CN112505719B - Doppler frequency correction secondary compensation laser wind-finding radar wind-finding method and system - Google Patents
Doppler frequency correction secondary compensation laser wind-finding radar wind-finding method and system Download PDFInfo
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
- G01S17/02—Systems using the reflection of electromagnetic waves other than radio waves
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
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- G—PHYSICS
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- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
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Abstract
The invention relates to a laser wind-finding radar wind-finding method and system, in particular to a Doppler frequency correction secondary compensation laser wind-finding radar wind-finding method and system, which solve the problems of low correction precision and complicated correction process when the existing frequency correction method is adopted for single correction. The wind measuring method comprises the following steps: step 1: collecting echo signals; step 2: a large amount of coherent accumulation is carried out on the echo signals to obtain an echo signal discrete sampling data sequence x (n); step 3: selecting L effective data points from x (N), adding zero to N points, and performing fast Fourier transform of the sampling points N to obtain an amplitude spectrum A (k); step 4: obtaining a position index value k 0 of the first correction frequency by using a discrete frequency spectrum correction method; step 5: calculating a first correction frequency f d1; step 6: setting a simulated ideal cosine signal by using the first correction frequency f d1 to obtain a second correction frequency f d2; step 7: calculating a twice corrected frequency difference Δf d; step 8: calculating a final correction frequency f d; step 9: wind speed is calculated.
Description
Technical Field
The invention relates to a laser wind-finding radar wind-finding method and system, in particular to a Doppler frequency correction secondary compensation laser wind-finding radar wind-finding method and system.
Background
The basic principle of laser wind radars is to apply the doppler effect. The doppler effect is the change in frequency of a received signal when there is radial relative motion between the source and the receiver. Since the Doppler frequency of the target echo is proportional to the speed of its radial relative movement, the speed of the radial relative movement of the target can be determined as long as the Doppler frequency of the target echo is accurately measured. When the echo signals scattered by aerosol in the atmosphere wind field received by the balance detector and the local oscillation light are subjected to beat frequency, data processing is needed. In the short-range detection data processing, the FFT periodogram method is generally used for processing data because the FFT ((Fast Fourier Transformation, FFT) periodogram method) has small calculation amount, short time and high efficiency.
When the FFT periodogram method is used for processing data, as the discrete spectrum analysis can only be carried out on a truncated finite-length time domain, energy leakage inevitably exists, so that the frequency spectrum obtained through the FFT (Fast Fourier transform, FFT) can generate larger errors in frequency, amplitude and phase, and further the application of the technology in engineering is limited to a great extent, and therefore, the correction technology of the discrete spectrum needs to be studied to eliminate or greatly reduce the errors and improve the analysis precision. For the discrete spectrum of single frequency components or more distant multi-frequency components, there are four main correction methods currently in common use: ratio method, energy gravity method, continuous refinement Fourier transform analysis method (FFT+FT) and phase difference method.
Taking an energy gravity center method as an example, the energy gravity center method is a general discrete spectrum correction method proposed on the basis of a three-point convolution amplitude correction method in 2001, ding Kang and the like. For common window functions, such as rectangular windows, hanning windows, hamming windows, etc., the discrete spectral energy center of gravity will tend to infinity as the number of spectral lines n approaches the origin. The energy gravity center method is a correction method with higher correction precision by utilizing the principle that the energy gravity center of a discrete window spectrum function is the origin of coordinates to calculate the frequency correction amount.
In Liu Changwen et al, "spectral correction in laser Doppler velocimetry and its application" ("China laser", volume 30, 7 th 2003), a method for improving the measurement accuracy of Doppler frequency using an energy gravity center method to meet the measurement requirements is disclosed. The method utilizes the energy gravity center of various window function discrete spectrums to approach the center of a main lobe or near the center of the main lobe, and through windowing a signal power spectrum, a window function is selected according to the distribution characteristics of the signal power spectrum, the number of sampling points is determined, and finally, the center coordinates of the main lobe are obtained by using a plurality of spectral lines with larger power spectrum values in the main lobe, so that the corrected Doppler frequency is obtained. The energy concentricity method does not depend on a window function, and the algorithm is simple to realize and has small calculated amount. However, the method still has the following defects: the Doppler frequency correction precision is related to the selection of window function types and sampling points, and when the window function selects a rectangular window or the number of the selected sampling points is small, the Doppler frequency correction precision is low, and the requirement of high speed measurement precision of the frequency-modulated continuous wave landing radar cannot be met.
The invention discloses a frequency modulation continuous wave landing radar speed measuring method based on Doppler frequency spectrum gravity center correction in Chinese patent with application publication number of CN 108535719A and application publication date of 2018.09.14. The method can be used for correcting the gravity center of the frequency modulation continuous wave Doppler frequency spectrum by using the gravity center correction coefficient, so that the calculated amount of speed measurement is reduced, and the speed measurement precision is improved. However, the method needs to calculate the frequency estimation value and the frequency spectrum width estimation value of the beam center difference frequency signal of the frequency modulation continuous wave landing radar antenna, and establishes a comparison table of the ground incidence angle of the radar antenna beam, the frequency spectrum width and the frequency ratio of the difference frequency signal, so that the process is complicated.
