Inverse synthetic aperture ladar imaging with the Range Instantaneous Doppler algorithm

Inverse synthetic aperture ladar (ISAL) is analogous to the inverse synthetic aperture radar but works at frequency 104–5 times higher. A method is proposed to process the ISAL data with stepped-frequency continuous waveform (SFCW). Due to the short wavelength used for ISAL and the relatively long transmission of the SFCW, the motion during the transmission and the relative range shift between adjacent sub-band chirps must be both taken into account while performing the bandwidth synthesis and imaging. An analytic signal model was derived to address this problem. In the proposed model, three additional terms, named the bulk range walk term, the stepped phase error term and the additional range shift, were formulated in ISAL imaging with SFCW for the first time. Based on the signal model, an effective method was proposed to coherently synthesise all the sub-band echoes and then focus the ISAL image. Experimental result was shown and the focused image of an airplane model demonstrated the effectiveness of the proposed method.

High quality linear laser frequency chirp of high chirp rate is critical to many laser ranging applications. In this paper, we describe a cost-effective chirp linearization approach implemented on our Inverse synthetic Aperture LADAR (ISAL) imaging testbed. Our approach uses a COTS PZT for external cavity laser frequency tuning and a common self-heterodyne fiber interferometer as a frequency monitor, with a two-step hardware and software chirp linearization procedure to achieve high quality chirp. First, the nominal triangle waveform input to PZT drive is modified through an iterative process prior to ISAL imaging acquisition. Several waveforms with chirp rates between 1 and 4THz/s have been acquired with residual chirp rate error ~ +/-2% in usable region. This process generally needs to be done only once for a typical PZT that has excellent repeatability but poor linearity. The modified waveform is then used during ISAL imaging acquisition without active control while the imperfection in transmitted frequency is monitored. The received imaging data is resampled digitally based on frequency error calculated from the frequency monitor data, effectively reduce chirp nonlinearity to ~+/- 0.2% in chirp rate error. The measured system impulse response from return signal shows near designed range resolution of a few mm, demonstrating the effectiveness of this approach.

Target vibration introduces unexpected vibration phases in the echo signal of inverse synthetic aperture LADAR (ISAL). The vibration phases cause the imaging results to be defocused. How to estimate the vibration phases has always been one of the difficulties in ISAL imaging, and there has not been an effective solution. In this paper, two methods are proposed to estimate the vibration phases in different situations. The method based on multi-channel along-track interferometry (MCATI method) is for some cases that the target velocity vector is parallel to some baselines of ISAL. The method based on orthogonal interferometry (OI method) is for other cases that the target velocity vector is not parallel to any baseline of ISAL. In addition, a high signal to noise ratio (SNR) is essential for the MCATI and OI methods to get high estimation accuracy. Therefore, a large power-aperture product should be carefully considered in the ISAL system design. Furthermore, on the condition of low SNR, range cell accumulation is also helpful to reduce the influence of noise. Take the ISAL, which is utilized to image the satellite in 500km orbit, as an example, the estimation accuracy can satisfy the demand of synthetic aperture imaging with 0dB single-pulse SNR and 300 range cell accumulation. The proposed methods are verified with simulation and experiment.

During the spaceborne inverse synthetic aperture ladar (ISAL) imaging, the radial velocity between the piggyback satellite of ladar and the observed satellite is usually relatively large, which will introduce the inner-pulse Doppler that cannot be ignored in the echo. In this case, the traditional “stop-and-go” assumption will no longer hold, and the compressed result along the range direction would be seriously broaden. Therefore, to obtain a well-focused ISAL image, this article proposed a spaceborne ISAL imaging algorithm for high-speed moving targets based on the inner-pulse Doppler compensation and residue video phase (RVP) correction. In the article, the estimate method of the radial velocity, the inner-pulse Doppler compensation method, and the RVP correction method are given. After inner-pulse Doppler compensation, RVP correction and translational motion compensation, the range-compressed result under the “stop-and-go” assumption can be obtained, and the target can then be imaged by the traditional ISAL imaging algorithm. The simulation results demonstrate the effectiveness of the proposed algorithm.

