HF Radio Signals Research Articles

Directional diversity of HF signals received over high latitude paths and the possibility of improved data throughput by means of spatial filtering

The high latitude ionosphere is a dynamic propagation medium in which HF radio signals associated with each propagation mode may arrive at the receiver over a range of angles in both azimuth and in elevation. Furthermore, ionospheric movements at the reflection points impose Doppler shifts and Doppler spreads onto the signals. Large Doppler spreads and multipath propagation in which the delay spread is large are particularly prevalent at these northerly latitudes and can result in a significant degradation in the performance of digital communication systems. This investigation suggests that the application of spatial filtering from simple antenna arrays comprising pairs or triplets of antennas can significantly reduce the level of apparent Doppler spreading, frequently to levels at which modern, high speed modems are expected to operate satisfactorily. The investigation concentrated on Doppler spread since this was the most significant factor in the data employed in this study; however, it is noted that reductions in effective multipath delay spread are also possible.

Behaviour of super-resolution direction finding algorithms for HF signals propagating through the high latitude ionosphere

The performance of high frequency direction finding systems is known to be related to the frequency of operation and to the propagation mode content of the incoming signal, which is dependent upon the electron density distribution in the ionosphere. In addition to large scale tilts that cause gross deviations of the signal from the great circle direction, irregularities in the ionospheric electron density distribution may be considered as providing a rough reflecting surface for HF radio waves. As a result, signals associated with each propagation mode arrive at the receiver over a range of angles in both azimuth and elevation. Measurements are presented of HF radio signals received over a disturbed, high latitude path. Multiple, closely separated traces were apparent in the time history of the calculated bearings, the precise nature of which depends upon the array geometry and algorithm employed. The presence of these multiple traces is attributed to the behaviour of the DF algorithms when the signals arrive over a range of azimuth and elevation angles due to the rough reflector nature of this disturbed region of the ionosphere.

Determining polarization parameters and angles of arrival of HF radio signals using three mutually orthogonal antennas

This paper presents results of an experimental verification of our earlier suggested spectral‐polarization method of measuring the polarization parameters and angles of arrival of HF electromagnetic waves by analyzing three mutually orthogonal projections of the radio signal field vector using a single receiving antenna. The initial stage of analysis involves calculating complex Doppler spectra of time variations of these projections. Thereupon, for each spectrum component, these data are used to determine polarization parameters and angle‐of‐arrival spectra. The measurements were made on a HF radio path about 100 km in length, with simultaneous monitoring of the ionospheric situation using the oblique‐incidence sounding chirp‐ionosonde. In an effort to eliminate multipath effects, in the analysis we used nighttime intervals, for which a stable one‐mode reflected radio signal was observed. It is shown that the proposed method gives mean values of the azimuth and zenith angle which differ by no more than 2°–5° from calculated values. Results of an experimental verification of the suggested technique are in agreement with published data obtained by classic methods.

Determining the drift velocity of ionospheric irregularities by analyzing three mutually orthogonal projections of the HF radio signal field vector

This paper presents results of an experimental verification of our earlier suggested spectral-polarization method of measuring the interference pattern velocity by analyzing three mutually orthogonal projections of the radio signal field vector using a single receiving antenna. The measurements were made on an HF radio path about 100 km in length, with a simultaneous monitoring of the ionospheric situation using an oblique-incidence sounding chirp–ionosonde. In an effort to eliminate multipath effects, in the analysis we used nighttime intervals, for which a stable one-mode reflected radio signal was observed. It is shown that the proposed method gives the mean values of the azimuth and zenith angle which differ by no more than 2 to 5° from calculated values. Mean values of the velocity of travelling ionospheric disturbances (of order 50 m/s) and propagation directions (north-westward changing to northward by the morning hours) obtained for these time intervals are consistent with existing published data.

Analysis of the response of an HF radio signal to ionospheric plasma disturbances caused by acoustic shock waves

We analyze the form of the response of an HF radio signal scattered by the nonstationary disturbances of the electron density Ne caused by acoustic shock-wave disturbances in the upper atmosphere from rockets in the active section of the trajectory. A typical picture of the manifestation of additional modes in the spectrum of an oblique Doppler HF sounding signal is presented. The evolution of the signal response under different conditions of the experiment is discussed.

