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Spaceborne Synthetic Aperture Radar (SAR) is a unique tool for large scale Earth observation, but the current generation of SAR sensors suffers from some fundamental limitations with regard to their imaging and mapping capabilities. To overcome these limitations, several new instrument architectures and SAR imaging modes have been suggested that em...
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... example is the combination of the displaced phase center technique of the previous section with a ScanSAR or TOPS mode (cf. Fig. 2, top left). As in classical ScanSAR, azimuth bursts are used to map several swaths. The associated resolution loss from sharing the synthetic aperture among different swaths is compensated by illuminating a wider Doppler spec- trum and collecting the radar echoes with multiple displaced azimuth apertures. Such a system is cur- rently ...
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... arriving simultane- ously from different directions. For this, multiple nar- row elevation beams are formed where each beam fol- lows the echo of a different pulse transmitted by a wide beam illuminator. This enables an increase of the coverage area without the necessity to either lengthen the antenna or to employ burst modes. The top right of Fig. 2 provides an illustration, where three narrow Rx beams follow the echoes from three simultane- ously mapped image swaths that are illuminated by a broad Tx beam. A sufficiently high antenna is needed to separate the echoes from the different swaths by digital beamforming on receive. An alternative is range variant null steering as ...
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... interesting alternative to a planar antenna is a reflector that is fed by a multichannel array as illus- trated on the lower left of Fig. 2. A parabolic reflector focuses an arriving plane wave on one or a small sub- set of feed elements. As the swath echoes arrive as plane waves from increasing look angles, one needs only read out one feed element after the other to steer a high gain beam in concert with the arriving echoes. This technique was originally suggested in a ...
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... mode are the blind ranges that are due to the fact that the radar cannot transmit and receive at the same time. This can be overcome in a bistatic SAR where the transmitter is sufficiently separated from the receiver. To avoid a separate transmit satellite, one can employ a variation of the PRF which shifts the blind ranges across the swath (Fig. 2, lower right). The PRF variation could either be implemented in discrete steps leading to a multiple beam ScanSAR mode or a pulse-to-pulse variation of the PRI. The latter provides better per- formance but requires a dedicated SAR processing which is currently under development. ...
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Citations
... SAR retrieves the wind fields by measuring the Normalized Radar Cross-Section (NRCS) variation of the sea surface, and it is the only satellite-based system that can offer wind information with a sub-kilometer spatial resolution (ranging from a few to a few hundred meters) [15]. However, as SAR operates at a single azimuth angle, external wind direction information is required to resolve the 180-degree ambiguity of the wind direction, which leads to the inversion precision being highly dependent on the accuracy of the auxiliary data [16][17][18]. Beyond wind field retrieval, SAR images can also be employed to estimate wave directional spectra based on the nonlinear mapping relationship between them. ...
Dedicated to synchronously acquiring large-area, high-precision, and multi-scale ocean wind and wave information, a novel concept of a spaceborne ocean microwave dual-function integrated sensor is proposed in this paper. It integrates the functions of a scatterometer and SAR by sharing a single phased-array antenna. An overview of the scientific requirements and motivations for the sensor are outlined firstly. In order to fulfill the observation requirements of both the functions, the constraints on the system parameters such as frequency, antenna size, and incidence angle are analyzed. Then, the selection principles of these parameters are discussed within the limitations of antenna area, bandwidth, available time, and cost. Additionally, the constraints on the time sequence of transmitting and receiving pulses are derived to ensure that there is no conflict when the two functions operate simultaneously. Subsequently, a method for jointly designing the pulse repetition frequency (PRF) of both the functions is introduced, along with zebra maps to verify its effectiveness. At the end of the paper, the system and performance parameters of the sensor are given for further insight into it.
