Exploitation of Synthetic Aperture Radar Targets Velocities

Dear Colleagues,

Velocity is a relative parameter that can be detected by synthetic aperture radar (SAR). The range of velocities estimable by SAR can vary from a few millimeters per year up to several meters per seconds and more. In this environment, the observations can be performed using several images or working on a single image. The estimation of target velocity can be assessed considering point targets that are detected inside a few resolution cells or using distributed targets that are detected over a huge number of resolution cells. This Special Issue has the objective of publishing works that explore the velocities generated by any nature of targets and spanned along a wide spectrum.

Millimeters and centimeters per year of velocity are usually generated by the Earth’s deformation, and this kind of activity can be detected by SAR in the Differential SAR Interferometry (DInSAR) configuration. In this environment, the persistent scatter interferometry (PSInSAR) signal processing technique can be successfully applied, processing long temporal series of InSAR data in order to track in time the Earth deformations in space, velocity, and acceleration. This technique is also very applicable for the monitoring of urban areas. The global interferometric SAR phase information is provided by different contributors due to topography, Earth deformation, atmospheric delays, orbit errors, phase ambiguities, and general noise. The estimation of the Earth’s deformations with millimeter precision is possible only by estimating a reliable atmospheric phase screen (APS) that is focused on the perform separation of the interferometric phase contribution due to the Earth’s deformation from all unwanted atmospheric effects. In the radar community, several techniques have been developed to retrieve this parameter, and estimation is possible only by processing a long temporal series of interferometric SAR images and under a special assumption of having temporal and spatial constraints.

Centimeter and decimeter per week velocities are hard to detect using the exploitation of the SAR interferometric phase only. Early detection and early warning are of great importance in giant landslide monitoring. In this context, the exploitation of hybrid-SAR techniques is very important. The technique combines both phase-based and amplitude-based information to detect and monitor large-scale landslides. This ratio of velocities is also held by glaciers, which are nowadays melting in a very fast ratio. In these environments, velocities can also be detected using the same hybrid solutions or non-tracking techniques.

Velocities of one or a few meters per day are detectable exploiting multi-temporal SAR data observed using interferometric geometry. Icebergs play an important role in the climate through the transfer of freshwater and heat between ice sheets and the oceans. In this case, synthetic aperture radar images also offer a robust means of observing and tracking in time icebergs in the often dark and cloud-covered polar areas. The tracking can be performed in some very rare cases by exploiting very short time DInSAR techniques and in most cases using tracking capabilities based on the magnitude exploitation.

Velocities of hundreds of meters to some kilometers per day determine the limit for migrating from a multi-temporal to a single-pass SAR acquisition approach. In this case, the targets can be coherent or distributed. In most cases, this gamma of velocities is estimable using along-track interferometry, which is able to detect the range component of the two-dimensional range-azimuth velocity vector of the targets. Measuring velocities in this spectrum can also be performed by Doppler sub-apertures and pixel tracking. Several applications can be exploited starting from maritime surveillance, terrain surveillance and border monitoring, sea current estimation, rivers velocity estimation, and many other applications.

Velocities of meters and several meters per seconds are produced by man-made objects moving on the ground, sailing on the sea or flying in the sky. The applications devoted to this are moving-target indication (MTI) or ground-MTI (GMTI) radars and data fusion observed by different sensors. Some typical applications are wide-area traffic monitoring, using the scan-MTI mode of some airborne radar sensors or using flexible GMTI mode radars, which may be designed for rapidly monitoring wide areas for moving targets. These kinds of remote sensing will scan environments with different natures to perform target detection from a different aspect and angles with a high revisit rate. An important role in this field is the micro-motion estimation of targets. Bistatic SAR (BSAR) provides strategic advantages in radar imaging. The motions and micro-motions of objects under SAR observations generate Doppler and micro-Doppler effects observable in the received radar echoes. The micro-Doppler effect is typically derived for some vibrating targets observed by bistatic SAR. The corresponding bistatic factor is shown to be a function of the bistatic acquisition geometry. The effect of the target vibration on the focused image has also been shown to be influenced by the acquisition geometry. Deriving reliable vibrational models is useful for the micro-Doppler classification of different targets. Micro-motion observations can be performed observing targets in different electromagnetic bands like X, Ku, Ka and more.

The kilometers per second gamma of velocities is devoted to air and space surveillance. In this environment applications can be well-fitted to track small targets like satellites and debris. Inverse SAR (ISAR) is also welcome in order to image satellites and other types of space equipment detected from the Earth and/or space.

In this SAR investigation environment, we welcome the exploitation of new and innovative techniques:

According to the high-resolution wide swath (HRWS) technique, the exploitation of different strategies for generating separation and the orthogonality of signal ambiguities is a crucial task. In this environment, synthetic aperture radars can be also tailored for future multiple-input multiple-output solutions equipped with multichannel antennas to enable wide-area and high-resolution imaging.

Low-PRF radars: According to the minimum antenna area constraint, synthetic aperture radar systems require a low-pulse repetition frequency (PRF) to image the wide swaths in ocean surface monitoring scenarios. This low PRF solution, if set lower than the Doppler bandwidth, will cause azimuth ambiguities. Several methods can be devoted to mitigating azimuth ambiguities when using an under-sampled SAR system for ship detection and velocity estimation over the open sea. In this context, it appears that several methods are only appropriate for detecting bright targets over the dark backgrounds in which residual energy loss occurs for useful signals. Innovative signal processing techniques are welcome to exploit this interesting task.

GeoSAR: This Earth observation technique uses the motion generated by the orbital interference existing on geosynchronous orbits. The nearly fixed position of the geosynchronous platforms makes GeoSAR systems suitable for continuous monitoring applications. However, using high transmitted powers and large antenna sizes over very long integration times, which can be hours long, can cause signals to decorrelate significantly due to atmospheric changes. As a result, an efficient and precise APS is highly necessary. A reliable strategy for avoiding signal aberrations due to atmospheric parameter variations can be the short-term periodic acquisitions based on Doppler sub-apertures.

This Special Issue invites contributions on the above processes and phenomena and include but are not restricted to the following topics:

• Earth deformation monitoring;
• Urban area 3D/4D reconstruction and deformation monitoring;
• Atmospheric phase screed estimation techniques;
• Glacier monitoring;
• Iceberg drift monitoring;
• Maritime surveillance;
• Micro-motion estimation of targets;
• Terrain surveillance and border monitoring;
• Data fusion;
• Space situational awareness (SSA);
• Satellite debris detection and tracking;
• Target detection and tracking.

Prof. Thierry Bouwmans
Dr. Filippo Biondi
Dr. Pietro Milillo
Guest Editors

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• Synthetic Aperture Radar (SAR)
• Inverse SAR (ISAR)
• Polarimetric SAR (PolSAR)
• Interferometric Synthetic Aperture Radar (InSAR)
• Polarimetric InSAR (PolInSAR)
• Differential InSAR (DInSAR)
• 4D SAR Tomography (4D-TomoSAR)
• Ice and Glaciers monitoring
• Pixel Tracking
• Target detection
• Along-Track SAR Interferometry ATI-SAR
• Multi Chromatic Analysis (MCA)
• Doppler Sub-apertures
• Space situational Awareness (SSA)