MIMO-SAR Interferometric Measurements for Structural Monitoring: Accuracy and Limitations

Abstract

Terrestrial Radar Interferometry (TRI) is a measurement technique capable of measuring displacements with high temporal resolution at high accuracy. Current implementations of TRI use large and/or movable antennas for generating two-dimensional displacement maps. Multiple Input Multiple Output Synthetic Aperture Radar (MIMO-SAR) systems are an emerging alternative. As they have no moving parts, they are more easily deployable and cost-effective. These features suggest the potential usage of MIMO-SAR interferometry for structural health monitoring (SHM) supplementing classical geodetic and mechanical measurement systems. The effects impacting the performance of MIMO-SAR systems are, however, not yet sufficiently well understood for practical applications. In this paper, we present an experimental investigation of a MIMO-SAR system originally devised for automotive sensing, and assess its capabilities for deformation monitoring. The acquisitions generated for these investigations feature a 180∘ Field-of-View (FOV), distances of up to 60 m and a temporal sampling rate of up to 400 Hz. Experiments include static and dynamic setups carried out in a lab-environment and under more challenging meteorological conditions featuring sunshine, fog, and cloud-cover. The experiments highlight the capabilities and limitations of the radar, while allowing quantification of the measurement uncertainties, whose sources and impacts we discuss. We demonstrate that, under sufficiently stable meteorological conditions with humidity variations smaller than 1%, displacements as low as 25m can be detected reliably. Detecting displacements occurring over longer time frames is limited by the uncertainty induced by changes in the refractive index.

Interferometric Processing of a Developed Mimo Gbsar for Displacement Monitoring

“…MIMO radar obtained 0.0065 mm displacement standard deviation (std), demonstrating better stability than MIMO GBSAR (0.021 mm). Accordingly, based on Gaussian modelling (Baumann-Ouyang et al, 2021), the developed MIMO GBSAR can detect a minimum displacement signal of 0.126 mm with a 99.7% confidence level. However, the lower displacement accuracy of our developed MIMO GBSAR could be due to the mechanical rail's fluctuations and coregistration errors.…”

“…The received signals need to be orthogonal that means their inner product have to zero in order to be distinguished. This could be done through time-division or frequency-division (Baumann-Ouyang et al, 2021). We performed the first approach, which is also known as time division multiplexing (TDM).…”

“…However, their capabilities make them attractive for use in other fields of applications (Yanik et al, 2020). Recent studies by (Baumann-Ouyang et al, 2021) and (Ciattaglia et al, 2020) approve the applicability of these radars for structural monitoring with nearly micrometer accuracy. Moreover, few studies applied W band MIMO radars to monitor real-world objects such as bridges and other large structures (Pieraccini and Miccinesi, 2019;Rodrigues and Li, 2021).…”

A Novel GB-SAR System Based on TD-MIMO for High-Precision Bridge Vibration Monitoring

“…However, the displacement measured by our radar system is the bridge deformation projected on the radar LOS direction. Equation (26) shows the deformation of the bridge in the vertical direction. To further verify the generality of our algorithm, a practical scenario experiment was carried out on the Hongqi Canal Bridge in Xi'an.…”

“…The implementation of this method is easier than WD and FD because it avoids the problem of waveform and frequency separation. The majority of MIMO radar systems operate in TDM mode, for example, the MIMO-based non-collinear array system [24], MELISSA system [25], 77G Millimeter wave system [26], and the Ku-Band MIMO GB-SAR system [19]. TDM-MIMO (referred to as MIMO except for special instructions) GB-SAR can compensate for the disadvantages of low data acquisition rate and motion error [27] in mechanical scanning GB-SAR.…”