A radar snow depth measurement method combining frequency and phase

Abstract

The present invention provides a radar snow depth measurement method combining frequency and phase, comprising: step 1: obtaining a transmission signal generated by a frequency modulated continuous wave radar, and an echo signal generated after the transmission signal encounters target snow; step 2: mixing the transmission signal and the echo signal to obtain a beat signal; step 3: determining the thickness of the snow according to the beat signal R=R2-R1; wherein R2 is the distance between the beat signal and the snow surface, R1 is the distance between the beat signal and the ground, and R is the actual depth of the snow. The method provided by the present invention combines phase and frequency to achieve high-precision data measurement.

Description

Technical Field

[0001] The present invention relates to the field of radar detection technology, and in particular to a radar snow depth measurement method combining frequency with phase. Background Art

[0002] At present, the methods of snow detection are mainly divided into two categories: manual and instrument observation. The traditional manual measurement method uses a snow ruler to measure the depth of snow in a flat area, or uses a special container to hold snow, waits for the snow to melt, measures the weight of the snow water, and infers the snowfall. This method is first limited by the measurement terrain. Under relatively complex terrain conditions, the data results measured by the snow ruler will have a large deviation from the actual situation; and the use of containers to hold snow water requires waiting until the snow melts before calculation, which takes a long time. The two main methods of snow depth detection in use are ultrasonic ranging and laser ranging. Ultrasonic ranging is greatly affected by the wind speed and temperature in the measurement environment. Any change in the above two variables may cause a large error in the measurement result; the cost of laser ranging is relatively high, and it is not suitable for large-scale deployment across the country.

[0003] Therefore, there is currently a lack of a snow detection method with high detection accuracy and wide applicability. Summary of the invention

[0004] The purpose of the present invention is to solve the defects of the above-mentioned prior art and provide a radar snow depth measurement method combining frequency and phase.

[0005] A frequency combined with phase radar snow depth measurement method, comprising:

[0006] Step 1: Acquire a transmission signal generated by a frequency modulated continuous wave radar and an echo signal generated when the transmission signal encounters target snow;

[0007] Step 2: Mixing the transmission signal with the echo signal to obtain a beat signal;

[0008] Step 3: Determine the thickness R1 of the snow according to the beat signal;

[0009] Wherein, R1=R2-R; R is the distance between the radar and the ground, and R2 is the distance between the radar and the snow surface.

[0010] Furthermore, in the above-mentioned frequency combined with phase radar snow depth measurement method, step 3 comprises:

[0011] Step 31: Perform FFT transformation on the beat signal to obtain a beat signal spectrum;

[0012] Step 32: determining the location of the maximum value of the spectrum according to the beat signal spectrum;

[0013] Step 33: determining an error quantization value q according to the position of the maximum value of the spectrum;

[0014] Step 34: Correct the error quantization value q by calculating the phase to obtain a corrected error quantization value q;

[0015] Step 35: Determine the distance R between the radar and the ground according to the positive error quantization value q.

[0016] Further, in the above-mentioned frequency combined with phase radar snow depth measurement method, step 35 comprises:

[0017] The R is determined according to the following formula:

[0018]

[0019] Among them, f S is the sampling frequency; c is the speed of light; T is the modulation period; N is the number of FFT points; B is the sweep bandwidth.

[0020] Beneficial effects:

[0021] The method provided by the present invention combines phase and frequency to achieve high-precision distance measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a schematic diagram of the position of the radar placed vertically above the snow surface;

[0023] Figure 2 The present invention is a flow chart of the radar snow depth measurement method combining frequency with phase;

[0024] Figure 3 This is the frequency change waveform of the sawtooth wave FMCW radar transmitting and receiving signals. DETAILED DESCRIPTION

[0025] In order to make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the present invention is described clearly and completely below. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0026] Figure 1 The schematic diagram of the radar being placed vertically on the snow surface is shown in Figure 2. Figure 1As shown, the FMCW radar is placed on the ground. The height of the FMCW radar from the snow surface is R2, and the distance from the ground is R1. The actual depth of the snow is calculated to be R=R2-R1. In actual detection, the FMCW will receive two echo signals from the snow surface and the ground. The values ​​of R1 and R2 can be calculated. This placement method requires the FMCW radar system to have sufficient distance resolution. When the depth change of the snow is at the millimeter level, the system can still detect it.

