CN113253301A - Multi-frequency pulse laser radar signal processing method and wind measuring radar system - Google Patents
Multi-frequency pulse laser radar signal processing method and wind measuring radar system Download PDFInfo
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- CN113253301A CN113253301A CN202110754595.XA CN202110754595A CN113253301A CN 113253301 A CN113253301 A CN 113253301A CN 202110754595 A CN202110754595 A CN 202110754595A CN 113253301 A CN113253301 A CN 113253301A
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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
- G01S17/88—Lidar systems specially adapted for specific applications
- G01S17/95—Lidar systems specially adapted for specific applications for meteorological use
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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
- G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
- G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
- G01S7/483—Details of pulse systems
- G01S7/484—Transmitters
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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
- G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
- G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
- G01S7/483—Details of pulse systems
- G01S7/486—Receivers
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
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Abstract
The application relates to a multi-frequency pulse laser radar signal processing method and a wind measuring radar system. The method comprises the following steps: the target is detected by emitting a plurality of frequency coherent laser pulse signals, a plurality of multi-frequency echo signals reflected by the targets at different distances are correspondingly received, and the multi-frequency echo signals are processed by adopting a microwave signal processing mode to obtain the moving speed of the target. By adopting the method, the problem that the speed/distance resolution of the traditional single-frequency laser radar is difficult can be solved, namely the speed resolution of the traditional laser radar is improved, the distance resolution is reduced, the distance resolution is improved, the speed resolution is reduced, the complex wind field is finely detected under the lower signal-to-noise ratio, and the frequency difference between multi-frequency signals is adjusted to adapt to various detection conditions.
Description
Technical Field
The application relates to the technical field of laser radar wind measurement, in particular to a multi-frequency pulse laser radar signal processing method and a wind measurement radar system.
Background
The complex wind field refers to a wind field which changes violently in time and space, and comprises medium and small-scale airflow fields such as wind shear fields, aircraft disturbance fields and the like, all-weather detection of the complex wind field is a key problem for researching the dynamics rule of a complex target, and the complex wind field is also a cross research direction of space information acquisition and meteorological detection, and has important application requirements in the aspects of military and civil aviation safety, environment monitoring, important weapon launching guarantee, anti-stealth and the like. The laser radar takes laser as an information bearing medium, and is widely applied to the fields of wind field measurement and the like due to the advantages of high monochromaticity, high coherence, high directivity, non-contact property and the like of the laser. The method has good detection capability under the clear air condition, and the main principle is that Doppler frequency shift is generated after interaction between laser carrier waves and aerosol particles in air, the Doppler frequency shift of scattering echoes is detected to obtain wind speed, and wind field information is indirectly sensed.
In the aspect of an existing single-frequency laser radar data processing mode, due to the fact that the laser coherence length is short, existing laser radars cannot perform coherent accumulation by using sampling between pulses like traditional microwave radars, and instead directly perform coherent processing by using adjacent sampling, namely spectrum analysis is performed on adjacent N samples in a certain pulse repetition Period (PRT), then incoherent accumulation of M pulses is performed to improve the signal-to-noise ratio, and finally the average Doppler speed of the adjacent N samples is obtained. According to the signal processing theory, the velocity resolution of the echo is proportional to the coherence time length, so that by using the adjacent sampling coherence mode, more sampling points are needed if high velocity resolution is obtained, and the additional influence is that the distance resolution (the range contained by adjacent N samples) is deteriorated; if a high distance resolution is desired, fewer sampling points are used, and the additional effect is a deterioration in the speed resolution. Therefore, the single-frequency laser radar system faces the situation of difficult speed resolution and distance resolution, and is difficult to meet the real requirements of fine wind field perception and the like. And since the width of a single sample is one pulse width (T)P) The central part of the distance resolution is sampled and overlapped for multiple times, the spectral resolution is higher, the sampling overlapping on the two sides is less, the integral spectral resolution is reduced, the obtained average Doppler velocity is the sparse average velocity of the particles in the distance resolution, and an error is inevitably generated between the obtained average Doppler velocity and the true average velocity of the distance resolution.
Disclosure of Invention
Therefore, it is necessary to provide a multi-frequency pulse laser radar signal processing method and a wind radar system capable of solving the problem of speed resolution and distance resolution of the conventional single-frequency laser radar.
A multi-frequency pulse laser radar signal processing method is applied to a wind measuring radar system, the wind measuring radar system comprises a multi-frequency continuous laser generating device, a first optical fiber beam splitter, a pulse modulator, a radio frequency driving source, a circulator, an optical transceiver, a continuous adjustable attenuator, a second optical fiber beam splitter, a balance detector, an analog-digital conversion acquisition card and a digital signal processing system,
generating a multi-frequency laser signal with high coherence of each frequency by the multi-frequency continuous laser generating device, dividing the multi-frequency laser signal into two beams of multi-frequency laser signals by the first optical fiber beam splitter, taking one beam as intrinsic reference light, and taking the other beam as signal light;
the signal light is modulated by a pulse modulator and a radio frequency driving source to obtain a multi-frequency pulse laser signal, the multi-frequency pulse laser signal is sequentially transmitted to an observation area through the circulator and the optical transceiver and is scattered with a plurality of target points in different radial distance units in the observation area, part of scattered echoes return to the optical transceiver, and a first multi-frequency pulse echo signal is formed through the circulator;
the intrinsic reference light passes through the continuous adjustable attenuator, then simultaneously passes through the second fiber beam splitter with the first multi-frequency pulse echo signal, and then passes through the balance detector, so that a second multi-frequency pulse echo signal with Doppler frequency shift is obtained;
inputting the second multi-frequency pulse echo signal into an analog-digital conversion acquisition card to be sampled according to a preset sampling frequency to obtain a first multi-frequency pulse sampling signal;
inputting a first multi-frequency pulse sampling signal into the digital signal processing system for signal processing, and performing square frequency detection and low-pass filtering on the first multi-frequency pulse sampling signal to obtain a second multi-frequency pulse sampling signal with each frequency difference corresponding to Doppler frequency shift, wherein each pulse repetition period comprises a plurality of sampling points, each sampling point corresponds to a distance unit, and the number of the sampling points in each pulse repetition period is obtained by the ratio of the pulse repetition period to the sampling time interval;
and carrying out coherent Doppler processing on the sampled data of a plurality of pulse repetition periods corresponding to the same distance unit to obtain the Doppler frequency spectrum of the distance unit, estimating the average speed of each target point in the distance unit by the frequency spectrum of the Doppler frequency spectrum, and sequentially calculating the average speed of each target point in different radial distance units.
