CN103792527A - Method for applying M sequence to phase encoding system imaging radar pulse compression - Google Patents

Abstract

The invention discloses a method for applying an M sequence to phase encoding system imaging radar pulse compression. The method is applied to a laser synthetic aperture radar imaging system. With the method adopted, the problem of imaging quality reduction and existence of virtual images which are caused by excessive sidelobe which is further caused by a situation that a traditional pulse compression system adopts a fast convolution method to perform pulse compression, can be solved. According to the method of the invention, phase codes are the M sequence, and the algorithm of pulse compression is a periodic convolution method. The method is characterized in that a matrix which is used for the pulse compression and is constructed in a manner that the M sequence is prolonged and is subjected to shifting operation so as to form the matrix is used so as to match the design of a filter. With the method adopted, sidelobe suppression can be realized under the premise that the complexity of the algorithm is not increased, and the sidelobe can be suppressed to be 1/P under the condition that the length of code elements is P.

Description

A kind of M sequence is applied to phase encoding system imaging radar impulse compression method

Technical field

The present invention relates to radar imagery technology, be specifically related to a kind of M sequence and be applied to phase encoding system imaging radar impulse compression method.

Background technology

From the theory of laser radar, the in the situation that of power limited, realize long-range detection, wide while requiring large pulse, and require high detection accuracy, and requiring high detective bandwidth, when simple pulse, wide bandwidth product is close to 1, Shi Kuanyu bandwidth is interrelated, wide and bandwidth can not simultaneously increase time.That is to say, to the simple laser pulse of constant wavelength, maximum operating range and range resolution exist contradiction.For solving this contradiction, transmit and must adopt the sophisticated signal form with wide bandwidth when large.

Synthetic Aperture Laser Radar (SAL) system is because operating distance is far away, and resolution requirement is higher, obtain transmitting of wide bandwidth when large, need to use complicated modulation waveform.Apply more modulator approach and have three kinds: linear frequency modulation, nonlinear frequency modulation and phase encoding.Wherein linear FM signal and phase-coded signal have more been used in SAL system.Owing to being easy to realize, recent years research direction mainly concentrates on linear frequency modulation system, but it is high that phase-modulation system has transponder pulse repetition frequency compared to linear frequency modulation system, and modulation system is independent of laser instrument, to not advantages of higher of flying platform stability requirement.What Luo Ke LMT of the U.S. had been successfully completed in 2011 that the airborne experiment of Synthetic Aperture Laser Radar adopts is exactly phase-modulation system.It is the highest once airborne experiment of SAL imaging index in the world at present

Although phase modulation system is easier to Project Realization compared to linear frequency modulation system, but process and exist distance to the pulse compression main lobe secondary lobe shortcoming higher than (PSL) by traditional matched filtering means, the result causing is just brought diplopia, image quality is declined, and the impact bringing is larger along with the expansion apart to imaging scale.

It is to use improved genetic algorithm to find the reasonable coding of autocorrelation that solution is mentioned in USAF laboratory in the paper of issuing for 09 year, by code optimization being reached to the object of Sidelobe Suppression.But effect of optimization is limited, cannot reach perfect condition to the inhibition of secondary lobe.

M sequence is a kind of coding conventional in the communications field, has unique autocorrelation performance: in the situation that the cycle repeats, do auto-correlation and can reach desirable autocorrelation function.That is to say, desirable autocorrelation function is can reach doing Cyclic autocorrelation, is fast convolution algorithm and conventional matched-filter uses.

The Cyclic autocorrelation character of M sequence can be explained like this:

$R\left(m\right)=\underset{j=1}{\overset{P}{\mathrm{\Σ}}}x{\left(j\right)}^{*}x(j-m)=\left\{\begin{array}{c}1m\≠1\\ \mathrm{Pm}=1\end{array}\right.;---\left(1\right)$

Wherein x (n) is the periodic sequence with M sequence length P circulation.

