CN112558037B - Terahertz radar signal atmosphere transmission distortion compensation method based on database matching - Google Patents
Terahertz radar signal atmosphere transmission distortion compensation method based on database matching Download PDFInfo
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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 provides a terahertz radar signal atmospheric transmission distortion compensation method based on database matching, which is characterized in that an echo signal one-dimensional range profile is used for identifying an independent scattering center to compensate a terahertz radar distortion signal after atmospheric transmission. And then, simulating the independent scattering center and the one-dimensional range profile under different atmospheric conditions by using the terahertz frequency band atmospheric transmission characteristic information database, and comparing the simulated independent scattering center and the one-dimensional range profile with the actual one-dimensional range profile. The parameters such as air pressure, air temperature and the like corresponding to the best matching result are estimated atmospheric condition parameters. And finally, dividing the imaging area into a plurality of parts, and completing the atmospheric transmission waveform distortion compensation by using the average distance of each part and the estimated atmospheric extinction and phase offset coefficient. The invention has the advantages that: the calibration body used in radar measurement can provide independent scattering centers, and the method is not limited by the atmospheric conditions during measurement; by comparing the actually measured and simulated one-dimensional range profile, the atmospheric condition parameters can be estimated, and the atmospheric parameter inversion is realized.
Description
Technical Field
The invention relates to the technical field of atmosphere transmission compensation and terahertz radar imaging, in particular to a terahertz radar signal atmosphere transmission distortion compensation method based on database matching.
Background
The terahertz frequency band (0.1 THz-10 THz) can be used for acquiring a high-resolution radar image. However, the earth atmosphere in the terahertz band belongs to a dispersive medium, compared to the conventional radar band. When the terahertz radar signal is transmitted in the atmosphere, the amplitude attenuation and the phase shift of the signal caused by the atmosphere transmission are more serious along with the increase of the transmission distance, and the detection distance of the terahertz radar system is influenced. In addition, because the amplitude attenuation and the phase shift of the signal are nonlinear changes along with the frequency, the waveform of the echo signal is distorted, so that the sidelobes of the one-dimensional range profile are increased, even false targets are generated, the radar image quality is influenced, and the target detection probability is reduced.
The terahertz pulse after atmospheric transmission can be expressed as:
E(f,l)＝E(f,0)·exp[-α(f)l]·exp[iβ(f)l] (1)
wherein E represents terahertz pulses; f is the frequency; l is the transmission distance; i is an imaginary symbol; alpha is the atmospheric absorption coefficient; beta is the phase offset coefficient.
Fig. 2 shows a one-dimensional range profile of a desired scattering center processed at a distance 161m in the 460-490GHz band obtained by simulation, assuming a radar cross-sectional area (Radar Cross Section, RCS) of the scattering center of 0 dbm. The dashed line in the figure is a one-dimensional range profile irrespective of atmospheric transmission, and the solid line is a one-dimensional range profile obtained by transmission at the altitude of 28m with reference to the atmosphere in US 76. As can be seen from fig. 2, the atmospheric transmission not only causes attenuation (peak reduction) of the echo signal, but also causes phenomena such as scattering center position shift and side lobe rise: the ideal condition targets 161m, and the difference value between the main lobe and the first side lobe is 13dB; the target is offset by one distance resolution unit (the highest distance resolution available for 30GHz bandwidth is 0.005 m), the difference between the main lobe and the first side lobe is 5dB, considering the atmospheric transmission effect.
Theoretically, if the atmospheric condition parameters are known, the atmospheric absorption coefficient and the phase shift coefficient can be accurately obtained through calculation (see [1]C.H.Townes and A.L.Schawlow.Microwave Spectroscopy[M ]. New York: doever,1975 and [2]Y.Yang,M.Mandehgar,and D.R.Grischkowsky.Understanding THz Pulse Propagation in the Atmosphere[J ]. IEEE Transactions on Terahertz Science and Technology,2012,2 (4): 406-415 ]) so as to compensate the terahertz waveform distortion caused by atmospheric transmission. However, in practical engineering application, it is difficult to accurately acquire all the atmospheric condition parameters, and it is also impossible to ensure that the atmospheric characteristics on the long-distance transmission path remain unchanged, so that it is necessary to analyze the echo signal, estimate the atmospheric condition parameters therefrom, and further compensate for the terahertz waveform distortion caused by atmospheric transmission.
