Aerial Synthetic Aperture Radar Research Articles

Backprojection SAR interferometry

Synthetic aperture radar (SAR) interferometry uses the phase difference between two SAR antennas to obtain an elevation estimate of the imaged terrain. Using an initial digital elevation model (DEM), the time-domain backprojection algorithm implicitly removes the terrain height phase from images during image formation. The use of a DEM during image formation adds additional information to the process of interferometry, resulting in different sensitivities to conventional interferometry. This article presents a novel method of SAR interferometry using backprojected imagery. It is shown that the sensitivity and performance of backprojection interferometry is significantly different to that of conventional methods. Specifically, it is shown that backprojection interferometry is much less sensitive to errors in measurement of the interferometric baseline length and angle. This comes at the expense of higher sensitivity to phase errors. We conclude that backprojection interferometry is best suited for airborne operation.

Application of a new algorithm using Doppler information to retrieve complex wind fields over the Black Sea from ENVISAT SAR images

Several algorithms have been proposed to retrieve near-surface wind fields from C-band synthetic aperture radar (SAR) images acquired over the ocean. They mainly differ in the way they retrieve the wind direction. Conventionally, the wind direction is taken from atmospheric models or is extracted from the linear features sometimes visible in SAR images. Recently, a new wind retrieval algorithm has been proposed, which also includes the Doppler shift induced by motions of the sea surface. In this article, we apply three wind retrieval algorithms, including the one using Doppler information, to three complex wind events encountered over the Black Sea and compare the SAR-derived wind fields with model wind fields calculated using the high-resolution weather research and forecasting (WRF) model. It is shown that the new algorithm is very efficient in resolving the 180° ambiguity in the wind direction, which is often a problem in the streak-based wind retrieval algorithms. However, the Doppler-based algorithm only yields good results for wind directions that have a significant component in the look direction of the SAR antenna. Furthermore, it is dependent on good separation of the contributions to the Doppler shift induced by surface currents and wind-related effects (wind drift and wind-sea components of the ocean wave spectrum). We conclude that an optimum wind retrieval algorithm should consist of a combination of the algorithms based on linear features and Doppler information.

Nonlinear Vibration Analysis of Membrane SAR Antenna Structure Adopting a Vector Form Intrinsic Finite Element

ABSTRACTThis study adopted the Vector Form Intrinsic Finite Element (VFIFE) method to study the nonlinear vibration of the membrane SAR (Synthetic Aperture Radar) antenna structure. As the dynamic characteristic of the antenna is mainly determined by the support frame, it can be simplified as an axially loaded cantilever beam. The linear and geometrically nonlinear models of the axially loaded cantilever beam are established. The beam is modeled as discrete mass points which are connected by deformable elements through VFIFE method. A statics analysis is first presented to verify the VFIFE method. Then effects of the geometrical nonlinearity and axial load are investigated. It is believed that the presented study is valuable for better understanding the influences of the geometrical nonlinearity and axial load of the cantilever beam on the structural vibration characteristics.

Joint synthetic aperture radar and space-time adaptive processing on a single receive channel

Wide-area persistent synthetic aperture radar (SAR) video offers the ability to track ground-moving targets via changes between video frames. A shortcoming is the difficulty in maintaining track when a Doppler shift places moving target returns colocated with strong clutter. We propose to augment a SAR modality with space-time adaptive processing (STAP). The wide-area SAR capability generates a large data rate, stressing on-board processing or a ground station data link; therefore, expansion to multiple radar channels for STAP can present a system bottleneck. Consequently, we adopt a multiple-transmit, single-receive (MISO) architecture that constrains the system data rate for down-link or on-board processing to that of a single antenna SAR system. A frequency division design for quasi-orthogonal transmit waveforms is presented; the approach maintains coherence on clutter, achieves the maximal unaliased band of radial velocities set by the pulse repetition frequency of the system, retains full resolution SAR images, and requires no increase in receiver data rate vis-a-vis the wide-area SAR modality. For N transmit antennas and Ns samples per pulse, the enhanced sensing provides a STAP capability with N times larger range bins than the SAR mode at the cost of O(Ns) increase in computation per pulse.

