Modified Omega-K algorithm for processing helicopter-borne frequency modulated continuous waveform rotating synthetic aperture radar data

With appropriate geometry configuration, helicopter- borne rotating synthetic aperture radar （ROSAR） can break through the limitations of monostatic synthetic aperture radar （SAR） on forward-looking imaging. With this capability, ROSAR has extensive potential applications, such as self-navigation and self-landing. Moreover, it has many advantages if combined with the frequency modulated continuous wave （FMCW） technology. A novel geometric configuration and an imaging algorithm for helicopter-borne FMCW-ROSAR are proposed. Firstly, by per- forming the equivalent phase center principle, the separated trans- mitting and receiving antenna system is equalized to the case of system configuration with antenna for both transmitting and receiving signals. Based on this, the accurate two-dimensional spectrum is obtained and the Doppler frequency shift effect in- duced by the continuous motion of the platform during the long pulse duration is compensated. Next, the impacts of the velocity approximation error on the imaging algorithm are analyzed in de- tail, and the system parameters selection and resolution analysis are presented. The well-focused SAR image is then obtained by using the improved Omega-K algorithm incorporating the accurate compensation method for the velocity approximation error. FJnally, correctness of the analysis and effectiveness of the proposed al- gorithm are demonstrated through simulation results.

Adaptive digital self-interference cancellation based on fractional order LMS in LFMCW radar

Adaptive digital self-interference cancellation(ADSIC)is a significant method to suppress self-interference and improve the performance of the linear frequency modulated continuous wave(LFMCW)radar.Due to efficient implementation structure,the conventional method based on least mean square(LMS)is widely used,but its performance is not sufficient for LFMCW radar.To achieve a better self-interference cancellation(SIC)result and more optimal radar performance,we present an ADSIC method based on fractional order LMS(FOLMS),which utilizes the multi-path cancellation structure and adaptively updates the weight coefficients of the cancellation system.First,we derive the iterative expression of the weight coefficients by using the fractional order derivative and short-term memory principle.Then,to solve the problem that it is difficult to select the parameters of the proposed method due to the non-stationary characteristics of radar transmitted signals,we construct the performance evaluation model of LFMCW radar,and analyze the relationship between the mean square deviation and the parameters of FOLMS.Finally,the theoretical analysis and simulation results show that the proposed method has a better SIC performance than the conventional methods.

基于激光调频连续波正反向调谐色散对消方法

为了减小宽带激光调频连续波测量中的色散失配效应,提出了一种外腔可调谐激光器正、反向调谐消除色散失配的方法。当外腔可调谐激光器与光纤光路结合时,系统将产生色散失配效应,表现为目标信号的谱峰展宽和峰值偏移,测距值随调谐带宽增加而变化,导致测量不稳定。为了解决这一问题,研究了外腔可调谐激光器正、反向调谐时的系统色散失配特性,结果表明,在正反向扫频时色散趋势具有对称分布的特点,建立了正反向扫频系统色散失配模型,在此基础上提出通过外腔激光器正、反向调谐实现色散对消。该方法不需要预先标定系统的色散系数,也不需要循环迭代补偿,单次测量即可完成系统色散补偿,从而为提高色散补偿效率提供了一种思路。

Range Ambiguity Elimination in a Short-Range FMCW Radar System

Modified implementation architecture for sinusoidal frequency modulation is introduced to extract the range information from the received radar echo. Range ambiguity problem arises because the range is calculated from the estimated phase of the received signal which is wrapped into （0, 2π]. By integrating Doppler frequency shifts, the variation of range can be estimated and used as an auxiliary information to help eliminating the corresponding range ambiguity. The performance of the new technique is evaluated by simulations. The results show that this technique is robust to sever phase noise and can be used effectively for ambiguity elimination of the modified sinusoidal frequency modulated continuous wave radar.

High-Precision Vital Signs Detection Method Based on Spectrum Refinement and Extended DCMA

In this paper,the spectral estimation algorithm is extended to the detection of human vi-tal signs by mm-wave frequency modulated continuous wave(FMCW)radar,and a comprehensive algorithm based on spectrum refinement and the extended differentiate and cross multiply al-gorithm(DCMA)has been proposed.Firstly,the improved DFT algorithm is used to accurately obtain the distance window of human body.Secondly,phase ambiguity in phase extraction is avoided based on extended DCMA algorithm.Then,the spectrum range of refinement is determ-ined according to the peak position of the spectrum,and the respiratory and heartbeat frequency information is obtained by using chirp z-transform(CZT)algorithm to perform local spectrum re-finement.For verification,this paper has simulated the radar echo signal modulated by the simu-lated cardiopulmonary signal according to the proposed algorithm.By recovering the simulated car-diopulmonary signal,the high-precision respiratory and heartbeat frequency have been obtained.The results show that the proposed algorithm can effectively restore human breathing and heart-beat signals,and the relative error of frequency estimation is basically kept below 1.5%.