In one embodiment, the pulse modulator further outputs an electrical signal synchronized with the multi-frequency pulse laser signal to an analog-to-digital conversion acquisition card as a trigger signal for sampling.
In one embodiment, the obtaining the average velocity of each target point in the range cell by performing coherent doppler processing on the sampled data of a plurality of pulse repetition periods corresponding to the same range cell includes:
performing autocorrelation calculation on the sampled data of a plurality of adjacent pulse repetition periods corresponding to one distance unit to obtain an autocorrelation calculation result;
performing Fourier transform according to the autocorrelation calculation result to obtain a Doppler frequency spectrum corresponding to the range cell, and obtaining a plurality of Doppler frequencies corresponding to the range cell through frequency spectrum estimation;
and calculating according to the frequency difference between the Doppler frequencies and the frequencies of the multi-frequency laser signals to obtain the average speed of each target point in the distance unit.
In one embodiment, the calculating the average velocity of each target point in the range unit according to the frequency difference between the doppler frequencies and the frequencies of the multi-frequency laser signal includes:
calculating according to each Doppler frequency and the corresponding frequency difference to obtain a plurality of average Doppler speeds of each target point in the distance unit;
and averaging the average Doppler velocities to obtain the average velocity of the target point in the range unit.
The application also provides a wind-finding radar system, including:
the multi-frequency continuous laser generating device is used for generating multi-frequency laser signals with high coherence of each frequency;
the first optical fiber beam splitter is used for splitting the multi-frequency laser signal into two beams of multi-frequency laser signals, wherein one beam is used as intrinsic reference light, and the other beam is used as signal light;
the pulse modulator is used for modulating the signal light into a multi-frequency pulse laser signal;
a radio frequency driving source for providing a pulse modulated radio frequency signal to modulate the width and repetition frequency of the multi-frequency pulse laser signal;
the circulator is used for sending the modulated multi-frequency pulse laser signal to the optical transceiver and receiving a first multi-frequency pulse echo signal from the optical transceiver;
the optical transceiver is used for transmitting the multi-frequency pulse laser signals to an observation area and receiving first multi-frequency pulse echo signals formed by scattering the multi-frequency pulse laser signals from a plurality of target points in different radial distance units in the observation area;
a continuously adjustable attenuator for adjusting the intensity of the intrinsic reference light;
the second optical fiber beam splitter is used for simultaneously inputting the intrinsic reference light after intensity adjustment and the first multi-frequency pulse echo signal into the balance detector;
the balance detector is used for carrying out square rate detection according to the intrinsic reference light and the first multi-frequency pulse echo signal to obtain a second multi-frequency pulse echo signal with Doppler frequency shift;
the analog-digital conversion acquisition card is used for sampling the second multi-frequency pulse echo signal according to a preset sampling frequency to obtain a first multi-frequency pulse sampling signal;
a digital signal processing system for performing signal processing on the first multi-frequency pulse sampling signal,
carrying out square rate detection and low-pass filtering on the first multi-frequency pulse sampling signal to obtain a second multi-frequency pulse sampling signal of which each frequency difference corresponds to Doppler frequency shift, wherein each pulse repetition period comprises a plurality of sampling points, each sampling point corresponds to a distance unit, and the number of the sampling points in each pulse repetition period is obtained by the ratio of the pulse repetition period to the sampling time interval;
and carrying out coherent Doppler processing on the sampled data of a plurality of pulse repetition periods corresponding to the same distance unit to obtain the Doppler frequency spectrum of the distance unit, estimating the average speed of each target point in the distance unit by the frequency spectrum of the Doppler frequency spectrum, and sequentially calculating the average speed of each target point in different radial distance units.
In one embodiment, the pulse modulator is further configured to output an electrical signal synchronized with the multi-frequency pulse laser signal to an analog-to-digital conversion acquisition card as a trigger signal for sampling.
In one embodiment, the pulse modulator comprises an acousto-optic modulator, or an electro-optic modulator.
In one embodiment, the rf driving source includes an arbitrary waveform generator, or a transistor logic level signal generator, or an rf signal generator.
According to the multi-frequency pulse laser radar signal processing method and the wind measuring radar system, multi-frequency pulse laser light is transmitted to the observation area, echo data are processed by adopting an inter-pulse coherence method to obtain the average speed of targets at different positions in the observation area, the problem that the traditional single-frequency laser radar is difficult to distinguish speed and distance can be solved by adopting the mode, and a new effective technical means is provided for fine detection of a complex wind field.
Drawings
FIG. 1 is a schematic structural diagram of a wind radar system according to an embodiment;
FIG. 2 is a schematic diagram of frequency spectra of position signals in the wind-measuring radar system in one embodiment;
FIG. 3 is a diagram illustrating a method for processing a multi-frequency pulsed lidar signal in a digital signal processing system according to an embodiment;
FIG. 4 is a diagram illustrating a single frequency pulsed lidar signal processing method in a digital signal processing system according to an embodiment;
fig. 5 is a velocity average velocity measurement error simulation diagram obtained by simulating a multi-frequency pulse laser radar signal processing method and a single-frequency pulse laser radar signal processing method in one embodiment.
Detailed Description
In order to make the objects, technical solutions and advantages of the present application more apparent, the present application is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the present application and are not intended to limit the present application.
