| Abstract: |
Root zone soil moisture (RZSM), is an essential variable in meteorology, hydrology, and agriculture. Passive and active microwave techniques are the most mature for soil moisture remote sensing, however their penetration depth is proportional to wavelength, typically around 5~cm at L-band (e.g. SMOS or SMAP). Measuring RZSM (the top meter) requires lower frequencies. (VHF and P-band),P-band presenting significant difficulties for spaceborne measurements, due to the required antenna size, presence of radio-frequency interference (RFI), and competition for spectrum allocations. Signal of Opportunity (SoOp) is a new approach to remote sensing, re-utilizing powerful signals in bands allocated for communications. Reflectometry uses a forward-scatter geometry, presenting a high signal to noise ratio and different response to vegetation than either backscatter or radiometry.
Recent airborne experiments have demonstrated the sensitivity of reflected signals from geostationary communication satellite transmissions in 230-270 MHz and 360-380 MHz. Reflectivity can be related to the dielectric constant and, through established semi-empirical models, temperature, salinity, soil texture, and soil moisture. Algorithms for retreival of surface reflectivity from the correlation of direct and reflected signals have been developed, incorporating instrument calibration through the use of antenna swapping and noise loads and vicarious calibration during over-water flights. We plan to present early results from the processing of these data and comparison vs. in-situ measurements.
The small power requirements of a passive SoOp receiver also make it an ideal payload for cubesats and unpiloted aerial vehicles (UAVs) and preliminary analysis of the feasibility of a cubesat mission have begun. In conclusion, we will present an evaluation of SoOp methods as a complement to microwave radiometry.
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