Excitation of small-scale waves in the F region of the ionosphere by powerful HF radio waves

Abstract Ionospheric small-scale waves in the F region, initiated by heating facilities in Nizhniy Novgorod, have been studied by the method of field-aligned scattering of diagnostic HF radio signals. Experimental data have been obtained on the radio path Kiev–N. Novgorod–St. Petersburg during heating campaigns with heater radiated power ERP = 20 MW and 100 MW. Observations of scattered HF signals have been made by a Doppler spectrum device with high temporal resolution. Analysis of the experimental data shows a relation between the heater power level and the parameters of ionospheric small-scale oscillations falling within the range of Pc 3–4 magnetic pulsations. It is found that the periods of wave processes in the F region of the ionosphere, induced by the heating facility, decrease with increasing heating power. The level of heating power also has an impact on the horizontal east-west component of the electric field E, the vertical component of the Doppler velocity V d and the amplitude of the vertical displacements M of the heated region. Typical magnitudes of these parameters are the following: E = 1.25 mV⧹m, V d = 6 m⧹s, M = 600–1500 m for ERP = 20 MW and E = 2.5–4.5 mV⧹m, V d = 11–25 m⧹s, M = 1000–5000 m for ERP = 100 MW. The results obtained confirm the hypothesis of excitation of the Alfven resonator by powerful HF radio waves which leads to the generation of magnetic field oscillations in the heated region giving rise to artificial Pc 3–4 magnetic pulsations and ionospheric small-scale wave processes. In this situation an increase of the heater power would lead to a growth of the electric field of hydromagnetic waves propagating in the ionosphere as well as the amplitude of the vertical displacements of the heated region.

Ionospheric phenomena produced by modification experiments

Ionospheric modifications produced by powerful high frequency radio waves are studied using the method of field-aligned scattering of diagnostic HF radio signals. Experimental data have been obtained in the course of two ionospheric heating campaigns carried out in November 1983-February 1984 and in May 1993 on the radio paths Kiev-Vasilsursk-St Petersburg and Krasnodar-Vasilsursk-St Petersburg. Observations of HF scattered signals have been made by the Doppler spectrum method with high time resolution. Analysis of the experimental data shows the appearance of quasiperiodic variations in Doppler frequency shift, fd, with periods of 30–60 s during the heating cycles. Powerful HF waves are assumed to excite the Alfvén resonator, generating oscillations of the magnetic field lines in the heated region and giving rise to artificial fd variations and magnetic pulsations. In the case of countinuous action of the powerful HF transmitter, artificial ionospheric waves are sometimes generated with periods corresponding to medium-scale travelling ionospheric disturbances (12–25 min), representative of internal gravity waves.

Traveling ionospheric disturbance diagnostics using HF signal trajectory parameter variations

In the model of a perfectly reflecting surface the inverse problem of restoring dynamic and statistical parameters of traveling ionospheric disturbances (TIDs) from variations of angles of arrival and Doppler frequency shift of HF radio signals on one‐hop oblique radio paths is solved. The technique for determining dispersion law, fluctuation spectrum, geometry, direction, and velocity of TID motion is described.

The investigation of long-distance HF propagation on the basis of a chirp sounder

Experimental data on round-the-world HF radio signals near the terminator are given. The critical frequency of the ionospheric waveguide is found to be F c ∼ 16–17 MHz. At frequencies F < F c the group delay has a negative dispersion τ dot g = ∂τ/∂F ⋍ −100 μs/MHz and τ dot g ⋍ 80 μs/MHz for frequencies f > f c. Ray-tracing calculations are carried out. It is found that the low frequency branch of round-the-world signals ( F < F c) is formed mainly by waveguide modes and the high frequency branch ( F > F c) by 0 ricochet and hop modes. Experiments on waveguide modes escaping from the ionospheric channel due to field-aligned scattering by artificial ionospheric turbulence are carried out. The conditions for trapping of radio waves in the ionospheric waveguide are investigated. It is shown that if the gradient of the critical frequency F 0F2 is less than minus 2 × 10 −2 MHz/100 km radio wave trapping takes place in the ionospheric waveguide at frequencies exceeding by 1–2 MHz the maximum observed frequency of the hop mode. The frequency time characteristics of the mode and the geophysical conditions for the effective control of radio waves escaping from the waveguide are defined.

