The use of THz frequencies has enabled the opportunity to perform synthetic-aperture radar (SAR) imaging at the sub-mm level. It is of great interest for applications where high-resolution remote sensing in short range is required. However, with the increase of the operating frequencies from microwave to THz, the SAR image formation algorithms work with a larger amount of data and become more sensitive to phase errors that can be caused by insufficient signal sampling rate or physical factors that cause platform deviations. This motivates the use of fast image formation capable to handle phase errors. In this paper, we present the experimental results on the performance of the local backprojection (LBP) algorithm for processing THz SAR signals. The LBP algorithm has been tested with the real data in the frequency range 0.325-0.5 THz. The results demonstrate the efficiency of the LBP algorithm for the SAR scene reconstruction and highlight the necessity of the use of two times higher signal upsampling to achieve reconstruction accuracy similar to the global backprojection algorithm. 

The resolution of a synthetic aperture radar (SAR) system depends not only on the operating frequency range of a radar but also on the geometry. A radar system operating at MHz frequencies facilitates SAR resolution to the meter (m) level with a linear aperture. With the same radar system, the realization of the circular aperture can further enhance the SAR resolutions to the submeter level. For GHz SAR systems, the realization of circular apertures helps to reduce the resolution from the centimeter (cm) level to the millimeter (mm) level. In this paper, a resolution equation for SAR systems synthesizing circular aperture (CSAR) is introduced. The equation is derived from the backprojection integral for CSAR, its Fourier transform, and the method of stationary phase. The accuracy of the derived equation is enhanced with a correcting factor that is numerically calculated with a cubic interpolation. Therefore, the equation can provide a more accurate, analytical, and practical estimate of the spatial resolution that can be reached with a CSAR system than the ones available in the literature. The resolution equation introduced in this paper is verified with simulations of a MHz SAR system and a GHz SAR system. The equation is further verified by an experiment with a THz inverse SAR (ISAR) system. The radar utilized in the ISAR experiment is a D-band radar system mounted on an antenna positioner that supports circular movement.

The emerge of Terahertz (THz) radar systems allows the short-range applications such as material characterization, hyper accuracy localization at cm or even better level, scattering analysis of rough surfaces, and material defect detection. The paper presents the experimental results about material defect detection based on synthetic aperture radar (SAR) operating at THz frequencies. For the experiments, an electrical insulator is damaged causing the defects in the orders of mm and sub-mm. A SAR testbed at THz frequencies built with a vector network analyzer and a frequency extender in the frequency range 325-500 GHz is used to measure the electrical insulator in the form of SAR with a two-dimensional (2D) aperture. The damaged parts inside the electrical insulator can be observed clearly in the three-dimensional (3D) SAR image. This supports the material defect detection that enhances monitoring industrial production processes and controlling product quality. 

The use of THz frequencies for radar remote sensing has provided a new set of applications, where short-range sensing for high-resolution imaging can be used. At the same time, radar imaging of physically large objects for understanding their radar signatures, which is based on long-range remote sensing, can be a cumbersome process and require significant financial resources. In this paper, we propose to explore the synthetic-aperture-radar (SAR) concept at sub-THz frequencies to scale a real-life scenario of long-range imaging of foreign objects down to short-range sensing in the indoor environment. The idea is experimentally studied on four downscaled marine vessels, where the SAR imaging system is based on a D-band FMCW radar that operates at frequencies 126-182 GHz, and the global backprojection algorithm with modified linear interpolator is used for the SAR scene reconstruction. The experimental results demonstrate the feasibility of the proposed idea. 

A solution for the three-dimensional (3D) hyper-accurate localization in indoor environment for mobile equipment problem can be based on radar systems. Mobile equipment with an integrated radar system is known as a joint radar-communication (JRC) system or a joint communication and sensing (JCAS) system. The paper proposes an approach for 3D hyper-accurate localization in indoor environment without modifications of cellular network infrastructure. The simulations and experiments show the feasibility of the proposal. © 2024 IEEE.

Exploration of THz frequencies has enabled new applications in the areas, where high-resolution short-range remote sensing is of great interest. At the high frequency ranges like THz or sub-THz, synthetic aperture radar (SAR) image formation algorithms are highly sensitive to platform deviations, which motivates the development of new algorithms that are capable to handle this problem.In this paper, we introduce a modified linear interpolation algorithm with the phase-control procedure for the three-dimensional (3D) global backprojection (GBP) algorithm. The procedure assigns the actual time needed for the signal to be transmitted to the given point in the defined image plane and collected by the receiver into the phase of the complex SAR data. The algorithm has been verified with the real monostatic and bistatic SAR data acquired in the 0.325-0.5 THz frequency band.

The expansion of the synthetic aperture radar (SAR) to the emerging THz spectrum has enabled a new era of applications in the areas of automobile, security, non-destructive testing, and material characterization. Thanks to the sub-mm wavelength, extraction of material surface properties is possible and of significant interest for the THz SAR applications. The properties define the surface scattering behavior, which is relational to the applied frequency. This study focuses on surface classification. We evaluate the scattering behavior of a rough surface and a smooth surface at 1.5 THz based on a SAR processing sequence that is introduced in this paper. First, we form the 3D SAR images of the metallic objects and then evaluate the surface properties based on the variation in the energy reflected by the object's surface.

The THz frequency spectrum provides an opportunity to explore high-resolution synthetic-aperture-radar (SAR) short-range imaging that can be used for various applications. However, the performance of THz SAR imaging is sensitive to phase errors that can be caused by an insufficient amount of data samples for image formation and by path deviations that can be practically caused by SAR platform vibrations, changes in speed, changes in direction, and acceleration. To solve the former problem, an improved interpolation procedure for backprojection algorithms has been proposed. However, to make these algorithms efficient in handling the latter problem, an additional autofocusing is necessary. In this paper, we introduce an autofocusing procedure based on compressed sensing that is incorporated into the backprojection algorithm. The reconstruction is based on the following calculated parameters: windowed interpolation sinc kernel, and range distances between SAR platform and image pixels in a defined image plane. The proposed approach is tested on real data, which was acquired by the 2\pi FMCW SAR system through outdoor SAR imaging. © 2023 IEEE.

Mobile equipment with an integrated radar system allows active localization by realizing monostatic, bistatic, multistatic and passive SAR. For mobile user localization, it is also possible by implementing backprojection at base station. In the paper, an approach for precise mobile user localization by implementing backprojection at base station is proposed. The simulation shows the feasibility of the proposal. Some technical issues such as accuracy and backprojection ambiguity, are also addressed. The practicality of the proposal is also demonstrated with a practical frequency-modulated continuous wave (FMCW) radar system and a monostatic radar measurement based on the system. © 2023 IEEE.

Precise mobile equipment localization in indoor environment is possible for mobile equipment with an integrated radar system. Deploying an omni-directional antenna at a base station allows localizing a single mobile unit at a time slot and a frequency resource block. With cell sectoring, an approach to cope with increasing capacity in a cell of a mobile network, helps to localize multiple mobile units at a time slot and a frequency resource block. Most importantly, cell sectoring helps to avoid localization ambiguity caused by the backprojection process. The paper presents the precise multiple mobile equipment localization approach in indoor environment based on cell sectoring. The simulation illustrates the benefit of the approach. The practicality of the approach is also addressed in the paper. © 2023, © ICST Institute for Computer Sciences, Social Informatics and Telecommunications Engineering.