High Resolution Harmonic Radar ISAR Experiments and System Design Strategies

Usually, in traditional Inverse Synthetic Aperture Radar (ISAR) systems design and mode selection for space satellite targets, coherent integration gain in azimuth direction hardly can be analyzed, which depends on target’s motion. In this study, we combine the target orbit parameters to determine its motion relative to radar and deduce coherent integration equation in ISAR imaging to realize the selection of imaging intervals based on coherent integration, which can ensure the resolution in azimuth direction. Meanwhile, we analyze the influence of target orbit altitude to echo power and imaging Signal-to-Noise Ratio (SNR) that provides a new indicator for space observation ISAR systems design. The result of simulation experiment illustrates that with target orbit altitude increasing, coherent integration gain in azimuth direction of large-angular observation offsets the decreasing of imaging SNR in a degree, which provides a brand-new perspective for space observation ISAR systems and signal processing design.

Bistatic inverse synthetic aperture radar (ISAR) is an ISAR system whose transmitter and receiver are placed in different places. The configuration mode makes it have some differences with monostatic ISAR. Combined with the reality of bistatic ISAR, the paper analyzed the two important factors, signal to noise ratio (SNR) and bistatic angle, who affect the imaging quality. The equal SNR curve of bistatic ISAR is Cassini's Oval whose focus are transmitter and receiver, and the equal bistatic angle curve is a series of circular arc whose center is on the vertical bisector of bistaic baseline and pass the bistatic radar. To ensure the imaging quality, if the echo has enough SNR, the imaging segment should choose the area far from the vertical bisector of based line. The research in this study provides significant evidence for the selection of imaging segment.

Earth observation from airborne and spaceborne platforms using synthetic aperture radar (SAR) has provided scientists and researchers with a deeper understanding of the planet for geological studies, weather forecasting, and disaster mitigation, among others. Aiming to develop our own spaceborne synthetic aperture radar for the purpose of environmental remote sensing, we have a program to test our system subcomponents on airborne and ground based systems. This paper presents the design and testing of a wideband inverse synthetic aperture radar (ISAR) system. This work will lead to the investigation of generating images from circularly polarized (CP) transmits and receive signals, improvement of related imaging algorithms for CP signals and suitable antenna design. Data measurement setup was constructed in an anechoic chamber with the use of a vector network analyzer (VNA) and automated rotary turntable to mount targets.

This paper addresses the estimation of the target translational motion by using a multistatic Inverse Synthetic Aperture Radar (ISAR) system composed of an active radar sensor and multiple receiving-only devices. Particularly, a two-step decentralized technique is derived: the first step estimates specific signal parameters (i.e., Doppler frequency and Doppler rate) at the single-sensor level, while the second step exploits these estimated parameters to derive the target velocity and acceleration components. Specifically, the second step is organized in two stages: the former is for velocity estimation, while the latter is devoted to velocity estimation refinement if a constant velocity model motion can be regarded as acceptable, or to acceleration estimation if a constant velocity assumption does not apply. A proper decision criterion to select between the two motion models is also provided. A closed-form theoretical performance analysis is provided for the overall technique, which is then used to assess the achievable performance under different distributions of the radar sensors. Additionally, a comparison with a state-of-the-art centralized approach has been carried out considering computational burden and robustness. Finally, results obtained against experimental multisensory data are shown confirming the effectiveness of the proposed technique and supporting its practical application.

Dual-polarized inverse synthetic aperture radar (ISAR) systems can provide more extra information than single-polarized ISAR systems, which can be used to increase image quality during the imaging process. However, there is limited research on the subject of dual-polarized ISAR. In this letter, an enhanced phase gradient autofocus (PGA) method was proposed for dual-polarized ISAR imaging. The Cloude-Pottier decomposition is employed to extract the polarization parameters <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$H$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\alpha $ </tex-math></inline-formula> from the dual-polarized primary image. Subsequently, the PGA is enhanced by selecting high-quality prominent scatterers based on these <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$H$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\alpha $ </tex-math></inline-formula> parameters. By making full use of the prominent scatterers selected by polarization information, the performance of PGA was improved, and thus, the imaging quality was significantly improved. Finally, both the real measurement data of aircraft targets and ship targets verified the effectiveness of the proposed method.

