CN110927687B - Meteor detection method based on incoherent scattering radar - Google Patents
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Abstract
The invention discloses a meteor detection method based on incoherent scattering radar, which comprises the following steps: (1) dividing two paths of orthogonal sampling data of an original baseband into two paths, wherein one path is used as input data for meteor head event detection, the other path is used as input data for meteor trail event detection, and the two event detections are used as independent steps to run in parallel; (2) meteor event detection: (3) and extracting meteor parameters. The meteor detection method based on the incoherent scattering radar disclosed by the invention overcomes the sensitivity to the arrival angle of a radar beam, widens the coverage range of a detectable meteor area, and can detect meteor events in real time and extract meteor parameters such as height, radial speed, radial deceleration and equivalent scattering sectional area.
Description
Technical Field
The invention belongs to the field of space environment detection, and particularly relates to an 80km-160km meteor detection method based on an incoherent scattering radar in the field.
Background
When entering the earth atmosphere (generally below 160km), the composite granular substance from the interplanetary space can generate violent friction with surrounding atmospheric molecules, and a large amount of heat, light and ionization phenomena are generated, so that a meteor event is formed. The meteor comprises a meteor body (generating meteor head echo) moving at a high speed at the front end and ionized gas (generating meteor trail echo) in a long thin paraboloid shape at the rear end. McKinley introduces a method for observing Meteor trail by using a VHF (very High frequency) frequency band Meteor radar in the book 'Meter Science and Engineering', and the basic principle is to complete the observation of Meteor by receiving Fresnel scattering echoes of incident electromagnetic waves from Meteor trail. The method has the disadvantages that the method is very sensitive to the measurement angle, and for a single-station radar, meteor trail and radar wave beam must be approximately orthogonal to ensure that the backscatter echo can be measured; and the meteor radar can only detect meteor trail with the initial diameter smaller than or equal to the radar wavelength, and the initial diameter of the meteor trail can increase along with the rise of the height. Researches indicate that the VHF meteor radar can only detect meteors with the theoretical detection quantity of 3.5 percent in practice; in addition, the meteor radar rarely observes meteor head echoes, and the defects limit the observation and research of meteors.
Disclosure of Invention
The invention aims to solve the technical problem of providing a meteor detection method based on incoherent scattering radar.
The invention adopts the following technical scheme:
the improvement of a meteor detection method based on incoherent scattering radar, which comprises the following steps:
(1) dividing two paths of orthogonal sampling data of an original baseband into two paths, wherein one path is used as input data for meteor head event detection, the other path is used as input data for meteor trail event detection, and the two event detections are used as independent steps to run in parallel;
(2) meteor event detection:
(21) meteor head event detection:
the motion of the meteor body relative to the observation station is represented as second-order deceleration motion with the distance changing along with time, and the meteor motion formula is as follows:
in the formula (1), r0Representing the starting distance of the target, v is the radial velocity of the target, a is the radial acceleration of the target, the rightmost term is the disturbance term, the influence in the high-speed movement of the meteors is negligible, and the coherent echo of the meteors can be represented as follows:
in the formula (2), j represents an imaginary number, TpFor the radar pulse repetition period, A (t) represents the echo voltage value of meteor at the receiver end, fdThe Doppler frequency shift of the meteoroid is the Doppler frequency shift of the meteoroid, if the radar wavelength is 0.6m, the Doppler frequency shift is between tens and hundreds of kHz, and the rightmost term is the initial phase of the meteoroid in different radar repetition periods;
n pulse samples of M IPPs represented by equation (2) are sequentially arranged and zero-padded, and assuming that the scattering power of the meteor body as it passes through the radar pulse is a constant value, equation (2) can be rewritten into the following discrete form:
wherein, ω isdAnd omegaaRespectively representing normalized Doppler phase shift and normalized phase drift, respectively corresponding to radial velocity and radial deceleration, wherein P is the number of sampling points between adjacent IPPs, an FFT (fast Fourier transform) is carried out on the formula (3) to obtain an accumulated power spectrum, the peak value of the power spectrum corresponds to the convergence of meteor echo energy under corresponding frequency, and the process is represented as follows:
