Mountain watersheds play an outsized role in global hydrology and function as “water towers” for 1/6th of the global population and its economy. Mountain snow provides a key source for agricultural and urban water supply, hydropower, recreation, and ecosystems. These historically reliable natural reservoirs are susceptible to both climate change and socioeconomic pressures. Despite the need for information about snow water equivalent (SWE) in the mountains, current spaceborne capabilities lack the ability to track the seasonal evolution of mountain snow across the necessary spatial and temporal scales. Properly capturing seasonal snow dynamics is necessary to constrain models and their projections of current and future snow-derived water resources. My presentation reports on the development of a new mission concept for the remote sensing of terrestrial snow. We propose a P-band Signal of Opportunity (SoOp) synthetic aperture radar (SAR) concept to take advantage of signals already being broadcast by existing communication satellites. The phase and amplitude of the signal reflected from terrestrial surfaces can be measured using receivers on a constellation of small satellites. P-band signals with wavelength 0.8 to 1.0 m are attractive because they penetrate deep snow and forest canopies found in mountain environments, with limited sensitivity to snow microstructure and stratigraphy that can confound retrievals at shorter wavelengths. Measured phase changes in the reflected signal yield SWE of dry snow independent of depth or density, or snow depth if the snow is wet. Moreover, the amplitude of the reflected signal yields retrievals of root zone soil moisture, which provides additional insight into the antecedent conditions before snow and the partitioning and fate of snow meltwater into soil water storage vs. runoff. The concept proposes using a formation flight of small satellites to allow the use of interferometric SAR processing to obtain a spatial resolution of a few hundred meters and a swath widt