Disclosure of Invention
The invention aims to provide a Doppler frequency correction secondary compensation laser wind-finding radar wind-finding method and system, which are used for solving the technical problems that when the Doppler frequency correction is carried out by adopting the existing Doppler frequency correction method for a single time, the correction precision is low, the requirement of high speed measurement precision of a frequency modulation continuous wave landing radar cannot be met, and a comparison table of the ground incidence angle of a radar antenna wave beam and the frequency spectrum width and frequency ratio of a difference frequency signal is required to be established in the correction process, so that the process is complicated.
The technical scheme adopted by the invention is that the laser wind-finding radar wind-finding method for Doppler frequency correction and secondary compensation is characterized by comprising the following steps:
step 1: the laser wind measuring radar measures an atmospheric wind field and acquires echo signals through the signal acquisition card;
step 2: and (3) performing a large amount of coherent accumulation on the echo signals acquired in the step (1) to obtain an echo signal discrete sampling data sequence:
x(n)，n＝0，1，…，K-1，
k is the number of sampling points of the echo signal single-frequency pulse signal, and the value of K is an integer power of 2;
Step 3: selecting L effective data points from an echo signal discrete sampling data sequence x (N), carrying out zero padding to N points, and carrying out fast Fourier transform processing of the sampling points N to obtain a magnitude spectrum A (k), wherein k is an independent variable of a frequency domain and represents frequency;
Step 4: substituting the amplitude spectrum A (k) obtained in the step 3 into a formula corresponding to the discrete spectrum correction method by using the discrete spectrum correction method, and performing primary frequency correction to obtain a position index value k 0 of primary correction frequency of Doppler frequency;
Step 5: the first corrected frequency f d1 of the Doppler frequency is calculated according to the following equation (1):
fd1＝k0·fs/N (1)；
in the formula (1): f s is the sampling frequency of the signal acquisition card in the step 1;
step 6: setting a simulated ideal cosine signal by using a first correction frequency f d1 of the Doppler frequency, performing fast Fourier transform processing again, and performing second frequency correction by using the discrete frequency spectrum correction method which is the same as that in the step 4 to obtain a second correction frequency f d2 of the Doppler frequency;
Step 7: the difference Δf d between the twice corrected frequencies of the doppler frequency is calculated according to the following equation (2):
Δfd＝fd1-fd2 (2)；
Step 8: the final corrected frequency f d of the doppler frequency is calculated according to the following equation (3):
fd＝fd1+Δfd＝2fd1-fd2 (3)；
Step 9: and (3) calculating the wind speed of the atmospheric wind field according to the final corrected frequency f d of the Doppler frequency calculated in the step (8).
Further, in step 6, when setting the simulated ideal cosine signal, the method further includes a step of setting a phase value set value of the simulated ideal cosine signal to be consistent with a phase value of the echo signal. The device is arranged because the simulation shows that the amplitude of the signal has little influence on the frequency error, and the phase has larger influence on the frequency error, so that the mean value and the root mean square error of the corrected frequency error can be reduced from the magnitude of thousands Hz, thousands Hz to the magnitude of hundreds Hz or even ten Hz, the wind speed measurement accuracy can be calculated through the Doppler frequency shift formula f dop＝2Vr/lambda, and the wind speed measurement accuracy is improved from 0.1m/s to 0.01m/s in theory.
Further, in step 6, the method for acquiring the phase value of the echo signal includes: the light beam carrying the phase information is acquired first, and then the phase is solved by the principle of quadrature mixing.
Further, the laser wind-finding radar in the step 1 is that pulse laser emitted by a laser seed source enters the atmosphere after passing through an acousto-optic frequency shifter and is used for measuring wind speed;
in step 6, the method for acquiring the light beam carrying the phase information includes the following steps:
step a.1: the collimator emits pulse laser to be incident on the surface of the optical lens to generate a reflected echo signal with the random initial phase being the same as the random initial phase introduced by the acousto-optic frequency shifter;
Step a.2: the initial phase of the periodic pulse of the reflected echo signal generated in the step a.1 is obtained through identification;
Step a.3: and d, taking the initial phase of the periodic pulse of the reflected echo signal obtained by the identification in the step a.2 as a reference, moving pulse waveforms in other periods to be aligned with the initial phase, eliminating random phases of the same pulse and different periodic waveforms, and performing effective coherent superposition to obtain the light beam carrying the phase information.
Or in step 6, the method for acquiring the light beam carrying the phase information comprises the following steps:
step b.1: an energy beam splitting coupling prism is adopted, laser emitted by a laser seed source enters an acousto-optic frequency shifter, and laser output from the acousto-optic frequency shifter is divided into two paths, and one path passes through the energy beam splitting coupling prism and then is transmitted for measuring wind speed; the other path is reflected by an energy beam splitting coupling prism;
Step b.2: acquiring a signal of a random initial phase generated by an acousto-optic frequency shifter from the reflected light reflected by the energy-splitting coupling prism in the step b.1;
step b.3: b.2, identifying and obtaining the initial phase of the periodic pulse of the signal with the random initial phase generated by the acousto-optic frequency shifter;
step b.4: and b.3, taking the initial phase of the periodic pulse of the signal with the random initial phase generated by the acousto-optic frequency shifter obtained by the identification in the step b.3 as a reference, moving pulse waveforms in other periods to be aligned with the initial phase, eliminating the random phase of the same pulse and different periodic waveforms, and performing effective coherent superposition to obtain the light beam carrying the phase information.