if the maximum value argmax () in the formula (4) is larger than the decision threshold, the meteor head event is considered to be detected, and the decision threshold is set as the noise average power spectral density according to the noise statistics
6 to 10 times of the total weight of the composition, wherein
Is calculated by the formula
(22) Meteoric trail detection:
calculating a time delay profile matrix on each height interval from an original echo sampling value obtained by post-detection filtering, and then calculating an autocorrelation function of an echo signal on each height area according to the matrix, wherein the calculation method of the autocorrelation function of the discrete signal comprises the following steps:
accumulating the autocorrelation function obtained by calculation, correcting by using a fuzzy function to obtain autocorrelation functions of the plasma in different height areas, and finally performing fast Fourier transform on the autocorrelation functions to obtain a power spectrum of the plasma; the time resolution is set as 2 minutes, the height resolution is 4km, the electron concentration and the ion component can be calculated from the plasma power profile, the neutral atmospheric wind speed can be calculated, and if a thin layer with the electron density 2 times higher than that of an ionized layer appears in an area from 80km to 160km and lasts for a certain time, meteor trail is considered to be detected;
(3) extracting meteor parameters:
(31) if a meteor head event is detected in step (21):
the radial velocity of the meteor body can be calculated from the frequency corresponding to the maximum value of the power spectrum by Vr＝-fdλ/2, wherein fdFrequency shift for meteoric bodies;
based on the obtained doppler shift and in combination with the transmit pattern, the time domain form of the received signal can be approximated:
H(t)＝Aψ(t)[cos(2fd·t)+isin(2fd·t)] (6)
in the above formula psi (t) refers to the transmission code pattern, H (t) is used as a matched filter to perform pulse compression on the received signals S (tau, t) of the continuous M IPPs, then the pulse pressure values are added to complete accumulation, and finally the velocity fuzzy function is used as the resolution ratio to respectively perform fdSelecting a certain number of frequency points on the left and right sides as new Doppler frequency shift values of formula (6), sequentially completing pulse compression by using an updated matched filter, comparing peak values of all pulse pressure values, selecting a frequency value corresponding to the maximum pulse pressure peak value as the updated Doppler frequency shift, reading the time delay tau of meteor echo signals from a pulse pressure time domain result, and further calculating the track height of meteor bodies, wherein the calculation formula is Alt-c-tau/2-sin theta, wherein c represents the light velocity, and theta is the radar beam elevation angle;
calibrating noise temperature T of radar receiving antennasysNoise power of the known system is Pnoise＝κTsysAnd B, the receiving power of meteor head echo is as follows:
in the above formula, G is the antenna gain, PTIs the peak power of the transmitter, λ is dayLine wavelength, σmIn order to coherently scatter the cross-sectional area, and then according to the real-time signal-to-noise ratio after pulse compression
K is pulse pressure gain, and scattering sectional area sigma can be calculatedmIt is also known that meteor echo is coherent scattering from N electrons surrounding the fluid at distances less than λ/4, and is formulated as
Wherein r iseFor the electron radius, the calculation formula is
So that sigma is obtainedmThen, the electron number in the coherent scattering area near the meteor body can be calculated by the formula;
if the meteor body is detected in not less than 6 radar pulse periods, enough observation values of radial velocity can be obtained, the velocity value is fitted in time by using a least square method to obtain the velocity change rate, namely the deceleration of the meteor body, the BP parameter of the meteor body, namely the meteor body-to-mass ratio can be obtained according to a momentum equation (8),
in the above formula, Γ is an air resistance coefficient, and is 1 here, ρ (z) is an air density that varies with height, and assuming that the meteor is an ideal sphere, the above formula can be rewritten as
Wherein r ismIs the equivalent radius of the meteor body, pmCalculating the equivalent size and mass of the meteor body after obtaining BP as the density of the meteor body;
(32) if a meteor trail event is detected in step (22):
the space when the ionosphere is detected is divided into height intervals with the width as the distance resolution, the average echo power on the corresponding height can be obtained by calculating the correlation function ACF (0) at the zero time delay position for echo signals in different intervals and carrying out fuzzy solution operation, and then the power profile in the whole detection height area is obtained, and the relation between the electron density and the echo power is as follows:
