Authors: Ashish Joshi, Shefali Agrawal, R. P. Singh

Glaciers are massive bodies of ice formed over many years from compacted snow that slowly flows downhill under the force of gravity. Glacial lakes are formed from melting glacier ice. They can be found on top of glaciers, inside or beneath them, or in front of and beside glaciers. These lakes are usually held back by glacier ice or by loose piles of rocks and sediments left behind by glaciers, called moraines. Studying glacial lakes using optical remote sensing is challenging for several reasons. Cloud cover over mountainous regions and seasonal snow cover over glacial lakes often obstruct satellite observations, making their detection and monitoring challenging using optical remote sensing. Synthetic Aperture Radar (SAR) has the capability to penetrate through clouds, light vegetation, and dry snow, allowing the Earth’s surface to be observed even in cloudy conditions. The penetration capability of SAR is primarily controlled by the wavelength of the transmitted signal, making SAR useful in mountainous and forested regions. Thus, due to its penetration capability, SAR is very effective for detecting glacial lakes, especially when they are covered by dry snow. SAR backscatter from snow is complex: dry snow is relatively transparent, allowing deeper penetration and producing scattering from the ground and within the snowpack (volume scattering), whereas wet snow strongly absorbs the radar signal due to the presence of liquid water, leading to a significant reduction in SAR backscatter. The SAR can penetrate the dry snow up to tens of meters, and thus, lakes buried under snow can be visualized in the SAR image. Earlier SAR Satellite missions like RISAT-1 and Sentinel-1 have detected this phenomenon and observed the glacial lakes under snow.

The NASA-ISRO Synthetic Aperture Radar (NISAR) mission was successfully launched on 30 July 2025 aboard ISRO’s GSLV-F16 from the Satish Dhawan Space Centre (SDSC) SHAR, Sriharikota, India. NISAR is a Multifrequency SAR mission with L-Band (24 cm wavelength) developed by NASA and S-band (9.4 cm wavelength) developed by ISRO. On 4 January 2026, NISAR acquired a satellite image of the glacial lake Masar Tal. Masar Tal is situated at 30.74540° N, 78.98710° E in the Tehri Garhwal district of Uttarakhand, India. It lies at an elevation of approximately 4,738 m above mean sea level within the Himalayas.

Figure 1: NISAR and Sentinel-2 image of Masar Tal, Uttarakhand, India a) NISAR S-band image acquired on 04 Jan 2026, b) Sentinel-2 image acquired on 05 Jan 2026, c) Sentinel-2 image acquired on 07 July 2025,

NISAR possesses both interferometric and polarimetric capabilities, enabling it to map the Earth’s surface for a wide range of applications, including deformation monitoring, ecosystem assessment, cryosphere studies, flood mapping, soil moisture measurement, sea-ice dynamics, and ocean studies. Higher wavelength of NISAR (L&S) allows more penetration capability and helps in delineating subsurface features like paleo channel and snow. The Range-Doppler Single Look Complex (RSLC) product of Masar Tal acquired in RH polarization was orthorectified and analysed. The orthorectified image of NISAR acquired on 4 January 2026 is then compared with the optical images of Sentinel-2 acquired on 5 January 2026 and 6 July 2025. Figure 1 presents a comparison of the NISAR image with the Sentinel-2 optical image. In Figure 1(a), the lake is clearly visible as a darker appearance in the NISAR image due to the interaction of the radar signal with lake water, whereas, in Figure 1(b), it cannot be visualized in the Sentinel-2 optical image as it is completely covered by snow and therefore appears white. In Figure 1(c), the lake is visible in the Sentinel-2 optical image during the snow-free season. Figure 2 shows the 3D visualization of the lake in the NISAR S-band image by overlaying the image over Digital Elevation Model (DEM).

Figure 2: NISAR image of Masar Tal, Uttarakhand, India acquired on 04 Jan 2026 (3D view)

References:

• Singh S. K., Rajawat, A. S., Rathore, B. P., Bahuguna, I. M., and Chakraborty, M. (2015), Detection of glacier lakes buried under snow by RISAT-1 SAR in the Himalayan terrain. Current Science, 109, 1728-1732.
• Singh R.P. (2024), https://science.iirs.gov.in/radar-detects-glacier-lake-buried-in-snow/