[2] The efficiency of communication systems in a significant degree depends on the coordination of the transmitted signals with the propagation channel. At the same time, the problems of coordination in the HF range can be resolved successfully only at understanding of the specifics of the impact of the propagation medium on the HF signals. Similar problems arise also at the development of models of the HF channel describing adequately the real features of HF propagation.
[3] Finally, the adaptation of both the channel models and real HF communication systems to current ionospheric conditions is possible only at the presence of radio channel sounding means with use of real-time data. Systems of both, pulse [Al'pert, 1972; Blagoveshchensky and Zherebtsov, 1987; Davies, 1969; Moller, 1974] and continuous [Filipp et al., 1991; Poole and Evans, 1985] vertical (VS), oblique (OS), and oblique backscatter (OBS) sounding are used for these purposes. The pulse methods, being simple and reliable, have, as it is widely known, one very important disadvantage: they require high power of emission in a pulse, the power seldom being below 1 kW.
[4] On the basis of the sounding methods using continuous wide-band signals at much lower emitting power (usually 10 or 100 W), one can measure not only the traditional parameters (the mode structure, relative delays, the ratio of ray amplitudes, the signal-to-noise ratio, etc.), but also quasi-instant transmission and pulse characteristics of ionospheric radio channel. The method of ionospheric channel diagnostics with the use of signals with the LFM, the so-called chirp sounding (R. B. Fenwick, Commun. News, 1974) is widely applied in practice of radio communication [Goodman, 1992]. On the other hand, the use of small-power wide-band signals at continuous sounding puts additional requirements both on creation of the sounding equipment and on the choice and development of the means and methods of processing of the obtained information.
[5] It is also important to note that the development of wide-band adaptive communication and direction-finding systems of the decameter range leads to an important role of the problems of studies of dynamical spectral and statistical characteristics of a nonstationary ionospheric radio channel. The nonstationarity of ionospheric channel is in a significant degree due to the influence of wave disturbances of different origin generated at passage of terminator and during geomagnetic disturbances.
[6] Medium-scale traveling ionospheric disturbances (TID) with dimensions of 100-500 km are typical ionospheric disturbances observed at middle latitudes [Danilov et al., 1987; Hocke and Schlegel, 1996]. Different methods and techniques are used for studying TID, including ionosondes of vertical [Bowman, 1990; MacDougall et al., 1997] and oblique [Cherkashin et al., 2003] sounding, Doppler measurements [Burmaka et al., 2003; Waldock and Jones, 1987], incoherent scatter radars [Burmaka et al., 2004; Huang et al., 2003; Hearn and Yeh, 2003], MU radars [Fukao et al, 1978], and transionospheric sounding with the help of signals of the GPS navigation satellites [Ho et al., 1998]. Beginning from the second half of the 1990s, optical observations of the nightglow of the ionosphere are widely used [Shiokawa et al., 2003; Taylor et al., 1998].
[7] In spite of considerable efforts and achievements in the field of wave disturbance studies, many important questions still are open. The mechanism of transformation of various types of energy in the atmosphere is still obscure; the agent stimulating the trigger mechanism of the accumulated energy release is not still found. The patchiness of the data and their discrepancy lead in some cases to erroneous conclusions.
[8] Because of the complexity and variety of relations in the solar wind-magnetosphere-ionosphere-atmosphere-Earth system and also the presence of various physical mechanisms responsible for generation of wave disturbances and their nonstationarity, carrying out of coordinated systematic studies of wave disturbances seems very actual. The entire set of the observation methods and equipments available should be attracted to such studies. In our mind, the network of the stations of oblique LFM sounding operating on the regular basis in automatic regime is one of effective methods of organizing systematic observations for manifestation of wave disturbances.
[9] In this paper, the results of regular observations of the conditions of ionospheric propagation of HF radio waves at radio paths of different length and orientation obtained with the help of the network of chirp sounders are presented. The measurements were conducted in 2005. Vast many-month continuous set of data on the frequency-time and frequency-amplitude display ionograms was obtained. The registration and processing of the data were performed in an automatic regime. It is shown that the method of oblique LFM sounding with high resolution in the group delay time and frequency makes it possible to detect reliably wave disturbances with a period from 15 min to a few hours and with an amplitude of variations of the MOF from 0.1 MHz and higher.
[10] Spectral analysis of variations in MOF and the effect of appearance of z-type features at the high-angle ray of ionograms of the oblique sounding are used for revealing of wave disturbances. The high-angle ray slides along the layer maximum and is considered as an unstable trajectory. The high-angle ray is a sensitive detector of various disturbances and due to this property is quite convenient for detecting of ionospheric wave disturbances [Erukhimov et al., 1997].
[11] The efficiency of using of the high-angle ray for detecting
disturbances in a significant degree is determined by the
availability of modern equipment for wide-band sounding of the
ionosphere with high resolution in frequency and group delay time,
as well as by proper choice of the path geometry. From the point of
view of the sounding by modernized automats the LFM ionosonde
modernized by the authors satisfies completely the above indicated
requirements. Such factors as the length and the orientation of the
path influence the detection of the wave disturbances by the
high-angle ray. At very long paths (multihop), the effect of
disturbance impact on the characteristics of the high-angle ray is
of an integral character, this fact making difficult identification
of the disturbance source. Moreover, at such paths the effect of the
impact of ionospheric disturbances on signal characteristics may be
masked by scattering of radio waves from the Earth. Scattering of
radio waves by intense irregularities (if the path passes in the
high-latitude ionosphere) can lead to the same effect. On the other
hand, at short paths with the length
500-700 km the frequency
range of the propagating HF waves shifts toward lower frequencies,
where the high level of stationary noise exists. This circumstance
causes an increase of the threshold of the TIDs detection. Starting
from these requirements, the most suitable for revealing effects of
the impact of wave disturbances on the characteristics of the
high-angle ray are midlatitude one-hop paths ~1500-3000 km long.
[12] In this paper a modeling of the frequency-time and frequency-amplitude display ionograms is performed for both, conditions of quiet ionosphere and at the presence of wave disturbances. Comparing the experimental and calculated ionograms of oblique sounding, we determined the parameters of wave disturbances causing such features on ionograms as z-type disturbances of the high-angle ray.
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