbance data produced in Korea : sunspots, geo-magnetograms, aurora, Ionogram, and Total Electron Content (TEC) map by GPS data. Finally, we introduce some examples of the satellite orbit and communication effects caused by these activities; e.g., the disturbances of the KOMPSAT-1 operational orbit and HF communication.

Operational Space Environment Network Display (OpSEND)

The Air Force Research Laboratory and Space and Missile Systems Center, Detachment 11, have implemented a new system of graphical products that provide easy‐to‐visualize displays of space weather effects on various radio systems operating through the ionosphere. By means of a three‐level Web page, the user can select the radio system (UHF, HF, GPS, or radar), the map area (global or regional theater where the radio system is located), the time (nowcast or forecast), and additional options. This system, the Operational Space Environment Network Display (OpSEND), was first installed at the 55th Space Weather Squadron, Schriever Air Force Base (AFB), Colorado, and is now producing its first four products with a new Web format at the Air Force Weather Agency, Offutt AFB, Nebraska. Radar auroral clutter maps specify (nowcast) the estimated position and strength of the auroral oval relative to radar coverage regions. UHF Satcom scintillation maps nowcast and 2‐hour forecast ionospheric scintillation and its potential degradation to UHF Satcom communication links. HF illumination maps nowcast and 1‐hour forecast HF communication performance. Estimated GPS single‐frequency error maps nowcast and 1‐hour forecast positioning errors resulting from GPS constellation geometry and inaccurate ionospheric correction. Details and examples are presented.

SuperDARN radar HF propagation and absorption response to the substorm expansion phase

Abstract. Coherent scatter HF ionospheric radar systems such as SuperDARN offer a powerful experimental technique for the investigation of the magnetospheric substorm. However, a common signature in the early expansion phase is a loss of HF backscatter, which has limited the utility of the radar systems in substorm research. Such data loss has generally been attributed to either HF absorption in the D-region ionosphere, or the consequence of regions of very low ionospheric electric field. Here observations from a well-instrumented isolated substorm which resulted in such a characteristic HF radar data loss are examined to explore the impact of the substorm expansion phase on the HF radar system. The radar response from the SuperDARN Hankasalmi system is interpreted in the context of data from the EIS-CAT incoherent scatter radar systems and the IRIS Riometer at Kilpisjarvi, along with calculations of HF absorption for both IRIS and Hankasalmi and ray-tracing simulations. Such a study offers an explanation of the physical mechanisms behind the HF radar data loss phenomenon. It is found that, at least for the case study presented, the major cause of data loss is not HF absorption, but changes in HF propagation conditions. These result in the loss of many propagation paths for radar backscatter, but also the creation of some new, viable propagation paths. The implications for the use of the characteristics of the data loss as a diagnostic of the substorm process, HF communications channels, and possible radar operational strategies which might mitigate the level of HF radar data loss, are discussed.Key words. Ionosphere (ionosphere-magnetosphere interactions). Magnetospheric physics (storms and substorms). Radio science (radio wave propagation)

The influence of high latitude off-great circle propagation effects on HF communication systems and radiolocation

A wealth of knowledge resulted from the investigations into gross deviations from the great circle path (GCP) outlined below. The magnitude and variances of the deviations from the GCP have been quantified and the associated geophysical conditions and signal characteristics identified. Research is currently being undertaken aimed at incorporating the knowledge gained from the experiments into techniques that will be of direct operational application. The main activities in this area include the development of ionospheric models coupled with ray tracing studies (some preliminary results of which were presented here) both to confirm the cause of the large bearing deviations and to assess the impact of the presence of the high latitude ionospheric features causing the deviations on paths other than those subjected to experimental investigation.

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.

The response of the high‐latitude ionosphere to the coronal mass ejection event of April 6, 2000: A practical demonstration of space weather nowcasting with the Super Dual Auroral Radar Network HF radars

The ionosphere at high latitudes is the site of important effects in space weather. These include strong electrical currents that may disrupt power systems through induced currents and density irregularities that can degrade HF and satellite communication links. With the impetus provided by the National Space Weather Program, the radars of the Super Dual Auroral Radar Network have been applied to the real‐time specification (“nowcasting”) of conditions in the high‐latitude ionosphere. A map of the plasma convection in the northern high‐latitude ionosphere is continually generated at the Johns Hopkins University Applied Physics Laboratory (JHU/APL) SuperDARN web site using data downloaded in real time from the radars via Internet connections. Other nowcast items include information on the conditions of HF propagation, the spatial extent of auroral effects, and the total cross polar cap potential variation. Time series of various parameters and an animated replay of the last 2 hours of convection patterns are also available for review. By comparing with simultaneous measurements from an upstream satellite, it is possible to infer the effective delay from the detection of changes in the solar wind at the satellite to the arrival of related effects in the high‐latitude ionosphere. We discuss the space weather products available from the JHU/APL SuperDARN web site and their uses by simulating a nowcast of the ionosphere on April 6, 2000, during the arrival of a coronal mass ejection (CME)‐related shock. The nowcast convection pattern in particular satisfies a critical need for timely, comprehensive information on ionospheric electric fields.