... Multi-mode imaging system is the main means for SAR to obtain different high-resolution target images, and how to 2 > REPLACE THIS LINE WITH YOUR MANUSCRIPT ID NUMBER (DOUBLE-CLICK HERE TO EDIT) < separate the received echoes of each mode is the key to study this imaging system. At present, two types of echo separation methods are widely used: band pass filter (BPF) method [22]- [23] and azimuth phase coding (APC) method [24]- [28]. The BPF method is based on the premise that each mode has a different carrier frequency, and the echo signals of each mode are separated by band-pass filters with different center frequencies at the receiving end. ...
This paper proposes a novel imaging mode which allows for the acquisition of SAR images with different resolution in a single imaging process, specifically designed for multiple-input multiple-output synthetic aperture radar (MIMO-SAR). To achieve this, a multi-mode array system model based on two-dimensional intra-pulse scanning is established, followed by the division of full aperture signals into sub-aperture signals. Improved azimuth phase coding technology is then employed to separate multi-mode echo signals, and spatial filtering technology and range digital beam forming technology are used to remove ambiguity in azimuth and range of the sub-aperture signals of each mode. Finally, sub-aperture image coherent fusion algorithm is used to generate high-resolution images corresponding to the full aperture of each mode. Simulation results show that the improved azimuth phase coding can effectively separate multi-mode echo signals, and the multi-resolution characteristics and imaging effect are verified.
... T HE next generation of spaceborne synthetic aperture radar (SAR) with high-resolution wide-swath (HRWS) imaging capability has become an acknowledged powerful electronic equipment for Earth observation [1]- [3]. High spatial resolution images with ultra swath coverage are essential to the identification, confirmation, and description of objects, which can be widely applied in the field of environmental monitoring, disaster management, sea and land surface imaging, and so on [4], [5]. ...
Staggered synthetic aperture radar (SAR), which operates with variable pulse repetition interval (PRI), staggers blind areas to solve the blind range problem caused by constant PRI in conventional high-resolution wide-swath SAR imaging. The PRI variation strategy determines the blind area distribution, and thus has a significant influence on the imaging performance in staggered mode. Generally, the existing strategies based on linear PRI variation can control the blind areas in a straightforward way, which has achieved impressive results. However, the linearity of the PRI variation imposes regularity or even periodicity on the locations of the blind areas, which limits the distribution of the blind areas. The imaging performance has the potential to be further improved by introducing much more irregularity into the PRI sequences. To this end, this paper proposes an optimized nonlinear PRI variation strategy for staggered SAR mode. First, a novel objective function is defined which quantitatively measures the uniformity of the blind area distribution along the slant range and the discontinuity of the blind area distribution along the azimuth. Subsequently, the optimum nonlinear PRI variation strategy is found using an optimization problem and the proposed objective function. A knowledge-guided genetic algorithm is proposed to solve the optimization problem. Comparisons with the existing linear variation strategies show that the proposed strategy can provide a superior imaging performance after reconstruction with a lower objective function value. Simulations and experiments on raw data generated in staggered SAR mode are performed to verify the effectiveness of the optimized nonlinear PRI variation strategy.
... To overcome this apparent contradiction, the multi-receive (sub-apertures) methods were first studied such as Digital Beamforming SAR (DBF-SAR) [4]. Then, the methods employing the MIMO array were considered such as MIMO-SAR, which gives multimodel mode operation including the aforementioned modes [5,6]. Different SAR modes, notably multi-channel Stripmap SAR, scan-SAR, and Terrain Observation with Progressive Scans SAR (TOPSAR), have been employed by using the Scan-On-Receive (SCORE) approach (primarily for spaceborne applications). ...
The study offers a Synthetic Aperture Radar (SAR) imaging concept called Strip-Spot SAR, which uses linear Multiple-Input Multiple-Output (MIMO) radar arrays, which are becoming increasingly attractive for short-range sensing in a variety of growing application sectors. The concept specifically employs Digital Beam-Forming (DBF) techniques, which are enabled in such systems to give contiguous azimuth imaging, as in Stripmap SAR, but with the fine spatial resolution of a Spotlight SAR. Its fundamental concepts are analytically derived and experimentally validated under laboratory conditions using calibrated and real targets. Finally, the performance and limitations of the concept are investigated.
