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An FPGA-based acoustic time-delay meter with the post-detection correlation processing of pseudo-noise signals is proposed. The measurement method is based on the real-time computation of the mutual correlation function between the reference signal that follows the modulation law and the received signal. The experimental results obtained using a prototype of the developed system are presented. The results validated the viability of the developed software and hardware solutions as applied to acoustic-, radio-, and hydroacoustic-location systems.

We estimate the accuracy of measurement of the instantaneous frequency ω(t) by the autonomous and mutual (i.e., using the reference signal) Wigner–Ville transform. It is shown that the estimation of the frequency ω(t) by the autonomous Wigner–Ville transform depends on the type of nonlinearity of the function ω(t) to a greater degree and on its time-variation rate to a lesser degree. The mechanism of increasing the frequency-time resolution of the mutual Wigner–Ville transform due to using the matched signal is described and the method for measuring the instantaneous frequency and the phase, which is based on revealing the character of modulation of the real and imaginary parts of the mutual power Pmut(t, ω) determined by the reference oscillation, is considered. Stability of the mutual Wigner–Ville transform with respect to normal white noise is estimated.

We consider the possibility of obtaining frequency–time distributions of signals by means of the cross Wigner–Ville transform in which the reference (test) signal is designed in accordance with the structure of the studied signal. It is shown that because of the complex nature of the Wigner–Ville distributions (WVDs) P(t, ω), information on the frequency–time characteristics of studied signals can be obtained by analyzing the modulus of the distribution |P(t, ω)|, as well as by the real Re[P(t, ω)] and imaginary Im[P(t, ω)] parts of the WVD. Their analysis by the method of selected sections permits one to disclose in more detail the spectral–time content of processed signals and thereby increase the frequency–time resolution. The efficiency of the proposed technique was verified by numerical modeling and in work with acoustic signals received by a mobile receiver from a fixed transmitter.

We compare the data on the atmospheric radio emission in two ozone lines with resonance frequencies 96,228 and 101,736 MHz, obtained at the N. I. Lobachevsky State University of Nizhny Novgorod in 1996–2001, with the corresponding results of measuring the number of neutrons in the atmosphere, obtained at the Institute of Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation (Troitsk, Moscow region) using the NM-64 monitor. It is shown that variations in both the optical depth and the intensity ratio of these lines correlate with variations in the number of neutrons. The maximum observed correlation coefficients for the optical-depth decrements of the lines amount to 0.5–0.6, which is about a factor of 1.5 higher than the maximum correlation coefficient for their intensity ratio. We analyze the dependence of the correlation coefficients on the time shift between data series for the ozone line intensity and the number of neutrons in the atmosphere. It is shown that correlation between these data series almost vanishes for a time shift of about 200 h. We discuss the features of the data on the number of neutrons in the atmosphere and the possibility of the solar origin of neutrons.

Low-frequency pulsations of 22 and 37 GHz microwave radiation detected during solar flares are analyzed. Several microwave bursts observed at the Metsahovi Radio Observatory are studied with time resolutions of 100 and 50 ms. A fast Fourier transformation with a sliding window and the Wigner-Ville method are used to obtain frequency-time diagrams for the low-frequency pulsations, which are interpreted as natural oscillations of coronal magnetic loops; the dynamical spectra of the pulsations are synthesized for the first time. Three types of low-frequency fluctuations modulating the flare microwave radiation can be distinguished in the observations. First, there are fast and slow magneto-acoustic oscillations with periods of 0.5–0.8 s and 200–280 s, respectively. The fast magneto-acoustic oscillations appear as trains of narrow-band signals with durations of 100–200 s, a positive frequency drift dν/dt=0.25 MHz/min, and frequency splitting δν=0.01–0.05 Hz. Second, there are natural oscillations of the coronal magnetic loops as equivalent electrical circuits. These oscillations have periods of 0.5–10 s and positive or negative frequency drift rates dν/dt=8×10−3 Hz/min or dν/dt=−1.3×10−2 Hz/min, depending on the phase of the radio outburst. Third, there are modulations of the microwave radiation by short periodic pulses with a period of 20 s. The dynamical spectra of the low-frequency pulsations supply important information about the parameters of the magnetic loops: the ratio of the loop radius to its length r/L≈0.1, the plasma parameter β≈10−3, the ratio of the plasma densities outside and inside the loop ρe/ρi≈10−2, and the electrical current flowing along the loop I≈1012 A.

We consider the characteristics of typical nonlinear distortions in the dynamical spectra of signals, obtained using the Wigner–Ville method, and discuss the existing techniques for attenuating these distortions. Certain methods are proposed to “clean” Wigner–Ville distributions of products of the nonlinear interaction between the signal component under study and its more powerful harmonic or pulsed components. The proposed techniques are mainly based on a preliminary linear processing (e.g., using the fast Fourier transform) and/or frequency filtering of the signal. A software package for calculating the Wigner–Ville distribution on a PC is developed. Using this package, we approved successfully the proposed algorithms for test and actual signals.