The prior art related to the present invention is presented as follows:
1.1 technical solution of the prior art
In the infrared and hyperspectral earth-to-earth remote sensing field, a statistical (empirical) model or an atmospheric radiation transmission model (see documents [3]M.K.Griffin and H.K.Burke.Compensation of Hyperspectral Data for Atmospheric Effects[J ]. Lincoln Laboratory Journal,14 (1): 29-54,2003 ] and documents [4] Tong Qingxi, zhang Bing, zheng Lanfen ] hyperspectral remote sensing-principle, technology and application [ M ]. Beijing: advanced education press, 2006.) is generally adopted to calculate the large-gas-path radiation and the transmittance in the spectral band, so as to solve the radiance or the reflectivity of the earth surface in the spectral band, and realize the atmospheric transmission compensation.
1.2 disadvantages of the prior art one
Infrared and hyperspectral sensors are typically passive remote sensing systems, and the effect of path radiation and transmittance on the radiation received by the sensor is typically considered in atmospheric compensation. For the terahertz frequency band, the earth atmosphere radiation is very weak and can be ignored, and the terahertz system is a coherent system, so that the atmospheric transmission not only affects the radiation intensity of a sensor receiving signal, but also affects the phase of the receiving signal, and the phenomena of scattering center offset, side lobe rise and the like are caused as shown in fig. 2. Therefore, only amplitude attenuation caused by atmospheric transmission is compensated, and the problem of terahertz radar signal distortion cannot be completely solved.
2.1 technical solution of the prior art
When the atmospheric transmission characteristics are not considered, the terahertz radar signal is linearly changed with time (chirp signal) in the frequency domain, and thus the received signal can be analyzed by taylor or fourier series expansion, from which the linear change coefficient (first-order coefficient) is extracted, to thereby realize the atmospheric dispersion compensation (see documents [5]M.Mandehgar and D.R.Grischkowsky.Understanding Dispersion Compensation of the THz Communication Channels in the Atmosphere[J ]. IEEE Photonics Technology Letters,27 (22): 2387-2390, 2015.)
2.2 disadvantages of the second prior art
Although taylor or fourier series expansion may be used to analyze the frequency domain echo signals and extract linear coefficients of variation (first order coefficients) therefrom. However, since the atmospheric transmission characteristics within the frequency band are highly likely to have components that vary linearly with frequency, the extracted first-order coefficients are still affected by the atmospheric radiation transmission characteristics. If a plurality of scattering centers exist, the echo signals in the frequency domain are superimposed by the echoes of the scattering centers, and the accurate extraction of the first-order coefficients is more difficult. Furthermore, the method does not take into account the effect of the phase offset.
3.1 technical solution of the third prior art
Inspired by the second technical scheme, the echo signals can be analyzed by adopting Taylor or Fourier series expansion, and as the components above the second order are all generated by atmospheric transmission, the atmospheric extinction coefficient and the phase offset coefficient can be respectively fitted by accumulating the components above the first order, so that the atmospheric transmission waveform distortion compensation can be realized.
3.2 disadvantages of the third prior art
As can be seen from the theoretical calculation result, there is no fixed period between the atmospheric extinction coefficient and the scattering coefficient according to the frequency, so that theoretically, an infinite number of stages are needed to realize the atmospheric transmission distortion compensation. When finite terms are used, a periodic signal is introduced in the amplitude and phase compensation, so that a periodic residual exists.
Disclosure of Invention
Aiming at the characteristics of the three classification schemes, the invention provides an atmospheric parameter estimation and transmission distortion compensation method based on waveform matching for compensating the atmospheric transmission distortion of terahertz radar signals. Firstly, utilizing the frequency domain radar echo data to generate a one-dimensional range profile, and distinguishing an independent scattering center. Then selecting any scattering center, and simulating one-dimensional distance images of the independent scattering centers under different atmospheric conditions by using distance information and a terahertz frequency band atmospheric transmission characteristic information database. And then, matching the one-dimensional distance image of the extracted independent scattering center with a simulation result, and selecting the best matching result. The parameters such as air pressure and air temperature corresponding to the best matching result are estimated atmospheric condition parameters, and the corresponding atmospheric extinction coefficient and phase offset coefficient can be used for atmospheric compensation. And finally, dividing the imaging area into a plurality of parts according to the distance, respectively performing fast Fourier transform, and completing atmospheric transmission compensation in a frequency domain.
