Inverse Synthetic Aperture Ladar Imaging Algorithm for Maneuvering Target Based on FRFT-CLEAN

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.

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.

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.

Inverse synthetic aperture ladar (ISAL), which operates at visible or infrared wavelength, is more sensitive to maneuver than inverse synthetic aperture radar (ISAR). In this paper, we propose the Range Instantaneous Doppler (RID) algorithm for very high resolution ISAL imaging of maneuvering targets. The Wigner-Ville distribution (WVD) and the Radon transform are used for computing the time-varying Doppler spectrum of maneuvering targets. The data processing of the RID algorithm is illustrated. The simulation shows that the RID algorithm can reconstruct a clearer ISAL image of the maneuvering target than the conventional rang Doppler (RD) algorithm. This is a crucial step in developing a real ISAL system.

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.

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

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.

Neural network based synthetic aperture ladar imaging through marine atmosphere

By extending synthetic aperture technology from a microwave band to laser wavelength, the synthetic aperture ladar (SAL) achieves extremely high spatial resolution independent of the target distance in long-range imaging. Nonlinear phase correction is a critical challenge in SAL imaging. To address the issue of phase noise during the imaging process, we first analyze the theoretical impact of nonlinear phase noise in imaging performance. Subsequently, a reconstruction and compensation method based on orthonormal complete basis functions is proposed to mitigate nonlinear phase noise in SAL imaging. The simulation results validate the accuracy and robustness of the proposed method, while experimental data demonstrate its effectiveness in improving system range resolution and reducing the peak side lobe ratio by 3 dB across various target scenarios. This advancement establishes a solid foundation for the application of SAL technology in ground-based remote sensing and space target observation.

In this paper, we discuss the application of the spatial correlation algorithm (SCA) (Attia and Steinberg, 1989; Attia, 1993; Attia, 1994), and some of its variants, to synthetic aperture LADAR (SAL) imaging. These algorithms (which derive the required phase corrections from range-resolved LADAR clutter received by the SAL sensor) are very robust, computationally efficient, perform very well in the presence of large uncorrelated phase errors, and do not require dominant scatterers of opportunity. Performance and limitations of the basic SCA are discussed. Dependence of the accuracy of the resulting phase corrections on the SNR per pulse and number of range cells available for estimating the corrections is presented in considerable detail. The impact of residual uncorrected phase errors on image SNR and dynamic range is statistically evaluated taking into account the total number of pulses coherently combined along the imaging baseline. Also, a method for exploiting special features of 3D interferometric SAL imaging for the purpose of improving phase correction accuracy is introduced.

High‐resolution imaging of distant, dynamic targets is fundamentally limited by finite‐aperture diffraction. Unlike radar, optical detectors record only intensity and lack the temporal resolution for direct phase measurement, hindering straightforward extension of synthetic aperture techniques to the optical domain. While Macroscopic Fourier ptychography (FP) offers a promising computational alternative, its reliance on mechanical scanning or engineered illumination restricts its use to cooperative, static scenes. In this paper, we present Inverse Synthetic Aperture Macroscopic Fourier ptychography (ISAMFP), which, to the best of our knowledge, is the first practical realization of inverse synthetic aperture imaging for diffusely reflecting targets within the Fourier ptychography framework. By transforming the natural target motion from a detrimental factor into an effective source of angular diversity, ISAMFP eliminates the need for mechanical scanning or illumination modulation and achieves resolution enhancement far beyond the aperture limit using intensity‐only measurements. Experimental validations on USAF resolution targets, unmanned aerial vehicles, and complex multi‐target scenes highlight both the robustness and versatility of ISAMFP, establishing it as a powerful tool for non‐interferometric synthetic aperture imaging of moving, non‐cooperative targets, with broad implications for surveillance, defense, and remote sensing.

The synthetic aperture imaging ladar (SAIL) systems typically generate large amounts of data difficult to compress with digital method. This paper presents an optical SAIL processor based on compensation of quadratic phase of echo in azimuth direction and two dimensional Fourier transform. The optical processor mainly consists of one phase-only liquid crystal spatial modulator(LCSLM) to load the phase data of target echo and one cylindrical lens to compensate the quadratic phase and one spherical lens to fulfill the task of two dimensional Fourier transform. We show the imaging processing result of practical target echo obtained by a synthetic aperture imaging ladar demonstrator. The optical processor is compact and lightweight and could provide inherent parallel and the speed-of-light computing capability, it has a promising application future especially in onboard and satellite borne SAIL systems.

Due to the short laser wavelength, almost all practical targets are rough. Surface elevations in rough targets will result in layovers in synthetic aperture ladar (SAL). High resolution SAL image with layovers will be different from the target picture taken by incoherent tools as digital camera. To investigate the layovers in SAL image, a simplified mathematical model is built by optics diffraction theory and a laboratory SAL is setup using 1550 nm tunable laser source. Layovers in SAL images, in both theoretical simulation and experimental demonstra- tion, are carefully observed. Detailed results on various targets are illustrated.

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.

One of the urgent nowadays problem is to determine the characteristics of space debris (SD) in near-Earth space. Non-functioning space vehicles and their remains, collectively referred to as SD, are located both at low altitudes of up to 2000 km, orbits, and in orbits between 19,000 and 22,000 km, and in the geostationary ring above 32,000 km. Most of these objects are in high-inclination orbits, the planes of which intersect, so the average relative speed of their mutual flight is about 10 km/s. Due to the huge supply of kinetic energy, the collision of any of these objects with an operating spacecraft can damage it or even disable it. Small fractions of space debris pose the main threat to the implementation of safe space activities, since the intensity of passage through a unit of surface for small-sized (less than 10 cm) fractions is about an order of magnitude greater than for fractions more than 10 cm. Therefore, effective information support for the safety of space activities is possible only in the organization of continuous operational control of not only large, but small-sized space objects, including small fractions of space debris. In To identify the geometric shape of space debris objects, to clarify their flight orbit, the presence of rotation, an optimal algorithm for the inverse synthetic aperture radar imaging of space debris objects in the multi-position radar system is proposed. In this paper, an analysis of the trajectory signal was carried out, potential resolutions were obtained in terms of range and azimuth, an estimate of the synthesizing time was made.