CN106054157A - Digital Dechirp wideband phased array radar Keystone transform algorithm - Google Patents

Abstract

The invention discloses a digital Dechirp wideband phased array radar digital beam forming technology based on Keystone transform, so as to solve the problems of a large computation amount and aperture fill exiting when multiple beams are formed in a wideband digital phased array radar wide angle range at the same time. Firstly, digital Dechirp mixing and extraction are carried out on wideband intermediate frequency digital echo signals of a single array element, and the signal bandwidth is greatly reduced; then, array element-level Keystone transform is adopted, echoes of different angles of all array elements are moved to a No.0 array element target echo reference position, broadband DBF computation amount is delicately reduced to the magnitude of narrowband DBF, and the range high resolution performance of the broadband DBF is kept at the same time. Compared with the traditional method, the algorithm of the invention has the advantages that the computation amount is lower, and engineering implementation is facilitated.

Description

Keystone transformation algorithm of digital Dechirp broadband phased array radar

Technical Field

The invention belongs to the field of radar signal processing, and particularly relates to a digital Dechirp broadband phased array radar Keystone transformation algorithm which can compensate the problem of aperture transit under wide-angle scanning of a broadband digital phased array radar.

Background

The time difference of the radar target echo wave front reaching the two ends of the phased array antenna array is called aperture transit time, the corresponding distance is called aperture transit distance, and the larger the deviation angle between the target and the antenna normal is, the longer the aperture transit time/aperture transit distance is. If the radar adopts a broadband signal, the aperture transit distance is larger than a plurality of distance resolution units, and at the moment, if the phased array radar adopts the traditional narrow-band digital beam forming technology, the signal-to-noise ratio is reduced, the beam side lobe is raised, the pointing accuracy is poor, and the distance image resolution capability is reduced.

Aiming at the problems of the broadband phased array radar, the current solutions are divided into the following four methods:

analog delay line method: the analog delay line mainly utilizes the time consumed by the transmission of electromagnetic waves in a microwave transmission line to realize the delay of signals, the delay amount is often quantized to be integral multiples of the period of a central frequency signal, and the analog delay line has the following problems: the device has the advantages of large volume, high cost, low quantization precision, high system complexity, sensitive temperature and the like, and is not used basically at present.

Digital delay line method: after a broadband intermediate frequency analog signal AD is sampled, the delay of the signal is realized through digital down-conversion and a digital delay filter; the digital delay line has flexible delay function, can be directly used in combination with a digital beam forming network to form flexible digital beams, is convenient for subsequent digital signal processing, has the advantages of small volume, small environmental influence, high precision and flexible change of delay quantity, and has the defects that if a system needs to generate simultaneous multi-beams, corresponding delay quantity needs to be generated for each beam direction, and the complexity is greatly improved;

the analog-digital hybrid method generally adopts an analog frequency modulation local oscillation technology to convert a broadband analog signal into a narrowband analog signal, and then performs distance delay compensation processing in a digital domain through AD sampling, wherein the digital delay compensation method has the above disadvantages.

Keystone method: the method comprises the steps of firstly carrying out fast time dimension FFT on broadband intermediate frequency echoes received by each array element, converting the broadband intermediate frequency echoes into a frequency domain, then carrying out Keystone conversion among the array elements, and finally carrying out airspace digital beam forming.

The invention firstly generates digital frequency modulation local oscillator near the distance of the target to realize digital frequency mixing, then aligns target echo envelopes of different distances, namely Dechirp processing, and finally carries out Keystone conversion among array elements to realize the alignment of the echoes of different incidence angles to reference distance envelopes, converts broadband digital beam forming into a narrow-band digital beam forming method, greatly reduces the complexity of system design in simultaneous multi-beam application, reduces beam side lobe level, improves the space coherent gain of the system, and improves the angle resolution and angle measurement precision, distance resolution and distance measurement precision of the system.

Disclosure of Invention

Technical problem to be solved

The invention provides a digital Dechirp broadband phased array radar Keystone transformation algorithm, aiming at solving the problems of large operation amount and aperture transition caused by the fact that multiple beams are simultaneously formed in a wide angle range of a broadband digital phased array radar. Firstly, digital Decirp mixing and extraction are carried out on a single array element broadband intermediate frequency digital echo signal, so that the signal bandwidth is greatly reduced; and then array element level Keystone conversion is adopted to move echoes of all array elements at different angles to a No. 0 array element target echo reference position, so that the calculation amount of the broadband DBF is skillfully reduced to the magnitude of the narrowband DBF, and the distance high-resolution performance of the broadband DBF is kept.

