CN116256718A - High-distance resolution imaging method based on miniaturized VHF radar system - Google Patents
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Abstract
The invention discloses a high-distance resolution imaging method based on a miniaturized VHF radar system, which comprises the following steps: step 1, transmitting Gaussian pulses of a plurality of frequency points by using a transmitter of a miniaturized VHF radar system; step 2, receiving scattered echoes from irregular bodies through the peripheral antennas, and transmitting echo signals to a receiver through an array formed by three groups of receiving antennas; and step 3, finally uploading signals in the receiver to a PC end, reading the original data, and after extracting baseband IQ data, expanding the baseband data by utilizing a high-distance resolution imaging technology to obtain a refined structure of the irregular body. The imaging method disclosed by the invention can carry out fine processing on the structure of the radar detection target, thereby obtaining the fine structure (layering phenomenon and physical structure) of the irregular body in the spatial distribution, improving the spatial resolution of the original imaging from 500m to 50m, and solving the problem of imaging inaccuracy caused by insufficient range resolution of the radar.
Description
Technical Field
The invention belongs to the field of space physics, and particularly relates to a high-range Resolution Imaging (RIM) method based on a miniaturized VHF radar system.
Background
Irregularities in the ionosphere, i.e. various dimensions of ionized "agglomerates" or "wavy" structures floating in the normal ionosphere structure, have a significant difference in electron density from that of the background ionosphere, where the electron density of the structure is higher or lower than the average electron density of the surrounding medium.
Disclosure of Invention
The invention aims to provide a high-range resolution imaging method based on a miniaturized VHF radar system.
The invention adopts the following technical scheme:
in a high range resolution imaging method based on a miniaturized VHF radar system, the improvement comprising the steps of:
step 1, transmitting Gaussian pulses of a plurality of frequency points by using a transmitter of a miniaturized VHF radar system;
step 2, receiving scattered echoes from irregular bodies through the peripheral antennas, and transmitting echo signals to a receiver through an array formed by three groups of receiving antennas;
and step 3, finally uploading signals in the receiver to a PC end, reading the original data, and after extracting baseband IQ data, expanding the baseband data by utilizing a high-distance resolution imaging technology to obtain a refined structure of the irregular body.
Furthermore, the center frequency of the VHF radar is 48.2MHz, the antenna adopts 1-transmit-3-receive-pair peripheral antennas, the peak power of a transmitter is 20Kw, the receiver is a 6-digital-channel receiving system, and the antenna is used for observing E-layer FAI.
Further, in step 1, the set experimental parameters include longitude and latitude coordinates of the station, carrier frequency of the multi-frequency point, bandwidth of the transmitted signal, number of coherent accumulation, pulse repetition frequency, detection distance, code system, sampling point number of the single-frequency point and original distance resolution.
Further, in step 3, the processing of expanding the baseband data by using the high-range resolution imaging technique is specifically:
the original baseband IQ data is read, namely echo scattering data with amplitude and phase, the distance gate data acquired by each frequency under each time resolution is processed, namely the baseband IQ data corresponding to the mth frequency is multiplied by the conjugation of the baseband IQ data of the nth frequency, and then the whole average is carried out to obtain the data R of the mth row and the nth column of the cross-correlation matrix mn Performing the above operation on each frequency to obtain the whole cross-correlation matrix R;
the scan vector e is found as follows:
in the above formula, j represents the jth basic scatterer, k n R represents a threshold distance and T represents a transposition operator for the wave number corresponding to the nth carrier frequency;
and substituting the correlation matrix R and the scanning vector e into the RIM technology based on the Capon algorithm to obtain the brightness distribution function of the subdivided irregular body, namely the processed refined space-time distribution result of the irregular body.
In step 3, the RIM technique has a frequency number of 5-11, a frequency interval of 10kHz-100kHz, a single frequency acquisition number of 256-1500 at a time resolution, and a data code system using 16-bit complementary codes.
The beneficial effects of the invention are as follows:
according to the imaging method disclosed by the invention, the miniaturized VHF radar system is utilized to detect the irregular body, and the structure of a radar detection target can be subjected to fine processing by transmitting pulse signals of a plurality of frequency points, so that the fine structure (layering phenomenon and physical structure) of the irregular body in spatial distribution can be obtained, the spatial resolution of original imaging can be improved from 500m to 50m, and the problem of imaging inaccuracy caused by insufficient range resolution of the radar is solved.
