The single-pair unwrapping ceiling is a property of the track, not of the radar: Sentinel-1 evidence from rock–ice avalanches at Langtang Lirung and Ronti Peak

Ajay Kumar Sah 

This study asks what single-pair satellite data can detect before rock–ice avalanches, such as the Langtang Lirung (Nepal) and Ronti Peak (Chamoli, India) failures. The single-pair line-of-sight limit of 0.42 m yr⁻¹ for 12-day Sentinel-1 pairs is treated as a property of the radar, but the downslope ceiling depends on the track. I measure sensitivity to downslope motion f across Copernicus GLO-30 elevation cells using 500 m source discs for four Sentinel-1 tracks and an optical detection floor using Sentinel-2 offset tracking over stable ground. Slope-parallel interferometric synthetic aperture radar ceilings are 0.46 versus 2.77 m yr⁻¹ at Langtang Lirung, and 0.97 versus 1.21 m yr⁻¹ at Ronti Peak. At Langtang Lirung, moving the disc centre gives a factor of 4.5–5.9, and the descending track remains lowest. Non-spanning Sentinel-2 pairs give median optical floors of 5.55 m yr⁻¹ at Langtang Lirung and 5.77 m yr⁻¹ at Ronti Peak, whereas pairs spanning the event cannot measure the floor. Between 2.77 and 5.55 m yr⁻¹, neither the radar pairs nor the median optical pair can measure motion. Monitoring programmes should select radar tracks based on viewing geometry, report the ceiling per track, and integrate Sentinel-1 with optical offset tracking.