Imaging of target with complicated motion using ISAR system based on IPHAF-TVA

In contrast to conventional radar, harmonic radar systems utilize two different frequencies for transmission and reception. Therefore, a desired target is equipped with a tag that includes a frequency doubler. It exclusively reflects the signal at twice the originally sent out frequency, making this specific target unique. Thus, enabling the key advantage of harmonic radar systems: clutter rejection. However, the range equation of such a system deviates significantly from conventional radar. This is mostly due to the power transfer function of the required doubler. Therefore, achieving sufficient range to satisfy a desired application is challenging. In this article, the measures with which state-of-the-art systems address those challenges are reviewed and explained with the help of this deviating range equation. Furthermore, this article highlights the potential of utilizing active tags as well as other research to increase realizable ranges and the frequencies of harmonic radar into the mmWaves, respectively.

In pest management research, harmonic radar systems have been largely used to study insect movement across open or vegetation-poor areas because the microwave signal is attenuated by the high water content of vegetation. This study evaluated whether the efficacy of this technology is sufficient to track insects in vegetative landscapes. Field efficacy data were collected using portable harmonic microwave radar and electronic dipole tags mounted on adults of three economically important pests: Leptinotarsa decemlineata (Say), Diabrotica virgifera virgifera (LeConte) [corrected] and Conotrachelus nenuphar Herbst. Detection and recovery of tagged Colorado potato beetles, plum curculios and western corn rootworms was high within and among potato plants, moderate within apple trees and high within, but not between, corn plants respectively. The efficacy of the radar depends on the ability of the operator to move around the host, scanning for a signal 'sightline' with the tagged insect among plant structures. The detection rate of tagged insects by harmonic radar systems is high enough to track the walking path of pests through low row crops such as potato, tall row crops such as corn or tall but well-separated trees of orchard-type crops by adapting the scanning procedure to the vegetative architecture.

W-band inverse synthetic aperture radar (ISAR) imaging systems are very useful for automatic target recognition and classification due to their high spatial resolution, high penetration and small antenna size. Broadband linear frequency modulated wave (LFMW) is usually applied to this system for its de-chirping characteristic. However, nearly all of the LFMW generated in electronic W-band ISAR system are based on multipliers and mixers, suffering seriously from electromagnetic interference (EMI) and timing jitter. And photonic-assisted LFMW generator reported before is always limited by bandwidth or time aperture. In this paper, for the first time, we propose and experimentally demonstrate a high-resolution W-band ISAR imaging system utilizing a novel logic-operation-based photonic digital-to-analog converter (LOPDAC). The equivalent sampling rate of the LOPDAC is twice as large as the rate of the digital driving signal. Thus, a broadband LFMW with a large time aperture can be generated by the LOPDAC. This LFMW is up-converted to W band with an optical frequency comb. After photonic-assisted de-chirping processing and data processing to the echo, a high-resolution two-dimension image can be obtained. Experimentally, W-band radar with a time-bandwidth product (TBWP) as large as 79200 (bandwidth 8 GHz; temporal duration 9.9 us) is established and investigated. Results show that the two-dimension (range and cross-range) imaging resolution is ~1.9 cm × ~1.6 cm with a sampling rate of 100 MSa/s in the receiver.

This paper will present the findings associated with data eclipsing effects in the inverse polar format algorithm (IPFA) for turntable spotlight inverse synthetic aperture radar (ISAR) imaging systems. Typical turntable ISAR systems illuminate a scene of interest with an electromagnetic source in order to obtain samples of the reflectivity function over a given set of aspect angles. These samples form a bandlimited portion of the spatial frequency (kx,ky) response of the scene, from which a focused image may be generated by using a simple 2D inverse fast Fourier transform (IFFT). However, the collected samples lie on a polar raster in (kx,ky) space, which cause defocusing effects in the final image unless interpolation to a rectangular grid of points is first accomplished. The IPFA implements a frequency selective waveform in order to sample the reflectivity function of the scene of interest so that the (kx,ky) samples lie on a rectangular grid of points. This is accomplished by designing a rectangular grid of (kx,ky) sample points based upon the desired image resolution, pixel spacing, and scene size. Once this is accomplished, a corresponding set of frequency/aspect angle (f,thetas) pairs can be generated for data collection. However, the (f,thetas) pairs must first be sorted in order of increasing aspect angle in order to get the correct frequency transmission schedule. This sorting results in a staggered transmission schedule that can result in pulse overlapping (eclipsing) if certain conditions are not met regarding the pulse length and turntable rotation rate. This paper will detail the requirements under which these conditions must be met and will provide simulation results showing the effect of pulse overlap in image formation.

