RESEARCH COMMUNICATIONS CURRENT SCIENCE, VOL. 109, NO. 9, 10 NOVEMBER 2015 1728 *For correspondence. (e-mail: sushil@sac.isro.gov.in) Detection of glacier lakes buried under snow by RISAT-1 SAR in the Himalayan terrain S. K. Singh*, A. S. Rajawat, B. P. Rathore, I. M. Bahuguna and M. Chakraborty Geo-Science and Applications Group, Space Applications Centre (ISRO), Ahmedabad 380 015, India Synthetic aperture radar (SAR) signals penetrate through the dry snow and cloud providing crucial data over the Himalayan temperate glaciers and comple- ment the optical images. In the present study, RISAT- 1 C band and AWiFS images of winter/ablation period over Samudra Tapu and Gepang Gath moraine dammed lakes (MDLs) in Himachal Pradesh have been analysed. Backscattering coefficient of the lake was observed to be low throughout the year. Penetra- tion depth of SAR into dry snowpack was calculated to vary from 4 to 22 m for a range of snow density (0.1–0.5 g/cm 3 ), whereas it was estimated to be 1.20– 2.01 m based on ground observations for 30 January and 24 February 2013. The present study provides results of RISAT-1 C-band penetration up to ~2 m through the snowpack to detect MDLs in the Himala- yan terrain. The detection of MDLs using the back- scattering images of winter season was validated with synchronous AWiFS sensor images. Keywords: Backscattering coefficient, glacier lakes, snow and cloud, synthetic aperature radar. SNOW/ CLOUD cover obscures the land-cover features in the Himalayan region leading to availability of appropriate satellite data being considerably restricted in the optical region. Active microwave synthetic aperture radar (SAR) sensors at lower frequencies are able to penetrate through the atmosphere and thus become important during cloudy conditions 1,2 . Snow is transparent at microwave wave- lengths, and depending on frequency radar penetration depth can reach up to tens of metres under dry snow con- ditions 3,4 . Backscatter received at SAR antenna is a sum of surface scattering at the air/snow interface, volume scattering within the snowpack, scattering at the snow/ soil interface and volumetric scattering from the underly- ing surface 5 . SAR data have provided useful information over the complex glaciated terrain; however, not much in- formation is available for understanding glacier signature in the Himalayan region 6,7 . ERS-1 data were studied over five lakes in Northern Montana region to understand the different processes of frozen lake and break-up date over a decade-scale time-frame 8 . TerraSAR-X and Radarsat-2 SAR data were successfully used to map and monitor glacial lake detection during snow- and ice-free seasons, emphasizing a need for integrated multi-level approach 9 . Lakes at high altitude fluctuate seasonally due to change in melt rate during accumulation and ablation period. The accessibility of these lakes is restricted in the optical spectrum due to the cloud cover in ablation period and snow cover in the accumulation period. RISAT-1 C band SAR sensor carries multi-mode SAR system at different resolution and swath, and has enhanced the imaging capacity in the microwave region along with other existing satellite systems 10,11 . The pre- sent communication presents a case study of the use of winter-time RISAT-1 SAR data in MRS mode to detect snow-buried glacial lakes using multi-temporal images covering Samudra Tapu and Gepang Gath Moraine dammed lakes (MDLs) in Chandra sub-basin, Lahaul and Spiti region, Himachal Pradesh, supported by synchro- nous AWiFS optical data. Figure 1 shows the location of the MDLs of Samudra Tapu and Gepang Gath glaciers. Field photograph (Figure 1) shows the extension of Samudra Tapu lake and associ- ated geomorphological features. Samudra Tapu is the sec- ond largest glacier in the Chandra sub-basin. The snout of the glacier is located at 3230N lat. and 7732 E long., at an altitude of 4200 m amsl and about 10 km SW of the famous Chandra Tal Lake 12 . Gepang Gath is a debris cov- ered glacier with an area of 13.1 sq. km, located at 3231N lat. and 7714E long. and at an altitude of 4300 m amsl. In the present study, RISAT-1 MRS data from winter period (January–February 2013) and ablation period (June–November 2013) have been used. Table 1 provides the technical specifications of RISAT-1 MRS mode. The synchronous optical AWiFS data were used to validate the existing conditions during SAR data acquisition dates. ERDAS Image processing software has been used to process the RISAT-1 and AWiFS data. Digital number (DN) of RISAT-1 MRS mode was converted into back- scattering coefficient for each pixel using meta data. Equation (1) is used to estimate the backscattering coeffi- cient ( 0 ) as given below: 0 10 db 10 centre sin 20log (DN ) 10 log , sin p p I K I             (1) where I p is the incidence angle at the pth pixel, I centre the incidence angle at the centre of the scene, K db is the abso- lute calibration constant and DN is the average pixel intensity. Area of interest (AOI) of backscattering coeffi- cients data over MDL was extracted using the aoi tool of ERDAS. Average  0 was calculated over the lake region for temporal RISAT-1 MRS data. Snowfall takes place due to the NW disturbances and the region remains under sub-zero temperature conditions during February in Chandra basin. Snow remains dry in nature, which is a mixture of air and ice crystals causing volume scattering losses more prominent during SAR interaction.