The application provides a multi-frequency pulse laser radar signal processing method, which is applied to a wind measuring radar system, wherein the wind measuring radar system comprises a multi-frequency continuous laser generating device, a first optical fiber beam splitter, a pulse modulator, a radio frequency driving source, a circulator, an optical transceiver, a continuous adjustable attenuator, a second optical fiber beam splitter, a balance detector, an analog-digital conversion acquisition card and a digital signal processing system,
generating a multi-frequency laser signal with high coherence of each frequency by a multi-frequency continuous laser generating device, dividing the multi-frequency laser signal into two multi-frequency laser signals by a first optical fiber beam splitter, taking one of the multi-frequency laser signals as intrinsic reference light, and taking the other multi-frequency laser signal as signal light;
the signal light is modulated by a pulse modulator and a radio frequency driving source to obtain a multi-frequency pulse laser signal, the multi-frequency pulse laser signal is transmitted to an observation area through a circulator and an optical transceiver in sequence, and is scattered with a plurality of target points in different radial distance units in the observation area, part of scattered echoes return to the optical transceiver, and then a first multi-frequency pulse echo signal is formed through the circulator;
the intrinsic reference light passes through the continuous adjustable attenuator, then simultaneously passes through a second optical fiber beam splitter with the first multi-frequency pulse echo signal, and then passes through a balance detector to obtain a second multi-frequency pulse echo signal with the frequency of Doppler frequency shift;
inputting the second multi-frequency pulse echo signal into an analog-digital conversion acquisition card to be sampled according to a preset sampling frequency to obtain a first multi-frequency pulse sampling signal;
inputting the first multi-frequency pulse sampling signal into a digital signal processing system for signal processing,
carrying out square rate detection and low-pass filtering on the multi-frequency pulse sampling signal to obtain a second multi-frequency pulse sampling signal of which each frequency difference corresponds to Doppler frequency shift, wherein each pulse repetition period comprises a plurality of sampling points, each sampling point corresponds to a distance unit, and the number of the sampling points in each pulse repetition period is obtained by the ratio of the pulse repetition period to the sampling time interval;
carrying out coherent Doppler processing on the sampled data of a plurality of pulse repetition periods corresponding to the same range cell to obtain the Doppler frequency spectrum of the range cell, estimating the average velocity of each target point in the range cell by the frequency spectrum,
and sequentially calculating the average speed of each target point in each different radial distance unit.
It can be seen that the method for processing the multi-frequency pulse laser radar signal in the application comprises a part of processing the signal by each component in the wind measuring radar and a part of processing in the digital signal processing system, and the two are processes of processing in sequence, and the method is a complete technical scheme. For better understanding of the method, the first part of the signal processing method will be described with reference to the wind-measuring radar system provided in the present application, and then the signal processing method in the digital signal processing system will be described.
As shown in fig. 1, the present application also provides a wind radar system comprising:
a multi-frequency continuous laser generating device 1 for generating multi-frequency laser signals with high coherence of each frequency;
the first optical fiber beam splitter 2 is used for splitting the multi-frequency laser signal into two multi-frequency laser signals, wherein one multi-frequency laser signal is used as intrinsic reference light, and the other multi-frequency laser signal is used as signal light;
the pulse modulator 3 is used for modulating the signal light into a multi-frequency pulse laser signal;
a radio frequency driving source 4 for providing a pulse-modulated radio frequency signal to modulate the width and repetition frequency of the multi-frequency pulse laser signal;
a circulator 5, configured to send the modulated multi-frequency pulse laser signal to the optical transceiver 6 and receive a first multi-frequency pulse echo signal from the optical transceiver 6;
the optical transceiver 6 is used for transmitting the multi-frequency pulse laser signals to an observation area and receiving first multi-frequency pulse echo signals formed by scattering from a plurality of target points which are located in different radial distance units in the observation area 7;
a continuously adjustable attenuator 8 for adjusting the intensity of the intrinsic reference light;
the second optical fiber beam splitter 9 is used for inputting the intrinsic reference light after intensity adjustment and the first multi-frequency pulse echo signal into the balance detector 10 at the same time;
the balance detector 10 is used for performing square detection according to the intrinsic reference light and the first multi-frequency pulse echo signal to obtain a second multi-frequency pulse echo signal with the frequency of Doppler frequency shift;
the analog-digital conversion acquisition card 11 is used for sampling the second multi-frequency pulse echo signal according to a preset sampling frequency to obtain a first multi-frequency pulse sampling signal;
a digital signal processing system 12 for performing signal processing on the first multi-frequency pulse sampling signal,
carrying out square rate detection and low-pass filtering on the multi-frequency pulse sampling signal to obtain a second multi-frequency pulse sampling signal of which each frequency difference corresponds to Doppler frequency shift, wherein each pulse repetition period comprises a plurality of sampling points, each sampling point corresponds to a distance unit, and the number of the sampling points in each pulse repetition period is obtained by the ratio of the pulse repetition period to the sampling time interval;
and carrying out coherent Doppler processing on the sampled data of a plurality of pulse repetition periods corresponding to the same distance unit to obtain the Doppler frequency spectrum of the distance unit, estimating the average speed of each target point in the distance unit by the frequency spectrum of the Doppler frequency spectrum, and sequentially calculating the average speed of each target point in different radial distance units.
Specifically, the connection relationship among each component of the wind-measuring radar system is as follows: output end and first of multi-frequency continuous laser generator 1The input ends of the optical fiber beam splitters 2 are connected, the first optical fiber beam splitter 2 divides the multi-frequency laser into two parts, wherein the first output end A1Output intrinsic reference light, and a second output terminal B1And outputting the signal light. First output end A of first optical fiber beam splitter 21Connected to the input of the continuously adjustable attenuator 8, the output of the continuously adjustable attenuator 8 and the first input A of the second fiber splitter 93And (4) connecting.