[0027] Radar operating band selection and parameter setting

[0028] In the snow detection of the present invention, K band (24 GHz) is selected as the detection scheme, and the detection distance R is set to 300 m, the sweep period T is set to 200 us, the wavelength λ is set to 1.24 cm in the 24 GHz band, and the Boltzmann constant k is set to 1.28×10 -23 , receiver ambient temperature T e =273.15K, radar cross-sectional area of ​​snow surface σ = 4.5e -5 m 2 . Other general parameter settings: Transmitting antenna gain G t =10dBi, receiving antenna gain G r =10dBi, signal-to-noise ratio (SNR) 3dB, receiver noise factor F = 6dB, receiver inherent loss L = 6dB.

[0029] The continuous wave radar detection equation can be expressed as:

[0030]

[0031] Among them, SNR is the minimum signal-to-noise ratio of the receiver (receiver sensitivity).

[0032] Since the peak power of the FMCW radar's transmitted signal is equal to the average power, the radar's transmit power equation can be obtained by processing formula (1):

[0033]

[0034] Among them, P cw is the transmitting power of the radar system, R is the detection range of the radar, k is the Boltzmann constant (1.38E-23J / K), T e represents the ambient temperature of the radar receiver (usually 300K), F represents the noise coefficient of the radar receiver, L loss Represents the loss of the radar receiver, L win represents the attenuation caused by windowing FFT in digital signal processing, SNR represents the signal-to-noise ratio of the radar receiver, T represents the sweep period of the frequency modulated continuous wave radar, and G tRepresents the transmit antenna gain, G r Represents the receiving antenna gain, A e represents the effective aperture of the antenna, λ represents the wavelength of the radar carrier frequency, and σ is the radar scattering cross-section of the target.

[0035] Frequency Modulated Continuous Wave Radar Ranging Principle

[0036] Generally, radar measures distance by calculating the time interval τ between the emission and reception of electromagnetic waves. Assuming that the propagation speed of electromagnetic waves is the speed of light c, and the distance between the target and the radar is r, the distance detection formula is:

[0037] r=cτ / 2 (3)

[0038] The frequency modulated continuous wave (FMCW) radar transmits a periodic signal, and its frequency changes linearly over time. Using sawtooth frequency modulation, the frequencies of the transmitted signal and the echo signal will also change. Previous studies have found that the FMCW radar ranging formula is:

[0039]

[0040] Where c is the speed of light, T is the modulation period, B is the sweep bandwidth, and f t is the transmitted signal frequency, f r is the echo signal frequency, f b1 is the frequency spectrum of the beat signal, f b1 =f t -f r .

[0041] It can be seen from formula (4) that as long as the spectrum information of the beat signal is determined, the distance can be calculated. To obtain the spectrum information of the beat signal, it is necessary to perform FFT operation on the collected beat signal. However, due to the existence of the FFT fence effect, there is a large error between the obtained spectrum information and the actual value. Although the frequency refinement method can reduce the distance resolution unit and improve the accuracy, it still cannot meet the design requirements. In order to solve this problem and achieve more accurate measurement at the same distance, the present invention proposes a high-precision ranging method combining frequency and phase.

[0042] The high-precision ranging method combining frequency and phase is described in detail below:

[0043] Figure 2 The flowchart of the radar snow depth measurement method combining frequency and phase of the present invention is as follows: Figure 2 Said method comprises the following steps:

[0044] Step 1: obtaining a transmission signal generated by a frequency modulated continuous wave radar and an echo signal generated when the transmission signal encounters target snow;

[0045] Step 2: Mixing the transmission signal with the echo signal to obtain a beat signal;

[0046] Step 3: Perform FFT transformation on the beat signal to obtain the beat signal spectrum;

[0047] Step 4: Determine the position of the maximum value of the spectrum according to the beat signal spectrum;

[0048] Step 5: Determine the error quantization value q according to the position of the maximum value of the spectrum;

[0049] Step 6: Correct the error quantization value q by calculating the phase to obtain a corrected error quantization value q;

[0050] Step 7: Determine the distance R between the radar and the ground according to the positive error quantization value q.

