Dual-frequency high-resolution mobile mapping of ground motion of the Brinzauls landslide in Switzerland with a car-based interferometric SAR system at L- and Ku-band
Authors: Frey Othmar , Werner Charles , Caduff Rafael 
Published: March 2024

Geohazards related to ground motion are widespread in mountainous regions. Time series of spaceborne SAR data are commonly used to retrieve maps of ground motion with extensive spatial coverage. However, there are situations where terrestrial radar systems are more suitable or even necessary for measuring ground motion. Such situations include slopes facing north or south, where the line of sight of current space-based SAR systems is nearly perpendicular to the prevalent direction of ground motion; slopes that lie in radar shadow or layover for current spaceborne SAR geometries; fast-moving landslides requiring shorter interferometric measurement intervals; and cases demanding higher spatial resolution or higher frequencies (e.g., Ku-band) with better sensitivity to line-of-sight motion.Terrestrial stationary radar/SAR systems, typically operating at Ku- or X-band, have been employed for many years to address these challenges. However, their limited synthetic aperture (or antenna size in the case of real-aperture radars) result in a constant angular resolution in the azimuth direction, leading to a &#160;reduced spatial azimuth resolution with increasing distance.Monitoring a landslide from a moving car or a UAV with a longer synthetic aperture allows using lower frequencies such as L-band, offering good spatial resolution at the meter level. Aperture synthesis from a car or UAV at higher frequencies (e.g., Ku-band) with smaller radar antennas can significantly improve the azimuth resolution to sub-meter or even decimeter level compared to stationary terrestrial radar systems which typically have azimuth resolutions in the order of 10m and more at range distances of several kilometers.In our previous work, we had demonstrated mobile mapping of ground motion using a compact repeat-pass L-band interferometric SAR system on a car and a UAV. Recently, a Ku-band SAR system (a modified version of the Gamma Portable Radar Interferometer (GPRI)) was added to the car-borne InSAR measurement setup. The new configuration allows simultaneous data acquisitions at both frequencies during repeat-pass SAR measurements while driving along a road.Frequency diversity proves to be advantageous in mountainous areas with varying land cover and motion processes with different velocities and scales. In this contribution, we present recent results from car-borne mobile mapping campaigns in the Swiss Alps showcasing the dual-frequency car-borne SAR setup (Gamma L-band SAR and modified GPRI at Ku-band). In particular, we present ground motion measurements of the Brinzauls landslide in Switzerland taken in fall 2023 at both frequencies, Ku- and L-band, and at different time intervals. The case study strikingly shows the complementary properties of the two frequencies in terms of sensitivity to motion and temporal decorrelation. The unprecedented high-resolution SAR imagery and interferograms obtained with the car-borne Ku-band SAR (decimeter-level azimuth resolution) allows discriminating different bodies of the landslide moving at different velocities in detail.