Beam broadening synthesis for SAR antenna array based on Projection Matrix Algorithm

A new beam broadening synthesis technique for Synthetic Aperture Radar (SAR) antenna array, namely Projection Matrix Algorithm (PMA) is presented. The theory of PMA is introduced firstly, and then the iterative renewed manner is improved to resolve the unbalance problem under amplitude and phase control. In order to validate the algorithm correct and effective, an actual engineering application example is investigated. The beam synthesis results of 1.0∼4.5 times broadening under the phase only control and the amplitude and phase control using improved PMA are given. The results show that the beam directivity, the beam broadening, and the side-lobe level requirements were met. It is demonstrated that the improved PMA was effective and feasible for SAR application.

위성용 전개형 SAR 안테나 형상 설계 및 툴킷 개발

최근 지구관측위성으로 전천후 사용가능한 영상레이더의 중요성이 매우 높아지고 있다. 또한 저비용 고효율 위성의 개발추세에 따라 경량화 SAR 안테나에 관한 연구가 활발히 이루어지고 있다. CFRP를 이용한 전개형 SAR 안테나는 여러 방법 중 장점이 많은 후보 중 하나이다. 본 논문에서는 전개형 SAR 안테나의 형상에 필요한 변수들을 찾아내 수치화 하였으며, 안테나 크기에 따른 최적값을 찾아 설계하였다. 설계된 형상은 유한요소 모델해석을 통해 구조적 안전성을 분석하였다. 마지막으로 체계요구규격의 변경에 따른 안테나 형상의 변경을 수월하게 진행할 수 있도록 툴킷을 개발하였다. 툴킷을 통해 변수에 따른 안테나 형상모델 설계 및 힌지 공차에 따른 안테나면의 오차분석을 수행하였으며, 전자파 해석 프로그램과의 호환성을 확인하였다. Significance of SAR(Synthetic Aperture Radar) satellite regadless of weather have grown for Earth observation. According to the cost-effective trend in satellite development, SAR antenna is actively studied. It's a competitive candidate to use deployable SAR antenna out of CFRP. In this study, variables for an antenna configuration model was researched and evaluated. The design of the antenna was structurally analyzed by FEM(Finite Element Model). Tool-kit was developed for modifying the SAR antenna model easily in accordance with system requirement change. In the tool-kit, antenna configuration design and error analysis of the antenna surface could be achieved. And compatibility of tool-kit results to CST, a RF analysis program, was confirmed.

전개형 위성 안테나 메커니즘 모델링 및 분석

지구 관측용 위성의 한 종류인 SAR(Synthetic Aperture Radar) 위성은 지구의 영상 획득 시, 주변 환경의 영향을 받지 않는다는 장점이 있어 현재까지 많은 수가 개발되었다. 높은 영상 품질을 얻기 위해 SAR 안테나는 큰 직경을 가져야 한다. 그러나 위성을 우주까지 운반하는 발사체의 내부 공간은 한정적이므로 SAR 안테나는 발사체 내부 배치 시에는 접어놓았다가 우주에서는 펼쳐질 수 있는 전개형으로 설계된다. 본 연구에서는 위성의 목표 임무를 고려하여 전개형 위성 안테나의 메커니즘과 관련된 설계 요소 값들을 선정하였다. 이를 바탕으로 전개형 위성 안테나의 형상을 설계하였으며, 시뮬레이션을 통해 위성 기동 시 전개형 위성 안테나에 발생하는 변화를 관찰하였다. Large number of SAR(Synthetic Aperture Radar) satellites, one type of earth observation satellite, have been developed as they have the advantage of not being affected by surrounding environment during the earth image acquisition. In order to gain high image quality, SAR antenna should have large diameter. However, internal space of satellite launch vehicle is limited and this leads SAR antenna to be designed deployable so that it can be folded in launch vehicle and unfolded in space. In this research, values of various design factors of deployable satellite antenna were chosen considering satellite's target mission. Configuration of deployable satellite antenna was designed by applying the chosen values of design factors, and variation in deployable satellite antenna during satellite maneuver was observed through simulation.

