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Figure 1 |
[14] Two-channel chirp sounder created on the basis of the "Katran'' R-399 A receiver was used in the measurements. The rate of the frequency sweeping was 100 kHz s -1. The range of emitting frequencies was 8-30, 4.2-30, and 4-30 MHz for Cyprus, Inskip, and the rest of the transmitters, respectively. Two 9-m collapsible-whip antennas were used for the reception. The time synchronization of the start of reception of LFM signals was provided with the help of GPS with the error less than 10 ms. The residual signal was digitized at intermediate frequency (IF) ( f = 215 kHz) using the 14-digit analog-digital transformer (ADT) with the frequency discreteness of 50000 Hz, the latter value exceeding considerably the used transmission band of the receiver at IF (3000 Hz).
[15] The signal transformation and its processing included the following stages. The received residual signal was put through the procedures of a transfer to the zero frequency with obtaining quadrature components (complex low-frequency envelope), low-frequency filtration by a digital filter with the transmission band of 500 Hz, and decimation with the reduction of the discreteness frequency down to 3000 Hz. As a result, the procedure described increased the dynamical range not less than by 10 dB. The procedures of digitalization, filtration, signal quadrature revealing, and decimation were organized in such a way that the entire preliminary processing was performed in real time in an automatic way and made it possible to obtain continuous recordings of unlimited duration in time. Further spectral processing of the differential signal was performed based on multitaper method [Thomson, 1982] with the aim to extract continuous and discrete components of spectral density of power. The determination of ray parameters (the amount of rays, their amplitudes and time delays) is based on the estimation of the discrete components in the spectrum of the residual signal. Such estimate is checked up based on the threshold statistical criterion (statistics of F distribution) [Thomson, 1982]. Noise spectral density and the ratio of the signal power to the noise power in the reception band consistent with the signal are determined by the histogram method assuming that the frequency band of the spectral analysis is much larger than the frequency band of the receiving signal. For such purpose the histogram of the power spectral density is obtained in the frequency band of the receiver. The maximal level of histogram corresponds to the probable value of the noise spectral density during the period of the data obtaining. After that the ratio of the signal power to the noise power is determined quite evidently. As a result of the processing in real time, the following parameters were determined: the level of the noise spectral density in the reception band, the number of detected propagation rays, amplitudes of all rays, the signal-to-noise ratio for each ray, and absolute delays of each of detected rays [Vertogradov, 2005].
[16] Further processing had the aim to obtain the frequency-time and frequency-amplitude display ionograms. Frequency-delay and frequency-amplitude display of the single propagating rays and modes are formed by comparison of the obtained parameters in the multivariate space frequency-time-amplitude. Each point that at the beginning was identified with some ray is characterized by three coordinates: fj - frequency, tjl - time delay, ajl - amplitude of the l th ray on the j th frequency. The amount of rays, nj, and their coordinates in the three-dimensional (3-D) space are determined according to the algorithm described above in the process of spectral processing based on multiple-taper method spectral analysis. In the 3-D space frequency-time-amplitude we introduce the distance between points according to the rule
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[17] Two points that correspond to the neighbor frequencies fj-1 and fj with the indices l and m are assumed to belong to the same frequency branch if the relation rj,l;j-1,m < 3 is realized. The parameters ef, et, and ea are selected empirically and assumed to be 500 kHz, 0.1 ms, and 3 dB correspondingly. If for some point the correspondence with the previous points is not established, then we assume that this point relates to a new branch. As a consequence at the end of the process of sounding many frequency branches are formed. After that the procedure of the secondary processing takes place. At this stage the regions restricted by the lowest (LOF) and the maximal (MOF) observed frequencies are found for each branch. The total amount of these regions for all frequency branches provides the opportunity to determine the intervals: the frequency intervals where one ray, two rays, three rays, etc., exist. From the MOF for all rays the MOF of the path is determined as the maximal frequency from all observed frequencies. From the lowest frequencies the LOF of the path is determined as the minimal frequency from all observed frequencies.
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Figure 2 |
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Figure 3 |
[19] The further processing of frequency-time and frequency-amplitude display ionograms at the studied paths was aimed at (1) obtaining of the diurnal variations of the maximum observed frequency of particular propagation modes, MOF was determined for extraordinary modes; (2) obtaining of averaged monthly mean variations in MOF of particular propagation modes; (3) comparison of the averaged monthly mean values of MOF with the forecasted values based on the International Reference Ionosphere IRI 2001; (4) spectral analysis of the time variations in MOF; (5) analysis of the frequency-time display and its variations caused by TID; and (6) analysis of the frequency-time display for particular propagation modes and comparison of the experimental dependencies with the results of modeling.
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