# Drone SAR Imaging for Monitoring an Active Landslide Adjacent to the M25 at Flint Hall Farm
## 5 Discussion
The zoomed-in image for Flint Hall Farm shows that the corner reflectors and other instrumentation appear as a grouped blur of bright pixels of approximately 50 \(\times\) 120 m, given the 5 \(\times\) 20 m spatial resolution of the Sentinel-1 sensor. It is also assumed that the very high radar return from the corner reflectors contributed to the saturation of nearby pixels to create this oversized vertical blur. Discrete target identification is not possible for small and closely spaced objects at this spatial resolution. Figure 19 provides a side-by-side comparison of the satellite and drone SAR images; the group of blurred pixels in the former is visible as individual targets in the latter due to the improved spatial resolution. Whilst this comparison of drone and satellite SAR utilises Sentinel-1 data for the latter, the spatial resolutions of satellite Stripmap and ScanSAR sensors range between 3 and 100 m [43; 44; 45; 46; 47; 48; 49; 50; 51]. The sub-meter spatial resolutions of COSMO-SkyMed Second Generation (CSG) and PAZ are only available in the spotlight imaging mode, which provides a narrower swath width, and with data only freely available for scientific research and application development upon acceptance of a project proposal, rather than commercial use [44; 46]. The landslide in December 2000 that initiated the ongoing monitoring of Flint Hall Farm was triggered by excessive winter rainfall. As stated, the Flint Hall Farm Landslide is a wedge failure within the Gault Clay activated by high groundwater levels, whilst the Flint Hall Farm South Landslide is a dormant creep movement with occasional reactivation during wet conditions ([PERSON], 2015). Seasonal landslide movements are often attributed to changes in the water level ([PERSON], 2015), as such it is desirable to monitor landslides pre-, during (if safe) and  
Figure 19: Side-by-side comparison of (**a**) average SAR amplitude for Flint Hall Farm (white boundary) from September 2021 to September 2023; the corner reflector and in situ instrumental pixels are circled in blue, and (**b**) drone SAR image from Flint Hall Farm, with circled targets, including the corner reflectors (red), and other fenced areas for in situ instrumentation (yellow, blue, and pink). Figure 18: Average SAR amplitude for Flint Hall Farm (white boundary) from September 2021 to September 2023, with annotated M25 and Godstone. The zoomed image boundary is denoted by the red box. The corner reflector and in situ instrumental pixels are circled in blue. post-rainfall. The drone radar system may be deployed on a sub-daily basis for the creation of a detailed deformation time-series surrounding weather events; this may inform early-warning prediction models and hazard mitigation procedures. The temporal resolution of satellite SAR is dictated by orbital patterns and is often restricted to days or weeks, thus preventing the creation of such a detailed time-series. Regarding the research and development timeline outlined in the Introduction, this case study demonstrates drone SAR imaging, which satisfies points one and two. For the monitoring of the landslide movement, ongoing and future work addresses points three and four of the timeline and includes repeat-pass SAR data collection at Flint Hall Farm, and developing the InSAR processing chain for the drone SAR data. Equation (4) outlines the phase change (\(\Delta\varphi\)) contributors in typical satellite InSAR processing:  
\[\Delta\varphi=\varphi_{flat}+\varphi_{topo}+\varphi_{orbit}+\varphi_{defo}+ \varphi_{tropo}+\varphi_{iono}+\varphi_{scat}+\varphi_{noise} \tag{4}\]  
where \(\varphi_{flat}\) (flat earth) and \(\varphi_{topo}\) (topographic) are geometric terms, which for satellite InSAR are typically corrected using a satellite-derived Digital Elevation Model (DEM). The commonly utilised National Aeronautics and Space Administration (NASA) Shuttle Radar Topography Mission (SRTM) DEM has a 30 m spatial resolution; for drone InSAR, a finer spatial resolution is required to match the drone SAR data resolution.