in the above formula, C is a constant, PtFor transmitter power, Δ Pr(r2) For receiving echo power within a unit distance range at a distance r, Δ r is the distance variation, k is the scattering wave number, λDFor the Debye length, the formula is
For the electron-ion temperature ratio, for the plasma parameter, generally k2 λ D 21, provided that
Equation (9) can be rewritten as:
the electron density of the formula (10) is irrelevant to plasma parameters, so that the electron density can be calculated from an echo power value, after an electron concentration profile of the whole detection interval is obtained, the time of occurrence of meteor trail and the height of a track can be visually found by comparing the electron concentration profile with the density of a background ionized layer, and the time difference between the start and the end of a meteor event is the whole residence time of the meteor trail in a radar beam;
(33) if no meteor head or meteor trail event is detected, updating a decision threshold through pure ionosphere echo data;
(4) and (4) repeating the steps (1) to (3) and processing the data collected at the next moment.
The invention has the beneficial effects that:
the meteor detection method based on the incoherent scattering radar disclosed by the invention overcomes the sensitivity to the arrival angle of a radar beam, widens the coverage range of a detectable meteor area, and can detect meteor events in real time and extract meteor parameters such as height, radial speed, radial deceleration and equivalent scattering sectional area.
Compared with the traditional low-power VHF meteor radar, the meteor detection method based on the incoherent scattering radar can detect meteor trail and meteor head echo, and the observation range of the meteor is expanded. This property can be used to help people study the coupling between meteors and ionosphere E and to count the mass flux (kg/year) of meteors into the atmosphere each year.
The method disclosed by the invention is used for detecting the meteor trail based on incoherent scattering excited by the plasma in a thermal equilibrium state, and the detection mode is insensitive to the radar beam arrival angle and is not influenced by the initial diameter of the meteor, so that the meteor trail with wider height distribution can be detected, and the detection efficiency of the meteor is improved.
The method disclosed by the invention can extract a plurality of meteor parameters, including orbit height, radial velocity and radial deceleration of meteor body, equivalent scattering sectional area, mass-to-surface ratio, size, mass, electron concentration of meteor trail, drift velocity, residence time and the like. In addition, compared with the common VHF meteor radar, the incoherent scattering radar has narrower detection beams, so that the incoherent scattering radar has higher transverse resolution, and the time resolution and the distance resolution are both superior to those of the VHF radar, so that the obtained parameters have better space-time precision.
When the meteor is observed, the incoherent scattering radar can simultaneously obtain plasma parameters of a meteor occurrence region (an ionized layer E layer), including electron concentration, electron temperature, ion temperature, plasma velocity and the like, so that the study on the coupling between the meteor body and the high-altitude atmosphere is facilitated, and the study on the space weather science is of great significance.
When the meteor body moves along the direction of radar beam, the method disclosed by the invention can completely detect the whole process of the meteor body from ablation to disappearance in the atmosphere, especially the change of the mass-to-area ratio (BP) is measured, which has important significance for researching how the meteor body deposits substances in the atmosphere.
Drawings
FIG. 1 is a schematic flow chart of the method disclosed in example 1 of the present invention;
FIG. 2 is an example diagram of meteor echo time domain sample data;
FIG. 3 is a schematic diagram of meteor echo detection according to the method disclosed in embodiment 1 of the present invention;
fig. 4 is a schematic diagram of meteor trail echo detection according to the method disclosed in embodiment 1 of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention will be described in further detail below with reference to the accompanying drawings and examples. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
The incoherent scattering radar is the strongest earth-based means for monitoring and researching the space environment in the world at present, has the technical characteristics of high power, large aperture, low noise and the like, can directly measure a plurality of ionospheric parameters such as electron temperature, ion temperature, electron density, plasma velocity and the like, and can also be used for observing and researching space objects such as meteorosomes, space fragments and the like.