Further, in step 4, the discrete spectrum correction method is a ratio method, an energy gravity method, a continuous refinement fourier transform analysis method or a phase difference method.
In step 2, in the process of performing a large number of coherent accumulation on the echo signals acquired in step 1, the phase of the echo signals is obtained by adopting the principle of quadrature mixing.
The invention also provides a Doppler frequency correction and secondary compensation laser wind-measuring radar wind-measuring system, which is characterized in that:
the device comprises a laser seed source, a beam splitter, an acousto-optic frequency shifter, an energy beam splitting coupling prism, a first balance detector, a first analog-to-digital converter, a first data acquisition and processing module, an optical fiber power amplifier, a circulator, an optical antenna, a first reflecting mirror, a second reflecting mirror, an optical coupler, a second balance detector, a second analog-to-digital converter and a second data acquisition and processing module;
the beam splitter is used for receiving incident light from the laser seed source and splitting the incident light into transmitted light and reflected light;
The acousto-optic frequency shifter and the energy splitting coupling prism are sequentially arranged on a transmission light path of the beam splitter; the energy beam splitting coupling prism splits incident light from the acousto-optic frequency shifter into reflected light and transmitted light;
the first balance detector is arranged on a reflection light path of the energy beam splitting coupling prism, and the output end of the first balance detector is connected with the first data acquisition and processing module through the first analog-to-digital converter and is used for acquiring the light beam waveform carrying the phase information;
the optical fiber power amplifier is arranged on a transmission light path of the energy beam splitting coupling prism, and the output end of the optical fiber power amplifier is connected with a transmitter interface of the circulator; the antenna interface of the circulator is connected with the optical antenna, and pulse laser for measuring wind speed is emitted through the optical antenna; the receiver interface of the circulator is connected with the optical coupler;
the first reflecting mirror is arranged on the reflecting light path of the beam splitter, and the reflecting surface of the first reflecting mirror faces the beam splitter;
The second reflector is arranged on the emergent light path of the first reflector, and the reflecting surface of the second reflector is opposite to the reflecting surface of the first reflector;
The optical coupler is arranged on an emergent light path of the second reflecting mirror and is used for coupling the pulse laser beam emergent from the second reflecting mirror with the reflected beam received by aerosol particles through an optical antenna output by a receiver interface of the circulator;
The output end of the optical coupler is connected with the second balance detector, the second analog-to-digital converter and the second data acquisition and processing module in sequence and is used for calculating the wind speed.
Further, the reflecting surface of the first reflecting mirror forms an angle of 45 degrees with the reflecting light path of the beam splitter;
The second mirror is parallel to the first mirror.
The beneficial effects of the invention are as follows:
(1) According to the Doppler frequency correction secondary compensation laser wind-finding radar wind-finding method, firstly, a great amount of coherent accumulation is carried out on echo signals, so that the influence of noise on frequency errors is not dominant, at the moment, errors caused by a discrete frequency spectrum correction method adopted in Fast Fourier Transform (FFT) processing and correction are dominant, and therefore, the first correction frequency f d1 errors of Doppler frequencies obtained after the first frequency correction can be compensated through the second frequency correction; secondly, in the method, the simulated ideal cosine signal is set by using the first correction frequency f d1 of the Doppler frequency, the second frequency correction is carried out, and the frequency error of the first frequency correction is compensated by the second frequency correction, so that the method does not belong to the unbiased estimation category and can not be bound by the lower boundary of the Kramer; after the method is adopted, the wind speed precision obtained by final measurement can be improved by one order of magnitude; furthermore, in the correction process of the method, a comparison table of the ground incidence angle of the radar antenna beam and the frequency spectrum width and frequency ratio of the difference frequency signal is not required to be established, and the process is simple; therefore, the invention solves the technical problems that when the existing Doppler frequency correction method is adopted for correcting Doppler frequency once, the correction precision is lower, the requirement of high speed measurement precision of the frequency modulation continuous wave landing radar cannot be met, and in the correction process, a comparison table of the ground incidence angle of a radar antenna beam and the spectrum width and frequency ratio of a difference frequency signal is required to be established, and the process is complicated.
(2) In the Doppler frequency correction secondary compensation laser wind measuring radar wind measuring method, preferably in the step 6, when the simulated ideal cosine signal is set, the method further comprises the step of setting the phase value set value of the simulated ideal cosine signal to be consistent with the phase value of the echo signal, so that the influence of the phase on the frequency error can be reduced, the mean value and the root mean square error of the corrected frequency error can be reduced to the magnitude of thousands Hz, hundred Hz or even ten Hz from the magnitude of thousands Hz, the wind measuring accuracy can be calculated through a Doppler frequency shift formula f dop＝2Vr/lambda, and the theoretical wind speed measuring accuracy is improved to 0.01m/s from 0.1 m/s.