Second output B of the first fiber splitter 21An output end A of the radio frequency driving source 4 is connected with the input end of the pulse modulator 34Connected with the input end of the driving signal of the pulse modulator 3, the output end of the pulse modulator 3 is connected with the input end of the circulator 5, and the first output end A of the circulator 52The light beam emitted by the optical transceiver 6 irradiates on a target, namely an observation area 7, and the back scattering echo of the target 7 is collected by the optical transceiver 6 and then returns to the first output end A of the circulator 52Transmitted inside the circulator 5 to the second output B2And a second input B of a second fiber splitter 93And (4) connecting. The output end of the second optical fiber beam splitter 9 is connected with the input end of the balance detector 10, the output end of the balance detector 10 is connected with the input end of the analog-digital conversion acquisition card 11, and the output end of the analog-digital conversion acquisition card 11 is connected with the input end of the digital signal processing system 12. Output terminal B of radio frequency driving source 44And outputting an electric signal synchronous with the pulse to the analog-digital conversion acquisition card 11 as a trigger signal for echo acquisition.
In one embodiment, the multi-frequency continuous laser generator 1 is configured to output multi-frequency continuous laser light with coherence, including but not limited to multi-frequency continuous laser light generated by frequency doubling, frequency shifting, electro-optical modulation, frequency selection of a mode-locked laser, and the like.
In one embodiment, the detection target is a fluid particle, including but not limited to an atmospheric aerosol, and the lidar is configured to detect an atmospheric wind speed if the detection target is an atmospheric aerosol.
In one embodiment, the pulse modulator includes, but is not limited to, an acousto-optic modulator (AOM), or an electro-optic modulator (EOM).
In one embodiment, the RF driving source includes, but is not limited to, an Arbitrary Waveform Generator (AWG), or a transistor logic level signal TTL generator, or an RF signal generator.
In one embodiment, the digital signal processing system 12 is a DSP data processing system.
When the multi-frequency laser signal emitted by the multi-frequency continuous laser generator 1 is set, it is necessary to realize a reading range that can cover a detection area to realize higher speed detection accuracy.
Wherein, the maximum Doppler frequency shift of the target echo can not exceed half of the pulse repetition frequency, the existing multi-frequency laser radar system is adopted, and the maximum Doppler frequency shift of the target
Then maximum Doppler absolute velocity
A larger speed range can be measured inversely proportional to the frequency difference, i.e. decreasing the frequency difference.
The accuracy of the velocity measurement depends on the frequency accuracy of the acquired power spectrum, the power spectrum is obtained by performing autocorrelation calculation on a series of samples of echoes and then performing fourier transform, and the corresponding velocity measurement accuracy is as follows:
wherein, the larger the coherent echo number M is, the larger the frequency difference delta f is, the smaller the pulse repetition frequency PRF is, and the velocity measurement precision delta V isrThe smaller the measurement, the more accurate the measurement.
Therefore, the magnitude of the frequency difference needs to be reasonably selected to balance the speed measurement range and the speed measurement precision to meet the detection requirement.
When the pulse modulator 3 and the rf drive source 4 are set, the pulse repetition frequency and the pulse width of the multi-frequency pulse laser signal need to meet certain requirements.
Wherein, the pulse repetition frequency PRF =1/PRT, where PRT is the pulse repetition period and refers to the time difference between the start of one pulse and the start of the next. The pulse laser radar adopts a flight time method for ranging, namely, the target distance R = the difference delta T multiplied by the light speed c/2 between the receiving time and the transmitting time of the echo, and the delta T cannot exceed the pulse repetition period PRT at the maximum, otherwise, the time for receiving the echo can be overlapped to the next pulse period, which is called range ambiguity, and a complicated means is additionally adopted for solving the range ambiguity. The maximum value of the measuring distance is R in order to obtain a longer measuring distance without causing distance ambiguitymax=c/2/PRF。
Because the scattering echo of aerosol and other particles is very weak, the wind lidar belongs to weak signal detection, if the energy of the transmitted detection pulse is too low, the signal-to-noise ratio of the echo is extremely low, and speed and distance measurement is difficult to realize, so if the set pulse width T isPNarrower, the peak power is amplified to a higher peak power to meet the minimum pulse energy requirement. However, too high peak power easily causes nonlinear effects, so that the optical frequency shifts during transmission, and the pulse width is not suitable to be too narrow. Meanwhile, the pulse width is not too wide, and the larger the pulse width is, the area (c.T) included by single measurement of echo isPThe larger the/2), the more complex the particle velocities contained therein, resulting in a broader calculated doppler spectrum. Due to the selection of multiple frequency probes, multiple frequency peaks (Δ) are obtained in the final Doppler spectrum2–Δ1, Δ3–Δ2, Δ3–Δ1…), it is difficult to clearly obtain the specific values of the frequency peaks if aliasing is highly likely to occur in the spectrally broadened peaks.
The intrinsic reference light is collimated at a continuously adjustable attenuator 8
Adjusted to give formula (1):
the first multi-frequency pulse echo signal and the intrinsic reference light passing through the second beam splitter
After the synthesis, the formula (2) is obtained:
echo in equation (2)
The phase terms of (2) have no influence in data processing, the unwritten phase terms are omitted, and for simplicity, the intensities of the frequencies at the same position in the system are written to be equal, so that the implementation process of the method is not influenced. Echo wave
And a reference light
And the two optical fibers enter a second optical fiber beam splitter 9 together and then are transmitted to a balance detector 10 for coherent detection and photoelectric conversion of signals.