The imaging method disclosed by the invention combines a Capon algorithm and a frequency domain interference technology, scans and refines the irregular body observation of the middle latitude ionosphere E region, ensures that a radar detection result has good distance resolution after eliminating a distance weighting effect, has higher received echo resolution, can effectively observe the middle and small-scale ionosphere irregular body fine structure, obtains more detail information, and can rapidly distinguish the space-time position corresponding to the irregular body from the graph, thereby providing favorable conditions for the research work of the E layer field on the irregular body and laying a technical foundation for the subsequent deeper research on the irregular body.
Drawings
FIG. 1 is a block flow diagram of the disclosed imaging method;
FIG. 2 is a flow chart of a process for data for the disclosed imaging method;
FIG. 3 (a) is a plot of echo power density as a function of time for a single frequency point for 1 minute;
FIG. 3 (b) is a plot of echo power density as a function of 1 minute after high range resolution;
FIG. 4 (a) is a plot of the power density of a single frequency point 1 hour echo;
fig. 4 (b) is a graph showing the echo power density function 1 hour after high range resolution.
Detailed Description
The present invention will be described in further detail with reference to the drawings and examples, in order to make the objects, technical solutions and advantages of the present invention more apparent. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the invention.
Based on the demonstrated miniaturized VHF radar system, the information of the irregular body radiation signal source is obtained through a multi-frequency interference experiment, namely, gaussian pulse signals with similar frequencies are emitted in a shorter time interval to observe the irregular body structure in the same range gate, and the optimal combination of a plurality of frequency signals is utilized, so that the height and thickness of the irregular body layer in the ionized layer can be estimated, and meanwhile, the fine-scale irregular body structure can be reconstructed in the resolution range. According to the invention, under the condition that the detection distance resolution of an original radar system is 500m, a cross-correlation matrix generated by utilizing multi-frequency signal combination is utilized through a high-distance resolution imaging technology, a threshold scanning vector of each distance gate is constructed, and the 500m distance gate is divided into 10 layers through the scanning vector and the cross-correlation matrix, so that an imaging result of 50m distance resolution can be realized.
For range resolution, the spatial resolution of a pulsed radar is determined by the radar resolution size, which in turn is determined by the radar beam width (angle) and the pulse width and receiver bandwidth (range). Due to the limited pulse power of the transmitter, the range resolution of radar detection is limited, which is a serious problem in radar research. The resolution is improved so that the structure of the ionosphere can be finely observed, the basic knowledge of the physical and aerodynamic of the ionosphere can be learned and understood, and the detection and prediction capability can be enhanced.
The standard method of improving the range resolution is to reduce the width of the transmitted pulse, however, the sensitivity of radar detection is also reduced due to the reduced transmission energy contained in one pulse. To reduce the resolution limit, a single thin layer within the same range gate can be observed by transmitting a pair of closely spaced frequencies, which is known as the frequency domain interference algorithm (FDI). On this basis, with an optimal combination of multiple frequency signals, fine-scale atmospheric structures can be reconstructed within the resolution range, a method known in the MST radar field as Range Imaging (RIM) or Frequency Interference Imaging (FII). The basic principles and specific application procedures of the FDI technology and the RIM/FII technology are given below.
Principle of multi-frequency interference distance imaging algorithm:
it is well known that single-station pulsed radar operates at a carrier frequency which makes it impossible to resolve an isolated single thin layer of horizontal turbulence embedded within the scattering volume defined by the pulse length. However, the advent of Frequency Domain Interference (FDI) technology has enabled us to detect such thin layers by transmitting two closely spaced signals. By using the technique, the thickness sigma of the thin layer can be estimated respectively according to the amplitude and the phase (i.e. the coherence) of the normalized complex correlation function obtained by the radar echoes with slightly different working frequencies r And a position r.
First, assume that there is a single primary scatterer at a distance r above the vertically pointing backscatter radar, at two different radar transmitter frequencies ω 1 And omega 2 The phase difference of the coherent detection of the scatterer echoes is 2 Δωr/c, where Δω=ω 1 -ω 2 In radians/second, c is the speed of light in free space. Since Δω is known, r can be accurately determined from the measured phase difference. Due to radar technology limitations, the phase difference measurement may be affected by integer ambiguity, resulting in confusion in the calculation of the distance r, which can be avoided by selecting Δω, since a rough estimate of r can be obtained from the radar detection threshold range. For example, using a 100kHz radar transmitter frequency difference limits the 1.5km thick scatterer detection zone to one lobe of the interference pattern, i.e., the measured radian phase angle varies by only 2π from the bottom to the top of the scatterer. Thus, this technique allows the location of the primary scatterer within the finite scattering volume to be accurately determined.