The Analysis of Moonborne Cross Track Synthetic Aperture Radar Interferometry for Global Environment Change Monitoring

Faced to the earth observation requirement of large scale global environment change, a SAR (Synthetic Aperture Radar) antenna system is proposed to set on Moon's surface for interferometry in this paper. With several advantages superior to low earth obit SAR, such as high space resolution, large range swath and short revisit interval, the moonborne SAR could be a potential data resource of global changes monitoring and environment change research. Due to the high stability and ease of maintenance, the novel system is competent for offering a long and continuous time series of remote sensing imagery. The Moonborne SAR system performance is discussed at the beginning. Then, the peculiarity of interferometry is analyzed in both repeat pass and single pass cases. The chief distinguishing feature which is worth to research the potentiality of repeat pass interferometry is that the revisit interval is reduced to one day in most cases, and in worst case one month. Decorrelation deriving from geometry variety is discussed in detail. It turns out that the feasibility of moonborne SAR repeat pass interferometry depends on the declination of Moon. The severity of shift effects in radar echoes increased as Moon approaches to the equatorial plane. Moreover, referring to the single pass interferometry, two antennas are assumed to set on different latitude of Moon. There is enough space on Moon to form a long baseline, which is highly related to the interferogram precision.

Compact waveguide broadband dual-polarized horn array with a novel orthomode transducer in Ku-band for high-power SAR systems

A novel compact waveguide-based broadband and dual-polarized Ku-band Synthetic Aperture Radar antenna with a high level of isolation between polarizations and high input power level has been designed for small platforms and unmanned aerial vehicles (UAV). The antenna design is composed of 8 dual-polarized horn antennas in an array configuration fed through a very efficient and novel structure to generate two orthogonal polarizations based on a orthomode transducer to achieve a large bandwidth and an excellent isolation between polarizations needed in a polarimetric system. Finally, as the antenna will be placed inside a pod under the UAV wings it must be lightweight, robust and of small size.

Design of 40-W AlGaN/GaN MMIC High Power Amplifiers for $C$-Band SAR Applications

Two C-band monolithic high power amplifiers (HPAs) have been designed and implemented exploiting a 0.25-μm AlGaN/GaN HEMT process on an SiC substrate. The circuits have been designed for use in transmit/recevie modules of satellite synthetic aperture radar antennas for Earth observation. The design was accurately focused on the HEMTs' electrical and thermal working conditions in order to guarantee the reliable operation required by space applications. The HPAs operate in pulsed conditions with typical pulsewidth of 50 μs and 10% duty cycle: in that regime, the circuits deliver about 40 W with more than 21-dB associated gain and 40% to 45% power-added efficiency in the 5-5.8-GHz band. The achieved performance clearly demonstrates the very high potentiality of this technology for the replacement of GaAs-based HPAs in new generations of this type of systems.

On the ballistic configuration during interferometric imaging

Types of ballistic configuration of the satellite formation for Earth remote sensing for interferometric imaging using a synthetic aperture radar antenna are considered. Possible configurations for multiposition satellite formations are provided.

Tri-Band Dual-Polarization Shared-Aperture Microstrip Array for SAR Applications

The design of a tri-band dual-polarization (TBDP) shared-aperture microstrip array antenna for synthetic aperture radar (SAR) applications is presented. It operates at L-, Sand X-bands with a frequency ratio of 1:2.8:8. This TBDP shared-aperture array is assembled by two L/S and L/X DBDP shared-aperture sub-arrays with one L-band dual-polarized sub-array. A prototype array has been fabricated and measured, which exhibits wide impedance bandwidth (VSWR ≤ 2) of 13.4%, 14.8%, and 16.8% in L-, S-, and X-bands, respectively. The measured array isolation is better than -27 dB in all three bands. The measured radiation patterns agree well with the simulation, confirming the cross-polarization level of below dB within the main lobe region, and the array scanning capability of ±27° in S- and X-bands. This array design approach can be further extended to shared-aperture arrays with more than three bands.