Because the photosensitive medium responds to the intensity of the light, the reference light detected by the detector and the light intensity Z after the echo is coherent2Also known as square law detection. When square law detection is performed, sum frequency signal containing frequency 2 is generatedf 1+Δ1, 2f 2+Δ2, 2f 3+Δ3, f 1+Δ1+f 2The iso-sum and difference frequency signals contain a frequency delta1, Δ2, Δ3, f 1+Δ1-f 2And the like. Limited by the bandwidth of the balanced detector, only the frequency being Δ1, Δ2, Δ3…, the high frequency term cannot be responded to by passing the frequency difference doppler shift term, and the high frequency is lost, i.e. the passing signal is the second multi-frequency pulse echo signal with the frequency of doppler shift:
inputting the above signals into the analog-digital conversion acquisition card 11 through the signal formula (3), wherein the analog-digital conversion acquisition card 11 adopts an electrical signal which is output by the radio frequency driving source 4 and is synchronous with the pulse as a trigger signal of sampling, and the electrical signal is in accordance with a preset sampling frequencyf SEquation (3) is sampled with a sampling interval Δ t =1 @f S。
Then, after sampling the low-frequency passing signal, that is, the formula (3), by using the analog-to-digital conversion acquisition card 11, a first multi-frequency pulse sampling signal is obtained and input into the digital signal processing system 12 for further processing, that is, another part of the method of the present application includes the following steps:
carrying out square rate detection and low-pass filtering on the first multi-frequency pulse sampling signal to obtain a second multi-frequency pulse sampling signal of which each frequency difference corresponds to Doppler frequency shift, wherein each pulse repetition period comprises a plurality of sampling points, each sampling point corresponds to a distance unit, and the number of the sampling points in each pulse repetition period is obtained by the ratio of the pulse repetition period to the sampling time interval;
and carrying out coherent Doppler processing on the sampled data of a plurality of pulse repetition periods corresponding to the same distance unit to obtain the Doppler frequency spectrum of the distance unit, estimating the average speed of each target point in the distance unit by the frequency spectrum of the Doppler frequency spectrum, and sequentially calculating the average speed of each target point in different radial distance units.
The coherent doppler processing of the sampled data of multiple pulse repetition periods corresponding to the same range cell to obtain the average velocity of each target point in the range cell includes: and carrying out autocorrelation calculation on the sampled data of a plurality of adjacent pulse repetition periods corresponding to one range cell to obtain an autocorrelation calculation result, carrying out Fourier transform according to the autocorrelation calculation result to obtain a Doppler frequency spectrum corresponding to the range cell, obtaining a plurality of Doppler frequencies corresponding to the range cell through frequency spectrum estimation, and calculating according to the frequency difference between the Doppler frequencies and each frequency of the multi-frequency laser signal to obtain the average speed of each target point in the range cell.
Wherein, calculating the average velocity of the target point in the range unit according to the frequency difference between the multiple doppler frequencies and the frequencies of the multi-frequency laser signal comprises: and calculating according to each Doppler frequency and the corresponding frequency difference to obtain a plurality of average Doppler speeds of each target point in the range cell, and averaging the average Doppler speeds to obtain the average speed of each target point in the range cell.
Specifically, when the first multi-frequency pulse sampling signal is input to the digital signal processing system 12 and is squared again, i.e. sum frequency and difference frequency terms are generated again, the sum frequency terms are filtered after low-pass filtering, and the remaining difference frequency terms, the second multi-frequency pulse sampling signal is obtained as follows:
in the formula (4), Δ2–Δ1, Δ3–Δ2, Δ3–Δ1… is the Doppler shift of the multiple sets of frequency differences.
Grouping the second multi-frequency pulse sampling signals according to coherence time, wherein the movement of the aerosol particles
The time less than one wavelength is considered to be within the coherence time (for example, assuming that under the conditions of 1G frequency difference, 20m/s aerosol particle motion speed, PRF =10KHz, the coherence time is
)。
Assuming that the data duration within a coherence time is M PRTs, take the ith sample data of M consecutive pulses from the 1 st PRT to the Mth PRT as an example, assume ZDSP(PRT,i) ~ ZDSP(m.prt, i), on which autocorrelation is performed:
in the formula (5), the first and second groups,
is a variable ranging from 1 to the mth PRT.
And then carrying out Fourier transform:
then, the frequency spectrum of the ith sampling unit is obtained, and the frequency spectrum of the n frequencies is obtained through the frequency spectrum analysis
A plurality of
Doppler frequency (Delta)2–Δ1, Δ3–Δ2, Δ3–Δ1…), and then apply this
The Doppler frequency is proportional to the frequency difference (f) of the multifrequency laser2–f1, f3–f2, f3–f1…), is made of
The sampled, i.e. distance-resolved, average Doppler velocity, f, can be inferreddIs the doppler frequency difference, Δ f is the frequency difference. And an average velocity can be obtained for each set of Doppler frequency difference and the frequency difference of the corresponding multi-frequency continuous laser signal, e.g.
，
And averaging the speeds of the several frequency differences to obtain the average speed of the target point.
Furthermore, in each pulse repetition period, each sampling data corresponds to the echo data of a plurality of target points at different positions, and the echo data are sequentially received from near to far according to the radial distance of each target point. Since the emission is performed according to the set pulse repetition frequency when the multi-frequency pulse laser signal is emitted to the observation area, echo data can be received once in each pulse repetition period for a plurality of target points in each range unit in a plurality of pulse repetition periods. Therefore, the method of correlation calculation, Fourier transform, frequency spectrum estimation and averaging is carried out on the M sampling point data of each target point in M pulse repetition periods, so that the average speed of each sampling unit can be obtained, and finally, the speed distribution which changes along with the distance is formed.
Next, taking a tri-band pulsed laser signal as an example, with reference to fig. 2, the signal spectrum variation in each position in the wind radar system is further explained.
First output end A of first optical fiber beam splitter 21And a second output terminal B1All output three-frequency laser signal f1, f2, f3Second output B of the first fiber splitter 21After the pulse is modulated by the pulse modulator 3, the pulse is outputted from a first output terminal A of the circulator 52Transmitted to a target, received in the original path and internally transmitted to a second output terminal B2When the three-frequency signal in the echo carries Doppler frequency shift proportional to respective frequency, the echo frequency is f1+δ1, f2+δ2, f3+δ3. First output end A of first optical fiber beam splitter 21The output three-frequency signal is adjusted in signal intensity by the continuous adjustable attenuator 8, and then passes through the first input end A of the second optical fiber beam splitter 93Transmitted to the second optical splitter 9, and the echo passes through the first input end B of the second optical splitter 93Transmitted to a second optical fiber beam splitter 9, the second optical fiber beam splitter 9 contains six frequencies f1, f2, f3, f1+δ1, f2+δ2, f3+δ3. The second optical fiber beam splitter 9 transmits six frequencies to a balanced detector 10 for square rate detection, and only a low-frequency signal delta meeting the bandwidth range of the detector is generated due to interference of good coherence of three-frequency signals1,δ2,δ3Leaving behind. The low-frequency signal of the balance detector 10 is collected by an A/D analog-digital conversion collecting card 11 and then transmitted to a DSP digital signal processing system 12 for processingThe squaring leaves a low frequency signal delta2-δ1，δ3-δ2，δ3-δ1I.e. proportional to the doppler shift of the frequency difference signal.