WIDE SWATH FMCW SAR DATA PROCESSING IN SQUINT MODE

This paper concentrates on the data processing of Frequency Modulation Continuous Wave(FMCW),Synthetic Aperture Radar(SAR)in the case of wide swath and squint mode.In the mode,the Doppler centroid dramatically varies along slant range compared to conventional pulsed-SAR.This poses a challenge for system design and signal processing since a very large azimuth bandwidth would be introduced.In the paper,we accommodate the Doppler centroid variations with range by an improved spectral-length extension method,where a bulk range shift and updated Doppler centroid variations are introduced to greatly reduce the azimuth aliasing with respective to the existing methods.Moreover,an image formation approach that integrates wave number domain algorithm is presented to focus the raw data of FMCW SAR in the case of wide swath and squint mode.Point target simulation experiment demonstrates the advantages of the presented method.

HIGH-RESOLUTION AUTOMOBILE FMCW SAR AND SIGNAL PROCESSING

The paper presents a high-resolution automobile Frequency Modulation Continuous Wave Synthetic Aperture Radar(FMCW SAR) named MiniSAR and the procedure of its signal processing.The imaging geometry of automobile SAR is very different from that of airborne SAR,leading to a different data processing method for automobile SAR.Therefore,in the paper,we propose an image formation approach that can well handle the focusing issues of automobile SAR.The effects of the strong reflected signal and the spatial-variant synthetic aperture length are analyzed.The processed results with automobile FMCW SAR read data validate the presented method.

Convolution neural network and 77 GHz millimeter wave radar based intelligent liquid classification system

An intelligent liquid classification system based on 77 GHz millimeter wave radar and convolution neural network are proposed in this paper.The data are collected by the AWR1843 radar platform and processed by the neural network on the host PC in real-time.The doppler heatmap generated by radar signal processing is tried for the first time as the input of the system.The information carried by the heatmap in 2 dimensions is analyzed and the reason why the doppler heatmap could be used for classification is explained.The feasible experiment proved that the proposed method can successfully classify 8 kinds of common liquid with high accuracy.The result of the experiment is explained and the limitations of the experiment are discussed.It can be drawn that the combination of FMCW millimeter wave radar and convolution neural network is a method with great potential for liquid classification.The advantages of real time,non-invasive and unilateral measurement can also be used for the detection of dangerous liquids.

Intrapulse coherence degradation suppressing method of echo signal in coherent lidar

Aiming at coherence degradation during target detection,a suppressing method based on frequency-modulated continuous wave coherent lidar is proposed.Combined with a random iteration algorithm,a long-pulse echo signal with coherent degradation is matched with random phase noise of a certain frequency and achieves coherence restoration.Simulation and field experiment results show that this proposed method can recover the intrapulse coherence in long-pulse echo signals.In addition,for the real target echo signal at 4.2 and 19.8 km,the peak signal-to-noise ratio processed by this method is increased by 0.35 times and 4 times after pulse compression,respectively.

A Downward-looking Three-dimensional Imaging Method for Airborne FMCW SAR Based on Array Antennas

With regard to problems in conventional synthetic aperture radar （SAR）, such as imaging distortion, beam limitation and failure in acquiring three-dimensional （3-D） information, a downward-looking 3-D imaging method based on frequency modulated continuous wave （FMCW） and digital beamforming （DBF） technology for airborne SAR is presented in this study. Downward-looking 3-D SAR signal model is established first, followed by introduction of virtual antenna optimization factor and discussion of equivalent-phase-center compensation. Then, compensation method is provided according to reside video phase （RVP） and slope term for FMCW SAR. As multiple receiving antennas are applied to downward-looking 3-D imaging SAR, range cell migration correction （RCMC） turns to be more complex, and corrective measures are proposed. In addition, DBF technology is applied in realizing cross-track resolution. Finally, to validate the proposed method, magnitude of slice, peak sidelobe ratio （PSLR）, integrated sidelobe ratio （ISLR） and two-dimensional （2-D） contour plot of impulse response function （IRF） of point target in three dimensions are demonstrated. Satisfactory performances are shown by simulation results.