... In STWE-SAR, the length of the echo window is constrained by the PRF, which limits the maximum continuous swath width and leads to the appearance of blind regions, as shown in Figure 1 [29]. Benefiting from the development of the variable PRF technique or Staggered-SAR [30,31], the problem of blind regions has been greatly improved. However, the echo separation in Staggered-SAR is still needed. ...
Space-time waveform-encoding (STWE)-synthetic aperture radar (SAR) is an effective way to accomplish high-resolution and wide-swath (HRWS) imaging. By designing the specific signal transmit mode, the echoes from several subswaths are received within a single receiving window and overlap each other in STWE-SAR. In order to separate the overlapped echoes, the linear-constrained minimum variance (LCMV) beamformer, a single-null beamformer, is typically used. However, the LCMV beamformer has a very narrow and unstable notch depth, which is not sufficient to accurately separate the overlapped echoes with large signal energy differences between subswaths. The issue of signal energy differences in STWE-SAR is first raised in this paper. Moreover, a novel echo separation scheme based on a second-order cone programming (SOCP) beamformer is proposed. The beam pattern generated by the SOCP beamformer allows flexible adjustment of the notch width and depth, which effectively improves the quality of separation results compared to the LCMV beamformer. The simulation results illustrate that the scheme can greatly enhance the performance of echo separation. Furthermore, the experimental results based on the X-band STWE-SAR airborne system not only demonstrate the scheme’s effectiveness but also indicate that it holds great promise for future STWE-SAR missions.
... I N THE past decade, increasing focus has been given to the development of new synthetic aperture radar (SAR) concepts capable of delivering both high resolution and wide coverage [1]- [5]. In order to overcome the fundamental limitation imposed by the direct relation between swath width and azimuth resolution, the solutions usually consider multiple elevation beams in combination with SCan-On-REceive (SCORE) or the acquisition by multiple subapertures in the along-track direction [2], [4], [6]- [11]. ...
... Finally, we make a note on the discrete implementation of the back-projection integral. The discrete back-projection integral for the complete staggered signal can also be described by (3) and (4). In (4), we considered the left-Riemann approximation (up to a multiplicative constant). ...
The combination of SCan-On-REceive with continuous variation of the pulse repetition interval during transmission (staggered operation) is a viable option for the acquisition of high-resolution synthetic aperture radar (SAR) data over wide areas. Since the acquired data are not uniformly sampled and contain gaps within the synthetic aperture mainly due to the interruption of reception during transmission (i.e., due to blockage), proper reconstruction strategies must be considered in order to minimize artifacts. Conventionally, large oversampling rates are required to ensure low ambiguity levels, but this is not always feasible due to data-rate constrains. This article presents a detailed analysis of the blockage recovery in the low-oversampling case. Moreover, we propose a data adaptive strategy to optimally perform this step, ensuring the best performance for point-like targets and avoiding degradation for distributed scatters. This paper also presents simulation results that show the impact of staggered data for interferometric applications.
... When making use of classical signal coding and correlation techniques, the interfering energy of the other waveforms is spread, but still completely present. To ensure signal orthogonality in SAR, it was firstly proposed in [5] to use chirp-like pseudo-orthogonal signals, which provide orthogonality only within a limited time frame. Outside this time frame, a time-variant Digital Beam-Forming (DBF) on receive is used as a spatial filter that suppresses all unwanted signals. ...
... One example of waveforms suitable for the OWB technique are the Segmental-Shifted Chirp (SSC) waveforms [5]. These pseudo-orthogonal waveforms are shifted versions of a chirp, which preserve the same modulation rate but require different sub-pulse durations and bandwidths. ...