Next, by comparing the method for processing the multi-frequency laser pulse signal in the present application with the conventional method for processing the single-frequency laser pulse signal, it is described that the reason why the speed/distance resolution of the current single-frequency laser radar is difficult is embodied, and the processing parts of the two methods in the digital signal processing system are taken as examples:
as shown in fig. 3, after receiving the echo signals transmitted by the a/D adc card 11, the DSP system 12 squares all the signals, performs low-pass filtering to obtain M continuous PRTs within the coherence time, sequentially performs autocorrelation-fourier transform-spectrum analysis-velocity averaging on the data of M continuous pulses within each sampling distance to obtain the average doppler velocity of each sampling distance, and finally forms the velocity distribution with the distance. Signal Z of ith sampling distanceD(0,i) ~ ZDPrt, i) as an example, power spectrum:
in equation (7), where the angular frequency
. Due to the inter-pulse coherence, the frequency resolution is proportional to the pulse repetition frequency PRF =1/PRT, and a distance resolution of every other sampling interval can be obtained as
The average power spectrum of (a). From the mean power spectrum
The Doppler velocity of the range cell is estimated, one way of doing this is by a piecewise weighted average of the power spectrum, i.e.
Considering that the movement of the particles is far smaller than the movement distance of the particle positions in adjacent sampling coherence of the single-frequency laser radar after (M +1) PRT (pulse repetition time), the multi-frequency laser radar can be considered to have no spectrum pollution caused by the movement of the sampling positions. The speed resolution can be improved under the condition of not influencing the distance resolution by increasing the number of coherent pulses, and the distance resolution can be improved by reducing the pulse width.
As shown in FIG. 4, within each PRT, T is addedsFor continuous sampling N times at intervals, each sampling obtains a time width TpThe particle echo signal is obtained by performing autocorrelation and Fourier transform on L continuous samples in a PRT, accumulating frequency spectrums calculated by M PRTs, performing frequency spectrum estimation to obtain Doppler frequency shift, and reversely deducing the average speed of the L samples with corresponding lengths. The same process is applied to all samples in the PRT in sequence to obtain a velocity profile with distance. To be located at time tmOf the kth pulse sample of the sequence of L successive echoes Zs(tm,k) ~ Zs(tmK + L) for example, with an average power spectral density over incoherent integration of M pulses of
In the formula (9), the angular frequency
，
For the sampling frequency, it can be seen that the spectral density calculated by adopting an adjacent sampling coherent mode comes from scattering particles at different positions, so that the wind speed obtained by inversion is a sparse average value of the wind speed in the whole detection area, the number of sampling points in the central area is large, the spectral resolution is high, and the number of sampling points closer to the two sides is lower, so that the overall spectral resolution is lowered. And the classical spectrum estimation method also stores itselfIn error, when the number of sampling points is small, the frequency resolution is low; frequency leakage from windowing can also lead to inaccurate estimated frequencies. Therefore, to achieve fine spectral resolution in a conventional single frequency lidar requires more sampling, distance resolution (NT)s+Tp) c/2 will inevitably increase.
Next, the multi-frequency laser pulse signal processing method (taking three frequencies as an example) in the present application and the conventional single-frequency laser pulse signal processing method are simulated respectively to obtain the average speed measurement error of the wind radar system under different methods, as shown in fig. 5.
According to the total number of particles in the wind field
Adding random noise to received echoes
Strength I thereofnThe pulse and system parameters in the simulation of the single/three-frequency laser radar are consistent with those described in fig. 5, and 65 sampling points are needed to cover the wind field range of 100m, which is comprehensively set by the given SNR and the strength of the detection signal. The average speed measurement error of each sampling point under different signal-to-noise ratios (SNR) is obtained by using a sampling and data processing mode of a single/three-frequency laser radar, namely the average error in the graph is shown as a dotted line (20 ns-single frequency) and a cross dotted line (20 ns-double frequency). The trend according to the curve can be divided into four stages:
region 1: when the signal-to-noise ratio is lower than minus 15 to minus 12dB, the average speed measurement errors of the single three-frequency laser radar are larger and are close to 6 m/s;
region 2: with the increase of the signal-to-noise ratio to-6 dB, the average speed measurement error of the three-frequency laser radar is rapidly reduced to about 0.2m/s, and the error of the single-frequency laser radar is slowly reduced to 5 m/s;
region 3: the signal-to-noise ratio is further increased to 4dB, the error of the three-frequency laser radar is stabilized at 0.2m/s, and the average error of the single-frequency laser radar is rapidly reduced to about 3.5 m/s;
region 4: the signal-to-noise ratio is larger, when the signal-to-noise ratio is higher than 4dB, the speed measurement errors of the single-frequency laser radar and the three-frequency laser radar are basically stable although slight fluctuation exists, but the speed measurement error of the three-frequency laser radar is smaller.
Therefore, the signal intensity of the three-frequency laser radar is improved greatly due to the excellent coherent accumulation mode between pulses, noise interference is eliminated, accurate speed measurement under weak signals is realized, and detection at a longer distance is expected to be achieved.
In the multi-frequency pulse laser radar signal processing method, a multi-frequency continuous laser source with high coherence is generated as a carrier (f)1, f2, f3…) the frequency difference is in the microwave band. Dividing a multi-frequency continuous carrier into two parts, emitting one path of modulated pulse as signal light to atmosphere to interact with aerosol particles, and generating Doppler frequency shift (delta) in direct proportion to frequency by multiple frequencies in the signal light1=v• f1/2c, Δ2= v• f2/2c, Δ3=v• f3And/2 c, …), and the other path is taken as a reference light and the echo to be sent to the photoelectric detector after being combined. The optical signal is converted into an electric signal by a photoelectric detector, and simultaneously, coherent beat frequency is carried out on reference light and signal light, the reference light and the signal light are acquired by A/D analog-digital conversion, and square law detection (delta) is carried out by a DSP information processor1+Δ2+Δ3+…)2Obtaining Doppler shift difference (Delta) of multiple frequencies2–Δ1, Δ3–Δ2, Δ3–Δ1…), i.e. a doppler shift in the frequency difference of the microwave band. The number of Doppler frequency differences finally obtained is related to the number of used multifrequency frequencies, if n frequencies are used, the obtained number is obtained
A difference in doppler shift.