Summary of the invention

The present invention, on the existing SAL imaging algorithm based on phase encoding system, has proposed a kind of new matched filtering method based on M sequence and has realized apart to pulse compression.Single M sequence is carried out to phase-modulation to laser, and the laser of phase-modulation is with the form transmitting of pulse.Utilize the autocorrelation of M sequence to detect to the echoed signal receiving, Cyclic autocorrelation applies to pulse compression with the form of matrix multiple.

Carry out periodic convolution if changed into from multiobject echo sequence s (r) with matched filter, can be expressed as so:

$\begin{array}{cc}y\left(p\right)=\underset{j=1}{\overset{j=R}{\mathrm{\Σ}}}s{\left(j\right)}^{*}h(p-j)& p\∈[0,P);---\left(2\right)\end{array}$

Wherein M represents the distance that the receives length to single sequence, and h (p) is the sequence take P as the cycle, in each cycle, is phase encoding sequence, the y (p) obtaining for long be the sequence of P.

In order to utilize the more intense characteristic of M sequence period autocorrelation, just need to be to utilizing formula (2) to calculate through the phase signal of different target aliasing in echo, because the matched filter now needing is Periodic filter, cannot carry out fast convolution computing at frequency domain by conventional method, so data processing method also should adjust.Echo data length is R, the window that is R by a length intercepts one piece of data as the first row in H in h (n), then this windowsill fixed-direction at the upper each slip data intercept of h (n), and its distance that departs from initial position is as columns corresponding in H.Slip total length is P.So just construct the matrix of a P*R.

With one-period in A (p) expression h (n), distance upwards target number is T, and matched filter will be expanded into the matrix H (p, r) of a P* (P+T-1) so, meet R=P+T-1.In Practical Project, be related to that P>T sets up, H (p, r) builds with such form so:

$H(p,r)=\left[\begin{array}{ccccccc}A\left(1\right)& A\left(2\right)& ......& A\left(P\right)& A\left(1\right)& ......& A(T-1)\\ A\left(P\right)& A\left(1\right)& ......& A(P-1)& A\left(P\right)& ......& A(T-2)\\ ......& ......& ......& ......& ......& ......& ......\\ A\left(3\right)& A\left(2\right)& ......& A\left(2\right)& A\left(3\right)& ......& A(T+1)\\ A\left(2\right)& A\left(1\right)& ......& A\left(1\right)& A\left(2\right)& ......& A\left(T\right)\end{array}\right];---\left(3\right)$

Each provisional capital of H (p, r) is the move right process of of lastrow.And the length of every a line matches to data sequence length with distance, namely exist and be related to R=P+T-1.The data of why having widened every row are in order to utilize formula (2), make the secondary lobe that comes from other targets drop to minimum in current goal position influence.

And matched filtering process has just become H (p, r) and data matrix and carries out matrix multiple and carry out convolution algorithm performance period.We do not use FFT computing in life cycle convolution algorithm, pulse compression process is matrix multiple computing, be that H matrix and s matrix carry out product calculation, the scale of H matrix is P*R, and P represents code length, a normally constant, here get 1023, N and represent that distance is to data scale, s matrix size is R*N, n represents Data in Azimuth Direction scale, and can to obtain our carrying out distance be 1023*N*R to the needed operand of compression to theoretical analysis so.

And if we use the frequency domain fast convolution method need to be through a FFT, once distance is to multiplication of vectors, an IFFT, through calculating its operand:

(log ₂R*R+R+log ₂R*R)*N＝(2*log ₂R+1)*R*N； （4）

See theoretically as seen, the computing scale that frequency domain fast convolution method needs is on M train pulse compression algorithm.Because cover ripple door size estimation is not very accurate, cause in engineering distance to tend to data volume larger, be larger so fast convolution method is compressed the operand expense of carrying out pulse compression compared to the matrix multiple of M sequence.In other words, we use M sequence to compensate to have reduced computational complexity.

Through MATLAB measured data, use lena figure as target image, utilize respectively two kinds of algorithms to solve statistical study, can find out and use fast convolution method to process used time 0.2162s and use matrix multiplication to process used time 0.2378s.The two differs and not quite.

The advantage of this method is:

1, this disposal route has effectively suppressed secondary lobe, and its effect equals the PSL under perfect condition.