Drawings
FIG. 1 is a schematic flow chart of the method of the present invention;
FIG. 2 is a schematic flow chart of an embodiment of the method for correcting the discrete spectrum in the method of the present invention when the energy gravity method is adopted;
FIG. 3 is a schematic diagram of quadrature mixing;
FIG. 4 is a schematic diagram of an embodiment of a Doppler frequency corrected, second compensated laser wind lidar wind measurement system of the present invention;
FIG. 5 is a graph showing the mean value of Doppler frequency errors of simulated single frequency signals according to the embodiment 1 of the method of the present invention;
Figure 6 is a graph of mean value versus frequency error for a simulated single frequency signal of method embodiment 2 of the present invention.
The reference numerals in the drawings are as follows:
The laser device comprises a 1-laser seed source, a 2-beam splitter, a 3-acousto-optic frequency shifter, a 4-energy beam splitting coupling prism, a 5-first balance detector, a 6-first analog-to-digital converter, a 7-first data acquisition and processing module, an 8-optical fiber power amplifier, a 9-circulator, a 10-optical antenna, an 11-first reflecting mirror, a 12-second reflecting mirror, a 13-optical coupler, a 14-second balance detector, a 15-second analog-to-digital converter and a 16-second data acquisition and processing module.
Detailed Description
The invention will be described in detail below with reference to the drawings and the detailed description.
Referring to fig. 1, the laser wind-finding radar wind-finding method with doppler frequency correction secondary compensation of the invention comprises the following steps:
step 1: the laser wind measuring radar measures an atmospheric wind field and acquires echo signals through the signal acquisition card;
step 2: and (3) performing a large amount of coherent accumulation on the echo signals acquired in the step (1) to obtain an echo signal discrete sampling data sequence:
x(n)，n＝0，1，…，K-1，
k is the number of sampling points of the echo signal single-frequency pulse signal, and the value of K is an integer power of 2;
Step 3: selecting L effective data points from an echo signal discrete sampling data sequence x (N), carrying out zero padding to N points, and carrying out fast Fourier transform processing of the sampling points N to obtain a magnitude spectrum A (k), wherein k is an independent variable of a frequency domain and represents frequency;
Step 4: substituting the amplitude spectrum A (k) obtained in the step 3 into a formula corresponding to the discrete spectrum correction method by using the discrete spectrum correction method, and performing primary frequency correction to obtain a position index value k 0 of primary correction frequency of Doppler frequency;
Step 5: the first corrected frequency f d1 of the Doppler frequency is calculated according to the following equation (1):
fd1＝k0·fs/N (1)；
In the formula (1): f s is the sampling frequency of the signal acquisition card in the step 1;
Step 6: setting a simulated ideal cosine signal by using a first correction frequency f d1 of the Doppler frequency, performing fast Fourier transform processing again, and performing second frequency correction by using the same discrete frequency spectrum correction method as the step 4 to obtain a second correction frequency f d2 of the Doppler frequency;
Step 7: the difference Δf d between the twice corrected frequencies of the doppler frequency is calculated according to the following equation (2):
Δfd＝fd1-fd2 (2)；
Step 8: the final corrected frequency f d of the doppler frequency is calculated according to the following equation (3):
fd＝fd1+Δfd＝2fd1-fd2 (3)；
Step 9: and (3) calculating the wind speed of the atmospheric wind field according to the final corrected frequency f d of the Doppler frequency calculated in the step (8).
The Doppler frequency correction secondary compensation laser wind-finding radar wind-finding method is suitable for Doppler frequency re-correction after correction by all discrete frequency spectrum correction methods including a ratio method, an energy gravity center method, a continuous refinement Fourier transform analysis method (FFT+FT) and a phase difference method.