Because the frequency difference of the multi-frequency signal corresponds to a longer equivalent wavelength and corresponds to a longer coherent time at the same speed, the multi-frequency pulse laser radar can continue to use the signal processing mode of the pulse microwave radar, namely the adjacent pulse coherent mode, to perform spectrum analysis and Doppler estimation on the sampled data of continuous M pulses in a certain sampling distance to obtain the distance unit (T)p• c/2) Average doppler velocity of (d).
Under the processing mode of adjacent pulse coherence, increasing the number M of coherent pulses can improve the speed resolution and reduce the pulse width TpThe distance resolution can be reduced, and the speed resolution and the distance resolution do not affect each other. Because the sampled particles are still substantially in current distance resolution (T) after M PRT times in the speed range of the normal anemometrypC/2), there is no obvious shift, it can be considered that there is no spectrum pollution caused by the shift of the sampling position in the multi-frequency lidar, and the obtained average doppler velocity can reflect the average velocity of the particles in the range resolution.
In the process of inverting the velocity by using the doppler shift, laser spectral lines are broadened by optical noise, system noise, electrical noise and the like, wherein speckle noise has a large influence. Speckle noise is in direct proportion to frequency, and because the multi-frequency difference signal is positioned in a microwave band, the frequency (less than hundred GHz) of the multi-frequency difference signal is far less than the laser frequency (hundred THz), the multi-frequency laser radar can inhibit Doppler spectral line broadening caused by the speckle noise.
Because the coherent accumulation of M continuous pulses by the multi-frequency laser radar can improve the spectrum intensity by M2Double, the traditional single-frequency laser radar can only improve the spectrum intensity by M-M by carrying out incoherent accumulation on the power spectrums of M pulses2By fold, the more nearly the number of accumulations is M times. Therefore, the coherent accumulation mode among the pulses of the multi-frequency pulse laser is benefited, the spectrum intensity of the echo signal is improved greatly, noise interference is favorably eliminated, the spectrum with high signal-to-noise ratio is obtained by processing under the lower received echo signal-to-noise ratio, the lower speed error is favorably obtained, the accurate speed measurement under the weak signal is realized, and the detection at a longer distance is hopefully achieved. And the multi-frequency difference provided by the multi-frequency laser radar ensures that the speed measuring range and the speed measuring precision are considered, the smaller the frequency difference is, the larger the speed measuring range is, the larger the frequency difference is, the higher the speed measuring precision is, and the frequency in the carrier wave is reasonably selectedf 1 , f 2 , f 3 …, different frequency differences can be combined, and high-precision results in a speed measurement range are expected.
According to the method, while the excellent detection capability of the laser radar under the clear sky condition is kept, the problem that the traditional single-frequency laser radar is difficult to distinguish in speed and distance is solved by adopting a microwave radar signal inter-pulse processing mode, namely the speed resolution is improved by reducing the distance resolution, and the speed resolution is improved by reducing the distance resolution. Increase coherent pulse number in this application and can improve speed and distinguish, reduce pulse width and can reduce the distance and distinguish, speed is distinguished and is not influenced each other with the promotion of distance distinguishing.
When the inversion speed of the power spectrum is calculated, the spectrum pollution caused by the movement of the sampling position does not exist, and more accurate wind speed measurement can be realized. Because the traditional laser radar adopts a method of adjacent sampling coherence, the obtained average Doppler velocity is the sparse average velocity of the particles in the range resolution, and the average velocity may deviate from the real average velocity of the range resolution center. The coherent signal processing method between pulses adopted in the method ensures that the coherent area is accurate to each range cell, and few particles move into and out of the range cell in the normal radar coherent time, thereby obtaining more accurate frequency spectrum and speed estimation in the range cell.
The signal processing mode of pulse coherent accumulation adopted in the method can improve the spectrum intensity of the echo signal to a greater extent, is beneficial to eliminating noise interference, processes the echo signal to noise ratio to obtain the spectrum with high signal to noise ratio under lower received echo signal to noise ratio, and is expected to realize fine detection of a complex wind field at a longer distance and a lower signal to noise ratio.
The method adopts the multi-frequency laser frequency difference signal in the microwave band to extract Doppler information, thereby effectively reducing shot noise caused by atmospheric turbulence and the like.
The method uses multi-frequency laser signals, the frequency difference of which can be flexibly selected to adapt to the requirements of different speed ranges and different detection accuracies of targets, for example, multi-frequency signals with smaller frequency difference can be selected for ultra-high-speed targets to reduce Doppler frequency shift and meet the bandwidth requirements which can be achieved when detectors and data are processed; for the low-speed target, the frequency difference is increased, and the measurement precision is improved; even when the target with high speed and low speed exists in the whole measuring range, the frequency difference of large and small are set simultaneously, and the composite detection requirement is met.
The technical features of the above embodiments can be arbitrarily combined, and for the sake of brevity, all possible combinations of the technical features in the above embodiments are not described, but should be considered as the scope of the present specification as long as there is no contradiction between the combinations of the technical features.
The above-mentioned embodiments only express several embodiments of the present application, and the description thereof is more specific and detailed, but not construed as limiting the scope of the invention. It should be noted that, for a person skilled in the art, several variations and modifications can be made without departing from the concept of the present application, which falls within the scope of protection of the present application. Therefore, the protection scope of the present patent shall be subject to the appended claims.