2, data processing method is simple, is easy to hardware and realizes, for real-time processing provides possibility.

3, the time complexity of matched filter operation reduces.

4, to processing the data that can remove redundancy, facilitate orientation to process to compression through distance, reduce on the whole imaging time.

Accompanying drawing explanation

Fig. 1 uses the method for traditional matched filtering to coming from target T=[11111222221111122222] the echo compression result of carrying out pulse compression.Although can find out and can compress target, outside target, side-lobe energy is very high.

Fig. 2 amplifies for target part in Fig. 1 compression result being carried out to part.Can find out and compress the target pulse energy obtaining because the impact of secondary lobe has very macrorelief.

Fig. 3 uses the M sequence of 1023 length as phase-modulation sequence, and the compression result of by matrix multiple method, same target being carried out pulse compression, can find out, almost there is no secondary lobe outside target.

Fig. 4 is that in Fig. 3 compression result, target part is carried out part amplification.Can find out and compress the target pulse energy obtaining without fluctuating.

Fig. 5 uses a part of lena figure as target, uses conventional matched-filter to process the imaging results obtaining: can see that figure middle distance upwards has a lot of diplopias.

Fig. 6 uses lena to scheme same part as target, uses the M sequence of 1023 length as phase-modulation sequence, processes the imaging results obtaining: can see that the middle distance of publishing picture does not upwards almost have diplopia with matrix multiple.

Embodiment:

M sequence is applied to the utilization of phase encoding system synthetic aperture laser radar imaging needs following steps:

1) use the M sequence of single length to carry out phase-modulation to laser waveform, sequential coding length P determines according to requirement of engineering.Modulation waveform is launched with impulse form.

2) reflect from target the echoed signal obtaining along orientation to gathering: return laser beam gathers through balance detection device and AD, will in echo-pulse, gather out with matrix S (r, n) through the phase encoding information of stack.R represents that N represents the upwards scale of data of orientation apart from the scale of the data that upwards collect.

3) utilize formula (3) to build matched filter matrix H (p, r), and executable operations y (p, n)=H (p, r) * S (r, n).

4) y (p, n) obtaining is the data that obtain to pulse compression through distance, next carries out orientation to process of pulse-compression again.

Claims (1)

1. M sequence is applied to a phase encoding system imaging radar impulse compression method, it is characterized in that comprising the following steps:

(1) the M sequence that is P by single length is carried out phase-modulation to laser, launches with the form of pulse through the laser of phase-modulation;

(2) Cyclic autocorrelation of M sequence is applied to pulse compression with the form of matrix multiple, the method that obtains pulse compression matrix H (p, r) is such:

After echo acquirement through Technologies Against Synthetic Aperture laser radar, we obtain the digital signal for imaging, its middle distance is R to data length, h (n) is the M sequence of the circulation take the cycle as P, the window that is R by a length intercepts one piece of data as the first row in H in h (n), then this windowsill fixed-direction at the upper each slip data intercept of h (n), and its distance that departs from initial position is as columns corresponding in H.Slip total length is P.So just construct the matrix of a P*R;

With one-period in A (p) expression h (n), distance upwards target number is T, so matched filter will be expanded into the matrix H (p of a P* (P+T-1), r), meet R=P+T-1, in Practical Project, be related to that P>T sets up, H (p, r) builds with such form so:

$H(p,r)=\left[\begin{array}{ccccccc}A\left(1\right)& A\left(2\right)& ......& A\left(P\right)& A\left(1\right)& ......& A(T-1)\\ A\left(P\right)& A\left(1\right)& ......& A(P-1)& A\left(P\right)& ......& A(T-2)\\ ......& ......& ......& ......& ......& ......& ......\\ A\left(3\right)& A\left(2\right)& ......& A\left(2\right)& A\left(3\right)& ......& A(T+1)\\ A\left(2\right)& A\left(1\right)& ......& A\left(1\right)& A\left(2\right)& ......& A\left(T\right)\end{array}\right];$

Every a line of H (p, r) is exactly the move right process of of lastrow, and the matrix being built by this quadrat method just can be realized distance to pulse compression with matrix multiplication.

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