[0051] Specifically, a frequency modulated continuous wave (FMCW) radar transmits a periodic signal with a frequency f t (t) will change linearly with time. Using sawtooth frequency modulation, the frequency changes of the transmitted signal and the echo signal are as follows: Figure 3 As shown, Figure 3 is the frequency variation waveform of the transmitted and received signals of the sawtooth FMCW radar; where f t represents the transmitted signal, f t represents the echo signal, the center frequency of the sawtooth wave modulation is f0, the sweep bandwidth is B, the modulation period is T, and the time delay between the transmitted signal and the echo signal is τ.

[0052] For sawtooth frequency modulation, according to the set parameters, the transmitted signal has:

[0053]

[0054] The instantaneous phase of the transmitted signal can be expressed as:

[0055]

[0056] In formula (6), Represents the initial phase of the transmitted signal.

[0057] Therefore, the instantaneous voltage of the transmitted signal can be expressed as:

[0058]

[0059] In formula (7), V T Represents the instantaneous voltage of the transmitted signal.

[0060] For a fixed target, there is no Doppler shift. Therefore, the echo signal is the time delay of the transmitted signal. If the time delay is τ, the echo signal is:

[0061]

[0062] In formula (8), V R Represents the instantaneous voltage of the received signal.

[0063] After mixing the transmit signal with the receive signal and performing low-pass filtering, the resulting beat signal can be expressed as:

[0064]

[0065] For close-range measurements, τ<<T, so the interval 0≤t<τ and The final beat signal is:

[0066]

[0067] It can be seen from formula (10) that the frequency of the radar beat signal is proportional to the time delay. Therefore, the frequency of the radar beat signal can be used to calculate the time delay τ, and then the distance information can be obtained (here an ideal basic FMCW radar model is used. The center frequency is 24GHz, and the sweep bandwidth B = 200M). To extract the frequency of the radar beat signal, it is necessary to perform Fourier transform on the beat signal obtained after mixing. Therefore, formula (10) can be written as:

[0068] V(t)=u(t)·v1(t), -∞<t<+∞ (11)

[0069] in:

[0070]

[0071]

[0072] According to the properties of Fourier transform, the Fourier transforms of v1(t) and u(t) are:

[0073]

[0074]

[0075] The Fourier transform of V(t) is the convolution of v1(t) and u(t), which gives:

[0076]

[0077] In formula (14), f0 represents the carrier frequency, and f represents the beat signal frequency.

[0078] From formula (14), we can see that the amplitude of V(f) is The maximum value is obtained at the point where the amplitude reaches the maximum value. b , and substitute it into formula (4) to get the distance information.

[0079]

[0080] The frequency of the beat signal obtained after mixing is f b =f t ±f r , here we only consider f b =f t -f r , f t Represents the frequency of the transmitted signal, f r Represents the received signal frequency, f b is the frequency of the beat signal, according to the triangle similarity form:

[0081]

[0082] The radar measures the distance by calculating the time interval τ between the emission and reception of electromagnetic waves. Assuming that the propagation speed of electromagnetic waves is the speed of light c, and the distance between the target and the radar is r, the mixed signal is substituted into the distance detection formula according to formula (3) to obtain:

[0083]

[0084] According to the mathematical expression of the beat signal obtained by formula (10), the discrete spectrum of the beat signal is calculated using discrete Fourier transform (DFT). The sampling frequency depends on the sampling time of AD and the time of discrete Fourier transform. Then the complex expression of the beat signal is:

[0085]

[0086] The frequency spectrum of the beat frequency signal is calculated using discrete Fourier transform. The mathematical expression of the beat frequency signal after Fourier transform is:

[0087]

[0088] In this formula, x k Represents the size of the kth spectral line of FFT, N represents the number of FFT points, k represents the kth spectral line of FFT, n represents the value of FFT from 0 to N-1, Δf represents the FFT fence effect error, and q represents the error quantization. is the phase of the intermediate frequency signal.

