Design of Vortex Forward-Looking SAR Imaging using OAM Beam Modulation

The traditional range Doppler (RD) imaging algorithm has become the most intuitive and classical method in synthetic aperture radar (SAR) imaging processing because of its easy implementation and high processing efficiency. However, owing to the poor quality of the image produced by RD algorithm, this traditional imaging method is growing increasingly unable to meet practical needs. In an effort to solve the problem of poor azimuth imaging quality, this study proposes a high-resolution RD azimuth imaging algorithm. The expression of the optimal order of the SAR azimuth signals using fractional Fourier transform (FrFT) is derived in detail herein. A theoretical analysis shows that this optimal order is unique and depends on the azimuth imaging parameters. The experimental results based on airborne SAR simulation data and spaceborne SAR measurement data indicate that the azimuth resolution of the proposed algorithm is improved by 31.6 or 46.8% as against that of the traditional RD algorithm, whereas the peak side lobe ratio (PSLR) and integrated side lobe ratio (ISLR) values obtained via the proposed algorithm are approximately equivalent to those obtained via the traditional RD algorithm.

The vortex electromagnetic (EM) wave carrying orbital angular momentum (OAM) is promising to benefit the higher degree of freedom for synthetic aperture radar (SAR) imaging. Combining the traditional two-dimensional (2D) SAR and the vortex EM wave, vortex SAR can obtain the three-dimensional (3D) information to realize 3D imaging. This letter proposes a novel 3D vortex SAR imaging method based on the vortex EM wave to obtain 3D target information. First, a vortex SAR imaging model is established by the proposed 3D imaging algorithm. Then, the modified range-doppler (RD) algorithm is therefore proposed to reconstruct the target's profiles. The objective of the 3D RD algorithm is to decompose the time-domain correlation operations of the SAR 3D space into three independent one-dimensional correlations and then perform separate focusing processing on each dimension. Finally, the 3D information of the targets can be obtained according to the modified RD algorithm. The point target simulation is performed in this paper. The simulation results establish the effectiveness of the proposed technique. This paper lays the groundwork for the development of vortex EM waves in the field of 3D radar imaging.

For a spaceborne or an airborne synthetic aperture radar (SAR) system, it may be easily interfered with radio frequency interferences (RFIs) due to the increase demand for frequency occupation. The RFIs introduce artifacts in the focused SAR imagery, resulting in high noise floor and erroneous interpretations. Previous works on narrowband RFI mitigation mainly focused on single-channel SAR systems, while with the development of SAR imaging technology, the high-resolution wide-swath (HRWS) imagery technology now reaches its maturity to finally take shape in current SAR systems. To obtain HRWS images, the multi-channel SAR (MC-SAR) system has been employed to tackle the contradictory requirements for both high resolution and low pulse repetition frequency (PRF). In this paper, we propose a novel interference-mitigation model and analyze the low-rank property of the narrowband RFI for HRWS SAR systems. Then, we employ an image-domain sparse regularization to protect the real echoes of the SAR system and mitigate the RFIs by solving the low-rank recovery problems of RFIs. The real airborne SAR data is used to demonstrate the effectiveness of the proposed method.

Bistatic radar possesses the forward-looking imaging performance that conventional synthetic aperture radar (SAR) lacks. Nevertheless, bistatic forward-looking imaging is seriously limited by the motion of the transmitter and receiver, resulting in a restrained resolution. In this work, the method for vortex missile-borne bistatic SAR (BSAR) is proposed, which combines the orbital angular momentum (OAM) beam with the traditional missile-borne BSAR technique to obtain a larger Doppler bandwidth. Higher azimuth resolution is achieved by compensating large spatial range cell migration (RCM) and serious range-azimuth coupling. First, the imaging scene of vortex missile-borne BSAR and the OAM-based echo model are established and deduced. Second, the preprocessing method for amplitude compensation is proposed with the design of the transmitted OAM beam, which improves the uncorrelation between different pulse echoes. Then, the improved range Doppler (RD) algorithm with azimuth compensation factor is proposed for vortex missile-borne BSAR to obtain the super-resolution 2-D reconstruction for the target. Simulation results show that compared with conventional missile-borne BSAR, the proposed vortex missile-borne BSAR possessing the extending Doppler bandwidth can obtain superior azimuth resolution performance with the identical synthetic aperture length situation. Furthermore, this work will contribute to the application for OAM-carrying in the field of radar forward-looking imaging and the development for high-resolution missile-borne BSAR imaging technology.

