InSAR analysis Infrastructure Monitoring Sentinel Imagery

InSAR Analysis and Corner Reflector Experiments for Infrastructure Stability Monitoring Using Sentinel-1 Imagery

Initially, paper abstracts were presented as a talk and as a poster at the Fringe 2023 workshop by European Space Agency.  Links to the full documents can be found at the end of this article. 

National Grid Energy Transmissions (NGET), which owns and maintains the high-voltage electricity transmission network in England and Wales, conducts invasive analysis annually to monitor the towers most at risk of movement. Moreover, the NGET inspection teams perform annual line walking activities and monthly substation inspections during which they visually assess the presence of asset motion.

These interventions are crucial to avoid issues which may cause expensive assets replacements or reconstruction. It costs NGET over £6 million per year to monitor only 1% of their most at risk assets.

National Grid Energy Transmission

Synthetic Aperture Radar Interferometry (InSAR) is an accurate Earth Observation method to monitor an asset’s stability at a much lower cost and without the need to have physical access to the assets. This technique uses SAR satellite datasets e.g., Sentinel-1 which is freely available from the European Space Agency (ESA).

Persistent Scatterer InSAR (PSI) is a novel technique to select strong, stable scatterers that remain coherent for the entire time series of radar acquisitions (Ferretti et al. 2001). In this study, we first applied conventional PSI analysis using SARPROZ software (Perissin et al.,2011) across three NGET areas of interest which each included 80 km of overhead lines (OHLs) and some underground cables and overground substations. To monitor an asset’s stability using the PSI technique, it must be possible to identify at least one data point that can be accurately and definitively assigned to the asset itself. Some assets, depending on their characteristics (e.g. size, orientation, roughness and material) and their scattering behavior are ‘natural reflectors’ providing a strong and coherent back scattered signal that can be used for PSI asset motion analyses, but not all.

One of the results of this study was an insight into the high % of towers that are not ‘natural reflectors’ and a study of how this issue could be solved by the installation of corner reflectors. Corner reflectors (CRs) can be used to enhance the backscattered signal from the target where the signal is not strong and coherent enough to be selected as a Persistent Scatterer (PS) point (Cigna et al., 206 and Kelevitz et al., 2022).

Therefore, in the next phase, we focused on designing and installing a number of CRs both on National Grid pylons, and a test site at Cranfield University, to assess their ability to make NGET assets, in particular tower monitorable when using PSI derived from free and open-source Sentinel-1 imagery. This project also set out to determine what the minimum distance between two installed CRs would need to be to still get two separate signals and therefore two distinct asset motion measurements from one single asset. We designed the CRs for these experiments to be clearly visible in images for a typical UK rural landscape away from woodland. Assuming a low vegetated background and a signal to noise ratio (SNR) equals to 10, five trihedral CRs with an inner side length of 70 cm were manufactured. In the installation phase, the CRs were fixed rigidly to a support and pointed towards the Sentinel-1 selected tracks using the local incidence angle and azimuth angle.

We mounted three CRs at the Cranfield University site, on 15th Dec 2022 in an open grassy area south-west of the runway in an L shaped formation (60 m along track and 20 m across- track), each with a 100% clear view to the Sentinel 1 satellites. We compared the amplitude time series of the pixel in which each CR was located in the three images taken before CR installation with four images taken after installation. Before doing the amplitude time series analysis, we co-registered all Sentinel-1 SLC images track 81 descending with respect to the first image after the first CR installation and georeferenced the images using a high-resolution LiDAR DEM, and finally manually corrected georeferencing using a visible feature in the SAR image.

Corner reflectors

The result of this analysis confirms that the installed CRs have a strong back scattering signal towards the satellite in comparison to the background before the installation which matches with what we expected. We then proceeded to reduce the distances between the CR’s in the North-South (along track) and East-West (across track) directions, with two experiments- 30 m along track and 10 m Across-track on 9 Jan 2023 and 5 m and 10 m in across track on 21 Jan 2023 (table 1). The results showed that as long as the spacing is more than approximately 30 m (N-S) or 7 m (E-W) then the reflectors should be visible in the amplitude images as distinct targets. As the targets merge, it is probably possible to detect that more than one reflector is present, but this may require more than a simple visual inspection to be confident of the result.

To better distinguish the signal of the two overlapping CRs in the amplitude image after installation, we applied amplitude time series analysis for all the pixels in the large bright area. The time series analysis confirmed a jump of amplitude after the CR installation for the pixels belongs to the CRs. Moreover, we applied an RGB colour composite analysis using the images before and after each installation which helped to distinguish the pixels corresponding to the installed CRs. 

The complete abstract, including tables and figures, is available for viewing and downloading here.  

The more comprehensive presentation is available for viewing and downloading here

Dr. Zahra Sadeghi
Dr. Zahra Sadeghi

Earth Observation-SAR Engineer

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