Referring to fig. 2, in this embodiment, taking an energy gravity center method as an example of a discrete spectrum correction method, a laser wind-finding radar wind-finding method with doppler frequency correction secondary compensation of this embodiment includes the following steps:
step 1: the laser wind measuring radar measures an atmospheric wind field and acquires echo signals through the signal acquisition card;
step 2: and (3) performing a large amount of coherent accumulation on the echo signals acquired in the step (1) to obtain an echo signal discrete sampling data sequence:
x(n)，n＝0，1，…，K-1，
k is the number of sampling points of the echo signal single-frequency pulse signal, and the value of K is an integer power of 2;
In the embodiment, in step 2, in a process of performing a large number of coherent accumulation on the echo signals acquired in step 1, a quadrature mixing principle is adopted to calculate the phases of the echo signals;
Step 3: selecting L effective data points from an echo signal discrete sampling data sequence x (N), carrying out zero padding to N points, and carrying out fast Fourier transform processing of the sampling points N to obtain a magnitude spectrum A (k), wherein k is an independent variable of a frequency domain and represents frequency;
Step 4: substituting the amplitude spectrum A (k) obtained in the step 3 into a formula corresponding to a discrete spectrum correction method by using an energy gravity center method, and performing primary frequency correction to obtain a position index value k 0 of a primary correction frequency of Doppler frequency, wherein the position index value k 0 is specifically:
setting an echo signal power spectrum S (k), wherein the main lobe power spectrum of a rectangular window is Z (k), and the echo signal power spectrum after adding the rectangular window is Y (k), wherein Y (k) =S (k) ×Z (k);
combining step 3, it can be known that performing data selection on the echo signal discrete sampling data sequence is equivalent to adding a rectangular window, where Y (k) =a (k) 2;
from the rectangular window energy centroid characteristics, it can be deduced that:
the energy center of gravity position of the rectangular window main lobe abscissa is deduced to be:
Wherein h is the number of spectral lines which participate in correction at the two sides of the maximum position of the power spectrum amplitude, and k 1 is the discrete frequency index of the maximum value of the power spectral line;
Step 5: the first corrected frequency f d1 of the Doppler frequency is calculated according to the following equation (1):
fd1＝k0·fs/N (1)；
In the formula (1): f s is the sampling frequency of the signal acquisition card in the step 1;
at this time, the first corrected frequency f d1 of the obtained doppler frequency belongs to the category of unbiased estimation, and is constrained by the lower bound of the caramerro; the lower bound of the caramerol is expressed as follows under the FFT periodogram method:
Wherein, the sampling time is T, the sampling point number is N, and the SNR is the signal-to-noise ratio;
Step 6: setting a simulated ideal cosine signal by using a first correction frequency f d1 of the Doppler frequency, performing fast Fourier transform processing again, and performing second frequency correction by using an energy gravity center method to obtain a second correction frequency f d2 of the Doppler frequency;
Step 7: the difference Δf d between the twice corrected frequencies of the doppler frequency is calculated according to the following equation (2):
Δfd＝fd1-fd2 (2)；
Step 8: the final corrected frequency f d of the doppler frequency is calculated according to the following equation (3):
fd＝fd1+Δfd＝2fd1-fd2 (3)；
Step 9: and (3) calculating the wind speed of the atmospheric wind field according to the final corrected frequency f d of the Doppler frequency calculated in the step (8).
The simulation shows that the amplitude of the signal has little effect on the frequency error and the phase has great effect on the frequency error, so in the method of the invention, preferably in the step 6, when setting the simulated ideal cosine signal, the step of setting the phase value set value of the simulated ideal cosine signal to be consistent with the phase value of the echo signal is also included. By the arrangement, the mean value and the root mean square error of the correction frequency error can be reduced from the magnitude of thousands Hz, thousands Hz to the magnitude of thousands Hz, hundred Hz or even ten Hz, and the correction frequency error can be calculated through a Doppler frequency shift formula f dop＝2Vr/lambda, so that the theoretical wind speed measurement accuracy is improved from 0.1m/s to 0.01m/s. The precondition for maintaining the phase is to acquire the beam carrying the phase information and then solve for the phase. The method of solving the phase may be implemented by the principle of quadrature mixing, the principle of which is shown in fig. 3.
In the laser wind measuring radar wind measuring system, if pulse laser emitted by a laser seed source directly enters the atmosphere without passing through an acousto-optic frequency shifter (AOM), the measured result is the absolute value of Doppler frequency, and whether the measured target approaches or is far away from the laser radar system cannot be determined by positive and negative. To solve this problem, an acousto-optic frequency shifter (AOM) is added to the optical path of the lidar anemometry system to add a frequency shift to the local oscillator signal. Namely, the laser wind-finding radar in the step 1 is used for measuring the wind speed by leading pulse laser emitted by a laser seed source to enter the atmosphere after passing through an acousto-optic frequency shifter; an acousto-optic frequency shifter (AOM) can effectively and rapidly modulate the frequency, direction or intensity of the output light. However, during this interaction, thermal noise, background light, etc. may cause a frequency shift error to the input signal, so that the phase of the transmitted signal becomes random. The random phase of the same pulse is fixed during the transmission of the transmitted signal light in the atmosphere channel, while their initial random phase is changed for different pulse periods.
The invention provides two methods for acquiring the light beam carrying the phase information:
a first method of acquiring a light beam carrying the phase information comprises the steps of:
Step a.1: the collimator emits pulse laser to be incident on the surface of the optical lens to generate a reflected echo signal with the same random initial phase as that introduced by the acousto-optic frequency shifter;
step a.2: the initial phase of the periodic pulse of the reflected echo signal generated in the step a.1 is obtained through identification;
Step a.3: and d, taking the initial phase of the periodic pulse of the reflected echo signal obtained by the identification in the step a.2 as a reference, moving pulse waveforms in other periods to be aligned with the initial phase, eliminating random phases of the same pulse and waveforms in different periods, and performing effective coherent superposition to obtain a light beam carrying the phase information.
