Indoor THz SAR Trajectory Deviations Effects and Compensation With Passive Sub-mm Localization System

Abstract

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 submm 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.

Short-Range SAR Imaging From GHz to THz Waves

“…Very-high positioning accuracy is required at THz. Since, for a coherent processing, positioning deviations in the synthetic aperture have to be very small in relation to the wavelength [15]. Especially, for the case of vertical movement against gravity, the mounting load is of critical importance.…”

“…The phase error results in image degradation, blurred images, and incorrect range and azimuth estimations. Based on the far-field condition, the positioning accuracy requirement is R ≤ λ/16, where R = R m − R as defined in [15]. It shows that the THz SAR is highly sensitive to motion errors due to the shorter-wavelengths.…”

“…Recently many THz SAR testbeds are proposed in the literature with focus on short-range applications in the domain of defense/security [10], [11], automobile [12], [13], non-destructive testing (NDT) [14], and indoor room profiling [15]. With electronic transceiver modules, the SAR imaging testbeds are based on radar frontends [10]- [13] and vector network analyzers (VNA) with extension modules [14]- [17].…”

Compensation of Sensor Movements in Short-Range FMCW Synthetic Aperture Radar Algorithms

“…Those existing publications deal with intrapulse motion compensation in FMCW systems using radar sensors with moderate (relative) bandwidths, ranging from 500 MHz in [27] up to 2 GHz in [26] and [28]. In contrast to those applications, millimeter-wave systems above 100 GHz, for instance, in the field of security imaging [30], nondestructive testing [31], or indoor localization [32], often provide a much larger bandwidth and they are therefore not covered by the aforementioned works. Additional examples are the systems presented in [32] and [33], with center frequencies of approximately 275 and 300 GHz and bandwidths of 50 and 40 GHz, respectively.…”

“…In contrast to those applications, millimeter-wave systems above 100 GHz, for instance, in the field of security imaging [30], nondestructive testing [31], or indoor localization [32], often provide a much larger bandwidth and they are therefore not covered by the aforementioned works. Additional examples are the systems presented in [32] and [33], with center frequencies of approximately 275 and 300 GHz and bandwidths of 50 and 40 GHz, respectively. On one side, the large bandwidth results in a high resolution but also introduces great demands at the positioning unit in an SAR system since the position error has to be a fraction of the wavelength.…”