In this paper, we propose a deep learning-based model to generate high-resolution inverse synthetic aperture radar (ISAR) images under a limited radar bandwidth. While traditional ISAR systems achieve a higher resolution with a wider bandwidth, this comes at the cost of increased system resource consumption. To address this issue, we propose a method to generate high-resolution images from low-resolution ISAR images using deep learning. The training data used in this study consist of ISAR image pairs generated by calculating the scattered field using physical optics. The images generated by the proposed model can achieve average performances of 0.08, 8.94, and 0.19 in terms of learned perceptual image patch similarity, natural image quality evaluator, and deep image structure and texture similarity, respectively, compared to the original images.

Inverse synthetic aperture radar (ISAR) images are frequently used in target classification and recognition applications. Nevertheless, the interpretation of ISAR images remains problematic for several reasons. One of these is the fact that the image plane cannot be defined by the user but instead depends on the target's own motions and on its relative position with respect to the radar. In order to overcome the problem of interpreting two-dimensional (2D) ISAR images, a method for three-dimensional (3D) reconstruction of moving targets is presented. This method is based on the use of a dual interferometric ISAR system. The interferometric phases measured from two orthogonal baselines are used to jointly estimate the target's effective rotation vector and the heights of the scattering centers with respect to the image plane. The scattering center extraction from the ISAR image is performed by applying a multichannel CLEAN technique. Finally, a 3D image of the moving target is reconstructed from the 3D spatial coordinates of the scattering centers. The effectiveness and robustness of the proposed algorithm is first proven theoretically and then tested against several radar-target scenarios as well as in the presence of noise.

From the perspective of radar signal processing, the pulse-Doppler radar signal model is mostly based on the stop-and-go model due to the short pulse propagation time. Unlike the pulse-Doppler radar, the frequency modulated continuous wave (FMCW) radar takes a long time in the process of signal transmission to echo reception. During this period, the target cannot be considered to be stationary during the acquisition of the entire sweep echo, so the stop-and-go assumption in the FMCW system is no longer valid, which means that the target's motion related to the fast time must be taken into account. In the inverse synthetic aperture radar (ISAR) imaging scenario of high-speed maneuvering targets, the frequency offset correction introduced by the target's motion becomes reasonably complicated that it is difficult to restore the usual ISAR signal processing. In this work, a new method based on phase retrieval and image contrast-based autofocus (ICBA) is proposed to address the complex problem of motion compensation for FMCW ISAR systems. To cope with the fast time-varying characteristics of the phase error and the higher number of phase terms relative to pulsed radars, the proposed algorithm performs fine compensation with higher accuracy, resulting in high-resolution and autofocus imaging. Experimental results demonstrate the effectiveness of the proposed method.

Harmonic radar technology has been researched and developed for studying the foraging behaviour of honey bees. This work presents the development of a working prototype of a narrow-band harmonic radar system, at 2.5/5GHz, using off-the-shelf components to detect the presence of Indian honey bees. Based on empirical tests on the tag, we estimate the radar probability of detection and probability of false alarms as 88% and 0.3% respectively. The maximum detectable range for a monostatic configuration of the radar is 3.84 m.

Monitoring technologies are needed to track insects and gain a better understanding of their behavior, population, migration, and movement. A portable microwave harmonic-radar tracking system that utilizes antenna miniaturization techniques was investigated to achieve this goal. The system mainly consists of a compact radar unit, a hand-held transmitter/receiver (Tx/Rx) patch antenna array panel, and a passive radio frequency (RF) transponder. The system transmits a signal at 5.882 GHz and receives returned signals in the 11.764 GHz band. The RF transponder to be mounted on insects is a 9.63 mm 9.63 mm flat plane weighing 6 mg. Tests were conducted to optimize the horizontal and vertical tracking ranges of the system by measurement of radio frequency signal strength from the transponder in open terrain, inside tree canopies, and underground at different tracking angles. In open terrain, a directional tracking range of over 60 m was achieved. However, obstructions from tree canopies and the high water content of leaves severely weakened the microwave signal strength and restricted the tracking range to below 20 m. To obtain maximum signal strength, the Tx/Rx antenna panel and the transponder must be in a mutually clear line of sight and their angles must be maintained within a 10 range. However, the use of harmonic radar technology to develop a portable tracking system with a small insect-borne transponder currently poses insurmountable challenges for antenna design due to the very stringent requirements to accommodate the tracking range, the transponder size and mass, and the methods used to attach the transponders to insects.