The invention adopts the technical scheme that: an atmospheric parameter estimation and transmission distortion compensation method based on waveform matching comprises the following steps:
step 1: acquiring a terahertz radar echo signal transmitted by the atmosphere, and generating an actual one-dimensional range profile;
step 2: identifying independent scattering centers in the one-dimensional range profile;
step 3: simulating one-dimensional distance images of the independent scattering centers under different atmospheric conditions by using a terahertz frequency band atmospheric transmission characteristic information database;
step 4: extracting an actual one-dimensional distance image near the independent scattering center, and comparing the actual one-dimensional distance image with the simulated one-dimensional distance image;
step 5: selecting atmospheric parameters corresponding to the best-matching one-dimensional range profile;
step 6: calculating estimated atmospheric transmittance and phase offset according to the best matching atmospheric parameters;
step 7: and compensating the terahertz radar echo signals transmitted by the atmosphere to obtain a compensated one-dimensional range profile.
Further, the step 2 specifically includes:
considering that an actual radar pulse is a band-limited signal, a side lobe in a one-dimensional range profile is determined as a scattering center by local peak search, so that an independent scattering center is identified by adopting a peak density clustering method, and the local density of a certain scattering center is defined as follows:
wherein ρ is k Is the local density of scattering centers k; d, d jk Is the distance between scattering center j and scattering center k; d, d c Is a distance threshold; χ is a decision function, have certaintyMeaning type:
the local densities of the scattering centers are arranged from large to small. Defining a minimum distance for a scattering center as:
δ k ＝min(d jk ),k∈ρ j ＞ρ k (6)
in delta k The minimum value of the distance from the pixel k to the pixel higher than the local density of the pixel k; min is the minimum value sign, if the distance is the minimum value delta k If the distance is smaller than the distance threshold, the scattering center is considered to be a side lobe;
local density ρ k Smaller and at a minimum distance delta k The larger scattering center is determined to be an independent scattering center, and the distance is acquired through a one-dimensional distance image.
Further, the step 3 specifically includes:
echo signals of the independent scattering centers and under different atmospheric transmission conditions are generated by using the following steps:
wherein E represents a radar echo signal; f (f) v The frequency of the v-th stepping pulse signal; a is scattering center intensity; l is the distance of the scattering center; c is the speed of light; i is an imaginary symbol; alpha and beta are respectively atmospheric absorption and phase offset coefficients, and are obtained by inquiring a terahertz frequency band atmospheric transmission characteristic information database;
generating a one-dimensional range profile of the signal (7) by using inverse fast Fourier transform, and evaluating the similarity between the one-dimensional range profile and an actual one-dimensional range profile by using a correlation coefficient r:
wherein E is x And E is connected with y Respectively interceptedActual and simulated one-dimensional distance images; cov (E) x ,E y ) Covariance of both; var (E) x ) With Var (E) y ) For their respective variances.
Further, the step 7 specifically includes:
the atmospheric parameter with the largest correlation coefficient is selected to calculate the atmospheric extinction coefficient and the phase offset coefficient, the one-bit range profile is divided into a plurality of parts, and after the parts are subjected to fast Fourier transform, the echo signals are compensated in the frequency domain by using the following steps:
E′ M (f v )＝E M (f v )·exp[α′(f v )l′]·exp[-iβ′(f v )l′] (9)
wherein E 'is' M Representing the compensated radar echo signal; e (E) M Representing an original radar echo signal; f (f) v The frequency of the v-th stepping pulse signal; alpha and beta are estimated atmospheric extinction coefficients and phase offset coefficients; l' is the reference distance of the imaging region;
and (3) performing inverse fast Fourier transform on the echo signals after compensation by using the method (9), and splicing all the parts to obtain the compensated one-dimensional range profile.