The method disclosed by the embodiment has been applied to a meridian engineering curved non-coherent scattering radar in a non-public way, and the main parameters of the radar are as follows:
(1) dividing two paths of orthogonal sampling data of an original baseband into two paths, wherein one path is used as input data for meteor head event detection, the other path is used as input data for meteor trail event detection, the two event detections are used as independent steps to run in parallel, the example of meteor echo time domain sampling data is shown in figure 2, the track height of a meteor event is about 100km, and the signal-to-noise ratio is about 20 dB;
(2) meteor event detection: the basic objective is to determine the presence or absence of meteor events from raw echo samples of the radar. Meteors are typically observed by detecting meteor bow echoes and/or meteor trail echoes at the receiving end.
(21) Meteor head event detection: the basic process is that firstly N sampling points (N depends on the sampling rate) in meteor regions (80-160km) in continuous M radar pulse periods (Inter-pulse Period-IPP, the corresponding duration is generally less than ten milliseconds) are combined into a data sequence, and zero is filled in the middle at time intervals. The data sequence is subjected to Fast Fourier Transform-FFT (Fast Fourier Transform-FFT) to obtain a corresponding power spectrum. The maximum value of the power spectrum is found and compared with a decision threshold. And if the judgment threshold is exceeded, determining that a meteor head event is detected. The decision threshold is calculated by a constant false alarm detection method and is updated in real time at intervals. An example of calculating the power spectrum in this step can be seen in fig. 3, where the power spectrum is obtained by performing fast fourier transform on the continuous radar echo sample data, where the abscissa represents the doppler velocity and the ordinate represents the relative echo power level. One case shown in the inset is the power spectrum of the meteor echo at the corresponding frequency when the signal-to-noise ratio is high. This step is specifically described below:
the motion of the meteor body relative to the observation station is represented as second-order deceleration motion with the distance changing along with the time, and the meteor motion formula is as follows:
in the formula (1), r0Representing the starting distance of the target, v is the radial velocity of the target, a is the radial acceleration of the target, the rightmost term is the disturbance term, the influence in the high-speed movement of the meteors is negligible, and the coherent echo of the meteors can be represented as follows:
in the formula (2), j represents an imaginary number, TpFor the radar pulse repetition period, A (t) represents the echo voltage value of meteor at the receiver end, fdThe Doppler frequency shift of the meteoroid is the Doppler frequency shift of the meteoroid, if the radar wavelength is 0.6m, the Doppler frequency shift is between tens and hundreds of kHz, and the rightmost term is the initial phase of the meteoroid in different radar repetition periods;
n pulse samples of M IPPs represented by equation (2) are sequentially arranged and zero-padded, and assuming that the scattering power of the meteor body as it passes through the radar pulse is a constant value, equation (2) can be rewritten into the following discrete form:
wherein, ω isdAnd omegaaRespectively representing normalized Doppler phase shift and normalized phase drift, respectively corresponding to radial velocity and radial deceleration, wherein P is the number of sampling points between adjacent IPPs, an accumulated power spectrum can be obtained after FFT transformation is carried out on the formula (3), and the peak value of the power spectrum corresponds to convergence of meteor echo energy under corresponding frequency, and the process is represented as follows:
if in formula (4)Is greater than a decision threshold, a meteor head event is considered to be detected, and the decision threshold is set as the noise average power spectral density according to the noise statistics
6 to 10 times of the total weight of the composition, wherein