Outer circular synthetic apeture radar imaging based on Maxwell’s equations

A general synthetic aperture radar (SAR) signal model based on the Maxwell's equations is derived, and three approximations are discussed for engineering applications. Based on this signal model, a novel operation of SAR, called outer circular synthetic aperture radar (Outer-CSAR), is investigated for wide observation. The Outer-CSAR works similarly to the general circular SAR, but the beam of the SAR antenna points at the outer of the circle instead of the inner. The signal model and imaging algorithm are presented for the Outer-CSAR, and furthermore, simulation is given to validate the signal model and imaging algorithm.

Large SAR Antenna Deployable Structure Design and Optimization

Remote sensing satellites equipped with large synthetic aperture radar (SAR) can image the earth in all weather and lighting conditions. For high packaging efficiency in launch and precise configuration on orbit, deployable mechanisms are designed to deploy the large SAR antenna panels. For the two-panel antenna, a six-bar-linkage deployable driving mechanism is designed. According to the design, a parameterized dynamic model is built in ADAMS software with secondary development technology to obtain its dynamics characteristics through simulation analysis. To improve the dynamic characteristics and the precision of deployment design of experiments and optimizations were carried through in this paper.

Virtual Antenna Array Analysis for MIMO Synthetic Aperture Radars

Multiple-input multiple-output (MIMO) synthetic aperture radar (SAR) that employs multiple antennas to transmit orthogonal waveforms and multiple antennas to receive radar echoes is a recently proposed remote sensing concept. It has been shown that MIMO SAR can be used to improve remote sensing system performance. Most of the MIMO SAR research so far focused on signal/data models and corresponding signal processing algorithm. Little work related to MIMO SAR antenna analysis can be found. One of the main advantages of MIMO SAR is that the degrees of freedom can be greatly increased by the concept of virtual antenna array. In this paper, we analyze the virtual antenna array for MIMO SAR high-resolution wide-swath remote sensing applications. The one-dimensional uniform and nonuniform linear antenna arrays are investigated and their application potentials in high-resolution wide-swath remote sensing are introduced. The impacts of nonuniform spatial sampling in the virtual antenna array are analyzed, along with a multichannel filtering-based reconstruction algorithm. Conceptual system and discussions are provided. It is shown that high operation flexibility and reconfigurability can be obtained by utilizing the virtual antenna arrays provided by the MIMO SAR systems, thus enabling a satisfactory remote sensing performance.

Bistatic Forward-Looking SAR: Results of a Spaceborne–Airborne Experiment

Forward-looking radar imaging continues to gain in significance due to a variety of convenient applications, like landing assistance for aircraft in poor visibility conditions. Synthetic aperture radar (SAR) techniques are typically used to achieve a high azimuth resolution, but conventional monostatic SAR is not applicable in forward direction because of azimuth ambiguities and poor Doppler resolution. To improve the Doppler resolution and to avoid azimuth ambiguities, bistatic SAR configurations can be used to obtain high-resolution radar images. This is demonstrated for the first time in a spaceborne-airborne SAR experiment by using TerraSAR-X as the illuminator and the Phased Array Multifunctional Imaging Radar as the receiver. For convenience, the receiver's SAR antenna was mounted on the aircraft's loading ramp and looked backward. Due to identical image properties and the same challenges for forward- and backward-looking sensors, this configuration also demonstrates the feasibility of forward-looking bistatic SAR. This letter describes the experimental setup, analyzes the performance, and presents the imaging results.

ECCM AUXILIARY ANTENNA SYSTEM FOR A MULTIBEAM SAR ARRAY

A SAR (Synthetic Aperture Radar) antenna typically consists of a large planar array to obtain a directive pattern with low side lobe level. A multibeam SAR system requires a set of secondary beams, implementing some additional features. To protect the SAR operation from possible jammers ECCM (Electronic Counter-Countermeasure) capabilities can be implemented requiring an auxiliary beam which cover the portion of main pattern to be protected. A possible architecture of this secondary antenna system is here presented by using a set of radiating elements combined together to obtain the ECCM channel. To minimize the feeding network complexity and to maximize the system reliability, the ECCM antenna is a cluster of circular corrugated horns, eventually realized as a single-block unit.