Compared with the existing terahertz radar signal atmospheric transmission compensation method, the atmospheric parameter estimation and transmission distortion compensation method based on waveform matching provided by the invention has the following advantages:
(1) The method of the invention has universality. The calibration body adopted in radar measurement can be regarded as an independent scattering center, and further the method is adopted to compensate echo signals, so that the method is not limited by the atmospheric conditions in measurement, and all atmospheric parameters do not need to be known;
(2) The method can realize the inversion of the atmospheric parameters. The atmospheric condition parameters can be estimated by comparing the actual measurement with the simulated one-dimensional range profile, and the atmospheric parameter inversion is realized by optimizing the atmospheric condition parameter combination.
Drawings
FIG. 1 is a flow chart of a terahertz radar signal atmospheric transmission distortion compensation method based on database matching;
FIG. 2 is a one-dimensional range profile of a single scattering center at a distance of 161m in the 460-490GHz band;
fig. 3 is a one-dimensional range profile obtained by taking atmospheric transmission characteristics into consideration in the 460-490GHz frequency band. Fig. 3 (a) is a region from 160.9 to 161.1m distance, containing 1 scattering center. Fig. 3 (b) is a region from 165.9-166.1m, containing 3 scattering centers. FIG. 3 (c) is a region of distance 166.8-167m, containing 1 scattering center;
fig. 4 is a one-dimensional range profile (linear coordinates) obtained by simulating a 161.0037m scattering center using a terahertz atmospheric transmission characteristic database. FIG. 4 (a) shows an air pressure of 1013mb, an air temperature of 281.14K, and a water vapor density of 4.968g/m 3 The results obtained. FIG. 4 (b) shows an air pressure of 1013mb, an air temperature of 283.14K and a water vapor density of 5.61g/m 3 The result obtained;
FIG. 5 shows the calculated attenuation coefficient and phase shift coefficient for estimating the atmospheric parameters;
FIG. 6 is a one-dimensional range profile after 460-490GHz atmospheric transmission compensation. Fig. 6 (a) is a region with a distance of 160.9-161.1 m. Fig. 6 (b) is a region of distance 165.9-166.1 m. FIG. 6 (c) shows the region of distance 166.8-167 m.
Detailed Description
The invention is further described below with reference to the drawings and detailed description.
The block diagram of the terahertz radar signal atmospheric transmission distortion compensation method based on database matching is shown in fig. 1, and the basic technical principle is as follows.
Step 1: and generating an actual one-dimensional distance image.
Assuming that the radar emits a step frequency signal, if there are M scattering centers along the radial distance, the radar echo model can be expressed as:
wherein E is M Representing radar echo signals; f (f) v The frequency of the v-th stepping pulse signal; a is scattering center intensity; l (L) k Distance from the kth scattering center; c is the speed of light; i is an imaginary symbol; alpha is the atmospheric absorption coefficient; beta is the phase offset coefficient.
A pair of discrete fourier transform relations (see documents [6] Xu Xiaojian, huang Peikang ] radar system and information processing [ M ] beijing: electronic industry press. 2010.) are formed between the target echo changing with the step frequency and the one-dimensional range profile of the target, so that the distribution of the scattering center of the target on each range cell can be obtained by one-dimensional inverse fourier transform:
wherein N is the number of frequency steps; the meaning of the other symbols is the same as that of formula (2).
In actual data processing, an inverse fast fourier transform is typically used to generate a one-dimensional range profile and zero values are added during processing to ensure that the range profile is smooth.
Step 2: and identifying independent scattering centers.
The scattering center appears as a local peak, and thus the scattering center can be obtained by a local peak search method. Considering that the actual radar pulse is a band-limited signal, local peak search will determine the side lobes in the one-dimensional range profile as scattering centers. Therefore, the independent scattering centers are identified by adopting a peak density clustering method.
Defining the local density of a scattering center as:
wherein ρ is k Is the local density of scattering centers k; d, d jk Is the distance between scattering center j and scattering center k; d, d c Is a distance threshold; χ is a decision function, having the formula:
the local densities of the scattering centers are arranged from large to small. Defining a minimum distance for a scattering center as:
δ k ＝min(d jk ),k∈ρ j ＞ρ k (6)
in delta k The minimum value of the distance from the pixel k to the pixel higher than the local density of the pixel k; min is the minimum sign. If the distance is the minimum value delta k If the distance is smaller than the distance threshold, the scattering center is considered as a side lobe.