Because of the influence of large range walk and geometrical distortion, traditional synthetic aperture radar (SAR) imaging algorithms, such as range doppler (RD) etc., cannot be used for three demensional (3-D) forward looking SAR (F-SAR) imaging directly. In this paper, we first derive the chirp scaling algorithm (CSA) for 3-D F-SAR. Next, we analyze the geometrical distortion analytically and provide an effective approach to reconstruct the scatters to their exact positions without interpolation. Moreover, we briefly discuss the point spread function's isosurface shape and how geometrical distortion correction (GDC) influence on it. Finally, the simulation experiments about single point and multipoint verify the algorithm's validity.

Bistatic synthetic aperture radar (SAR) operateswith spatially separated transmit and receive antennas that are mounted on separated platforms. Provided that there is an overlap of both antenna footprints, the platforms can move with different velocities in arbitrary directions. A special configuration is given, when the receive antenna looks in forward direction, which is called bistatic forward-looking SAR. Besides the well known advantages of bistatic SAR like the increased information content of the data because of different RCS and scattering characteristics, such a configuration enables high resolution imaging in forward direction, which is not possible with conventional monostatic SAR systems. This paper analyzes a bistatic forward-looking configuration and demonstrates the capability and feasibility of imaging in forward or backward direction using the radar satellite TerraSAR-X as transmitter and the airborne SAR system PAMIR as receiver.

Electromagnetic (EM) vortex wave carrying orbital angular momentum (OAM) can potentially be utilized to achieve azimuthal super-resolution in synthetic aperture radar (SAR) imaging realms. This contribution proposes an imaging algorithm based on the compressed sensing (CS) theory for the side-view EM vortex strip-map SAR. Firstly, the observation geometry and the echo signal model are described and established. Subsequently, the imaging algorithm, including Bessel function compensation, range compression and azimuth process, is carried out to realize the two dimensional (2D) joint detection of the point target in the range and azimuth domain. Simulation results validate the effectiveness of the proposed algorithm, along with the analysis the different CS reconstruction algorithms for multi-target imaging. Compared with the existing traditional Range Doppler (RD) algorithm, the proposed method can achieve superior azimuth resolution for target identification, which not only solve the problem of target's azimuth profile at high side-lobe levels, but also reduce the cost of radar hardware system. The work and results provide suggestions to the development of forthcoming and new-generation EM vortex SAR imaging technology.

As having the imaging ability of area in front of flight direction, forward-looking synthetic aperture radar (SAR) has become a hot topic in areas of SAR research. Nevertheless, existing imaging algorithms of forward-looking SAR are all based on the assumption that the platform is moving in low velocity. When this assumption is not satisfied, the imaging result of existing algorithms will show focus out effect and deviation of imaging position from real position. Aiming at this problem and taking airborne forward-looking SAR system using single transmitting and multiple receiving antennas as the research object, this paper analyzes the feasibility of forward-looking SAR imaging of high velocity platform, and proposes a forward-looking SAR imaging algorithm suitable for high velocity platform according to its spatial geometry and the model of echo signal. Finally, the raw echo data of an array of point targets is simulated, and the focused images are obtained using the algorithm proposed in this paper. Imaging results demonstrate the validity of the algorithm. (6 pages)

Ultrahigh frequency ultrawideband (UHF UWB) synthetic aperture radar (SAR) system can penetrate the foliage, get the high-resolution image, and add the scattered information, which can be used to detect the concealed area under the foliage. In fall, the monostatic and bistatic campaign with the airborne UHF UWB SAR system has been conducted. This experiment operates the airborne SAR system, in conjunction with a ground-based stationary receiver, which can simultaneously collect the monostatic and bistatic scattered data. The fast factorized back projection (FFBP) algorithm has been applied to process the monostatic and bistatic SAR scattered data, as well as the subaperture spectrum-equilibrium method. Focused monostatic and bistatic SAR images prove the feasibility and property of the developed airborne monostatic and bistatic UHF UWB SAR.