Radar systems with multiple transmit and multiple receive channels (MIMO) are capable of advanced imaging modes that can benefit the next generation of Earth science and planetary science Synthetic Aperture Radar (SAR) missions. Apart from an increased flexibility, MIMO SAR enables simultaneous full-polarized acquisitions without any reduction of the swath width or resolution. By making use of an innovative orthogonal-waveform beamforming technique, sufficient suppression of cross-correlation interference from orthogonal signals can be achieved. For verification of this MIMO SAR imaging mode, the airborne radar sensor DBSAR-2 operated by NASA will serve as a test bed.
... In recent years, increasing focus has been given to the development of new SAR modes capable of imaging wide swaths with high resolution [1,2,3,4,5]. The solutions are usually based on SCan-On-REceive (SCORE) [6] and/or on the acquisition by multiple sub-apertures in the along-track direction in order to overcome the limitations imposed by the direct relation between swath width and azimuth resolution of conventional SAR acquisition modes. ...
... Digital beamforming will become a key technology for future systems and boost out the performance of current SAR sensors by at least one order of magnitude. Other technological advances such as large reflector antennas will allow for ultra-wide swath imaging by simultaneously providing very high resolution in the metre regime (Krieger et al., 2012). As a result, a continuous spatio-temporal coverage of the whole globe will become feasible. ...
Advanced-DInSAR time-series methods have demonstrated to be a powerful instrument for the detection of millimetre ground displacements. Due to the dense spatial sampling, the measurements provide unique information of the extent and dynamics of surface deformation. The vast increase in systematically available SAR data from recently launched satellite constellations will dramatically improve the regular coverage around the globe. This allows for regular geodetic measurements that assure the data supply for the operational assessment of very small displacement rates associated with tectonic processes preceding earthquakes.
The study proposes a methodological framework for the generation of atmospherically- corrected A-DInSAR time-series products and its integration with complementary geodetic measurements from ground-based GPS for the retrieval of the full three-dimensional velocity field. The method has been tested over the Hyblean Plateau in south-east Sicily, Italy. The surface deformation of the area was thereby mapped in an unprecedented spatial detail and attests the contraction along the northern rim caused by the convergence of the Nubia-Eurasian plates. Horizontal displacements along the Scicli-Ragusa fault belt and around the Avola fault indicate that tectonic stress is accommodated over a broader network of active faults and that the Hyblean Plateau acts as a discrete tectonic regime.
In order to allow for a more rapid generation of such products, a processing chain for the time-series DInSAR products is presented as well. In particular, it addresses the possibility of parallelisation under the big-data paradigm.
... However, the transmissions at each transmit array element needs to be distinct and the transmissions should remain identifiable in the receiver side. Excessive correlation noise of MIMO channels can impair the receiver's ability to distinguish between signals, thus lowering the contrast and accuracy of the resulting radar image [9], there exist a multitude of methodologies to ensure the transmission quality, including the techniques used for MIMO noise mitigation [10] and the design of high efficient waveforms [11][12]. Other methods have focused on stochastic processing for MIMO SAR [13]. ...
High resolution and wide-swath imaging can be obtained by Multiple-Input Multiple-Output (MIMO) synthetic aperture radar (SAR) with the state of the art technologies. The time division multiple access (TDMA) MIMO SAR mimics the motion of the antenna of SAR systems by switching the array channels to transmit the radar signals at different time slots. In this paper, we develop a simulation tool with ray tracing techniques to retrieve high resolution and accurate SAR images for development of MIMO SAR imaging methods. Without loss of generality, in the proposed simulator, we apply a TDMA MIMO SAR system with 13 transmitting antennas and 8 receiving antennas, where all transmitting antennas share a single transmitter and the receiving antennas share a single receiver. By comparing with the normal simulation MIMO SAR strategies, the simulation image using ray tracing results validate that the proposed method provides more accurate and higher resolution SAR images.