The digital analyzer is a part of a multichannel modulation-type radiometric detector intended for studying radiation lines of impurity gases in the Earth's atmosphere in a frequency range of 96–110 GHz. The signal passing through one of the radiospectrometer's analog filtering channels is digitized at an intermediate frequency of 30 MHz. A digital filtering algorithm based on the quadrature conversion is described. Software implementing this algorithm was developed. The structure of the input system and the interface between the analog-to-digital converter and the control computer are described. A distinct feature of the interface is its 32-Kbyte buffer memory. The test results are presented.

Using the Wigner-Ville transform, we study the dynamic spectra of low-frequency modulation of the intensity of microwave emission from several solar flares and detect the microwave bursts modulated by chirp signals (i.e., signals whose frequencies ω=ω0± kt, where k is a constant and t is the time) with a positive chirp rate. Such a modulation corresponds to the process of energy accumulation in the corresponding coronal magnetic loop (CML). We also detect chirp modulation with a negative chirp rate corresponding to powerful dissipation of electric current in the CML during a solar flare. The chirp modulation of the intensity of microwave emission from a CML arises from the excitation of eigenoscillations of the loop as an equivalent electric circuit. In this case, the modulation frequency is proportional to the loop electric current and varies with the variation of the latter. The electric-current values found for a few events using the dynamic spectra of chirp signals lie in the range 1011-1012A, and the rate of energy increase (decrease) is estimated to be 1.4·1026-3·1029 erg/sec for the characteristic time scale τ≃ 103-4·104 sec. The events studied give evidence for the possibility of realizing the “circuit” model of solar flares under solar-corona conditions.

We analyze low-frequency intensity fluctuations of the microwave emission from solar flares at frequencies 22 and 37 GHz. The three microwave bursts of durations of about 1 h, observed at the Metsahovi Radio Observatory (Finland) with the time resolution of 0.1 and 0.05 s, are studied. To obtain spectral-temporal characteristics of the low-frequency fluctuations, we apply the Wigner-Ville method, i.e., the time-lag Fourier transform of the “local” autocorrelation function of an analytical signal. As a result, we obtain for the first time the dynamical spectra of the low-frequency fluctuations, which are identified as MHD eigenoscillations of coronal magnetic loops. The features of the dynamical spectra testify that several types of low-frequency pulsations are excited in coronal magnetic loops during solar flares: 1) Fast and slow magnetosonic oscillations with periods of 1-1.5 s and 200-280 s, respectively. Fast magnetosonic oscillations appear as pulse trains of duration 100-200 s and have the positive frequency drift dν/ dt≈ 0.125 Hz/min and the frequency splitting δν≈ 0.05 Hz; 2) The eigenoscillations of a coronal magnetic loop as an equivalent electric circuit. The period of these oscillations is about 1 s during the initial stage of a microwave burst and increases gradually up to 4 s during the decay stage of the radio emission; and 3) Intensity modulation of the microwave radiation by a periodic pulse sequence with a period of about 1 s at the burst onset and about 2 s at its end. The parameters of the dynamical spectra and identification of the MHD pulsations allow us to obtain information on the loop parameters, such as the ratio of the loop radius to its length (r/L≈ 0.1), the ratio of the gas pressure to the magnetic-field pressure inside the loop (β ≈ 3· 10-3), the ratio of plasma densities outside and inside the loop, and the electric current in the coronal loop (I≈ 1.5 · 1012 A).

An IBM PC-based control system for the multichannel modulation-type radiometric receiver with the series-parallel filtering spectral analysis in theK a band is developed. A special-purpose instrument interface with a single-board controller linked to the computer via ISA bus is used for controlling receiver components and transferring data. Primary attention is drawn to the electromagnetic compatibility of the control system and receiver. Appropriate circuit designs and test results are provided.

We analyze the results of observations of the radio emission of atmospheric N2O in the line J=3→4 at frequency near 100.5 GHz. Observations aimed at monitoring the nitrous oxide content at altitudes h>20 km were performed in Nizhny Novgorod from March to May, 1997. We discover variations of the optical-depth decrement in the line, which can be explained by the redistribution of nitrous oxide over the altitude range h=20–55 km. This assumption does not contradict published data on in-situ measurements of N2O density at h=5–35 km. Using the obtained data we calculate the rotational-line spectra of atmospheric N2O. We point out that the measured decrements of the optical depths are greater than the calculated ones. We discuss observations of the N2O lines in the mm-wave band as a promising method for monitoring the vertical mixing in the atmosphere.