능동형 SAR 시스템의 송수신 모듈 출력 보정 방법 연구

본 논문에서는 기 개발된 능동형 SAR(Synthetic Aperture Radar) 안테나의 송수신 모듈 출력에서 발생하는 이득과 위상 오차를 보정하는 방법에 대해 소개한다. 먼저 오차에 대해 정의하고, 시범 측정을 통해 획득한 측정값을 바탕으로 오차의 특성을 분석한 뒤, 이를 보상하기 위한 송수신 모듈의 제어 방법을 제안하였다. 16조의 송수신 모듈로 구성된 안테나에서 보정 전 이득은 28.2~29.0 dBm, 위상은 <TEX>$101.7^{\circ}{\sim}165.2^{\circ}$</TEX>의 분포를 보였으나, 보정후 이득은 27.4~28.0 dBm, 위상은 <TEX>$116.1^{\circ}{\sim}120.0^{\circ}$</TEX>의 분포를 나타내었다. 이러한 송수신 모듈의 출력 분포를 바탕으로 배열 이론을 적용하여 안테나 빔 패턴을 생성하고 비교함으로써 제안한 보정 방법이 능동형 SAR 시스템의 성능 만족에 효과적임을 검증하였다. In this paper, a compensation method of the amplitude and phase errors from the T/R(Transmit/Receive) modules in an active SAR(Synthetic Aperture Radar) system is introduced. The errors are defined and classified, and characterized by analyzing the measurement data acquired from the pilot test. To compensate these errors, a control methodology of T/R modules output is proposed. Before the compensation is applied, 16 T/R modules integrated on the active SAR antenna show the amplitude in 28.2~29.0 dBm and the phase in <TEX>$101.7^{\circ}{\sim}165.2^{\circ}$</TEX>. After the compensation, the amplitude and phase are distributed in 27.4~28.0 dBm and <TEX>$116.1^{\circ}{\sim}120.0^{\circ}$</TEX> respectively. The antenna beam patterns generated by the array theory with the distributions are compared, and the proposed method is verified as good to apply for the active SAR system.

FA-SCANSAR: FULL APERTURE SCANNING PULSE BY PULSE FOR THE NEARSPACE SLOW-MOVING PLATFORM BORNE SAR

Because the nearspace slow-moving platform borne synthetic aperture radar (SAR) can realize high resolution imaging using low pulse repetition frequency (PRF), a full-aperture ScanSAR (FA-ScanSAR) operation, which switches the range beam pulse by pulse, was proposed for wide swath imaging. This operation separates the wide swath into several sub-swaths, and each of which can be illuminated by a narrow range beam. The SAR antenna switches the range beam to point at each of the sub-swaths in turn, transmits pulses and receives echoes pulse by pulse. The design method of main system parameters and the calculating expressions of the performance indexes are addressed in the paper. A design example is given to compare the performance of the conventional strip operation, ScanSAR and FA- ScanSAR operation. The results show that FA-ScanSAR operation can obtain high resolution by full aperture accumulation in wide swath and improve the signal-to-noise ratio of SAR images for the nearspace slow-moving platform borne SAR.

Joint Regularization of Phase and Amplitude of InSAR Data: Application to 3-D Reconstruction

Interferometric synthetic aperture radar (SAR) images suffer from a strong noise, and their regularization is often a prerequisite for successful use of their information. Independently of the unwrapping problem, interferometric phase denoising is a difficult task due to shadows and discontinuities. In this paper, we propose to jointly filter phase and amplitude data in a Markovian framework. The regularization term is expressed by the minimization of the total variation and may combine different information (phase, amplitude, optical data). First, a fast and approximate optimization algorithm for vectorial data is briefly presented. Then, two applications are described. The first one is a direct application of this algorithm for 3-D reconstruction in urban areas with very high resolution images. The second one is an adaptation of this framework to the fusion of SAR and optical data. Results on aerial SAR images are presented.