Local density ρ k Smaller and at a minimum distance delta k The larger scattering center is determined to be an independent scattering center, and the distance is acquired through a one-dimensional distance image.
Step 3: and simulating one-dimensional distance images of the independent scattering centers under different atmospheric conditions.
Selecting any scattering center, and then rewriting formula (2) into
Wherein E represents a radar echo signal; f (f) v The frequency of the v-th stepping pulse signal; a is scattering center intensity; l is the distance of the scattering center; c is the speed of light; i is an imaginary symbol; alpha and beta are respectively atmospheric absorption and phase offset coefficients, and are obtained by inquiring a terahertz frequency band atmospheric transmission characteristic information database.
The one-dimensional range profile of the independent scattering center under different atmospheric conditions can be simulated by inverse fast fourier transform.
Step 4: and extracting a range profile near the independent scattering center and comparing the range profile with the simulation range profile.
Taking the scattering center positions selected in the step 3 as references, intercepting actual and simulated one-dimensional distance images with the same length, and evaluating the similarity of the two images through a correlation coefficient r:
wherein E is x And E is connected with y The intercepted actual and simulated one-dimensional distance images are respectively; cov (E) x ,E y ) For the co-ordination of the twoVariance; var (E) x ) With Var (E) y ) For their respective variances.
Step 5: and selecting the atmospheric parameters corresponding to the best-matching one-dimensional range profile.
And calculating the correlation coefficient between each group of simulation results in the terahertz frequency band atmospheric transmission characteristic information database and the actual one-dimensional range profile by using the formula (8), and selecting the atmospheric parameter with the largest correlation coefficient, and the corresponding atmospheric extinction coefficient and phase offset coefficient. By refining the values of the variables in the atmospheric transmission characteristic information database, the accuracy of atmospheric parameter estimation can be improved.
Step 6: an estimated atmospheric transmittance and phase offset are calculated.
As can be seen from equation (2), the radar echo signal is a superposition of the scattering centers, and the atmospheric transmission characteristics are related to the distance between the scattering centers, so it is difficult to realize atmospheric transmission compensation for each scattering center in the frequency domain. Thus, the reference distance of the imaging region can be used, or the non-blurring window can be divided into several segments in the time domain (one-dimensional range profile), and the average distance of each segment can be used to calculate the atmospheric transmittance and the phase shift.
Step 7: compensating the radar echo signal.
The radar echo signal compensation can be realized by using the estimated atmospheric extinction coefficient, the estimated phase offset coefficient and the estimated reference distance. Referring to formula (2), there is an atmospheric compensation calculation formula:
E′ M (f v )＝E M (f v )·exp[α′(f v )l′]·exp[-iβ′(f v )l′] (9)
wherein E 'is' M Representing the compensated radar echo signal; e (E) M Representing an original radar echo signal; f (f) v The frequency of the v-th stepping pulse signal; alpha and beta are estimated atmospheric extinction coefficients and phase offset coefficients; l' is the reference distance of the imaging region.
Or dividing a one-bit range profile into a plurality of parts, performing fast Fourier transform on each part, compensating echo signals in a frequency domain, and then performing inverse fast Fourier transform to splice data of each part. Compensation of radar echo signals can also be achieved.
Examples
The present invention will be further described below using simulation data. The simulated signal is a 460-490GHz step frequency pulse train with pulse frequency increments of 12.5MHz (distance window size of 12 m). Assuming that no distance aliasing occurs, 5 scattering centers of equal intensity are set at 161m, 166m, 166.02m, 166.03m, and 166.9m positions, respectively. The transmission path is set at the position of 280m of the standard atmospheric altitude of US76, the corresponding air pressure is about 1005mb, the air temperature is about 287.6K, and the water vapor content is about 5.78g/m 3 。
Step 1: and generating an actual one-dimensional distance image.