An attempt to model the influence of the trough on HF communication by using neural networks

Trough is an interesting phenomenon in characterizing the behavior of the ionosphere, especially during disturbed conditions. The subject, which was introduced around the 1970s, is still attracting attention, especially during recent years. In HF communication, in particular, over the midlatitude ionospheric regions the electron density trough exhibits a phenomenon of abrupt gradients of electron densities in space and time which are directly reflected to foF2. Thus the performances of HF communications are directly affected. In this work an attempt has been made for the modeling to quantify the influence of the ionospheric midlatitude electron density trough on the ionospheric critical frequency foF2 by using neural networks. Data sets are used from the ground stations that include observations in the trough region. It has been demonstrated that the neural‐net based approaches are promising in modeling of the ionospheric processes. Data generated by using statistical relationships are used to train the neural network. Then the trained neural network is used to forecast the ionospheric critical frequency, foF2, values 1 hour in advance for the cases when the probability of influence of the trough is high. Preliminary results will be presented to discuss the suitability of the neural‐network‐based approach in the modeling of complex processes such as the influence of the trough on foF2. The basic contributions of this work are 1) generation and organization of significant data for teaching complex processes, 2) neural‐network‐based modeling of a highly complex nonlinear process such as the influence of the trough on foF2 forecasting, and 3) general demonstration of learning capability by calculating cross correlations and general demonstration of reaching a proper operating point by calculating errors (that is, during the optimization process the neural network reaches the global minimum by using the gradient descent method).

Wideband scattering functions for HF ionospheric propagation channels

A physically based method has been developed to simulate the wideband HF ionospheric propagation channel relevant to the case of wideband spread spectrum HF communications and also other HF applications such as digital broadcasting and over-the-horizon radar. It is based on the consideration and solution of the equations governing pulse signal propagation through a fluctuating time varying random ionosphere. The wideband scattering function has been constructed as the appropriate Fourier transform of the correlation function of a channel impulse response. Numerical codes have been written, which allow numerical simulation of the wideband scattering function of the HF sky wave ionospheric fluctuation channel for any given model of the background ionosphere and time varying ionospheric turbulence with an anisotropic inverse power law spatial spectrum and frozen drift of the ionospheric inhomogeneities. When employed in the simulation of the scattering function for real conditions of propagation, the method provides the possibility of analysing the propagation effects for different relative bandwidths of the background channel, fluctuating channel and transmitted pulse. The effects of the transmitted pulse bandwidth and anisotropy of the irregularities have been studied. The numerical results have been obtained and presented, which demonstrate the contribution of the effects enumerated in the wideband scattering function of the HF ionospheric channel.

Effect of horizontal gradients on ionospherically reflected or transionospheric paths using a precise homing-in method

A homing-in method is presented for determining ionospheric reflected or transionospheric paths between fixed transmitter and receiver locations in the presence of ionospheric gradients or ripples. Both initial elevation and azimuth are automatically adjusted to find the path that arrives exactly at the receiver. The method can be used for any 3D ionospheric model to find precise ray paths and phase and group delays for both magneto-ionic modes. The method takes full account of path location, geomagnetic field orientation and the bending of the ray path resulting from horizontal as well as vertical gradients of electron density. It can also find multiple paths e.g. low and high angle, 1- and 2-hops for both ordinary and extraordinary modes. Examples of its use are given for both terrestrial HF links and Earth to Satellite paths. For paths reflected from the ionosphere, the effect of gradients of both critical frequency and height of maximum electron density are determined and the comparative effect of gradients on high and low angle and 1- and 2-hops paths for both magneto-ionic modes investigated. Path variation with frequency for a fixed link is also studied and the bandwidth of the ionospheric background channel (dispersive bandwidth) and its reciprocal (the pulse rise time), important for wideband digital HF broadcasting or spread spectrum HF communications, is estimated for a range of frequencies, for high- and low-angle rays and 1- and 2-hop paths. For Earth–satellite paths, the effect of the ionosphere and horizontal ionospheric gradients is determined for a range of frequencies and elevation angles. It is shown that the method can also enable the determination of second-order errors in satellite navigation methods, such as GPS, due to ionospheric gradients and the effect of the geomagnetic field.

Predictions of HF communication MUF in the region of the South China Sea

In this paper, an artificial-neural-network (ANN) method is employed to predict the HF-communication MUF (maximum useable frequency) in the region of the South China Sea. Historical observation data of f/sub 0/F/sub 2/ from six ionospheric observatories around the South China Sea are used to train the network. The comparison of ANN predictions and International Reference Ionosphere (IRI) models with the observations shows that the ANN predictions are closer to the observed results than are the IRI predictions.

Multiple scattering of HF radio waves propagating across the sea surface

A problem that is of considerable importance to the HF communications and radar communities is the calculation of the spatio-temporal modulations imposed on a radio wave as a result of propagating across a rough sea surface. We derive equations, in the form of a multiple-scattering series, describing the spatio-temporal structure of a radio wave at a point on the rough sea surface after propagation from an antenna at another point on the surface. We present calculations illustrating the sensitivity of the resultant Doppler spectra to ocean surface conditions, as described by well known parametric models. The generality of the approach means that it can easily accommodate user-specified antenna patterns, arbitrary coastline geometries, arbitrary sea-wave spectrum models, wave-field inhomogeneity and a wide range of seawater conductivities.

Determination of f 0F2 Short-Term Variations from GPS Time Delay Observations

Reliable HF communications require propagation assessment. Such assessment could be facilitated with the monitoring of ionospheric characteristics by continuously available passive means, i.e., measurements of the total electron content (TEC) using satellite-emitted signals without a need for burdening the electromagnetic spectrum. With the Global Positioning System (GPS) providing instantaneous time delay, or equivalently, TEC, values when needed, an assessment of HF propagation may be available on a near realtime basis.