The ground‐airborne transient electromagnetic method with a galvanic source has such advantages as high efficiency, great exploration depth, high signal to noise ratio and good performance in mountainous areas. However, so far its interpretation system has not been established, which restricts the development of this method. This paper aims at building up a complete ground‐airborne electromagnetic detection system to enrich the theory of this method. This paper consists of three parts: (1) full field apparent resistivity definition; (2) Kirchhoff immigration of TEM virtual wave field; and (3) inverse synthetic aperture imaging. Magnetic field intensity is selected for full field apparent resistivity definition. The results of theoretical models show that the first and last part of the apparent resistivity curve can reach the resistivity of the first and the last layer, respectively. Thus, full space and full time apparent resistivity definition is achieved. Based on previous study, Kirchhoff migration of ground‐airborne transient electromagnetic method with a long wire source is realized. A correlation method is adopted for multiple survey point synthesis, which pushes the traditional single point processing forward point‐by‐point for multi‐coverage inverse synthetic aperture imaging.The results of layered models show that full field apparent resistivity can reveal the underground electric change smoothly, completely and gradually. When the resistivity of the middle layer changes, the differentiation of full field apparent resistivity is obvious, which can easily and intuitively identify the electric layer. As receiver altitude, offset and time are considered simultaneously, full field apparent resistivity can be calculated in full space and full time. The results of the complex model with a water‐bearing gob shows that the further the offset from the gob is, the weaker the resistivity anomaly on full field apparent resistivity section is. There are two interfaces from the Kirchhoff immigration results, of which the upper one indicates surface. As the electrical difference between earth and air is large, the wave field signal of this interface is strong, which spreads over the whole area. While the anomalies of the lower interface are mainly distributed in the middle area and weaken outwards, which indicate the water bearing gob. The upper interface changes little after inverse synthetic aperture imaging, while the gob anomaly narrows and has a clear boundary. The anomaly boundary matches well with the designed model. Inverse synthetic aperture imaging of ground‐airborne transient electromagnetic survey with a long wire source is realized on the basis of inverse synthetic aperture radar (ISAR). Full field apparent resistivity is put forward with inverse function and iteration, which can realize calculation of this variable in the full zone and full time. Pseudo‐seismic migration of transient electromagnetism is used in 3D transient field imaging. Inverse synthetic aperture radar is introduced in inverse synthetic aperture imaging of the ground‐airborne transient electromagnetic method with a long wire source, which can help enhance resolution. The results of designed models show that correlation can strengthen useful signal and improve signal to noise ratio and resolution, which prove the efficiency of inverse synthetic aperture imaging of the ground‐airborne transient electromagnetic method with a long wire source.

Inverse synthetic aperture radar (ISAR) imaging is typically useful when there is a need to classify, recognize, or identify a moving target of interest. In fact, an ISAR image highlights two-dimensional (2-D) geometric features of a target, which can provide indications of target's type, size, and other salient information. Such information can be then used for target classification, recognition, and identification. In this chapter, we will provide five case studies on applications of ISAR imaging. Such case studies are chosen to diversify the type of application and to give additional indications about how to interpret ISAR images. Specifically, in the first case study (Section 12.1), we show an example of ground-based ISAR images of a noncooperative sailing ship. The ISAR techniques used to form the ISAR images are the range-Doppler (Chapter 3) and the ICBA (Chapter 5) techniques. Different time windows are chosen to show the effects of the coherent processing interval (CPI) length on the ISAR image. Since the radar is ground based, this represents a case where the ISAR image is formed by exploiting only the sea surface induced target's motions. The second case study (Section 12.2) refers to a scenario where the radar is carried by an aircraft and the target is a ship at sea. In this case, both the target and the platform motions concur to the ISAR image formation. Also in this case, the range-Doppler and ICBA are used to form the ISAR image sequence. The third case study (Section 12.3) concerns dual ground-based/satellite ISAR imaging of a noncooperative sailing ship. In this experiment two radars are used to image a sailing ship at the same time. Two techniques are here used to form the ISAR images: a refocus technique (see Chapter 8) to form the satellite ISAR image; and a range-Doppler with ICBA to form the ground-based ISAR image. In this case study, we show how a dual system is able to form ISAR images of the same target with different target's views and projections. This is a simple case of multiple-perspective ISAR images, which may be looked at as a way to improve a target's classification and recognition [1]. The fourth case study (Section 12.4) shows ISAR images of four aircrafts obtained by processing data acquired with a ground-base radar. This case study may be regarded as the classic ISAR imaging example. Four different airplanes have been selected to show how their characteristics are mapped onto ISAR images. Such characteristics can then be used by classifiers for recognition purposes.

Because of the high frequency and large bandwidth of the transmitted signal of inverse synthetic aperture Ladar (ISAL), effect as dispersion in range profile exists when imaging uncooperative targets, which leads to the distortion on one-dimensional range profile. The echo model of ISAL is established for uncooperative target. Research shows that, for uniform motion target and uniformly accelerated target, ISAL signals after optical heterodyne processing can be considered as multi-component linear frequency modulation (LFM) signals with the same chirp rate. An ISAL motion compensation method based on fractional Fourier transform (FRFT) is proposed here. Simulation results demonstrate the validity of this method. (5 pages)

The maneuverability of targets cannot be neglected in many circumstances for Inverse Synthetic Aperture Radar (ISAR) imaging procedure. For these circumstances, the Range Instantaneous Doppler (RID) algorithm is required to form the well-focused ISAR images. Simultaneously, as the inevitable means of obtaining the target’s cross-range size, the cross-range scaling approach for the RID images also needs to be considered consequently. Since there are few cross-range scaling methods focused on the RID images, we propose a novel method in this paper based on the phase coefficients estimation and scatterers pairing process to solve the problem. Without the special requirements for the scatterers or complicated optimization process, the proposed approach is widely adaptive and efficient. Otherwise, besides the novel estimation method for the rotation parameters, we also propose the novel technique for calculating the cross-range scaling factor of the RID images, which is quite different from that of the Range Doppler (RD) images. Finally, by using the recommended RID imaging algorithm and the proposed cross-range scaling approach, the results of ISAR imaging and cross-range scaling experiments on the simulation and real-data experiment are represented, which demonstrate the effectiveness of the proposed approach in this paper.