Ultra-wideband FMCW ISAR imaging with a large rotation angle based on block-sparse recovery

Ultra-wideband frequency modulated continuous wave （FMCW） radar has the ability to achieve high-range resolution. Combined with the inverse synthetic aperture technique, high azimuth resolution can be realized under a large rotation angle. However, the range-azimuth coupling problem seriously restricts the inverse synthetic aperture radar （ISAR） imaging performance. Based on the turntable model, traditional match-filter-based, range Doppler algorithms （RDAs） and the back projection algorithm （BPA） are investigated. To eliminate the sidelobe effects of traditional algorithms, compressed sensing （CS） is preferred. Considering the block structure of a signal at high resolution, a block-sparsity adaptive matching pursuit algorithm （BSAMP） is proposed. By matching pursuit and backtracking, a signal with unknown sparsity can be recovered accurately by updating the support set iteratively. Finally, several experiments are conducted. In comparison with other algorithms, the results from processing the simulation data, some simple targets, and a complex target indicate the effectiveness and superiority of the proposed algorithm.

FMCW-InISAR imaging for high-speed target based on bistatic configuration

Interferometric inverse synthetic aperture radar(InISAR)imaging has been proved to be a powerful means for obtaining threedimensional(3-D)space shape of noncooperative targets.Frequency modulated continuous wave(FMCW)InISAR(FMCWInISAR)has unique advantages of low power,low cost,and small volume compared with traditional coherent pulsed InISAR.However,FMCW-InISAR imaging has two additional issues to consider,the one is the invalidation of the assumption of stop&go,which is caused by the relatively long sweep interval of FMCW;the other is the isolation of the transmitting and receiving antennas,which is the inherent issue of the transmitter-receiver community radar systems.To solve these two problems,a bistatic FMCW-InISAR imaging algorithm for high-speed targets is proposed in this paper.For improving the isolation of the transmitting and receiving antennas,a bistatic configuration based FMCW-InISAR system is designed.According to the characteristics of bistatic,a bistatic equivalent motion model and corresponding signal model are established.Since the assumption of stop&go is invalid in the case of FMCW,indicating that the target cannot be viewed as motionless during a sweep repetition interval(SRI),a parametric estimation based quadratic phase factor(QPF)compensation method is investigated to eliminate the range walk caused by the radial motion of the target during the SRI.In addition,considering the farfield trait of the target and combining the traditional InISAR imaging process,a combined QPF compensation technique is proposed to reduce the computational burden of the algorithm.Finally,the effectiveness and the robustness of the proposed algorithm are evaluated by some simulations.

A novel scheme for processing FMCW-ladar data acquired with low sampling rate

The frequency-modulated continuous wave(FMCW)ladar is a useful sensor for remote sensing applications and has been widely used in both the military and civilian fields for precise geospatial data acquisition.Typically,the FMCW-ladar signal is collected by use of the hetero-dyne detection through the dechirping so as to decrease the sampling rate commensurate with the bandwidth.In this manuscript,we proposed a novel scheme for processing the FMCW-ladar signal acquired through the simplified heterodyne detection,and the sampling rate was the same as that in the dechirp detection.Based on the time-frequency relation of the FMCW-ladar signal,two algorithms were proposed to post-process the echo acquired by the simplified heterodyne detection with the sub-Nyquist sampling rate.For the FMCW-ladar echo data acquired by the simplified heterodyne detection with the same sampling rate as in the traditional dechirp detection,the algorithms can achieve the unambiguous range image retaining the range resolution commensurate with the transmitted chirp bandwidth.The effectiveness of the scheme was validated by simulation and real data processing experiments.The capability of the proposed scheme provides an alternative for the FMCW-ladar system without use of the dechirp detection,which can benefit the future FMCW-ladar and microwave photonics radar applications in the remote sensing.

High-speed ground moving target detection research using triangular modulation FMCW

The frequency modulated continuous wave(FMCW)radar has the characteristics of low probability of interception,good hidden property and the ability to counter anti-radiation missiles.This paper proposes a new method for high-speed ground moving target detection(GMTD)using triangular modulation FMCW.According to the characteristic of the opposite range shift induced by the upslope and downslope modulation FMCW,the upslope and downslope are imaged,respectively.After compensation of continuous motion of the platform and time difference between upslope and downslope signals for imaging,the moving target can be detected through displaced phase center antenna(DPCA)technology.When the moving target is detected,the moving target image is extracted,and correlation processing is used to obtain the range shift,which can be used to estimate the target radial velocity,and further to find the real position of the target.The effectiveness of this method is verified by the result of computer simulation.