Technical scheme

A digital Dechirp broadband phased array radar Keystone transformation algorithm is characterized by comprising the following steps:

step 1: intermediate frequency digital echo obtained by single PRT of mth array element of phased array radarAnd complex intermediate frequency digital linear frequency modulation local oscillator signalAre mixed to obtainWherein tau is_refIs a reference distance R_refCorresponding delay, f₀At the center frequency of the intermediate frequency, f_cIs the radio frequency center frequency, mu is the chirp rate, R_{△_m}For the m-th array element target distance R_TDistance R from reference_refDifference of difference, T_pIs the chirp width, t is the fast time, c is the speed of light, τ_{T_m}Target echo received for mth array elementDelay, T_refThe local oscillation time is digital;

step 2: will s_{b_m}(t, m) decimating a reservation every K-1 sample points by an anti-aliasing FIR filter, saidTo s_{b_m}(t, m) performing fast time dimension digital filtering extraction to obtain new baseband signalWhere n is the fast time dimension, t_sThe' is the sampling rate after extraction, and B is the bandwidth of the linear frequency modulation signal;

and step 3: to s_b(nt_s', m) performing array element dimension Keystone interpolation, and obtaining the interpolation by adopting a sinc functionWherein,is a virtual array element position;

and 4, step 4: computing multiple beamsWherein,θ_ii-1, I is the number of beams that need to be formed;

and 5: FFT is carried out on each beam in a fast time dimension to complete pulse pressure, different window functions are selected according to requirements, and a high-resolution one-dimensional range profile s of a target is obtained_{hrrp_i}(n)。

The interpolation in step 3 can also adopt DFT + FFT or CZT algorithm.

Advantageous effects

The digital Dechirp broadband phased array radar Keystone transform algorithm provided by the invention fully utilizes characteristics of broadband linear frequency modulation signals and broadband phased array radar echoes, and realizes aperture transition compensation through time domain and space domain processing. The time domain broadband signal is changed into a narrow band signal by a digital Dechirp method, so that the signal bandwidth and the baseband sampling rate are greatly reduced; by Keystone conversion, the airspace broadband DBF is changed into the narrow-band DBF, the operation amount can be greatly reduced in the application of simultaneous multi-beam, and engineering application is facilitated.

According to the algorithm, digital Dechirp frequency mixing is carried out on broadband echo signals, then data rate is greatly reduced through extraction, then echoes in different directions of all array elements are moved to the reference position of the No. 0 array element target echo through an array element-level Keystone conversion method, broadband digital beam forming is ingeniously changed into narrow-band digital beam forming, and space coherent accumulation gain, angle/distance resolution and angle/distance precision are effectively improved.

Drawings

Fig. 1 shows that the Dechirp processing and extraction are completed by the intermediate frequency digital linear frequency modulation local oscillator, and the fast time high sampling rate is changed into the low sampling rate: (a) a single-array-element digital linear frequency modulation local oscillation block diagram; (b) echo spectrum before mixing; (c) echo spectrum after frequency mixing; (d) digitally extracting the echo spectrum;

fig. 2 is an array element dimension interpolation process, which completes the Keystone transformation, aligns the wave in different directions of the space to the 0 th array element, and realizes the aperture transit compensation: (a) array element dimension Keystone transformation block diagram; (b) a data distribution diagram before and after array element dimension Keystone transformation interpolation;

fig. 3 is a digital multi-beam implementation block diagram;

FIG. 4 is a diagram illustrating the completion of pulse compression of each beam and the final acquisition of an array element-distance plane;

FIG. 5 is a diagram of the overall signal processing of the present invention

Detailed Description

The invention will now be further described with reference to the following examples and drawings:

firstly, carrying out digital frequency modulation and mixing on the single pulse repetition period digital baseband echo of each array element to obtain a difference frequency echo; secondly, array element dimension Keystone interpolation is carried out to realize transit compensation of the airspace aperture; and performing digital beam forming again to generate a beam with a desired direction, and finally performing weighted FFT (fast Fourier transform), namely pulse compression, on the desired beam to obtain a beam distance plane.

The input signal of the convention algorithm is a digital baseband signal of an array element channel after AD sampling. The main labels are: the dimension of the array elements is recorded as M, and the number of the array elements is M; the fast time dimension is recorded as N, the total number N; the beam dimension is I and the total number of beams is denoted as I.

The invention has the following implementation steps:

step 1, intermediate frequency linear frequency modulation local oscillation digital frequency mixing, referring to fig. 1(a), fig. 1(b) and fig. 1 (c).

1a) Parameter design: the radar operating signal baseband waveform is represented as:t is fast time, chirp slopeB is the bandwidth of the chirp signal, T_pFor chirp width, design sampling rate f_sWherein f is_s<B, sampling interval of

The target echo received by the mth array element is delayed toTarget intermediate frequency echo isM is 0: M-1, M is the number of array elements, f_cIs the carrier frequency. An intermediate frequency digital chirp local oscillator signal is generated as a reference,τ_refis a reference distance R_refCorresponding delay, selection of reference distance independent of array element, T_refThe digital local oscillator time width. See fig. 1 (a).

Mixing the intermediate frequency digital echo with a complex digital frequency modulation local oscillator, wherein the mixing result is as follows:wherein R is_{△_m}For the m-th array element target distance R_TDistance R from reference_refDifference of difference, T_pFor chirp width, T is required_ref>T_pIn the normal case T_ref≈T_pThe third quadratic phase term can be ignored, at this timeSee fig. 1 (b).