Based on analysis of a point target imaged by different synthetic aperture radar (SAR) systems, the commonly used impulse response function in SAR Imaging (IRF-SAR)-a two-dimensional (2-D) sinc function-is shown to be inappropriate for ultrawideband-ultrawidebeam (UWB) SAR systems utilizing a large fractional signal bandwidth and a wide antenna beamwidth. As a consequence, the applications of the 2-D sinc function such as image quality measurements and spatial resolution estimations are limited to narrowband-narrowbeam (NB) SAR systems exploiting a small fractional signal bandwidth and a narrow antenna beamwidth. In this paper, a more general IRF-SAR, which aims at UWB SAR systems, is derived with an assumption of flat two-dimensional (2-D) Fourier transform (FT) of a SAR image and called IRF-USAR. However, the derived IRF-USAR is also valid for NB SAR systems.

Synthetic aperture radar (SAR), as a wideband radar system, may be subject to strong interferences with a variety of signals. The narrowband interference (NBI) exemplifies the most typical of its kind, such as the form of radio frequency interference (RFI). With the development of SAR imaging technology, the high-resolution wide-swath (HRWS) imagery technology now reaches its maturity to finally take shape in current SAR systems. To obtain HRWS images, the multichannel SAR (MC-SAR) system has been employed to tackle the contradictory requirements for both high resolution and low pulse repetition frequency (PRF). Previous interference methods focused on single-channel SAR systems and few research works for MC-SAR systems. In this article, we first derive a new interference-mitigation model for HRWS SAR systems and conclude that the low-rank property of the NBI is suitable for MC-SAR systems. Then we employ an image-domain sparse regularization to protect the real echoes of the SAR system and mitigate the NBIs by solving the low-rank recovery problems of NBIs. Also, the MC-SAR system errors are further taken into account as a measure for our method’s practical applicability. Finally, the real SAR data is used to demonstrate the effectiveness of the proposed method.

Synthetic aperture radar (SAR) raw data do not have a direct application; therefore, SAR raw signal processing algorithms are used to generate images that are used for various required applications. Currently, there are several algorithms focusing SAR raw data such as the range-Doppler algorithm, Chirp Scaling algorithm, and Omega-k algorithm, with these algorithms being the most used and traditional in SAR raw signal processing. The most prominent algorithm that operates in the frequency domain for focusing SAR raw data obtained by a synthetic aperture radar with large synthetic apertures is the Omega-k algorithm, which operates in the two-dimensional frequency domain; therefore, in this paper, we used the Omega-k algorithm to produce SAR images and modify the Omega-k algorithm by adding the Doppler factor to improve the accuracy of SAR raw data processing obtained by the continuous wave and pulsed frequency modulated linear frequency modulated radar system from the surfaces of interest. On the other hand, for the case of unmanned aerial vehicle-borne linear frequency modulated continuous wave (LFM-CW) SAR systems, we added motion compensation to the modified Omega-k algorithm. Finally, the testing and validation of the developed Omega-k algorithm used simulated and real SAR raw data for both pulsed synthetic aperture and continuous wave radars. The real SAR raw data used for the validation of the modified Omega-k algorithm were the raw data obtained by the micro advanced synthetic aperture radar (MicroASAR) system, which is an LFM-CW synthetic aperture radar installed on board an unmanned aerial system and the raw data obtained by European remote sensing (ERS-2) satellite with a synthetic aperture radar installed.