Is calculated by the formula
(22) Meteoric trail detection: when the meteor trail is formed and diffused for tens of seconds, the plasma in the meteor trail is in an equilibrium state. There are two modes of electron fluctuation in the plasma at this time: respectively, ion sound waves and langeuir waves. The waves of the two modes have isotropy and are not influenced by the arrival angle of the radar beam. The plasma now produces incoherent scattering of the incident electromagnetic wave, which can be detected by radar. The basic flow is to carry out post-detection filtering on input data respectively, generate a time delay profile matrix, calculate an autocorrelation function, solve a fuzzy function, perform incoherent accumulation and other operations. And (4) obtaining the echo power of the corresponding height from the finally accumulated autocorrelation function at the zero time delay position, and further obtaining the power profile of the whole meteor region (80-160 km). The parameters of the ionospheric electron concentration and the like can be calculated from the power profile and combined with the formula (10). And finally, comparing the electron concentration profile in the current radar pulse repetition period with the background ionosphere concentration. And if the concentration of the ionized layer is 2 times or more than the background ionized layer concentration, determining that a meteoric trail event is detected. The background ionosphere concentration is calculated from the echo data without meteor trail and is updated in real time at intervals. An example of calculating the power profile in this step can be seen in fig. 4, where the abscissa represents the number of different accumulation periods and the ordinate represents the track height. As can be seen, in the 3 rd to 10 th accumulation periods, meteor trail echoes which are obviously higher than the ionosphere echo power exist at the track height of 110 km. This step is specifically described below:
the ionospheric incoherent scattering signal processing method used in detecting meteor trail is as follows: calculating a time delay profile matrix on each height interval from an original echo sampling value obtained by post-detection filtering, and then calculating an autocorrelation function of the echo signal on each height area according to the matrix, wherein the calculation method of the autocorrelation function of the discrete signal comprises the following steps:
accumulating the autocorrelation function obtained by calculation, correcting by using a fuzzy function, eliminating the influence of a modulation mode and a filter, obtaining autocorrelation functions of the plasma in different height areas, and finally performing fast Fourier transform on the autocorrelation functions to obtain a power spectrum of the plasma; the time resolution depends on the accumulation duration, is generally set to be 2 minutes, the height resolution is determined by a fuzzy function and is generally about 4km, physical parameters such as electron concentration and ion components can be calculated from a plasma power profile, the neutral atmospheric wind speed can be calculated, and if a thin layer with the electron density 2 times higher than that of an ionized layer appears in an area from 80km to 160km and lasts for a certain time, meteor trail is considered to be detected;
(3) extracting meteor parameters: if meteor (meteor head/meteor trail) events are detected in the step (2), executing a step (3) to extract relevant parameters of meteor from the echo: the orbit height and the radial velocity of the meteor body can be directly measured from the data of the meteor head echo. The radial deceleration of the meteor body can be obtained by carrying out linear fitting on the change value of the radial velocity along with the time. The ballistic parameter-BP of the meteor body, i.e. the ratio of the mass to the surface area ratio of the meteor body (mass-to-surface ratio), can be calculated from the equation of momentum when the meteor body enters the atmosphere. If the meteor shape is assumed to be spherical and the source is asteroid, the equivalent radius and mass of the meteor can be further calculated.