A second method of acquiring a light beam carrying the phase information comprises the steps of:
step b.1: an energy beam splitting coupling prism is adopted, laser emitted by a laser seed source enters an acousto-optic frequency shifter, and laser output from the acousto-optic frequency shifter is divided into two paths, and one path passes through the energy beam splitting coupling prism and then is transmitted for measuring wind speed; the other path is reflected by an energy beam splitting coupling prism;
Step b.2: acquiring a signal of a random initial phase generated by an acousto-optic frequency shifter from the reflected light reflected by the energy-splitting coupling prism in the step b.1;
step b.3: b.2, identifying and obtaining the initial phase of the periodic pulse of the signal with the random initial phase generated by the acousto-optic frequency shifter;
Step b.4: and b.3, taking the initial phase of the periodic pulse of the signal which is obtained through identification and passes through the acousto-optic frequency shifter and is generated by the random initial phase as a reference, moving pulse waveforms in other periods to be aligned with the initial phase, eliminating the random phase of the same pulse and different periodic waveforms, and performing effective coherent superposition to obtain a light beam carrying the phase information.
Referring to fig. 4, the present invention further provides a doppler frequency correction secondary compensation laser wind-finding radar wind-finding system, and when the doppler frequency correction secondary compensation laser wind-finding radar wind-finding system of this embodiment is adopted, the method for obtaining the beam waveform carrying the phase information is the second method. The Doppler frequency correction secondary compensation laser wind-finding radar wind-finding system comprises a laser seed source 1, a beam splitter 2, an acousto-optic frequency shifter 3, an energy beam splitting coupling prism 4, a first balance detector 5, a first analog-to-digital converter 6, a first data acquisition and processing module 7, an optical fiber power amplifier 8, a circulator 9, an optical antenna 10, a first reflecting mirror 11, a second reflecting mirror 12, an optical coupler 13, a second balance detector 14, a second analog-to-digital converter 15 and a second data acquisition and processing module 16; the beam splitter 2 is used for receiving incident light from the laser seed source 1 and splitting the incident light into transmitted light and reflected light; the acousto-optic frequency shifter 3 and the energy splitting coupling prism 4 are sequentially arranged on a transmission light path of the beam splitter 2; the energy splitting coupling prism 4 splits the incident light from the acousto-optic frequency shifter 3 into reflected light and transmitted light; the first balance detector 5 is arranged on a reflection light path of the energy beam splitting coupling prism 4, and the output end of the first balance detector is connected with the first data acquisition and processing module 7 through the first analog-to-digital converter 6 and is used for acquiring a light beam waveform carrying the phase information; the optical fiber power amplifier 8 is arranged on a transmission light path of the energy beam splitting coupling prism 4, and the output end of the optical fiber power amplifier is connected with a transmitter interface of the circulator 9; the antenna interface of the circulator 9 is connected with an optical antenna 10, and pulse laser for measuring wind speed is emitted through the optical antenna 10; the receiver interface of the circulator 9 is connected with an optical coupler 13; the first reflecting mirror 11 is disposed on the reflection light path of the beam splitter 2, and the reflection surface of the first reflecting mirror 11 faces the beam splitter 2; the second reflecting mirror 12 is arranged on the outgoing light path of the first reflecting mirror 11, and the reflecting surface of the second reflecting mirror is arranged opposite to the reflecting surface of the first reflecting mirror 11; the optical coupler 13 is disposed on the outgoing optical path of the second reflecting mirror 12, and is used for coupling the pulse laser beam emitted by the second reflecting mirror 12 with the reflected beam received by the aerosol particles through the optical antenna 10 output by the receiver interface of the circulator 9; the output end of the optical coupler 13 is connected with a second balance detector 14, a second analog-to-digital converter 15 and a second data acquisition and processing module 16 in sequence and is used for calculating the wind speed. In this embodiment, for the sake of simple structure, it is preferable that the reflecting surface of the first reflecting mirror 11 forms an angle of 45 ° with the reflecting light path of the beam splitter 2; the second mirror 12 is parallel to the first mirror 11.
The following are two embodiments of doppler frequency correction:
Example 1
The simulation signal setting parameters are as follows: signal amplitude a=10, initial phaseThe pulse width is 1024/400000000s, the signal center frequency f 0 =30 MHz, the sampling frequency f s =400 MHz, the number of observation sequence effective points L=256, zero padding is carried out to N=1024 points, and FFT is carried out, wherein the frequency resolution Δf=f s/N= 0.39063MHz. And (3) selecting an energy gravity center method with corrected spectral line quantity h=3 to perform frequency correction. Different signal to noise ratios are simulated from-10 dB to 10dB, frequency correction is carried out once every 0.5dB signal to noise ratio, and in order to make the result more general, the simulated frequency error of each signal to noise ratio is respectively subjected to 1000 times of accumulation and averaging.
The simulation results are shown in fig. 5, and the doppler frequency root MEAN square error rmse=2737 Hz, the doppler frequency MEAN value mean=2727 Hz, the doppler frequency root MEAN square error rmse=371 Hz and the doppler frequency MEAN value mean=119 Hz are not optimized.
Example 2
The simulation signal setting parameters are as follows: signal amplitude a=10, initial phaseThe pulse width is 1024/400000000s, the signal center frequency f 0 =80 MHz, the sampling frequency f s =400 MHz, the number of observation sequence effective points L=256, zero padding is carried out to N=1024 points for FFT, and the frequency resolution Δf=f s/N= 0.39063MHz. And (3) selecting an energy gravity center method with the correction spectral line quantity h=3 to perform frequency correction. Different signal to noise ratios are simulated from-10 dB to 10dB, frequency correction is carried out once every 0.5dB signal to noise ratio, and in order to make the result more general, the simulated frequency error of each signal to noise ratio is respectively subjected to 1000 times of accumulation and averaging.