Bistatic inverse synthetic aperture radar (ISAR) can help image the high-speed target for its advantage in tracking and more observation angle to provide more information than monostatic ISAR. However, the complex image geometry makes it difficult to achieve a clear image of the target with geometry distortion correction and calibration. Furthermore, high-speed motion will make the image blurred or defocussed. To address these problems, a bistatic ISAR (B-ISAR) imaging method for high-speed motion target with geometric distortion correction and calibration is proposed. According to the motion decomposition idea, we established the B-ISAR echo model of the high-speed motion target. Then, based on the range Doppler algorithm, we deduce the analytic formula of the geometric distortion factor and calibration factor, and transform the imaging problem into a parameter estimation problem. With the sparsity of the scattering points, the required parameters are solved using the expectation maximization algorithm based on the maximum a posteriori probability criterion. With the estimated parameters, a clear B-ISAR image of a high-speed motion target with geometric correction and calibration is obtained. The simulations show that the proposed method has better resolution and simultaneously attains geometric distortion correction and calibration of the image.

In response to the application requirement of wide-range high-resolution imaging of non-cooperative moving real targets by inverse synthetic-aperture ladar (ISAL), experiments were conducted on the depolarization effect of target materials, and the polarization selection of ISAL receiving and transmitting channels was discussed. Considering the impact of target depolarization and the demand for along-track interferometry, combined with beam-broaden and high-gain amplifiers, an ISAL system design method that can stably image multiple non-cooperative real targets has been proposed. Under the condition of broadening the transmitting and receiving beams to 3° in the elevation direction for non-cooperative moving vehicles, echo data with a duration of 1 s is obtained. The spatial correlation algorithm combined with along-track interferometry is used to estimate the vibration phase error. The sub-aperture Range-Doppler algorithm is used for imaging. The ISAL imaging results of the moving vehicle validated the high-resolution imaging ability of ISAL and its potential for stable imaging of non-cooperative moving real targets.

Inverse Synthetic Aperture Radar (ISAR) is generally used for classification of ship targets after detection and tracking by the Radar. There are various methods for high resolution 2D imaging of maritime targets in ISAR mode like Range Doppler Algorithm and Polar Format Algorithm. After the raw image is formed, it has to pass through the post processing algorithm suite before an acceptable image can be displayed to observer. This paper presents an efficient algorithm suite for the post processing of ISAR image.

逆合成孔径激光雷达(Inverse Synthetic Aperture Ladar, ISAL)对机动目标成像时,其回波信号存在距离向色散和方位向多普勒时变的问题。对于机动性满足二阶运动近似的目标,分析了其ISAL回波信号特征,该文提出一种基于FRFT-CLEAN的ISAL成像算法。该算法利用分数阶Fourier变换(FRactional Fourier Transform, FRFT)消除距离色散。在运动补偿后,利用FRFT并结合CLEAN技术(FRFT-CLEAN)实现对机动目标的方位成像。仿真实验证明了该方法的有效性。

Multi-Channel Inverse Synthetic Aperture Ladar Imaging Detection Technology and Experimental Research

For targets with complex motions, the time-varying Doppler frequency deteriorates inverse synthetic aperture radar (ISAR) images. After range alignment and phase adjustment, azimuth echoes in a range cell can be modeled as multicomponent cubic phase signals (CPSs). The chirp rate and the quadratic chirp rate of the CPS are identified as the causes of the time-varying Doppler frequency; thus, it is necessary to estimate these two parameters correctly to obtain a well-focused ISAR image. The parameter-estimation algorithm based on the modified chirp rate-quadratic chirp rate distribution (M-CRQCRD) is proposed for the CPS and applied to the ISAR imaging of targets with complex motions. The computational cost of M-CRQCRD is low, because it can be implemented by the fast Fourier transform (FFT) and the nonuniform FFT easily. Compared to two representative parameter-estimation algorithms, the M-CRQCRD can acquire a higher antinoise performance due to the introduction of an optimal lag-time. Through simulations and analyses for the synthetic radar data, the effectiveness of the M-CRQCRD and the imaging algorithm based on the M-CRQCRD are verified.

Inverse synthetic aperture radar (ISAR) imaging of a non-cooperative manoeuvring target is a challenging task due to its time-varying orientation and rotation velocity which denies accurate measurement. This paper investigates the general principles of ISAR imaging of manoeuvring targets, and proposes an algorithm for application in situations when the manoeuvrability is not too serious and the Doppler variation of sub-echoes from scatterers can be approximated as a first order polynomial. The imaging results using real data show the effectiveness of the new method.