Fig. 3 shows that a one-dimensional range profile is obtained by taking into account the simulation of the atmospheric transmission characteristics, without windowing to suppress side lobes. Fig. 3 (a), 3 (b) and 3 (c) are enlarged one-dimensional distance images of three regions, respectively. As can be seen from fig. 3 (a) and fig. 3 (c), the atmospheric transmission causes the scattering center to shift in position, and the side lobe increases; as can be seen from fig. 3 (b), when several scattering centers are closer, the sidelobe is raised due to atmospheric transmission, and a false scattering center appears in the one-dimensional range profile.
Step 2: and identifying independent scattering centers.
The length of the wave data can be calculated to be 2402 sampling points according to the bandwidth and the frequency increment. To ensure one-dimensional range profile quality, an inverse fast fourier transform was performed using 32772 samples, each corresponding to a distance of approximately 0.36mm (distance window length divided by number of samples). Setting a distance threshold d c At 15 times the oversampling rate, i.e., 195 sampling points. The final scattering centers are divided into three groups, wherein the distances of the two independent scattering centers are 161.0037m and 166.0033m, respectively.
Step 3: and simulating one-dimensional distance images of the independent scattering centers under different atmospheric conditions.
And selecting 161.0037m scattering centers to simulate one-dimensional range profile. Fig. 4 shows two sets of one-dimensional range profiles (linear coordinates) of 161.0037m scattering centers obtained by simulation using a terahertz atmospheric transmission characteristic database. Wherein FIG. 4 (a) shows an air pressure of 1013mb, an air temperature of 281.14K, and a water vapor density of 4.968g/m 3 The resulting junctionFruit; FIG. 4 (b) shows an air pressure of 1013mb, an air temperature of 283.14K and a water vapor density of 5.61g/m 3 The results obtained.
Step 4: and extracting a range profile near the independent scattering center and comparing the range profile with the simulation range profile.
130 sampling points are selected by taking the scattering center as a midpoint, and the correlation coefficient between the actual one-dimensional distance image and the simulated one-dimensional distance image is calculated. Fig. 4 shows two sets of matching processes, wherein the correlation coefficients of fig. 4 (a) and fig. 4 (b) are 0.9990 and 0.9999, respectively. FIG. 4 (b) shows the best results obtained from the terahertz band atmospheric transfer characteristics database, so that the estimated atmospheric condition parameters are atmospheric pressure 1013mb, air temperature 283.14K, and water vapor density 5.61g/m 3 With the set atmospheric conditions (air pressure 1005mb, air temperature 287.6K, water vapor content 5.78 g/m) 3 ) There is a difference.
Step 5: an estimated atmospheric transmittance and phase offset are calculated.
The atmospheric attenuation coefficient and the phase shift coefficient obtained by calculating the estimated atmospheric condition parameter are shown in fig. 5. The 12m distance window is divided into 60 parts in 0.2m units, and the atmospheric transmittance and the phase shift are calculated for each part, and then the echo signal is compensated by the equation (9).
Step 6: compensating the radar echo signal.
Fig. 6 shows a one-dimensional range profile after atmospheric transmission compensation, in which fig. 6 (a), 6 (b) and 6 (c) are respectively enlarged one-dimensional range profiles of three regions. As can be seen, the atmospheric transmission compensation compensates for waveform distortion and transmission attenuation, and the side lobe drop in fig. 6 (b) makes it easier to identify the true scattering center.
The invention can also adopt alternative schemes to accomplish the aim of the invention: compensating atmospheric transmission distortion of the linear frequency modulation waveform radar signal; in identifying the individual scattering centers (step 2), spectral estimation or other clustering methods may be employed; when the one-dimensional distance image of the independent scattering center under different atmospheric conditions is simulated (step 3), a part of atmospheric parameters are obtained through measurement, and then the other atmospheric parameters are utilized to establish an atmospheric transmission database simulation one-dimensional distance image; when the one-dimensional distance images are matched (step 4 and step 5), the similarity of the two curves is judged by utilizing other criteria such as Euclidean distance and the like; the frequency domain atmospheric transmission compensation can be realized by adopting time domain-frequency domain transformation methods such as Fourier transformation, discrete Fourier transformation and the like.