(31) If a meteor head event is detected (meteor body passes the radar beam) in step (21), the Doppler shift of the meteor body can be derived from the power spectrum. And generating a matched filter bank by taking the frequency point value in the frequency shift range as a variable through a formula (6), and respectively performing pulse compression with the echo data. And updating the Doppler frequency shift value by the frequency corresponding to the maximum response value, and calculating the radial velocity of the meteor body. In addition, the orbit height of the meteor can be calculated through time delay. And calculating the number of electrons generating coherent scattering according to the signal-to-noise ratio of the echo. If the radial velocity of the meteor is calculated in 6 or more echo data, the deceleration of the meteor can be calculated by least square fitting. The mass-to-surface ratio of the meteoroid can be calculated through a momentum equation (8), the equivalent diameter and the mass can be further obtained, and finally the result is stored. This step is specifically described below:
the radial velocity of the meteor body can be calculated from the frequency corresponding to the maximum value of the power spectrum obtained in the step (2), and the calculation method is Vr＝-fdλ/2, wherein fdFrequency shift for meteoric bodies;
based on the obtained doppler shift and in combination with the transmit pattern, the time domain form of the received signal can be approximated:
H(t)＝Aψ(t)[cos(2fd·t)+isin(2fd·t)] (6)
in the above formula psi (t) refers to the transmission code pattern, H (t) is used as a matched filter to perform pulse compression on the received signals S (tau, t) of the continuous M IPPs, then the pulse pressure values are added to complete accumulation, and finally the velocity fuzzy function is used as the resolution ratio to respectively perform fdSelecting a certain number of frequency points on the left and right sides as new Doppler frequency shift values of formula (6), sequentially completing pulse compression by using an updated matched filter, comparing peak values of all pulse pressure values, selecting a frequency value corresponding to the maximum pulse pressure peak value as the updated Doppler frequency shift, reading the time delay tau of meteor echo signals from a pulse pressure time domain result, and further calculating the track height of meteor bodies, wherein the calculation formula is Alt-c-tau/2-sin theta, wherein c represents the light velocity, and theta is the radar beam elevation angle;
measurement of equivalent scattering cross section area of meteor head echo requires calibration of noise temperature T of radar receiving antennasysNoise power of the known system is Pnoise＝κTsysAnd B, the receiving power of meteor head echo is as follows:
in the above formula, G is the antenna gain, PTFor transmitter peak power, λ is antenna wavelength, σmIn order to coherently scatter the cross-sectional area, and then according to the real-time signal-to-noise ratio after pulse compression
K is pulse pressure gain, and scattering sectional area sigma can be calculatedmIt is also known that meteor echo is coherent scattering from N electrons surrounding the fluid at distances less than λ/4, and is formulated as
Wherein r iseFor the electron radius, the calculation formula is
So that sigma is obtainedmThen, the electron number in the coherent scattering area near the meteor body can be calculated by the formula;
if the meteor is detected in not less than 6 radar pulse periods, enough observation values of radial velocity can be obtained, the velocity value is fitted in time by using a least square method to obtain the velocity change rate, namely the deceleration of the meteor, after the deceleration of the meteor is obtained, the BP parameter of the meteor, namely the meteor mass-to-surface ratio can be obtained according to a momentum equation (8),
in the above formula, gamma is an air resistance systemWhere 1 is assumed here, ρ (z) is the air density which varies with height, and assuming that the meteor is an ideal sphere, the above formula can be rewritten as
Wherein r ismIs the equivalent radius of the meteor body, pmAccording to the previous research results, the meteors measured by the incoherent scattering radar are mainly from small planets, and the density of the meteors is 1g/cm3. Therefore, after the BP is obtained, the equivalent size and the mass of the meteor can be further calculated;
(32) if a meteor trail event is detected in step (22), the orbit height and the time of appearance and disappearance of the meteor trail can be obtained from the electron density profile. The drift velocity of the meteor trail relative to the radar beam can be calculated from the change of the meteor trail orbit height along with the time, and the result is finally stored. This step is specifically described below:
the electron concentration, the track height and the residence time of the meteor trail can be directly measured from the echo power of the meteor trail. The drift velocity (atmospheric wind speed) of the meteor trail can be calculated through nonlinear inversion. The measurement method of electron density is described below: the space when the ionosphere is detected is divided into height intervals with the width as the distance resolution, the average echo power on the corresponding height can be obtained by calculating the correlation function ACF (0) at the zero time delay position for echo signals in different intervals and carrying out fuzzy solution operation, and then the power profile in the whole detection height area is obtained, and the relation between the electron density and the echo power is as follows:
in the above formula, C is a constant, PtFor transmitter power, Δ Pr(r2) For receiving echo power within a unit distance range at a distance r, Δ r is the distance variation, k is the scattering wave number, λDFor the Debye length, the formula is
For the electron-ion temperature ratio, for the plasma parameter, generally k2 λ D 21, provided that
Equation (9) can be rewritten as:
the electron density of the formula (10) is irrelevant to plasma parameters, so that the electron density can be calculated from an echo power value, after an electron concentration profile of the whole detection interval is obtained, the time of occurrence of meteor trail and the height of a track can be visually found by comparing the electron concentration profile with the density of a background ionized layer, and the time difference between the start and the end of a meteor event is the whole residence time of the meteor trail in a radar beam;
(33) if no meteor head or meteor trail event is detected, updating a decision threshold through pure ionosphere echo data;
(4) and (4) repeating the steps (1) to (3) and processing the data collected at the next moment.