The simulation results are shown in fig. 6, wherein the root MEAN square error rmse=1407hz of the doppler frequency is different from the signal to noise ratio before the optimization, the MEAN value MEAN of the doppler frequency is= -1401Hz, the root MEAN square error rmse=251 Hz of the doppler frequency after the optimization, and the MEAN value MEAN of the doppler frequency is= -4Hz.
The Doppler frequency correction secondary compensation laser wind-finding radar wind-finding method of the invention has the principle that:
The signal-to-noise ratio of the scattered echo signal received by the laser wind lidar each time is low. The precondition for extracting doppler shift when processing data is that a useful signal can be extracted, namely, the ratio between the useful signal and noise is improved, namely: signal to noise ratio. Coherently accumulating scattered echo signals received by a balanced detector is a common method to improve the signal-to-noise ratio. The signal to noise ratio can be improved because the scattered echo signals are coherently accumulated, and the Doppler signal components in the scattered echo signals have correlation, so that the intensity of the Doppler signal components is continuously increased along with the increase of the accumulated times; the noise is generally gaussian, has randomness in different pulse periods and is independent, so that the noise counteracts each other with increasing accumulated times and tends to be smooth. Therefore, the scattered echo signals of K periods are superimposed, and in theory, the signal to noise ratio can be increased to K times of the signal in a single period under the condition of complete phase alignment.
In the research of the laser wind-finding radar, the parameter estimation problem is involved in the processing of the echo signals, and the lower boundary of the Clamet-Rao Low Bound (CRLB) is a key parameter. In statistics, parameter estimation theory is mostly established on the relation between observed sample data and parameters to be estimated, and belongs to data processing of random signals. The parameter estimation criteria may be a minimum variance criterion, a maximum likelihood criterion, a least squares criterion, etc. The obtained estimators are different by different estimation methods, and the standard for measuring the performance of the estimators is the mean and variance of the estimators. In practical applications, unbiased, effective and consistent are often used as metrics. When the value of the estimator fluctuates around the true value, if their average error is zero, this estimator is called an unbiased estimator, where the smaller the estimator variance, the more efficient the estimator. The variance is different for different estimation methods for a particular estimation method, but the variance has a lower bound, i.e. the lower bound of the caramerro, which is independent of the estimation method and reflects only the best effect of the parameters that can be estimated using the existing information. In the echo signal data processing process, after Doppler frequency shift extraction, frequency estimation correction is needed, namely, the problem of parameter estimation is solved. In the method, the simulated ideal cosine signal is set by using the first correction frequency f d1 of Doppler frequency, the second frequency correction is carried out, and the frequency error of the first frequency correction is compensated by the second frequency correction, so that the method does not belong to the unbiased estimation category and can not be bound by the lower boundary of the Kramer; therefore, by adopting the method, the wind speed precision obtained by final measurement can be improved by one order of magnitude.
Claims (7)
1. A Doppler frequency correction secondary compensation laser wind-finding radar wind-finding method is characterized in that:
The laser wind-finding radar comprises a laser seed source (1), a beam splitter (2), an acousto-optic frequency shifter (3), an energy splitting coupling prism (4), a first balance detector (5), a first analog-to-digital converter (6), a first data acquisition and processing module (7), an optical fiber power amplifier (8), a circulator (9), an optical antenna (10), a first reflecting mirror (11), a second reflecting mirror (12), an optical coupler (13), a second balance detector (14), a second analog-to-digital converter (15) and a second data acquisition and processing module (16);
The beam splitter (2) is used for receiving incident light from the laser seed source (1) and splitting the incident light into transmitted light and reflected light;
The acousto-optic frequency shifter (3) and the energy beam splitting coupling prism (4) are sequentially arranged on a transmission light path of the beam splitter (2); the energy beam splitting coupling prism (4) splits incident light from the acousto-optic frequency shifter (3) into reflected light and transmitted light;
The first balance detector (5) is arranged on a reflection light path of the energy beam splitting coupling prism (4), and the output end of the first balance detector is connected with the first data acquisition and processing module (7) through the first analog-to-digital converter (6) and is used for acquiring a light beam waveform carrying the phase information;
The optical fiber power amplifier (8) is arranged on a transmission optical path of the energy splitting coupling prism (4), and the output end of the optical fiber power amplifier is connected with a transmitter interface of the circulator (9); the antenna interface of the circulator (9) is connected with the optical antenna (10), and pulse laser for measuring wind speed is emitted through the optical antenna (10); the receiver interface of the circulator (9) is connected with an optical coupler (13);
the first reflecting mirror (11) is arranged on a reflecting light path of the beam splitter (2), and the reflecting surface of the first reflecting mirror (11) faces the beam splitter (2);
the second reflecting mirror (12) is arranged on the emergent light path of the first reflecting mirror (11), and the reflecting surface of the second reflecting mirror is opposite to the reflecting surface of the first reflecting mirror (11);
