Modeling Soil Moisture Retrieval Errors in the Time-Series Ratio Method

The use of a "time-series ratio" soil moisture retrieval approach is under consideration for the upcoming NASA/ISRO synthetic aperture radar (NISAR) mission's soil moisture product. As such, it is of interest to characterize the algorithm's anticipated error budget as part of prelaunch activities. This letter develops an error model to estimate retrieval errors for the proposed algorithm. The model accounts for error contributions from speckle and thermal noise as well as uncertainties that arise as part of the retrieval process. Spatial and temporal behaviors of the retrieval errors are determined using a soil moisture active passive (SMAP)-based soil moisture climatology. The results show that soil moisture retrieval errors from the time-series ratio method meet the 0.06 $\text{m}<^>{3}/\text{m}<^>{3}$ unbiased root mean square error (URMSE) performance metric established for the NISAR soil moisture product.