Claims (1)
1. A meteor detection method based on incoherent scattering radar is characterized by comprising the following steps:
(1) dividing two paths of orthogonal sampling data of an original baseband into two paths, wherein one path is used as input data for meteor head event detection, the other path is used as input data for meteor trail event detection, and the two event detections are used as independent steps to run in parallel;
(2) meteor event detection:
(21) meteor head event detection:
the motion of the meteor body relative to the observation station is represented as second-order deceleration motion with the distance changing along with the time, and the meteor motion formula is as follows:
in the formula (1), r0Representing the starting distance of the target, v is the radial velocity of the target, a is the radial acceleration of the target, the rightmost term is a disturbance term, the influence in the high-speed motion of the meteors is ignored, and the coherent echo of the meteors is represented as follows:
in the formula (2), j represents an imaginary number, TpFor the radar pulse repetition period, A (t) represents the echo voltage value of meteor at the receiver end, fdThe Doppler frequency shift of the meteoroid is the Doppler frequency shift of the meteoroid, if the radar wavelength is 0.6m, the Doppler frequency shift is between tens and hundreds of kHz, and the rightmost term is the initial phase of the meteoroid in different radar repetition periods;
n pulse samples of M IPPs represented by equation (2) are sequentially arranged and zero-padded, and assuming that the scattering power of the meteor body as it passes through the radar pulse is a constant value, then equation (2) is rewritten to the discrete form:
wherein, ω isdAnd omegaaRespectively representing normalized Doppler phase shift and normalized phase drift, respectively corresponding to radial velocity and radial deceleration, wherein P is the number of sampling points between adjacent IPPs, performing FFT (fast Fourier transform) on the formula (3) to obtain an accumulated power spectrum, wherein the peak value of the power spectrum corresponds to convergence of meteor echo energy under corresponding frequency, and the process is represented as follows:
if the maximum value argmax () in the formula (4) is larger than the decision threshold, the meteor head event is considered to be detected, and the decision threshold is set as the noise average power spectral density according to the noise statistics
6 to 10 times of the total weight of the composition, wherein
Is calculated by the formula
(22) Meteoric trail detection:
calculating a time delay profile matrix on each height interval from an original echo sampling value obtained by post-detection filtering, and then calculating an autocorrelation function of an echo signal on each height area according to the matrix, wherein the calculation method of the autocorrelation function of the discrete signal comprises the following steps:
accumulating the autocorrelation functions obtained by calculation, correcting by using a fuzzy function to obtain autocorrelation functions of the plasma in different height areas, and finally performing fast Fourier transform on the autocorrelation functions to obtain a power spectrum of the plasma; setting the time resolution as 2 minutes and the height resolution as 4km, calculating the electron concentration and the ion composition from the plasma power profile, and calculating the neutral atmospheric wind speed, and if a thin layer with the electron density 2 times higher than that of an ionized layer appears in an area of 80km to 160km and lasts for a certain time, considering that meteor trail is detected;
(3) extracting meteor parameters:
(31) if a meteor head event is detected in step (21):
the radial velocity of the meteor body is calculated from the frequency corresponding to the maximum value of the power spectrum by the method Vr＝-fdλ/2 ofIn fdFrequency shift for meteoric bodies;
and according to the obtained Doppler frequency shift and by combining the transmitting code pattern, approximately giving the time domain form of the received signal:
H(t)＝Aψ(t)[cos(2fd·t)+i sin(2fd·t)] (6)