Synthetic aperture radar (SAR) is an active sensor that is operated using the moving platforms such as satellite, aircraft, or unmanned aerial vehicle (UAV). As a kind of imaging radar, it uses microwave to detect the target in remote area. Due to the characteristics of microwave, the SAR can be operated regardless of the weather conditions. Also, it offers highresolution images of targets by utilizing the wide-bandwidth transmit signal. Due to the features of SAR, it is widely used in several area such as surveillance, land monitoring, urban managing, disaster monitoring etc. To conduct the continuous monitoring for some specific areas, the SAR payloads are loaded on UAV and operated recently. Several UAV-SARs exist already in research fields and practical uses (e.g. AirMOSS of NASA). These platforms usually perform its missions on 1-2 km above the ground. The specifications of the recent UAV-SARs and the conceptual design of the proposed UAV-SAR are listed in Table 1 [1]. To achieve the high-resolution, the SAR system uses a linear frequency modulated (LFM) signal called chirp. As the bandwidth of transmit signal is inverse proportional to the resolution, the SAR system requires wide-bandwidth signal to offer the better performance. There are several types of chirp signal generators: memory-map based type, signal generators using frequency multipliers, direct digital frequency synthesizer (DDFS) type, and parallel DDFS type. The memory-map based chirp signal generator stores the predefined chirp waveform in the PROM or other memory devices and loads the signal using the counter. It has the advantage of high-precision output. However, it requires the large sized ROM to store the wide-bandwidth signal and hard to update the predefined signal especially the SAR is operated in space mission. Compared to the previous one, the DDFS generates only the phase of the signal using registers and full-adders. Next, it combines the phase signal and the amplitude signal that is stored look-up table (LUT). When DDFS generates the desired signal, it controls the frequency control word (FCW). The fast switching speed, less memory dependency, simple structure, easy configurability, and etc. are the features of DDFS. However, as this system truncates the I/O bits between the phase signal generator and LUT, the signal characteristics are degraded due to the spurs. The signal generator using frequency multiplier is proposed to realize the wide-bandwidth signals. First, it generates the relatively narrow band signal. Next, using the frequency multiplier and band-pass filter (BPF), it expands the bandwidth of transmit signal. Using the frequency multiplier, SAR system can achieve the wide-bandwidth with simple structure. However, as the bandwidth of signal is multiplied with the hardware, also the residual noise of the signal gets larger. In this paper, the parallelized DDFS type chirp signal generator is presented. The signal generators are operated based on a digital circuit. Therefore, the baseband output bandwidth of the presented signal generation methods is limited due to the clock frequency of the devices. However, by using the multiple memory and the high-speed multiplexer, the parallelized DDFS can generate the wide-bandwidth chirp signal with relatively low clock frequency. As the recent SAR system requires the high-resolution images, the SAR system equipped with wideband signal generator is also needed. In this paper, the conceptual design for X-band UAV SAR and the parallelized DDFS chirp signal generator for the wideband chirp pulse (up to 800 MHz) have been proposed. Using the circular polarization, the proposed SAR system can be used to full-polarimetric SAR. Also, due to the wideband chirp pulse, the SAR system can offer the images of sub-meter resolution.

Synthetic Aperture Radar (SAR) was invented by Carl Wiley at Goodyear Aircraft Company in Goodyear, Arizona, in 1951. From that time forward, as the company became Goodyear Aerospace Corporation, Loral Corporation, and finally Lockheed Martin Corporation, the Arizona employees past and present played a long and storied role in numerous SAR firsts. These include the original SAR patent (known as Simultaneous Doppler Buildup), the first demonstration SAR and flight test, the first operational SAR system, the first operational SAR data link, the first 5-foot resolution operational SAR system, the first 1-foot resolution SAR system, and the first large scale SAR digital processor. The company has installed and flown over five hundred SAR systems on more than thirty different types of aircraft for numerous countries throughout the world. The company designed and produced all of the evolving high performance SAR systems for the U. S. Air Force SR-71 “Blackbird” spy plane throughout its entire operational history, spanning some twenty-nine years. Recent SAR accomplishments include long-range standoff high performance SAR systems, smaller high resolution podded SAR systems for fighter aircraft, and foliage penetration (FOPEN) SAR. The company is currently developing the high performance SAR/MTI (Moving Target Indication) radar for the Army Aerial Common Sensor (ACS) system.

Intensive research is currently concentered at the field of Space-borne Reflector Synthetic Aperture Radar (SAR) Systems for high resolution wide-swath application. To meet the requirement of high resolution wide-swath SAR system, multichannel systems based on Reflector SAR system is introduced, for example, Tandem-L/NISAR Mission. This paper introduces an air-borne SAR system based on Reflector SAR system, and takes several flight experiments. The results show that using Digital beam forming (DBF) algorithm, a simultaneous improvement in azimuth resolution and wide swath can be achieved.