Synthetic aperture radar (SAR) is a well-known imaging technique and most commonly used up to the microwave frequency spectrum (below 30 GHz) which provides spatial resolution in the sub-m range. To enhance the resolution, higher frequency spectra such as millimeter-wave (mmWave) and terahertz (THz) regions are being investigated. The mmWave and THz spectral ranges extend the SAR applications to non-destructive testing (NDT), material characterization, and sub-mm resolution imaging. However, the higher frequency spectrum suffers from higher path loss and potentially higher atmospheric absorption that limits the propagation distance. Nevertheless, the mmWave/THz spectrum is suitable for short-range applications such as indoor room profiling. From theoretical analysis, it can be summarized that the higher frequency spectrum provides better resolution but a comparative study on the impact on the image quality of the frequency spectrum ranging from GHz to THz has not been presented. Besides, as of the hardware complexity of the THz devices, the optimum range of the spectrum is always under investigation. The optimum range is defined where no strong improvements in the image quality are achievable with further increases in the frequency spectrum. Therefore, this paper presents an overview of electronics-based imaging using the SAR technique for the frequency spectrum ranging from GHz to THz with the focus on NDT and high-resolution imaging. Seven frequency bands: 5-10 GHz, 68-92 GHz, 75-110 GHz, 0.122-0.168 THz, 0.22-0.33 THz, 0.325-0.5 THz, and 0.85-1.1 THz are selected for a comparative analysis. The results are presented for 2D and 3D imaging using the backprojection algorithm. Additionally, state-of-the-art imaging based on SAR technique with electronics transceiver modules has only been demonstrated up to the sub-0.75 THz, whereas in this paper the spectrum up to 1.1 THz has been addressed.

In radar remote sensing, the Terahertz (THz) spectrum is presently being investigated worldwide with focus on short-range indoor and outdoor applications. The spectrum broadens the unmanned aerial vehicle (UAV) based synthetic aperture radar (SAR) applications to indoor room profiling with sub-mm resolution and material characterization as many materials have unique fingerprints at this spectrum. SAR technique requires precise localization information of the mobile radar sensor, which in conventional SAR is achieved using an existing localization infrastructure, such as a global positioning system (GPS) and inertial measurement unit (IMU). For the indoor THz SAR, the GPS does not provide coverage in indoor complex environments, and also the state-of-art compact IMU does not provide the required sub-mm accuracy. These limitations can be overcome by utilizing an indoor localization system. Therefore, this paper presents an indoor THz simultaneous localization and mapping (SLAM) system. The system comprises of passive tags based radio frequency identification (RFID) localization system and SAR that provides the UAV localization and mapping of the in-room objects. Another challenge for the UAV based indoor THz SAR that is addressed in this paper is motion compensation (MOCO). At the THz, MOCO requires special consideration due to very small trajectory deviation is in the range of carrier wavelength. Therefore, to study the effects of the sub-mm translational errors, a testbed has been set up, and measurement results are presented in this paper along with the 3D electromagnetic simulation results for a carrier frequency of 275 GHz and bandwidth of 50 GHz. Further, to compensate these errors, the sub-mm localization system is used and the results are presented to validate the proposed solution for indoor THz SAR MOCO.

In this paper, new concepts for continuous 24/7 real-time monitoring of plants and insects with miniaturized terahertz (THz) systems are described and experimentally tested. Thus, for the first time, small-scale insights into the instantaneous plant health but also in their long-term growth can be obtained. Using such compact THz systems, e.g. water uptake, insect infestation and the behavior of pollinators (i.e. honey bees) and pests can be measured dynamically and non-invasively at virtually any position in the close biotope surrounding them. In addition to general understanding, this can be used to optimize crop yield and reduce resource consumption as well as for identifying characteristics of insect-plant interactions induced by potential environmental stressors. Given such holistic a approach, the proposed concepts may provide a significant advancement in environmental monitoring technology.

Imaging in the terahertz frequency range has attracted growing interests since the first image of a leaf more than 20 years ago, due to its countless applications in basic and applied research, medical imaging, and nondestructive testing. However, most terahertz imaging approaches rely on focusing optics which require knowledge about the imaging scene before the actual imaging takes place. Further, imaging is mostly restricted to short distances and high resolution is only achieved for systems with a high bandwidth. Here, we present a method that enables high-resolution imaging of small metallic and dielectric objects at distances up to 2 m based on a synthetic aperture. We derive a simple approximation for the resolution of partial circular synthetic apertures with limited bandwidth. The bandwidth limitation is encountered by replacing the measured signals with replica signals of high bandwidth and equal round-trip time so that the resolution is only limited by the carrier frequency and signal-to-noise ratio of the measurement system.

This paper proposes a novel single-layer, low-profile harmonic transponder to be integrated with the printable diodes based on Si particles. The introduced prototype consists of two bowtie dipoles that are directly matched to the harmonic generation element at a fundamental $2.45\mathrm{GHz}$ frequency and also at the corresponding second harmonic $4.9\mathrm{GHz}$ frequency. Therefore, the antennas and T-matching parts can be manufactured as separate inlays using a single layer-substrate. Besides the new prototype, the harmonic conversion loss (CL) is theoretically and experimentally investigated. In this regard, the equivalent circuit is driven and utilized to analyze the CL performance with variations in fundamental frequency and input power. The introduced transponder outperforms the state-of-the-art work from the printability, size, and CL point of views.

This paper addresses indoor environment mapping by employing the synthetic aperture radar (SAR) technique at millimeter wave (mmWave) frequencies. The mmWave-based SAR can provide a high-resolution map for example of an emergency scenario like a burning room. The high-resolution map drives a new era of SAR applications such as object detection, classification, characterization, and precise localization. A major requirement at high frequencies is the precise knowledge of SAR trajectory, where radar sensors are mounted on a mobile platform such as a drone or unmanned aerial vehicle (UAV). State-of-the-art localization methods such as GPS-aided inertial measurement units are not valid due to limited coverage and accuracy. One of the primary solutions could be the SAR assisted with an indoor localization system, which is exploited in the work. The presented indoor localization system comprises two types of passive chipless frequency-coded tags, based on dielectric resonators and frequency-selective surfaces. In this work, firstly, the proposed method of integrating SAR and localization systems is evaluated in a single-tag environment. Further, a version of a room equipped with a multi-tag system is considered for real-time applications, and a successful demonstration of indoor environment mapping for the frequency spectrum of 75-110 GHz is presented.