in the above formula psi (t) refers to the transmission code pattern, H (t) is used as a matched filter to perform pulse compression on the received signals S (tau, t) of the continuous M IPPs, then the pulse pressure values are added to complete accumulation, and finally the velocity fuzzy function is used as the resolution ratio to respectively perform fdSelecting a certain number of frequency points on the left and right sides as new Doppler frequency shift values of formula (6), sequentially completing pulse compression by using an updated matched filter, comparing peak values of all pulse pressure values, selecting a frequency value corresponding to the maximum pulse pressure peak value as the updated Doppler frequency shift, reading a meteor echo signal time delay tau from a pulse pressure time domain result, and further calculating to obtain the orbit height of a meteor body, wherein the calculation formula is Alt-c-tau/2-sin theta, wherein c represents the light velocity, and theta is the radar beam elevation angle;
calibrating noise temperature T of radar receiving antennasysNoise power of the known system is Pnoise＝κTsysAnd B, the receiving power of meteor head echo is as follows:
in the above formula, G is the antenna gain, PTFor transmitter peak power, λ is antenna wavelength, σmIn order to coherently scatter the cross-sectional area, and then according to the real-time signal-to-noise ratio after pulse compression
K is pulse pressure gain, and scattering sectional area sigma is calculatedmIt is also known that meteor echo is coherent scattering from N electrons surrounding the fluid at distances less than λ/4, and is formulated as
Wherein r iseFor the electron radius, the calculation formula is
So that sigma is obtainedmThen, calculating by the formula to obtain the number of electrons in a coherent scattering region near the meteor body;
if the meteor is detected in not less than 6 radar pulse periods, enough observation values of radial velocity can be obtained, the least square method is used for fitting the velocity value in time to obtain the velocity change rate, namely the deceleration of the meteor, the BP parameter of the meteor, namely the meteor mass-to-surface ratio is obtained according to the momentum equation (8),
in the above equation, Γ is an air resistance coefficient, and is 1 here, ρ (z) is an air density that varies with height, and assuming that the meteor is an ideal sphere, the above equation is rewritten as
Wherein r ismIs the equivalent radius of the meteor body, pmCalculating the equivalent size and mass of the meteor after obtaining BP for the density of the meteor;
(32) if a meteor trail event is detected in step (22):
the space when the ionosphere is detected is divided into height intervals with the width as the distance resolution, the average echo power on the corresponding height is obtained by calculating a correlation function ACF (0) at the zero time delay position for echo signals in different intervals and carrying out fuzzy solution operation, and then the power profile in the whole detection height area is obtained, wherein the relationship between the electron density and the echo power is as follows:
in the above formula, C is a constant, PtFor transmitter power, Δ Pr(r2) For receiving echo power within a unit distance range at a distance r, Δ r is the distance variation, k is the scattering wave number, λDFor the Debye length, the formula is
For the electron-ion temperature ratio, for the plasma parameter, generally k2λD 21, provided that
Equation (9) is rewritten as:
the electron density of the formula (10) is irrelevant to plasma parameters, so that the electron density is calculated from the echo power value, after an electron concentration profile of the whole detection interval is obtained, the time of occurrence of meteor trail and the height of a track are visually found by comparing the electron concentration profile with the density of a background ionized layer, and the time difference between the start and the end of a meteor event is the whole stay time of the meteor trail in a radar beam;
(33) if no meteor head or meteor trail event is detected, updating a decision threshold through pure ionosphere echo data;
(4) and (4) repeating the steps (1) to (3) and processing the data collected at the next moment.
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| CN117665810B (en) * | 2024-01-31 | 2024-04-09 | 中国科学院地质与地球物理研究所 | Ionosphere electron density detection method, system and equipment for linear frequency modulation signals |
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