[0089] Among them, the phase of the intermediate frequency signal The relationship with distance R can be expressed by the following formula

[0090]

[0091] Formula (19) decomposes the maximum frequency value corresponding to the frequency domain beat signal into m+q frequency resolutions (Δf).

[0092] f b =(m+q)·Δf (21)

[0093] In an ideal situation, f b The corresponding spectral line falls exactly on the maximum value of the spectrum (X m ) where the spectrum line is located, at this time f b =m·Δf. But this is usually impossible. Therefore, it is necessary to determine q to improve the measurement accuracy. q is the error caused by the FFT fence effect in frequency. In order to determine the value of q, the discrete frequency amplitude is studied.

[0094] f b =mΔf+qΔf|X m |=max|X n | (22)

[0095] |X m | represents the maximum value on the FFT spectrum, |X n |Represents a collection of FFT results.

[0096] The position of q should be on both sides of the maximum value of the spectrum and the value range should be between -1 / 2 and 1 / 2:

[0097]

[0098] According to formula (19) and formula (23), we can get:

[0099]

[0100] Since the number of sampling points N>>π, equation (24) can be simplified as:

[0101]

[0102] Then the value of q can be expressed by the following formula:

[0103]

[0104] Phase ranging algorithm

[0105] From formula (14), we can see that the phase at the maximum amplitude of the beat signal can be expressed as:

[0106]

[0107] This value is equal to the initial phase of the beat signal, and we can use this feature to perform accurate distance calculation. Perform discrete Fourier transform on equation (10), assuming that the number of sampling points in a frequency sweep cycle is N, then the sampling interval Δt is expressed as:

[0108] Δt=T / N (28)

[0109] Sampling frequency f s It is expressed as:

[0110] f s =1 / Δt=N / T (29)

[0111] The sampled signal can be expressed as follows after DFT:

[0112]

[0113] Assuming that the discrete frequency at which V(k) has the maximum amplitude in the half interval (0, N / 2) is k1, the phase here can be expressed as:

[0114]

[0115] Bring-in and Can be transformed into:

[0116]

[0117] Let Δr = r-k1ΔR, which represents the difference between spectral lines in the same distance library. is the initial phase of the beat signal, The phase error caused by the discrete Fourier transform of the beat signal. This error is caused by the sampling interval between the spectra, that is, when the frequency of the beat signal is not equal to the frequency of the corresponding spectrum line at the maximum DFT amplitude, an additional phase difference will be generated.

[0118] Since the value of N is relatively large, formula (32) can be simplified as:

[0119]

[0120] Bring-in and Available Finally, we get:

[0121]

[0122] From formula (34), we can see that within a distance resolution unit, the phase and distance of the beat signal are linearly related. Based on this characteristic, we can calculate the precise distance.

[0123] Frequency combined with phase ranging algorithm

[0124] We know that the phase of the radar IF signal It is proportional to the measured distance R. In order to avoid the 2π phase ambiguity problem, the following is introduced:

[0125]

[0126]

[0127]

[0128] The value of q is finally determined as:

[0129]

[0130] Assume that the sampling rate is set to f s , then the higher precision distance measurement formula can be written as:

[0131]

[0132] The method provided by the present invention cannot improve the accuracy to the level required by this article by using only the frequency refinement ranging algorithm, and the phase ranging algorithm is limited to only one distance unit, which will inevitably introduce 2π phase ambiguity and has limited application scenarios. Therefore, a new frequency combined with phase ranging algorithm is proposed to improve the ranging accuracy. This method can perform continuous and long-term measurements; it realizes automation, intelligence, and remote observation.

