Properties of seasonal snow cover, such as the extent and duration of snow cover, as well as snow mass or SWE (Snow Water Equivalent), are required for monitoring and modelling e.g. the water and energy balance of the surface of the Earth. State-of-the-art methods for retrieving SWE from Earth Observing satellites rely on passive microwave sensors . Although these provide practically daily global coverage over snow-covered areas and a long history of measurements, the observational requirements for many applications are not met. In particular, observational needs in terms of spatial and temporal resolution, as well as product accuracy, cannot be achieved by present satellites and satellite data products on SWE.
For many hemispheric scale applications, there is a need for timely, regular observations of global snow mass at an intermediate (500 m to 1 km) spatial resolution with 1-2 day revisit time in support of operational environmental prediction. On the other hand, water resource management over alpine watersheds require even higher spatial resolution and revisit time. The user needs for SWE products are diverse and cover a wide range of spatial resolution and temporal revisit. No single sensor can match all the user needs consistently.
CoReH2O (Cold Regions Hydrology High-resolution Observatory), a candidate for ESAâ€™s 7th Earth Explorer , aimed to address some of the observational gaps by providing information on SWE, as well as snow accumulation over glaciers, at a resolution ranging from 200 to 500 meters and a revisit time between 3 and 15 days. The sensor envisaged for CoReH2O was a dual polarization, dual frequency Synthetic Aperture Radar (SAR). However, CoReH2O was finally not selected for further development after Phase A. With the non-selection of CoReH2O, the snow science community has been investigating alternate options for global snow mass retrieval. Dedicated mission concept studies have been launched by Space Agencies in Europe and Canada. The ESA SnowConcepts study, led by FMI with partners including CNR-IFAC (Italy), DLR (Germany), Environment and Climate Change Canada, Gamma Remote Sensing (Switzerland) and ENVEO IT GmbH (Austria), aims to assess the scientific justification for improved snow mass retrieval, the capabilities of present Earth Observing systems, and different technological options for to address a range of geophysical observation needs. The investigated concepts include several options exploiting active microwave observations, ranging from multi-frequency scatterometry to concepts exploiting single- and multiple pass InSAR techniques.
 Takala M., K. Luojus, J. Pulliainen, C. Derksen, J. Lemmetyinen, J-P. KÃ¤rnÃ¤, J. Koskinen, and B. Bojkov, 2011. Estimating northern hemisphere snow water equivalent for climate research through assimilation of space-borne radiometer data and ground-based measurements. Remote Sens. Environ., 115(12): 3517-3529
 Rott, H., S.H. Yueh, D.W. Cline, C. Duguay, R. Essery et al., 2010. Cold Regions Hydrology High-resolution Observatory for Snow and Cold Land Processes. Proc. IEEE, 98(5): 752-765.