  Spaceborne radar remote sensing of the earth system is essential to study
natural and man-made changes in the ecosystem, water and energy cycles, weather
and air quality, sea level, and surface dynamics. A major challenge with
current approaches is the lack of broad spectrum tunability due to narrow band
microwave electronics, that limit systems to specific science variable
retrievals. This results in a significant limitation in studying dynamic
coupled earth system processes such as surface and subsurface hydrology, where
broad spectrum radar remote sensing is needed to sense multiple variables
simultaneously. Rydberg atomic sensors are highly sensitive broad-spectrum
quantum detectors that can be dynamically tuned to cover micro-to-millimeter
waves with no requirement for band-specific electronics. Rydberg atomic sensors
can use existing transmitted signals such as navigation and communication
satellites to enable remote sensing. We demonstrate remote sensing of soil
moisture, an important earth system variable, via ground-based radar
reflectometry with Rydberg atomic systems. To do this, we sensitize the atoms
to XM satellite radio signals and use signal correlations to demonstrate use of
these satellite signals for remote sensing of soil moisture. Our approach
provides a step towards satellite-based broad-spectrum Rydberg atomic remote
sensing.