Measurements of direction of arrival of HF sky wave signals by means of a wide aperture antenna array and two super resolution direction finding algorithms

From measurements of the amplitude and phase of ionospherically propagated HF radio signals received at several spaced antennas, the direction of arrival of the signals in both azimuth and elevation can be determined. A series of measurements has been undertaken using a wide aperture antenna array on signals received from several European broadcast stations (in the 9-15 MHz range) at distances of up to about 1400 km, and the directions of arrival estimated by means of two super-resolution algorithms; MUSIC and an iterative null steering method (an approximate maximum likelihood estimator) based on the IMP and DOSE algorithms. A detailed comparison of the performance of the two algorithms has been undertaken, and it is concluded that the null steering algorithm is significantly better than MUSIC at resolving the various propagation modes, particularly for short signal analysis periods.

Ionospheric wave processes during HF heating experiments

Effects of ionospheric modification produced by powerful high frequency radio waves are studied using the method of field-aligned scattering of diagnostic HF radio signals. Observations of scattered HF signals have been made by the Doppler spectrum method. Analysis of the experimental data shows the appearance of quasiperiodic variations in Doppler frequency shift f d, with periods 30–60 s during the heating cycles. Powerful HF waves are assumed to excite the Alfvén resonator generating oscillations of the magnetic field lines in the heated region and giving rise to f d artificial variations and magnetic pulsations. In the case of continuous action of the powerful HF transmitter ionospheric waves are sometimes observed with periods 12–25 min, typical of medium-scale travelling ionospheric disturbances.

Study of effectiveness of adaptive processing of hf radio signals using three-element receiving antennas

Space processing of hf signals for the elimination of interference and fading is investigated. Working prototypes of an adaptive antenna system consisting of electric dipoles and magnetic loops are described, as are the results of full-scale experiments in the elimination of interference distributed through space and in polarization with the desired-signal source. The possibility of a increase of up to 20–30 dB in the signal-to-noise ratio, which is achieved using a compact antenna system, is demonstrated experimentally.

Measurements of the direction of arrival of HF sky wave signals and single site location using a seven element wide aperture (294 m) interferometer array

From measurements of the amplitude and phase of ionospherically propagated HF radio signals received at several spaced antennas, the direction of arrival (DOA) of the signal in both azimuth and elevation can be determined. Furthermore, by combining the DOA with reflection height measurements, the transmitter location can be estimated from a single receiving site. The direction of arrival has been measured for signals received from several European broadcast stations at distances of up to about 1400 km and the position fixes obtained are discussed. The performance of the system was severely affected by multimoded propagation and techniques for recognising periods of single moded propagation, when accurate measurements are to be expected, are described. Good position fixes were obtained when measurements were restricted to these periods provided that accurate height information was available. A period of data is also discussed for which the signal was found to be propagated predominantly by a 2-hop F-region reflection. A wide spread of fixes for the ground reflection point was found and this is assumed to be caused by the roughness of the ground surface at the reflection point.

Uniformly spaced field‐aligned ionization ducts

MF and HF radio wave signals transmitted from topside sounders can often become trapped in magnetic‐field‐aligned ionization ducts located approximately vertically below the satellite. The resulting traces observed on ionograms are called combination mode traces. A number of cases of combination mode traces observed with the ISIS topside sounders were studied for passes over mid‐ and low‐latitude regions which exhibited distinguishable, nearly uniformly spaced traces on the ionograms typical of this type of propagation in the frequency range above 1 MHz. Associated extraordinary vertical and ducted traces were also evident in all cases. The ducted trace is interpreted as due to propagation in a duct encompassing the satellite at the time of measurement, whereas the combination mode traces are interpreted as due to ducts located well below the satellite. The simultaneous presence of a ducted trace and combination mode traces suggests the presence of a parallel system of ducts that are field aligned. On interpreting the ionograms for events studied by ray tracing, it was found that the ducts are separated by as much as 70 km transverse to their field alignment. The data indicate that, at times, the ducts may exist down to the F peak and may extend into the conjugate region as well. A multiplicity of ducts with such large, relatively uniform separations was not previously appreciated, and the mechanism for creating them in this fashion is not clear. One possibility for the even spacing of these irregularities is that they result from acoustic gravity waves at lower altitudes. Possible relationships to other geophysical phenomena, particularly equatorial phenomena, are discussed.

