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Introduction
Published in Hai Deng, Zhe Geng, Radar Networks, 2020
Compared with optical imaging, radar imaging is weather-independent and could be implemented day and night. Moreover, radar waveforms could penetrate ground, water, and walls to generate images of the target. In radar imaging, the two primary figures of merit are spatial resolution and dynamic range (Richards, 2014). Currently, 2D high-resolution images of static ground scenes are often acquired by SAR, while moving targets such as aircrafts and missiles could be imaged using inverse SAR (ISAR). The basic theory behind SAR is that it uses a small antenna array on a moving platform to mimic a much larger antenna array, hence archiving radar images with higher spatial resolution.
Geomorphological Studies from Remote Sensing
Published in Prasad S. Thenkabail, Remote Sensing Handbook, 2015
James B. Campbell, Lynn M. Resler
Likewise, SAR, developed over several decades beginning in the 1970s, plays a significant, and growing, role in geomorphological, geophysical, and geologic inquiry. SAR, a specific form of imaging radar, collects imagery by broadcasting a beam of microwave energy (wavelengths in the range approximately 1 cm to 1 m, as selected for specific systems), then recording the energy backscattered from the earth’s surface to form images. As active systems, SAR can measure the time delay between transmission of the original signal and receipt of its echo, as well as differences in phase, and polarization. These differences provide a basis for characterization of terrain surfaces, to include roughness, moisture status, subsurface features, vegetation cover and structure, and drainage systems.
Radar Monitoring of Volcanic Activities
Published in Ramesh P. Singh, Darius Bartlett, Natural Hazards, 2018
SAR is an imaging radar system designed, as the name implies, to take advantage of a large ‘synthetic’ antenna to produce images of much better resolution than would be possible otherwise. SAR systems operate on the same principles as Doppler radars, but have additional capability to distinguish among return signals from individual resolution elements within a target footprint. SARs are side looking, that is, they direct signals to the side of their path across the surface rather than straight down. As a result, the arrival path of the radar signal is oblique to the surface being imaged. Return signals from near-range parts of the target (the part closest to the ground track of the radar) generally arrive back at the radar sooner than return signals from far-range areas, so the relationship between round-trip travel time and range can be used to organize return signals in the across-track, or range, direction. In the along-track, or azimuth, direction, the Doppler principle comes into play. Signals returned from areas that are ahead of the radar as it travels along its path are shifted to slightly higher frequencies, while returns from trailing areas are shifted to slightly lower frequencies. An imaging radar uses the relationship between return signal frequency and relative velocity between radar and target to organize return signals in the azimuth direction. In this way, the returns from each resolution element on the ground can be assigned unique coordinates in range and azimuth. The resulting data can be processed into an image of the target area, which contains information about topography and radar reflective properties of the surface.
Dual circularly polarized aperture-coupled metasurface antennas with high-isolation for X-band synthetic aperture radars
Published in Electromagnetics, 2022
Thi Ngoc Hien Doan, Khac Kiem Nguyen, Son Xuat Ta
Since the early 1950s, synthetic aperture radar (SAR) was introduced as an imaging radar, which utilizes relative motion between an antenna and the target under observation to synthesize a very long antenna via signal processing (Skolnik 1970). Today, SAR has become an important device for microwave remote sensing because of its capability to operate day and night, and in nearly all-weather conditions (Ulaby, Moore, and Fung 1981). The SAR system has been applied in many fields such as: sea monitoring (Drinkwater, Kwok, and Rignot 1990), mining (Lynne and Taylor 1986), oil pollution monitoring (Hovland, Johannessen, and Digranes 1994), terrain classification (Kong et al. 1990), and so on. As compared to the single polarization system, the SAR with dual linear polarization can be receive more target information, and consequently, improve the capability of target detection an identification. Accordingly, a larger number of dual-polarized antennas (Capece et al. 2014; Di Bari et al. 2011; Mao et al. 2015; Wang et al. 2021; Zhao et al. 2017) have been devoted for the SAR applications. These antennas targeted for reducing complexity, size, mass, and cost, while achieving broadband, dual polarization, high isolation, high gain, and high efficiency. Recently, circularly polarized (CP) SAR has been received more attention due to the CP-SAR mitigates polarization mismatch losses caused by the Faraday rotation effects and antenna misalignment (Sumantyo et al. 2021). Moreover, dual-CP, including both right- and left-hand CP (RHCP and LHCP), is expected for the SAR because of the dual-CP modes can provide more polarimetric information (Izumi et al. 2017). However, the SAR antennas with dual-CP, e.g., see (Ravindra et al. 2017), are usually bulky, large mass, and high realization cost. For achieving planar configuration, light weight, and low cost, microstrip patch antennas (Aloni and Kastner 1994; Bouça et al. 2020; Chen et al. 2017; Garcia-Aguilar et al. 2012; Narbudowicz, Bao, and Ammann 2013) are a preferred choice for dual-CP systems. The patch antennas can be fed by different feeding structures to generate a dual-CP radiation. The feeding structures, for instance, includes microstrip-lines traversed a cross-slot (Aloni and Kastner 1994), hybrid couplers (Bouça et al. 2020; Garcia-Aguilar et al. 2012), exploiting even and odd modes in a coplanar waveguide (Narbudowicz, Bao, and Ammann 2013), microstrip-line through proximity coupling (Chen et al. 2017). Most of the dual-CP patch antennas suffer a common drawback of narrow bandwidth. To broaden the bandwidth, but keeping a low-profile, metasurface-based antennas (Liu, Yang, and Pan 2019; Yang et al. 2018) have been designed for generating dual-CP radiation. Their port-to-port isolation values are, however, limited to 10-dB.