Claims (4)
1. A terahertz radar signal atmospheric transmission distortion compensation method based on database matching is characterized by comprising the following steps of: the method comprises the following steps:
step 1: acquiring a terahertz radar echo signal transmitted by the atmosphere, and generating an actual one-dimensional range profile;
step 2: identifying independent scattering centers in the one-dimensional range profile;
step 3: simulating one-dimensional distance images of the independent scattering centers under different atmospheric conditions by using a terahertz frequency band atmospheric transmission characteristic information database;
step 4: extracting an actual one-dimensional distance image near the independent scattering center, and comparing the actual one-dimensional distance image with the simulated one-dimensional distance image;
step 5: selecting atmospheric parameters corresponding to the best-matching one-dimensional range profile;
step 6: calculating estimated atmospheric transmittance and phase offset according to the best matching atmospheric parameters;
step 7: and compensating the terahertz radar echo signals transmitted by the atmosphere to obtain a compensated one-dimensional range profile.
2. The method for compensating the atmospheric transmission distortion of the terahertz radar signal based on database matching according to claim 1, wherein the method comprises the following steps: the step 2 specifically includes:
considering that an actual radar pulse is a band-limited signal, a side lobe in a one-dimensional range profile is determined as a scattering center by local peak search, so that an independent scattering center is identified by adopting a peak density clustering method, and the local density of a certain scattering center is defined as follows:
wherein ρ is k Is the local density of scattering centers k; d, d jk Is the distance between scattering center j and scattering center k; d, d c Is a distance threshold; χ is a decision function, having the formula:
arranging the local densities of all scattering centers from large to small, and defining the minimum distance value of a certain scattering center as follows:
δ k ＝min(d jk ),k∈ρ j ＞ρ k (6)
in delta k The minimum value of the distance from the pixel k to the pixel higher than the local density of the pixel k; min is the minimum value sign, if the distance is the minimum value delta k If the distance is smaller than the distance threshold, the scattering center is considered to be a side lobe;
local density ρ k Smaller and at a minimum distance delta k The larger scattering center is determined to be an independent scattering center, and the distance is acquired through a one-dimensional distance image.
3. The method for compensating the atmospheric transmission distortion of the terahertz radar signal based on database matching according to claim 1, wherein the method comprises the following steps: the step 3 specifically includes:
echo signals of the independent scattering centers and under different atmospheric transmission conditions are generated by using the following steps:
wherein E represents a radar echo signal; f (f) v The frequency of the v-th stepping pulse signal; a is scattering center intensity; l is the distance of the scattering center; c is the speed of light; i is an imaginary symbol; alpha and beta are respectively atmospheric absorption and phase offset coefficients, and are obtained by inquiring a terahertz frequency band atmospheric transmission characteristic information database;
generating a one-dimensional range profile of the signal (7) by using inverse fast Fourier transform, and evaluating the similarity between the one-dimensional range profile and an actual one-dimensional range profile by using a correlation coefficient r:
wherein E is x And E is connected with y The intercepted actual and simulated one-dimensional distance images are respectively; cov (E) x ,E y ) Covariance of both; var (E) x ) With Var (E) y ) For their respective variances.
4. The method for compensating the atmospheric transmission distortion of the terahertz radar signal based on database matching according to claim 1, wherein the method comprises the following steps: the step 7 specifically includes:
the atmospheric parameter with the largest correlation coefficient is selected to calculate the atmospheric extinction coefficient and the phase offset coefficient, the one-bit range profile is divided into a plurality of parts, and after the parts are subjected to fast Fourier transform, the echo signals are compensated in the frequency domain by using the following steps:
E′ M (f v )＝E M (f v )·exp[α′(f v )l′]·exp[-iβ′(f v )l′] (9)
wherein E 'is' M Representing the compensated radar echo signal; e (E) M Representing an original radar echo signal; f (f) v The frequency of the v-th stepping pulse signal; alpha and beta are estimated atmospheric extinction coefficients and phase offset coefficients; l' is the reference distance of the imaging region;
and (3) performing inverse fast Fourier transform on the echo signals after compensation by using the method (9), and splicing all the parts to obtain the compensated one-dimensional range profile.
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