Time delay variations in HF propagation

An experiment has been established to determine the range of environmentally induced fluctuations in the propagation of HF radio signals. Data are collected with a fully digitized system that is stablized with a cesium beam standard. Initial efforts have been directed toward monitoring at San Diego, California, the time of arrival of WWV transmissions from Fort Collins, Colorado, for the frequencies 5, 10, 15, and 20 MHz. Variations in these arrival times from 25 to 175 μs have been observed depending upon frequency, mode of propagation, time of day, and solar activity.

A tsunami warning system using an ionospheric technique

Earthquakes generate Rayleigh waves which travel along the surface of the earth about 20 times faster than the potentially destructive tsunami (if one is also generated) and reach distant points several hours before the tsunami. The Rayleigh wave excites an upward-propagating acoustic wave which, because of the exponential decrease in air density with height, increases to a large amplitude at ionospheric heights and produces ionization density changes in the ionosphere proportional to the amplitude and form of the wave. HF radio signals reflected from the ionosphere are Doppler-shifted by these motions and can be processed to produce a record which is a faithful reproduction of the Rayleigh wave. By analyzing the ionospheric record and tracing the Rayleigh wave back to the earthquake's epicenter, the characteristics of the earthquake can be determined. This information can then be used to deduce the probability that the earthquake generated a tsunami Instrumentation for a tsunami warning system based on this method is simple and inexpensive.

Irregularity motions in the polar ionosphere

The frequency spectrum of a CW HF radio signal transmitted from an aircraft and propagated over a 1‐hop path inside the polar cap was recorded for about 50 nighttime hr during August 1967. Two different general types of disturbance were observed in these records: (a) a diffuse spectral broadening which was always present to some extent, even under quiet conditions, but which on some occasions was increased by as much as a factor of 2; and (b) discrete frequency anomalies, which were recorded at a rate of about two per hour, and which generally appeared above the carrier frequency (about 9 MHz) and decreased in frequency with a median slope of —2.2 Hz min−1. These discrete anomalies can be interpreted most satisfactorily in terms of Doppler‐shifted reflections from moving ionospheric irregularities. On the basis of this interpretation, the observations are taken to show that irregularity velocities are often in the range from 0.5 to 1.0 km sec−1, with a preferred direction of motion roughly toward the quadrant from geographic E to S and centered on geomagnetic E. However, the measurements were relatively insensitive to geomagnetic N‐S movements and to low velocities.

Beyond the horizon ionospheric propagation experiment from an equatorial orbiting satellite to a middle latitude station

Results of the long range propagation experiment carried out with the HF radio signals emitted by the San Marco 2 satellite, and received at the Florence station, Italy, are presented. The satellite, launched in April 1967, had a low equatorial orbit with a perigee of 204 km and an apogee of 632 km. From June 26, for a period of 11 days, an emission on 20.005 MHz was utilized for the experiment. Relative reception occurrence observations are presented as a function of subsatellite-station range, satellite local time and ground station local time. Due to the orbit and station position, the explored ranges are 5000–15,000 km. During the total of 172 orbits, the signal was received at the Florence station (43°48'N; 11°12'E) for 82 per cent of the orbits from ranges of 5000–10,000 km and 51 per cent of the orbits from ranges of 10,000–15,000 km. A typical reception occurrence of the order of 70–80 per cent was found at both satellite and station sunset (with satellite at low altitude) over ranges of about 10,000km. During the experiment, some measured values of path losses vary between 132 and 147.5 dB. Comments are given on the factors influencing the possibility of ionospheric long range propagation.

Quasiperiodic scintillation in the ionosphere

The hypothesis is advanced that a regular cyclic type of fading seen occasionally on chart recordings of HF and VHF radio frequency signals that have penetrated the ionosphere is a special case of the more normal irregular ionospheric scintillation. Analysis of satellite signals containing this special type of anomaly leads to a better understanding of ionospheric scintillation in general. The paper shows how properties of normal scintillation are consistent with the two ray interference formula derived for the more orderly pattern. As an illustration we have included the analysis of an ionospheric irregularity appearing over Thule that provides height and electron density measurement. Emphasis is given to the part reflection plays in scintillation, a process generally considered non-existent in HF and VHF propagation.

Guided propagation of HF radio waves in the ionosphere

A review is given of the work done in the field of ionospheric guided propagation of HF radio signals from artificial satellites. The possible theoretical approaches to the problem are summarized with more details on the analysis in terms of ray-tracing based on a geometric-optics treatment.