the optical coupler (13) is arranged on an emergent light path of the second reflecting mirror (12) and is used for coupling the pulse laser beam emergent from the second reflecting mirror (12) with the reflected beam received by aerosol particles through the optical antenna (10) output by the receiver interface of the circulator (9);
The output end of the optical coupler (13) is connected with a second balance detector (14), a second analog-to-digital converter (15) and a second data acquisition and processing module (16) in sequence and is used for calculating the wind speed;
The wind measuring method comprises the following steps:
step 1: the laser wind measuring radar measures an atmospheric wind field and acquires echo signals through the signal acquisition card;
step 2: and (3) performing a large amount of coherent accumulation on the echo signals acquired in the step (1) to obtain an echo signal discrete sampling data sequence:
x(n)，n＝0，1，…，K-1，
k is the number of sampling points of the echo signal single-frequency pulse signal, and the value of K is an integer power of 2;
Step 3: selecting L effective data points from an echo signal discrete sampling data sequence x (N), carrying out zero padding to N points, and carrying out fast Fourier transform processing of the sampling points N to obtain a magnitude spectrum A (k), wherein k is an independent variable of a frequency domain and represents frequency;
Step 4: substituting the amplitude spectrum A (k) obtained in the step 3 into a formula corresponding to the discrete spectrum correction method by using the discrete spectrum correction method, and performing primary frequency correction to obtain a position index value k 0 of primary correction frequency of Doppler frequency;
Step 5: the first corrected frequency f d1 of the Doppler frequency is calculated according to the following equation (1):
fd1＝k0·fs/N (1)；
in the formula (1): f s is the sampling frequency of the signal acquisition card in the step 1;
step 6: setting a simulated ideal cosine signal by using a first correction frequency f d1 of the Doppler frequency, performing fast Fourier transform processing again, and performing second frequency correction by using the discrete frequency spectrum correction method which is the same as that in the step 4 to obtain a second correction frequency f d2 of the Doppler frequency;
Step 7: the difference Δf d between the twice corrected frequencies of the doppler frequency is calculated according to the following equation (2):
Δfd＝fd1-fd2 (2)；
Step 8: the final corrected frequency f d of the doppler frequency is calculated according to the following equation (3):
fd＝fd1+Δfd＝2fd1-fd2 (3)；
Step 9: and (3) calculating the wind speed of the atmospheric wind field according to the final corrected frequency f d of the Doppler frequency calculated in the step (8).
2. The doppler frequency corrected, secondarily compensated, laser wind lidar wind method of claim 1, wherein: in step 6, when setting the simulated ideal cosine signal, the method further comprises the step of setting the phase value set value of the simulated ideal cosine signal to be consistent with the phase value of the echo signal.
3. The doppler frequency corrected, secondarily compensated, laser wind lidar wind method of claim 2, wherein: in step 6, the method for acquiring the phase value of the echo signal includes: the light beam carrying the phase information is acquired first, and then the phase is solved by the principle of quadrature mixing.
4. A doppler frequency corrected, secondarily compensated, laser wind lidar wind method according to claim 3, wherein:
The laser wind-finding radar in the step 1 is that pulse laser emitted by a laser seed source enters the atmosphere after passing through an acousto-optic frequency shifter and is used for measuring wind speed;
in step 6, the method for acquiring the light beam carrying the phase information includes the following steps:
step b.1: an energy beam splitting coupling prism is adopted, laser emitted by a laser seed source enters an acousto-optic frequency shifter, and laser output from the acousto-optic frequency shifter is divided into two paths, and one path passes through the energy beam splitting coupling prism and then is transmitted for measuring wind speed; the other path is reflected by an energy beam splitting coupling prism;
Step b.2: acquiring a signal of a random initial phase generated by an acousto-optic frequency shifter from the reflected light reflected by the energy-splitting coupling prism in the step b.1;
step b.3: b.2, identifying and obtaining the initial phase of the periodic pulse of the signal with the random initial phase generated by the acousto-optic frequency shifter;
step b.4: and b.3, taking the initial phase of the periodic pulse of the signal with the random initial phase generated by the acousto-optic frequency shifter obtained by the identification in the step b.3 as a reference, moving pulse waveforms in other periods to be aligned with the initial phase, eliminating the random phase of the same pulse and different periodic waveforms, and performing effective coherent superposition to obtain the light beam carrying the phase information.
5. The doppler frequency corrected, secondarily compensated, lidar anemometry method of any one of claims 1 to 4, wherein: in the step 4, the discrete spectrum correction method is a ratio method, an energy gravity method, a continuous refinement fourier transform analysis method or a phase difference method.
6. The doppler frequency corrected, secondarily compensated, laser wind lidar wind method of claim 5, wherein: in step 2, in the process of performing a large number of coherent accumulation on the echo signals acquired in step 1, the phase of the echo signals is calculated by adopting the principle of quadrature mixing.
7. The doppler frequency corrected, secondarily compensated, laser wind lidar wind method of claim 6, wherein:
The reflecting surface of the first reflecting mirror (11) forms an angle of 45 degrees with a reflecting light path of the beam splitter (2);
the second mirror (12) is parallel to the first mirror (11).
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