Generalizing to the real case, the scatterer is at frequency ω 1 The correlation detection signal at this point can be expressed as:
wherein phi is j Is the random phase angle corresponding to a specific basic scatterer, A j Is the corresponding amplitude. Also at frequency omega 2 The correlation detection signal at this point can be expressed as:
wherein r is j And g is a possible gain for the distance value of the jth basic scatterer.
When phi is j In interval [0,2 pi ]]When independently and uniformly distributed, the aggregate is averaged<V 1 >＝<V 2 >=0, but
Wherein represents complex conjugate. The original normalized complex cross-correlation is defined as:
after the gain and amplitude terms are cancelled, it now becomes:
for a reasonable model of a sufficiently thin local scattering layer, r j With mean r and variance
Gaussian distribution, sigma r Representing the scattering layer thickness, the normalized cross-correlation thereof will be reduced to S 12 ＝|S 12 |exp (iΦ), where equation (6) represents the coherence amplitude and equation (7) is the coherence phase angle.
Φ＝2Δωr/c (7)
If the power signal to noise ratio (S/N) of the radar echo is limited, the right side of equation (6) should be divided by 1+ (N/S). When the actual measurement values of the coherence and the phase angle are available, the parameter sigma obtained by the formulas (6) and (7) r And r is a reasonable estimate of the local scattering layer thickness and distance. When the scattering layer thickness is small, the coherence is large and the average layer distance is measured relatively accurately. The statistical uncertainty of the parameter r is given by equation (8):
ε＝(c/2Δω)(2K) -1/2 (|S 12 | 2 -1) 1/2 (8)
where K is the integration length of the cross-correlation estimator, which is significantly smaller when the coherence is close to 1. When the scattering layer is wider or a plurality of scattering layers exist in the scattering body, the unimodal distribution is not real any more, the coherence is smaller, and the position information is not reliable any more. In summary, the FDI cross correlation technique provides a higher vertical resolution when the distance resolution is the least stable, which is ineffective when the scattering layer is dispersed, whereas the usual doppler technique is sufficient for all practical purposes to investigate the layer characteristics.
The normalized cross-correlation analysis described above may be further refined by first performing a discrete fourier transform using two separate time sequences for each frequency. The normalized cross-correlation of the two Doppler frequency sequences gives the normalized cross-spectrum commonly used in radar interferometry data analysis. The doppler sorting achieved in this way helps to resolve the position of different scattering layers with different line-of-sight velocities. Furthermore, the variation of the cross-spectrum phase angle with doppler frequency can in principle provide additional information about the direction of the scattering layer.
Early FDI frequency domain interferometry used only a single thin layer of turbulence based on two closely spaced frequencies to observe within the same range gate. In fact, the result of the FDI mode is ambiguous since only two frequencies are used, and complex atmospheric structures can also produce multiple FDI laminas. To eliminate the effects of the localized assumption of a single gaussian layer in FDI, further resolution improvement, palmer et al developed Range Imaging (RIM) that utilized an optimal combination of multiple frequency signals, allowing fine-scale atmospheric structures to be reconstructed within the resolution range.
RIM and FII techniques were originally introduced into multi-receiver Coherent Radar Imaging (CRI) for determining the observed multiple echo centers in a radar detection area. By a similar process, these algorithms can help us identify multiple ionospheric irregularities in the radar detection area, and then estimate the height and thickness of these irregularities, thus greatly improving the range resolution of the echo profile. By using the information such as layer height, layer thickness, echo distribution and the like, a great amount of ionosphere E region irregularity research can be carried out. In this chapter, we continue to use the VHF radar of the armed han region for radar frequency scanning detection Range Imaging (RIM), with the best combination of multiple adjacent frequency signals, to obtain fine-scale irregular structures in the radar detection area.
RIM/FII radar imaging is a process that utilizes inversion algorithms such as fourier, capon, maximum entropy, etc., to obtain an estimate of the so-called power density or brightness function associated with spatial refractive index fluctuations. The complexity of RIM/FII is the inversion process, in which the Capon method is extremely robust and convenient in the inversion algorithm used by RIM/FII. Without regard to the doppler frequency ordering of echoes, the Capon equation can be expressed simply as:
wherein P (r) is the scattered imaging power at distance r after distance scan (subdivision) processing, i.e. the power density function after RIM imaging, the superscript H and-1 in equation (9) represent Hermite (Hermitian) operator and inverse operator, respectively, and T in equation (11) represents the transposed operator, k 3 For the wave number corresponding to the nth carrier frequency, vector e represents the scan vector of the n-dimensional frequency at the threshold distance r. R is R mn The frequency domain signal non-normalized cross-correlation function calculated for the mth frequency and the nth frequency at zero time delay represents the total set average of the two signals. R is R mn Can be expressed simply as:
wherein,,<·>represents the ensemble average, r m And r n Indicating the detection distance of the scatterer,
and->
Phase terms related to the system response to different transmit frequencies. The phase angle of equation (12) is averaged together and is referred to herein as the FDI phase.