The signal bandwidth is changed from the original mu T_pDecrease to mu (T)_ref-T_p) The analysis bandwidth is greatly reduced and therefore a fast time dimension filtering decimation can be performed, see fig. 1 (c).

1b) To s_{b_m}(t, m) performing fast time dimension digital filtering decimation, see FIG. 1(d)

The extraction rate selection method comprises the following steps:to s_{b_m}(t, m) performing fast time dimension digital filtering extraction to obtain new baseband signal s_b(t, m), where t is nt_s'，t_s' is the sample rate after decimation.

In order to improve the signal-to-noise ratio, an anti-aliasing FIR filter must be designed before extraction, the filter characteristic is a linear phase characteristic, and the passband bandwidth is larger than mu (T)_ref-T_p) The in-band fluctuation is less than 0.2dB, and the stop band rejection is more than 50 dB. The anti-aliasing FIR filter can be optimally designed through an MATLAB own fdatool tool box. The specific implementation method of the filtering extraction comprises the following steps: will s_{b_m}(t, m) through the anti-aliasing FIR filter, then extracting and reserving every K-1 sampling points, namely realizing the fast time dimension digital filtering extraction and obtaining a new baseband signal s_b(t,m)。

The echo signal after time-dimensional digital filtering and extraction is represented as:for simplicity, s is_b(nt_s', m) is denoted as s_b(n,m)。

And 2, performing array element dimension Keystone transformation, and referring to fig. 2(a) and fig. 2 (b).

To array element echo signal s_b(n, m), array element dimension Keystone interpolation processing is carried out. For the uniform linear array, assuming that the unit pitch is d, the incident angle is θ, the number of array elements is M, and taking the 0 th array element as a reference, the wave path difference of the M-th array element is: r_mD · M · sin θ, M ranges from 0,1 …, M-1, R is substituted_mReplacement of R_{△_m}Then the echo becomesDefining the positions of the virtual array elements as follows:namely, the original (n-m) plane is changed into the (n-k) plane, namely, the signal sampling is changed from the original rectangular format point into the trapezoidal format point, and a new data plane s is obtained_keystone(N, k), the dimension is constant, still N × M, and the interpolation can be performed by a sinc function, i.e.It can also be implemented by DFT + FFT or CZT algorithms.

Step 3, digital beam forming, see fig. 3.

3a) Steering vector calculation

Knowing the desired signal angle of incidence θ_iI-1, I is the number of beams to be formed and the array element spacing wavelength ratioDesigning the desired signal steering vector toDimension M × I.

3b) Simultaneous multi-beam forming

ComputingAnd obtaining a frequency domain result of the expected signal beam, wherein the dimension is N × I, and if the number of the beam is equivalent to that of the input array elements, FFT can be carried out between the array elements to realize space domain multi-beam.

And 4, compressing the frequency domain pulse, and referring to fig. 4.

Then FFT is carried out on the fast time dimension of each wave beam to finish pulse pressure, different window functions are selected according to requirements to obtain a high-resolution one-dimensional range profile s of the target_{hrrp_i}(n)。

Claims (2)

1. A digital Dechirp broadband phased array radar Keystone transformation algorithm is characterized by comprising the following steps:

step 1: intermediate frequency digital echo obtained by single PRT of mth array element of phased array radarAnd complex intermediate frequency digital linear frequency modulation local oscillator signalAre mixed to obtainWherein tau is_refIs a reference distance R_refCorresponding delay, f₀At the center frequency of the intermediate frequency, f_cIs the radio frequency center frequency, mu is the chirp rate, R_{△_m}For the m-th array element target distance R_TDistance R from reference_refDifference of difference, T_pIs the chirp width, t is the fast time, c is the speed of light, τ_{T_m}Delay of target echo received for mth array element, T_refThe local oscillation time is digital;

step 2: will s_{b_m}(t, m) decimating a reservation every K-1 sample points by an anti-aliasing FIR filter, saidTo s_{b_m}(t, m) performing fast time dimension digital filtering extraction to obtain new baseband signalWhere n is the fast time dimension, t_sThe' is the sampling rate after extraction, and B is the bandwidth of the linear frequency modulation signal;

and step 3: to s_b(nt_s', m) performing array element dimension Keystone interpolation, and obtaining the interpolation by adopting a sinc functionWherein,is a virtual array element position;

and 4, step 4: computing multiple beamsWherein,θ_ii-1, I is the number of beams that need to be formed;

and 5: FFT is carried out on each beam in a fast time dimension to complete pulse pressure, different window functions are selected according to requirements, and a high-resolution one-dimensional range profile s of a target is obtained_{hrrp_i}(n)。

2. The digital Decirp wideband phased array radar Keystone transform algorithm as claimed in claim 1, wherein the interpolation in step 3 can also adopt DFT + FFT or CZT algorithm.