[0133] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for measuring snow depth by radar combining frequency with phase, characterized in that: include: Step 1: Acquire a transmission signal generated by a frequency modulated continuous wave radar and an echo signal generated when the transmission signal encounters target snow; Step 2: Mixing the transmission signal with the echo signal to obtain a beat signal; Step 3: Determine the thickness R of the snow according to the beat signal; Wherein, R = R1-R2; R2 is the height of the FMCW radar from the snow surface; R1 is the distance of the FMCW radar from the ground; The step 3 comprises: Step 31: Perform FFT transformation on the beat signal to obtain a beat signal spectrum; Step 32: determining the location of the maximum value of the spectrum according to the beat signal spectrum; Step 33: determining an error quantization value q according to the position of the maximum value of the spectrum; Step 34: Correct the error quantization value q by calculating the phase to obtain a corrected error quantization value q; Step 35: determining the distance R between the radar and the ground according to the corrected error quantization value q; The steps 31-34 also include: The spectrum of the beat frequency signal is calculated using discrete Fourier transform. The mathematical expression of the beat frequency signal after Fourier transform is: In this formula, x k represents the size of the kth spectral line of FFT, N represents the number of FFT points, k represents the kth spectral line of FFT, n represents the value of FFT from 0 to N-1, Δf represents the FFT fence effect error, q represents the error quantization, is the phase of the intermediate frequency signal; where the phase of the intermediate frequency signal The relationship with distance R can be expressed by the following formula Formula (19) decomposes the maximum frequency value corresponding to the frequency domain beat signal into m+q frequency resolutions Δf; f b =(m+q)·△f (21) In an ideal situation, f b The corresponding spectral line falls exactly on the spectrum maximum value X m At the spectral line, f b =m·Δf; however, this is usually impossible, so it is necessary to determine q to improve the measurement accuracy. q is the error caused by the FFT fence effect in frequency. In order to determine the value of q, the discrete frequency amplitude is studied: f b =mΔf+qΔf |X m |=max|X n | (22) |X m | represents the maximum value on the FFT spectrum, |X n | represents a collection of FFT results; The position of q should be on both sides of the maximum value of the spectrum and the value range should be between -1 / 2 and 1 / 2: According to formula (19) and formula (23), we can get: Since the number of sampling points N>>π, equation (24) can be simplified as: The value of q can be expressed by the following formula: From formula (14), we can see that the phase at the maximum amplitude of the beat signal can be expressed as: This value is equal to the initial phase of the beat signal, and this feature is used to perform accurate distance calculation. Perform discrete Fourier transform on equation (10). Assuming that the number of sampling points in a frequency sweep cycle is N, the sampling interval Δt is expressed as: Δt=T / N (28) Sampling frequency f s It is expressed as: f s =1 / Δt=N / T (29) The sampled signal can be expressed as follows after DFT: Assuming that the discrete frequency at which V(k) has the maximum amplitude in the half interval (0, N / 2) is k1, the phase here can be expressed as: Bring-in and Can be transformed into: Let Δr = r-k1ΔR, which represents the difference between the spectral lines in the same distance library, where: is the initial phase of the beat signal, The phase error caused by the discrete Fourier transform of the beat signal is caused by the sampling interval between the spectra. That is to say, when the frequency of the beat signal is not equal to the frequency of the spectrum line corresponding to the maximum DFT amplitude, an additional phase difference will be generated. Since the value of N is relatively large, formula (32) can be simplified as: Bring-in and Available Finally, we get: It can be seen from formula (34) that within a distance resolution unit, the phase and distance of the beat signal are linearly related. Based on this characteristic, the accurate distance is calculated; Since the phase of the radar IF signal It is proportional to the measured distance R. In order to avoid the 2π phase ambiguity problem, the following is introduced: The value of q is finally determined as: Assume that the sampling rate is set to f s , then the higher precision distance measurement formula can be written as: Among them, f S is the sampling frequency; c is the speed of light; T is the modulation period; N is the number of FFT points; B is the sweep bandwidth.

Images