In embodiment 1, the embodiment discloses a high-range resolution imaging method based on a miniaturized VHF radar system, wherein the center frequency of the VHF radar is 48.2MHz, 1-transmission and 3-reception pair-circumference antennas are adopted as antennas, the peak power of a transmitter is 20Kw, a receiver is a 6-digital channel receiving system, and 3 antennas for E-layer FAI observation are adopted. As shown in fig. 1, the method comprises the following steps:
step 1, transmitting Gaussian pulses of a plurality of frequency points by using a transmitter of a miniaturized VHF radar system; the set experimental parameters comprise site longitude and latitude coordinates, carrier frequency of multi-frequency points, transmission signal bandwidth, coherent accumulation times, pulse repetition frequency, detection distance, code system, single-frequency point sampling point number and original distance resolution.
Step 2, receiving scattered echoes from irregular bodies through the peripheral antennas, and transmitting echo signals to a receiver through an array formed by three groups of receiving antennas;
and step 3, finally uploading signals in the receiver to a PC end, reading the original data, and after extracting baseband IQ data, expanding the baseband data by utilizing a high-distance resolution imaging technology to obtain a refined structure of the irregular body.
The base band data expansion processing by using the high-distance resolution imaging technology comprises the following steps:
the original baseband IQ data is read, namely echo scattering data with amplitude and phase, the distance gate data acquired by each frequency under each time resolution is processed, namely the baseband IQ data corresponding to the mth frequency is multiplied by the conjugation of the baseband IQ data of the nth frequency, and then the whole average is carried out to obtain the data R of the mth row and the nth column of the cross-correlation matrix mn Performing the above operation on each frequency to obtain the whole cross-correlation matrix R;
the scan vector e is found as follows:
in the above formula, j represents the jth basic scatterer, k n R represents a threshold distance and T represents a transposition operator for the wave number corresponding to the nth carrier frequency;
the scan vector e is mainly characterized by multiplying the wave numbers corresponding to different frequencies by the layer number distance subdivided in the range gate to achieve the purpose of scanning finer layered data in the original range gate. If the set is 11 frequency points, the original 500m distance gate is layered into 10 layers, and then the scan vector e is an 11-dimensional column vector under each layer in the distance gate.
And substituting the correlation matrix R and the scanning vector e into the RIM technology based on the Capon algorithm to obtain the brightness distribution function of the subdivided irregular body, namely the processed refined space-time distribution result of the irregular body.
The RIM technology has frequency number of 5-11, frequency interval of 10kHz-100kHz, single frequency acquisition number of 256-1500 in time resolution, and 16-bit complementary code.
FIG. 2 is a flow chart of a process for data for the disclosed imaging method.
And performing frequency scanning preliminary exploration by utilizing VHF radars in the Wuhan region. The wuhan VHF radar mainly operates at a center frequency of 48.2MHz, and the carrier frequency can be varied around the center frequency while satisfying the transmission bandwidth. The carrier frequencies are generated by the same oscillator and can be changed from one pulse to another so as to meet the basic requirement of multi-frequency scanning, namely, echoes of different carrier frequencies are received almost simultaneously, and the radar pulse shape can be Gaussian pulse or rectangular pulse for improving the signal-to-noise ratio.
Radar frequency scan tests were performed on ionosphere E using range imaging RIM techniques at 2017, 2-11, 2021, 3, 2022, 6-8. In the current RIM test 11 frequencies between 48.15MHz and 48.25MHz are used, the frequency interval is set to 0.01MHz, the bandwidth of the transmitted signal is 320kHz, the selection and use of the above radar parameters is limited by the capability of the martial arts VHF radar system and for the reason of protecting the radar system. Table 2 lists some of the important radar parameters used in the test. The pulse repetition frequency of these test transmit signals was 525Hz, the sampling time was about 2ms, the number of coherent accumulation was 8, and the Rmn non-normalized value in equation (12) was estimated using 256 data points per carrier frequency, so the time resolution of the signals was about 1 minute.