Claims (8)
1. The method is characterized in that the method is applied to a wind measuring radar system, and the wind measuring radar system comprises a multi-frequency continuous laser generating device, a first optical fiber beam splitter, a pulse modulator, a radio frequency driving source, a circulator, an optical transceiver, a continuous adjustable attenuator, a second optical fiber beam splitter, a balance detector, an analog-digital conversion acquisition card and a digital signal processing system;
generating a multi-frequency laser signal with high coherence of each frequency by the multi-frequency continuous laser generating device, dividing the multi-frequency laser signal into two beams of multi-frequency laser signals by the first optical fiber beam splitter, taking one beam as intrinsic reference light, and taking the other beam as signal light;
the signal light is modulated by a pulse modulator and a radio frequency driving source to obtain a multi-frequency pulse laser signal, the multi-frequency pulse laser signal is sequentially transmitted to an observation area through the circulator and the optical transceiver and is scattered with a plurality of target points in different radial distance units in the observation area, part of scattered echoes return to the optical transceiver, and a first multi-frequency pulse echo signal is formed through the circulator;
the intrinsic reference light passes through the continuous adjustable attenuator, then simultaneously passes through the second fiber beam splitter with the first multi-frequency pulse echo signal, and then passes through the balance detector to obtain a second multi-frequency pulse echo signal with Doppler frequency shift;
inputting the second multi-frequency pulse echo signal into an analog-digital conversion acquisition card to be sampled according to a preset sampling frequency to obtain a first multi-frequency pulse sampling signal;
inputting the first multi-frequency pulse sampling signal into the digital signal processing system for signal processing, and performing square frequency detection and low-pass filtering on the first multi-frequency pulse sampling signal to obtain frequency differences among frequencies, namely second multi-frequency pulse sampling signals corresponding to Doppler frequency shift, wherein each pulse repetition period comprises a plurality of sampling points, each sampling point corresponds to a distance unit, and the number of the sampling points in each pulse repetition period is obtained by the ratio of the pulse repetition period to the sampling time interval;
and carrying out coherent Doppler processing on the sampled data of a plurality of pulse repetition periods corresponding to the same distance unit to obtain the Doppler frequency spectrum of the distance unit, estimating the average speed of each target point in the distance unit by the frequency spectrum of the Doppler frequency spectrum, and sequentially calculating the average speed of each target point in different radial distance units.
2. The method according to claim 1, wherein the pulse modulator further outputs an electrical signal synchronized with the multi-frequency pulse laser signal to an analog-to-digital conversion/acquisition card as a trigger signal for sampling.
3. The method of claim 2, wherein the obtaining the average velocity of each target point in the range cell by performing coherent doppler processing on the sampled data of a plurality of pulse repetition periods corresponding to the same range cell comprises:
performing autocorrelation calculation on the sampled data of a plurality of adjacent pulse repetition periods corresponding to one distance unit to obtain an autocorrelation calculation result;
performing Fourier transform according to the autocorrelation calculation result to obtain a Doppler frequency spectrum corresponding to the range cell, and obtaining a plurality of Doppler frequencies corresponding to the range cell through frequency spectrum estimation;
and calculating according to the frequency difference between the Doppler frequencies and the frequencies of the multi-frequency laser signals to obtain the average speed of each target point in the distance unit.
4. The method according to claim 3, wherein calculating the average velocity of each target point in the range unit according to the frequency difference between the doppler frequencies and the frequencies of the multi-frequency laser signal comprises:
calculating according to each Doppler frequency and the corresponding frequency difference to obtain a plurality of average Doppler speeds of each target point in the distance unit;
and averaging the average Doppler velocities to obtain the average velocity of the target point in the range unit.
5. A wind lidar system, comprising:
the multi-frequency continuous laser generating device is used for generating multi-frequency laser signals with high coherence of each frequency;
the first optical fiber beam splitter is used for splitting the multi-frequency laser signal into two beams of multi-frequency laser signals, wherein one beam is used as intrinsic reference light, and the other beam is used as signal light;
the pulse modulator is used for modulating the signal light into a multi-frequency pulse laser signal;
a radio frequency driving source for providing a pulse modulated radio frequency signal to modulate the width and repetition frequency of the multi-frequency pulse laser signal;
the circulator is used for sending the modulated multi-frequency pulse laser signal to the optical transceiver and receiving a first multi-frequency pulse echo signal from the optical transceiver;
the optical transceiver is used for transmitting the multi-frequency pulse laser signals to an observation area and receiving first multi-frequency pulse echo signals formed by scattering back from a plurality of target points in different radial distance units in the observation area;
a continuously adjustable attenuator for adjusting the intensity of the intrinsic reference light;
the second optical fiber beam splitter is used for simultaneously inputting the intrinsic reference light after intensity adjustment and the first multi-frequency pulse echo signal into the balance detector;
the balance detector is used for carrying out square rate detection according to the intrinsic reference light and the first multi-frequency pulse echo signal to obtain a second multi-frequency pulse echo signal with Doppler frequency shift;
the analog-digital conversion acquisition card is used for sampling the second multi-frequency pulse echo signal according to a preset sampling frequency to obtain a first multi-frequency pulse sampling signal;
the digital signal processing system is used for carrying out signal processing on the first multi-frequency pulse sampling signal, carrying out square frequency detection and low-pass filtering on the first multi-frequency pulse sampling signal, and obtaining frequency difference among frequencies, namely second multi-frequency pulse sampling signals corresponding to Doppler frequency shift;
and carrying out coherent Doppler processing on the sampled data of a plurality of pulse repetition periods corresponding to the same distance unit to obtain the Doppler frequency spectrum of the distance unit, estimating the average speed of each target point in the distance unit by the frequency spectrum of the Doppler frequency spectrum, and sequentially calculating the average speed of each target point in different radial distance units.
6. The system according to claim 5, wherein the pulse modulator is further configured to output an electrical signal synchronized with the multi-frequency pulse laser signal to an analog-to-digital conversion acquisition card as a trigger signal for sampling.
7. The lidar system of claim 6, wherein the pulse modulator comprises an acousto-optic modulator, or an electro-optic modulator.
8. The lidar system of claim 6, wherein the radio frequency drive source comprises an arbitrary waveform generator, or a transistor logic level signal generator, or a radio frequency signal generator.
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