VHF radar frequency scanning mode observation parameter in Wuhan region
Taking observation data within one minute as an example, 241 x 2816 data corresponding to each frequency of each channel are recorded, 11 groups of data are processed by using a formula (10) on baseband IQ data of a single channel, an R matrix of 11 x 11 in each threshold value is obtained, and an inverse matrix R of the R matrix is calculated -1 . According toFrequency calculation wave number k n The threshold scan for each Δr=500 m is subdivided into 10 layers, i.e. the spacing between adjacent r after subdivision is 50m. Then, a threshold scan vector e and a conjugate transpose matrix e are calculated by using the formula (11) H Substituting the difference into formula (9) to obtain scattered imaging power P (r), and finding the standard deviation sigma of the distance weight function corresponding to the minimum value of the error ER by using formula (13) z The distance weighting effect on P (r) is eliminated using a gaussian distance weighting function.
The final result is shown in fig. 3 (a), 3 (b), 4 (a) and 4 (b), wherein 3 (a) and 4 (a) are echo detection results of single frequency points, 3 (b) and 4 (b) are results processed by a high-distance resolution imaging technology, and it can be obviously found that the echo duration is longer and the coverage range is wider in a multi-frequency scanning mode, the echo signal is more continuous in the distance range, the echo distribution in the single mode is blurred and is in a dispersion state, and the echo in the multi-frequency scanning mode has distinct layers and clear structure. It is not difficult to find that applying multi-frequency scanning imaging to the observation of ionosphere field to irregularities (FAI), after appropriate scanning and refinement, the distance resolution of the echo detection area is improved after the distance weighting effect is eliminated, more fine irregularities can be observed, so that some fine structures of FAI can be resolved, and the space-time position corresponding to FAI echo can be rapidly resolved from the echo power map, which is helpful for researching physical characteristics and motion rules of the ionosphere E irregularities, and more fine researches can be performed on the irregularities.
Claims (5)
1. A high range resolution imaging method based on a miniaturized VHF radar system, comprising the steps of:
step 1, transmitting Gaussian pulses of a plurality of frequency points by using a transmitter of a miniaturized VHF radar system;
step 2, receiving scattered echoes from irregular bodies through the peripheral antennas, and transmitting echo signals to a receiver through an array formed by three groups of receiving antennas;
and step 3, finally uploading signals in the receiver to a PC end, reading the original data, and after extracting baseband IQ data, expanding the baseband data by utilizing a high-distance resolution imaging technology to obtain a refined structure of the irregular body.
2. The high range resolution imaging method based on a miniaturized VHF radar system according to claim 1, characterized in that: the center frequency of the VHF radar is 48.2MHz, the antenna adopts 1-transmit-3-receive-pair peripheral antennas, the peak power of a transmitter is 20Kw, the receiver is a 6-digital-channel receiving system, and the antenna is used for observing E-layer FAI.
3. The high range resolution imaging method based on a miniaturized VHF radar system according to claim 1, characterized in that: in step 1, the set experimental parameters include longitude and latitude coordinates of a station, carrier frequency of multiple frequency points, transmission signal bandwidth, coherent accumulation times, pulse repetition frequency, detection distance, code system, single frequency point sampling point number and original distance resolution.
4. The high range resolution imaging method based on a miniaturized VHF radar system according to claim 1, characterized in that: in step 3, the processing of the baseband data expansion by using the high-distance resolution imaging technology specifically includes:
the original baseband IQ data is read, namely echo scattering data with amplitude and phase, the distance gate data acquired by each frequency under each time resolution is processed, namely the baseband IQ data corresponding to the mth frequency is multiplied by the conjugation of the baseband IQ data of the nth frequency, and then the whole average is carried out to obtain the data P of the mth row and the nth column of the cross-correlation matrix mn Performing the above operation on each frequency to obtain the whole cross-correlation matrix R;
the scan vector e is found as follows:
in the above formula, j represents the firstj basic scatterers, k n R represents a threshold distance and T represents a transposition operator for the wave number corresponding to the nth carrier frequency;
and substituting the correlation matrix R and the scanning vector e into the RIM technology based on the Capon algorithm to obtain the brightness distribution function of the subdivided irregular body, namely the processed refined space-time distribution result of the irregular body.
5. The miniaturized VHF radar system-based high range resolution imaging method according to claim 4, characterized in that: in the step 3, the frequency number selection range of the RIM technology is 5-11, the frequency interval is 10kHz-100kHz, the single frequency acquisition number under the time resolution is 256-1500, and the data code system adopts 16-bit complementary codes.
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